KH2PO4 Crystals Research Articles

Influence of driving power on the performance of UV KDP-based acousto-optical tunable filters

Spectral imaging is a powerful tool for remote sensing applications. Among other valuable options, acousto-optical tunable filters (AOTF) offer interesting properties to this class of instruments. When sensitivity to ultra-violet (UV) light is required, KH2PO4 crystal (KDP) is a good choice for the acousto-optic (AO) interaction medium as its AO figure of merit is relatively large compared to other materials transparent in UV. However, it generally requires more acoustic power than well-known TeO2-based devices to achieve the highest diffraction efficiency. We report on measurements performed with a KDP-based AOTF (9-cut) driven with RF signals up to 3 W. It has been found that owing to various effects (Joules, acoustic wave attenuation,…) the heating of the crystal by a few degrees yields severe alteration of the AOTF spectral transmission function (STF): shift of the central wavelength, increase of the bandwidth and decrease of the peak filter efficiency have been observed. These three effects degrade the AOTF overall performance if not taken into account. This paper presents the experimental evidences and shows how a temperature-dependent AOTF model allows to derive the inner temperature of the device. Possible reasons for the flattening of the STF are also discussed.

β′-Yb3+:Gd2(MoO4)3 crystal – A promising self-frequency doubling laser material

A β′-Yb3+:Gd2 (MoO4)3 crystal with dimensions of 22×22×25mm3 was grown successfully by the Czochralski method. Its relative efficiency of second harmonic generation was measured by Kurtz power method, the intensity is about 3 times as large as that of KH2PO4 crystal. The thermal expansion coefficients of the crystal are 14.7×10−6K−1, 13.5×10−6K−1 and −20.1×10−6K−1 along a-, b- and c-axes, respectively. The thermal conductivity along the c-axis is 1.44W/mK at room temperature. The absorption cross section at 977nm is 0.48×10−20cm2 with full width at half maximum of 20nm. The emission cross section at 1030nm is 1.13×10−20cm2. The fluorescence lifetime is 722μs. The results indicate that β′-Yb:Gd2(MoO4)3 crystal is a promising candidate of self-frequency doubling laser material.

Influence of deuteration on the KH2PO4 crystal micro-defects characterization by using positron annihilation spectroscopy

Deuterated potassium dihydrogen phosphate (K(DxH1-x) 2PO4) crystals with different deuteration levels (x=0, 0.51, 0.85) were grown by conventional cooling method from deuterated solutions at Shandong University. Positron annihilation spectroscopy has been widely used to the study on micro-defects of semiconductors and other materials, which is very sensitive to the crystal structure, defect types, defect concentrations, and so on. In this paper, positron annihilation spectroscopies (positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy), combined with X-ray diffraction (XRD) are used to investigate micro-defects characterization in K(DxH1-x) 2PO4 crystals. Influences of deuteration degree on the crystal structure characteristics, defect types and concentrations are discussed. It can be concluded from XRD experiments that the lattice parameters of a and b increase with the increase in deuteration levels, while no obvious change occurs on the lattice parameter c. KH2PO4(KDP) crystals at low deuteration level and high deuteration level could be regarded as low deuterium-doped KDP crystal and low hydrogen-doped DKDP crystal respectively. It is indicated that the higher the replacement ratio in the crystals, the weaker the diffraction peak they show. Positron annihilation lifetimes increase clearly in the highly-deuterated KDP crystals. It is found that neutral interstitial defects and oxygen defects in the KDP crystal increase with increasing deuteration degree. And these types of defects can be attributed to lattice distortion effect. From positron annihilation lifetime results we can arrive at another conclusion that the compound defects will form and defects concentration is declined, when hydrogen vacancies, K vacancies and substitutional impurity defects continue to react by means of association reactions. These phenomena suggest that high deuteration plays a significant role in promoting association reaction of internal defects in the crystals. Furthermore, the polymerization reaction of the clusters and micro-cavities continue to occur, therefore defect concentrations will show a constant decrease. Doppler broadening spectra show that the internal defects in the crystals increase integrally with an increase of deuteration level; this agrees well with the results of positron annihilation lifetime. Moreover, Doppler broadening spectra indicate that the proportional change of these defects is synchronous and consistent with the actuality. To sum up, our experimental results suggest that the defect reaction is weak in low degree of KDP crystal deuteration growth (less than 50%), while reaction is enhanced in the high degree of deuteration growth (higher than 50%).

