Strong nonlinear optical response and transient symmetry switching in Type-II Weyl semimetal $\beta$-WP2

TL;DR

This study investigates the strong nonlinear optical response and ultrafast symmetry switching in the Type-II Weyl semimetal $eta$-WP$_2$, revealing its potential for topological optoelectronic applications.

Contribution

It provides the first detailed static and time-resolved SHG analysis of $eta$-WP$_2$, demonstrating ultrafast symmetry switching and strong nonlinear optical effects in a Type-II Weyl semimetal.

Findings

$eta$-WP$_2$ exhibits strong SHG response despite high conductivity.

Ultrafast laser pulses induce symmetry switching to an inversion symmetric state.

Nonlinear optical response can be tuned on ultrafast timescales.

Abstract

The topological Weyl semimetals with peculiar band structure exhibit novel nonlinear optical enhancement phenomena even for light at optical wavelengths. While many intriguing nonlinear optical effects were constantly uncovered in type-I semimetals, few experimental works focused on basic nonlinear optical properties in type-II Weyl semimetals. Here we perform a fundamental static and time-resolved second harmonic generation (SHG) on the three dimensional Type-II Weyl semimetal candidate $\beta$-WP$_2$. Although $\beta$-WP$_2$ exhibits extremely high conductivity and an extraordinarily large mean free path, the second harmonic generation is unscreened by conduction electrons, we observed rather strong SHG response compared to non-topological polar metals and archetypal ferroelectric insulators. Additionally, our time-resolved SHG experiment traces ultrafast symmetry switching and reveals that polar metal $\beta$-WP$_2$ tends to form inversion symmetric metastable state after photo-excitation. Intense femtosecond laser pulse could optically drive symmetry switching and tune nonlinear optical response on ultrafast timescales although the interlayer coupling of $\beta$-WP$_2$ is very strong. Our work is illuminating for the polar metal nonlinear optics and potential ultrafast topological optoelectronic applications.