Single- and two-photon up-converted stimulated emission in monolithic organic single crystals

We evidence optically pumped spectral narrowing in ultra thin (3.5 microns) single crystals of (alpha) -octithiophene ((alpha) -8T), the luminescent conjugated octamer of thiophene. Experiment is achieved in a transverse pumping mode. It occurs at a pump energy threshold of approximately 0.1 (mu) J per pulse (33 ps, 10 Hz) to yield an intense and narrow emission line peaking at 700 nm. At pump energies higher than 15 (mu) J per pulse, a second narrow line of weaker intensity emerges at 640 nm. Gain narrowing of these two lines can be easily described in the usual framework of stimulated emission (SE). Light amplification originates from the combination of a net dipole alignment and efficient wave-guiding towards the edges of the crystal. The (alpha) -8T molecules provide both the emitter and the optical cavity. Spectral SE selection of the two SE lines at 640 and 700 nm can be monitored by simply scanning the spatial position of the pump beam onto the surface of the crystal. Similar SE phenomena are also observed in the (alpha) -4T and (alpha) 6T crystals, showing that conjugated oligothiophenes ((alpha) -nT) constitute a family of monolithic organic single crystals naturally adapted to laser action. Additionally, in a longitudinal pumping gain guiding situation, nanojoule-threshold infrared two-photon pumped up- converted stimulated emission is demonstrated in the oligothiophene crystals.