Terahertz air photonics for standoff explosive detection

Terahertz wave generation and detection using air as both the terahertz wave emitter and sensor media through nonlinear optical processes similar to four-wave mixing (named terahertz air photonics) were introduced. Due to the unique features of this technique, it is possible to find applications for terahertz air photonics in remote sensing and identification of explosives by sending optical beams rather than terahertz beams far away and generating and detecting terahertz waves in the vicinity of the target. As one of the most important steps for the standoff explosive detection application, we demonstrated standoff intense terahertz wave generation by sending an 800-nm femtosecond laser pulse and its second harmonic at a distance of 17 meters, which is the current limit allowed by the laser pulse energy and the length of our laboratory. A phase compensator was specifically designed to compensate for the temporal walk-off between the 800-nm pulse and its second harmonic travelling in dispersive air.