Two-color laser wavelength effect on intense terahertz generation in air☆☆Project supported by the National Natural Science Foundation of China (Grant Nos. 51132004, 11604205, and 11474096), Science and Technology Commission of Shanghai Municipality, China (Grant No. 14JC1401500), Shanghai Municipal Education Commission, China (Grant No. ZZGCD15066), and Foundation of Fujian Educational Committee, China (Grant No. JAT160412).

Abstract

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser–plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model. We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.