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LPA Online Seminars

Introduction to Optically Triggered Spin Dynamics: Applications of Intense (< 1mJ) Laser Pulses in Solid-State Physics

by Mr Fangzhou Wang (Forschungszentrum Jülich, PGI-6)

Building 04.8, room 142 (FZJ)

Building 04.8, room 142



Numerous fundamental limits have been set for industrial applications in information technology. Approaches like Heat-Assisted Magnetic Recording (HAMR) have been suggested, that can reverse a magnetic bit with a relatively small magnetic field by locally heating the storage medium with a laser beam. However, this technique still faces problems of cooling and switching with a low energy efficiency.

In the course of the rapid development of laser technology, modifications and excitation of microscopic states in magnetic materials can be realized within hundreds of femtoseconds by ultrashort laser pulses at a moderate laser light intensity[1]. Importantly, such ultrafast laser pulses can be used not only for the excitation, but also for a detection of ultrafast processes with time resolution given by the laser pulse duration.. This method is commonly referred to as the pump-probe technique, and is the state-of-the-art method which is used for studies in solid-state physics. To be more specific, for the investigation of magnetic systems the time-resolved magneto optical Kerr effect (TR-MOKE) is employed.

The discovery of ultrafast magneto-optic phenomena suggests a new reversal mechanism, the so called all-optical switching (AOS) [2]. The magnetic bit can be manipulated by an intense laser pulse depending on its polarization without applying the external field. This new mechanism is believed to involve highly nonequilibrium electronic states [3,4] and might lead to a break-through  in optically induced spin reversal times. Therefore, a comprehensive understanding of the spin and magnetization dynamic on different timescale becomes a subject of vital importance for both research and information-processing technologies.

In this seminar, a general introduction will be given on the application ultrafast laser pulses in the field of spin dynamics for a solid-state ferromagnetic material. Depending on the applied laser pulse energy, we succeeded to excite a ferromagnetic material from the perturbative regime towards the non-linear regime. Multiple coherent physical effects can be triggered, such as optically induced ultrafast demagnetization and relaxation via domain wall motion, e.g. AOS and THz generation [5].

[1]             E. Beaurepaire, J. C. Merle, A. Daunois, and J. Y. Bigot, Physical Review Letters 76, 4250 (1996).
[2]             C. H. Lambert et al., Science 345, 1337 (2014).
[3]             K. Vahaplar et al., Physical Review Letters 103, 117201 (2009).
[4]             J. Hohlfeld, E. Matthias, R. Knorren, and K. H. Bennemann, Physical Review Letters 78, 4861 (1997).
[5]             T. Kampfrath et al., Nature Nanotechnology 8, 256 (2013).