Potassium Dihydrogen Phosphate Crystals Research Articles

Room temperature noncritical phase matching fourth harmonic generation properties of ADP, DADP, and DKDP crystals

Noncritical phase-matching (NCPM) fourth harmonic generation (FHG) of a 1053 nm laser was realized at room temperature on an ammonium dihydrogen phosphate (ADP), 35% deuterated DADP and 62% deuterated potassium dihydrogen phosphate (DKDP) crystals grown by point-seed rapid growth method. The NCPM temperature is 33.7°C for ADP, 32.1°C for DADP, and 31°C for DKDP crystal. The temperature, angle tuning properties, and performance in NCPM FHG of ADP DADP and DKDP crystals are calculated and compared in detail. The 2ω-to-4ω conversion efficiency has been demonstrated up to 67% with ADP crystal, 63.45% with DADP crystal, and 63.78% with DKDP crystal. Large-aperture high-efficiency fourth harmonic generation would be realized by improving the temperature control and crystal quality with all these crystals and all these results could be a good reference in crystal chosen of NCPM FHG.

Research in minimum undeformed chip thickness and size effect in micro end-milling of potassium dihydrogen phosphate crystal

Micro-milling is a promising approach to repair the micro-defects on the surface of potassium dihydrogen phosphate (KDP) crystal. However, KDP crystal is difficult to machine due to its properties of soft-brittle and easy deliquescence. This study investigates the minimum undeformed chip thickness hm and the size effect in micro end-milling of KDP crystal by comprehensively analyzing cutting force, specific cutting force and machined surface quality. A model of the chip formation, which is capable of connecting the minimum undeformed chip thickness, the undeformed chip thickness and the periodicity of cutting force together, is developed to predict the value of minimum undeformed chip thickness. The normalized minimum undeformed chip thickness λe in terms of the ratio of the minimum undeformed chip thickness to the cutting edge radius re, is estimated to be 0.43 ≤ λe ≤ 0.48. The significantly non-proportional increase of specific cutting force indicates the existence of size effects when the ratio of feed per tooth to cutting edge radius ft/re is less than 0.7. The machined surface quality also reflects severe size effect by the phenomenon that the micro cracks and brittle pits appear on the groove base, and the value of surface roughness Ra is large when the ratio ft/re is less than 0.5. Furthermore, the surface quality deteriorates and the brittle cutting appears when the ratio ft/re is much larger than 1 which seems similar to macro-milling. The perfect machined surface with almost no ploughing effect and brittle cutting is achieved at a ratio ft/re of 0.7. Therefore, a feed per tooth, slightly larger than the minimum undeformed chip thickness but smaller than cutting edge radius, is recommended for micro-milling of KDP crystal or soft-brittle crystal.

Numerical simulation of the hydrodynamics and mass transfer in 3D spiral motion system for KDP crystal growth

A numerical study of the unsteady turbulent flow and mass transfer involved in the potassium dihydrogen phosphate crystal growth from an aqueous solution with a three-dimensional spiral motion configuration was performed using the standard k–ε model with an enhanced wall treatment. The three-dimensional spiral motion is composed of a uniform circular motion on the horizontal plane and a reciprocating motion in the vertical direction. The simulation indicates that the crystal spiral motion will drive the mainstream solution rotating around the axis of the growth vessel and form an oscillatory flow near the crystal, which results in a periodical reversal of the supersaturation distribution on the crystal faces and effectively suppresses the morphological instability of the crystal surfaces. With the increase of the orbital radius R or the characteristic rotation rate ω, the line speed V0 of crystal in the horizontal plane will increase, leading to a decline of the thickness of the diffusional boundary layer and an increase of the time-averaged supersaturation on the crystal faces. At the same time, the homogeneity of the surface supersaturation will be improved too, which would benefit the morphological stability of the crystal surfaces. However, when the crystal size increases, the time-averaged supersaturation on the crystal faces will decrease and its homogeneity will deteriorate. Therefore, with crystal growth, increasing the orbital radius, R, and reducing the period, T1, of the crystal circular motion are necessary.

