Theory for Nonlinear Spectroscopy of Vibrational Polaritons

TL;DR

This paper develops a quantum-mechanical theory for pump-probe spectroscopy of vibrational polaritons, aligning well with experimental data and offering insights into their nonlinear optical properties and vibrational anharmonicities.

Contribution

It introduces a comprehensive quantum theory based on Langevin and input-output formalisms for vibrational polaritons, aiding interpretation of complex spectroscopic signals.

Findings

Good agreement with experimental spectra when considering vibrational anharmonicities

Provides an intuitive mode-coupling interpretation of spectroscopic data

Establishes a theoretical framework for analyzing nonlinear optical properties

Abstract

Molecular polaritons have gained considerable attention due to their potential to control nanoscale molecular processes by harnessing electromagnetic coherence. Although recent experiments with liquid-phase vibrational polaritons have shown great promise for exploiting these effects, significant challenges remain in interpreting their spectroscopic signatures. In this letter, we develop a quantum-mechanical theory of pump-probe spectroscopy for this class of polaritons based on the quantum Langevin equations and the input-output theory. Comparison with recent experimental data shows good agreement upon consideration of the various vibrational anharmonicities that modulate the signals. Finally, a simple and intuitive interpretation of the data based on an effective mode-coupling theory is provided. Our work provides a solid theoretical framework to elucidate nonlinear optical properties of molecular polaritons as well as to analyze further multidimensional spectroscopy experiments on these systems.