text: THz radiation has several distinct advantages for use in spectroscopy. Many materials are transparent at terahertz wavelengths, and this radiation is safe for biological tissue being non-ionizing (as opposed to X-rays). Many interesting materials have unique spectral fingerprints in the terahertz range that may be used for identification. Demonstrated examples include several different types of explosives, dynamic fingerprinting of DNA and protein molecules using polarization varying anisotropic terahertz microspectroscopy, polymorphic forms of many compounds used as active pharmaceutical ingredients (API) in commercial medications as well as several illegal narcotic substances.
  Since many materials are transparent to THz radiation, underlying materials can be accessed through visually opaque intervening layers.
  Though not strictly a spectroscopic technique, the ultrashort width of THz radiation pulses allows for measurements (e.g., thickness, density, defect location) on difficult-to-probe materials like foam. These measurement capabilities share many similarities to those of pulsed ultrasonic systems as the depth of buried structures can be inferred through timing of their reflections of these short terahertz pulses.