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.
