Latest Development of Laser - driven Tunable Circularly Polarized Terahertz Radiation

The terahertz spectrum and related control have applications in many fields such as physics, biology and medicine. The frequency of terahertz radiation generated by gas and solid plasmas reaches 100 THz. Recent studies have shown that the formation of pump light and its second harmonic Will effectively generate terahertz radiation up to MV / cm intensity, the linearly polarized laser-driven radiation will change its polarization from linear to elliptical due to the modulation of the gas plasma. In order to obtain controlled polarization radiation, elliptical polarization, circular polarization and other laser pulses are used to generate the required terahertz radiation. In this paper, the static magnetic field is added to the transmission axis of two-color linearly polarized laser and the direction of the radiation can be controlled by the direction of the magnetic field. When the magnetic field strength is 100T, the frequency of the cyclotron oscillation is much higher than the plasma oscillation frequency, And by the magnetic field intensity adjustment. Under this condition, the magnetic field controls the plasma oscillation and affects the trajectories of the plasma electrons, which in turn produces such radiation characteristics. Since the cyclotron frequency is much greater than the plasma frequency and the radiation is a multi-period envelope, the radiation is narrowband. Shown in Figure 1 is the spatial distribution of terahertz radiation propagating in -x direction in vacuum, the magnetic field is 178T along the + x direction. In contrast, radiation without a magnetic field is indicated by a dotted line, where radiation is consistent with other research work and are single-cycle waveforms. With the addition of a magnetic field, the radiation has components in both y and z directions, with the two components having the same frequency and having phase differences. Shown in Figure 2 is the time waveform of the radiation, the visible radiation is 5THz, and the gyro oscillation frequency is equal, y and z components are different pi / 2 phase, when the magnetic field direction flip, the phase is changed to -pi / 2, as shown in Figure 2 shows. The main terahertz radiation process occurs as follows: First, plasma and electrostatic currents are generated by ionization, which further drives the electrostatic oscillating field in the plasma, which field is converted to electromagnetic radiation at the plasma boundary. In the absence of a static magnetic field, electrons have The velocity in the direction of the laser polarization, the z-direction, and the addition of the x-direction magnetic field, the electrons rotate in the yz plane so the radiation also has the y and z components. In the future, all-optical controlled magnetically controlled two-color field can generate THz radiation source.