All-optical modulation in crystalline organic semiconductor waveguides

We demonstrate all-optical modulation in crystalline organic semiconductor waveguides grown by the ultrahigh vacuum process of organic molecular-beam deposition. Two light beams with wavelengths of 1.06 and 0.514 μm were used as the guided and the pump light sources, respectively. A refractive index change of 5.4×10−5 at 1.06 μm was observed at a pump intensity of 1.0 W/cm2. This large nonlinear effect is attributed to free electron-hole pairs produced by the dissociation of excitons generated by the short wavelength beam. A carrier lifetime of (17±1) μs which determines the modulator switching time is in good agreement with theoretical predictions. To our knowledge, this is the first observation of free-carrier-induced index modulation in crystalline organic waveguides.