Coherent Rayleigh-Brillouin Scattering (CRBS)

Like two waves in water (Fig. 1), light which is also a wave, although electromagnetic, can interfere destructively and constructively creating areas of low and high intensity, respectively. Particles (be it atoms, molecules or nanoparticles) in such an electric field distribution would usually feel an attraction towards the high intensity regions. This attraction is proportional to the polarizability of the particles, that is the redistribution of the charge of the particles and the succeeding interaction of those now-polar particles with the field that created them.

With the use of two laser beams then (which we call the pumps ), we can create such interference patterns which ultimately perturb the medium in which they interfere, creating so-called optical lattices, which are periodic density modules within the medium. The frequency of these periodic structures will be proportional to the frequency difference (Δω) of the two pump beams which create it.

The same way that X-Rays, when incident upon a crystal at a specific angle (called the Bragg angle) can be diffracted from it revealing the distance between the crystallographic planes, a 3rd laser beam (which we call the probe) can be diffracted from the laser induced optical lattice, creating a 4th beam, called the signal. By carefully tailoring then the Δω and by recording the intensity of the resulting signal beam, we obtain the characteristic coherent Raleigh-Brillouin spectra (Fig. 2). Because of the thermal motion of the particles, for small values of Δω, the beam would be Doppler shifted, resulting in the central Rayleigh peak. When Δω reaches a resonance in the medium, which is none other but the speed of sound in the medium, then acoustic waves will be launched and we will essentially have phonon scattering: the equishifted from the Rayleigh, Brillouin peaks.