Microlens Hollow-Core Fiber Probes for Operando Raman Spectroscopy

We introduce a flexible microscale all-fiber-optic Raman probe which can be embedded into devices to enable operando in situ spectroscopy. The facile-constructed probe is composed of a nested antiresonant nodeless hollow-core fiber combined with an integrated high refractive index barium titanate microlens. Pump laser 785 nm excitation and near-infrared collection are independently characterized, demonstrating an excitation spot of full-width-half-maximum 1.1 μm. Since this is much smaller than the effective collection area, it has the greatest influence on the collected Raman scattering. Our characterization scheme provides a suitable protocol for testing the efficacy of these fiber probes using various combinations of fiber types and microspheres. Raman measurements on a surface-enhanced Raman spectroscopy sample and a copper battery electrode demonstrate the viability of the fiber probe as an alternative to bulk optic Raman microscopes, giving comparable collection to a 10 objective, thus paving the way for operando Raman studies in applications such as lithium battery monitoring.

Optical transmission of the NANF probe  S1 show the properties of the internal structure of the NANF used in this work, Fibre B from Sakr et al.. Figure S2 shows the ARROW model predictions for this fibre, given its strut thickness, clearly showing its suitability for 785 nm Raman applications.Figure S3 shows the actual attenuation data for the fibre, as seen in Sakr et al.. 29 At 10m in length, the resonance window of the fibre is measured to be 645-1021 nm.Hence, this fibre guides from the pump wavelength of 785 nm up to Raman shifts of 2945 cm -1 , covering the Raman fingerprint region.

Preparation of NPoM samples
The Au manoparticle on a mirror (NPoM) samples (discussed in Figure 7), were fabricated following the procedure outlined in Ref [S1].

Figure S1 and Table
Figure S1 and TableS1show the properties of the internal structure of the NANF used in this work, Fibre B from Sakr et al.. FigureS2shows the ARROW model predictions for this fibre, given its strut thickness, clearly showing its suitability for 785 nm Raman applications.FigureS3shows the actual attenuation data for the fibre, as seen in Sakr et al..29  At 10m in length, the resonance window of the fibre is measured to be 645-1021 nm.Hence, this fibre guides from the pump wavelength of 785 nm up to Raman shifts of 2945 cm -1 , covering the Raman fingerprint region.

Figure S1
FigureS1 An illustration of the parameters of fibre internal structure, using an SEM image to derive the glass structure outline.

Figure
Figure S2 ARROW model predictions for the NANF with strut thickness 580 ±10 nm.785 nm pump wavelength for Raman spectroscopy is shown as a black horizontal line, and relevant Raman scattering wavelengths are shown as a red shaded region (0-1900 cm -1 ).

Figure
Figure S3 The measured transmission and loss of the NANF used, as seen in Sakr et al.. (29) The red shaded region corresponds to wavenumbers in the Raman fingerprint region 0-1900 cm -1 .

Figure
FigureS4shows the optical setup for the collection characterisation experiments.For Figure5and 6, the excitation set-up is the same and shown in FigureS4a.The 405 nm laser diode is filtered by a 405 nm bandpass filter and launched into single mode fibre with a 4× objective, by placing the SMF on a translation stage.To collect the signals shown in Figures5 and 6, a 250 mm focal length lens launched light into a MMF.This 200 m MMF delivers light to a fibre-coupled Ocean Optics spectrometer, as shown by Figure S4 b -d.When collecting the 780 nm QD emissions, as in Figure 5 d and Figure 6 b, 450 nm and 650 nm long pass filters are used.

Figure
Figure S4 Optical set-up variations for characterising the signal collection of the probe.(a) The 405 nm laser diode is filter by a 405 nm bandpass, F1, and coupled into the unmodified SMF facet with a 4× objective.Depending on the sample, (b) uncoated SMF, (c) a SMF coated with QDs, or (d) the signal of the coated SMF collected by the fibre-probe, a 60× or 10× objective collects the transmitted signal.The signal is focused into a MMF by lens L1 with focal length 250 mm.The MMF delivers the light to the fibre-coupled spectrometer.The QD emission signal is filtered by F2, 650 nm long pass filter, to remove residual 405 excitation.

Table SI 1
The parameters of the fibre internal structures, as shown in FigureS1.