Nonlinear spectroscopy of collective modes in excitonic insulator

TL;DR

This paper investigates how nonlinear optical responses in excitonic insulators reveal the dominant interactions—electron-electron or electron-lattice—by analyzing phase and amplitude modes and their resonances.

Contribution

It introduces a theoretical framework linking nonlinear optical signatures to the underlying origin of the insulating phase in excitonic insulators.

Findings

Resonant excitation of the phase mode induces a large second harmonic response.

Different nonlinear effects are observed depending on whether electron-electron or electron-lattice interactions dominate.

A Landau-Ginzburg analysis explains the origin of photo-induced modes and their dependence on the dominant interaction.

Abstract

The nonlinear optical response of an excitonic insulator coupled to lattice degrees of freedom is shown to depend in strong and characteristic ways on whether the insulating behavior originates primarily from electron-electron or electron-lattice interactions. Linear response optical signatures of the massive phase mode and the amplitude (Higgs) mode are identified. Upon nonlinear excitation resonant to the phase mode, a new in-gap mode at twice the phase mode frequency is induced, leading to a huge second harmonic response. Excitation of in-gap phonon modes leads to different and much smaller effects. A Landau-Ginzburg theory analysis explain these different behavior and reveals that a parametric resonance of the strongly excited phase mode is the origin of the photo-induced mode in the electron-dominant case. The difference in the nonlinear optical response serve as a measure of the dominant mechanism of the ordered phase.