| KDP and DKDP is a kind of superior nonlinear optical crystal,which has been attracted more attention because of its superior piezoelectric property,electro-optic performance and frequency conversion performance,and has been applied as sonar device,frequency conversion device,electro-optic conversion device and piezoelectric transducer.With the development of inertial confinement fusion(ICF),the requirement has become more advanced,especially for the crystals as frequency conversion devices.So far,KDP and DKDP are the sole crystals to apply in ICF system due to its larger scale.In addition,under high-power laser irradiation,lower influence of Stimulated Raman Scattering(SRS)could be induced for DKDP crystal as third-harmonic conversion device.Even so,crystal damage also decreases frequency conversion efficiency and service cycle,and becomes a key factor in the aspect of limiting ICF application.Crystal damage for KDP and DKDP crystals is a relative complex process,and the mechanism of its generation is always the focus of most concern.Moreover,nonlinear absorption and "self-focusing" effect are two critical factors influencing crystal applications.In particular,nonlinear absorption effect may be induced in high power laser system.On the one hand,the existence of nonlinear absorption will result in the reduction of crystal conversion efficiency.On the other hand,lattice stability may decrease with the increase of absorption,and even cause laser-induced damage.As the frequency conversion device,self-phase modulation(SPM)and cross-phase Modulation(XPM)may be induced for KDP and DKDP crystals in high-power laser field.More importantly,phase-mismatch,pulse broadening and lower frequency conversion efficiency may be produced.With the increase of laser irradiation,the generation of "self-focusing effect" results in the enlargement of laser power near the focus,and even causes crystal damage.Moreover,SPM,XPM and self-focusing effect are closely associated with nonlinear refraction effect.At present,the research on nonlinear absorption and nonlinear refraction of KDP and DKDP are relatively few,so better understanding the characteristics of nonlinear absorption and nonlinear refraction is necessary,and should be to the benefit of KDP and DKDP applications.Z-scan technique is a simple and incredibly effective way to measure nonlinear absorption and refraction of materials.In addition,the types of nonlinear absorption,the sign of nonlinear refraction,nonlinear absorption coefficient and nonlinear refractive index could be calculated through this method.In this paper,nonlinear absorption and nonlinear refraction of KDP and DKDP crystals were explored by picosecond Z-scan technique.From the points of wavelength,laser power density,crystal orientation,the variation of deuterium content and crystal quality,the characteristics of nonlinear absorption and nonlinear refraction were studied in detail and provided a vital reference for the applications of KDP and DKDP crystals.The main contents of this paper are as follows:1.KDP,Fe3+ doped KDP and DKDP(12%,70%and 80%-deuterium content)were grown by conventional and rapid growth method.In addition,Ca2+ and Naa ions doped KDP crystals were also grown by this aqueous solution method.The results showed that:Through the conventional and rapid growth method,high-quality KDP and DKDP single crystals were obtained.For ions doped KDP crystals,high-quality single crystals with low-concentration dopant were grown,while crystal quality gradually decreased with the increase of ions concentration,especially the influence of Fe3+ ions.Compared to crystals grown by the conventional method,Fe3+ doped KDP crystals by rapid growth method was more easily to adsorb metal ions.Moreover,the impact of Fe3+ in prism area was larger than pyramid area.What' more,the mechanism of Z-scan and theoretical calculation of nonlinear absorption and nonlinear refraction were in detail elaborated.2.Using Z-scan technique,the characteristics of nonlinear absorption and nonlinear refraction were explored for KDP crystals grown by conventional and rapid growth method,especially the influence of crystal orientation,wavelength,laser power density and growth pattern.The results showed that:no obvious nonlinear absorption and nonlinear refraction of KDP crystals was discovered at ?=1064nm,while nonlinear absorption and nonlinear refraction effects were induced at ?,=532nm.At X,=532nm,lower nonlinear absorption and nonlinear refraction effects was discovered at the directions of phase mismatching including ? and ?-direction compared to z-direction.The sign of nonlinear refractive index for KDP crystal is positive,meaning the generation of "self-focusing" effect,and the values along z-direction was larger than I,II-type.The values of ? and n2 were about 10-11 cm/W?10-10cm/W and 10-13esu?10-12esu,respectively.This means that the distortion of the electron cloud played great role in nonlinear refraction effect,especially the distribution of H2PO4-.In addition,nonlinear absorption and nonlinear refraction effect increased with the increase of laser power density.The above results implied crystal orientation,wavelength and laser power density were the key factors in nonlinear absorption and nonlinear refraction of KDP crystal.3.Nonlinear absorption and nonlinear refraction were explored for 12%,70%,80%-DKDP crystals grown by conventional and rapid growth method using picosecond Z-scan technique.The laws of nonlinear absorption and nonlinear refraction effects were explored from the views of crystal orientation,wavelength,laser power density and growth pattern.The results showed that:At ?=1064nm,no obvious nonlinear absorption and nonlinear refraction for DKDP crystals was discovered,while nonlinear absorption and nonlinear refraction effects were induced at ?=532nm.Nonlinear absorption and nonlinear refraction effects along ? and ?-type were lower than z-direction,indicating that the phenomenon was correlation with crystal orientation and the distribution of H2PO4-groups.Nonlinear refractive index for DKDP crystal is positive meaning "self-focusing" effect,and the values of ? and n2 were about 10-11cm/W?10-10cm/W and 10-13esu?10-12esu,respectively.In addition,nonlinear absorption and nonlinear refraction effect increased with the increase of laser power density.In addition,the variation of deuterium content could effect on nonlinear absorption and nonlinear refraction of DKDP crystals,and growth methods including conventional and rapid growth have some difference in nonlinear absorption and refraction effects.4.The effect of crystal quality on nonlinear absorption and nonlinear refraction was explored for the Fe3+ and Na+-doped KDP crystals.Firstly,Fe3+ ions played an important role in nonlinear absorption and nonlinear refraction effect,especially II-type.Nonlinear absorption and nonlinear refraction effect increase with the increase of Fe3+content,and the effect on prism area was larger than pyramid area.This implied that crystal quality played a decisive role in nonlinear absorption and nonlinear refraction.In addition,nonlinear absorption and nonlinear refraction of KDP crystals were associated with the content of Na+ ions.With the increase of Na+ ions,nonlinear absorption and nonlinear refraction along ? or z-direction take on a increasing trend. |
PDF Full Text Request