Two-Color Spatially Resolved Tuning of Polymer-Coated Metasurfaces

For the realization of truly reconfigurable metasurface technologies, dynamic spatial tuning of the metasurface resonance is required. Here we report the use of organic photoswitches as a means for the light-induced spatial tuning of metasurface resonances. Coating of a dielectric metasurface, hosting high-quality-factor resonances, with a spiropyran (SPA)-containing polymer enabled dynamic resonance tuning up to 4 times the resonance full-width at half-maximum with arbitrary spatial precision. A major benefit of employing photoswitches is the broad toolbox of chromophores available and the unique optical properties of each. In particular, SPA and azobenzene (AZO) photoswitches can both be switched with UV light but exhibit opposite refractive index changes. When applied to the metasurface, SPA induced a red shift in the metasurface resonance with a figure of merit of 97 RIU–1, while AZO caused a blue shift in the resonance with an even greater sensitivity of 100 RIU–1. Critically, SPA and AZO can be individually recovered with red and blue light, respectively. To exploit this advantage, we coated a dielectric metasurface with spatially offset SPA- and AZO-containing polymers to demonstrate wavelength-dependent, spatially resolved control over the metasurface resonance tuning.


Polymer Film Preparation
The resulting film thicknesses produced with a spin rate of 1500 rpm are outlined in Table S1.When these films were found to be too thick, the spin rate was increased 3000 rpm (acceleration of 1000 rpm/s) for 5 mins, producing the film thicknesses reported in Table S1.Where both polymers were applied to a single metasurface, the pAZO thin film was prepared first to cover half of the metasurface arrays.The pSPA thin film was then prepared covering the second half of the arrays.Figure S1 depicts the typical separation of the polymer films immediately after the spin coating procedure, as well as after UV and red light irradiation.

Ellipsometry
The dispersive refractive indices determined via ellipsometry are shown in Figure S2.

Metasurface Transmission Measurements
The polarisation dependent transmittance spectra, described in detail in the Experimental Methods section of the manuscript is depicted in Figure S3.

Spatial Patterning of Polymer-Coated Metasurface
A schematic diagram depicting the apparatus for the spatial patterning of the metasurface using a digital micromirror device (DMD) is depicted in Figure S4.Full experimental details are provided in the Experimental Methods section of the manuscript. sample

3) Synthesis of 2-(3' ,3' -Dimethyl-6-nitro-3'H-spiro[chromene-2,2' -indol]-1' -yl)-ethanol
9,9,9a-trimethyl-2,3,9,9a-tetrahydro-oxazolo[2,3-a]indole (step 2) (0.85 g, 4,2 mmol, 1 eq) and 2-hydroxy-5-nitrobenzaldehyde (1.054 g, 6.3 mmol, 1.5 eq) were mixed in 10 mL of ethanol and heated under reflux for 3 h.After cooling to room temperature, the remaining solution was filtered and washed with ethanol.The red crystals (0.6319 g, 33%) were analysed via 1 H NMR: (4   The calculated transmission spectra of the metasurface when coated with 500 nm polymer layer with refractive index varying from 1.0 to 1.7 are presented in Figure S7.The top row presents data for x-polarised incident light and the bottom row is for y-polarised incident light.In both cases, the shifts of the four main resonances are indicated with a dashed grey line.For xpolarised light, the most significant shift is observed for X3, the anti-parallel electric dipole depicted in the near field profiles of Figure S7b and S7c.This transmission dip associated with resonance X3 shifts from l = 1235 nm when n = 1.0 to l = 1483 nm when n = 1.7, (average sensitivity of 354 nm RIU -1 ) highlighting the sensitivity of this resonance to the local environment.The resonances X1 and X4, arise from the in-plane anti-parallel electric and magnetic dipoles, respectively.Both these resonances are highly localised within the nanobars, and hence only experience a weak shift in response to the refractive index change.Similarly, the transmission dip attributed to X2 arises from the parallel-out of plane electric dipoles located within the nanobars, which are largely insensitive to the polymer refractive index change.
The bottom row of Figure S7, shows the transmission of y-polarised light through the polymer coated metasurface.The main resonance of interest to this work is the parallel out-of-plane magnetic dipole resonance Y3 located between the two bars of the unit cell.The transmission dip attributed to this resonance undergoes a significant shift from l = 1313 nm when n = 1.0 to l = 1644 nm when n = 1.7, corresponding to an average sensitivity of 473 nm RIU -1 , significantly larger than that of resonance X3.The Q factor of this resonance also increases as the refractive index increases, and the resonance becomes localised between the nanobars.Resonance Y1 is physically the equivalent to resonance Y3, but is located between the nanobars of neighbouring unit cells, as opposed to between the two nanobars within a single unit cell.As a result, this resonance also undergoes a significant shift with refractive index change.
Resonance Y2 and Y4 are anti-parallel in-plane and out-of-plane magnetic dipole resonances, respectively.These resonances are both highly localised within the nanobars and hence show minimal shift with refractive index change.

