Theoretical Study On The Process Of Terahertz Emission From Femtosecond Laser-Induced Gas Plasma

The method of generating terahertz (THz) waves from air plasma has many advantages, such as broader bandwidth, simple structure and so on. This paper uses photocurrent model to simulate this process and study the dependence of ultimate THz output on the corresponding parameters. Note that, the gas under discussing is air. Besides, this paper builds a physical model for the process of detecting THz waves from acoustic enhancement (TEA). The details of my work are listed at the following:(1) Making some revisions to the photocurrent model by taking into account the second order ionization and the scattering between the electrons and the around ions, and a new set of theories is deduced.(2) The THz emission enhancement when adding some 400nm laser intensity to the former 800nm is studied, and also the dependence of THz emission on this relative phase between 800nm and 400nm laser pulses. And then the THz emission when pumping with laser of different wavelengths is discussed.(3) Keeping the pump intensity invariant, but varying its bandwidth, the ionization processes of the air will be different, and the accelerating processes of ionized electrons in the laser field are also different, thus the corresponding THz emissions are different. Using photocurrent model, we study the dependence of the THz emission on the pulse widths.(4) Using photocurrent model, we study the dependence of THz emission on the total pump intensity, to know the potential of enlarging the pump intensity in obtaining larger THz emission.(5) In the process of generating THz waves from air plasma irradiated by two-color laser pulses, the introducing of 400nm can greatly enlarge the THz output. Using photocurrent model, we study the dependence of THz output on the 400nm intensity. By keeping the total intensity invariant, varying the intensity distribution between 800nm and 400nm, we study the corresponding THz emission magnitude.(6) The THz emission when pumping with few-cycle laser pulses of 800nm is discussed. At this case, the electric field is asymmetric, and leading to the enhancement of the ultimate THz emission.(7) In the process of laser induced air plasma, there is some acoustic emission accompanying. If using a THz wave irradiating the plasma, the ultimate acoustic emission can be enhanced. Thus, by detecting the enhancement the THz magnitude can be known. This paper will build a physical model to interpret this detecting process, and can also obtain some useful conclusions.