Phase-controlled, second-harmonic-optimized terahertz pulse generation in nitrogen by infrared two-color laser pulses

Broadband terahertz radiation can be efficiently produced by mixing laser pulses of different colors in the mid-infrared (MIR) and longwave-infrared (LWIR) spectral region. In this paper, we report on a numerical investigation of ultrashort terahertz pulse generation from plasmas created in nitrogen gas by two-color laser pulses with the fundamental laser pulse wavelength between 2.15 and 15.15 mu m, in order to explore the efficiency of the terahertz pulse generation process. The results show that the electron acceleration efficiency increases monotonically with the fundamental laser pulse wavelength. The most intense terahertz pulse generation is observed at 12.30 mu m with four optical-cycle laser pulses with 2.5 GW peak power. The results show that the terahertz pulse generation with a MIR laser is one order of magnitude and with a LWIR laser is two orders of magnitude more efficient than the terahertz pulse generation with Ti:Sapphire lasers using the exact same pulse parameters. The terahertz pulse generation efficiency is also known to be very sensitive to the relative phase between the components of the two-color laser pulses. One of the most useful tools to control the relative phase and optimize the terahertz pulse intensity is thin dielectric plates. It has been shown that alkaline halides and alkaline earth halides have suitable optical properties for the relative phase control for efficient terahertz pulse generation in the MIR spectral range. (C) 2020 Optical Society of America