Abstract

In a photosensitive material, such as As₂S₃, light beams of appropriate wavelengths can permanently increase the refractive index. Using this property, Bragg gratings and photoinduced integrated optical devices¹ have already been fabricated by near bandgap light exposure (~500 nm). We have recently observed that As₂S₃ is also photosensitive at longer wavelengths (~ 800 nm), corresponding to the transparency region where two-photon absorption dominates. The main advantage of working in this wavelength region is that the penetration depth (~ 2 cm) is much longer than at wavelengths near or in the band edge (few µm). This fact is of paramount importance when working with waveguides. It has recently been shown theoretically and experimentally in germano-silicate glass² that a channel waveguide can be self-written in a planar photosensitive waveguide. An incident gaussian beam causes an increase in the refractive index, the largest index change occurring where the intensity is the highest. This increase in the refractive index will counteract diffraction and the incident field will self-tran. Eventually, an induced waveguide will be formed along the propagation direction. We present here the self-writing of channel waveguides in As₂S₃ thin films, by two-photon absorption of femtosecond pulses near 800 nm: the strong two photon absorption induced photosensitivity of these glasses revealed that the waveguide formation is fast, of the order of minutes. To verify the refractive index change, we have observed the image of the output of the waveguide, presented in Figure 1, as a function of time. At the beginning, the light is spread in the planar slab. As a channel begins to form, more light is guided along the central axis, indicated by the narrowing of the output beam, as is clearly shown in the following frames of fig. 1. The output power is shown in fig. 2. The oscillations are likely due to the interference occurring between the various modes of the induced waveguide, meaning that higher order modes are being excited, which is a clear indication that a waveguide is formed and that the refractive index change is reaching saturation. We thus observed the formation of a self-written waveguide in As₂S₃, due to a permanent refractive index increase induced by two-photon absorption near 800 nm. This result clearly suggests that this material is photosensitive to two-photon absorption which can be used in applications such as optical storage. We are working on a better characterization of the process, such as the effect of the guide length, the input beam width, power, the input pulse width and the exposure time.

© 1998 IEEE