Demonstration of Optimal Control of Laser Induced Spin-Orbit Mediated Ultrafast Demagnetization

TL;DR

This paper demonstrates that laser-induced ultrafast demagnetization via spin-orbit coupling can be actively controlled using quantum optimal control theory, enabling tailored magnetic responses on femtosecond timescales.

Contribution

It introduces a method to optimize laser pulses for controlling ultrafast demagnetization, integrating TDDFT simulations with optimal control for the first time.

Findings

Optimal laser pulses can maximize demagnetization within constraints.

Controlled pulses can reduce laser power while maintaining effectiveness.

The approach enables future tailored ultrafast magnetic control.

Abstract

Laser induced ultrafast demagnetization is the process whereby the magnetic moment of a ferromagnetic material is seen to drop significantly on a timescale of $10-100$s of femtoseconds due to the application of a strong laser pulse. If this phenomenon can be harnessed for future technology, it offers the possibility for devices operating at speeds several orders of magnitude faster than at present. A key component to successful transfer of such a process to technology is the controllability of the process, i.e. that it can be tuned in order to overcome the practical and physical limitations imposed on the system. In this paper, we demonstrate that the spin-orbit mediated form of ultrafast demagnetization recently investigated [arXiv:1406.6607] by ab-initio time-dependent density functional theory (TDDFT) can be controlled. To do so we use quantum optimal control theory (OCT) to couple our TDDDT simulations to the optimization machinery of OCT. We show that a laser pulse can be found which maximizes the loss of moment within a given time interval while subject to several practical and physical constraints. Furthermore we also include a constraint on the fluence of the laser pulses and find the optimal pulse that combines significant demagnetization with a desire for less powerful pulses. These calculations demonstrate optimal control is possible for spin-orbit mediated ultrafast demagnetization and lay the foundation for future optimizations/simulations which can incorporate even more constraints.