Observation of strong magnetic effects in visible-infrared sum frequency generation from magnetic structures

Abstract

We have observed very strong magnetization-induced changes of the infrared-visible sum-frequency generation ͑SFG͒ intensity from thin magnetic films using a free electron laser as a tunable infrared source. With the help of a magnetic grating a clear resonance is observed due to the excitation of surface plasmonpolaritons. The magnetization dependence of this resonance opens the way for nonlinear magneto-optical SFG studies of magnetic surface excitations.In recent years, magnetization-sensitive second harmonic generation ͑MSHG͒ has been developed into a valuable method to study magnetic surfaces and multilayers. [1][2][3][4][5][6] This nonlinear magneto-optical technique combines the convenience of an optical technique with intrinsic surface/interface sensitivity 3,4 and strongly enhanced magneto-optical effects. 5 For example, MSHG allowed to detect enhanced magnetic moments at atomic steps, 7 to correlate interface roughness with interface anisotropy 8 and to obtain very high contrast domain images for thin films. 9 In addition, nonlinear magneto-optical effects have been discovered. 10,11 Second harmonic generation is just a degenerate case of the general nonlinear optical process of three-wave mixing, 1 ϩ 2 ϭ 3 . The argument of symmetry breaking at interfaces, which yields the interface specificity of MSHG, should equally apply to magnetization-induced sumfrequency generation ͑MSFG͒. Though SFG has been used for surface studies for more than a decade, 12,13 nothing is known on the magnetic properties of SFG. Compared to MSHG, MSFG would allow for much more spectroscopic opportunities, for example to probe magnetic excitations at surfaces and interfaces.In this paper we demonstrate that such a generalization can indeed be carried out and leads to very strong (Ͼ50%) effects on ultrathin films of iron on a GaAs͑001͒ single crystal surface as well as on sandwiched Pt/CoNi/Pt layers and nanostructure arrays. By syncronizing the output of a frequency doubled Nd:YLF laser with the tunable IR output of a free electron laser we obtained an SFG signal that was strongly dependent on the magnetization. On an array of magnetic lines ͑magnetic grating͒ a clear resonance was observed as a function of the infrared wavelength, which is an indication of the excitation of surface plasmon-polariton modes. The dependence of this resonant feature on the magnetization direction suggests a considerable interaction of these modes with the magnetization.The nonlinear optical polarization induced in a sample by two incident optical fields can be written aswhere s f g , ir , and vis are the frequencies of the SFG, infrared and visible beams, respectively, and i jk (2) is the second-order nonlinear optical susceptibility tensor similar as used for the description of MSHG. 1 Both SHG and SFG are described by a third rank tensor that vanishes in media with inversion symmetry, yielding the sensitivity of both SHG and SFG for symmetry breaking interfaces. In the same way one can also immediately derive the nonzero i jk (2) elements that a...

Magnetization-induced-second-harmonic generation from surfaces and interfaces

“…The following experiments 98 were done at the FreeElectron Laser for Infrared eXperiments (FELIX) in Nieuwegein, The Netherlands, which delivers tunable IR radiation with wavelengths in the range of 5-240 11m. 'rhis IRradiation comes in bursts, so-called macro pulses, typically 5 /lS long at a 5-Hz repetition rate.…”

Energy conservation in partially coherent wave fields from polarization and magnetization

“…However, unusual magnetic (meta)materials have been investigated recently [15][16][17][18][19], although the frequencies of the electromagnetic waves could be mostly in the infrared or the far-infrared regions. For some cases in astronomy, the magnetization effect may be comparable or even dominant.…”

Giant Magneto-refractive Effect in Second Harmonic Generation from Plasmonic Antennas in the Mid-infrared

“…We employed the FELIX free-electron laser as a source of fundamental radiation (about 5 mJ energy in the macropulse, 10 Hz repetition rate, and 25 MHz repetition rate of the picosecond micropulses within every macropulse) tunable in the wide range of wavelengths in the infrared (5−100 µm). Further details about FELIX can be found elsewhere [23,24]. By means of a 2 ′′ parabolic mirror the beam was focused onto the sample into a spot of about 300 µm in diameter.…”