Terahertz time-domain spectroscopy (THz-TDS) is a promising new technology which provides a relatively simple means of generating and detecting single-cycle pulses of far-infrared (or terahertz) radiation. One of the most interesting aspects of this system is its insensitivity to the thermal background. This obviates the need for cryogenic apparatus; as a result, this may be the first portable far-infrared spectrometer. Recent work has demonstrated the possibility of tomographic imaging using THz-TDS. In this imaging mode, a reflected pulse train is used to construct a three-dimensional representation of a composite material, using the timing between reflected pulses to determine the spacing between adjacent dielectric interfaces. Here, the transverse resolution is determined by the diffraction-limited focus of the THz beam, and is typically ～300 microns. The longitudinal (depth) resolution of ～100 microns is determined by the coherence length of the radiation, although the location of isolated surfaces can be determined with far higher precision. Since many common packaging materials have high transparency in the THz range, this suggests the possibility of exploiting this new imaging system for non-invasive testing and on-line monitoring. The operation of the THz "T-ray" imaging system will be described, and several examples will be provided which illustrate its capabilities and limitations.
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