A terahertz spectrometer on a chip, at the thermodynamical limits

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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.