Phase Matching Effect On The Second Harmonic And Terahertz Generations In β-Barium Borate

Nonlinear optics is of fundamental importance for optical frequency conversion and nonlinear optical spectroscopy.Second harmonic(SH)and terahertz(THz)generations are secondorder nonlinear processes.The physical mechanisms of the SH and THz generations are attributed to the same second-order nonlinear susceptibility tensor.There are a variety of optical methods for generating sufficiently intense and tunable THz sources.For instance,THz can be obtained from laser-generated air plasmas,which can radiate THz pulse with a spectrum from around 0.2 to more than 30 THz.Optical rectification in the nonlinear crystal is a widely applied method to generate THz for spectroscopy since it requires less complex optical setups.The crystal emits THz pulse pumped with an intensity ultrashort laser pulse.The THz spectrum of pulses generated by optical rectification depends on the laser pulse duration,material properties,crystal thickness,and the phase matching between the propagation speed of the laser pulse and the THz pulse inside the crystal.Generally,thinner crystals can generate THz bearing a broader spectrum because smaller thickness satisfies the phase matching between pump laser and THz in the wider spectrum.Barium borate(BBO)crystal has tremendously impacted on harmonic generation,frequency doubling,tunable optical parametric oscillators,and optical parametric amplifiers since its discovery in 1984.BBO crystals possess the merits such as high transparency,high nonlinear susceptibility parameters,and high damage threshold.It was demonstrated that BBO has huge birefringence in the 0.1-1.1 THz frequency region.Type-Ⅰ phase matching is widely employed to generate SH with β-BBO crystal.Type-Ⅰ phase matching for SH generation in β-BBO crystal requires the pump laser’s polarization in the crystal’s slow axis(ordinary wave)and SH’s polarization in the crystal’s fast axis(extraordinary wave).Different crystal geometrical cutting is necessary for different pump wavelengths to fulfill type-Ⅰ phase matching.This thesis investigates the phase matching effect on the SH and THz generation in type-Ⅰ β-BBO.The crystal applied in the experimental processes the geometrical cutting to satisfy the type-Ⅰ phase matching for efficient SH generation with pump wavelength of 800 nm.This type-Ⅰ β-BBO crystal can simultaneously generate SH and THz when pumped with optical pulse whose central wavelength is 800 nm and pulse duration is 50 fs.The SH and THz generations are attributed to the crystal’s same second-order nonlinear susceptibility tensor.However,the individual mechanisms underlying the SH and THz generations are different.The SH generation is based on frequency doubling(sum-frequency generation)and the THz generation is based on optical rectification(difference-frequency generation).The first chapter of the thesis introduces basic concepts in nonlinear optics.The geometrical cutting fundamentally affects the phasing matching through the cutting-dependent refractive index and effective susceptibility tensor.Different crystal geometrical cuttings are required to satisfy the phase matchings for different kinds of nonlinear processes such as frequency doubling,optical rectification,sum-frequency generation and difference-frequency generation.Furthermore,even for the same nonlinear process,different crystal geometrical cutting is in need for different wavelengths.Therefore,the second chapter first deduces the effective susceptibility tensor with arbitrary crystal’s geometrical cutting and then calculates the phase matching for SH and THz generations in type-Ⅰ β-BBO.The third chapter describes the experimental setup employed in the experiment.The fourth chapter experimentally investigates the effect of phase matching on SH and THz generations in type-Ⅰ β-BBO with the aid of theoretical results obtained in the second chapter.The fourth chapter shows how the THz peak electric field and THz power varies with the pump laser’s polarization.Then,the fourth chapter compares the polar plots of SH and THz powers versus pump laser’s polarization angle.The comparison shows that the polar plot of SH power versus pump laser’s polarization angle processes two lobes while the THz polar plot exhibits four lobes.Finally,the fifth chapter shows the conclusion and outlook.