Research On Nonlinear Properties And Laser Induced Damage Performance Of ADP Crystals At Short Wavelengths

KDP(KH2PO4/)/DKDP(K(DxHi-x)2PO4)crystal is a non-linear optical and electro-optic crystal material with superior performance,such as low half-wave voltage,large linear electro-optic coefficient,wide light transmission band and the ability to grow large-size single crystal,becoming the only nonlinear optical crystal material used in Inertial Confinement Fusion(ICF).However,the laser-induced damage of DKDP crystals seriously restricts the output of laser and the service life of components,which is the bottleneck restricting the development and application of inertial confinement nuclear fusion.Compared with DKDP crystals,ADP(NH4H2PO4)crystals have larger effective nonlinear coefficients,higher laser induced damage thresholds especially at short wavelengths and can be obtained large size ADP single crystals.Besides,Non-critical phase matching quadruple frequency can improve the laser output energy and reduce the phase mismatch caused by dispersion,being a potentially feasible technical route.Therefore,ADP crystals have great potential in short-wavelength applications in high power laser systems.In general,the Z-seed rapid growth of ADP crystals provides high-quality triple-frequency crystals.As the application direction of the non-critical phase matching quadruple frequency is(90°,45°),the Z-plate seed crystals may have insufficient size in the[100]or[001]direction.Therefore,directional growth ultilizing a quadruple-frequency seed can ensure the maximum growth of the crystal in the desired direction,thereby ensuring sufficient size.Therefore,the effects of seed crystal orientation and impurity ions on the growth mechanism of ADP crystals were studied systematically from different seed orientations.The multiphoton nonlinear absorption process is an important mechanism in laser-induced damage of DKDP/ADP crystals,mostly assisted by the gap energy state of the defects.A modulated electric field appears under laser irradiation,resulting in higher-order nonlinear effects such as nonlinear absorption and nonlinear refraction.Nonlinear absorption reduces the conversion efficiency and crystal stability,causing damage to the crystal;the crystal exhibits self-phase modulation(SPM),cross-phase modulation(XPM),and "self-focusing" effects under the strong laser,causing phase mismatch,pulse broadening and frequency conversion efficiency reduction,while the latter causes local light field enhancement,resulting in internal damage of the crystal.These three phenomena are closely related to the nonlinear refraction of the crystal.Therefore,investigating the nonlinear effects and damage of crystals under short-wavelength laser irradiation can provide an important reference for revealing the damage mechanism of DKDP/ADP crystals.The nonlinear properties of(D)ADP crystals under short-wavelength lasers were systematically investigated.The defect-induced damage behavior and mechanism of ADP and DKDP crystals irradiated by 355nm laser were compared and analyzed,which provided a basis for the practical application of ADP and DKDP crystals.The main contents of this paper are as follows:1.A series of small-scale Z-seed and(90°,45°)-seed growth ADP crystals were grown by point seed crystal rapid growth method.The difference in controlling the growth rate of Z-direction and oriented ADP crystals was found.The growth of ADP crystals in medium-scale Z-direction and directional seed under different conditions(such as raw materials,temperature interval and growth rate)were investigated.Growth experiments with large-diameter ADP crystals have shown that stricter and finer control of supersaturation is required.2.The effects of supersaturation on the microscopic growth morphology of(100)surface of Z-seed,oriented seed and doped Cr3+ oriented seed growth ADP crystal were studied by off-line AFM.The growth mechanism of the(100)plane of the ADP crystal with the Z-seed,oriented seed and doped Cr3+ oriented seed growth ADP crystals changes with the same trend as supersaturation decreases;the two-dimensional nuclear growth on the(100)surface of the ADP crystal is anisotropic and the growth rate in the[010]direction is greater than that of the[001]direction,?*and ?d indicate that the cylindrical growth rate of the oriented growth ADP crystal is smaller than the cylindrical growth rate of the Z-growth ADP crystal.There are void defects caused by "pinning" on the surface by Cr3+,which affects the growth and quality of the crystal.3.The effects of laser pulse width,wavelength,crystal orientation and thickness on the nonlinear properties of ADP crystals were studied by Z-scan method.The effects of different thicknesses on the nonlinear properties of KDP crystals at 355 nm wavelength were studied.The influence of D content on the nonlinearity of the DADP crystal is analyzed.The relationship between the laser damage threshold and the nonlinear effect of the ADP crystal is analyzed.It provides an important reference for revealing the damage process of the crystal and the practical application of the crystal.It was found the ADP/KDP/DKDP crystals exhibited reverse