Measurement Techniques of Low Symmetry Polaritons in the Monoclinic Crystal Bismuth Triborate Through Infrared Spectroscopy

Low symmetry anisotropic material interaction with incident light results in highly directional propagation of polaritons that exhibit exotic properties. The light-matter coupling (polaritons) in anisotropic material can have hyperbolic light propagation, and at the lowest symmetry of the crystal axis, the sheer effect acts on the hyperbolic polaritons. Probing these remarkable effects from anisotropy has been challenging, relying on specialized free electron lasers. Our work has developed techniques and implemented novel tools to observe these hyperbolic/hyperbolic sheer polaritons in low symmetry anisotropic crystals in mid to far infrared regions.<br/>We explore surface phonon polariton propagation (SPhPs) in low symmetry anisotropic material through two different developed techniques of Fourier transform infrared (FTIR) spectroscopy that allow us to measure crystal orientation-dependent optical properties. The techniques are applied to Bismuth Triborate (BiB₃O₆ ) crystals with (100) and (010) orientations. Primarily, we performed the infrared reflectance measurement of the samples; the measurement allowed us to study the anisotropic Reststrahlen bands of the material which further helped in understanding the phonon bands present in the material. From the phonon band structure, we could better determine where the polaritonic activity occurs while also exploring the differences in the phonon band structure of the two crystal samples. Lastly, we developed Otto-type prism coupling measurements using ZnSe prism to measure the polariton propagation. The approach utilizes evanescent fields produced from the incident light on the prism to propel SPhPs. With variations of incident beam angle and azimuthal crystal orientation angle, we were able to observe the polariton propagation of BiB₃O₆. Through the results of our study, BiB₃O₆ greatly demonstrates low symmetry polaritonic behavior that is influenced vastly by its incident crystal axis and is a great candidate for further studying the nature of polaritonic behavior of low symmetry materials.