Terahertz Radiation Generated by an Asymmetric Current in a Laser-Produced Plasma in a Gaseous Medium

Abstract

When atoms are exposed to an intense laser field, they can be tunnel-ionized. The liberated electrons are accelerated in the laser field and gain a transverse velocity. If the liberated electrons are driven asymmetrically, the net current would not be canceled out, and the THz radiation can be emitted. The THz radiation has been extensively used in various applications, including pump-probe experiments, medical image processing, non-destructive testing, security, ultrafast technologies, and nonlinear THz spectroscopy. Thus, it is important to understand the generation mechanism and the optimal conditions for the generation of intense THz radiation.

One way to induce the asymmetric current is to use an ultrashort laser pulse. We theoretically studied the generation of THz radiation using ultrashort laser pulses. The dependence of the THz generation on the laser pulse duration is analyzed in single-cycle and few-cycle regimes. It is shown that the single-cycle or few-cycle laser pulses can be used to generate intense THz emissions by the asymmetric current. Also, the intensity of the THz radiation strongly depends on the carrier-envelope phase (CEP) of the driving laser pulse. It is shown that the intensity of the THz radiation is pronounced at the CEP of 90-degree, and it decreases with increasing the laser pulse duration. Therefore, the use of the ultrashort laser pulse with the right CEP is critical for efficient THz radiation.

The other approach to induce the asymmetric current is to use two-color laser fields. The two color laser fields were obtained by superimposing the fundamental laser field with its second harmonic. We analyzed the THz radiation generated using the two-color laser fields theoretically by using the semi-classical theory and by solving the time-dependent Schrodinger equation. These analyses provide detailed electron dynamics in the intense two-color laser field. We also performed THz generation experiments using the two-color laser fields in a gaseous medium. Our experimental results exhibit a strong dependence on the relative phase and the intensity of the two-color laser field, showing that the asymmetric current can be controlled using these laser parameters.

Our analysis provides a detailed description of the asymmetric plasma current in a gaseous medium, which is critical to understand the electron dynamics in the laser-produced plasma. In addition, we also found the optimal conditions for the generation of intense THz radiation. These findings will be highly useful in both the fundamental research and the THz applications.