Efficient excitation of nonlinear phonons via chirped mid-infrared pulses: induced structural phase transitions

TL;DR

This paper explores efficient methods for exciting nonlinear phonons using chirped mid-infrared pulses to induce structural phase transitions, focusing on a detailed model of coupled nonlinear oscillators in materials like KTaO3.

Contribution

It introduces a dynamical mechanism involving frequency sweep driving to effectively excite low-frequency phonons, enabling phase transitions without strain.

Findings

Counterintuitive amplification at detuned frequencies

Effective excitation via frequency sweep (capture into resonance)

Applicable to realistic femtosecond pulse experiments

Abstract

Nonlinear phononics play important role in strong laser-solid interactions. We discuss nonlinear dynamical protocols which allow for efficient excitation and control of nonlinear phonons. We consider recent inspiring proposals: inducing ferroelectricity in paraelectric material such as KTaO$_3$ and inducing structural deformations in cuprates like La$_2$CuO$_4$ [A. Subedi et.al, Phys. Rev. B 89,220301 (2014), A.Subedi, Phys. Rev. B 95, 134113 (2017)]. High-frequency phonon modes are driven by mid-infrared pulses, and coupled to lower-frequency modes those indirect excitation causes structural deformations. Such proposals are in line with a series of recent experiments on light-induced phase transitions. We study in a more detail the case of KTaO$_3$ without strain, where (at first glance) it was not possible to excite the needed low frequency phonon mode by resonant driving of the higher frequency one. Behaviour of the phonon system is explained using a reduced model of coupled driven nonlinear oscillators. We find a dynamical mechanism which prevents effective excitation at resonance driving, and show that for certain detunings of driving frequency from the exact resonance the system response is, counterintuitively, greatly amplified. In order to induce ferroelectricity in KTaO$_3$ without a strain we employ driving with sweeping frequency, realizing so called capture into resonance. The method works for realistic femtosecond pulses. Our approach can be applied to other related systems, e.g. laser driven orthorombic perovskites like ErFeO$_3$.