GROWTH MECHANISM OF KDP CRYSTALS FROM AQUEOUS SOLUTIONS

The results of the effect of the growth history on Potassium Dihydrogen Phosphate (KDP) crystals growth mechanism are presented in the paper. Crystals were grown in temperature range of from aqueous solutions, saturated at . Two types of the experiments were performed. In both types, after the nucleation at crystals were grown at the same temperature for about 1.5 hour and then dissolved at temperature for about 15 min. After refaceting, in the first type, the crystal growth started at , followed by the temperature increasing in steps of to . In the second type, after refaceting the crystal growth started at , followed by the temperature decreasing in steps of to . Obtained results indicate that KDP crystals growth mechanisms do not depend on growth history. They are discussed in accordance with the current theories.

Comparative study on cracking behavior of sol-gel silica antireflective coating for high-powered laser system

Abstract Potassium dihydrogen phosphate (KDP) crystal is a kind of very crucial nonlinear optics for high-powered laser equipment as frequency conversion (doubler, tripler or quadrupler) components. In this paper, investigation on cracking behavior of sol-gel silica antireflective (AR) coatings has been performed using KDP crystal as substrate. Cracking induced by heating or moisturizing post-treatment is respectively studied, in which distinct cracking behaviors have been observed. It has been interestingly found that either temperature or humidity saltation can result in coating crack generation, but the corresponding residual stress demonstrates in completely reverse ways, giving rise to distinctive crack morphology on both crack edge and bottom. The origination and the derivation of such stress manner have been stated based on the intrinsic porous structure of sol-gel silica coating, and the critical cracking condition has been suggested dependent on the relationship between the accumulated residual stress against elastic recovery capability of such coating.

Figuring process of potassium dihydrogen phosphate crystal using ion beam figuring technology.

Currently, ion beam figuring (IBF) technology has presented many excellent performances in figuring potassium dihydrogen phosphate (KDP) crystals, such as it is a noncontact figuring process and it does not require polishing fluid. So, it is a very clean figuring process and does not introduce any impurities. However, the ion beam energy deposited on KDP crystal will heat the KDP crystal and may generate cracks on it. So, it is difficult directly using IBF technology to figure KDP crystal, as oblique incident IBF (OI-IBF) has lower heat deposition, higher removal rate, and smoother surface roughness compared to normal incident IBF. This paper studied the process of using OI-IBF to figure KDP crystal. Removal rates and removal functions at different incident angles were first investigated. Then heat depositions on a test work piece were obtained through experiments. To validate the figuring process, a KDP crystal with a size of 200 mm×200 mm×12 mm was figured by OI-IBF. After three iterations using the OI-IBF process, the surface error decreases from the initial values with PV 1.986λ RMS 0.438λ to PV 0.215λ RMS 0.035λ. Experimental results indicate that OI-IBF is feasible and effective to figure KDP crystals.

Optimization of morphological parameters for mitigation pits on rear KDP surface: experiments and numerical modeling.

In high power laser systems, precision micro-machining is an effective method to mitigate the laser-induced surface damage growth on potassium dihydrogen phosphate (KDP) crystal. Repaired surfaces with smooth spherical and Gaussian contours can alleviate the light field modulation caused by damage site. To obtain the optimal repairing structure parameters, finite element method (FEM) models for simulating the light intensification caused by the mitigation pits on rear KDP surface were established. The light intensity modulation of these repairing profiles was compared by changing the structure parameters. The results indicate the modulation is mainly caused by the mutual interference between the reflected and incident lights on the rear surface. Owing to the total reflection, the light intensity enhancement factors (LIEFs) of the spherical and Gaussian mitigation pits sharply increase when the width-depth ratios are near 5.28 and 3.88, respectively. To achieve the optimal mitigation effect, the width-depth ratios greater than 5.3 and 4.3 should be applied to the spherical and Gaussian repaired contours. Particularly, for the cases of width-depth ratios greater than 5.3, the spherical repaired contour is preferred to achieve lower light intensification. The laser damage test shows that when the width-depth ratios are larger than 5.3, the spherical repaired contour presents higher laser damage resistance than that of Gaussian repaired contour, which agrees well with the simulation results.

