Subcycle Transient Scanning Tunneling Spectroscopy with Visualization of Enhanced Terahertz Near Field
- Shoji YoshidaShoji YoshidaFaculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, JapanMore by Shoji Yoshida,
- Hideki HiroriHideki HiroriInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, JapanMore by Hideki Hirori,
- Takehiro TachizakiTakehiro TachizakiDepartment of Optical and Imaging Science and Technology, Tokai University, Hiratsuka-shi, Kanagawa 259-1292, JapanMore by Takehiro Tachizaki,
- Katsumasa YoshiokaKatsumasa YoshiokaDepartment of Physics, Graduate School of Engineering, Yokohama National University, Yokohama, 240-8501, JapanMore by Katsumasa Yoshioka,
- Yusuke ArashidaYusuke ArashidaFaculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, JapanMore by Yusuke Arashida,
- Zi-Han WangZi-Han WangFaculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, JapanMore by Zi-Han Wang,
- Yasuyuki SanariYasuyuki SanariInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, JapanMore by Yasuyuki Sanari,
- Osamu TakeuchiOsamu TakeuchiFaculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, JapanMore by Osamu Takeuchi,
- Yoshihiko KanemitsuYoshihiko KanemitsuInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, JapanMore by Yoshihiko Kanemitsu, and
- Hidemi Shigekawa*Hidemi Shigekawa*E-mail: [email protected]Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, JapanMore by Hidemi Shigekawa
Copyright © 2019 American Chemical Society
Abstract
The recent development of optical technology has enabled the practical use of a carrier-envelope phase-controlled monocycle electric field in the terahertz (THz) regime. By combining this technique with metal nanostructures such as nanotips, which induce near-field enhancement, the development of novel applications is anticipated. In particular, THz scanning tunneling microscopy (THz-STM) is a promising technique for probing ultrafast dynamics with the spatial resolution of STM. However, the modulation of the THz waveform is generally accompanied by an enhancement of the electric field, which is unknown in actual measurement environments. Here, we present a method enabling direct evaluation of the enhanced near field in the tunnel junction in THz-STM in the femtosecond range, which is essential for the use of the THz near field. In the tunneling regime, it was also demonstrated that the transient electronic state excited by an optical pulse can be evaluated using the THz-STM, and the ultrafast carrier dynamics in 2H-MoTe2 excited by an optical pulse was reproducibly probed.
When a nanoscale metal structure is irradiated by electromagnetic waves, the near field formed on the material surface is enhanced in a confined space far beyond the diffraction limit.(1) With a metal probe (nanotip), it is possible to obtain an intense terahertz (THz) near field in a local region directly beneath the nanotip owing to the field enhancement effect.(2) Therefore, this near field not only allows THz nanoscopy of structures much smaller than the wavelength of light beyond the diffraction limit(3) but also provides a platform for studying strong-field-driven dynamics.(4,5) Furthermore, the practical use of light with a controlled carrier-envelope phase (CEP) has recently become possible. In particular, in the case of THz waves, it is possible to spontaneously obtain CEP-stable light consisting of almost monocycle pulses through optical rectification by irradiating ultrashort optical pulses to a THz antenna and various nonlinear optical crystals.(6)
The application of this technology is expected to open new research areas, such as nanodevices that coherently control tunneling currents,(2,7) THz electron guns,(8,9) and new measurement and processing techniques as a result of combining THz waves with near-field scanning optical microscopy (NSOM) and scanning tunneling microscopy (STM).(2,10−15) For example, an ultrashort-pulse electron beam obtained by controlling the photoelectron emission from a metal probe in the time domain of a THz subcycle has realized new ultrafast electron microscopy, which enables the observation of ultrafast dynamics at the atomic scale.(9,16) By irradiating the tunnel junction with THz pulses, an ultrafast tunneling current has been driven, and its direction can be coherently manipulated by changing the CEP of the THz pulse.(15)
Recently, THz-STM has been developed using the above-described mechanism, in which the tip-enhanced THz near field is operated as a transient tunnel voltage VTHz, and the THz-induced tunneling current ITHz is measured as a signal. Since the tip enhancement reaches a factor of 1 × 105, it is possible to obtain the electric field strength necessary for the ultrafast driving of tunneling current with a relatively weak incident THz pulsed electric field.(2) In THz-STM, since the thermal expansion of the tip and sample is small,(2,10) time-resolved measurement can be performed by measuring the current at the moment the bias voltage is applied. The carrier dynamics of optically pumped InAs nanodots and molecular orbitals of a vibrating pentacene single molecule were visualized in real space with an atomic-level locality of ITHz and a high temporal resolution of sub-picoseconds.(10,11)
With the precise control and utilization of the electric field inside optical pulses, further extension toward the extreme limits of measurement and control can be expected. However, although THz electromagnetic waves are enhanced by coupling the antenna with the nanotip, as described above, the electric field waveform is also modulated simultaneously, as schematically shown in Figure 1a. This has been experimentally confirmed in NSOM and THz-STM.(15,17) Although the theoretical description of the antenna effect and other models has been discussed,(17) to realize precise measurement and control, it is essential to develop a new method for directly measuring and quantitatively evaluating the waveform in actual measurement environments.
Figure 1
Figure 1. Explanation of measurement principle. (a) Incident electric field and its modulation by a nanotip. (b, c) Schematic illustrations of measurement methods in photoemission (photofield emission and above-threshold photoemission) regime and tunneling regime, respectively. The barrier height is modulated by the THz electric field, and photoelectron emission (hot electron tunneling) is induced by the simultaneous irradiation by the probe light of 517 nm (1035 nm). (d) Dependence of photocurrent on the dc bias voltage VDC applied to the HOPG sample with respect to the nanotip while the tip was irradiated by the probe light and the THz electric field was turned off. The photocurrent observed for VDC ≤ 0 was due to above-threshold photoelectron emission caused by the process of three-photon absorption.
Here, we present a new method enabling the visualization of the enhanced THz near-field waveform over a nanotip and the tunnel junction by controlling the photoelectron emission and tunneling current in the femtosecond range. In the tunneling regime, it was also demonstrated that the transient electronic state excited by an optical pulse can be evaluated using the THz field as the instantaneous bias voltage, and the ultrafast carrier dynamics in 2H-MoTe2 excited by an optical pulse was reproducibly probed.
Results
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Experimental Setup
In this study, we performed experiments using a system constructed by combining an ultrashort-pulse laser and an ultrahigh-vacuum STM system (base pressure: <1 × 10–7 Pa), which enables precise manipulation of the nanotip position (Figure S1). A tungsten (W) STM tip coated with PtIr was used as the nanotip for measurement. As shown in Figure 1b,c, the nanotip was irradiated with two types of optical pulses: a THz light and a probe light. The THz pulse was generated by irradiating a LiNbO3 crystal with an infrared (IR) pulse (1035 nm, 309 fs, 1 MHz), and it was almost a monocycle with a large first peak of 1 ps width, as shown in Figure 2g. For the probe light, IR light and 517 nm light, which was generated by converting the IR light into a second-harmonic wave using a β-BaB2O4 (BBO) crystal, were prepared for the two types of measurements in which the photoelectron emission current (Figure 1b) and hot electron tunneling current (Figure 1c) were detected as a signal.
Figure 2
Figure 2. THz electric field modulation by nanotip. (a) Optical micrograph of the ultrasharp PtIr-coated W nanotip (tip 1). (b) Map of photoelectrons obtained by scanning the probe light (517 nm) on the nanotip. The spot position was moved by 31 steps of 4.5 μm in the x direction; then, after being shifted by 4.5 μm in the y direction, it was moved back along the x direction in 31 steps. This procedure was repeated, until seven steps had been completed in the y direction (the entire scanned area was 139.5 × 31.5 μm2). (c) Map of ITHz at zero delay time. (d) Optical micrograph of the PtIr-coated W nanotip fabricated by ordinary chemical etching (tip 2). (e) Map of photoelectrons obtained by scanning the probe light (517 nm) on the nanotip. (f) Map of ITHz at zero delay time. (g) Waveform of the incident THz light. (h–j) THz near-field waveforms measured at A, B, and C in (c) and (f), respectively. Red and blue lines show the results for tips 1 and 2, respectively. (k) Waveform obtained by integrating the electric field in (g). Vs = +10 V, IVIS = 14 mW, ETHz,pk = 4.1 kV/cm (electric field of the maximum peak), frep = 200 kHz, and tip–sample distance ≈ 1000 nm. Here, an HOPG sample was used as the counter electrode.
In a previous work, the THz near field at the metal tip apex was evaluated using the THz-field-driven photoemission (photofield emission) from the nanotip.(9,18) In our case, by using the STM setup, we realized two-dimensional (2D) analysis of the enhanced near field, including measurements in the tunnel regime. As an initial stepwise comparison, highly oriented pyrolytic graphite (HOPG), a typical material, and Bi2Se3, which has been well-studied by photoelectron spectroscopy,(19) were used as samples. The relaxation of the excited electronic states in these materials is sufficiently fast compared with temporal oscillation of the THz electric field, enabling probing of the THz pulse waveform. In the case of the Bi2Se3 sample, there are fast (<1 ps) and slow (∼10 ps) relaxation components for high-energy (E – EF > 1 eV) and low-energy regions, respectively. Hot electrons excited up to the high-energy bulk and surface states are cooled to a lower state within 1 ps. Such an ultrafast response of the hot electrons allows us to probe the THz near-field waveform in the STM tunnel junction.
In the last part of this paper, we show the result of time-resolved measurement using 2H-MoTe2 as a sample. We excited (pumped) the sample using IR light and measured the dynamics of the excited state using a THz pulse as the instantaneous bias voltage to be applied between the STM tip and sample.
Visualization of Terahertz Near-Field Waveform by Photoelectron Emission
First, we present the method and results when photoelectron emission was used for measurement. When irradiating a nanotip, photoelectron emission is induced by multiphoton absorption of the probe light (517 nm). The generated photoelectrons flow from the nanotip to an HOPG sample upon applying direct-current (dc) bias voltage VDC to the sample with respect to the nanotip and are detected by a current preamplifier. Since the photoelectron generation efficiency depends on the height of the potential barrier at the W nanotip surface, the photocurrent intensity changes with the profile of the barrier height, which is modulated by VDC and the THz near field generated on the nanotip, as shown in Figure 1b.
Figure 1d shows a current–voltage (I–V) curve obtained by varying the applied sample bias voltage VDC, while the tip was irradiated by the probe light with the THz electric field turned off. The photocurrent observed for VDC ≤ 0 was due to above-threshold photoelectron emission caused by the process of three-photon absorption. The photoelectron current increased monotonically as a function of VDC. It was also confirmed that the THz-induced photoelectron current ITHz, which was generated by the bias voltage produced by the THz pulse in addition to VDC, linearly changed with the incident THz field strength, ITHz ∝ ETHz, in this bias voltage range (Figure S2). Therefore, it is considered possible to investigate a THz near-field waveform by measuring ITHz while varying the delay time td between the THz pulse and the probe pulse. To detect the weak signal, the pump laser generating the THz pulses was chopped at 430 Hz, and ITHz(td) was measured by the lock-in detection method. The delay time was varied at a rate of 1 ps/s. All experiments were performed at room temperature.
