| Popis: |
Plasma is considered as a perspective nonlinear medium for generating narrow band terahertz (THz) pulses of high power because of its ability to maintain long-lived oscillations with extremely high electric field amplitudes. Since emission occurs at harmonics of the plasma frequency ω p ~n 1/2 , varying a value of plasma density allows one to easily change the radiation frequency in a wide range, thereby covering most of the THz band. An effective way to excite intense plasma oscillations is the injection of a short laser pulse into a plasma. By pushing plasma electrons away, a laser pulse forms a wake wave behind. Generated by such a mechanism in homogenies plasmas are predominantly longitudinal plasma waves which cannot be further converted into vacuum electromagnetic emission. One of the ways to transform laser energy to THz emission energy is nonlinear interaction of counter propagating laser wakefields. These wakes should have differing profiles of electrostatic potential and produce intense radiation at the doubled plasma frequency 1 , 2 . In the case of Gaussian laser beams, this condition as well as the existence of optimal beam size limit the ability to use the most powerful laser systems with high energy contents. In this work, we study whether the efficiency of the scheme can be kept high if focal intensities of thick laser beams with high absolute energies are modulated in transverse directions with the period close to the optimal size of thin Gaussian beams. Theoretical and particle-in-cell simulation studies on THz generation via the collision of thick laser beams focused by axicons will be reported. |
