The Theoretical Study Of Polarization Property Of THz Wave Generated From Two-color Laser-induced Gas Plasma

These years the technology of the THz air photonics has developed and is maturing. The physical quantities to describe the THz wave pulse have got the very extensive research. However the study about THz polarization is scarce. Linear polarized THz wave can be obtained from a gas plasma channel induced by two-color femtosecond laser pulses. The azimuthal orientation of the THz polarization can be coherent controlled by changing the optical delay of the two-color femtosecond lasers. This phenomenon about the coherent controllable THz polarization orientation angle has drawn much interest of the science researchers.This dissertation presents two way to explain the phenomenon about the coherent controlled THz polarization orientation angle. One way is to analyze the problem in macroscopic view, and the other way is to construct a physical model in microscopic view. From the two views the unification analytical expression is obtained to explain the phenomenon about the coherent controllable THz polarization angle in the end.In macroscopic view, a new method has been developed to analyze the physical system of turning by the same axis, and the conservation property of the azimuthal orientation angles turning by the same axis of the system has been found. With this conservation property it has been sucsessfully analyzed that the phenomenon of the controllable THz polarization generated from two-color laser-induced gas plasma. And then it has been found that the reason about the coherent controlled THz polarization is neither the optical delay, nor the relative phase of the two-color lasers. The reason is the relative rotation angle of the two-color lasers in the optical direction. When the fundamental light and the second-harmonic light are both right circularly polarized, the relative rotation angle of the two-color lasers in the optical direction is equivalent to the optical delay and then be equivalent to the relative phase, because these three physical quantities at this time are proportional to each other. However, when the fundamental light and the second-harmonic light are both linear polarized, the relative rotation angle is equivalent to the relative polarization angle of the two-color lasers, because the relative rotation angle has nothing to do with the optical delay or the relative phase. In microscopic view, based on the quantum optics, a seven-level quantum model has been constructed to state the microscopic mechanism of the phenomenon of the controllable THz polarization generated from two-color laser-induced gas plasma. This seven-level system is constructed of two four-level systems, while the two four-level systems share the same ground state. One four-level system is to generate the right-circularly polarized THz photon, and the other is to generate the left-circularly polarized THz photon. At last the polarization of the THz photon is the superposition of the two processes. We ues the average of the creation operator for photon to reflect the quantum amplitude of the superposition of the quantum state. In the end the unification analytical expression with the macroscopic view is obtained.Then the conservation property of the azimuthal orientation angles turning by the same axis of the system has been used to analyze the property of the vector superposition. In another way, the conservation property has been proved to be correct in this aspect.At last the conservation property has been used to analyze the generation of elliptically polarized THz wave with double helix electrodes. Based on the conservation property, guidance programmes for the experiment has been provided.