The oscillation frequency of terahertz (THz) waves is between 0.1 THz and 10 THz. THz time-domain spectroscopy (THz-TDS) systems usually measure samples at the focus of THz beams. The Gouy phase shift has attracted widespread attention since it was discovered and has an important connection between the beam propagation dynamics and the dispersion of light waves. In this case, the Gouy phase shift affects the inversion dielectric spectrum of the sample. In order to more conveniently detect the phenomenon of Gouy phase shift, we propose a method of using a water column as a probe to detect the distribution of THz beams when they converge and directly observe the Gouy phase shift, owing to the sensitivity of this detection method to THz amplitude and phase. The Gouy phase shift in the THz band can be observed with simple processing by detecting the THz time domain signal under different water column position using THz-TDS. This method is of great significance for better selection of THz sources and coherence tomography of samples in the future. At the same time, it provides more help to control the Gouy phase shift to develop phase-shift interference technology and enhance the depth resolution of THz imaging.
This research uses a two-dimensional photocurrent model to propose and theoretically demonstrate an effective method for generating polarization-controllable terahertz wave from gaseous plasma according to experimental conditions. The simulation results show that in a combined field composed of a linear two-color laser field of 1600 nm+800 nm and a third circularly polarized laser pulse, the phase of the third pump laser is the key factor to control the polarization of the THz wave generated by the emitted air plasma. Theoretical simulation also shows that based on the two-color field, by selecting the appropriate available laser wavelength for the third pulse, the terahertz output can be effectively increased, and the terahertz conversion efficiency is significantly improved, which improves the production efficiency of the lasers. For such an incommensurate three-color femtosecond laser field, the intensity ratios of the fundamental beam and the third laser can also be used to control the polarization state and the direction of the terahertz radiation. Using current laser technology, our scheme can be implemented in experiments.
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