Anisotropic optical properties of β-CuGaO2 single crystal studied via momentum transfer resolved EELS

β-CuGaO2 has attracted attention as a photovoltaic material because of its p-type semiconducting nature and strong-visible-light absorption capability. Because ion-exchange methods yield only micron-sized crystals and random orientations, conventional optical characterization of their anisotropic properties has been largely inaccessible. Herein, the dielectric function of β-CuGaO2 was determined as a function of momentum transfer (q) using q-resolved electron energy-loss spectroscopy (q-EELS) on single crystals in transmission electron microscopy (TEM). EELS spectra were acquired along the q// 100, q// 010, and q// 001, revealing a pronounced dependence of the visible-light-region spectral intensity on q. The dielectric functions derived from the EELS spectra indicate anisotropic absorption, which is stronger for q// 100 and q// 001 and weaker for q// 010. This anisotropic absorption feature is attributable to interband transitions from localized Cu 3d states at the valence band maximum and localized O 2p states at the conduction band minimum, consistent with theoretical studies. These results demonstrate that TEM-based q-EELS enables the evaluation of anisotropic dielectric properties even from micron-sized crystals, providing insight into the electronic structure of β-CuGaO2.