Figure 2 shows the results obtained using an HOPG sample as the counter electrode of the nanotip. Figure 2a,d shows optical micrographs of the two types of nanotips used in this study. Both tip 1 and tip 2 were fabricated by electropolishing, but tip 1 was sharper and had a longer tapered area (see Figure S1b for more detail). The spot position of the probe light pulse was two-dimensionally scanned over the nanotip, and the I–V curve (corresponding to Figure 1d) and waveform (Figure 2) measurements were performed at each pixel position.
First, we verified that the scanning system operated correctly. When an electric field is applied, the distance between the positions probed on the tip and sample affects the measurement. Therefore, we measured the photoelectron current for VDC = 0 at each pixel, that is, the current obtained by photoelectrons reaching the HOPG sample without a THz electric field and sample bias voltage VDC (corresponding to the value at VDC = 0 in Figure 1d). Figure 2b,e shows the results of mapping. Each map approximately reflects the shape of the nanotip, but the intensity is low at the tip apex, because the diameter of the nanotip is smaller than the light spot size. THz waveforms were obtained by measuring the THz current ITHz(td) at each pixel position while changing td and maintaining VDC at +10 V. Since the photoelectron current was almost linear with respect to the voltage around VDC = +10 V, as shown in Figure 1d, ITHz only depends on ETHz when photoelectron emission occurs. Thus, ITHz(td) provides a direct reflection of the waveform of the THz near field.
In ref (9), the strength of the incident THz field was 100 kV/cm, and the enhanced near field obtained by time-of-flight measurement was 9 MV/cm, which was much higher than the dc voltage (VDC = 30 V/(3 mm)). Therefore, photoelectrons were steered back to the nanotip side, and only half the shape of the THz field was not obtained. They obtained the whole shape of the THz electric field by reversing the orientation of the setup. In our case, the strength of the incident THz field was 4.1 kV/cm, and the effective bias voltage between the STM tip and sample estimated by using the I–V curve was 0.6 V, which was lower than the applied bias voltage of 10 V. In addition, by setting the tip–sample distance close to ∼1 μm and VDC to +10 V, the strong dc electric field between the tip and sample produced a photoelectron current larger than ITHz(td). Therefore, it was possible to measure the full change in the current, while the THz electric field direction was changed from the nanotip to the sample and vice versa.
Figure 2h–j shows the THz near-field profiles obtained for the two types of nanotips by performing measurements at A, B, and C in Figure 2c,f, respectively. On the one hand, when measuring the THz waveform, VDC was fixed at +10 V. On the other hand, as shown by the I–V curve in Figure 1d, the photocurrent changed almost linearly with the voltage in the vicinity of VDC = +10 V. Therefore, from the slopes (dI/dV) of the photoelectron I–V curves obtained at each point for VDC = +10 V, it was possible to convert each waveform to an effective THz voltage VTHz using the relationship of dI/dV = ITHz/VTHz, which is plotted on the vertical axis of each figure. Figure 2c,f shows maps of ITHz at zero delay time for tips 1 and 2, respectively. At the tip apex C, the peak values of VTHz for both tips were larger than those at A and B owing to the enhancement effect of the nanotip; in particular, the peak value was approximately twice as large for tip 1. Figure 2g shows the incident THz waveform obtained by electro-optic (EO) sampling (see Figure S1). In the simple model of antenna theory, the electric field generated by nanotip enhancement is obtained by integrating the original electric field,(17) which is shown in Figure 2k. However, as shown in the figures, the measured waveform is different from the calculated one, emphasizing the importance of directly measuring the modulated electric field (see Figures S3 and S4 for more detail). By using STM tips with different tapering, it was shown that the near-field waveforms generated at the two different types of tips were different, even for the same incident THz electric field. Recently, this tip dependence has been more clearly observed using various tips of different shapes.(15)
These results indicate that it is essential to directly evaluate the near field in the actual measurement environment to obtain a deeper understanding of the phenomena and for the practical use of THz techniques.
Measurement of THz Near-Field Waveform in the Tunnel Regime
We showed that the modulation of the electric field can be characterized using the photoelectronic emission. On the one hand, depending on various parameters such as the nanotip–sample distance, vacuum barrier height, THz field strength, and optical excitation energy, we can theoretically consider the regimes of photoelectronic emission, field emission, and tunneling;(9,18) however, this has not been sufficiently confirmed experimentally. On the other hand, since the present setup employs an STM system, measurement in the tunnel regime is possible. In the measurement by THz-STM with electronic control by an electric field, evaluation of the electric field waveform under the measurement conditions becomes indispensable. To proceed, we used the probe that yielded the near-field profile of the tip (through the photoelectronic emission) to perform the measurement in the tunnel regime, that is, THz-STM measurement.
Figure 3a shows a schematic illustration of the experimental setup. VDC and the current were set to 2 mV and 2 pA, respectively, to bring the nanotip closer to the tunnel region. The bias voltage was set to this low value instead of increasing the current to avoid the possibility of the sample breaking when the current increases. To suppress photoelectron emission, the original IR light (1035 nm) was used for the probe. When the tip was retracted 100 nm, the signal, which was observed in the range of 100 μm for the case of photoelectrons, disappeared, thus confirming that the signal originated from the tunnel current. HOPG and Bi2Se3 were used as samples. The signal was significantly weaker for HOPG, approximately 1/100 of that for Bi2Se3, and a detailed analysis shown in Figure 3 was performed for Bi2Se3 (see Figure S5 and its caption for more detail). The Bi2Se3 sample was prepared by cleavage in vacuum.
Figure 3
Figure 3. Measurement in tunnel regime. (a) Schematic illustration of the measurement setup. The IR pulse wavelength was set to 1035 nm. The direction of the THz pulse generated by a lithium niobate crystal was inverted by CEP control using the HWP. The THz field strength was continuously controlled using a wire grid polarizer. (b) Tunnel junction band diagrams (I) without and (II) with IR pulse and THz pulse irradiation. (c) Electrical over-stress measurement results of the incident THz waveform. By controlling the HWP angle α and adjusting the phase of the CEP, ϕCEP, the electric field waveform can be reversed. (d) Tip-enhanced near-field waveform obtained by photoelectron measurement. (e) ITHz as a function of delay time (VS = 2 mV, IT = 2 pA, IIR = 5 mJ/cm2, frep = 0.5 MHz). (f) ITHz–VTHz curves obtained for td = −5 ps and 0. (g) ITHz as a function of delay time simulated using the ITHz–VTHz curve. (h) Schematic to explain the method of simulation to obtain the spectra shown in (g). Analysis was performed for Bi2Se3.
Figure 3b shows schematic illustrations of the measurement mechanism in the tunnel regime, (I) without and (II) with irradiation. Bi2Se3 used for the analysis has a narrow band gap of ∼0.2 eV in the bulk,(19) and a metallic surface state called a Dirac cone is formed because of its topological property. Since Bi2Se3 is a degenerate n-type semiconductor owing to Se vacancies, the Fermi level is located above the bulk conduction band minimum.(20) Therefore, although downward band bending occurs at the surface, it is not modulated by optical excitation because of the metallic characteristics of Bi2Se3.
Here, the tunnel current was measured instead of photoelectrons, which were observed for Figure 2. To detect the weak signal of the THz-induced tunnel current, the IR pulse used for THz generation was chopped at 430 Hz, and ITHz(td) was measured by the lock-in detection method, as was performed for the photoelectron measurement. If an IR light pulse is irradiated while the tunnel barrier is changed owing to THz pulse irradiation, hot electrons are produced and enhance the tunnel current ITHz, where ITHz depends on the delay time between the IR pulse and the THz pulse [Figure 3b, (II)]. Since the relaxation of hot electrons in a high-energy state occurs faster than the oscillation of the THz electric field, the delay time dependence of ITHz was expected to represent a THz near-field waveform similar to that obtained by photoelectron emission measurement; however, the nonlinearity of the I–V curve affects the spectrum, which will be discussed later.
To probe the photoexcited electronic state, the VTHz dependence of ITHz was measured with the delay time td fixed to zero. For this purpose, a λ/2 wave plate (HWP) for the THz range and a wire grid polarizer (WGP) were introduced in this experiment, which enabled VTHz to be continuously changed in both the positive and negative voltage ranges.(15,21) When the angle of the HWP, α, shown in Figure 3a, is rotated by π/2, the phase of the CEP, ϕCEP, changes by π, and the direction of the electric field is reversed.
Figure 3c shows the waveform of the incident THz pulse measured at HWP angles of α = 0 and α = π. The THz waveform was inverted vertically as expected. Thereby, the near-field waveform at the tip apex was also inverted, as shown in Figure 3d. These near fields worked as the transient bias voltages of VTHz1 (ΔϕCEP = 0) and VTHz2 (ΔϕCEP = π) applied to the tunnel junction. Since the tip was different from those shown in Figure 2, the waveform was slightly different from those shown in the figures. However, a common feature is that the near-field waveform is modified from the incident THz waveform. The strength of the near field shown in Figure 3d is slightly asymmetric, which may be due to the slight change in the spot position while changing the angle. However, a linear relationship was obtained between the strength of the incident THz field and that of the obtained near field (see Figure S6).
Next, the delay time dependence of ITHz induced by these two near-field waveforms was measured in the tunnel region, and the results are shown in Figure 3e. Measurements were performed for VTHz = VTHz1 and VTHz2. The THz-pulse-induced tunnel current ITHz flowed from the sample to tip when α = 0 and from the tip to the sample when α = π/2, but a spectral shape similar to the near-field waveform was obtained. The fact that a delay time dependence of ITHz was observed implies that the tunneling probability transiently changed with the change in the electronic state induced by the IR pulse irradiation. To confirm this effect, transient I–V curve measurements were performed for td = −5 ps and 0 by measuring ITHz while continuously sweeping VTHz by rotating the WGP and HWP.
The obtained results are shown in Figure 3f. Since the transient I–V curve for td= −5 ps represents the transient electronic state 5 ps after IR excitation, where the photoexcited state of both the tip and sample are almost completely relaxed, the shape of the transient I–V curve is considered to be similar to that of the static I–V curve without the effect of the optical pulse (Figure S7). In fact, a semiconducting electronic state was obtained. In contrast, the slope of the I–V curve increased for the case of td = 0, and the I–V curve became similar to that for a metallic electronic structure. As shown in the band diagram (II) in Figure 3b, this change corresponds to the reduction of the effective vacuum barrier by the excitation of hot electrons; the I–V curve reflects the optically excited transient electronic state.
By using the ultrafast electronic response in the tunnel junction, we can evaluate the electric field waveform by the tunnel current. The transient I–V curve depends on the delay time in accordance with the dynamics of the photoexcited electronic state. Since the relaxation of the hot electrons was fast, the excited state was almost relaxed at td = −5 ps, and ΔIexc (VTHz) [=ITHz (VTHz, 0 ps) – ITHz (VTHz, −5 ps)] corresponds to the change in the I–V curve owing to photoexcitation. Therefore, ITHz (td) shown in Figure 3g was simulated from the relationship shown in Figure 3h, with the near field being considered to be the voltage applied between the probe and sample. (See Figure S8 for more detail.) Unlike the case of photoelectrons, the time-resolved waveform measured by the tunneling current has a different form from the near-field waveform, because the I–V curve is nonlinear.
