Study Of Strong Terahertz Radiation Based On Ultra Short And Ultra Intense Laser Interacting With Matter And Its Applications

Powerful terahertz(THz)radiation sources are crucial to the development of THz science.High-energy strong-field THz pulses have many significant applications such as the ultrafast control of matter and the THz-driven electron acceleration.In recent years,ultraintense laser-plasma interactions have been proposed as a novel approach to strong-field THz generation.Comparing to strong-field THz source based on electron acceleration and nonlinear crystals,transition radiation(TR)based on laser-solid interaction has unique advantages.When a relativistic laser irradiates onto the surface of a solid target,massive hot electrons will be generated via electron heating mechanisms such as×and vacuum heating.When the electrons cross the interface between rear surface of target and vacuum,transition radiation is generated due to the change of dielectric coefficients.In this thesis,broadband and energetic THz pulse is generated by laser-solid interaction and characterized.Preliminary applications are performed on the transition radiation THz source.In the second chapter,experimental results are presented on the generation of THz radiation from a solid foil irradiated by a 10-TW femtosecond laser pulse.The THz energy as a function of the laser energy and defocusing is studied.It is found that both the THz energy and the laser-to-THz conversion efficiency increase nonlinearly with the laser energy.At the maximum laser energy?270 m J,the measured THz pulse energy is 458?J,corresponding to a laser-to-THz energy conversion efficiency of 0.17%.No sign of saturation is observed in the experiment,implying that stronger THz radiation could be achieved with higher laser energy.By simultaneously monitoring the backward scattered laser light spectrum,it is qualitatively understood that the observed THz radiation as a function of the laser energy and laser defocus distance is closely relevant to the electron heating mechanisms at different laser intensities.The THz spectrum and polarization are characterized with different band-pass filers and a wire-grid polarizer,respectively.The THz radiation covers a ultrabroad band ranging from0.2 THz to 30 THz,and shows a radially polarized distribution.By fitting the measured THz spectrum with the theory of coherent transition radiation,the THz pulse duration is inferred to be?30 fs.In the THz focal spot of?1 mm size,the THz field strength is evaluated to be 3.68 GV/m.Such a strong-field THz source will enable the study of extreme THz-matter interactions.Besides,THz generation from different target structures is investigated.Experiment results show that multiple layer metal-plastic targets can yield THz radiation more efficiently than single layer metal target.Strong-filed THz radiation is widely used in areas such as matter control,nonlinear phononics and ultrafast manipulation of electron.In the third chapter,THz-matter interaction is studied via picosecond laser interacting with solid target.In the experiment,THz radiation generated by the first laser beam is focused onto4)₂crystal.Then,X-ray generated by another beam incidents on the sample to probe the ultrafast response to THz electric field.Besides,repetition solid target system is investigated in order to achieve practical TR source.A preliminary copper tape target system is designed and will be built in the next step,which will be utilized for further THz applications.