Progress on electro-optic crystals for Q-switches

Electro-optic Q-switched laser systems with high peak power have been widely applied in medical, cosmetology, measurement, processing, military industry, and other fields, while the crystal for electro-optic Q-switch is the core optoelectronic material for outputting high peak power pulse laser. With the technological progress and the development of short pulse, high repeat frequency and high power laser system application, the performance of crystals for electro-optic Q-switch is constantly improved. The paper reviews the research progress on basic characteristics, crystal preparation technology and electro-optic Q-switch application technology of several important electro-optic crystals for Q-switches including lithium niobate (LN) crystal, deuterated potassium dihydrogen phosphate (DKDP) crystal, low temperature phase barium metaborate (BBO) crystal, rubidium oxygen titanium phosphate (RTP) crystal, langasite (LGS) crystal and potassium titanium oxide phosphate (KTP) crystal, and propose that the research on practical electro-optic crystals for Q-switches is urgent to make breakthroughs. For a long time to come, exploring the electro-optic crystal materials, with high electro-optic coefficient, high laser damage threshold, stable performance, high temperature applicability and low cost, will be still an important issue in the field of crystal materials research.

Phase Equilibrium in the Aqueous Ternary System KH2PO4–NaH2PO4–H2O at 288.15 and 318.15 K

Phase equilibrium in the system KH2PO4–NaH2PO4–H2O at 288.15 and 318.15 K was determined using methods of isothermal solution saturation and moist residues. The equilibrium phase solid was also verified by X-ray diffraction (XRD). Furthermore, the crystallization areas in the phase diagram were analyzed and discussed. In the phase diagram of the two temperatures, there is one invariant point, two single saturated liquid curves, and two crystallization fields corresponding to the single salts NaH2PO4·2H2O, KH2PO4 at 288.15K and NaH2PO4·H2O, KH2PO4 at 318.15K. The density of saturated solutions for the systems studied at 288.15 and 318.15 K was measured and calculated to test the precision of the chosen model. This research filled the blank of phase diagram in this ternary system and can be applied to the production of potassium dihydrogen phosphate process to optimize the crystallization and separation process of potassium dihydrogen phosphate.

Cutting surface quality analysis in micro ball end-milling of KDP crystal considering size effect and minimum undeformed chip thickness

Potassium dihydrogen phosphate (KH2PO4 or KDP) crystal is a typical soft-brittle optical crystal, and the size effect and brittle cutting mode are easy to appear in micro ball end-milling of KDP crystal. In this paper, micro-grooving experiments are conducted to study the size effect and brittle cutting in micro ball end-milling of KDP crystal with different feed rate and depth of cut. The cutting force, machined groove base quality and chip morphology are collected and analyzed carefully. The size effect is discovered by the phenomena of the existence of oscillations and relaxations in cutting force and hyper-proportional increase of specific cutting force, when the ratio of feed per tooth to cutting edge radius ft/re is less than 1. While the brittle cutting mode is detected through the existence of sharp fluctuations in cutting force and cracks on the groove base when the ratio ft/re is larger than 2. From the further comprehensive analysis of cutting force, specific cutting force, machined groove base quality and chip morphology, the cutting parameters with ratios of the maximum undeformed chip thickness in one cutting circle to cutting edge radius hmax/re around 0.14, 0.2 and 0.4 are regarded as size effect, optimal and brittle cutting points, respectively. The size effect, ductile cutting and brittle cutting zones are divided by the size effect and brittle cutting boundaries (points). Among the optimal points, the depth of cut of 2μm with the ratio ft/re of 1 is the optimal cutting parameter for micro ball end-milling of KDP crystal.

Full-band error control and crack-free surface fabrication techniques for ultra-precision fly cutting of large-aperture KDP crystals

Large-aperture potassium dihydrogen phosphate (KDP) crystals are widely used in the laser path of inertial confinement fusion (ICF) systems. The most common method of manufacturing half-meter KDP crystals is ultra-precision fly cutting. When processing KDP crystals by ultra-precision fly cutting, the dynamic characteristics of the fly cutting machine and fluctuations in the fly cutting environment are translated into surface errors at different spatial frequency bands. These machining errors should be suppressed effectively to guarantee that KDP crystals meet the full-band machining accuracy specified in the evaluation index. In this study, the anisotropic machinability of KDP crystals and the causes of typical surface errors in ultra-precision fly cutting of the material are investigated. The structures of the fly cutting machine and existing processing parameters are optimized to improve the machined surface quality. The findings are theoretically and practically important in the development of high-energy laser systems in China.

