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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/SiO 2 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 Δ α 1550 were 2.15, 1.7 and 0.88 cm −1 , respectively, for pump emitting at 590, 612 and 644 nm.

Based on the total internal reflection (TIR) phenomenon and the thermo-optic effect in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si), a symmetric rib optical waveguide integrated switch is proposed and theoretically discussed. The device exploits the similar refractive index coupled to the different thermo-optic coefficient in the two materials. The possibility of alloying and doping for the band-gap engineering of a-Si:H, by means of the gas phase composition during the modern plasma enhanced chemical vapour deposition process, which takes place at temperatures as low as 220 degrees C, makes this semiconductor ideal for this type of application. In particular the refractive index at room temperature of the amorphous film can be properly tailored to match that of c-Si in order to achieve the light switching when the device experiences a given temperature change. TIR may be achieved however at the interface by acting on the temperature, because the two materials have different thermo-optic coefficient. The integrated single-mode rib waveguide is 4 μm wide and 3 μm high. The substrate is a SOI wafer with an oxide thickness of 500 nm. The switch has a quite short operation length of about 280 μm. The device performance is analyzed at the wavelength of 1.55 μm. It shows that the output crosstalk and insertion loss are less than -26.9 dB and 3.5 dB, respectively.

A novel amorphous-silicon-based photonic micromodulator is proposed which can be easily integrated into silicon low-cost optoelectronic appliances. The device is designed to operate at the two infrared communication wavelengths of 1300 and 1550 nm, either on broad or narrow spectrum sources, and is activated by visible light pulses controlling the infrared light absorption coefficient through the generation of free carriers. A planar waveguide integrated prototype has been fabricated on a silicon wafer by plasma enhanced chemical vapour deposition, and tested at bit rates up to 200 kbit/s.