Laser-driven quantum magnonics and THz dynamics of the order parameter in antiferromagnets

TL;DR

This paper presents a quantum mechanical model of laser-induced two-magnon dynamics in antiferromagnets, revealing femtosecond entanglement of magnon pairs and experimentally observing coherent THz oscillations.

Contribution

It introduces a quantum approach to describe femto-nanomagnons and their macroscopic entanglement, supported by ultrafast magneto-optical experiments.

Findings

Observation of 22 THz and 16 THz coherent oscillations in antiferromagnets.

Experimental data fit two-magnon excitation models up to the Ne9el temperature.

Demonstration of femtosecond-scale entanglement of magnon pairs.

Abstract

The impulsive generation of two-magnon modes in antiferromagnets by femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent longitudinal oscillations of the antiferromagnetic order parameter that cannot be described by a thermodynamic Landau-Lifshitz approach. We argue that this dynamics is triggered as a result of a laser-induced modification of the exchange interaction. In order to describe the oscillations we have formulated a quantum mechanical description in terms of magnon pair operators and coherent states. Such an approach allowed us to} derive an effective macroscopic equation of motion for the temporal evolution of the antiferromagnetic order parameter. An implication of the latter is that the photo-induced spin dynamics represents a macroscopic entanglement of pairs of magnons with femtosecond period and nanometer wavelength. By performing magneto-optical pump-probe experiments with 10 femtosecond resolution in the cubic KNiF$_3$ and the uniaxial K$_2$NiF$_4$ collinear Heisenberg antiferromagnets, we observed coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The detected frequencies as a function of the temperature ideally fit the two-magnon excitation up to the N\'eel point. The experimental signals are described as dynamics of magnetic linear dichroism due to longitudinal oscillations of the antiferromagnetic vector.