Terahertz Modulations Induced By Femtosecond Filaments Interaction In Gaseous And Liquid Media And Its Applications

Terahertz（THz）radiation has attracted intense interests from academic institutes and industrial communities,owing to its intrinsic properties including specific spectrum,low photon energy,strong penetration,ultrashort pulses,high resolution and ultrabroad bandwidth.Moreover,it has ever-growing applications in fundamental science,biomedicine,nondestructive testing,spectroscopy,wireless communication and space exploration.Terahertz science and technology,as a newly emerging field,are still facing many challenges and need to be improved in the study of THz spectra modulation,THz radiation enhancement in new ways and THz nonlinearities of traditional materials.In this dissertation,THz modulations induced by femtosecond laser filaments interaction in gaseous and liquid media were investigated and THz nonlinearities in phase change materials and two-dimension materials were studied as well.The details of this dissertation can be summarized as follows:1,An all-optical approach was proposed to modulate the spectra of THz radiation from two color filamentation with the assistance of the plasma grating in gaseous media.Noncollinearly interacting two color filaments with another single color filament,free electron densities would be spatially modulated in the intersection region,leading to the existence of plasma grating.Affected by plasma photo current and four wave mixing effect,the high frequency in the THz radiation（>0.75 THz）would be effectively increased.In the experiment,the dependences of THz modulation on the parameters of single pulse energy and relative polarization angle were investigated.It was found that filaments with high pulse energy and parallel polarization would help to enhance the high frequency component of the THz spectra.The high frequency above 0.75 THz covers most of the crystalline phonon vibrations,torsional deformations and intermolecular bonding.This approach will extend THz time domain spectroscopy and increase the detection sensitivity,providing new techniques for qualitative and quantitative analysis of crystalline and amorphous materials.2,We proposed and experimentally verified that THz generation from femtosecond laser pulse induced filamentation in liquid was dependent on 533 nm frequency in the supercontinuum.And the THz radiation amplitude could be efficiently enhanced by plasma grating formed by two noncollinear interacting femtosecond filaments.The dependences of THz amplitude on pump pulse energy and relative polarization angle were investigated.It is found that interacting filaments with energy higher than initial pulse and parallel polarization would result in highest amplitude enhancement.The enhancement was interpreted by increasing short wavelength in the supercontinuum induced by plasma grating in liquid.THz radiation from femtosecond filamentation in liquid has high conversion efficiency up to 10^-3 and could be enhanced by plasma grating.This approach would meet the urgent need in material science application where resonant excitation at 5-20 THz with moderate to high strength is needed,enabling novel avenues of control in complex material systems by targeting low energy collective phenomena directly.3,Nonlinear absorption coefficients of phase change material Ge₂Sb₂Te₅ and two-dimension material graphene were measured via Z-scan technique.And increased nonlinear transmittance was found in Ge₂Sb₂Te₅ and it could apply to THz sources with broad bandwidth and moderate to high THz power.A new approach was proposed to measure the THz pulse duration exploiting the saturable absorption of the graphene.The study of THz interaction with conventional material helps to interpret the nonlinearity of material at THz frequency and enable new applications.Ge₂Sb₂Te₅could be employed in THz coating technology and have compatibility with conventional optical anti-reflective films.As a saturable absorber,graphene could be used in the development of passive mode-locked THz semiconductor and pulse measurement techniques.