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, 54 (27), 7971-5

Enhanced Raman Scattering From Vibro-Polariton Hybrid States


Enhanced Raman Scattering From Vibro-Polariton Hybrid States

Atef Shalabney et al. Angew Chem Int Ed Engl.


Ground-state molecular vibrations can be hybridized through strong coupling with the vacuum field of a cavity optical mode in the infrared region, leading to the formation of two new coherent vibro-polariton states. The spontaneous Raman scattering from such hybridized light-matter states was studied, showing that the collective Rabi splitting occurs at the level of a single selected bond. Moreover, the coherent nature of the vibro-polariton states boosts the Raman scattering cross-section by two to three orders of magnitude, revealing a new enhancement mechanism as a result of vibrational strong coupling. This observation has fundamental consequences for the understanding of light-molecule strong coupling and for molecular science.

Keywords: Raman scattering; optical cavity; strong coupling; vibrations; vibro-polariton states.


Figure 1
Figure 1
Schematic illustration of strong coupling between a molecular resonance with frequency ωm and a cavity mode with frequency ωc, generating new light–matter hybrid states P+ and P−, separated by the vacuum Rabi splitting ħωVR.
Figure 2
Figure 2
a) Energy-level diagram under vibrational strong coupling showing the scheme for the Raman scattering from the new vibro-polariton states. b) Schematic description of the FP cavity used in this study composed of two thin Ag mirrors spaced by a PVAc layer. Raman excitation performed using a micro-Raman system with an objective lens. The inset shows the 3D structure of the PVAc monomer emphasizing the C=O bond, the target for VSC, with the exciting and Stokes scattered photons generating Raman scattering from the coupled vibration. |E1|2 (black line) shows qualitatively the intensity distribution of the first cavity mode with which the C=O resonators are coupled.
Figure 3
Figure 3
a) Red and dashed black curves are the measured and calculated transmission of a thin PVAc film (about 2 µm) deposited on Ge substrate. The green dashed line shows the fundamental cavity mode after deactivating all the absorption bands of the PVAc, leaving only the background refractive index of the cavity. The blue curve is the transmission of the coupled cavity. The inset shows the C=O band and the uncoupled cavity mode without the C=O resonators (white dashed curve) superimposed on the dispersion of the cavity after adding the C=O resonators. b) Raman scattering from the cavity that is not in the VSC regime given in (a) (red spectrum) and the reference sample (black spectrum). c) Cavity transmission without C=O resonators (dashed green) and with C=O resonators (solid blue) showing the formation of VP+ and VP− in the VSC regime. The inset shows in color plot the dispersion of VP+ and VP− with the dispersion of the uncoupled C=O resonators (dashed white line) and the uncoupled cavity mode (dashed parabola). d) Raman scattering from the cavity given in (c) showing in the inset the new density of states at 1966 cm−1 and 1600 cm−1 which correspond to the generation of VP+ and VP−, respectively. The Raman signals are vertically shifted for clarity.
Figure 4
Figure 4
a) Transmission spectra of different cavities tuned around the C=O absorption band (black dotted curve). The detuning from the absorption can be seen by slightly shifting the uncoupled cavity modes (dashed colored curves) with respect to the C=O band at 1740 cm−1. b) Raman scattering from the same cavities with corresponding numbers as in (a), showing the change in Raman intensity from VP+ and VP− versus fine-tuning the cavity mode around the C=O absorption band. The spectra in (a) and (b) are shifted for clarity.

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