用于红外探测的极度非简并非线性光学

Extremely Nondegenerate Nonlinear Optics for IR Detection

半导体中的双光子吸收(2PA)与能隙的三次方的倒数(Eg-3)成正比,这一关系限制了在宽带隙半导体中获得双光子吸收系数。但是已知在输入波长迥异的高度非简并情况下,双光子吸收率较简并状态会有大幅提升。在飞秒和皮秒脉冲下的泵浦探测传输实验中证实了几种直接能隙半导体具有这种性质。发现在许多直接能隙半导体中,非简并双光子吸收率较简并情况下可增强约100倍。在GaAs中,观察到双光子吸收系数达到1cm/MW,这么大的系数以前只在窄带半导体如InSb中观察到过。基于这种作用可以利用传统的半导体光电二极管获得敏感的选通探测。采用标准的GaN和GaAs光电二极管和高达14∶1的能量比的极度非简并光子证实了此技术。此外,还采用强紫外选通脉冲探测弱红外辐射,如采用GaN光电二极管,最小可探测红外脉冲能量可低至20pJ,而现有的标准制冷型MCT探测器的最小可探测能量为200pJ。我们现在还证实这种方法也可在连续波探测中应用。值得注意的是,这个过程没有用到类似χ(2)上转换的红外晶体或相位匹配。在本报告中将介绍怎样将这种探测技术用于其他半导体,以及如何优化器件的几何结构以满足实用化探测要求。

Two-photon absorption（2PA） in semiconductors has long been known to scale as the inverse third power of the energy gap, i.e. ETga , which limits the 2PA coefficients available in large gap semiconductors. However it is also known that in the highly nondegenerate case, where the input wavelengths are very different, the 2PA rate can be greatly enhanced over the degenerate case. We have recently verified this for several direct gap semiconductors in pump-probe transmission ex- periments with femtosecond and picosecond pulses, where we showed that, in many direct-gap semiconductors, nondegener- ate 2PA coefficients are enhanced approximately 100-fold over the degenerate case. In GaAs, we observed 2PA coefficients around 1 cm/MW. Coefficients this large were previously only observed in narrow-gap semiconductors such as InSb. Based on this effect, one may obtain sensitive gated detection using conventional semiconductor photodiodes. We have demonstra- ted this with standard GaN and GaAs photodiodes using extreme non-degenerate photon pairs with up to 14 ： 1 energy ratio. We can also detect weak IR radiation by employing intense UV gating pulses. The minimum detected IR pulse energy in GaN is as low as 20 pJ energy while for a standard cooled MCT detector, the minimum detectable energy is 200 pJ. We have also now demonstrated cw detection using this method. It is worth noting that this process does not use IR crystals or phase- matching, as employed by ./.{2~ upconversion detection. In this talk, we will show how this detection scales to other semicon- ductors and how one may optimize device geometries for practical detectioru