Development of Low-Cost and Portable Semiconductor Laser Based Evanescent-Wave THz Sensors

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Many molecules have characteristic spectral fingerprints at terahertz frequencies, which makes this frequency range unique for applications in spectroscopic sensing of chemical and biological species. However, progress in development of terahertz sensors is lacking due to the challenges associated with generation and detection of terahertz waves. Quantum-cascade lasers are the most powerful solid-state sources of terahertz radiation that have been developed in the past decade. However, they are required to be cooled much below room-temperature for operation, which makes it challenging to implement conventional optical sensing techniques with such lasers. This award will support development of a low-cost and compact terahertz sensor instrument based on arrays of chip-scale terahertz lasers for broadband spectroscopic sensing of solid or liquids. A unique "evanescent-wave" sensing scheme will allow measurement of terahertz properties of the analyte placed in ambient conditions while the laser chip is cooled inside a portable cryocooler for operation. A successful demonstration has the potential to spawn development of affordable commercial sensors at terahertz frequencies that would find applications in cell biology, process engineering, crystal engineering, pharmaceutical quality control, identification of counterfeit drugs, identification of explosives, and even drug discovery. Educational opportunities in the area of photonics and quantum-mechanics will be facilitated at various levels including that for elementary and middle-school students and engineering undergraduates. The project will also enable opportunities for interdisciplinary research across different academic departments of the university.This project will develop an intracavity evanescent-wave terahertz spectroscopic sensor with integrated arrays of terahertz quantum-cascade lasers on a semiconductor chip that would interact with an analyte in either liquid or solid-phase. A key characteristic of such lasers is that the evanescent-mode of the laser cavity can extend few millimeters above the appropriately designed cavities, which could therefore interact with analyte placed in ambient conditions while it is not in contact with the cooled laser chip. The resonant frequency of the laser interrogates the complex permittivity of the analyte. The fact that electronic non-linear mixing still works at terahertz frequencies, on-chip mixing will be utilized to efficiently down-convert the sensed terahertz signal to microwave frequency region. Hence, all that is needed to record the sensed data is a hand-held microwave spectrum analyzer. The need for expensive terahertz detectors as well as a terahertz spectrometer is completely eliminated, whereas the functionality attained can be similar to or better than that from many advanced spectroscopy techniques.