  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.