The peak around td = 0 is slightly wider in the experimental curve than in the simulated curve. This is thought to be due to the relaxation time of hot electrons not being taken into consideration in the simulation. It is considered that we can measure the relaxation process of hot electrons by measuring the delay-time dependence of the transient I–V curve in more detail, which is difficult at present. Since the THz near-field waveform has a tail of over 1 ps in the region of td < 0, the response of an electronic state with a similar relaxation time cannot be measured separately from the effect of the tail. It is expected to be possible to evaluate the excited state of a sample by conducting experiments using light pulses and THz pulses of a shorter time width in the future.
In addition, as clearly shown in Figure 3e, the waveform obtained by the tunnel current has a large positive–negative asymmetry (e.g., the ratio between the intensities of the first and second peaks) compared with the waveforms obtained by photoelectron emission shown in Figure 2. This is because the THz near field itself has positive–negative asymmetry, as shown in Figure 3d, and the tunnel current induced by the near field is further rectified (the asymmetry is enhanced) by the nonlinearity in the I–V curve, as schematically shown in Figure 3h. Although accurate characterization of the THz waveform with the tunnel current requires a correction using the I–V curve, as shown here, it was confirmed that the THz near-field profile can be observed even in the tunnel current regime.
As has been shown, transient I–V measurement is potentially a very powerful technique for elucidating the excited electronic states and dynamics of materials with atomic spatial resolution. For this purpose, it is indispensable to evaluate the tip-enhanced near field waveform. Since the tip-enhanced near field can be changed by adjusting the experimental conditions, environmental conditions, and the shape of the tip, it is extremely important to directly confirm the near-field waveform using the method described in this paper, in addition to by simulation such as by the finite difference time domain (FDTD) method.
Measurement of Time-Resolved Signal using THz Near Field as a Bias Voltage
In the last part, we show the result of time-resolved measurement using 2H-MoTe2 for a sample. We excited (pumped) the sample using IR light and measured the dynamics of the excited state using the THz electric field as the instantaneous bias voltage applied between the STM tip and sample. The bulk 2H-MoTe2 used in this study has an indirect band structure, but since it has direct band gap of 1.1 eV at the k point,(22) it is possible to induce the optical transition with the IR laser of our system (1035 nm).
Figure 4 shows the experimental scheme and a typical result. Figure 4a shows an STM image of the sample surface. The bright spots observed in the image are attributed to intrinsic defects such as vacancies.(23)Figure 4b shows the delay-time dependence of ITHz (red line). The enhanced near field of the THz probe pulse used for measurement, whose waveform was evaluated by the photoelectron method (blue line), is superimposed. The insets show the relationship between the pump and probe lights for td < 0 and td > 0. On the one hand, for td > 0, similar to the case of Bi2Se3, the THz near-field waveform was observed through the fast response of the sample such as hot electrons, although the signal was small. On the other hand, for td < 0, since the THz pulse was irradiated after the IR pulse, as shown in the inset, ITHz reflects the dynamics of the excited state due to photocarrier generation. Since 2H-MoTe2 is a conventional semiconductor, in contrast to Bi2Se3, photocarriers excited by optical pulses induce a surface photovoltage,(24,25) leading to a change in ITHz. Thus, the delay-time dependence of ITHz for td < 0 represents the carrier decay process in 2H-MoTe2. Figure 4e illustrates energy band diagrams of the STM tunneling junction in the steady state (without illumination) and the THz pulse-induced nonequilibrium state at the peak of the THz field (I–III). In the nonequilibrium state, when both THz and IR pulse illuminate the tunnel junction, photocarriers generated by IR pulse screen the THz field in semiconductor, and THz field is fully applied to the vacuum gap, as shown in II. Without photocarriers, THz near field partially penetrates the semiconductor surface and induces surface band bending shown in I, which reduces effective THz field applied to the vacuum gap and suppresses ITHz. Therefore, in the case of III, ITHz decreases as td becomes longer where photocarriers were decreases due to carrier recombination.
Figure 4
Figure 4. Transient electronic dynamics obtained by THz-STM for 2H-MoTe2. (a) STM image of the sample (sample bias VS = 1.2 V, tunnel current It = 10 pA). Bright spots indicated by D in the magnified image are defects. (b) Delay-time dependence of ITHz (red line). Near-field waveform at the STM tip apex used for measurement, which was obtained by the photoelectron method (blue line) is superimposed. (insets) The relationship between the pump and probe lights for td < 0 and td > 0. (c) Example of spectrum obtained over a wide delay-time range. The black line is a fitting curve with an exponential function of two components. (d) Light-intensity dependence of ITHz. ITHz has two components at 1.5 mJ/cm2, but a one-component fitting is in good agreement with ITHz at 1.1 and 0.7 mJ/cm2. (VS = −3 mV, IT = 0.6 pA, frep = 1 MHz). (e) Schematic illustrations of (left) without illumination, (I) THz illumination without IR, (II) THz illumination just after IR excitation, and (III) THz illumination after IR excitation with rather large td in the range of τc (carrier recombination lifetime).
Figure 4c shows the results of measurement over a wide delay-time range, focusing on the td < 0 side. Because of the limited range of the delay time, the appearance of the entire long decay component was not clear. Here, we assumed a simple model, and the relaxation of the excited state was well-fitted with a two-component exponential function with the lifetimes of τfast = 18 ps and τslow = 606 ps. On the one hand, τslow, which was considered to be the carrier recombination lifetime, was also confirmed by optical pump–probe measurement.(22) On the other hand, the fast relaxation time τfast appeared when the pump fluence was increased more than 1.5 mJ/cm2 as shown in light intensity dependence (Figure 4d). The result suggests that the fast carrier recombination was due to the capture of electrons and holes by defects via Auger processes, as was observed in other transition-metal dichalcogenide monolayer (TMDC) materials,(26) because the mechanism is dominant when the photocarrier density is high.(27) Since the signal originated from the hot electrons in the case of the Bi2Se sample, the conductance increased for positive and negative bias voltages to give similar results. In the case of 2H-MoTe, since band bending and other surface characteristics are involved, the bias-voltage dependence of signal may become more complicated. We leave a detailed analysis to future work.
Conclusion
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The combination of CEP-controlled monocycle pulses in the THz regime with metal nanostructures such as nanotips has interesting novel applications, which require a precise understanding of the THz waveform. However, when a THz electric field is irradiated to a nanostructure, the electric field is amplified, although its waveform is also modulated. Therefore, it is urgently necessary to develop a new method to directly measure and evaluate near-field waveforms in actual measurement environments.
We have succeeded in developing a new spectroscopy technique capable of visualizing the spatially resolved THz near field at a nanotip by controlling the photoelectron emission from the nanotip with subcycle time resolution. In addition, using the STM system in our experimental setup, we successfully demonstrated that the waveform can also be observed in the tunnel regime. In the tunnel regime, it was demonstrated that the transient electronic state excited by an optical pulse can be measured using the THz field as the instantaneous bias voltage. The ultrafast carrier dynamics in 2H-MoTe2 excited by an optical pulse was reproducibly probed.
Considering the rapid development of current research on the measurement and control of the extreme quantum limit via light, CEP technology will be a key to obtaining a deeper understanding of quantum processes and for the development of new devices such as to investigate the dynamics of hot electrons(28) and spins(29−32) and to perform single-molecule measurement,(33,34) for example. Our method, which enables the evaluation and accurate control of the CEP, is thus expected to play an essential role in the further advancement of science and technology.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.9b00266.
Schematic of experimental setup, data plots showing dependence of THz waveform at tip apex on electric field strength, frequency dependence of THz electric field modulated by the two types of nanotips, FDTD simulations of near field, measurement results for HOPG, relationship between tip-enhanced near field strength and THz intensity, tunnel spectrum of Bi2Se3, tip–sample distance dependence of the near-field waveform at the tip apex (PDF)
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Author Information
ARTICLE SECTIONS
- Hidemi Shigekawa - Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan;
http://orcid.org/0000-0001-9550-5148;
- Yoshihiko Kanemitsu - Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan;http://orcid.org/0000-0002-0788-131X
S.Y., H.H., and T.T. contributed equally to this research by advancing the THz-STM system and performing experiments. K.Y. contributed to the transient spectroscopy measurement, and FDTD simulations were performed by H.H., T.T., and Y.S., while Z.W. performed OPP measurement for comparison. Y. A., O. T. and Y.K. provided technical advice. H.S. organized and supervised the project and edited the paper with S.Y. and the other authors.
The authors declare no competing financial interest.
Acknowledgments
ARTICLE SECTIONS
H.S. and T.T. acknowledge the financial support of a Grant-in-Aid for Scientific Research (17H06088 and 19H02562, respectively) from Japan Society for the Promotion of Science. H.H. acknowledges the financial support of PRESTO (No. JPMJPR1427) grants from JST and Murata Science Foundation. We thank Y. Kawada, H. Takahashi, and Hamamatsu Photonics K. K. for fabricating the achromatic terahertz quarter-wave plate.
References
ARTICLE SECTIONS
This article references 34 other publications.
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- 9Wimmer, L.; Herink, G.; Solli, D. R.; Yalunin, S. V.; Echternkamp, K. E.; Ropers, C. Terahertz Control of Nanotip Photoemission. Nat. Phys. 2014, 10, 432– 436, DOI: 10.1038/nphys2974[Crossref], [CAS], Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXos1eqsL0%253D&md5=ee6727d84c1ac46e2894e5415d1eb034Terahertz control of nanotip photoemissionWimmer, L.; Herink, G.; Solli, D. R.; Yalunin, S. V.; Echternkamp, K. E.; Ropers, C.Nature Physics (2014), 10 (6), 432-436CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)The active control of matter by strong electromagnetic fields is of growing importance, with applications all across the optical spectrum from the extreme-UV to the far-IR. In recent years, phase-stable terahertz fields have shown tremendous potential for observing and manipulating elementary excitations in solids. In the gas phase, on the other hand, driving free charges with terahertz transients provides insight into ultrafast ionization dynamics. Developing such approaches for locally enhanced terahertz fields in nanostructures will create new means to govern electron currents on the nanoscale. Here, we use single-cycle terahertz transients to demonstrate extensive control over nanotip photoelectron emission. The terahertz near-field is shown to either enhance or suppress photocurrents, with the tip acting as an ultrafast rectifying diode. We record phase-resolved sub-cycle dynamics and find spectral compression and expansion arising from electron propagation within the terahertz near-field. These interactions produce rich spectro-temporal features and offer unprecedented control over ultrashort free electron pulses for imaging and diffraction.
