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

中红外光谱代表了一种材料的指纹，其吸收峰对应于组成该材料的原子键之间的振动频率，从而可以识别和定量各种材料。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。优选的红外光谱方法是傅里叶变换红外(FTIR)光谱。然而，带有移动组件的传统FTIR系统体积大、重量重，并且对环境波动(振动等)敏感。这些缺点使得它主要是一种仅限实验室使用的工具，需要广泛的人工参与，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提议使用集成的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，尺寸可以减少到不到1平方厘米。不再需要移动部件。凭借这些革命性的改进，FTIR可以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以及独立的环境监测。在过去的几年里，集成光子学经历了爆炸性的增长。虽然许多组件和系统已经展示了令人印象深刻的性能，但由于芯片上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，芯片上光谱学的发展缓慢。为了解决这些问题，建议的片上FTIR涉及两个主要创新。首先，首次将亚波长光栅超材料光波导作为吸收增强介质。它解决了引导(需要将光场限制在高折射率介电区域内)和吸收(倾向于光场传播到波导外部)之间的两难境地。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，由非对称MZI阵列形成的片上FTIR受到波长带宽极其有限的影响，两个臂之间的路径差异越来越大。在本项目中，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，这是以前从未展示过的。作为概念验证，本项目将在蓝宝石硅平台上设计、制造并实验演示一种以3.4微米为中心的亚波长光栅超材料光波导，用于甲烷检测。这一概念也可以很容易地扩展到其他波长范围。这一奖项反映了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。

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.