We report on experimental evidence for gain−induced guiding and refractive−index antiguiding along the junction plane of stripe−geometry double−heterostructure GaAs lasers. It had previously been found that carrier diffusion out of the active region leads to a gain profile along the junction plane that can be approximated by a parabolic variation. This results in a lowest−order mode having a Gaussian profile, a cylindrical phase front, and a constant radius of curvature. This curvature accounts for the astigmatism always observed in the output beam from stripe−geometry GaAs lasers, where gain guiding dominates index guiding. Experimental determinations of the far−field diffraction angle, ϑ, and the beam width, 2w, at the laser mirror in the junction plane enable us to calculate the parameters characterizing the gain distribution responsible for mode confinement, as well as a negative refractive−index increment which would tend to defocus the mode. The negative index increment, as measured between the center and the edges of the stripe, appears to be related to a competition between a negative free−carrier effect and a positive thermal focusing mechanism. Above threshold, changes in ϑ and w with current imply changes in the gain−guiding mechanism. The theory implies in particular a decrease in gain at the center of the stripe and an increase at the edges with an increase in current.
Amorphous silicon has been used extensively in electro-optical applications. Its use as a gate electrode material for advanced CMOS devices is currently being developed, as it offers certain desirable characteristics compared to the commonly used polycrystalline silicon. The properties of amorphous silicon, including optical and electronic, are highly variable depending on process conditions, i.e. deposition temperature, etching conditions, and chemistry. The variable optical properties present a challenge for broad-band reflected intensity optical thickness measurement techniques. This is because the constant angle reflection interference spectroscopy (CARTS) technique requires the knowledge of a film's dispersion (the change in refractive index with wavelength) before the film thickness can be determined'. An updated feature in Prometrix CARIS-based spectrophotometers allows the user to easily determine or verify dispersion for both transparent and absorbing films.As a demonstration of this technique, we have studied a series of amorphous silicon films deposited at varying temperatures. The deposition temperature ranged from 54(Y C to 570' C. The nominal thickness was approximately 1575A to 3170A. Due to the differences in deposition temperature, one would expect the optical properties to vary slightly. Using software that allows analysis of the spectral information, the dispersion was examined for each sample. With this knowledge, the film thicknesses could be reliably measured.
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