Electromagnetic Radiation And Transmission Characteristics Of Plasma Filaments Produced By Femtosecond Laser

Scientists first discovered that the femtosecond laser can act on air to form long-distance plasma filaments at the end of the 20 th century.Domestic and foreign researchers have carried out extensive and in-depth research on femtosecond laser plasma filaments.And femtosecond laser plasma filaments have been widely used in various fields.Femtosecond laser plasma filament is conductive,so it can be regarded as a virtual transmission line.Compared with the traditional antenna,it has the characteristics of stealth,fast reconfiguration,relatively simplified design and undamped oscillation.Therefore,the research and application of femtosecond laser plasma filaments in electromagnetic wave transmission have become one of the hotspots of current research.First of all,the finite difference method is used to simulate the beam intensity and electron density distribution of femtosecond laser plasma filaments with different laser energy and wavelength.The results show that with higher intensity of laser energy,the electron density and the beam intensity are higher.In addition,we find that the electron density and beam intensity of femtosecond laser plasma filament at 800 nm wavelength are the highest.Based on the theory,we built the experimental device of femtosecond laser plasma filament radiating and transmitting electromagnetic wave signal,and geted the influences of laser energy,focusing lens focal length,laser wavelength,polarization state and different positions on the electromagnetic radiation signal produced by femtosecond laser plasma filament.The results show that the electromagnetic wave radiation in plasma filament produced by high laser energy is stronger,and the electromagnetic wave radiation in plasma channel produced by 800 nm laser is stronger than that produced by 400 nm femtosecond laser,which is consistent with the theoretical simulation results.In addition,compared with the circularly polarized light,when the laser is linearly polarized light,the efficiency of multi-photon ionization is higher.And the diameter of the induced femtosecond laser plasma filament is larger,the defocusing effect of free electrons is stronger in a large range,so the electromagnetic wave radiation in the plasma filament generated by linearly polarized light is stronger,and there is no refocusing.In order to increase the intensity and lifetime of filaments efficiently,we set an external voltage along with the filament.When the external voltage is constant,the square longitudinal electrode has the best effect on enhancing the intensity of electromagnetic radiation signal generated by femtosecond laser plasma filament.When the external voltage is 300 V,the intensity of electromagnetic radiation signal increases by 1075 mV compared with that without voltage using square longitudinal electrode.Besides that,we have built an experimental device to study the transmission characteristics of femtosecond laser plasma filament.When the transmission distance is 6.5 cm,the energy of electromagnetic wave transmitted through femtosecond laser plasma channel is 2.85 times higher than that transmitted freely in air.We also calculated the electron density of plasma filament by comparing the transmission of electromagnetic wave by copper wire.