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.

中文翻译：

---

几何相关的双光子吸收，然后是 AlGaAs 波导中的自由载流子吸收

非线性吸收会限制非线性光学器件的效率。然而，它也可以用于光限制或开关应用。因此，光子器件中非线性吸收的表征对于设计有用的器件是必不可少的。这项工作使用非线性透射技术来测量带状加载、纳米线和半芯几何形状的 AlGaAs 波导在 1480 到 1560 nm 波长范围内的双光子吸收系数 ( ${\alpha_2}$ )。最高的${\alpha _2}$值分别为 2.4、2.3 和$1.1 \;{\rm{cm}}/{\rm{GW}}$是在 1480 nm 处测得的 0.8 µm 宽半核，分别为 0.6 µm 宽的纳米线和 0.9 µm 宽的带状加载波导，${\alpha_2}$随波长增加而减小。自由载流子吸收截面也从非线性透射率数据估计约为$2.2 \times {10^{- 16}}\; {\rm{cm}}^2$适用于所有三个几何图形。我们的结果有助于更好地理解不同截面几何形状的异质结构波导中的非线性吸收。我们讨论了异质结构不同层中的电场分布如何导致与几何相关的有效双光子吸收系数。更具体地说，除了通过改变材料成分来调整带隙能量之外，我们还将三阶非线性限制因子确定为估计有效非线性吸收强度的设计参数。