Measurement of Refractive Index and Elastic Modulus via Orthogonal Brillouin Imaging System

We developed an orthogonal Brillouin imaging system that allows simultaneous measurement of the refractive index, longitudinal modulus and shear modulus. To achieve this, we designed and experimentally demonstrated a novel setup utilizing two orthogonal Brillouin scattering interactions, which are coupled into a spectrometer equipped with a three-stage virtually imaged phased arrays (VIPAs). The refractive index can be determined from the difference in Brillouin frequency shifts of longitudinal phonons under orthogonal scattering conditions. Also, the elastic modulus can be obtained by utilizing the longitudinal and transverse phonon velocities. Based on this, the refractive index, longitudinal and shear moduli of polystyrene and polymethyl methacrylate are measured. This proposed method significantly enhances the capability of VIPA-based Brillouin imaging for evaluating the longitudinal and shear moduli of materials.