Research Of Nonlinear Optical Properties Of Vacuum-ultraviolet Fluoride Crystal-BaMgF₄

There is a great need for ultraviolet （UV）, especially vacuum ultraviolet（VUV） laser, owing to its promising applications in high-resolutionspectroscopy, micromachining, UV lithography, medical synthesis, and manyother fields. Compared with conventional excimer lasers, UV/VUVall-solid-state lasers through cascaded frequency conversion using UV/VUVnonlinear optical （NLO） crystals present great advantages for its simplicity,stability, narrow spectral bandwidth, better beam quality, and continuoustunability in the UV and VUV spectral regions. Thus, seeking for a kind ofsuitable and high-performance crystals is the focus of attention. Recently, theferroelectric crystal of BaMgF₄becomes a promising candidate for VUVfrequency conversion since it has a shorter cut-off wavelength （125nm） andcan use quasi-phase-matching （QPM） technique to realize the frequencyconversion in the VUV area. Therefor, the nonlinear optical properties ofBaMgF₄crystal is investigated in this thesis.Firstly, we have studied the second-order nonlinear optical properties of BaMgF₄. The second-order nonlinear coefficients are measured by Makerfringe method. The results ared₃₁=0.15pm/V,d₃₂=0.36pm/V andd₃₃=0.12pm/V. Besides, we calculate the angel tunable proprieties of secondharmonic generation （SHG） and sum frequency generation （SFG） in BaMgF₄crystal. The optimal phase-matching angle and effective nonlinear coefficientof BaMgF₄are obtained according to the energy and momentum conversionlaws in the type-I and type-II. Furthermore, we have carried out SHG andSFG in type-I for the fundamental wave of the Nd:YAG laser, using BaMgF₄crystal in experiment. The maximum conversion efficiency of SHG is1.4%and the very small harmonic with the wavelength of355nm is obtainedthrough SFG, which is the shortest harmonic in BaMgF₄crystal so far.Secondly, we have investigated the three-order nonlinear opticalproperties of BaMgF₄crystal. The nonlinear refraction index and nonlinearabsorption coefficient are measured in BaMgF₄single crystal using Z-scantechnique in ultraviolet region with a pulsed laser at400nm which is obtainedby SHG of BBO crystal using Ti: Sapphire laser. The experimental resultshow both nonlinear refraction and nonlinear absorption have large values,which indicate the potential applications of BaMgF₄crystal in VUV nonlinearprocess. In addition, The remarkable nonlinear absorption behavior isidentified to be three-photon absorption under the experimental conditions. Besides, the ultraviolet supercontinuum spectrum resulting fromself-phase modulation effect is observed by focusing400nm laser intoBaMgF₄crystal and it is correlated to the large third-order nonlinearcoefficients of BaMgF₄crystal.