Mechanism of micro-waviness induced KH₂PO₄ crystal laser damage and the corresponding vibration source

The low laser induced damage threshold of the KH2PO4 crystal seriously restricts the output power of inertial confinement fusion.The micro-waviness on the KH2PO4 surface processed by single point diamond turning has a significant influence on the damage threshold.In this paper,the influence of micro-waviness on the damage threshold of the KH2PO4 crystal and the chief sources introducing the micro-waviness are analysed based on the combination of the Fourier modal theory and the power spectrum density method.Research results indicate that among the sub-wavinesses with different characteristic spatial frequencies there exists the most dangerous frequency which greatly reduces the damage threshold,although it may not occupy the largest proportion in the original surface.The experimental damage threshold is basically consistent with the theoretical calculation.For the processing parameters used,the leading frequency of micro-waviness which causes the damage threshold to decrease is between 350-1 μm-1 and 30-1 μm-1,especially between 90-1 μm-1 and 200-1 μm-1.Based on the classification study of the time frequencies of microwaviness,we find that the axial vibration of the spindle is the chief source introducing the micro-waviness,nearly all the leading frequencies are related to the practical spindle frequency(about 6.68 Hz,400 r/min) and a special middle frequency(between 1.029 Hz and 1.143 Hz).     

Temperature dependence of the far infrared reflectivity spectra of a KD2PO4

Complete data on far infrared reflectivity spectra of a KD₂PO₄ single crystal in the temperature range 7–300 K, from the millimeter wave up to 400 cm^–1, for all crystallographic orientations of the crystal, in the ferro- and paraelectric phase, are given for the first time. The temperature dependence of the phonon spectra is similar to that we have observed and published for KH₂PO₄.     

Ferroelastic crystals and their nonlinear elastic properties observed in frequency range from gigahertz to milihertz

The experiments reported in this article have been performed to unify the results of earlier and recent determinations of parameters describing the elastic properties of Rb₄LiH₃(SO₄)₄, LiCsSO₄ (LCS) and KH₂PO₄ crystals measured by different experimental methods. The above crystals undergo a second-order phase transition, incommensurate phase transition (PT) and a first-order but close to second-order phase transition, respectively. To investigate the elastic properties of the crystals in the frequency range from 10^?1 to 10¹⁰ Hz, five experimental methods were applied: dynamic mechanical analysis, piezoelectric resonance, composite oscillator bar, ultrasonic wave propagation and Brillouin light scattering. The results of the experiments allowed identification of the processes contributing to the elastic response of the crystals investigated. Moreover, for the LCS crystal a model of the incommensurate PT was presented.     

Formation energies and electronic structures of native point defects in potassium dihydrogen phosphate

The formation energies and the equilibrium concentration of vacancies,interstitial H,K,P,O and antisite structural defects with P and K in KH 2 PO 4 (KDP) crystals are investigated by ab initio total-energy calculations.The formation energy of interstitial H is calculated to be about 2.06 eV and we suggest that it may be the dominant defect in KDP crystal.The formation energy of an O vacancy (5.25 eV) is much higher than that of interstitial O (0.60 eV).Optical absorption centres can be induced by defects of O vacancies,interstitial O and interstitial H.We suggest that these defects may be responsible for the lowering of the damage threshold of the KDP.A K vacancy defect may increase the ionic conductivity and therefore the laser-induced damage threshold decreases.     

Detection and characterization of absorption heterogeneities in KH2PO4 crystals

The SOCRATE facility is used for the detection of heterogeneous absorption in large and rapidly grown KH₂PO₄ (KDP) crystals. The different regions are subsequently characterized by spectrophotometry and laser-damage measurement. In accordance with previous results from the literature we found that, by using the SOCRATE damage measurement set-up with a large beam (300 ∗ 600 μm²), the absorption level does not play a major role in the damage mechanism at 355 nm. However, by testing the damage resistance of the same KDP component with a focused beam (9 μm diameter) at 355 nm, we highlight for the first time significantly different damage resistances between highly and weakly absorbing regions from a KDP crystal. Indeed, with a focused beam, we demonstrate that the laser-damage threshold may be lower in some sectors cut in the pyramidal part than in sectors from the prismatic part. This means that the absorbing defects at high concentration are not the predominant damage precursors at 355 nm.     

Modification of the dielectric properties of copper-doped CdIn2S4 single crystal

The study of the dielectric properties of a CdIn₂S₄〈3 mol % Cu〉 single crystal in alternating-current (ac) electric fields with frequencies f = 5 × 10⁴?3.5 × 10⁷ Hz has revealed the origin of dielectric loss (relaxation loss that is changed by the through current loss at high frequencies). It has been found that CdIn₂S₄〈Cu〉 has permittivity increment Δ?′ = 123, relaxation frequency f _r = 2.3 × 10⁴ Hz, and relaxation time τ = 43 μs. The doping of CdIn₂S₄ single crystal with copper (3 mol %) is established to substantially increase the permittivity (?′), dielectric loss tangent (tanδ), and ac conductivity (σ_ac). In this case, the frequency dispersion of ?′ and tanδ increases and that of σ_ac decreases.     