Simulations of Polymer Switching
The expected resonance shifts caused by photoswitch isomerisation, simulated in CST Studio Suite, are presented in Figure S8.The metasurface parameters were identical to those outlined in Section 3.1.The polymer films were assumed to be 500 nm thick, with refractive indices determined from ellipsometry measurements reported in Section 1.3.The top row of Figure S8 contains the data from pAZO coated metasurface (pAZO-MS) and the bottom row presents data from pSPA coated metasurface (pSPA-MS).
For pAZO-MS, the simulations predict a weak blue shift in the resonance position, qualitatively confirming the experimental results.The transmission dips seen here are weaker than those observed in experiments due to small variations in the k values obtained from fits to ellipsometry data.Forcing k to be 0 for l > 600 nm increased the magnitude of this dip.For pSPA a strong redshift in the resonance is observed upon switching.The magnitude of the resonance shift is approximately equal to that observed in experiments.

LED Spectral Analysis
The LEDs used in this work are Thorlabs M340L4, M365L3, M415L4, M625L4, which were all driven by a LED driver Thorlabs LEDD1B.An additional 450 nm (22 cd) from Roithner (Vienna, Austria) was powered by an external power supply.LED emission spectra were recorded using a ocean Insight Flame-T spectrometer and are presented in Figure S9.

Resolution of Spatial Patterning
The resolution of the spatial patterning was assessed by generating periodic patterned 450 nm irradiation using the DMD.First the image size on the camera was calibrated using white light illumination of a single metasurface array with known dimensions of 600 x 600 μm.
A thin film of pSPA was coated onto a glass coverslip using the standard spin coating procedure and was placed into the optical setup outlined in Section 1.7.The entire sample was exposed to 365 nm UV irradiation for 1 minute, and then subsequently exposed to the periodically patterned 450 nm light for 5 minutes.The spatial resolution was determined based on the transmission of the white light arm through the pSPA film (in its initial state SPA will transmit light and after UV irradiation it becomes opaque in the visible region).Based on this analysis the minimum feature resolution was found to be 11 μm.The spatial resolution reported here was limited by the optical setup and choice of objective, and is independent of the properties of the polymer.It is expected that higher resolutions can be achieved with a higher NA objective in place of L3 in Figure S4.

Metasurface Resonance Switching with Thick Polymer Film
Figure S11 summarises the initial attempts to achieve metasurface resonance tuning with polymer layers spin coated at 1500 rpm (resulting in polymer coatings 680 ± 10 nm thick).Transmission measurements of the polymer-coated metasurface arrays, under various irradiation conditions, are presented in Figure S11a.For ease of interpretation, the same data is presented in Figure S11c normalised to the wavelength of the initial resonance minima.