- 10Cocker, T. L.; Jelic, V.; Gupta, M.; Molesky, S. J.; Burgess, J. A. J.; Reyes, G. D. L.; Titova, L. V.; Tsui, Y. Y.; Freeman, M. R.; Hegmann, F. A. An Ultrafast Terahertz Scanning Tunnelling Microscope. Nat. Photonics 2013, 7, 620– 625, DOI: 10.1038/nphoton.2013.151[Crossref], [CAS], Google Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtVKhtbjE&md5=6076a70e7da02eb7cfc895f0f9694c64An ultrafast terahertz scanning tunnelling microscopeCocker, Tyler L.; Jelic, Vedran; Gupta, Manisha; Molesky, Sean J.; Burgess, Jacob A. J.; De Los Reyes, Glenda; Titova, Lyubov V.; Tsui, Ying Y.; Freeman, Mark R.; Hegmann, Frank A.Nature Photonics (2013), 7 (8), 620-625CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)Ultrafast studies of excitations on the nanometer scale are essential for guiding applications in nanotechnol. Efforts to integrate femtosecond lasers with scanning tunnelling microscopes (STMs) have yielded a no. of ultrafast STM techniques, but the basic ability to directly modulate the STM junction bias while maintaining nanometer spatial resoln. was limited to ∼10 ps (refs 7,8) and has required specialized probe or sample structures. Here, without any modification to the STM design, we modulate the STM junction bias by coupling terahertz pulses to the scanning probe tip of an STM and demonstrate terahertz-pulse-induced tunnelling in an STM. The terahertz STM (THz-STM) provides simultaneous subpicosecond (<500 fs) time resoln. and nanometer (2 nm) imaging resoln. under ambient lab. conditions, and can directly image ultrafast carrier capture into a single InAs nanodot. The THz-STM accesses an ultrafast tunnelling regime that opens the door to subpicosecond scanning probe microscopy of materials with at. resoln.
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- 14Shigekawa, H.; Yoshida, S.; Takeuchi, O. Nanoscale Terahertz Spectroscopy. Nat. Photonics 2014, 8, 815– 817, DOI: 10.1038/nphoton.2014.272[Crossref], [CAS], Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitV2nsrrP&md5=ce03eff54e21fa6fc06c85164638f48eSpectroscopy Nanoscale terahertz spectroscopyShigekawa, Hidemi; Yoshida, Shoji; Takeuchi, OsamuNature Photonics (2014), 8 (11), 815-817CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)The advent of terahertz spectroscopy schemes that offer single-photon sensitivity, femtosecond time resoln. and nanometer spatial resoln. is creating new opportunities for investigating ultrafast charge dynamics in semiconductor structures.
- 15Yoshioka, K.; Katayama, I.; Arashida, Y.; Ban, A.; Kawada, Y.; Konishi, K.; Takahashi, H.; Takeda, J. Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields. Nano Lett. 2018, 18, 5198– 5204, DOI: 10.1021/acs.nanolett.8b02161[ACS Full Text], [CAS], Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhtlKqurbO&md5=7574be4a18cc18eaf170342baa416180Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric FieldsYoshioka, Katsumasa; Katayama, Ikufumi; Arashida, Yusuke; Ban, Atsuhiko; Kawada, Yoichi; Konishi, Kuniaki; Takahashi, Hironori; Takeda, JunNano Letters (2018), 18 (8), 5198-5204CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the at.-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip-sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription-in situ tailoring of single-cycle THz near fields in a tunnel junction-will offer unprecedented control of electrons for ultrafast at.-scale electronics and metrol.
- 16Kealhofer, C.; Schneider, W.; Ehberger, D.; Ryabov, A.; Krausz, F.; Baum, P. All-Optical Control and Metrology of Electron Pulses. Science 2016, 352, 429– 433, DOI: 10.1126/science.aae0003[Crossref], [PubMed], [CAS], Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XmsFOksr4%253D&md5=ec3f7928ad5030415fa1ef146dadb2b0All-optical control and metrology of electron pulsesKealhofer, C.; Schneider, W.; Ehberger, D.; Ryabov, A.; Krausz, F.; Baum, P.Science (Washington, DC, United States) (2016), 352 (6284), 429-433CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Short electron pulses are central to time-resolved at.-scale diffraction and electron microscopy, streak cameras, and free-electron lasers. Phase-space control and characterization are demonstrated of 5-pm electron pulses using few-cycle THz radiation, extending concepts of microwave electron pulse compression and streaking to THz frequencies. Optical-field control of electron pulses provides synchronism to laser pulses and offers a temporal resoln. that is ultimately limited by the rise-time of the optical fields applied. Few-cycle waveforms carried at 0.3 THz were used to compress electron pulses by a factor of 12 with a timing stability of <4 fs (root mean square) and measure using field-induced beam deflection (streaking). Scaling the concept toward multi-THz control fields holds promise for approaching the electronic time scale in time-resolved electron diffraction and microscopy.
- 17Wang, K.; Mittleman, D. M.; Van Der Valk, N. C. J.; Planken, P. C. M. Antenna Effects in Terahertz Apertureless Near-Field Optical Microscopy. Appl. Phys. Lett. 2004, 85, 2715– 2717, DOI: 10.1063/1.1797554[Crossref], [CAS], Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXos1Cgs7s%253D&md5=c2d8a860b7b4324b912c416f054f2d95Antenna effects in terahertz apertureless near-field optical microscopyWang, Kanglin; Mittleman, Daniel M.; Van der Valk, Nick C. J.; Planken, Paul C. M.Applied Physics Letters (2004), 85 (14), 2715-2717CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)We have performed measurements on terahertz (THz) apertureless near-field microscopy that show that the temporal shape of the obsd. near-field signals is approx. proportional to the time-integral of the incident field. Assocd. with this signal change is a bandwidth redn. by approx. a factor of 3 which is obsd. using both a near-field detection technique and a far-field detection technique. Using a dipole antenna model, it is shown how the obsd. effects can be explained by the signal filtering properties of the metal tips used in the expts.
- 18Herink, G.; Wimmer, L.; Ropers, C. Field Emission at Terahertz Frequencies: AC-Tunneling and Ultrafast Carrier Dynamics. New J. Phys. 2014, 16, 123005, DOI: 10.1088/1367-2630/16/12/123005[Crossref], [CAS], Google Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVyksLs%253D&md5=f9e58636e1837110918640f87c83a610Field emission at terahertz frequencies: AC-tunneling and ultrafast carrier dynamicsHerink, G.; Wimmer, L.; Ropers, C.New Journal of Physics (2014), 16 (Dec.), 123005/1-123005/9CODEN: NJOPFM; ISSN:1367-2630. (IOP Publishing Ltd.)We demonstrate ultrafast terahertz (THz) field emission from a tungsten nanotip enabled by local field enhancement. Characteristic electron spectra which result from acceleration in the THz near-field are found. Employing a dual frequency pump-probe scheme, we temporally resolve different nonlinear photoemission processes induced by coupling near-IR (NIR) and THz pulses. In the order of increasing THz field strength, we observe THz streaking, THz-induced barrier redn. (Schottky effect) and THz field emission. At intense NIR-excitation, the THz field emission is used as an ultrashort, local probe of hot electron dynamics in the apex. A first application of this scheme indicates a decreased carrier cooling rate in the confined tip geometry. Summarizing the results at various excitation conditions, we present a comprehensive picture of the distinct regimes in ultrafast photoemission in the near- and far-IR.
- 19Sobota, J. A.; Yang, S.; Analytis, J. G.; Chen, Y. L.; Fisher, I. R.; Kirchmann, P. S.; Shen, Z. X. Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3. Phys. Rev. Lett. 2012, 108, 117403, DOI: 10.1103/PhysRevLett.108.117403[Crossref], [PubMed], [CAS], Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XpvVyrurw%253D&md5=602c08fb800783e2853134f5b66d745bUltrafast optical excitation of a persistent surface-state population in the topological insulator Bi2Se3Sobota, J. A.; Yang, S.; Analytis, J. G.; Chen, Y. L.; Fisher, I. R.; Kirchmann, P. S.; Shen, Z.-X.Physical Review Letters (2012), 108 (11), 117403/1-117403/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)Using femtosecond time- and angle-resolved photoemission spectroscopy, we investigated the nonequil. dynamics of the topol. insulator Bi2Se3. We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk conduction bands are unoccupied. Optical excitation leads to a metastable population at the bulk conduction band edge, which feeds a nonequil. population of the surface state persisting for >10 ps. This unusually long-lived population of a metallic Dirac surface state with spin texture may present a channel in which to drive transient spin-polarized currents.
- 20Bahramy, M. S.; King, P. D. C.; de la Torre, a.; Chang, J.; Shi, M.; Patthey, L.; Balakrishnan, G.; Hofmann, P.; Arita, R.; Nagaosa, N. Emergent Quantum Confinement at Topological Insulator Surfaces. Nat. Commun. 2012, 3, 1157– 1159, DOI: 10.1038/ncomms2162
- 21Kawada, Y.; Yasuda, T.; Takahashi, H. Carrier Envelope Phase Shifter for Broadband Terahertz Pulses. Opt. Lett. 2016, 41, 986– 989, DOI: 10.1364/OL.41.000986[Crossref], [PubMed], [CAS], Google Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC28fgt1ymuw%253D%253D&md5=86f463517a19fa7cb51b5df29da2651eCarrier envelope phase shifter for broadband terahertz pulsesKawada Yoichi; Yasuda Takashi; Takahashi HironoriOptics letters (2016), 41 (5), 986-9 ISSN:.We demonstrated controlled shifting of the internal phase of broadband terahertz (THz) pulses. The internal phase of an ultrashort pulse is called the carrier envelope phase (CEP), which is an important parameter in the interaction of few-cycle light pulses and matter. Our CEP shifter utilizes the ultra-broadband feature of prism wave plates. We analytically derived the amount of CEP shift achievable by the CEP shifter using Jones matrixes. THz time-domain measurements clearly showed the shift of the CEP, and the results agreed well with the calculated values. The CEP shift was as high as 2π, indicating that any CEP values can be chosen using our CEP shifter.
- 22Chen, K.; Roy, A.; Rai, A.; Movva, H. C. P.; Meng, X.; He, F.; Banerjee, S. K.; Wang, Y. Accelerated Carrier Recombination by Grain Boundary/Edge Defects in MBE Grown Transition Metal Dichalcogenides. APL Mater. 2018, 6, 056103, DOI: 10.1063/1.5022339[Crossref], [CAS], Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXptFSitbc%253D&md5=9c96c69bc72b7baf47b2ac3780ee429bAccelerated carrier recombination by grain boundary/edge defects in MBE grown transition metal dichalcogenidesChen, Ke; Roy, Anupam; Rai, Amritesh; Movva, Hema C. P.; Meng, Xianghai; He, Feng; Banerjee, Sanjay K.; Wang, YaguoAPL Materials (2018), 6 (5), 056103/1-056103/7CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)Defect-carrier interaction in transition metal dichalcogenides (TMDs) plays important roles in carrier relaxation dynamics and carrier transport, which dets. the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in mol. beam epitaxy (MBE)-grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, the carrier recombination rate in MBE-grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE-grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also obsd. coherent acoustic phonons in both exfoliated and MBE-grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with the previously reported result of theor. calcn. Our findings provide a useful ref. for the fundamental parameters: carrier lifetime and sound velocity and reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics. (c) 2018 American Institute of Physics.
