A terahertz spectrometer on a chip, at the thermodynamical limits

太赫兹光谱分析为远程无损检测痕量化学品和气体、爆炸物、病原体和生物制剂提供了一个平台。这些分子和危险物质在太赫兹频率上具有独特的吸收光谱，能够从遥远的距离识别隐藏的危险物质。外差式光谱仪是高光谱分辨率太赫兹光谱技术的前沿。通过许多不同的技术，如肖特基二极管中的非线性混频，超导-绝缘体-超导体结构，或热电子测辐射热计，它们的线宽与中心频率比低至百万分之一。然而，这些最先进的外差式光谱仪和混音器体积庞大(重达数十公斤)，受到远离基本噪声限制的电子噪声的限制，通常需要低温冷却才能达到明显的灵敏度。本项目提出了一种基于芯片级激光频率梳和芯片级光混合器的模块化集成的芯片级太赫兹光谱仪。激光频率梳由离散的光学频率线组成，在一个倍频程内可广泛调谐。光电混合器基于等离子体增强的吸收衬底，直接耦合到太赫兹天线以收集入射太赫兹辐射。提出的单片太赫兹接收器具有1-8太赫兹的光谱带宽，光谱分辨率优于1千赫，并且工作在接近热力学噪声极限的地方。高性能、低成本、紧凑的太赫兹光谱仪在空间科学、生物分析、环境研究、制药和工业质量控制中具有重要的应用价值。拟议的科学努力与外展和教育计划相结合。这涉及到面向代表不足的高中生和教师，改进研究生和本科课程，以及面向公众，重点关注代表不足的女性和少数族裔学生在本科生的暑期研究经历。外差芯片规模光谱仪的拟议进展包括三个交叉相关的推力。在推力1中，该项目将展示一种芯片上频率梳振荡器，其调谐范围为1-8THz，当参考光学载波时，梳状线到线的不均匀性为0.2ppm。在推力2号项目中，该项目将开发一种芯片上集成的泵浦激光器和放大器，用于与光电混合器和频率梳进行异质集成。在《推力3》中，该项目将展示基于天线耦合等离子体光混合器的集成芯片级外差接收器。该团队试图测量一种具有接近基本边界的双边带噪声敏感度的外差式光电倍增器。频率捷变方法是通过他们最近的初步研究和接近或在热力学噪声极限下的测量而实现的。提出的太赫兹光谱架构和科学推进器可以改变太赫兹波的平台，用于大气研究、空间探索和安全-工业-环境质量控制系统。科学推动力与教育宣传和跨学科培训工作相结合。这个为期三年的项目将培养新一代科学家，他们将在精密芯片级频率梳和等离子体光混合器的界面上，用于接近基本边界的变革性太赫兹光谱分析。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Terahertz spectrometry offers a platform for long-distance and non-destructive detection of trace chemicals and gases, explosives, pathogens, and biological agents. These molecules and hazardous agents have unique absorption spectra in the terahertz frequencies, enabling identification of concealed hazardous substances from remote distances. Heterodyne spectrometers are at the frontier for high spectral resolution terahertz spectrometry. Achieved through a number of different techniques such as nonlinear frequency mixing in Schottky diodes, superconductor-insulator-superconductor structures, or hot electron bolometers, they have linewidth-to-center frequency ratios down to one part in a million. These state-of-the-art heterodyne spectrometers and mixers, however, are bulky (weighing tens of kilograms), are bounded by electronic noise far from the fundamental noise limits, and often require cryogenic cooling to reach appreciable sensitivities. This project proposes a chip-scale terahertz spectrometer based on modular integration of a chip-scale laser frequency comb with a chip-scale photomixer. The laser frequency comb consists of discrete optical frequency lines, widely tunable over an octave. The photomixer is based on a plasmonically-enhanced absorbing substrate, directly coupled to a terahertz antenna to collect the incident terahertz radiation. The proposed single-chip terahertz receiver has spectrometry bandwidth of 1-8 THz with spectral resolution better than a kHz, and operates close to the thermodynamical noise limits. The high-performance, low-cost, and compact terahertz spectrometer brings valuable applications in space sciences, biological analysis, environmental studies, pharmaceuticals, and industrial quality control. The proposed scientific efforts are coupled with an outreach and education plan. This involves outreach to underrepresented high-school students and teachers, improvements to the graduate and undergraduate curriculum, and outreach to the general public with focus on underrepresented women and minority students in summer research experiences for undergraduates. The proposed advancement on the heterodyne chip-scale spectrometer consists of three cross-related Thrusts. In Thrust 1, the project will demonstrate an on-chip frequency comb oscillator with wide tuning range of 1-8 THz and comb line-to-line non-uniformity at 0.2 parts per quadrillion when referenced to the optical carrier. In Thrust 2, the project will develop an on-chip integrated pump laser and amplifier, for heterogeneous integration with the photomixer and frequency comb. In Thrust 3, the project will demonstrate the integrated chip-scale heterodyne receiver based on an antenna-coupled plasmonic photomixer. The team seeks to measure a heterodyne photomixer with double-sideband noise sensitivities close to the fundamental bound. The frequency-agile approach is enabled by their recent preliminary studies and measurements close to or at the thermodynamical noise limits. The proposed terahertz spectrometry architecture and scientific Thrusts can transform the platform of terahertz waves for atmospheric studies, space explorations, and safety-industrial-environmental quality control systems. The scientific Thrusts are integrated with educational outreach and cross-disciplinary training efforts. This three-year project will educate a new generation of scientists at the interface of precision chip-scale frequency combs and plasmonic photomixers for transformative terahertz spectrometry near the fundamental bounds.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.