μm
Further, Gaussian fits were performed to the transmission valleys, so the shift in the minimum wavelength can be easily visualised in Figure S11d.
UV irradiation with a 365 nm LED induced only minor red-and blueshifts of ~1 nm for both the pSPA coated metasurface (pSPA-MS) and pAZO coated metasurface (pAZO-MS), respectively.5 minutes of 625 nm red light irradiation recovered the pSPA-MS resonance position, but also weakly shifted the pAZO resonance towards its' initial position.415 nm blue light had the most significant impact, recovering the pAZO-MS resonance position within 2 minutes, but simultaneously caused a significant redshift in the pSPA-MS resonance position.
It was this observation that helped to identify the polymer thickness as the primary cause of the weak shifts.Since the light absorption of pSPA at 415 nm is much weaker than at 365 nm (see Figure 2a of the manuscript), better light penetration, and hence switching, is produced deep into the polymer film where the metasurface is located.At 365 nm, the absorption coefficient of the pSPA film is ~6 μm -1 , meaning 90% of the light is attenuated before reaching the top of the nanoresonators located 400 nm below the polymer surface.At 415 nm the absorption coefficient reduces to 0.1 allowing 87% of the incident light to penetrate through to the nanoresonators.Reducing the height of the polymer layer, by increasing the speed of the spin coating procedure, while changing no other parameters produced the results presented in Figure 5 of the manuscript.

Resonance Tuning with 1:1 pSPA:pAZO Mixture
Initally it was planned to spincoat the metasurface with a polymer film containing a 1:1 weight ratio of pSPA and pAZO.For this, separate 5 % wt.stock solutions of pSPA and pAZO were prepared in CHCl3 and THF, respectively and passed through a 200 μm PTFE filter.300 μL from each stock solution was added to a fresh vial, which was subsequently placed in an ultrasonic bath for 5 minutes.The absorption properties and switching rates of thin films of each photoswitches prepared with CHCl3 or THF were measured and found to be comparable, as seen in Figure S11.It was therefore deemed that the effect of mixing solvents on the switching behaviour was negligible.A thin film of the polymer mixture was prepared on the metasurface array using the usual spin coating procedure at 1500 rpm.Transmission measurements of the polymer coated metasurface with both x-and y-polarised incident light were performed and are presented in Figure S12.UV light induced an 11 nm and 9 nm redshift in the X3 and Y3 resonances, respectively.No irradiation conditions were identified which produced a significant blueshift from the original resonance position.

Figure S1 .
Figure S1.Sample images (A) before polymer coating was applied, (B) after both polymers were applied (note that pSPA is transparent), (C) after irradiation with UV light and (D) after irradiation with red light.(E) Diagram of metasurface arrays with varying EBL doses.

Figure S2 .
Figure S2.Ellipsometry Data.Full spectrum ellipsometry data of (top) pSPA and (bottom) pAZO, measured before (black) and after (coloured) irradiation with 365 nm LED.Solid lines indicate the real component of the refractive index (left axis) and dashed lines indicate the imaginary component (right axis).

L 6 LEDFigure S4 .
Figure S4.Optical apparatus for spatial patterning of metasurface resonance tuning.Light from a 450 nm CW laser initial passes through beam expansion optics (L1, f = 20 mm and L2, f = 50 mm) before being directed onto the DMD.The pattern is then imaged onto the sample and the transmitted light is incident on a CMOS camera.A removable mirror couples in white light for imaging purposes.M indicates a mirror and the focal lengths of the marked lenses are: fL1 = 20 mm, fL2 = 50 mm, fL3 = 200 mm, fL4 = 30 mm, fL5 = 25 mm, fL6 = 100 mm, and fL7 = 12 mm.