- 23Guguchia, Z.; Uemura, Y. J.; Pasupathy, A. N.; Edelberg, D.; Shermadini, Z.; Billinge, S. J. L.; Kerelsky, A.; Morenzoni, E.; Shengelaya, A.; Augustin, M. Magnetism in Semiconducting Molybdenum Dichalcogenides. Sci. Adv. 2018, 4, eaat3672 DOI: 10.1126/sciadv.aat3672
- 24Terada, Y.; Yoshida, S.; Takeuchi, O.; Shigekawa, H. Real-Space Imaging of Transient Carrier Dynamics by Nanoscale Pumpg-Probe Microscopy. Nat. Photonics 2010, 4, 869– 874, DOI: 10.1038/nphoton.2010.235[Crossref], [CAS], Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsVygs7nL&md5=7ec24b08732baaf9c0c2c5666f688fdeReal-space imaging of transient carrier dynamics by nanoscale pump-probe microscopyTerada, Yasuhiko; Yoshida, Shoji; Takeuchi, Osamu; Shigekawa, HidemiNature Photonics (2010), 4 (12), 869-874CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)Smaller and faster are key concepts underlying the progress of current nanoscience and nanotechnol. The development of a method of exploring the transient carrier dynamics in organized nanostructures with pinpoint accuracy is therefore highly desirable. Here, we present a new microscopy that enables real-space measurement of the spatial variation of ultrafast dynamics. It is a pulse-laser-combined scanning tunnelling microscopy with a novel delay-time modulation method based on a pulse-picking technique. A non-equil. carrier distribution is generated with ultrashort laser pulses, and its relaxation processes are obsd. by scanning tunnelling microscopy using a pump-probe technique. We have directly analyzed the recombination of excited carriers via the gap states assocd. with a cobalt nanoparticle/GaAs structure in real space. Through the site dependence of the decay time on the tunnelling current injection from the scanning tunnelling microscopy tip, the hole capture rate at the gap states has been imaged on the nanoscale for the first time.
- 25Mogi, H.; Kikuchi, R.; Yoshida, S.; Takeuchi, O.; Shigekawa, H. Externally Triggerable Optical Pump-Probe Scanning Tunneling Microscopy. Appl. Phys. Express 2019, 12, 025005, DOI: 10.7567/1882-0786/aaf8b2[Crossref], [CAS], Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXotlKjtLw%253D&md5=a260204f582e13816906aff2791214d3Externally triggerable optical pump-probe scanning tunneling microscopyMogi, Hiroyuki; Wang, Zi-han; Kikuchi, Ryusei; Yoon, Cheul Hyun; Yoshida, Shoji; Takeuchi, Osamu; Shigekawa, HidemiApplied Physics Express (2019), 12 (2), 025005/1-025005/4CODEN: APEPC4; ISSN:1882-0786. (IOP Publishing Ltd.)Optical pump-probe scanning tunneling microscopy (OPP-STM) has enabled the measurement of ultrafast dynamics in real space. However, the use of a pulse picker to ext. selected laser pulses to realize delay-time modulation, which efficiently suppresses the thermal expansion problems, limits the availability of time-resolved measurement. Here, we present a more applicable type of OPP-STM that we have developed. Two externally triggerable pulse lasers were used to produce pump and probe pulses, and wide-range delay-time modulation was simply realized by adjusting the timing of the pulses. The performance of this new type of OPP-STM was demonstrated by measuring the carrier dynamics in WSe2.
- 26Wang, H.; Zhang, C.; Chan, W.; Tiwari, S.; Rana, F. Ultrafast Response of Monolayer Molybdenum Disulfide Photodetectors. Nat. Commun. 2015, 6, 17– 20, DOI: 10.1038/ncomms9831
- 27Kar, S.; Su, Y.; Nair, R. R.; Sood, A. K. Probing Photoexcited Carriers in a Few-Layer MoS2 Laminate by Time-Resolved Optical Pump-Terahertz Probe Spectroscopy. ACS Nano 2015, 9, 12004– 12010, DOI: 10.1021/acsnano.5b04804[ACS Full Text], [CAS], Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslCqsbvE&md5=52013568453f688c93965122ade62e18Probing Photoexcited Carriers in a Few-Layer MoS2 Laminate by Time-Resolved Optical Pump-Terahertz Probe SpectroscopyKar, Srabani; Su, Y.; Nair, R. R.; Sood, A. K.ACS Nano (2015), 9 (12), 12004-12010CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)The dynamics of photoinduced carriers in a free-standing MoS2 laminate consisting of a few layers (1-6 layers) using time-resolved optical pump-terahertz probe spectroscopy is reported. Upon photoexcitation with the 800 nm pump pulse, the terahertz cond. increases due to absorption by the photoinduced charge carriers. The relaxation of the nonequil. carriers shows fast as well as slow decay channels, analyzed using a rate equation model incorporating defect-assisted Auger scattering of photoexcited electrons, holes, and excitons. The fast relaxation time occurs due to the capture of electrons and holes by defects via Auger processes, resulting in nonradiative recombination. The slower relaxation arises since the excitons are bound to the defects, preventing the defect-assisted Auger recombination of the electrons and the holes. The results provide a comprehensive understanding of the nonequil. carrier kinetics in a system of unscreened Coulomb interactions, where defect-assisted Auger processes dominate and should be applicable to other 2D systems.
- 28Wu, S. W.; Ogawa, N.; Ho, W. Atomic-Scale Coupling of Photons to Single-Molecule Junctions. Science 2006, 312, 1362– 1365, DOI: 10.1126/science.1124881[Crossref], [PubMed], [CAS], Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XltVOhtrg%253D&md5=38cf87ad4a29eea54e48c53a586686ccAtomic-Scale Coupling of Photons to Single-Molecule JunctionsWu, S. W.; Ogawa, N.; Ho, W.Science (Washington, DC, United States) (2006), 312 (5778), 1362-1365CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Spatial resoln. at the at. scale was achieved in the coupling of light to single mols. adsorbed on a surface. Electron transfer to a single mol. induced by green to near-IR light in the junction of a scanning tunneling microscope (STM) exhibited spatially varying probability that is confined within the mol. The mechanism involves photo-induced resonant tunneling in which a photoexcited electron in the STM tip is transferred to the mol. The coupling of photons to the tunneling process provides a pathway to explore mol. dynamics with the combined capabilities of lasers and the STM.
- 29Wang, Z.; Yoon, C.; Yoshida, S.; Arashida, Y.; Takeuchi, O.; Ohno, Y.; Shigekawa, H. Surface-Mediated Spin Dynamics Probed by Optical-Pump–Probe Scanning Tunneling Microscopy. Phys. Chem. Chem. Phys. 2019, 21, 7256, DOI: 10.1039/C8CP07786J[Crossref], [CAS], Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhvFelsr0%253D&md5=3b865077c003909246150c1a3231f0abSurface-mediated spin dynamics probed by optical-pump-probe scanning tunneling microscopyWang, Zi-Han; Yoon, Cheul-Hyun; Yoshida, Shoji; Arashida, Yusuke; Takeuchi, Osamu; Ohno, Yuzo; Shigekawa, HidemiPhysical Chemistry Chemical Physics (2019), 21 (14), 7256-7260CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)In current materials science and technologies, surface effects on carrier and spin dynamics in functional materials and devices are of great importance. In this paper, we present the surface-sensitive probing of electron spin dynamics, performed by optical-pump-probe scanning tunneling microscopy (OPP-STM). Time-resolved spin lifetime information on a manganese (Mn)-deposited GaAs(110) surface was successfully obtained for the first time. With increasing Mn d. via in situ evapn., a nonlinear change in the spin lifetime in the picosecond range was clearly obsd., while directly confirming the Mn d. by STM. In comparison with the results obtained by the conventional OPP method, we have also demonstrated that the obsd. nonlinear spin lifetime behavior was surface-mediated, which can be characterized using only the surface-sensitive OPP-STM technique.
- 30Loth, S.; Etzkorn, M.; Lutz, C. P.; Eigler, D. M.; Heinrich, A. J. Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution. Science 2010, 329, 1628– 1630, DOI: 10.1126/science.1191688[Crossref], [PubMed], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtFOqsr3F&md5=bb3d82531937f4f884e1a91f1ac516ebMeasurement of Fast Electron Spin Relaxation Times with Atomic ResolutionLoth, Sebastian; Etzkorn, Markus; Lutz, Christopher P.; Eigler, D. M.; Heinrich, Andreas J.Science (Washington, DC, United States) (2010), 329 (5999), 1628-1630CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resoln. of spin relaxation measurements to the at. scale was a challenge. A scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers vary 50-250 ns. The method can in principle be generalized to monitor the temporal evolution of other dynamical systems.
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- 32Yoshida, S.; Aizawa, Y.; Wang, Z. H.; Oshima, R.; Mera, Y.; Matsuyama, E.; Oigawa, H.; Takeuchi, O.; Shigekawa, H. Probing Ultrafast Spin Dynamics with Optical Pump-Probe Scanning Tunnelling Microscopy. Nat. Nanotechnol. 2014, 9, 588– 593, DOI: 10.1038/nnano.2014.125[Crossref], [PubMed], [CAS], Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVKls7%252FJ&md5=ed99367f0c33d5c7aac42e896eb174eaProbing ultrafast spin dynamics with optical pump-probe scanning tunnelling microscopyYoshida, Shoji; Aizawa, Yuta; Wang, Zi-han; Oshima, Ryuji; Mera, Yutaka; Matsuyama, Eiji; Oigawa, Haruhiro; Takeuchi, Osamu; Shigekawa, HidemiNature Nanotechnology (2014), 9 (8), 588-593CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Optical pump-probe scanning tunneling microscopy is demonstrated, which enables the nanoscale probing of spin dynamics with the temporal resoln. corresponding, in principle, to the optical pulse width. Spins are optically oriented using circularly polarized light, and their dynamics are probed by scanning tunnelling microscopy based on the optical pump-probe method. Spin relaxation in a single quantum well with a width of 6 nm was obsd. with a spatial resoln. of ∼1 nm. In addn. to spin relaxation dynamics, spin precession, which provides an estn. of the Land´e g factor, was obsd. successfully.
- 33Li, S.; Chen, S.; Li, J.; Wu, R.; Ho, W. Joint Space-Time Coherent Vibration Driven Conformational Transitions in a Single Molecule. Phys. Rev. Lett. 2017, 119, 176002, DOI: 10.1103/PhysRevLett.119.176002[Crossref], [PubMed], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhs1WlsLnP&md5=af5ca44d02cb52e8c390a5450dcf8b78Joint space-time coherent vibration driven conformational transitions in a single moleculeLi, Shaowei; Chen, Siyu; Li, Jie; Wu, Ruqian; Ho, W.Physical Review Letters (2017), 119 (17), 176002/1-176002/5CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)We report single-mol. conformational transitions with joint angstrom-femtosecond resoln. by irradiating the junction of a scanning tunneling microscope with femtosecond laser pulses. An isolated pyrrolidine mol. adsorbed on a Cu(001) surface undergoes reversible transitions between two conformational states. The transition rate decays in time and exhibits sinusoidal oscillations with periods of specific mol. vibrations. The dynamics of this transition depends sensitively on the mol. environment, as exemplified by the effects of another mol. in proximity.
