High-harmonic generation by electric polarization, spin current, and magnetization

TL;DR

This paper theoretically investigates high-harmonic generation (HHG) from electric polarization, spin current, and magnetization in magnetic insulators under terahertz or gigahertz electromagnetic waves, using spin chain models and quantum simulations.

Contribution

It introduces a theoretical framework for HHG in magnetic insulators involving electric polarization, spin current, and magnetization, including control mechanisms and experimental feasibility.

Findings

HHG exhibits clear harmonic peaks in electric polarization, spin current, and magnetization.

Even-order HHG by magnetization can be controlled via static magnetic fields.

Estimated electric field strengths for experimental detection are feasible for current setups.

Abstract

High-harmonic generation (HHG), a typical nonlinear optical effect, has been actively studied in electron systems such as semiconductors and superconductors. As a natural extension, we theoretically study HHG from electric polarization, spin current and magnetization in magnetic insulators under terahertz (THz) or gigahertz (GHz) electromagnetic waves. We use simple one-dimensional spin chain models with or without multiferroic coupling between spins and the electric polarization, and study the dynamics of the spin chain coupled to an external ac electric or magnetic field. We map spin chains to two-band fermions and invoke an analogy of semiconductors and superconductors. With a quantum master equation and Lindblad approximation, we compute the time evolution of the electric polarization, spin current, and magnetization, showing that they exhibit clear harmonic peaks. We also show that the even-order HHG by magnetization dynamics can be controlled by static magnetic fields in a wide class of magnetic insulators. We propose experimental setups to observe these HHG, and estimate the required strength of the ac electric field $E_0$ for detection as $E_0\sim100$kV/cm--1MV/cm, which corresponds to the magnetic field $B_0\sim0.1$T--1T. The estimated strength would be relevant also for experimental realizations of other theoretically-proposed nonlinear optical effects in magnetic insulators such as Floquet engineering of magnets.