Third Harmonic Enhancement Harnessing Photoexcitation Unveils New Nonlinearities in Zinc Oxide

TL;DR

This paper introduces a method to dynamically enhance and control third harmonic generation in zinc oxide films using photoexcitation, revealing a new nonlinear optical mechanism with real-time, reversible modulation capabilities.

Contribution

The study demonstrates a novel, switchable approach to enhance and manipulate third harmonic generation in zinc oxide, uncovering a previously unreported quadratic scaling mechanism.

Findings

Third harmonic generation can be enhanced up to 50 times.

The enhanced THG follows quadratic scaling with incident power.

The nonlinear response can be switched and modulated at picosecond timescales.

Abstract

Nonlinear optical phenomena are at the heart of various technological domains such as high-speed data transfer, optical logic applications, and emerging fields such as non-reciprocal optics and photonic time crystal design. However, conventional nonlinear materials exhibit inherent limitations in the post-fabrication tailoring of their nonlinear optical properties. Achieving real-time control over optical nonlinearities remains a challenge. In this work, we demonstrate a method to switch third harmonic generation (THG), a commonly occurring nonlinear optical response. Third harmonic generation enhancements up to 50 times are demonstrated in zinc oxide films via the photoexcited state generation and tunable electric field enhancement. More interestingly, the enhanced third harmonic generation follows a quadratic scaling with incident power, as opposed to the conventional cubic scaling, which demonstrates a previously unreported mechanism of third harmonic generation. The THG can also be suppressed by modulating the optical losses in the film. This work shows that the photoexcitation of states can not only enhance nonlinearities, but can create new processes for third harmonic generation. Importantly, the proposed method enables real-time manipulation of the nonlinear response of a medium. The process is switchable and reversible, with the modulations occurring at picosecond timescale. Our study paves the way to boost or suppress the nonlinearities of solid-state media, enabling robust, switchable sources for nonlinear optical applications.