Defects and Their Distribution in KH2PO4 Crystals with Embedded TiO2 Nanoparticles

ABSTRACTResults from the successful growth of high quality KH2PO4 (KDP) crystals with incorporated TiO2 anatase nanoparticles and the characterization of these crystals using several complementary methods are presented. Transmission and scanning electron microscopy have shown that the anatase nanoparticles were captured mainly by the pyramidal growth sector and, to a considerably lesser extent, by the prismatic growth sector. Energy dispersive x-ray analysis confirms that the growth layer stacks contain the TiO2 particles. Significant variation in the imaginary and real parts of the cubic nonlinear optical susceptibilities and refractive index changes at continuous wave excitation were found in prism and pyramid parts of pure KDP and KDP:TiO2 samples. The identified lines of electron paramagnetic resonance belong to four different centers FeA3+, FeB3+, CrR3+ and CrGB3+. From analysis of line intensities it was concluded that the concentration of non-controlled impurities in nominally pure KDP samples is several times larger than in KDP:TiO2, and that the concentration of non-controlled impurities in the prismatic part of the KDP:TiO2 boule is larger than in the pyramidal part.

Coupled Computational Fluid Dynamics–Discrete Element Method Simulations of a Pilot-Scale Batch Crystallizer

Computational fluid dynamics (CFD) coupled with the discrete element method (DEM) has been used to investigate numerically crystal dynamics in an existing pilot-scale batch crystallizer. The CFD-DEM combination provides a detailed description of crystal dynamics considering a four-way coupling. In a previous analysis,1 CFD had been coupled with a discrete phase model (DPM) using a simple one-way coupling. The corresponding predictions are then compared with those obtained through four-way coupling considering KH2PO4 crystals in water. From the CFD-DEM simulation, it is possible to investigate quantitatively the driving force controlling crystal growth and the interaction of crystals with reactor walls, baffles, and impellers. This delivers essential data for process improvement. Different seeding procedures were also compared. The seed crystals have been injected either within the complete liquid volume or, as in the experiments, through a funnel. By varying the most important crystallization process parameters, we found optimal conditions for a liquid phase volume in the crystallizer of 24 L, for injection through a funnel above the baffle, and for an initial seed crystal size of 0.5 mm.

Investigation on the Influence of Machine Tool Dynamics on the Wavefront Gradient of KH2PO4 Crystals

The root mean square gradient (GRMS) is a key parameter for KH2PO4 crystal of low-frequency wavefront, which affects the focusing performance in the inertial confinement fusion (ICF) system. The large wavelength waviness on the machined surface has significant influence on the GRMS. In this paper, the influence of the large wavelength waviness on the GRMS and the main sources resulting in the large scale ripples are analyzed based on dynamics analysis of the machine tool. The dynamic performance of machine tool is optimized and the reasonable dynamic stiffness region is obtained. Furthermore, the machined and the tested results validate the usefulness of the theoretical analysis.

Broadband second-harmonic and sum-frequency generation in KH2PO4 crystals doped with anatase nanocrystals

Broadband second-harmonic (SH) and sum-frequency generation was studied in KH2PO4 crystals with incorporated anatase modification of titania nanocrystals. A tiny amount (10−5 wt.%) of titania nanoparticles in the anatase phase exhibits a pronounced (up to 15%) increase of the SH efficiency in comparison with the nominally pure potassium dihydrogen phosphate (KDP) crystal. Higher anatase fraction (10−4 wt.%) results in a reduction of the SH conversion efficiency. The rise of the SH signal is attributed to resonant excitation of the defect states at the nanocrystal interface, and its subsequent decrease at higher concentration is likely to be related to a light scattering in the crystal bulk.

