A NEW PHASE-COHERENT LIGHT-SCATTERING METHOD - FIRST OBSERVATION OF COMPLEX BRILLOUIN SPECTRA

We demonstrate here a new phase-coherent light scattering method. Spon taneous Brillouin scattering is caused by thermally excited phonons wh ose phases have ns physical meaning because of their random, incoheren t nature. However, coherent phonons having phase information can be ge nerated resonantly by an optically induced scanning interference patte rn, when the dispersion relation of phonons is satisfied. By a phase-s ensitive detection of the light scattered by optically generated coher ent phonons, we have succeeded in measuring complex resonance spectra of acoustic phonons, which we call complex Brillouin spectra, with an ultimate signal-to-noise ratio.