The Study Of Terahertz Generation And Detection With Laser-Induced Gas Plasma

Terahertz(THz)waves have various unique physical and optical properties,such as transparent,low photon energies,special fingerprint spectrum and strong water absorption.These properties enable many potential scientific and commercial applications over various fields of physics,environment,medicine,astronomy,nondestructive testing,safety inspection,satellite communication,radar imaging and so on.Thanks to the advents of ultrashort laser technology and THz science and technology,many intense broadband THz generation and detection methods have been reported.Compared with traditional methods,the methods using laser induced gas-plasma are more efficient and advantageous.Focused on the THz air photonics,main contents of this thesis are the new methods and physical processes of THz generation and detection using laser induced gas-plasma.This thesis can be summarized into four parts:Firstly,we extend the transient photocurrent(PC)model to the case that a static magnetic field is imposed on the gas plasma.Based on two-color laser induced magnetized gas-plasma,we present a polarized THz pulse generation scheme.We theoretically investigate the mechanism that linear polarization translates into elliptical and circular polarization.Moreover,we propose an experimental realization.A laser-driven capacitor-coil target is used to generate a quasi-static magnetic field around gas plasma.The magnetic field strength can be tuned by the pump laser energy.And by changing magnetic field strength,the polarization of the THz radiation can be continuously tuned.This scheme offers an extremely significant way to obtain tunable polarized broadband THz radiation.Secondly,the mechanism of THz wave detection with one-color laser induced gas plasma is theoretically investigated.With a low probe energy,the waveform of detected signal is unipolar,losing the phase information of THz wave and the measurement is incoherent.In order to coherently detect THz waves with low probe energies,we present a new coherent detection scheme without a high voltage bias,avoiding its security hidden danger.Besides theoretical analysis,we have experimentally demonstrated the feasibility of this coherent detection scheme with a low probe energy.Thirdly,the mechanism of THz pulse coherent detection via two-color laser pulses of various frequency ratios in gas plasma is theoretically investigated and we extend the transient photocurrent model to this two-color scheme.Our investigations demonstrate that except for the commonly used frequency ratio of 2,other uncommon frequency ratios can also be utilized to detect THz pulse,such as 2n,n+1/2(n is a positive integer).Besides,the simulation results are analyzed from the point of electron ionization and electron acceleration in detail.We find that the frequency ratios of 2n and n+1/2 significantly intensify the asymmetry of both electron ionization and electron acceleration,making the modulating effects of THz wave more evident than that with other frequency ratios.This two-color detection method with uncommon frequency ratios is an alternative detection method in experimental researchesFourthly,we theoretically investigate the influence of carrier envelope phase(CEP)to the symmetry of few-cycle laser pulses and the THz detection with gas plasma.We find that there are two optimum CEPs for a few-cycle laser to perfectly reproduce a real THz waveform.Based on this result,we propose an THz coherent detection scheme using two few-cycle laser pulses with opposite CEPs,i.e.,their CEPs are differed by ?.This coherent detection scheme can both eliminate the influence of the CEP's instability and probe energy's fluctuation.Furthermore,by changing probe eneryies,this coherent detection scheme can be used to exactiy measure the CEP of few-cycle lasers.