Research On The Spatial Confinement Effect Of Terahertz Wave During Femtosecond Laser Filamentation

During the last two decades,terahertz(THz)science and technology has attracted intense attention from all over the world,for its promising applications in increasingly wide variety of fields,such as biomedical science,material science,homeland security,information and communications technology,earth and space science,etc.However,before entering the maturity period of THz science and technology,severe challenges still need to be overcome,including THz remote sensing,high-resolution THz imaging,THz nonlinear optics and THz remote enhancement.Firstly,as for THz remote sensing,femtosecond laser filamentation in air,being able to create a long plasma channel(filament)and generate THz wave at a remote location up to hundreds of meters away,has been suggested to suppress the THz loss during propagation in the atmospheric environment.However,the fundamental nature of the THz wave propagation during the filamentation is barely known.In this thesis,the superluminal propagation of THz wave during femtosecond laser filamentation has been observed,which indicates that,the THz pulse may propagate inside the plasma filament,rather than undergoing natural diffraction in air.This has significantly enriched the theories of THz wave generation and propagation during femtosecond laser filamentation.On the other hand,two models of the plasma filament in THz band,namely,Fabry-Perot-like(F-P-like)resonator and negative dielectric(ND)pin optical waveguide,have been built to shed light on the basic nature of the strong spatial confinement effect of THz wave during femtosecond laser filamentation.For the first time,THz mode field distribution inside the plasma filament has been analytically discovered.Secondly,as for THz imaging,due to the long THz wavelength,the obtained resolution of THz imaging is normally a few hundred microns(even in the scale of millimetre),which constitutes a major obstacle for the application of THz imaging since the interested samples are often much smaller than the THz wavelength.THz scanning near-field optical microscopy(THz-SNOM)has been suggested in order to optimize the THz imaging resolution.Nevertheless,THz-SNOM takes little use of the THz energy,thus sacrificing the signal to noise ratio(SNR).In this thesis,based on the aforementioned strong spatial confinement effect of THz wave during femtosecond laser filamentation,a novel THz imaging method with sub-wavelength resolution has been reported.By using this technique,THz imaging resolution beyond diffraction limit and better performance of SNR could be prospected,while retaining the exceptional high THz field strength achieved by the femtosecond laser pumping experiment.Thirdly,in field of THz nonlinear optics,in order to control matter and light,it requires that the electric field of THz at least exceeds 1 MV/cm.However,the reported strong THz electric field generation technologies are mostly carried out on state-of-art intense laser systems,or pumped with extremely high laser energy,which obviously cannot be widely adopted in ultrafast laser labs in the community.In this thesis,the THz electric field inside the filament has been calculated.Due to the existence of strong spatial confinement effect,it is found that the amplitude of THz pulse within the confinement region is much larger than that of the diffraction region,which makes femtosecond laser filament a potential new tool to achieve high THz wave amplitude with low pumping laser energy.At last,intense THz wave generation is key to the application of THz remote sensing.Femtosecond laser filamentation has been proposed as a THz remote source,since the plasma filament can be located at a remote distance with intense THz emission.Efforts have been made to enhance THz radiation from femtosecond laser filament,most of which rely on meticulous optical alignment in time and space.This will obviously become a major challenge for these methods to be implemented in out-lab atmosphere,especially when THz generation is aimed at a remote distance.In this thesis,based on the strong spatial confinement effect of THz wave during femtosecond laser filamentation,a simple method to enhance THz radiation from femtosecond laser filament array with a step phase plate has been presented.This scheme leads to significantly reduced complexity for the practical experimental setup of THz remote generation and further enhancement.