Abstract Semiconductors thin films are the foundation of modern technology. While the nonlinear optical (NLO) properties of bulk semiconductors have been systematically studied in the last three decades, it is still a great challenge to obtain them for semiconductors thin films, as the high laser irradiance in NLO experiments tends to irreversibly damage the thin films. In addition, tuning the NLO response of semiconductor thin films by alloying and doping has not been explored yet. Here, we study the influence of the Aluminum content in AlxGa1-xN thin films and the n-type doping concentration in GaN thin films on their two-photon absorption (2PA) coefficients. For this, we investigate five different GaN-based thin films: an unintentionally doped one with no Al as a reference, two n-type doped GaN films with distinct concentrations of silicon impurities, and two AlxGa1-xN alloys with an aluminum content of 5.5% and 9.0%, respectively. The femtosecond 2PA spectra reveal that doping impurities reduce the non-linear coefficients (∼10%), while alloying with Al enhances the 2PA coefficient up to 30%. We use the model of Brandi and Araujo to determine Kane’s energy parameter related to the transition matrix element for each sample and compare them with recent theoretical studies based on the k·p theory where an excellent agreement is found.

中文翻译：

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飞秒激光诱导 GaN 和 AlxGa1-xN 薄膜的双光子吸收：通过合金化和掺杂调节非线性光学响应

摘要 半导体薄膜是现代技术的基础。虽然在过去的 30 年里已经系统地研究了体半导体的非线性光学 (NLO) 特性，但获得它们对于半导体薄膜仍然是一个巨大的挑战，因为 NLO 实验中的高激光辐照度往往会不可逆地损坏薄膜. 此外，尚未探索通过合金化和掺杂来调整半导体薄膜的 NLO 响应。在这里，我们研究了 AlxGa1-xN 薄膜中的铝含量和 GaN 薄膜中的 n 型掺杂浓度对其双光子吸收 (2PA) 系数的影响。为此，我们研究了五种不同的基于 GaN 的薄膜：一种没有 Al 作为参考的无意掺杂的薄膜，两种具有不同硅杂质浓度的 n 型掺杂 GaN 薄膜，以及两种铝含量分别为 5.5% 和 9.0% 的 AlxGa1-xN 合金。飞秒 2PA 光谱显示掺杂杂质降低了非线性系数（~10%），而与 Al 合金化将 2PA 系数提高了 30%。我们使用 Brandi 和 Araujo 的模型来确定与每个样品的过渡矩阵元素相关的 Kane 能量参数，并将它们与最近基于 k·p 理论的理论研究进行比较，发现了极好的一致性。