High resolution, large spectral range on-chip Mid-infrared Fourier transform spectroscopy

中红外光谱代表了材料的指纹，其吸收峰对应于构成材料的原子键之间的振动频率，从而可以识别和量化各种材料。中红外光谱在制药、生物技术、工业化学、食品安全、环境监测等领域有着广泛的应用。首选的红外光谱方法是傅里叶变换红外光谱（FTIR）。然而，传统的FTIR系统的移动部件体积庞大，重量大，对环境波动（振动等）很敏感。这些缺点使得它主要是一个实验室工具，需要大量的人工参与，因此不适合现场应用。在这个项目中，德克萨斯大学奥斯汀分校的团队建议使用集成光子学技术在芯片上构建FTIR。FTIR的重量可以惊人地减少到几克，尺寸不到1平方厘米。不再需要活动部件。随着这些革命性的改进，FTIR可以应用于许多前所未有的领域，如战场上的有毒气体检测、机载平台上的温室气体监测和独立环境监测。集成光子学在过去几年中经历了爆炸式的增长。虽然许多元件和系统已被证明具有令人印象深刻的性能，但由于芯片上的吸收长度和强度有限，以及缺乏高分辨率，宽波长范围的光谱仪，片上光谱的发展缓慢。为了解决这些问题，提出的片上FTIR涉及两个主要创新。首先，首次将亚波长光栅超材料波导作为吸收增强介质。它解决了引导（要求光场被限制在高折射率介电区内）和吸收（更倾向于光场在波导外传播）之间的困境。与传统的条形波导相比，分析物对光的吸收可以提高400倍以上。其次，片上ftir由一组不对称的mzi组成，两臂之间的路径差越来越大，波长带宽非常有限。在这个项目中，每个MZI都增加了一个热光移相器。热移相器和增量波导长度差的结合使得同时实现高分辨率和大光谱范围成为可能，这在以前从未被证明过。作为概念验证演示，该项目将设计、制造并实验演示亚波长光栅超材料波导，在蓝宝石上硅平台上以3.4微米为中心的片上FTIR用于甲烷检测。这个概念也可以很容易地扩展到其他波长范围。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The mid-infrared spectrum represents a fingerprint of a material with absorption peaks corresponding to the frequencies of vibrations between the bonds of the atoms making up the material, and thus can identify and quantify all kinds of materials. Mid-infrared spectroscopy has a vast range of applications including pharmacy, biotechnology, industrial chemistry, food safety, and environmental monitoring. The preferred infrared spectroscopy method is Fourier transform infrared (FTIR) spectroscopy. However, conventional FTIR systems with moving components are bulky, heavy, and sensitive to environment fluctuations (vibration, etc). These disadvantages make it mainly a laboratory-only tool requiring extensive human involvement and therefore unsuitable for field applications. In this project, the team at the University of Texas at Austin proposes to use integrated photonics technology to build the FTIR on a chip. The weight of the FTIR can be amazingly reduced to a few grams and the size to less than 1 cm2. Moving parts are no longer needed. With these revolutionary improvements, FTIR can be used in many unprecedented areas such as toxic gas detection in battle fields, greenhouse gas monitoring on airborne platform, and standalone environment monitoring. Integrated photonics has been experiencing explosive growth in the past few years. While many components and systems have been demonstrated with impressive performance, the development of on-chip spectroscopy is slow due to the limited absorption length and strength on a chip, and the lack of high resolution, wide wavelength range spectrometers. To address these issues, the proposed on-chip FTIR involves two major innovations. First, subwavelength grating metamaterial waveguide is used as an absorption enhancement medium for the first time. It solves the dilemma between guiding (which requires the optical field to be constrained inside the high index dielectric region) and absorption (which prefers that the optical field propagates outside of the waveguide). The absorption of light by the analyte can be enhanced over 400 times compared to a conventional strip waveguide. Second, on-chip FTIRs formed by an array of asymmetric MZIs with increasing path differences between the two arms suffers from the extremely limited wavelength bandwidth. In this project, a thermo-optic phase shifter is added to each MZI. The combination of thermal phase shifters and incremental waveguide length differences makes it possible to achieve high resolution and large spectral range simultaneously, which has never been demonstrated before. As a proof-of-concept demonstration, this project will design, fabricate and experimentally demonstrate a subwavelength grating metamaterial waveguide enhanced, on-chip FTIR centered at 3.4 microns on the silicon-on-sapphire platform for Methane detection. The concept can also be readily extended to cover other wavelength ranges.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.