Observation of Dyakonov Surface Waves

Summary form only given. A special type of surface wave was predicted by M. I. Dyakonov in 1988. Such waves exist under special conditions at the interface of transparent anisotropic materials, and hens they are lossless. This surface waves require at least one of the two media to be an anisotropic medium, either biaxial or positive uniaxial, and various configurations have been investigated. In the case of the interface of a biaxial anisotropic medium with refractive indices n_x, n_y and n_z, and an isotropic medium with refractive indices n_c, surface waves form only when the refractive indices fulfill the condition n₁ > n_c > n₂ > n₃, where n₁ = max (n_x ; n_y ; n_z), n₃ = min (n_x ; n_y ; n_z) and, n₂ is in between n1 and n₃,. For a positive uniaxial medium, the condition becomes n_c > n_c > n_o, where n_o and n_c are the ordinary and extraordinary refractive indices, respectively. When such a condition is fulfilled, hybrid surface waves containing both ordinary and extraordinary field components propagate within a narrow angular band, Deltathetas, with respect to the crystal optical axis. The allowed propagation angles are referred to as the angular existence domain. The condition is difficult to meet with standard available crystals, making difficult the observation of Dyakonov waves . Here we report on their first observation. Our experiment was conducted in a structure based on a potassium titanyl phosphate (KTP) crystal at lambda = 632.8 nm and index matching liquids with refractive indices that satisfy the given condition. We used a modified Otto-Kretchmann configuration which is composed of a prism, an isotropic liquid layer, and KTP substrate to excite the surface wave at the liquid-KTP interface. Surf- ace wave excitation was detected by imaging the reflected beam. We conducted experiments with five different index matching liquids, and measured the cutoff angle, thetas_max, where the reflected peak associated with the Dyakonov surface wave disappears. The paper illustrates a potential application of Dyakonov surface waves as high sensitive refractive index sensors. The influence of the different structural parameters, such as the thickness of the isotropic layer as on the excitation is discussed.