Raman Probing the Local Ultrastrong Coupling of Vibrational Plasmon Polaritons on Metallic Gratings

Strong coupling of molecular vibrations with light creates polariton states, enabling control over many optical and chemical properties. However, the near-field signatures of strong coupling are difficult to map as most cavities are closed systems. Surface-enhanced Raman microscopy of open metallic gratings under vibrational strong coupling enables the observation of spatial polariton localization in the grating near field, without the need for scanning probe microscopies. The lower polariton is localized at the grating slots, displays a strongly asymmetric line shape, and gives greater plasmon-vibration coupling strength than measured in the far field. Within these slots, the local field strength pushes the system into the ultrastrong coupling regime. Models of strong coupling which explicitly include the spatial distribution of emitters can account for these effects. Such gratings enable exploration of the rich physics of polaritons, its impact on polariton chemistry under flow conditions, and the interplay between near- and far-field properties through vibrational polariton-enhanced Raman scattering.

Figure 1

Strong coupling in open gratings. (a) Raman microscopy (pump 532 nm) of grating (period $\mathrm{\Lambda }=4.7\mathrm{\mu }\mathrm{m}$, slot width $1\mathrm{\mu }\mathrm{m}$, height $h=0.1\mathrm{\mu }\mathrm{m}$, with $1\mathrm{\mu }\mathrm{m}$ thick PMMA layer). (b) Grating IR reflectance spectra in experiment (expt, left) and theory (RCWA, right). (c) Raman scattering from ground state $|g⟩$ to excited state $|e⟩$ or VibPERS to lower $|\mathrm{L}\mathrm{P}⟩$ and upper $|\mathrm{U}\mathrm{P}⟩$ 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 ($z=1\mathrm{\mu }\mathrm{m}$) showing integrated peak area of lower polariton ($A_{\mathrm{L}\mathrm{P}}$). (c, d) Raman maps at grating surface ($z=0\mathrm{\mu }\mathrm{m}$) for (c) LP ($A_{\mathrm{L}\mathrm{P}}$) and (d) PMMA ($A_{\mathrm{P}\mathrm{M}\mathrm{M}\mathrm{A}}$) modes with shared spectrally integrated intensity scale. White scale bars are $2\mathrm{\mu }\mathrm{m}$. (e)–(h) Laterally averaged $A_{\mathrm{L}\mathrm{P}}$ and ${\omega }_{\mathrm{L}\mathrm{P}}\text{−}{\omega }_0$ across the grating for (e),(f) $z=1$ and (g),(h) $z=0\mathrm{\mu }\mathrm{m}$.

Figure 3

Detuning of grating modes. (a) VibPERS spectra of lower polariton mode for different grating periods ($\mathrm{\Lambda }$). (b) Laterally averaged lower polariton mode (integrated area) vs $x$ position, which is localized at the grating slots (vertical bar is $100\mathrm{cts}·{\mathrm{cm}}^{-1}{\mathrm{mW}}^{-1}{\mathrm{s}}^{-1}$). 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 permittivity from Ref. [26]. (b) Coupled oscillator fit to RCWA simulations of grating scattering for upper and lower polariton modes vs momentum ($k_x$). Colors show vibrational Hopfield coefficient fraction (Supplemental Material, Sec. S2 [24]). (c) Molecular density-of-states (corrected and angle-averaged) showing asymmetric broadening of LP peak (gray bar indicating extinction of 0.1). (d) Polariton states showing scattering from ground state to bright (strongly coupled) and dark states, labeling Raman scattering ($\omega$) and infrared absorption ($\nu$). (e) Plasmon-vibration coupling strength $g$ vs normalized position $x/\mathrm{\Lambda }$, with red points from Fig. 2. (f) Map of spatial plasmon-vibration coupling strength $g$ along the $\mathrm{\Lambda }=4.7\mathrm{\mu }\mathrm{m}$ grating (modeled surface profile in white).