Coherent Acoustic Perturbation of Second-Harmonic-Generation in NiO

TL;DR

This paper investigates the ultrafast second-harmonic generation response in NiO, revealing an acoustic origin related to strain waves rather than magnetic reorientation, with implications for understanding nonlinear optical phenomena.

Contribution

It proposes a new mechanism for SHG response in NiO based on optically induced strain and acoustic waves, challenging previous magnetic reorientation explanations.

Findings

Magnetic structure changes are limited to 1.5% within 30 ps.

SHG response shows wavelength dependence and echoes indicative of acoustic effects.

Oscillation periods correspond to acoustic strain wave traversal times.

Abstract

We investigate the structural and magnetic origins of the unusual ultrafast second-harmonicgeneration (SHG) response of femtosecond-laser-excited nickel oxide (NiO) previously attributed to oscillatory reorientation dynamics of the magnetic structure induced by d-d excitations. Using time-resolved x-ray diffraction from the (3/2 3/2 3/2) magnetic planes, we show that changes in the magnitude of the magnetic structure factor following ultrafast optical excitation are limited to $\Delta<F_m>/<F_m>$ = 1.5% in the first 30 ps. An extended investigation of the ultrafast SHG response reveals a strong dependence on wavelength as well as characteristic echoes, both of which give evidence for an acoustic origin of the dynamics. We therefore propose an alternative mechanism for the SHG response based on perturbations of the nonlinear susceptibility via optically induced strain in a spatially confined medium. In this model, the two observed oscillation periods can be understood as the times required for an acoustic strain wave to traverse one coherence length of the SHG process in either the collinear or anti-collinear geometries.