The resonant acousto-optic effect

• Main Author: Poolman, Rhys
• Published: Cardiff University 2012
• Subjects: 534; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.567262

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.