# Extraction of higher-order nonlinear electronic response to strong field excitation in solids using high harmonic generation

## TL;DR

This paper demonstrates a method to extract high-order nonlinear susceptibilities from solids using high harmonic generation, revealing their angular dependence and potential for ultrafast optoelectronic applications.

## Contribution

It establishes a direct link between high harmonic generation and high-order nonlinear susceptibilities in solids, enabling characterization of electronic responses to strong fields.

## Key findings

- Extraction of 7th, 9th, and 11th order susceptibilities from sapphire.
- Identification of angular-resolved periodicities in susceptibilities.
- Potential for petahertz-scale signal manipulation in optoelectronics.

## Abstract

State-of-the-art experiments employ strong ultrafast optical fields to study the nonlinear response of electrons in solids on an attosecond time-scale. Notably, a recent experiment retrieved a 3rd order nonlinear susceptibility by comparing the nonlinear response induced by a strong laser field to a linear response induced by the otherwise identical weak field. In parallel, experiments have demonstrated high harmonic generation (HHG) in solids, a highly nonlinear process that until recently had only been observed in gases. The highly nonlinear nature of HHG has the potential to extract even higher order nonlinear susceptibility terms, and thereby characterize the entire response of the electronic system to strong field excitation. However, up till now, such characterization has been elusive due to a lack of direct correspondence between high harmonics and nonlinear susceptibilities. Here, we demonstrate a regime where such correspondence can be clearly made, extracting nonlinear susceptibilities (7th, 9th, and 11th) from sapphire of the same order as the measured high harmonics. The extracted high order susceptibilities show angular-resolved periodicities arising from variation in the band structure with crystal orientation. Nonlinear susceptibilities are key to ultrafast lightwave driven optoelectronics, allowing petahertz scaling manipulation of the signal. Our results open a door to multi-channel signal processing, controlled by laser polarization.

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Source: https://tomesphere.com/paper/1904.11103