Refractive Index Dispersion and Anisotropic Group Refractive Indices of Rubrene Crystals

Abstract

We have determined refractive indices of rubrene crystals in the wavelength regions where their emissions were observed. For this purpose, we grew in a vapor phase the rubrene crystals having both horizontal and vertical pairs of parallel crystal facets. The facets functioned as optical resonators that produced interference fringes in the emission and reflectance spectra. From the fringes in the emission spectrum, we estimated the dispersion of the phase refractive index along the crystal a-axis. The anisotropic group refractive indices were evaluated along the b-and c-axes from the fringes in the reflectance spectra. The experimental indices were compared with those computed from the density functional theory.

Iodine insertion and dispersion of refractive index in organic single crystal semiconductor

“…The resonant absorption wavelength or the parameter C , which is closely related to the energy band gap, was obtain as 537 ± 40, 526 ± 60, and 542 ± 44 nm at 270, 210, and 80 K, respectively. The nominal value of C ≈ 535 nm corresponding energy gap of an order of 2.3 eV is close to that of the single crystal rubrene 8 , one of the most intensively studied organic materials.…”

“…The frequency of the intensity modulations which overlay a monotonic emission spectrum for the broad range of wavelength is highly dependent on the sample thickness. Data also indicate that there exist considerable surface emissions in the TMTSF system, which is somewhat unusual considering the fact that the surface emissions were negligible in similar measurements on common organic single crystals such as rubrene and thiophene/phenylene co-oligomers 4 , 8 , 9 . The propagation of the molecular light emission is associated with the orientation of the molecular transition dipole moments (TDM) 14 , 15 .…”

“…The wavelength spacing between adjacent modulation peaks (Δλ), the so-called free spectral range, is inversely proportional to the thickness of the single crystal or, more precisely, the separation between two end-facets along the detecting direction of the light emission. In fact, a pair of parallel facets functions as a Fabry-Perot type resonator within which multiple reflections generate interferences resulting in the intensity modulation 4 , 8 , 9 , 14 . Only standing waves at discrete wavelength can exist in the self-cavity resonator.…”

“…With the substitution of the Sellmeier formula for the phase refractive index in Eq. ( 2 ), the dispersion of the group refractive index in a function of the three fitting parameters is obtained 8 , 9 . The thin lines through the data points shown in upper panel of Fig.…”

“…It would require accurate knowledge of both electric and optical parameters from various organic systems with many different structures 1 , 7 . Among the various optical properties of organic semiconductors, an important but seldom studied issue is that how the electromagnetic wave propagates through the medium depending on wavelength below absorption edge 8 , 9 . The dispersion of refractive index, essentially linked by absorption spectrum through Kramers-Kronig relation, can provide fundamental information on the energy band structure.…”