Phase-Sensitive Brillouin Light Scattering Spectroscopy

We report on the first implementation of phase sensitivity into Brillouin light scattering (BLS) spectroscopy. Combined with a time-and space-resolved BLS setup this allows for access to quantities such as the phase profile and the wave front of spin wave packets and beams. In order to access phase information, interference between the light inelastically scattered by the spin wave under investigation and a frequency shifted coherent reference beam was used. To achieve the necessary frequency shift of the reference light, a microwave electro-optical resonance modulator based on a Lithium Niobate single crystal was constructed. As this modulator is driven by the same microwave source that is used for the excitation of the spin waves, coherency between the frequency shifted and the inelastically scattered light guaranteed. Light inelastically scattered from spin waves is rotated by 90 deg in polarization. In order to have interference with the light shifted in frequency by the electro-optical modulator, an optical lambda/4 plate is used to convert the laser light into circularly polarized light before it enters the modulator. Designing an optical interferometer setup, mechanical and thermal stability is of paramount importance to yield reliable phase information. We have solved the problem of stability by spatially combining the optical reference and the signal beam along one single axis.