- 34Yoshida, S.; Taninaka, A.; Sugita, Y.; Katayama, T.; Takeuchi, O.; Shigekawa, H. Revealing the Conformational Dynamics in a Single-Molecule Junction by Site- and Angle-Resolved Dynamic Probe Method. ACS Nano 2016, 10, 11211– 11218, DOI: 10.1021/acsnano.6b06278[ACS Full Text], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvFOiurnF&md5=56488c790bcb22dd0bb3f2141d47e47fRevealing the Conformational Dynamics in a Single-Molecule Junction by Site- and Angle-Resolved Dynamic Probe MethodYoshida, Shoji; Taninaka, Atsushi; Sugita, Yoshihiro; Katayama, Tomoki; Takeuchi, Osamu; Shigekawa, HidemiACS Nano (2016), 10 (12), 11211-11218CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Single-mol. junctions were extensively studied because of their high potential for future nanoscale device applications as well as their importance in basic studies for mol. science and technol. However, since the bonding sites at an electrode and the mol. tilt angles, for example, cannot be detd. exptl., analyses were performed assuming the structures of such interactive key factors, with uncertainties and inconsistencies remaining in the proposed mechanisms. The authors have developed a methodol. that enables the probing of conformational dynamics in single-mol. junctions simultaneously with the direct characterization of mol. bonding sites and tilt angles. This technique revealed the elemental processes in single-mol. junctions, which were not clarified using conventional methods. The mechanisms of the mol. dynamics in 1,4-benzenedithiol and 4,4'-bipyridine single-mol. junctions, which, for example, produce binary conductance switching of different types, were clearly discriminated and comprehensively explained.
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Abstract
Figure 1
Figure 1. Explanation of measurement principle. (a) Incident electric field and its modulation by a nanotip. (b, c) Schematic illustrations of measurement methods in photoemission (photofield emission and above-threshold photoemission) regime and tunneling regime, respectively. The barrier height is modulated by the THz electric field, and photoelectron emission (hot electron tunneling) is induced by the simultaneous irradiation by the probe light of 517 nm (1035 nm). (d) Dependence of photocurrent on the dc bias voltage VDC applied to the HOPG sample with respect to the nanotip while the tip was irradiated by the probe light and the THz electric field was turned off. The photocurrent observed for VDC ≤ 0 was due to above-threshold photoelectron emission caused by the process of three-photon absorption.
Figure 2
Figure 2. THz electric field modulation by nanotip. (a) Optical micrograph of the ultrasharp PtIr-coated W nanotip (tip 1). (b) Map of photoelectrons obtained by scanning the probe light (517 nm) on the nanotip. The spot position was moved by 31 steps of 4.5 μm in the x direction; then, after being shifted by 4.5 μm in the y direction, it was moved back along the x direction in 31 steps. This procedure was repeated, until seven steps had been completed in the y direction (the entire scanned area was 139.5 × 31.5 μm2). (c) Map of ITHz at zero delay time. (d) Optical micrograph of the PtIr-coated W nanotip fabricated by ordinary chemical etching (tip 2). (e) Map of photoelectrons obtained by scanning the probe light (517 nm) on the nanotip. (f) Map of ITHz at zero delay time. (g) Waveform of the incident THz light. (h–j) THz near-field waveforms measured at A, B, and C in (c) and (f), respectively. Red and blue lines show the results for tips 1 and 2, respectively. (k) Waveform obtained by integrating the electric field in (g). Vs = +10 V, IVIS = 14 mW, ETHz,pk = 4.1 kV/cm (electric field of the maximum peak), frep = 200 kHz, and tip–sample distance ≈ 1000 nm. Here, an HOPG sample was used as the counter electrode.
Figure 3
Figure 3. Measurement in tunnel regime. (a) Schematic illustration of the measurement setup. The IR pulse wavelength was set to 1035 nm. The direction of the THz pulse generated by a lithium niobate crystal was inverted by CEP control using the HWP. The THz field strength was continuously controlled using a wire grid polarizer. (b) Tunnel junction band diagrams (I) without and (II) with IR pulse and THz pulse irradiation. (c) Electrical over-stress measurement results of the incident THz waveform. By controlling the HWP angle α and adjusting the phase of the CEP, ϕCEP, the electric field waveform can be reversed. (d) Tip-enhanced near-field waveform obtained by photoelectron measurement. (e) ITHz as a function of delay time (VS = 2 mV, IT = 2 pA, IIR = 5 mJ/cm2, frep = 0.5 MHz). (f) ITHz–VTHz curves obtained for td = −5 ps and 0. (g) ITHz as a function of delay time simulated using the ITHz–VTHz curve. (h) Schematic to explain the method of simulation to obtain the spectra shown in (g). Analysis was performed for Bi2Se3.
Figure 4
Figure 4. Transient electronic dynamics obtained by THz-STM for 2H-MoTe2. (a) STM image of the sample (sample bias VS = 1.2 V, tunnel current It = 10 pA). Bright spots indicated by D in the magnified image are defects. (b) Delay-time dependence of ITHz (red line). Near-field waveform at the STM tip apex used for measurement, which was obtained by the photoelectron method (blue line) is superimposed. (insets) The relationship between the pump and probe lights for td < 0 and td > 0. (c) Example of spectrum obtained over a wide delay-time range. The black line is a fitting curve with an exponential function of two components. (d) Light-intensity dependence of ITHz. ITHz has two components at 1.5 mJ/cm2, but a one-component fitting is in good agreement with ITHz at 1.1 and 0.7 mJ/cm2. (VS = −3 mV, IT = 0.6 pA, frep = 1 MHz). (e) Schematic illustrations of (left) without illumination, (I) THz illumination without IR, (II) THz illumination just after IR excitation, and (III) THz illumination after IR excitation with rather large td in the range of τc (carrier recombination lifetime).
References
ARTICLE SECTIONSThis article references 34 other publications.
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- 10Cocker, T. L.; Jelic, V.; Gupta, M.; Molesky, S. J.; Burgess, J. A. J.; Reyes, G. D. L.; Titova, L. V.; Tsui, Y. Y.; Freeman, M. R.; Hegmann, F. A. An Ultrafast Terahertz Scanning Tunnelling Microscope. Nat. Photonics 2013, 7, 620– 625, DOI: 10.1038/nphoton.2013.151[Crossref], [CAS], Google Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtVKhtbjE&md5=6076a70e7da02eb7cfc895f0f9694c64An ultrafast terahertz scanning tunnelling microscopeCocker, Tyler L.; Jelic, Vedran; Gupta, Manisha; Molesky, Sean J.; Burgess, Jacob A. J.; De Los Reyes, Glenda; Titova, Lyubov V.; Tsui, Ying Y.; Freeman, Mark R.; Hegmann, Frank A.Nature Photonics (2013), 7 (8), 620-625CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)Ultrafast studies of excitations on the nanometer scale are essential for guiding applications in nanotechnol. Efforts to integrate femtosecond lasers with scanning tunnelling microscopes (STMs) have yielded a no. of ultrafast STM techniques, but the basic ability to directly modulate the STM junction bias while maintaining nanometer spatial resoln. was limited to ∼10 ps (refs 7,8) and has required specialized probe or sample structures. Here, without any modification to the STM design, we modulate the STM junction bias by coupling terahertz pulses to the scanning probe tip of an STM and demonstrate terahertz-pulse-induced tunnelling in an STM. The terahertz STM (THz-STM) provides simultaneous subpicosecond (<500 fs) time resoln. and nanometer (2 nm) imaging resoln. under ambient lab. conditions, and can directly image ultrafast carrier capture into a single InAs nanodot. The THz-STM accesses an ultrafast tunnelling regime that opens the door to subpicosecond scanning probe microscopy of materials with at. resoln.
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- 12Eisele, M.; Cocker, T. L.; Huber, M. a.; Plankl, M.; Viti, L.; Ercolani, D.; Sorba, L.; Vitiello, M. S.; Huber, R. Ultrafast Multi-Terahertz Nano-Spectroscopy with Sub-Cycle Temporal Resolution. Nat. Photonics 2014, 8, 841– 845, DOI: 10.1038/nphoton.2014.225[Crossref], [CAS], Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhs1yht7%252FK&md5=4db21d34ab9c689af085a3167050c946Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolutionEisele, M.; Cocker, T. L.; Huber, M. A.; Plankl, M.; Viti, L.; Ercolani, D.; Sorba, L.; Vitiello, M. S.; Huber, R.Nature Photonics (2014), 8 (11), 841-845CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)A review. Phase-locked ultrashort pulses in the rich terahertz spectral range have provided key insights into phenomena as diverse as quantum confinement, first-order phase transitions, high-temp. supercond. and carrier transport in nanomaterials. Ultrabroadband electro-optic sampling of few-cycle field transients can even reveal novel dynamics that occur faster than a single oscillation cycle of light. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielec. functions averaged over the size, structure, orientation and d. of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM). We trace the time-dependent dielec. function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.
- 13Jelic, V.; Iwaszczuk, K.; Nguyen, P. H.; Rathje, C.; Hornig, G. J.; Sharum, H. M.; Hoffman, J. R.; Freeman, M. R.; Hegmann, F. A. Ultrafast Terahertz Control of Extreme Tunnel Currents through Single Atoms on a Silicon Surface. Nat. Phys. 2017, 13, 591– 597, DOI: 10.1038/nphys4047[Crossref], [CAS], Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXislGltbc%253D&md5=da4821d78390ac5c0e7ec8e3bfe826e4Ultrafast terahertz control of extreme tunnel currents through single atoms on a silicon surfaceJelic, Vedran; Iwaszczuk, Krzysztof; Nguyen, Peter H.; Rathje, Christopher; Hornig, Graham J.; Sharum, Haille M.; Hoffman, James R.; Freeman, Mark R.; Hegmann, Frank A.Nature Physics (2017), 13 (6), 591-598CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)Ultrafast control of current on the at. scale is essential for future innovations in nanoelectronics. Extremely localized transient elec. fields on the nanoscale can be achieved by coupling picosecond duration terahertz pulses to metallic nanostructures. Here, we demonstrate terahertz scanning tunnelling microscopy (THz-STM) in ultrahigh vacuum as a new platform for exploring ultrafast non-equil. tunnelling dynamics with at. precision. Extreme terahertz-pulse-driven tunnel currents up to 107 times larger than steady-state currents in conventional STM are used to image individual atoms on a silicon surface with 0.3 nm spatial resoln. At terahertz frequencies, the metallic-like Si(111)-(7 × 7) surface is unable to screen the elec. field from the bulk, resulting in a terahertz tunnel conductance that is fundamentally different than that of the steady state. Ultrafast terahertz-induced band bending and non-equil. charging of surface states opens new conduction pathways to the bulk, enabling extreme transient tunnel currents to flow between the tip and sample.
- 14Shigekawa, H.; Yoshida, S.; Takeuchi, O. Nanoscale Terahertz Spectroscopy. Nat. Photonics 2014, 8, 815– 817, DOI: 10.1038/nphoton.2014.272[Crossref], [CAS], Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitV2nsrrP&md5=ce03eff54e21fa6fc06c85164638f48eSpectroscopy Nanoscale terahertz spectroscopyShigekawa, Hidemi; Yoshida, Shoji; Takeuchi, OsamuNature Photonics (2014), 8 (11), 815-817CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)The advent of terahertz spectroscopy schemes that offer single-photon sensitivity, femtosecond time resoln. and nanometer spatial resoln. is creating new opportunities for investigating ultrafast charge dynamics in semiconductor structures.
