We present full-band structure calculations of temperature-and wavelength-dependent two-photon absorption coefficients and free-carrier absorption cross sections in GaAs, InP, and 0.92 eV-band gap Ga 64 In 36 As and InP 60 As 40 alloys. The calculated coefficient decreases with increasing wavelength and band gap but increases with temperature. Using detailed band structure analysis, we identify various contributions to the free-carrier absorption in GaAs and InP. Although the free-carrier absorption is found to arise predominantly from hole absorption, we show that direct absorption by excited electrons is possible, leading to an enhanced free-carrier absorption coefficient. This excited state absorption could be exploited to modulate the transmission of light at communication wavelengths ͑of 1.33 or 1.55 m͒ with, for example, the more commonly available 0.8 m diode laser. We further show that the high-intensity transmission calculated with our values of nonlinear parameters in GaAs agrees very well with the measured values.

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A z-stretching finite difference method is developed for simulating the paraxial light beam propagation through a lens in a cylindrically symmetric domain. By introducing a domain transformation in the z-direction, we solve the corresponding complex difference equations containing an interface singularity over a computational space for great simplicity and efficiency. A specially designed matrix analysis is constructed to the study the numerical stability. Computational experiments are carried out for demonstrating our results.

Nonlinear optical properties of a graphene-polyvinyl alcohol composite are studied using an irradiance-scan setup. The measurements are carried out at 785 nm and 1064 nm in the nanosecond and picoseconds temporal regimes, respectively. It is shown that the output fluence readily clamps with increasing input and graphene concentration due to nonlinear absorption and scattering. Furthermore, the nonlinear transmission demonstrates weak saturable absorption followed by strong optical limiting. The nonlinear coefficients βeff and Isat calculated via numerical fitting show that βeff depends on the graphene content and increases significantly in nanosecond regime due to high degree of thermally induced nonlinear scattering.