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Abstract
Nonlinear absorption can limit the efficiency of nonlinear optical devices. However, it can also be exploited for optical limiting or switching applications. Thus, characterization of nonlinear absorption in photonic devices is imperative for designing useful devices. This work uses the nonlinear transmittance technique to measure the two-photon absorption coefficients (${\alpha _2}$) of AlGaAs waveguides in strip-loaded, nanowire, and half-core geometries in the wavelength range from 1480 to 1560 nm. The highest ${\alpha _2}$ values of 2.4, 2.3, and $1.1 \;{\rm{cm}}/{\rm{GW}}$ were measured at 1480 nm for 0.8-µm-wide half-core, 0.6-µm-wide nanowire, and 0.9-µm-wide strip-loaded waveguides, respectively, with ${\alpha _2}$ decreasing with increasing wavelength. The free-carrier absorption cross section was also estimated from the nonlinear transmittance data to be around $2.2 \times {10^{- 16}}\; {\rm{cm}}^2$ for all three geometries. Our results contribute to a better understanding of the nonlinear absorption in heterostructure waveguides of different cross-sectional geometries. We discuss how the electric field distribution in the different layers of a heterostructure can lead to geometry-dependent effective two-photon absorption coefficients. More specifically, we pinpoint the third-order nonlinear confinement factor as a design parameter to estimate the strength of the effective nonlinear absorption, in addition to tailoring the bandgap energy by varying the material composition.
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