GaP、ZnSe、GaSe 和 ZGP 晶体中的高阶（N=4-6）多光子吸收与中红外克尔非线性
High-order (N=4-6) multi-photon absorption and mid-infrared Kerr nonlinearity in GaP, ZnSe, GaSe, and ZGP crystals
AI总结：
本文利用 Z 扫描和 2.35 μm 飞秒脉冲研究了 GaP、ZnSe、GaSe 和 ZGP 四种中红外晶体的高阶多光子吸收与非线性折射，发现吸收阶数 N=4-6 与带隙吻合，并揭示了自由载流子吸收异常增强及两种方法一致的非线性折射率测量结果。
摘要：
我们报告了一项关于四种重要中红外 χ(2) 晶体——GaP、ZnSe、GaSe 和 ZGP——中高阶多光子吸收、非线性折射及其各向异性的研究，采用 Z 扫描方法和 2.35 μm 飞秒脉冲，峰值强度超过 200 GW/cm²。我们发现非线性吸收遵循微扰模型，多光子吸收（MPA）阶数从 N = 4 到 6，与这些晶体的带隙高度吻合。通过在保持恒定峰值强度的同时将脉冲宽度在 30 至 70 fs 之间变化，对自由载流子吸收的作用进行了研究，结果表明在我们的强度水平下，MPA 过程中产生的自由载流子的吸收远强于根据其线性吸收截面所预期的结果。可能的机制包括高场效应，例如导带内的谷间散射以及向更高能带的吸收。非线性折射率通过（i）闭孔 Z 扫描和（ii）自相位调制引起的光谱展宽两种方法测量，两种方法的结果相互吻合良好。
英文摘要：
We report a study of high-order multi-photon absorption, nonlinear refraction, and their anisotropy in four notable mid-infrared \c{hi}(2) crystals: GaP, ZnSe, GaSe and ZGP using the Z- scan method and 2.35-μm femtosecond pulses with peak intensity in excess of 200 GW/cm2. We found that the nonlinear absorption obeys a perturbation model with multi-photon absorption (MPA) orders from N = 4 to 6, in good agreement with the bandgaps of the crystals. A study of the role of free carrier absorption, performed by varying the pulse duration between 30 and 70 fs while maintaining a constant peak intensity showed that at our intensity levels, free carriers generated in the process of MPA absorb much stronger than would be expected from their linear absorption cross section. Possible mechanisms include high-field effects, such as intra-valley scattering in the conduction band and absorption to higher lying bands. Nonlinear refractive indices were measured using (i) closed aperture Z-scan and (ii) spectral broadening due to self-phase modulation, both methods agreeing well with each other.
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