saturation absorption and self-focusing effects.When z0>L under picosecond laser irradiation,the nonlinear absorption coefficient ?,nonlinear refractive index n2 and third-order polarizability x1(3)of ADP crystal under 355 nm and 266 nm laser irradiation are anisotropic.The nonlinear critical optical power density I0*under 355 nm and 266 nm decreases with the photon energy increases from 3.49eV to 4.66eV,and the third-order nonlinear effect of the ADP crystal sharply increases.When z0>L under picosecond laser irradiation,the ? of DADP crystal increases with the increase of D content,because the optical band gap of DADP crystal decreases as D content increases,resulting in the two-photon absorption coefficient gradually increasing;When I0*?I0<I0**,the nonlinear absorption effect of the ADP crystal is in the measurable interval and the nonlinear absorption coefficient increases nonlinearly with the increase of the optical power density;As the D content increases,the self-focusing distance decreases.When z0<L under picosecond laser irradiation,the order of nonlinear parameters is:quadruple frequency<X<Z,and the LIDTs of thick samples order:Z<X<[110].Compared with the ordering of nonlinear parameters,it can be seen that the weaker ?,n2 and x1(3)correspond to the larger LIDT and both the nonlinearity and the LIDT are anisotropic.When z0>L under nanosecond laser irradiation,the third-order nonlinear absorption effect still exists in the ADP crystal.4.The Intrinsic damage characteristics and damage growth characteristics of DKDP crystals and ADP crystals under 355nm laser irradiation were investigated and compared.The influence of defects on the damage characteristics provides a reference for revealing the mechanism of damage induced by DKDP crystals and ADP crystal.The bulk damage density of DKDP crystals increases exponentially with the laser irradiation fluence.The damage density of DKDP crystals decreases with the same fluence after pretreatment,and the increase of laser conditioning energy can reduce the damage density under the same fluence.There are low,medium and high damage threshold defects in the low damage threshold DKDP samples,while only the medium and high damage threshold defects exist in other crystals.The damage points induced by low damage threshold defects in DKDP crystals are small,while those of medium damage threshold and high damage threshold are larger,mostly with the morphology of radiation-extended multi-directional cracks around the central cavity.The direction of cracks is consistent.Ultraviolet absorption spectroscopy shows that the defect induced by deep ultraviolet(less than 230nm)absorption is the key factor affecting the LIDT.With the increase of the maximum energy of laser pretreatment,the defects induced by deep ultraviolet(less than 230nm)absorption in the crystal are pretreated with an effective benign modification or elimination in the process.Fluorescence spectroscopy tests show the types of oxygen vacancies and defects represented by 623 nm peak have a definite correlation with LIDT;the types of impurities Ce,Sr,Ba,Fe3+,hydrogen/D vacancies and defects represented by 596 nm peak and 720 nm peak may be relevant.The positron annihilation spectrum test shows the laser conditioning can decompose large-scale vacancy cluster defects into smaller-scale vacancy cluster defects.Online scatter tests have shown there are two types of initial scattering defects in the low damage threshold crystal:one is the large-scale occlusion defect;the other is a micron-level point-like scattering defect.The correlation between large-scale inclusion defects and laser-induced initial damage is 100%,while that between spot-like scattering defects and laser-induced initial damage is 0%.There is only one orthogonal short tubular scattering defect in the medium,high damage threshold and laser conditioning crystals which LIDT of is around 8J/cm2,becoming one of the key types of defects in engineering applications.Laser conditioning can effectively improve LIDTs of the rapid growth ADP crystals,especially their prismatic sections.The damage growth test shows there are three types of defects in the ADP crystal according to the LIDTS:low(1-5J/cm2),medium(about 6-12J/cm2)and high LIDT defects(14J/cm2<).The damage morphology induced by low LIDT defects of ADP crystals is more common in point.The damage morphology induced by the medium and high LIDT defects is all mostly the morphology of shock wave interface around the central cavity.Ultraviolet absorption spectroscopy tests show the higher overall UV absorption of ADP crystals show a lower LIDT.The laser conditioning can effectively modify or eliminate the UV absorption defects;positron annihilation spectrum tells as the number and density of micro-defects such as single-vacancy point defects and vacancy clusters in the crystal are small,the LIDTs are high;the on-line scattering test shows there is only one type of fine short tubular scattering defect in all the ADP crystals,which has only a 10%probability of being the lowest threshold damage precursor in the conventional growth ADP crystal,while not the lowest threshold damage precursor in the rapid growth ADP crystal.