Oneand two-photon stimulated emission in oligothiophenes single crystals

We report on the observation of stimulated emission (SE) in plane-parallel single crystals of quaterthiophene (4T), sexithiophene (6T) and octithiophene (8T) at room temperature. One-photon transverse pumping at normal incidence induces gain narrowing and leads to two SE lines emerging at respectively low (50 μJ/cm2) and high (500 μJ/cm2) pumping levels. Two-photon excitation is realized in a longitudinal geometry, i.e. the near-IR beam of a Ti:sapphire laser being directed parallel to the crystal faces. Intense and highly directional emission of visible light (green for 4T, orange for 6T and red for 8T) is generated along the pump pathway throughout the crystal over lengths of several millimeters. Since in the solid state oligothiophenes also possess carrier mobilities as high as 0.1 cm2/V.s, they appear as potential materials in view of an organic laser diode.