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Figure 1. Strong coupling in open gratings. (a) Raman microscopy (pump 532 nm) of grating (period \Lambda=4.7 µm, slot width 1 µm, height h=0.1 µm, with 1 µm thick PMMA layer). (b) Grating IR reflectance spectra in experiment (expt, left) and theory (RCWA, right). (c) Raman scattering from ground state \left.\left|g\right.\right\rangle to excited state \left.\left|e\right.\right\rangle or VibPERS to lower \left.\left|LP\right.\right\rangle and upper \left.\left|UP\right.\right\rangle polariton states. (d) VibPERS spectra of slot and mesa in grating compared to flat regions. Left: Optical image of grating. Middle: Background-corrected Raman spectra. Right: Raman spectra of slot and mesa after subtracting flat spectrum.

Figure 2. Lower polariton enhanced Raman and localization in grating slots. (a) Raman maps acquired at two different heights above the grating surface. (b) Raman map at PMMA surface (z=1 µm) showing integrated peak area of lower polariton (A_{LP}). (c,d) Raman maps at grating surface (z=0 µm) for (c) LP (A_{LP}) and (d) PMMA (A_{PMMA}) modes with shared spectrally-integrated intensity scale. White scale bars are 2µm. (e-h) Laterally averaged A_{LP} and \omega_{LP}-\omega_0 across the grating for (e,f) z=1 µm and (g,h) z=0 µm.

Figure 3. Detuning of grating modes. (a) VibPERS spectra of lower polariton mode for different grating periods (\Lambda). (b) Laterally-averaged lower polariton mode (integrated area) vs x-position, which is localised at the grating slots (vertical bar is 100 cts\cdotcm-1mW-1s-1). Horizontal gray bars indicate slot positions.

Figure 4. Polariton-enhanced Raman scattering in gratings. (a) Field distribution (E_x/E_0) for the lower polariton under near-normal incidence (\theta=0.1°) transverse-magnetic (TM) excitation perpendicular to the grating grooves (dashed). Gold’s permittivity from Ref.25. (b) Coupled oscillator fit to RCWA simulations of grating scattering for upper and lower polariton modes vs momentum (k_x). Colours show vibrational Hopfield coefficient fraction (SI Section S2). (c) Molecular density-of-states (corrected and angle-averaged) showing asymmetric broadening of LP peak (grey bar indicating extinction of 0.1). (d) Polariton states showing scattering from ground state to bright (strongly coupled) and dark states, labelling Raman scattering (\omega) and infrared absorption (\nu). (e) Plasmon-vibration coupling strength g vs normalised position x/\Lambda, with red points from Fig. 2. (f) Map of spatial plasmon-vibration coupling strength g along the \Lambda=4.7 µm grating (modelled surface profile in white).