Nonresonant nonlinear magnonics in an antiferromagnet

TL;DR

This study demonstrates highly efficient, helicity-dependent magnon generation in Sr$_2$IrO$_4$ using below-gap circularly-polarized mid-infrared pulses, revealing a novel one-photon two-magnon coupling mechanism for ultrafast antiferromagnetic control.

Contribution

It uncovers a new one-photon two-magnon coupling mechanism enabling efficient, helicity-dependent magnon excitation without electron photoexcitation in an antiferromagnet.

Findings

Below-gap excitation yields at least 100 times greater magnon generation efficiency than above-gap.

Helicity dependence observed only with below-gap excitation, indicating a unique coupling mechanism.

Coherent 0.5 THz magnon mode excited in Sr$_2$IrO$_4$ following circularly-polarized laser pulses.

Abstract

Antiferromagnets exhibit rapid spin dynamics in a net zero magnetic background which enables novel spintronic applications and interrogation of many-body quantum phenomena. The layered antiferromagnet Sr$_2$IrO$_4$ hosts an exotic spin one-half Mott insulating state with an electronic gap arising from on-site Coulomb repulsion and strong spin-orbit coupling. This makes Sr$_2$IrO$_4$ an interesting candidate to interrogate dynamical attributes of the magnetic order using ultrafast laser pulses. We investigate the magnetization dynamics of Sr$_2$IrO$_4$ following circularly-polarized photoexcitation with below-gap mid-infrared (mid-IR -- 9 $\mu m$) and above-gap near-infrared (near-IR -- 1.3 $\mu m$) pulses. In both cases, we observe excitation of a zone-center coherent magnon mode featuring a 0.5 THz oscillation in the pump-induced Kerr-rotation signal. However, only below-gap excitation exhibits a helicity dependent response and linear (quadratic) scaling of the coherent magnon amplitude with excitation fluence (electric field). Moreover, below-gap excitation has a magnon generation efficiency that is at least two orders of magnitude greater in comparison to above-gap excitation. Our analysis indicates that the helicity dependence and enhanced generation efficiency arises from a unique one-photon two-magnon coupling mechanism for magnon generation. Thus, preferential spin-photon coupling without photoexcitation of electrons permits extremely efficient magnon generation. Our results reveal new possibilities for ultrafast control of antiferromagnets.