We report the sensing of explosive materials by using terahertz (THz-TDS) time domain spectroscopy at standoff distance. The 0.82 THz absorption peak of RDX is observed at a distance up to 30 m away from the emitter and receiver. This result supports the application of THz-TDS technique in remote sensing and detection of explosives.
A simple, compact CW sub-THz imaging system, utilizing a 0.2 and 0.6 THz Gunn diode source is presented. A silicon beam lead diode detector and a Golay cell are used for the detection. Various results are presented, which show that the CW THz imaging modality is suitable for diverse applications, such as non-destructive testing and security. The key components of the system include the Gunn diode assembly, an optical chopper, a polyethylene lens, a detector, a lock-in amplifier, and two translation stages. The beam from the Gunn diode is focused on the sample being imaged by the polyethylene lens, the transmitted or reflected beam is measured by the detector. The energy transmitted through the sample at each point in the plane of the sample is detected. Since the system has relatively few components compared to pulsed THz imaging systems, it is less expensive and easier to design and operate, although it does not provide depth or spectral information about the sample. Since no time-delay scans take place, scanning can be done quickly compared to a time-domain system, limited by the maximum velocity of the translation stages and response of the detectors. It provides information about the macroscopic features of hidden structures within materials that are transparent to sub THz radiation, such as space shuttle insulating foam, articles of clothing, and luggage.
Pulsed THz imaging is a promising non-destructive technology based on its high transmission through selected dielectric materials and its capability to provide time-of-flight and spectral information. The traditional method of the pulsed THz imaging is a point-to-point reflective scanning system. The image is acquired by analyzing the peak amplitude information of the THz pulse in the time-domain at each pixel. It requires the THz beam or sample scanned. In this paper, we present our approach of large scale, focal plane THz wave imaging. In our 2-D focal plane THz wave imaging, the THz beam is expanded to be 60 mm in diameter. The THz beam illuminates the target in a reflective manner, in which a polyethylene lens projects the image onto a 40 mm by 40 mm by 2 mm ZnTe sensor crystal. The probe beam is expanded to be 40 mm in diameter and overlap with the THz beam on the sensor. The modulated probe beam profile carrying the image information is captured by a CCD camera. This technique enables us to view the objects which are optically opaque but transparent in THz frequency and shows feasibility in remote sensing, security inspection, and military defense applications.
Pulsed THz imaging systems have a number of potential advantages
in inspection applications. They provide amplitude and phase
information across a broad spectral range in the far-infrared, and
many common packaging materials are relatively transparent in this
frequency range. We use T-ray imaging to allow the identification
of different powdered materials concealed inside envelopes. Using
the terahertz spectral information we show that different powders
may be uniquely identified.
Different thicknesses of the powders are imaged to investigate the
influence of scattering on the measured THz pulses and the
classification model is extended to allow it to identify different
materials independent of the material thickness.