- 15Yoshioka, K.; Katayama, I.; Arashida, Y.; Ban, A.; Kawada, Y.; Konishi, K.; Takahashi, H.; Takeda, J. Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields. Nano Lett. 2018, 18, 5198– 5204, DOI: 10.1021/acs.nanolett.8b02161[ACS Full Text], [CAS], Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhtlKqurbO&md5=7574be4a18cc18eaf170342baa416180Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric FieldsYoshioka, Katsumasa; Katayama, Ikufumi; Arashida, Yusuke; Ban, Atsuhiko; Kawada, Yoichi; Konishi, Kuniaki; Takahashi, Hironori; Takeda, JunNano Letters (2018), 18 (8), 5198-5204CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the at.-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip-sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription-in situ tailoring of single-cycle THz near fields in a tunnel junction-will offer unprecedented control of electrons for ultrafast at.-scale electronics and metrol.
- 16Kealhofer, C.; Schneider, W.; Ehberger, D.; Ryabov, A.; Krausz, F.; Baum, P. All-Optical Control and Metrology of Electron Pulses. Science 2016, 352, 429– 433, DOI: 10.1126/science.aae0003[Crossref], [PubMed], [CAS], Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XmsFOksr4%253D&md5=ec3f7928ad5030415fa1ef146dadb2b0All-optical control and metrology of electron pulsesKealhofer, C.; Schneider, W.; Ehberger, D.; Ryabov, A.; Krausz, F.; Baum, P.Science (Washington, DC, United States) (2016), 352 (6284), 429-433CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Short electron pulses are central to time-resolved at.-scale diffraction and electron microscopy, streak cameras, and free-electron lasers. Phase-space control and characterization are demonstrated of 5-pm electron pulses using few-cycle THz radiation, extending concepts of microwave electron pulse compression and streaking to THz frequencies. Optical-field control of electron pulses provides synchronism to laser pulses and offers a temporal resoln. that is ultimately limited by the rise-time of the optical fields applied. Few-cycle waveforms carried at 0.3 THz were used to compress electron pulses by a factor of 12 with a timing stability of <4 fs (root mean square) and measure using field-induced beam deflection (streaking). Scaling the concept toward multi-THz control fields holds promise for approaching the electronic time scale in time-resolved electron diffraction and microscopy.
- 17Wang, K.; Mittleman, D. M.; Van Der Valk, N. C. J.; Planken, P. C. M. Antenna Effects in Terahertz Apertureless Near-Field Optical Microscopy. Appl. Phys. Lett. 2004, 85, 2715– 2717, DOI: 10.1063/1.1797554[Crossref], [CAS], Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXos1Cgs7s%253D&md5=c2d8a860b7b4324b912c416f054f2d95Antenna effects in terahertz apertureless near-field optical microscopyWang, Kanglin; Mittleman, Daniel M.; Van der Valk, Nick C. J.; Planken, Paul C. M.Applied Physics Letters (2004), 85 (14), 2715-2717CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)We have performed measurements on terahertz (THz) apertureless near-field microscopy that show that the temporal shape of the obsd. near-field signals is approx. proportional to the time-integral of the incident field. Assocd. with this signal change is a bandwidth redn. by approx. a factor of 3 which is obsd. using both a near-field detection technique and a far-field detection technique. Using a dipole antenna model, it is shown how the obsd. effects can be explained by the signal filtering properties of the metal tips used in the expts.
- 18Herink, G.; Wimmer, L.; Ropers, C. Field Emission at Terahertz Frequencies: AC-Tunneling and Ultrafast Carrier Dynamics. New J. Phys. 2014, 16, 123005, DOI: 10.1088/1367-2630/16/12/123005[Crossref], [CAS], Google Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVyksLs%253D&md5=f9e58636e1837110918640f87c83a610Field emission at terahertz frequencies: AC-tunneling and ultrafast carrier dynamicsHerink, G.; Wimmer, L.; Ropers, C.New Journal of Physics (2014), 16 (Dec.), 123005/1-123005/9CODEN: NJOPFM; ISSN:1367-2630. (IOP Publishing Ltd.)We demonstrate ultrafast terahertz (THz) field emission from a tungsten nanotip enabled by local field enhancement. Characteristic electron spectra which result from acceleration in the THz near-field are found. Employing a dual frequency pump-probe scheme, we temporally resolve different nonlinear photoemission processes induced by coupling near-IR (NIR) and THz pulses. In the order of increasing THz field strength, we observe THz streaking, THz-induced barrier redn. (Schottky effect) and THz field emission. At intense NIR-excitation, the THz field emission is used as an ultrashort, local probe of hot electron dynamics in the apex. A first application of this scheme indicates a decreased carrier cooling rate in the confined tip geometry. Summarizing the results at various excitation conditions, we present a comprehensive picture of the distinct regimes in ultrafast photoemission in the near- and far-IR.
- 19Sobota, J. A.; Yang, S.; Analytis, J. G.; Chen, Y. L.; Fisher, I. R.; Kirchmann, P. S.; Shen, Z. X. Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3. Phys. Rev. Lett. 2012, 108, 117403, DOI: 10.1103/PhysRevLett.108.117403[Crossref], [PubMed], [CAS], Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XpvVyrurw%253D&md5=602c08fb800783e2853134f5b66d745bUltrafast optical excitation of a persistent surface-state population in the topological insulator Bi2Se3Sobota, J. A.; Yang, S.; Analytis, J. G.; Chen, Y. L.; Fisher, I. R.; Kirchmann, P. S.; Shen, Z.-X.Physical Review Letters (2012), 108 (11), 117403/1-117403/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)Using femtosecond time- and angle-resolved photoemission spectroscopy, we investigated the nonequil. dynamics of the topol. insulator Bi2Se3. We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk conduction bands are unoccupied. Optical excitation leads to a metastable population at the bulk conduction band edge, which feeds a nonequil. population of the surface state persisting for >10 ps. This unusually long-lived population of a metallic Dirac surface state with spin texture may present a channel in which to drive transient spin-polarized currents.
- 20Bahramy, M. S.; King, P. D. C.; de la Torre, a.; Chang, J.; Shi, M.; Patthey, L.; Balakrishnan, G.; Hofmann, P.; Arita, R.; Nagaosa, N. Emergent Quantum Confinement at Topological Insulator Surfaces. Nat. Commun. 2012, 3, 1157– 1159, DOI: 10.1038/ncomms2162
- 21Kawada, Y.; Yasuda, T.; Takahashi, H. Carrier Envelope Phase Shifter for Broadband Terahertz Pulses. Opt. Lett. 2016, 41, 986– 989, DOI: 10.1364/OL.41.000986[Crossref], [PubMed], [CAS], Google Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC28fgt1ymuw%253D%253D&md5=86f463517a19fa7cb51b5df29da2651eCarrier envelope phase shifter for broadband terahertz pulsesKawada Yoichi; Yasuda Takashi; Takahashi HironoriOptics letters (2016), 41 (5), 986-9 ISSN:.We demonstrated controlled shifting of the internal phase of broadband terahertz (THz) pulses. The internal phase of an ultrashort pulse is called the carrier envelope phase (CEP), which is an important parameter in the interaction of few-cycle light pulses and matter. Our CEP shifter utilizes the ultra-broadband feature of prism wave plates. We analytically derived the amount of CEP shift achievable by the CEP shifter using Jones matrixes. THz time-domain measurements clearly showed the shift of the CEP, and the results agreed well with the calculated values. The CEP shift was as high as 2π, indicating that any CEP values can be chosen using our CEP shifter.
- 22Chen, K.; Roy, A.; Rai, A.; Movva, H. C. P.; Meng, X.; He, F.; Banerjee, S. K.; Wang, Y. Accelerated Carrier Recombination by Grain Boundary/Edge Defects in MBE Grown Transition Metal Dichalcogenides. APL Mater. 2018, 6, 056103, DOI: 10.1063/1.5022339[Crossref], [CAS], Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXptFSitbc%253D&md5=9c96c69bc72b7baf47b2ac3780ee429bAccelerated carrier recombination by grain boundary/edge defects in MBE grown transition metal dichalcogenidesChen, Ke; Roy, Anupam; Rai, Amritesh; Movva, Hema C. P.; Meng, Xianghai; He, Feng; Banerjee, Sanjay K.; Wang, YaguoAPL Materials (2018), 6 (5), 056103/1-056103/7CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)Defect-carrier interaction in transition metal dichalcogenides (TMDs) plays important roles in carrier relaxation dynamics and carrier transport, which dets. the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in mol. beam epitaxy (MBE)-grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, the carrier recombination rate in MBE-grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE-grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also obsd. coherent acoustic phonons in both exfoliated and MBE-grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with the previously reported result of theor. calcn. Our findings provide a useful ref. for the fundamental parameters: carrier lifetime and sound velocity and reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics. (c) 2018 American Institute of Physics.
- 23Guguchia, Z.; Uemura, Y. J.; Pasupathy, A. N.; Edelberg, D.; Shermadini, Z.; Billinge, S. J. L.; Kerelsky, A.; Morenzoni, E.; Shengelaya, A.; Augustin, M. Magnetism in Semiconducting Molybdenum Dichalcogenides. Sci. Adv. 2018, 4, eaat3672 DOI: 10.1126/sciadv.aat3672
- 24Terada, Y.; Yoshida, S.; Takeuchi, O.; Shigekawa, H. Real-Space Imaging of Transient Carrier Dynamics by Nanoscale Pumpg-Probe Microscopy. Nat. Photonics 2010, 4, 869– 874, DOI: 10.1038/nphoton.2010.235[Crossref], [CAS], Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsVygs7nL&md5=7ec24b08732baaf9c0c2c5666f688fdeReal-space imaging of transient carrier dynamics by nanoscale pump-probe microscopyTerada, Yasuhiko; Yoshida, Shoji; Takeuchi, Osamu; Shigekawa, HidemiNature Photonics (2010), 4 (12), 869-874CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)Smaller and faster are key concepts underlying the progress of current nanoscience and nanotechnol. The development of a method of exploring the transient carrier dynamics in organized nanostructures with pinpoint accuracy is therefore highly desirable. Here, we present a new microscopy that enables real-space measurement of the spatial variation of ultrafast dynamics. It is a pulse-laser-combined scanning tunnelling microscopy with a novel delay-time modulation method based on a pulse-picking technique. A non-equil. carrier distribution is generated with ultrashort laser pulses, and its relaxation processes are obsd. by scanning tunnelling microscopy using a pump-probe technique. We have directly analyzed the recombination of excited carriers via the gap states assocd. with a cobalt nanoparticle/GaAs structure in real space. Through the site dependence of the decay time on the tunnelling current injection from the scanning tunnelling microscopy tip, the hole capture rate at the gap states has been imaged on the nanoscale for the first time.