Flatness improving method of KDP crystal in ICF system and its implementation in machine tool design

The flatness of the KH2PO4 (KDP) crystal has important effect on the inertial confinement fusion system. The method to improve the flatness of KDP crystal and its implementation in the machine tool design is presented in this paper. The finite element model of the whole machine tool is built up to describe the tool tip displacement with the cutting force along the cutting path. The influence of the axial and radial bearing stiffness of the aerostatic spindle on the machined surface flatness is discussed. Furthermore, a novel adjusting mechanism which is used to adjust the squareness between the spindle and the slide is designed, and a new machining process is proposed to improve the flatness of the machined surface. The machining trials are carried out to evaluate and validate the effectiveness of the presented approach and simulation.

Photoluminescence characterization of KH₂PO₄ crystal: application to three-dimensional growth-sector identification.

In this work, rapidly grown KH2PO4 (KDP) crystals extracted from the prismatic and the pyramidal growth sectors of crystal boules were analyzed using photoluminescence measurements. From the spectra, we deduced a robust criterion to discriminate between both growth sectors in an unknown KDP plate. Moreover, spatially resolved photoluminescence was shown to enable a local probing of different planes in the bulk of the material leading to accurate and nondestructive three-dimensional mapping of the sector boundary, which is often the weakest point in terms of laser-damage resistance in rapidly grown KDP crystals.

An integrated method for waviness simulation on large-size surface

The surface waviness of the KH2PO4 crystal has important influence on its optical performance, but it is difficult to simulate by the current simulation models. This article presents an integrated machining performance forecasting method, which considers the interaction between manufacturing process and machine tool and integrates the cutting simulation, machine tool dynamic performance simulation and the control system. The profile and waviness prediction on the large area are achieved by this method with short time and high accuracy. The machining trials on a fly cutting machine tool are carried out to demonstrate the effectiveness of the proposed approach.

Monte carlo simulations of the pseudo-spin ising model for hydrogen-bonded ferroelectrics

Monte Carlo (MC) simulations are performed for hydrogen-bonded-type ferroelectrics by using a three dimensional pseudo-spin Ising model. The pseudo-spin interaction strengths V and U are related to the Slater-Takagi energy parameters ɛ0 and ɛ1 via U = (ɛ0 − ɛ1)/2 and V = (2ɛ1 − ɛ0)/4. The phase transition temperature and the longitudinal and transverse dielectric constants are calculated as functions of interaction ratio R = − U/V, in the range of R from 0 to 1. According to the MC simulations, the longitudinal dielectric constant exhibits a ferroelectric phase transition while the transverse constant exhibits behavior associated with an antiferroelectric transition. The transverse dipole moments obtained from a mean-field approximation and the domain motion in the microscopic structure are discussed in view of hydrogen-ion bombardment experiments on KH2PO4 crystals.

Structural, electronic and optical properties of novel carbonate fluorides ABCO3F (A=K, Rb, Cs; B=Ca, Sr)

In the present study, we performed first principles calculations on the electronic and optical properties of layered alkali–alkaline earth carbonate fluorides which attract attention in the domain of nonlinear optics. The calculated lattice parameters and volumes with and without inclusion of van der Waals (vdW) correction methods to standard density functional methods were compared with experiments. We observed that vdW interactions are predominant in RbCaCO3F and CsCaCO3F as compared with other computed compounds. The calculated bulk modulus from single crystal elastic constants reveals that these materials are all relatively harder than the KH2PO4 (KDP) crystal. We also found that these materials are wide band gap insulators as obtained from Tran–Blaha modified Becke–Johnson potential. The linear optical properties such as dielectric function, refractive indices, birefringence and absorption spectra are presented. Finally, the calculated birefringence values indicate that these crystals could be promising for producing phase matching in the deep ultra-violet region.

