Frequency-angular distributions of optical signals generated via spontaneous parametric downconversion with the Stokes idler frequency shifts 0.2–5 THz are studied simultaneously with the same distributions in the anti-Stokes range while the nonlinear Mg:LiNbO3 crystal is cooled from 300 K to 4.2 K. The temperature dependencies of the angular distributions at fixed idler frequencies are analyzed using a theoretical Klyshko–Kirchhoff approach with account of thermal field fluctuations and inherent crystal absorption at terahertz frequencies. Although all the measurements are performed in the optical range, we demonstrate how such analysis enables to predict the temperature behavior of the total number of spontaneous parametric downconversion-generated idler photons in the terahertz range, the temperature variation of the optical-terahertz biphoton function, and to study the contributions of classical thermal and pure quantum field fluctuations to parameters of biphotons. It is shown that the temperature-induced growth of the numbers of signal and idler photons and their non-normalized correlation function is provided by overwhelming increase in classical fluctuations, while the pure quantum contributions to these parameters are not so sensitive to the crystal temperature.

中文翻译：

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研究 SPDC 光谱以揭示光学太赫兹双光子的温度依赖性

在非线性 Mg:LiNbO3 晶体从 300 K 冷却到 4.2 K 时，同时研究了通过自发参数下变频产生的光信号的频率角分布，其中斯托克斯闲散频移为 0.2-5 THz，反斯托克斯范围内的相同分布. 使用理论 Klyshko-Kirchhoff 方法分析固定惰轮频率下角分布的温度依赖性，并考虑了太赫兹频率下的热场波动和固有晶体吸收。尽管所有测量都是在光学范围内进行的，但我们展示了这种分析如何能够预测太赫兹范围内自发参数下变频产生的闲散光子总数的温度行为，光-太赫兹双光子函数的温度变化，并研究经典热和纯量子场涨落对双光子参数的贡献。结果表明，信号和闲散光子及其非归一化相关函数的温度诱导增长是由经典波动的压倒性增加提供的，而对这些参数的纯量子贡献对晶体温度不太敏感。