SURFACE BRILLOUIN LIGHT SCATTERING: 

An Introduction

Prediction of light scattering from acoustic waves was made by Brillouin[i] and, independently, by Mandelstam[ii] in the twenties, while a few years later Gross[iii] gave the experimental confirmation of such a prediction in liquids. The invention of laser in the sixties gave a new impulse to this kind of optical technique, but it was still impossible to detect acoustic as well as spin waves in opaque solids until the advent of a new class of spectrometers designed and fabricated for the first time by Sandercock in the seventies. He demonstrated that the sensitivity of a Fabry-Perot interferometer could be dramatically increased by passing the scattered light several times through the same interferometer.[iv] Such an improvement led to the observation of light scattering from surfacece acoustic waves as well as from surface spin waves in both transparent and opaque magnetic materials.

MECHANISM OF INTERACTION

For both acoustic as well as spin waves the interaction mechanism is based on the modulation of the dielectric constant of the medium through the elasto-optical and magneto-optical constants. 

Magneto-optic coupling:

Elasto-optic coupling:

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  In the case of phonons in opaque media, also the rippling induced at the free surface gives an important contribution to the scattering cross-section.

Both Stokes and anti-Stokes processes can take place, with roughly identical probability at room temperature. 

                         Stokes Process :  Magnon or phonon creation 

                      Anti-Stokes precess :  Magnon or phonon annihilation

       This is because both phonon and magnon frequencies are in the range 1-100 GHz (thir energy is much lower than thermal energy at RT)  so that an experimental apparatus with very high resolution is required, such as the tandem Fabry-Perot Interferometer.

 Measurements are usually taken in the backscattering interaction geometry, varying the applied magnetic field between zero and a few thousands Oe