We report the first systematic study of broadband THz wave generation by using the focused femtosecond laser beams in ambient air. Generations of pulsed THz waves using air as the nonlinear media have been previously demonstrated by Cook et al. and Hartmut et al. We measured dependence of generated THz wave on the polarization, amplitude and phase of the individually controlled fundamental and second harmonic beams. Our results confirms that four-wave-mixing rectification is the major mechanism of THz wave generation with mixing the fundamental and the SH beams in air. This work is significant by providing the feasibility of standoff distance detection greater than 50 meters.
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.