Laser-Induced Transmission on Ultrafast Timescales via Vibrational Strong Coupling

Strong coupling between vibrational modes and cavity optical modes leads to the formation of vibration-cavity polaritons, separated by the vacuum Rabi splitting. The splitting depends on the square root of the concentration of absorbers confined in the cavity. This dependence has important implications on the response of the coupled system after ultrafast infrared excitation. In this work, we report on solutions of W(CO)₆ in hexane with concentration chosen to access a regime that borders on weak coupling. Under these conditions, huge fractions of the W(CO)₆ oscillators can be excited, and the anharmonicity of the molecules leads to a commensurate reduction in the Rabi splitting. We report excitation fractions >0.8 and show drastic increases in transmission that can be modulated on the picosecond timescale. In comparison to previous experiments, the transient spectra we observe are much simpler because excited-state transitions lie outside the transmission spectrum of the cavity and, so, give much smaller contributions to the spectra. We find that the Rabi splitting recovers with the vibrational relaxation lifetime of uncoupled W(CO)₆, implying that polaritons are not directly involved in the relaxation after the first few ps.