Figure S7 .
Figure S7.Polymer-coated metasurface simulations.(A,D) simulations showing expected electromagnetic resonances when the two bar metasurface is coated with a 500 nm polymer layer with varying refractive index and illuminated with (A) x-and (D) y-polarised light.Grey dashed lines are a guide to the eye only, highlighting the shift of each resonance.(B,E) Electric near-field profiles and (E,F) magnetic near-field profiles in the unit cell of the metasurface when coated with a polymer of refractive index (B,E) n=1 and (C,F) n=1.7.

Figure S8 .
Figure S8.Simulated polymer-coated metasurface tuning.(A,D) structures of photoswitch homopolymers including diagram of polymer coated two bar metasurface.(B-F) Simulations of metasurface resonance position before (solid) and after (dashed) irradiation with UV light, measured with (B,E) x-polarised light and (C,F) y-polarised incident light.(B-C) pAZO coated metasurface, (E-F) pSPA coated metasurface.The insets in (B-F) show close-ups of the transmission within the spectral tuning range of the respective high-Q resonances.

Figure S9 .
Figure S9.Experimentally recorded spectra of LEDs used in this work.

Figure S10 :
Figure S10: Resolution of spatial patterning.(A) White light image of metasurface array with known dimensions of 600 x 600 µm used to calibrate the scale of the image.(B) white light image of periodic pattern projected onto pSPA film.Inset shows 450 nm DMD projection.(C) greyscale image analysis showing resolution of irradiated lines.

Figure S11 .
Figure S11.Metasurface resonance tuning with thick polymer film.(A) Transmission measurements of x-polarised light incident on pSPA-MS (purple) and pAZO-MS (orange) measured prior to irradiation (black solid), in the areas exposed to UV light only (solid coloured), red light (dot coloured) and finally blue light (dash-dot coloured).The inset shows a close-up of the transmission within the spectral tuning range of the X3 resonances.(B) schematic diagram of metasurface unit cell indicating height of polymer layer.(C) Metasurface resonance shifts after various irradiation conditions.Data here is the same as in (A) but with the wavelength normalised to the position of the initial resonance minima.(D) Shift in wavelength minima, determined from Gaussian fits to data in (C), after various irradiation conditions.

Figure S11 :
Figure S11: Solvent dependent photoswitching.(A) Optical density spectra of a thin film of pSPA dispersed in THF (solid) and CHCL3 (dotted) before (grey) and after (black) irradiation with a UV LED.Inset shows the chemical structure of pSPA.Arrow indicates wavelength used for kinetic measurements.(B) kinetic optical density measurements of pSPA prepared in CHCl3 at 570 nm during irradiation with various coloured LEDs indicated by shaded regions.(C) same as B, but for pSPA prepared in THF.(D-E) same as A-C, but for pAZO thin films.

Figure S12 :
Figure S12: Metasurface resonance tuning with 1:1 polymer mixture.(A) chemical structures and schematic diagram indicating polymer mixture spin coat onto metasurface array.(B-C) Transmission measurements of (B) x-polarised and (C) y-polarised light incident on the metasurface array coated with a 1:1 polymer mixture.Measurements were recorded prior to irradiation (solid black), and after irradiation with UV (solid maroon), Red (dashed maroon) and Blue (dotted marron) LEDs.Insets show zoomed-in region of X3 and Y3 resonances.The insets in (B-C) show close-ups of the transmission within the spectral tuning range of the respective high-Q resonances.

Table S1 .
Spin coat parameters and resulting film thickness determined from ellipsometry measurements.
The product was purified via column chromatography with chloroform to yield 107.6 mg (0.265 mmol, 18.6%).
g, 1.42 mmol) and trimethylamine (0.197 mL, 1.42 mmol) were dissolved in dichloromethane, purged with argon, and cooled to −35 •C.A solution of acryloyl chloride in dichloromethane was added slowly and the reaction mixture was heated up to room temperature overnight.The solution was extracted with saturated NaHCO3 (2 × 20 mL) and water (2 × 20 mL), and the organic phases were combined, dried over MgSO4, filtered, and evacuated under reduced pressure.