Crystal growth, nonlinear frequency-doubling and spectral characteristic of Nd:Ca9La(VO4)7 crystal

The pure Ca9La(VO4)7 and the Nd:Ca9La(VO4)7 crystals with various Nd3+-doped concentrations were grown by Czochraski method. The effective segregation coefficient of Nd3+ ion in Ca9La(VO4)7 crystal was measured as 1.07, which is favorable to the even distribution of Nd3+ ions. The frequency-doubling experiment and the crystal structure analyses show that the presence of La3+ ion in crystal can improve the SHG effect of Ca9La(VO4)7 crystal. The intensity of SHG effect, measured by the Kurtz powder method, is found to be about 4 times as large as that of KH2PO4 (KDP) crystal. The polarized absorption spectra and the polarized fluorescence spectra are analyzed based on Judd–Ofelt theory, which show that the Nd:Ca9La(VO4)7 crystal is hopeful to generate laser light at the wavelengths of about 921, 1069 and 1349nm. The luminescence decay kinetics are analyzed based on the migration model, and the results indicate that Nd3+ ion in Ca9La(VO4)7 crystal has a high quantum efficiency, about 98.2% in 3.2at.% Nd:Ca9La(VO4)7 crystal, and a weak concentration quenching effect. Therefore, Nd:Ca9La(VO4)7 crystal may be a high efficiency laser material and a candidate of self-frequency-doubling (SFD) laser crystal.

Crystalline perfection and optical properties of rapid grown KH2PO4 crystal with chromate additive

Potassium dihydrogen phosphate (KDP) crystals were grown in the presence of a series of chromate (CrO\(_{4}^{2-}\)) additive concentrations via rapid growth method. CrO\(_{4}^{2-}\) made KDP crystals were coloured by yellow-green, suggesting CrO\(_{4}^{2-}\) had entered into the crystal lattice. The elemental analysis indicated that Cr element in KDP crystal was at ppm level. High resolution X-ray diffraction data revealed that the crystalline perfection of these as-grown KDP crystals was destroyed after CrO\(_{4}^{2-}\) entered into crystal lattice, embedded in the full width at half maximum was broadened and satellite peaks appeared. Additionally, the extinction ratio was decreased with rise of CrO\(_{4}^{2-}\) concentration. CrO\(_{4}^{2-}\) introduced two absorption peaks centred at 360 and 280 nm and enhanced the intrinsic absorption near 220 nm, which were at the same band positions compared with the CrO\(_{4}^{2-}\) or HCrO\(_{4}^{-}\) transmittance spectra. Additionally, CrO\(_{4}^{2-}\) could increase the size of light scattering, which was attributed to the point defects and microscopic defects by the replacement by CrO\(_{4}^{2-}\) at PO\(_{4}^{3-}\) position.

Glassy domain wall matter in KH2PO4 crystal: Field-induced transition

We have investigated the domain wall (DW) dielectric response of potassium dihydrogen phosphate (KH2PO4) crystal under 0–500 V dc-bias electric field. Activated DW-contribution onsets freezing at Tf(ω, V), some 30 K below the ferroelectric TC; timescale τf(T, V) exhibiting Vogel-Fulcher (VFT) divergence. Sharply distinct low- and high-field behaviors of TC(V), DW-Tg(V), VFT-T0(V), barrier energy Ua(V), and DW glass-fragility m(V) signify a field-induced transition from randomly pinned/vitreous to clustered/glass-ceramic phases of domain wall matter. Field-hysteresis (ε′poled>ε′unpoled) observed at high dc-bias indicates coexistent unclustered DW phase, quenched-in during the field-cooling. We construct a paradigm T-E phase diagram depicting the complex glassy patterns of domain wall matter.

Study on the optical performance and characterization method of texture on KH2PO4 surface processed by single point diamond turning

The analysis based on the Fourier model method indicates that the steep profile (smaller wave period or larger depth) tends to decrease the laser induced damage threshold (LIDT) of KH2PO4 (KDP) crystal. The parallel or perpendicular texture directions on the front and back surfaces have different influences on the LIDT of optical components. Power spectrum density (PSD) method can realize the information of texture period, depth and direction on certain surface, especially the angular spectrum and radial spectrum. The fractal theory covers the shortage of PSD method in charactering the texture roughness or surface processing quality. The larger fractal dimension means that the surface fluctuates sharply and lots of brittle defects exist, this is consistent with the experimental results. The circumferential profile fractal dimension distribution also can characterize texture direction, but its directing ability decreases or even is lost when the sampling extent becomes small or the surface texture is not obvious any more. The angular spectrum is more accurate than the fractal dimension in charactering texture direction.

