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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Sunday, September 28, 2014

When Magnetism Meets Optics

S. Mangin (Left) and E. E. Fullerton

Authors: 

C.H. Lambert, M. Salah, N. Bergeard, G. Malinowski, M. Hehn, S. Mangin,
Equipe Nanomagnetisme et Electronique de Spin de l’Institut Jean Lamour UMR CNRS 7198, Université de Lorraine, France

Y. Fainman, E. E. Fullerton,
Center For Magnetic Recording Research, University of California San Diego (UCSD), USA 

M. Cinchetti, M. Aeschlimann, 
Department of Physics and Research Center OPTIMAS, University of Kaiserlautern- Allemagne, Germany 

B. Varaprasad, Y. Takahashi, K. Hono, 
National Institute for Materials Science, Japan

With the fast development of mass storage units all around the world (clouds, data centers…) the pressure to increase the density, speed and energy efficiency of conventional hard disk drives is becoming stronger and stronger. The discovery of “All-optical control of ferromagnetic thin films and nanostructures” might open up new technological horizons in magnetic recording. This work is the results of a collaboration between scientists and engineers from University of California San Diego, Universite de Lorraine, Kaiserlauter Universitat and National Institute for Materials Science in Tsukuba, Japan published in Science on September 14th 2014 [1].

 The authors found that they could control the final state of the magnetization of a broad range of magnetic materials using laser pulses of circularly polarized light instead of an applied magnetic fields. In particular these researchers find out that the magnetization of some magnetic material similar to those used in the recording industry can be manipulated directly with a laser beam. The ability to optically control magnetic materials the density and access time of data on hard drives could be increased dramatically.

Image: Writing with a laser on a magnetic thin film.

The first observation of “all optical switching” of magnetic materials was performed in 2007 by the group from T. Rasing in Nijmegen on a very particular ferrimagnetic alloy GdFeCo [2]. Since this discovery there has been extensive studies of optical switching of this material class including detailed studies of the magnetic response to optical excitations of both the rare-earth (Gd) and transition metal (Fe and Co) elements. Based on these studies a detailed understanding has emerged of the ultra-fast physics of rare-earth-transition-metal alloys [3,4]. However, the extent of the practical impact of this research is limited by the materials that are not compatible with many modern technologies. By extending these exciting studies to new classes of materials such as ferromagnets, the “all-optical” magnetization switching has made a significant step to demonstrate its potential for technological impact.

These results further show that theoretical understanding of all-optical switching needs to be re-examined. Most recent theories predicted that the all-optical reversal should only occur in ferrimagnetic materials, where the overall magnetization is the result of the competition between two magnetic sub-lattices that are antiferromagnetically coupled. Our results show that all-optical switching is not exclusive to ferrimagnetic materials and therefore antiferromagnetic exchange coupling between two magnetic sublattices is not required. The results do suggest that heating near the Curie point is important for the all-optical switching in ferromagnetic materials. Near the Curie point then a small symmetry-breaking from circularly polarized light (e.g. the inverse Faraday effect or transfer of angular momentum from the light to the magnetic system) can deterministically determine the magnetization direction. However details of this process still need to be determined.


Video: Writing with a laser on a magnetic thin film : Micrometer size "Etch A Sketch".

References:
[1] C-H. Lambert, S. Mangin, B. S. D. Ch. S. Varaprasad, Y. K. Takahashi, M. Hehn, M. Cinchetti, G. Malinowski, K. Hono, Y. Fainman, M. Aeschlimann, E. E. Fullerton, "All-optical control of ferromagnetic thin films and nanostructures".  Science, 345, 1337-1340 (2014). Abstract.
[2] C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, Th. Rasing, "All-optical magnetic recording with circularly polarized light". Physical Review Letters, 99, 047601 (2007). Abstract.
[3] Andrei Kirilyuk, Alexey V Kimel, Theo Rasing, "Laser-induced magnetization dynamics and reversal in ferrimagnetic alloys". Reports on Progress in Physics, 76, 026501 
(2013). Abstract.
[4] S. Mangin, M. Gottwald, C-H. Lambert, D. Steil, V. Uhlíř, L. Pang, M. Hehn, S. Alebrand, M. Cinchetti, G. Malinowski, Y. Fainman, M. Aeschlimann, E.E. Fullerton, "Engineered materials for all-optical helicity-dependent magnetic switching".  Nature Materials, 13, 286–292 (2014). Abstract.

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