Production and Characterization of High Repetition Rate Terahertz Radiation in Femtosecond-Laser-Induced Air Plasma

摘要

The purpose of this research was to produce and characterize high repetition rate terahertz radiation in ionized air plasma. An 800 nanometer, 50 femtosecond, 0.35 Watt, 40 KHz, pulsed Ti:Sapphire laser system was used as the source infrared beam. This beam was focused onto a second harmonic generation crystal to produce a collinear, perpendicularly polarized secondary beam at 400 nm. After realigning the polarization of the fundamental to the second harmonic and compensating for group velocity dispersion introduced by the optics, both beams were recombined and focused by a 3.75 cm focal length mirror to form an air plasma. An electrical bias of up to 27 kV/cm was applied across the plasma to enhance the terahertz and plasma signal. The air plasma intensity was measured using a 40 kHz ultrasonic transducer, while the terahertz radiation was measured by a silicon bolometer. Terahertz of reasonable power was detected and characterized using this method. Both the terahertz and the plasma were characterized concurrently throughout this experiment in an effort to determine the feasibility of using the ultrasonic transducer as an alignment aid. The characterization included the effects of polarization, bias, and delay on the plasma and terahertz signals. During the course of this research it was also discovered that silicon, often used as a filter to isolate the detector from the laser in terahertz research, was a significant source of terahertz as well. This experiment represents the first time THz was generated using the two color interaction modified by an external applied bias.