Growth, thermal and spectral properties of Nd3+:YCa9(VO4)7 crystal

A new crystal of Nd3+:YCa9(VO4)7 was grown successfully by the Czochralski method. Its intensity of second harmonic generation is about 1.5 times as large as that of KH2PO4 crystal. The thermal physical properties and the spectral properties of the crystal were investigated in detail. Based on the Judd–Ofelt theory, the intensity parameters were obtained. From the measured principal refractive indices, the Sellmeir equations and the theoretical phase-matching angles of the crystal were obtained. The results showed that Nd3+:YCa9(VO4)7 crystal is expected to be a new self-frequency-doubling laser crystal.

Influence of Light Intensification by Subsurface Cracks on KH2PO4 Crystal's Laser Induced Damage Threshold

The subsurface damage introduced by single point diamond turning (SPDT) is an important precursor that decrease the laser induced damage threshold (LIDT) of KH2PO4 (KDP) crystal. In this paper, based on the Fourier model theory, the modulation property of subsurface crack is analyzed. Research results indicate that light intensification by subsurface crack is an important mechanism of KDP failure. When the feed speed increases under the certain cutting depth, the subsurface damage depth and LIDT have the inverse changing trend. Namely that the intention of obtaining smaller damage depth by choosing suitable cutting parameters, will be also beneficial to improve the surface LIDT of KDP crystal. The combination of feed and cutting depth decides the damage depth together under the given spindle speed and tool geometry. The small feed is beneficial for improving the LIDT when cutting depth is large, while the situation is opposite when cutting depth is small.

The development of the depletion zone during ceiling crystallization: phase shifting interferometry and simulation results

The growth of high quality protein crystals is essential for the determination of their structure. This process is governed by many physical factors such as mass transport and solution flow. The quality of the crystals is usually better under diffusion-limited growth conditions, where a depleted zone of the solution encapsulates the crystal. We developed a Mach–Zehnder-based phase shifting interferometer coupled to image processing software to study the concentration gradients which develop around a crystal during its growth or dissolution. The depletion zones and the diffusion boundary layers around growing and dissolving KH2PO4 crystals are monitored and processed by a MATLAB based algorithm. Our main emphasis was to analyze the ceiling crystallization conditions in which the crystal is placed at the very top of the growth cell and therefore the solute transport is largely diffusion-limited. The experimental results are compared with simulations using finite element-based numerical calculations. The combined results clearly demonstrate the positive effect of the ceiling crystallization approach on crystal growth.

Growth and spectral properties of a new nonlinear laser crystal of Nd3+:Ca9Y0.5La0.5(VO4)7

A Nd3+-doped Ca9Y0.5La0.5(VO4)7 crystal with dimensions of ø23×35mm3 has been grown by the Czochralski method. The effective segregation coefficient of Nd3+ ion is measured as 1.3. Its relative efficiency of second harmonic generation has been measured by the Kurtz power method, the intensity is 1.5times as large as that of KH2PO4 crystal. Its Vickers hardness of (100) and (001) faces are 408.31 and 451.72N/mm2, respectively. The spectroscopic properties of Nd3+:Ca9Y0.5La0.5(VO4)7 crystal were investigated. The Judd–Ofelt theory was applied to calculate the spectral parameters. The absorption cross sections at 810nm are 4.35×10−20cm2 with full width half maximum (FWHM) of 12nm for π-polarization, and 5.61×10−20cm2 with FWHM of 11nm for σ-polarization, respectively. The emission section is 5.02×10−20cm2 for π-polarization and 7.45×10−20cm2 for σ-polarization, respectively. The fluorescence lifetime is 147μs. The results indicate that the Nd3+:Ca9Y0.5La0.5(VO4)7 crystal may be a promising candidate of self-frequency-doubling laser material.