This study investigates the complex refractive index ( \(\:\stackrel{\sim}{n}=n-ik\) ) of twenty-four twisted bilayer graphene (tBLG) samples on SiO₂/Si substrates using microscopic reflection spectroscopy. By constructing optical contrast (OC) spectra from the interference patterns between tBLG and the underlying SiO₂/Si substrate, we identified anomalous absorption peaks at wavelengths that depend on the twist angle (θt). A multilayer dielectric structure transfer matrix model was employed to correlate the OC spectra with the complex refractive index of tBLG. The complex refractive index was calculated using two methods, with the thicknesses of tBLG (d1) and SiO₂ (d2) as adjustable parameters. The results reveal significant variations in both n and k values at wavelengths corresponding to the anomalous absorption peaks, underscoring the substantial impact of θt on the complex refractive index of graphene.
