Geometry-dependent two-photon absorption followed by free-carrier absorption in AlGaAs waveguides

TL;DR

This study measures and analyzes the geometry-dependent two-photon absorption and free-carrier absorption in AlGaAs waveguides, revealing how waveguide structure influences nonlinear optical properties for device optimization.

Contribution

It provides the first detailed comparison of two-photon absorption coefficients across different AlGaAs waveguide geometries and introduces the nonlinear confinement factor as a key design parameter.

Findings

Highest $eta_2$ at 1480 nm for half-core waveguides

Free-carrier absorption cross-section estimated at $2.2\times10^{-16}$ cm$^2$

Two-photon absorption decreases with increasing wavelength

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. This work used the nonlinear transmittance technique to measure the two-photon absorption coefficients ($\alpha_2$) of AlGaAs waveguides in the strip-loaded, nanowire, and half-core geometries in the wavelength range from $1480$ to $1560~\text{nm}$. The highest $\alpha_2$ values of $2.4$, $2.3$, and $1.1~\text{cm}/\text{GW}$ were measured at $1480~\text{nm}$ for a $0.8$-nm-wide half-core, $0.6$-nm-wide nanowire, and $0.9$-nm-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 $2.2\times10^{-16}~\text{cm}^2$ 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 material composition.