Effect of motor rotor eccentricity on aerostatic spindle vibration in machining processes

In ultra-precision machining field, the air motorized spindle which is composed of a motor and an air bearing, plays a major role. In air motorized spindle, the motor eccentricity between the stator and the rotor is inevitably introduced during the manufacturing process, which directly affects the machining results of workpiece surface, and this phenomenon is particularly unwanted in machining. However, little attention has been paid to the motor eccentricity of air motorized spindle. In this paper, a new integrated electromechanical coupling method for estimating unbalanced force in air motorized spindle is presented, and the effects of motor rotor eccentricity on surface topography in ultra-precision processes are analyzed. An electromagnetic-mechanical method is used to study the coupling effects between the motor rotor and the aerostatic spindle. Meanwhile, the motor rotor and the aerostatic spindle are analyzed as a whole. In order to clearly describe the electromagnetic–mechanical method, the ultra-precision spindle for potassium dihydrogen phosphate crystal machining tool is selected as the research object, and the model of air motorized spindle and motor rotor eccentricity are presented. Besides, in order to assess the impact of the radial magnetic force caused by motor rotor eccentricity on the spindle performance, a range of rotor eccentricities is calculated. Additionally, the influence of the motor rotor eccentricity on the dynamic response of spindle is further analyzed. It is found that motor rotor eccentricity has a significant influence on the spindle vibration, which dramatically reduces the processing quality. Finally, the machining experiments are carried out, and the flatness errors of the workpiece caused by the motor rotor eccentricity are obtained by the wavelet method. The experimental results are consistent with the analysis results, which verifies the reliability of this method. This study is quite meaningful for deeply understanding the influence of motor rotor eccentricity on the machined surface.

Study on Micro Morphology of Potassium Dihydrogen Phosphate Crystals Grown at Elevated Temperatures

Potassium dihydrogen phosphate crystals were grown around 65 °C. The growth rate of KDP (KH2PO4) crystal prismatic faces was measured by laser polarization interference system. The surface micro morphology of KDP crystals grown at various supersaturation were comparatively observed by atomic force microscope systematically. The results show that the growth rate increased with the rise of supersaturation, the aspect ratio reduced. The steps bunching height occurred at σ = 0.03 and 0.04. At this point, the steps height reached maximum value about 11.3~24.4 nm. The step bunching was almost constant at higher supersaturation. A few pits and holes appeared on the crystal surface at high supersaturation. In our opinion, the forming of these pits and holes might be related with the step movement and 2D nuclei mechanism.

Aperture scalable, high-average power capable, hybrid-electrode Pockels cell.

We demonstrate a hybrid-electrode Pockels cell (HEPC) using a thin z-cut deuterated potassium dihydrogen phosphate (DKDP) crystal. The device is a reflective, longitudinally driven, longitudinally cooled Pockels cell, constructed with a plasma chamber providing the incident-side electrode and a silicon-substrate mirror serving the three purposes of a mirror, a back-side electrode, and a heat sink. The mirror and the DKDP are thermally coupled through a sub-millimeter, inert gas-filled gap. A time-multiplex pass-by driving method is proposed to favor low-voltage and fast-response operation. The experimental results support that the HEPC would be a competent device for high-energy and high-repetition-rate lasers.

Scalable process for mitigation of laser-damaged potassium dihydrogen phosphate crystal optic surfaces with removal of damaged antireflective coating.

We investigate an approach for the recycling of laser-damaged large-aperture deuterated potassium dihydrogen phosphate (DKDP) crystals used for optical switching (KDP) and for frequency conversion (DKDP) in megajoule-class high-power laser systems. The approach consists of micromachining the surface laser damage sites (mitigation), combined with multiple soaks and ultrasonication steps in a coating solvent to remove, synergistically, both the highly adherent machining debris and the laser-damage-affected antireflection coating. We identify features of the laser-damage-affected coating, such as the "solvent-persistent" coating and the "burned-in" coating, that are difficult to remove by conventional approaches without damaging the surface. We also provide a solution to the erosion problem identified in this work when colloidal coatings are processed during ultrasonication. Finally, we provide a proof of principle of the approach by testing the full process that includes laser damage mitigation of DKDP test parts, coat stripping, reapplication of a new antireflective coat, and a laser damage test demonstrating performance up to at least 12 J/cm2 at UV wavelengths, which is well above current requirements. This approach ultimately provides a potential path to a scalable recycling loop for the management of optics in large, high-power laser systems that can reduce cost and extend lifetime of highly valuable and difficult to grow large DKDP crystals.

