Wideband Terahertz Sensing And Spectroscopy with Electronic Sources and Detectors

A growing variety of reflection and transmission spectroscopy in the 1- 1000 GHz regime can be done with wideband all-electronic terahertz (THz) spectrometers. We have measured gas absorption spectra and energetic material reflection spectra with such instruments using phase-locked microwave sources to drive picosecond GaAs nonlinear transmission lines, enabling measurement of both wideband spectra and single lines with Hertz- level precision, a new mode of operation not readily available with optoelectronic THz techniques. We take two approaches to coherent measurements: (1) spatially combining the freely propagating beams from two coherent picosecond pulse generators (which have discrete spectra ranging from ~ 6 to < 500 GHz), or (2) using a more conventional coherent source/detector arrangement with sampling detectors. The first method employs a dual-source interferometer (DSI) modulating each harmonic of one source with a precisely-offset harmonic from the other source—both sources being driven with stable phase-locked synthesizers—the resultant beat frequency can be low enough for detection by a standard composite bolometer. Room-temperature detection possibilities for the DSI include antenna-coupled Schottky diodes. Finally, we have recently introduced a reflectometer based on serrodyne modulation of a linearized delay line, using a technique that is process-compatible with pulse generator circuits. Keywords: wideband electronic terahertz techniques, gas spectroscopy, reflection spectroscopy, nonlinear transmission lines, samplers, coherent measurements, dual source interferometer

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