Spectrometers are widely used tools in chemical and biological sensing, material analysis, and light source characterization. Mid-infrared (mid-IR) spectrometers are of particular importance to sensing and imaging applications, since many biochemical molecules have spectral "fingerprints" in the mid-IR frequency. However, the large size, weight, and cost of existing mid-IR spectrometers has limited their adoption despite the broad range of potential applications. In this work, the investigators will leverage advances in integrated photonics to develop chip-scale mid-IR spectrometers which are low-cost, compact, and lightweight. The chip-scale mid-IR spectrometers will utilize a new design paradigm based on "complex photonic structures" to achieve the same performance as the existing tabletop mid-IR spectrometers. The chip-scale mid-IR spectrometers will have wide-spread applications from trace-gas sensing for pollution control and environmental monitoring to combustion analysis and medical diagnostics. The investigators will integrate the research with education including training graduate and undergraduate students, curriculum development and outreach activities.
The development of a high-resolution chip-scale spectrometer could enable portable, low-cost spectroscopy for field sensing as well as enhancing lab-on-a-chip functionality. Among the different types of on-chip spectrometers that have been developed, none of them is applicable to the mid-infrared (IR) frequency because of the long wavelength and large detector noise. Current IR spectrometers rely on the multiplex advantage (the so-called Fellgett's advantage) to achieve an acceptable signal-to-noise ratio. These spectrometers operate by scanning a mirror over a long distance, which cannot be easily adopted on-chip. The primary goal of this proposal is to develop novel on-chip spectrometers capable of utilizing the Fellgett's advantage to achieve the level of sensitivity required for mid-IR sensing applications, while maintaining small footprint, high-resolution, broad-bandwidth, and low-loss. The investigators will leverage the unique characteristic of complex photonic structures, which provide broadband, non-resonant enhancement of optical pathlengths in a limited footprint to develop chip-scale spectrometers. They propose three spectrometer designs based on (i) a disordered structure, (ii) a chaotic cavity, (iii) a spiral waveguide, which provide different spectral resolution, footprint and sensitivity, and are therefore optimized for different applications. Each spectrometer consists of a complex photonic structure with a single input waveguide and a single output waveguide. The transmitted light intensity is recorded as the refractive index of the complex structure is modulated (e.g., via the thermo-optic or electro-optic effect). The input spectrum is then recovered with the calibration data describing the system response to different wavelengths and refractive indices. This approach can achieve the Fellgett?s advantage in a chip-scale spectrometer without moving components while maintaining a small footprint. The complex systems such as disordered structures and chaotic cavities can have extremely sensitive response to changes in the probe wavelength and refractive index modulation, thus enhancing the spectral resolution and operation bandwidth. Finally, the insights gained through the proposed work into novel photonic structures with broadband pathlength enhancement could enable additional applications in on-chip sensing and imaging.
