Advancements in photomixing and photoconductiveswitching for THz spectroscopy and imaging

Advancements in photomixing and photoconductiveswitching for THz spectroscopy and imaging

Author(s): Elliott R. Brown, Wright State Univ. (United States) and PhysicalDomains, LLC (United States)
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Ultrafast photoconductive devices have been an important breakthrough in THztechnology during the past two decades. Photoconductive switches have becomethe workhorse in moderate-resolution time-domain systems, and photomixers havebeen widely implemented in high-resolution spectrometers of various types. Theprimary photoconductive material has been low-temperature-grown GaAs. Morerecently, this has been rivaled by ErAs-GaAs: a nanocomposite consisting ofErAs nanoparticles embedded in a GaAs matrix. ErAs-GaAs photomixers haveproduced very useful THz output power levels between 1.0 and 10.0 microwattwhen pumped by low-cost DFB lasers operating around 780 nm. ErAs-GaAsphotoconductive switches have produced average output power approaching 1 mW,and peak power exceeding 1 W when pumped by frequency-doubled fibermodel-locked lasers. The photomixer performance has been utilized in the firstU.S. commercial THz photomixing spectrometer which has already beendemonstrated on a variety of interesting materials including polar vapors,solid explosives, polysaccharides, nucleic acids, and nonlinear-opticalcrystals. The photoconductive switch performance has been utilized in a THzimpulse radar having a broadband power spectrum centered at around 500 GHz, anda relatively simple gated-receiver to provide pulse averaging and noisereduction. The impulse radar has been used for several applications inbiomedical imaging, such as imaging of skin burns, skin cancer, and the ocularcornea.