Generation of phonons from electrostriction in small-core optical waveguides

We investigate the generation of acoustic phonons from electrostriction of optical waves in small core waveguides. We specifically consider simple step-index strip waveguides composed of silica or silicon in air, with sub-micron lateral dimensions. Such waveguides support one or a few optical modes, but a rich spectrum of acoustic phonons that becomes densely populated as the phonon frequency increases. We evaluate rigorously the phonon energy density that results from the electrostriction of two frequency detuned guided optical waves, that are either coor contra-propagating, including phonon loss. Plotting this energy density as a function of frequency detuning reveals the phonon wave packets that are electrostrictically active and gives a quantitative estimation of the energy transfer from optical waves to particular phonons. Furthermore, in the backward interaction geometry, the dispersion relation of such phonons can be accessed directly by varying the optical wavelength.