In-guide measurement of the infra red absorption variation induced in hydrogenated amorphous silicon by visible radiation

Several authors characterized the photo-induced absorption (PA) in a-Si:H by performing suitable physical models describing the optical modifications of the illuminated material. Usual investigation techniques study the phenomenon by analysing transmitted or reflected probe light directed on the film surface. Alternatively, we report an in-guide analysis performed through properly designed planar a-Si:H/SiO2 waveguides, fabricated on a c-Si substrate by plasma enhanced chemical vapour deposition (PECVD). The technique allows a wider interaction region between the probe and the pump beams, then the experimental setup sensitivity improves highly. As probe, we used a coherent beam emitting at 1550 nm wavelength and different pump lights in the visible spectrum. We observed the amplitude modulation induced over the probe radiation traveling through the waveguide when an intermittent visible pump was shone over it. LEDs emitting at 644, 612 and 590 nm wavelengths, at intensities up to 0.85 mW/cm(2), were alternatively used. The PA effect shows a linear increasing on the pump intensity and is stronger for longer wavelength pumps. A numerical analysis of the optical propagation was performed to evaluate the absorption variation: at the highest pump intensity the maxima values for Deltaalpha(1550) were 2.15, 1.7 and 0.88 cm(-1), respectively, for pump emitting at 590, 612 and 644 nm.