The significant scientific and technological potential of terahertz (THz) wave sensing and imaging has been attracted
considerable attention within many fields of research. However, the development of remote, broadband THz wave
sensing technology is lagging behind the compelling needs that exist in the areas of astronomy, global environmental
monitoring, and homeland security. This is due to the challenge posed by high absorption of ambient moisture in the
THz range. Although various time-domain THz detection techniques have recently been demonstrated, the requirement
for an on-site bias or forward collection of the optical signal inevitably prohibits their applications for remote sensing.
The objective of this paper is to report updated THz air-plasma technology to meet this great challenge of remote
sensing. A focused optical pulse (mJ pulse energy and femtosecond pulse duration) in gas creates a plasma, which can
serve to generate intense, broadband, and directional THz waves in the far field.
We apply THz imaging technology to evaluate fire damage to a variety of carbon fiber composite samples.
The majority of carbon fiber materials have polarization-dependent reflectivities in the THz frequency
range, and we show how the polarization dependence changes versus the burn damage level. Additionally,
time domain information acquired through a THz time-domain spectroscopy (TDS) system provides further
information with which to characterize the damage. The technology is discussed in terms of nondestructive
testing applications to the defense and aerospace industries.
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.