Surface Micromorphology and Growth Rate Related to Supersaturation of KDP Crystals Grown at Different Temperature

A series of potassium dihydrogen phosphate (KDP) crystals are grown from various saturation temperature solutions by “point seed” technique. The prismatic faces growth rate of KDP crystals are measured by laser polarization system. The micromorphology of the steps on (100) face are comparatively researched by atomic force microscope (AFM) systematically. The results show that step bunching decreases with the supersaturation rise from 0.01 to 0.03 when the saturation temperature is around 35°C, the bunching increases in the case around 45°C, 55°C and 65°C, which is more significant around 65°C. The terrace width has the similar trend, it increases with the rise of supersaturation at different temperatures. The step slope has various trend with the change of supersaturation at various temperature. The normal growth rate and tangential velocity both increase with the rise of the supersaturation. In addition, at the same supersaturation, the higher the temperature, the faster the growth rate.

Vickers Microhardness Studies of Pure and Amino Acids Doped Potassium Dihydrogen Phosphate Crystals

Vickers microhardness study was carried out to understand the mechanical behaviour of pure and different amino acids (L-arginine, L-lysine, L-alanine) doped potassium dihydrogen phosphate (KDP) single crystals grown by slow solvent evaporation technique. Indentations were made on smooth {100} plane of pure and different amino acid doped crystals for loads varying from 0.298 N to 0.98 N. Vickers microhardness decreased as doping level of amino acid was increased. Indentation size effect (ISE), Kick's law, Hays and Kendall's law and Proportion specimen resistance (PSR) model were verified for pure and different amino acids doped crystals. The values of the load independent hardness and Newtonian pressure were higher in pure KDP crystals than the amino acids doped KDP crystals. It was found that as doping concentration of amino acids increased the load independent hardness and Newtonian pressure decreased. The results indicated that the doping of amino acids made KDP crystals softer.

Noncritical phase-matched fourth harmonic generation properties of traditional grown large-size DKDP crystal

Noncritical phase-matched (NCPM) fourth harmonic generation (FHG) of 1053nm was conducted at different regions of a traditional grown large-size 65.8% deuterated potassium dihydrogen phosphate (DKDP) crystal. NCPM FHG was realized at the temperature of 32.1–32.4°C. The 0.3°C variation of the temperature is induced by the 0.12% inhomogeneity of deuterium content. Comparing with 2.3°C of temperature acceptance, the 0.3°C variation is acceptable. Thus, the traditional grown DKDP crystal is in good homogeneity of deuterium content in view of NCPM FHG. On the other hand, the linear absorption coefficient varies from 0.08cm−1 to 0.12cm−1. When driven with 0.85GW/cm2 of second harmonic radiation, the FHG conversion efficiency reaches 68.9%. Larger linear absorption leads to 3% reduction of the conversion efficiency. Therefore, measures should be taken to suppress the linear absorption.

Thermo-optical characteristics of DKDP crystal

This letter presents a theoretical and experimental investigation of thermally induced polarization distortions occurring in an optical element made of c-cut tetragonal crystals. Two material characteristics were defined for this class of crystals: the optical anisotropy parameter ξ and the thermo-optical constant Q. These were generalized with analogous characteristics of elastically isotropic cubic crystals. The experimental investigation of these characteristics for popular tetragonal deuterated potassium dihydrogen phosphate (DKDP) crystal was carried out.

Effect of cyclohexane diamine tetraacetic acid on micro morphology of rapidly grown potassium dihydrogen phosphate crystals

Potassium dihydrogen phosphate (KDP) crystals were grown from aqueous solutions with different concentrations of cyclohexane diamine tetraacetic acid (CDTA) by the “point seed” rapid growth technique.