Generation and Characterization of Strong Terahertz Fields From kHz Laser Filamentation

We report the generation of intense broadband THz fields (>16 MV/cm) at a 1-kHz repetition rate via two-color femtosecond laser filamentation in air. We find that the output THz energy can be enhanced by optimizing the plasma filament length via weak focusing. More importantly, the radiated THz beam can be refocused to near its diffraction limit even in long-length filamentation. We also examine the divergence angle of THz radiation emitted from short- and long-length filamentation. A real-time lock-in imaging technique is also applied to characterize frequency-dependent THz radiation profiles by using a microbolometer along with THz bandpass filters.