Optically Pumped Floquet States of Magnetization in Ferromagnets

TL;DR

This paper demonstrates the creation of Floquet states in ferromagnets through periodic optical excitation, revealing tunable magnetization dynamics that could enhance ultrafast magnetic control and quantum information applications.

Contribution

It introduces a novel method of inducing and controlling Floquet states in ferromagnets using femtosecond laser pulses, bridging condensed matter physics and ultrafast optics.

Findings

Floquet states observed in ferromagnetic iron-garnet films.

Magnetization precession amplitude increased by an order of magnitude.

External magnetic field enables tuning of Floquet states.

Abstract

Floquet states have been subject of great research interest since Zel'dovich's pioneering work on the quasienergy of a quantum system subject to a temporally periodic action. Nowadays periodic modulation of the system Hamiltonian is mostly achieved by microwaves leading to novel exciting phenomena in condensed matter physics: Floquet topological insulators, chiral edge states etc. On the other hand, nonthermal optical control of magnetization at picosecond time scales is currently a highly appealing research topic for potential applications in magnetic data storage. Here we combine these two concepts to investigate Floquet states in the system of exchange-coupled spins in a ferromagnet. We periodically perturb the magnetization of an iron-garnet film by a train of circularly-polarized femtosecond laser pulses hitting the sample at 1 GHz repetition rate and monitor the magnetization dynamics behaving like a Floquet state. An external magnetic field allows tuning of the Floquet states leading to a pronounced increase of the precession amplitude by one order of magnitude at the center of the Brillouin zone, i.e. when the precession frequency is a multiple of the laser pulse repetition rate. Floquet states might potentially allow for parametric generation of magnetic oscillations. The observed phenomena expand the capabilities of coherent ultrafast optical control of magnetization and pave a way for their application in quantum computation or data processing.