Indium tin oxide (ITO) films have recently emerged as a new class of functional materials for nonlinear optical (NLO) devices due to their exotic properties around epsilon-near-zero (ENZ) wavelength. Here, we experimentally investigated and tailored the NLO absorption properties of ITO films. The NLO absorption response of ITO films is investigated by using the femtosecond Z-scan measurement technique at two different wavelengths of 1030 nm (out of ENZ region) and 1440 nm (within ENZ region). Interestingly, we observed conversion behavior from saturable absorption (SA) to reverse saturable absorption (RSA) at 1030 nm with the increasing incident laser intensity, whereas only SA behavior was observed at 1440 nm. We demonstrate that SA behavior was ascribed to ground-state free electrons bleaching in the conduction band, and RSA was attributed to three-photon absorption. Moreover, results reveal that ITO film shows more excellent SA performance at 1440 nm with a nonlinear absorption coefficient of $\sim -23.2\mathrm{cm}/\mathrm{GW}$ and a figure of merit of $\sim 1.22\times {10}^{-16}\mathrm{esu}·\mathrm{cm}$. Furthermore, we tailored the SA and RSA behaviors of ITO films at 1030 and 1440 nm wavelengths via post-annealing treatment. The modulatable NLO absorption was ascribed to the changing of free-carrier concentration in ITO films via annealing treatment. The experimental findings offered an inroad for researchers to tailor its NLO absorption properties by changing the free-carrier concentration through chemical modification such as annealing, oxidation, or defect implantation. The superior and tunable nonlinear optical response suggests that ITO film might be employed as a new class material with potential applications in novel optical switches or optical limiters to realize the all-optical information process.

中文翻译：

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调整ε接近零的材料的饱和和反饱和吸收的原理

铟锡氧化物（ITO）膜由于其在ε近零附近（ENZ）波长的奇异特性，最近已成为一种用于非线性光学（NLO）器件的新型功能材料。在这里，我们通过实验研究和定制了ITO膜的NLO吸收特性。通过使用飞秒Z扫描测量技术，在1030 nm（在ENZ区域外）和1440 nm（在ENZ区域内）的两个不同波长下，研究了ITO膜的NLO吸收响应。有趣的是，我们观察到随着入射激光强度的增加，在1030 nm处从饱和吸收（SA）到反向饱和吸收（RSA）的转换行为，而在1440 nm仅观察到SA行为。我们证明了SA行为归因于导带中漂白的基态自由电子，RSA归因于三光子吸收。此外，结果表明，ITO膜在1440 nm处具有更优异的SA性能，其非线性吸收系数为$〜-23.2\mathrm{厘米}/\mathrm{毛重}$ 并具有以下优点 $〜1.2\mathrm{2个}\times {10}^{-\mathrm{1个}6}\mathrm{su}·\mathrm{厘米}$。此外，我们通过后退火处理来调整ITO膜在1030和1440 nm波长下的SA和RSA行为。可调节的NLO吸收归因于通过退火处理的ITO膜中自由载流子浓度的变化。实验结果为研究人员通过化学修饰（例如退火，氧化或缺陷注入）改变自由载流子浓度来调整其NLO吸收特性提供了一条途径。优异且可调谐的非线性光学响应表明，ITO薄膜可被用作新型材料，在新型光学开关或光学限制器中具有潜在的应用前景，可实现全光信息处理。