- 25Mogi, H.; Kikuchi, R.; Yoshida, S.; Takeuchi, O.; Shigekawa, H. Externally Triggerable Optical Pump-Probe Scanning Tunneling Microscopy. Appl. Phys. Express 2019, 12, 025005, DOI: 10.7567/1882-0786/aaf8b2[Crossref], [CAS], Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXotlKjtLw%253D&md5=a260204f582e13816906aff2791214d3Externally triggerable optical pump-probe scanning tunneling microscopyMogi, Hiroyuki; Wang, Zi-han; Kikuchi, Ryusei; Yoon, Cheul Hyun; Yoshida, Shoji; Takeuchi, Osamu; Shigekawa, HidemiApplied Physics Express (2019), 12 (2), 025005/1-025005/4CODEN: APEPC4; ISSN:1882-0786. (IOP Publishing Ltd.)Optical pump-probe scanning tunneling microscopy (OPP-STM) has enabled the measurement of ultrafast dynamics in real space. However, the use of a pulse picker to ext. selected laser pulses to realize delay-time modulation, which efficiently suppresses the thermal expansion problems, limits the availability of time-resolved measurement. Here, we present a more applicable type of OPP-STM that we have developed. Two externally triggerable pulse lasers were used to produce pump and probe pulses, and wide-range delay-time modulation was simply realized by adjusting the timing of the pulses. The performance of this new type of OPP-STM was demonstrated by measuring the carrier dynamics in WSe2.
- 26Wang, H.; Zhang, C.; Chan, W.; Tiwari, S.; Rana, F. Ultrafast Response of Monolayer Molybdenum Disulfide Photodetectors. Nat. Commun. 2015, 6, 17– 20, DOI: 10.1038/ncomms9831
- 27Kar, S.; Su, Y.; Nair, R. R.; Sood, A. K. Probing Photoexcited Carriers in a Few-Layer MoS2 Laminate by Time-Resolved Optical Pump-Terahertz Probe Spectroscopy. ACS Nano 2015, 9, 12004– 12010, DOI: 10.1021/acsnano.5b04804[ACS Full Text], [CAS], Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslCqsbvE&md5=52013568453f688c93965122ade62e18Probing Photoexcited Carriers in a Few-Layer MoS2 Laminate by Time-Resolved Optical Pump-Terahertz Probe SpectroscopyKar, Srabani; Su, Y.; Nair, R. R.; Sood, A. K.ACS Nano (2015), 9 (12), 12004-12010CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)The dynamics of photoinduced carriers in a free-standing MoS2 laminate consisting of a few layers (1-6 layers) using time-resolved optical pump-terahertz probe spectroscopy is reported. Upon photoexcitation with the 800 nm pump pulse, the terahertz cond. increases due to absorption by the photoinduced charge carriers. The relaxation of the nonequil. carriers shows fast as well as slow decay channels, analyzed using a rate equation model incorporating defect-assisted Auger scattering of photoexcited electrons, holes, and excitons. The fast relaxation time occurs due to the capture of electrons and holes by defects via Auger processes, resulting in nonradiative recombination. The slower relaxation arises since the excitons are bound to the defects, preventing the defect-assisted Auger recombination of the electrons and the holes. The results provide a comprehensive understanding of the nonequil. carrier kinetics in a system of unscreened Coulomb interactions, where defect-assisted Auger processes dominate and should be applicable to other 2D systems.
- 28Wu, S. W.; Ogawa, N.; Ho, W. Atomic-Scale Coupling of Photons to Single-Molecule Junctions. Science 2006, 312, 1362– 1365, DOI: 10.1126/science.1124881[Crossref], [PubMed], [CAS], Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XltVOhtrg%253D&md5=38cf87ad4a29eea54e48c53a586686ccAtomic-Scale Coupling of Photons to Single-Molecule JunctionsWu, S. W.; Ogawa, N.; Ho, W.Science (Washington, DC, United States) (2006), 312 (5778), 1362-1365CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Spatial resoln. at the at. scale was achieved in the coupling of light to single mols. adsorbed on a surface. Electron transfer to a single mol. induced by green to near-IR light in the junction of a scanning tunneling microscope (STM) exhibited spatially varying probability that is confined within the mol. The mechanism involves photo-induced resonant tunneling in which a photoexcited electron in the STM tip is transferred to the mol. The coupling of photons to the tunneling process provides a pathway to explore mol. dynamics with the combined capabilities of lasers and the STM.
- 29Wang, Z.; Yoon, C.; Yoshida, S.; Arashida, Y.; Takeuchi, O.; Ohno, Y.; Shigekawa, H. Surface-Mediated Spin Dynamics Probed by Optical-Pump–Probe Scanning Tunneling Microscopy. Phys. Chem. Chem. Phys. 2019, 21, 7256, DOI: 10.1039/C8CP07786J[Crossref], [CAS], Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhvFelsr0%253D&md5=3b865077c003909246150c1a3231f0abSurface-mediated spin dynamics probed by optical-pump-probe scanning tunneling microscopyWang, Zi-Han; Yoon, Cheul-Hyun; Yoshida, Shoji; Arashida, Yusuke; Takeuchi, Osamu; Ohno, Yuzo; Shigekawa, HidemiPhysical Chemistry Chemical Physics (2019), 21 (14), 7256-7260CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)In current materials science and technologies, surface effects on carrier and spin dynamics in functional materials and devices are of great importance. In this paper, we present the surface-sensitive probing of electron spin dynamics, performed by optical-pump-probe scanning tunneling microscopy (OPP-STM). Time-resolved spin lifetime information on a manganese (Mn)-deposited GaAs(110) surface was successfully obtained for the first time. With increasing Mn d. via in situ evapn., a nonlinear change in the spin lifetime in the picosecond range was clearly obsd., while directly confirming the Mn d. by STM. In comparison with the results obtained by the conventional OPP method, we have also demonstrated that the obsd. nonlinear spin lifetime behavior was surface-mediated, which can be characterized using only the surface-sensitive OPP-STM technique.
- 30Loth, S.; Etzkorn, M.; Lutz, C. P.; Eigler, D. M.; Heinrich, A. J. Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution. Science 2010, 329, 1628– 1630, DOI: 10.1126/science.1191688[Crossref], [PubMed], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtFOqsr3F&md5=bb3d82531937f4f884e1a91f1ac516ebMeasurement of Fast Electron Spin Relaxation Times with Atomic ResolutionLoth, Sebastian; Etzkorn, Markus; Lutz, Christopher P.; Eigler, D. M.; Heinrich, Andreas J.Science (Washington, DC, United States) (2010), 329 (5999), 1628-1630CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resoln. of spin relaxation measurements to the at. scale was a challenge. A scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers vary 50-250 ns. The method can in principle be generalized to monitor the temporal evolution of other dynamical systems.
- 31Krause, S.; Sonntag, A.; Hermenau, J.; Friedlein, J.; Wiesendanger, R. High-Frequency Magnetization Dynamics of Individual Atomic-Scale Magnets. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 064407, DOI: 10.1103/PhysRevB.93.064407
- 32Yoshida, S.; Aizawa, Y.; Wang, Z. H.; Oshima, R.; Mera, Y.; Matsuyama, E.; Oigawa, H.; Takeuchi, O.; Shigekawa, H. Probing Ultrafast Spin Dynamics with Optical Pump-Probe Scanning Tunnelling Microscopy. Nat. Nanotechnol. 2014, 9, 588– 593, DOI: 10.1038/nnano.2014.125[Crossref], [PubMed], [CAS], Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVKls7%252FJ&md5=ed99367f0c33d5c7aac42e896eb174eaProbing ultrafast spin dynamics with optical pump-probe scanning tunnelling microscopyYoshida, Shoji; Aizawa, Yuta; Wang, Zi-han; Oshima, Ryuji; Mera, Yutaka; Matsuyama, Eiji; Oigawa, Haruhiro; Takeuchi, Osamu; Shigekawa, HidemiNature Nanotechnology (2014), 9 (8), 588-593CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Optical pump-probe scanning tunneling microscopy is demonstrated, which enables the nanoscale probing of spin dynamics with the temporal resoln. corresponding, in principle, to the optical pulse width. Spins are optically oriented using circularly polarized light, and their dynamics are probed by scanning tunnelling microscopy based on the optical pump-probe method. Spin relaxation in a single quantum well with a width of 6 nm was obsd. with a spatial resoln. of ∼1 nm. In addn. to spin relaxation dynamics, spin precession, which provides an estn. of the Land´e g factor, was obsd. successfully.
- 33Li, S.; Chen, S.; Li, J.; Wu, R.; Ho, W. Joint Space-Time Coherent Vibration Driven Conformational Transitions in a Single Molecule. Phys. Rev. Lett. 2017, 119, 176002, DOI: 10.1103/PhysRevLett.119.176002[Crossref], [PubMed], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhs1WlsLnP&md5=af5ca44d02cb52e8c390a5450dcf8b78Joint space-time coherent vibration driven conformational transitions in a single moleculeLi, Shaowei; Chen, Siyu; Li, Jie; Wu, Ruqian; Ho, W.Physical Review Letters (2017), 119 (17), 176002/1-176002/5CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)We report single-mol. conformational transitions with joint angstrom-femtosecond resoln. by irradiating the junction of a scanning tunneling microscope with femtosecond laser pulses. An isolated pyrrolidine mol. adsorbed on a Cu(001) surface undergoes reversible transitions between two conformational states. The transition rate decays in time and exhibits sinusoidal oscillations with periods of specific mol. vibrations. The dynamics of this transition depends sensitively on the mol. environment, as exemplified by the effects of another mol. in proximity.
- 34Yoshida, S.; Taninaka, A.; Sugita, Y.; Katayama, T.; Takeuchi, O.; Shigekawa, H. Revealing the Conformational Dynamics in a Single-Molecule Junction by Site- and Angle-Resolved Dynamic Probe Method. ACS Nano 2016, 10, 11211– 11218, DOI: 10.1021/acsnano.6b06278[ACS Full Text], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvFOiurnF&md5=56488c790bcb22dd0bb3f2141d47e47fRevealing the Conformational Dynamics in a Single-Molecule Junction by Site- and Angle-Resolved Dynamic Probe MethodYoshida, Shoji; Taninaka, Atsushi; Sugita, Yoshihiro; Katayama, Tomoki; Takeuchi, Osamu; Shigekawa, HidemiACS Nano (2016), 10 (12), 11211-11218CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Single-mol. junctions were extensively studied because of their high potential for future nanoscale device applications as well as their importance in basic studies for mol. science and technol. However, since the bonding sites at an electrode and the mol. tilt angles, for example, cannot be detd. exptl., analyses were performed assuming the structures of such interactive key factors, with uncertainties and inconsistencies remaining in the proposed mechanisms. The authors have developed a methodol. that enables the probing of conformational dynamics in single-mol. junctions simultaneously with the direct characterization of mol. bonding sites and tilt angles. This technique revealed the elemental processes in single-mol. junctions, which were not clarified using conventional methods. The mechanisms of the mol. dynamics in 1,4-benzenedithiol and 4,4'-bipyridine single-mol. junctions, which, for example, produce binary conductance switching of different types, were clearly discriminated and comprehensively explained.
Supporting Information
ARTICLE SECTIONSThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.9b00266.
Schematic of experimental setup, data plots showing dependence of THz waveform at tip apex on electric field strength, frequency dependence of THz electric field modulated by the two types of nanotips, FDTD simulations of near field, measurement results for HOPG, relationship between tip-enhanced near field strength and THz intensity, tunnel spectrum of Bi2Se3, tip–sample distance dependence of the near-field waveform at the tip apex (PDF)
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