High-order Harmonic Generation and its Unconventional Scaling Law in the Mott-insulating $\rm{Ca_2RuO_4}$

TL;DR

This study reveals that high-order harmonic generation in Mott insulator Ca2RuO4 is strongly enhanced at low temperatures and follows a unique scaling law linked to electron correlations, highlighting the influence of many-body physics on nonlinear optics.

Contribution

It demonstrates a novel empirical scaling law for harmonic generation in strongly correlated materials, connecting nonlinear optical response to many-body electronic structure.

Findings

Harmonic generation is significantly enhanced at 50 K.

The enhancement follows a new scaling law based on gap energy.

Simple single-electron models cannot explain the observed phenomena.

Abstract

Competition and cooperation among orders is at the heart of many-body physics in strongly correlated materials and leads to their rich physical properties. It is crucial to investigate what impact many-body physics has on extreme nonlinear optical phenomena, with the possibility of controlling material properties by light. However, the effect of competing orders and electron-electron correlations on highly nonlinear optical phenomena has not yet been experimentally clarified. Here, we investigated high-order harmonic generation from the Mott-insulating phase of Ca2RuO4. Changing the gap energy in Ca2RuO4 as a function of temperature, we observed a strong enhancement of high order harmonic generation at 50 K, increasing up to several hundred times compared to room temperature. We discovered that this enhancement can be well-reproduced by an empirical scaling law that depends only on the material gap energy and photon emission energy. Such scaling law cannot be explained by a simple two-band model under the single electron approximation. Our results suggest that the highly nonlinear optical response of strongly correlated materials is deeply coupled to their electron-electron correlations and resultant many-body electronic structure.