Dispersion of the refractive indices of thiophene/phenylene co-oligomer single crystals

We have determined the wavelength dependence of group refractive indices for crystals of thiophene/phenylene co-oligomers (TPCOs). We calculated the group indices from the broadband interference modulation of the emission spectra. The interference is caused by a pair of parallel crystal edges that function as an optical resonator. These indices steeply increased with decrease in wavelengths and were in good agreement with those estimated from the mode intervals of the longitudinal multimode at the laser oscillation. Furthermore, we derived the dispersion of phase refractive indices on the basis of the Sellmeier dispersion equation. These refractive indices gradually increased with decrease in wavelengths and ranged 2.5–3.2 for the materials. The implications of the experimental results are discussed.