基于频率变换技术的超高灵敏度太赫兹相干探测研究

Terahertz (THz) technology has proven its great potentials in public security、material recognition、life science etc. Now it is rather than a toy in the lab. Recently, as scientists become more familiar with this new spectrum, it is believed that several Nobel Prizes will be born related to THz science. Compared to the long history of the R&D activities of THz source, THz detection suffers from a big delay due to the technology barrier. Nowadays, high-power THz source is quite rare all over the world, making highly-sensitive THz detection extremely important. Most THz detectors in the market are based on thermal detection, which are slow、non-sensitive and they cannot supply phase information. Similar to the successful heterodyne detection in radio science, we can realize THz detection based on frequency conversion technique by changing it to mature visible/infrared light regime, thus it is possible to make use of excellent existing components. In this way, fast、broadband and coherent THz detection is feasible. In this project, we are going to solve the wave coupling equations numerically by applying full-wave calculations of electromagnetic fields, breaking down general small-signal approximations and plane wave assumptions. Moreover, we are going to visualize the wave coupling process by taking practical beams into account. Based on such a powerful tool, we will be able to discuss the theoretical limit by taking exact crystals and possible waveguide structures into account. Extremely-high sensitivity will be pursued in experiments by using excellent equipments and resorting to possible optical amplifications. Phase measurement will be pursued, so as to explore new applications of such frequency-domain measurement systems. We hope to supply cutting-edge THz detection approach towards the best THz systems and we are glad to explore the feasibility of such an approach for practical applications with a reasonable cost. We believe the research above is undoubtedly important in both theoretical studies and actual applications.

本世纪太赫兹技术已走出实验室并展示出其在公共安全、生命科学等领域的巨大应用潜力，被认为是下一个诺贝尔奖的诞生地。相对于波源的发展，太赫兹探测技术具有滞后性。当前大功率太赫兹源普遍欠缺，因此高灵敏度探测尤为重要。现有的太赫兹探测器多为热探测，响应慢、灵敏度较低，不能提供相位信息。类似于微波超外差探测技术，基于频率变换技术我们可在成熟的可见光、红外频段实现太赫兹波探测，该方法响应速度快、灵敏度高、调谐范围宽。本项目中，我们将采用全波计算，打破小信号近似和平面波入射的限制精确求解波耦合过程，结合实际波束传播物理过程建立完善的太赫兹频率变换探测的物理模型，可视化频率变换的整个过程；在实际系统中结合光放大技术实现超高灵敏度太赫兹相干探测，为尖端科学研究提供高性能探测手段，对太赫兹系统中的高性能低成本探测部分探索新的技术路线。该工作无论在太赫兹探测的理论研究还是实际应用方面都具有重要意义。

太赫兹技术在公共安全、生命科学等领域展示出巨大应用潜力。当前大功率太赫兹源普遍欠缺，因此高灵敏度探测尤为重要。类似于微波超外差探测技术，基于频率变换技术我们可在成熟的可见光、红外频段实现太赫兹波探测，该方法响应速度快、灵敏度高、调谐范围宽。本项目从时变耦合波方程组出发，分析了探测过程中上转换光的增益特性，为系统性能优化和换算标定提供了理论依据。搭建了太赫兹上转换探测实验系统，捕捉到ns量级上转换光脉冲信号和上转换光的光谱信息，灵敏度较热探测器Golay Cell高3个数量级。针对实验中观察到的新现象，开展了对共腔双参量竞争效应和级联放大效应的研究，国际上首次报道了不少于7阶的级联差频，为根本上提高差频转换效率提供了可能。针对系统中波束整形和耦合部分，设计、加工并测试了消像差非球面太赫兹透镜，聚焦效果优于商用的球面透镜。针对1 THz以下频段的探测，补充了CMOS探测技术研究，实现了220-299 GHz频段的探测。上述成果对生命科学、医学、物理化学等基础研究领域具有重要的理论和实际意义，也将推动太赫兹技术在环境监测、安全检查等领域的应用，为后续高性能太赫兹系统的开发及实用化打下坚实的基础。