Optical Properties of LiNbO$_2$ thin films

The complex dielectric functions of LiNbO$_2$ were determined using optical transmittance and reflectance spectroscopies at room temperature. The measured dielectric function spectra reveal distinct structures at several bandgap energies. The bandgaps (exciton resonances) in the spectrum were observed at ca. 2.3, 3.2, 3.9, and 5.1 eV, respectively. These experimental data have been fit using a model dielectric function based on the electronic energy-band structure near critical points plus excitonic effects. The features of measured dielectric functions are, to some extent, reproduced quantitatively by an ab-initio calculation including the interaction effects between electrons and holes.