Terahertz-induced second-harmonic generation in quantum paraelectrics: hot-phonon effect

TL;DR

This paper presents a temperature-dependent dynamic model incorporating hot-phonon effects to explain terahertz-induced second-harmonic generation in quantum paraelectrics, challenging the interpretation of transient ferroelectric order.

Contribution

It introduces a novel hot-phonon based model that accurately reproduces experimental SHG features in quantum paraelectrics under terahertz excitation.

Findings

The hot-phonon effect explains the long-lived non-oscillatory SHG response.

The model reproduces SHG oscillations at twice the soft-mode frequency.

Temperature and field-strength dependencies match experimental observations.

Abstract

Recent terahertz-pump second-harmonic-generation(SHG)-probe measurements of quantum paraelectrics observed a significant long-lived non-oscillatory SHG component following an ultrafast resonant excitation of the soft mode, which was interpreted as a signature of terahertz-induced transient ferroelectric order. Here we propose a temperature-dependent dynamic model incorporating the hot-phonon effect to simulate the soft-mode behaviors under ultrafast terahertz excitation. Its application to paraelectric KTaO3 produces quantitatively most of the features exhibited in our time-resolved SHG measurements and those in existing literature, including a long-lived non-oscillatory SHG response, SHG oscillations at twice the soft-mode frequency, SHG dampings as well as temperature and field-strength dependencies. We conclude that the observed terahertz-induced non-oscillatory SHG response in quantum paraelectrics is a consequence of the induced nonequilibrium hot-phonon effect, offering an alternative to its existing interpretation as a signature of transient ferroelectric order.