The resonant acousto-optic effect

This dissertation is theoretical investigation of the resonant acousto-optic effect in ionic crystals and thin metal foils. The optical properties of these types of materials, in the presence of coherent acoustic pump excitation, are numerically modelled and compared with analytical results. The resonant acousto-optic effect in bulk ionic materials is shown to be dependent on the coupling of a bulk acoustic wave to the TO-phonon component of a TO-phonon polariton. This requires that the material used is not only an ionic crystal but also has a strongly anharmonic interatomic potential. It is also demonstrated that the process “TO phonon ± one (two) transverse acoustic phonon(s)→ TO phonon” is responsible for the cubic (quartic) resonant acousto-optic effect. The role of acoustic intensity and frequency in the optical properties of CuCl and TlCl is considered. Higher order transitions are also investigated. It is shown that, in the ferroelectric material LiNbO3, both cubic and quartic scattering channels are sufficiently strong enough to consider the resonant acousto-optic effect associated with them on an equal footing. The coupling strength of both scattering channels is estimated to the nearest order of magnitude. The cubic coupling is found be σ3 = 5 meV and the quartic coupling strength is found to be σ4 = 0.3 meV both for the acoustic intensity Iac = 25 kWcm−2. The effect the phase difference between the two anharmonic terms has on the optical properties of LiNbO3 is then investigated. A tunable THz filter is proposed, based on the resonant acousto-optic effect in LiNbO3. A numerical method is developed to calculated the partial wave amplitudes and optical properties of metal foils with acoustically excited, propagating sinusoidally corrugated surfaces. It is then used on a system of a thin acoustically perturbed Au foil on a glass substrate. The effects of varying the angle of incidence, acoustic wavevector, corrugation amplitude and foil thickness are investigated. The numerical method is shown to remain stable even for strong coupling between the acoustic wave and surface plasmon polariton

Far-infrared response of acoustically modulated transverse optical-phonon polaritons

The authors propose a scheme to achieve strong modification of the light properties in the terahertz (THz) range and in particular up to 70% changes in the THz reflectivity of CuCl, TlCl and LiNbO3 crystals. This is realised by using transverse optical (TO) phonons as a mediator in the interaction between an acoustic wave (AW) and a THz light field, via the strong anharmonicities of the interatomic potential. Their numerical modeling of CuCl, TlCl and LiNbO3 crystals also predicts that these effects are tunable by applying various coherent AWs from sub-GHz to few GHz frequency. The length of the interaction between electromagnetic and acoustic fields is also greatly reduced compared to conventional acousto-optics. The modifications of the reflectivity spectrum are because of single and multiple intra-branch phonon transitions within the TO-phonon polariton dispersion branches