EPR evidence for local pseudo freeze out of impurity dynamics in KH2PO4 type ferroelectrics and antiferroelectrics

The temperature dependence of the reorientation rates of the AsO₄^4? centers in KH₂AsO₄ and NH₄H₂AsO₄ as well as of the CrO₄^3? centers in KH₂PO₄ shows a strong deviation from an Arrhenius behavior which can be quantitatively described as a result of the local “pseudo freeze-out” of the dynamics of these centers and the associated polarization cloud far above the transition temperature T_C of the host crystal.     

Order parameter fluctuation spectrum and central peak in partially deuterated H-bonded ferroelectrics

The order parameter fluctuation spectrum in an isotopically disordered KH₂PO₄ type crystal has been evaluated as a function of the deuterium content using Mori's continued fraction representation. In contrast to a pure system we have now two narrow central peaks and two overdamped soft modes. The results show that the static central peak observed by light scattering in paraelectric KH₂PO₄ cannot be produced by naturally abundant deuterium.     

The nonlinear optical coefficient,phasematching, and optical damage in the chalcopyrite AgGaSe2

We have made a direct measurement of the absolute nonlinear coefficient for AgGaSe₂ by phasematched second harmonic generation. The measured value is d₃₆ = (3.24 ± 0.50) × 10^?11 m/V. For CO and Co₂ lasers the observed phasematching angles for second harmonic generation and frequency mixing are within one degree of the values calculated by fitting the reported index of refraction data to Sellmeier equations. At 1.06 μm the optical damage threshold depends on the number of incident laser pulses. For 1000 pulses damage occurs at 11 MW/cm². The samples were cut from crack-free single crystal boules with absorption coefficient smaller than 0.1 cm^? at 10.6 μm.     

-OHO- vibrations in the Raman spectra of paraelectric CsH2PO4 crystal

Polarised Raman spectra in the frequency range of 1500 cm^-1 to 400 cm^-1 are reported for paraelectric CsH₂PO₄ crystal. The spectral features recorded are interpreted by means of factor group analysis applied to the triatomic vibrational units -OHO-.     

Proton spin-lattice relaxation by critical polarization fluctuations in KH2PO4

The observed anomalous decrease in the proton spin-lattice relaxation timeT ₁ on approaching the Curie point in a rather pure KH₂PO₄ single crystal is explained by magnetic dipolar coupling to the ferroelectric mode. The isolated “non-interacting” O?H...O proton flipping time is estimated from theT ₁ data as τ=0.66·10^?12 sec for the paraelectric phase and τ=2.24·10^?12 sec for the ferroelectric phase, in good agreement with the results obtained from other methods.     

Dielectric relaxation in single crystal NH4H2PO4 in the high-temperature regime

The dispersion curves of the dielectric response in single crystal NH₄H₂PO₄ were obtained in the radio frequency range and below the high-temperature transition at T_p−160 °C. The results reveal dielectric relaxation at low frequency, which is about 10⁵ Hz at 70 °C, and it shifts to higher frequencies (∼3×10⁶ Hz) as the temperature increases. The relaxation frequency was determined from the peak obtained in the imaginary part of the permittivity as well as from the derivative of the real part of the permittivity. The activation energy E_a=0.55 eV, obtained from the relaxation frequency is very close to that derived from the dc conductivity. We suggest that this dielectric relaxation could be due to the proton jump and phosphate reorientation that cause distortion and change the local lattice polarizability inducing dipoles like      

Light scattering spectra of polarization fluctuations and models of the phase transition in KDP type ferroelectrics

Abstract

Recent light scattering studies in KH₂PO₄ type hydrogen-bonded crystals are reviewed. Dynamical properties of these crystals have been discussed in terms of the coupled tunneling proton-optical phonon model up to date. Light scattering spectra in several GHz and cm^?1 regions with various scattering geometries observed in KH₂PO₄, KD₂PO₄ and their mixed crystals, however, revealed that all these spectra are well analysed in terms of a coupled polarization relaxation mode-acoustic phonon system. Since most of the spontaneous polarization is due to shifts of K, P and O ions, relaxation motions of H₂PO₄ dipoles arc concluded to be the origin of these low-frequency spectra. Models of the phase transition mechanism are discussed on the basis of these results and other recent experimental studies including the explanation of large isotope effects in these crystals.     

A study of growth mechanism of KDP and ADP crystals by means of quantum chemistry

Impurity adsorption, crystal growth by adsorption of growth unit and step-pinning mechanism of metal ion adsorption were investigated for potassium dihydrogen phosphate (KDP; KH₂PO₄) and ammonium dihydrogen phosphate (ADP; NH₄H₂PO₄) by quantum chemistry. In this study, the ideal crystal morphologies, the growth unit and the crystal surface with and without metal ions were calculated and analyzed by using electrostatic property. It is found that the computational results based on electrostatic potential distribution can account for the observed behaviours on KDP and ADP crystal growth.     

