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Strong exciton-photon coupling has been observed in a highly oriented organic single crystal microcavity. This microcavity consists of a thiophene/phenylene co-oligomer (TPCO) single crystal laminated on a high-reflection distributed Bragg reflector. In the TPCO crystal, molecular transition dipole was strongly polarized along a certain horizontal directions with respect to the main crystal plane. This dipole polarization causes significantly large anisotropies in the exciton transition and optical constants. Especially the anisotropic exciton transition was found to provide the strong enhancement in the coupling with the cavity mode, which was demonstrated by a Rabi splitting energy as large as ∼100 meV even in the “half-vertical cavity surface emitting lasing” microcavity structure.

We have fabricated optical devices using an organic semiconductor crystal as an emission layer in combination with a two-dimensional (2D) inorganic diffraction grating used as an optical cavity. We formed the inorganic diffraction grating by wet etching of aluminum-doped zinc oxide (AZO) under a 2D cyclic olefin copolymer (COC) diffraction grating used as a mask. The COC diffraction grating was fabricated by nanoimprint lithography. The AZO diffraction grating was composed of convex prominences arranged in a triangular lattice. The organic crystal placed on the AZO diffraction grating indicated narrowed peaks in its emission spectrum under ultraviolet light excitation. These are detected parallel to the crystal plane. The peaks were shifted by rotating the optical devices around the normal to the crystal plane, which reflected the rotational symmetries of the triangular lattice through 60°.

We have fabricated ambipolar light-emitting field-effect transistors made of 1,4-bis(5-phenylthiophen-2-yl)benzene (AC5) single crystals, which have 35% fluorescent quantum efficiency. The obtained hole and electron mobilities were 2.9 x 10(-1) cm(2)/V s and 6.7 x 10(-3) cm(2)/V s, respectively. These are the highest values among AC5 transistors. Importantly, although the light emission from the crystal surface was less than the detection level of the camera, we observed bright and polarized light emission from the edge of the single crystals. This polarized edge emission is attributed to the strong self-assembled light-confining nature and perfectly aligned transition dipole moments, which are advantageous for future laser devices.

Vertical-cavity organic lasers are fabricated with melt-grown crystals of a cyano-substituted thiophene-phenylene co-oligomer. Due to lying molecular orientation, surface-emitting lasing is achieved even in the half-cavity crystal grown on a distributed Bragg reflector (DBR) under optical pumping at room temperature. Anticrossing splits in angle-resolved photoluminescence spectra suggest the formation of exciton-polaritons between the cavity photons and the confined Frenkel excitons. By constructing the full-cavity structure sandwiched between the top and bottom DBRs, the lasing threshold is reduced to one order, which is as low as that of the half cavity. Around the threshold, the time profile of the full-cavity emission is collapsed to a pulsed shape accompanied by a finite turn-on delay. We discuss these observed characteristics in terms of a polariton contribution to the conventional photon lasing.

We have achieved to observe vertical cavity surface emitting lasing (VCSEL) from an optically pumped organic single crystal. A vapor-grown thiophene/phenylene co-oligomer (TPCO) single crystal with in-plane polarization component was used as the gain medium in an optical microcavity. Since the top and bottom surfaces of the TPCO crystal serve as high-quality resonators, well-defined Fabry-Perot resonance is achieved. Furthermore, by using high reflectivity distributed Bragg reflector as the cavity resonator, we have observed lasing emissions with a line width of 0.9 nm and a threshold of ∼3 mJ/cm2. The VCSEL-type optical feedback configuration demonstrated in this study is expected to be one of the available means to realize the electrically pumped organic laser device.

We report the improvement of electron transport in 5,5'-bis(biphenylyl)-2,2':5',2 ''-terthiophene (BP3T) single crystals on a field-effect transistor configuration by systematically investigating the effects of device aging under pure nitrogen and optimizing the organic dielectric layer-fabrication process. We reduced the effect of electron traps and achieved extremely high current densities up to 10 kA/cm(2), which is one or two orders of magnitude greater than the current densities achieved in previous devices.