Terahertz Radiation Induced By Near Infrared Femtosecond Laser Pulse In Monoclinic Crystal BaGa₄Se₇

The electromagnetic?EM?responses of crystals come from lattice and electrons,and can be described by permittivity e and nonlinear susceptibility c,which are usually the functions of frequency and are spatially anisotropic determined by symmetry of crystal's structures.The characteristic energy of many lattice oscillations is about a few me V,corresponding to terahertz?THz?frequencies,which results in that the contribution weight of lattice oscillations to electromagnetic responses in THz frequency is much higher than that in visible frequency range.It is also the reason why the THz refractive index and absorption coefficient of many crystals are higher than optical values.Therefore,using THz spectroscopy to study lattice oscillation related EM wave propagation and nonlinear interaction processes with matter has some unique advantages.Compared to usually used cubic or uniaxial crystals,the second order permittivity tensor of monoclinic crystals is expressed as a complex symmetric matrix when absorption can not be ignored.There is no way to diagonalize the matrix by spatial rotation operation and the refraction ellipsoid can not be built and be used to describe the propagation of EM waves in monoclinic crystals.The nonlinear responses also become much more complicated.The situation greatly restricts the study on linear and nonlinear optical properties of monoclinic crystals and relevant optoelectronics applications.Especially in THz frequency range,the absorption of lattice always exists in general,which makes the study on the THz responses of monoclinic crystals very difficult.Monoclinic crystal BaGa₄Se₇?BGS?is a newly developed nonlinear optical material,which has excellent infrared nonlinear optical properties and abundant lattice oscillation modes in THz frequencies.Studying THz generation process from nonlinear excitation of monoclinic crystals will promote the development of novel terahertz source,elucidate the interaction of lattice oscillation and EM field,and provides technique for measurement and characterization of second order nonlinear susceptibility ??#?as well.In this dissertation,terahertz radiation from BaGa₄Se₇ crystal induced by near infrared laser is measured and analyzed to investigate the second-order nonlinear optical properties of monoclinic crystals.The main research contents and obtained results are shown as follows.?1?To investigate the influence of lattice vibration no linear and nonlinear optical properties of monoclinic crystals,polarized Raman scattering spectra are measured to analyze phonon modes of BaGa₄Se₇ crystal.The results show that the phonon modes near 2.6 THz and 6.0 THz carry dipoles indicated by obvious direction dispersion observed in spectral data.These dipoles can couple EM wave effectively to form phonon plaritons,which influence and even dominate the optical properties of crystals in frequency above 1.7 THz.?2?Eigen modes for EM wave propagating in monoclinic crystals are deduced from Maxwell equations.The two eigen modes are both elliptically polarized,and have the same chirality and ellipticity,but the principal axes of the two modes are orthogonal with each other.Complex dielectric tensor elements and complex refractive index are extracted from transmission experiment data obtained using a terahertz polarized measurement system.These parameters lay the foundation for further analyzing terahertz radiation from BGS under optical excitation.?3?The Electric field and polarization state of terahertz radiation from BGS are measured by a polarization sensitive terahertz time-domain measurement system.The second-order nonlinear susceptibility tensor involving lattice vibration is obtained.The expression for generated THz electric fields is deduced based on difference frequency generation mechanism,from which we propose a method to extract ??#?of BGS.The possible reasons why the generated THz signals hardly change with crystal thickness is also discussed.