Synthesis, structure, and dielectric properties of KH2PO4 fractal aggregates

Macroscopic fractal aggregates of KH₂PH₄ (KDP) measuring up to 500 μm have been obtained. The fractal structure forms as a result of the precipitation of KDP particles from a supersaturated aqueous solution in the presence of a temperature gradient followed by a diffusioncontrolled mechanism of aggregation. The electron-microscopic analysis performed has shown that the fractals are formed predominantly from crystallites of the tetragonal modification measuring ∼1 μm. The dielectric constant (ɛ) of fractal KH₂PO₄ has been measured in the temperature range 80–300 K. A characteristic anomaly has been discovered on the ɛ(T) curve in the vicinity of 122 K, which attests to a ferroelectric phase transition. The absolute value of ɛ is significantly smaller than the components ɛ ₁₁ and ɛ ₃₃ for KH₂PO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 2059–2061 (November 1999)     

Measurement of orientation crystallization rates of linear polymers by means of dynamic X-ray diffraction technique. II. Frequency dispersion of strain-induced crystallization coefficient of natural rubber vulcanizates in subsonic range

The orientation crystallization behavior of natural rubber vulcanizates was investigated by means of a dynamic X-ray diffraction technique utilizing the half-circle sector technique. The frequency dependence of dynamic diffraction intensities from diatropic and paratropic crystal planes, (002) and (200) planes, was observed over a frequency range from 10^?3 to 10¹ Hz as a function of temperature, degree of cross-linking, static extension ratio, and dynamic strain amplitude.

The frequency dependence of the dynamic X-ray diffraction gives two dispersion regions around 10^?2 and 10^?1 Hz. The phase angle between the dynamic X-ray diffraction and dynamic strain of a bulk specimen is definitely positive; i.e., the dynamic strain of the specimen is always behind the dynamic orientation crystallization.

The frequency dispersion at low frequencies, around 1WZ Hz is very obvious for the (200) crystal plane and shifts somewhat to higher frequencies under those conditions that increase the mobility of self-diffusion of rubber molecules, i.e., increase of temperature and decreases of extension ratio and degree of cross-linking. The frequency dispersion at high frequencies, around 10¹ Hz, is rather obvious for the (002) crystal plane and does not shift appreciably under the experimental conditions.     

Structure of a high temperature phase of potassium dideuteriophosphate (DKDP)

We previously determined the crystal structures of the high temperature phases of KH₂PO₄ [J.A. Subramony, S. Lovell, B. Kahr, Chem. Mater. 10 (1998) 2053. [6]]. These triclinic and monoclinic phases were obtained by heating the room temperature tetragonal form until new crystal phases were identified by polarization microscopy. These samples were subsequently cooled to room temperature thereby preserving the metastable high temperature phases for analysis. KD₂PO₄ is distinct from KH₂PO₄ in that it crystallizes at room temperature in a monoclinic phase unknown for its isotopomer, therefore the question as to whether it would support the corresponding high temperature phases remained open until our report here of the transformation of monoclinic KD₂PO₄ to an isomorphous triclinic high temperature phase: space group P with a=7.475(1) Å, b=7.440(1) Å, c=7.184(1) Å, α=88.53(1) Å, β=86.81(1)°, γ=88.09(1)°, V=398.58 Å³. This is the first known high temperature phase of DKDP, a material coveted at room temperature for its optical properties.     

Raman spectral of low-lying longitudinal mode of KH2PO4 and KD2PO4 in ferroelectric phase

The origin of the low-lying modes of KH₂PO₄ and KD₂PO₄ in the ferroelectric phase has been clarified by the z(xy)z? Raman scattering experiments. The “S-mode”, which has been usually assigned to the proton tunneling mode in KH₂PO₄ at T<T_c, has been found in the z(xy)z? spectrum of KD₂PO₄ in contrast to the x(xy)y spectrum. It has been found that the frequency of the “S-mode” of KD₂PO₄ is higher than that of KH₂PO₄. These results have shown that the “S-mode” is far from the proton tunneling mode nor the proton/deuteron mode at all. From the present Raman spectroscopy, it is concluded that the “S-mode” is assigned to the libration mode of the PO₄ tertahedrons.     

Dynamics of muons in the ferroelectric material KDP

Muon spin relaxation in zero field and longitudinal field was measured in single crystal samples of KH₂PO₄ (KDP) and KD₂PO₄ (DKDP) over a temperature range of 5 K to 300 K. At low temperatures, diamagnetic muons and muon substituted radicals with nuclear hyperfine coupling can be observed. For both KDP and DKDP, a minor change was observed in the dynamics of the muon below 140 K. Above 140 K, the mobility of the muon appears to increase and the diffusion rate becomes faster with increasing temperature. Only a small increase in the relaxation rate is observed in KDP due to the presence of theH ⁺, suggesting that the relaxation effects probably originate from the³¹P.     

Frequency-dependent protonic conduction and high resolution 1H NMR of KDP around 298 K

Measurements of the dc conductivity, polarisation conductivity, and high resolution ¹H NMR have been carried out on KDP (KH₂PO₄) around 298 K. Comparisons between the different results are reported and an interpretation is proposed in order to clarify the protonic motion within the bulk material.