Resonant Acousto-optics and Acousto-plasmonics

The resonant acousto-optic (RAO) effect is, in general, a mediation of the interaction between an acoustic wave and a light field by a solid state excitation, when the light field and the excitation are in resonance.

For example, in ionic crystals with soft TO-phonon modes, the interaction between coherent acoustic wave and terahertz (THz) light field is mediated by a TO phonon. The TO phonon is coupled to the acoustic wave via the anharmonicity present in the interatomic potential of the crystal lattice.

The acoustically induced grating propagate with the sound velocity producing a dynamical photonic crystal structure. The incoming light in vacuum and polariton waves in the crystal experience multiple Bragg scattering that opens band gaps in the folded light dispersion. The reflectivity is enhanced considerably at such acoustically induced gaps, their spectral positions being effectively controlled by the acoustic frequency.

The dramatic spectral changes observed in soft ionic crystals have led us to the idea of a widely tunable THz filter. The proposed design of the filter represents a slab of LiNbO3 placed between narrow doped semiconductor contacts producing a resonator for ultrasonic waves but at the same time transparent for THz light. The extinction spectra show a high contrast in THz light filtering as well as its wide-range tunability that demonstrates a large figure of merit of the proposed device.

Activities

We aim to study the RAO effect also in the optical range. In semiconductor structured, a surface acoustic wave (SAW) can lead to dynamical light trapping and quantisation near the interface with vacuum or substrate. In case of metallic films, SAW mediates the interaction between light and surface plasmon. In other words, with the help of the SAW a surface plasmon polariton can be excited by an external laser source and can emit light in the direction normal to the surface. This can be used in particular for an efficient optical image transfer using SAW envelopes which ripple the metallic films in a controlled way.