Phonon-Polaritons in Non-Centrosymmetric Systems: Theory of Terahertz Pump-Optical Probe Spectroscopy

TL;DR

This paper develops a theoretical framework to understand phonon-polaritons in non-centrosymmetric systems, emphasizing their roles in THz pump-optical probe spectroscopy and expanding experimental phase space.

Contribution

It introduces a many-body nonlinear optical model and perturbative Maxwell's equations to describe phonon-polariton interactions in various experimental schemes.

Findings

Broadband THz pumps enhance accessible phonon-polariton dispersion.

The framework accurately describes existing experiments and guides future research.

It offers a general approach for modeling light-lattice hybridization in time-resolved spectroscopy.

Abstract

Hybrid lattice-light modes, known as phonon-polaritons, represent the backbone of advanced protocols based on THz pumping of infrared modes. Here we provide a theoretical framework able to capture the different roles played by phonon-polaritons in experimental protocols based either on Raman-like pump and probe schemes, typical of four-wave-mixing processes, or on THz pump-visible probe three-wave mixing protocols. By using a many-body description of the nonlinear optical kernel, along with a perturbative solution of nonlinear Maxwell's equations, we highlight the advantages of exploiting broadband THz pumps to enlarge the phase space of the phonon-polariton dispersion accessible in a single experiment. Besides providing a quantitative description of existing and future experiments, our results offer a general framework for the theoretical modeling of the hybridization between light and lattice degrees of freedom in time-resolved experiments.