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

Abstract

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.

Refractive Index Dispersion and Anisotropic Group Refractive Indices of Rubrene Crystals

“…Because the emission polarized parallel to the a-axis was ~12 times stronger than that to the b-axis, the emission was polarized parallel to the a-axis. We determined the wavelength dependence of group refractive index n g for every ~5 nm from the fringes following the reported procedures [11,13]. For this we used the variation in mode order of fringes δN, its wavenumber variation δν, and the resonator length L: n g = δN/2Lδν.…”

“…We measured the interference fringes in both the reflectance and emission spectra using the methods described in the literature [11][12][13]. For both the measurements we irradiated incident lights perpendicularly to the substrate plane.…”

“…a pair of reflecting mirrors). Using a SOLAR TII MS7504 monochromator spectrograph we acquired the emission spectra occurring from the crystal along the direction perpendicular to the resonator mirrors [11,13].…”

“…From the viewpoint of the optoelectronic device applications, however, the dispersions in the emission wavelength regions (~600−800 nm) are essential. In this connection, we have recently developed a simple and useful method to determine the dispersions and anisotropic group refractive indices of the organic crystals [11,12]. In the method, we used interference fringes that were observed in the emission and reflectance spectra.…”

“…Broadband nature of these spectra enables us to investigate the refractive index relevant to the emission wavelength regions. By applying this method to crystals of thiophene/phenylene co-oligomers, we have determined their refractive indices [11][12][13]. In the present studies, on the basis of the above method, we have evaluated the dispersions and anisotropic group refractive indices of the rubrene crystals.…”