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Lasers & Sources

Femtosecond Lasers Offer Promise for Increased Data Storage

From oemagazine September 2005
31 September 2005, SPIE Newsroom. DOI: 10.1117/2.5200509.0002

Femtosecond lasers used for ultra-fast nonthermal control of magnetization offer promise for faster processing speeds and increased data storage following recent research by Radboud University in the Netherlands and its research partners.

Ever-increasing demand for information storage and data-manipulation speeds has driven an intense search to find alternate and faster means to control the magnetization of a medium, other than through the use of magnetic fields. Researchers at Radboud University, in collaboration with the Ioffe Physico-Technical Institute in Russia and the Moscow Power Engineering Institute in Russia, have discovered an alternate mechanism for ultra-fast coherent spin control, which offers new prospects for increased information storage density and manipulation speed.

Previous experiments on laser-induced demagnetization and spin reorientation using ultra-fast lasers have accessed time scales on the order of a picosecond or less. The observed magnetic excitation produces a rapid temp-erature increase following optical absorption. The thermal origin of spin excitation limits the application areas, however, since the repetition frequency is limited by the cooling time.

Magnetic excitations in DyFeO3 were probed by the magneto-optical Faraday effect. The studied DyFeO3 samples were prepared from x-ray-oriented boules grown by a floating-zone method. Graph courtesy Andrei Kirilyuk, Radboud University, Netherlands.

In the experiments performed by Radboud University and its partners, the team was able to nonthermally excite and coherently control the spin dynamics in dysprosium orthoferrite (DyFeO3) using circularly polarized femto-second laser pulses and the inverse Faraday effect. The studied DyFeO3 samples were prepared from x-ray-oriented boules grown by a floating-zone method. They were cut perpendicular to the z and x crystal axes and were 60 µm thick. The high-intensity laser radiation induced a static magnetization, which was determined by the magneto-optical susceptibility function. The advantage offered by using the inverse Faraday effect is that it does not require absorption, thus the effect of the light on the magnetization is nonthermal and the process is considered instantaneous, limited in time by the pulse width of the laser. The helicity of the optical beam controls the sign of the photo-induced magnetization. The instantaneous changes in the Faraday rotation were followed by oscil-lations, which were assigned to the oscillations of the magnetization.

Theo Rasing of Radboud University says "the results are very significant, as they show we can manipulate magneti-zation states at femtosecond time scales - four to six orders of magnitude faster than present-day nanosecond speeds." The advantage of this technique over other data storage methodologies is the ability to directly manipulate the magnetization of the medium. "As the medium is not heated, the speed is not limited by the cooling processes, which can be a serious problem with other approaches," Rasing says.

In addition to improving data storage, application areas include quantum computing and the emerging field of spintronics, which involves the storage and transport of electronic spin in semiconductors. The prospects for commercialization using this technique depend on the development of compact femtosecond lasers, which have shown a significant reduction in size during the last few years.