Spectroscopy with electronic terahertz techniques

We report gas absorption spectra and energetic material reflection spectra measured with an all-electronic terahertz (THz) spectrometer. This instrument uses phase-locked microwave sources to drive picosecond GaAs nonlinear transmission lines, enabling measurement of both broadband 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 similar to 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.