Baoan Liu

Non-critical phase-matching conditions for fourth harmonic generation of DKDP crystal

The non-critical phase matching (PM) conditions of deuterated dihydrogen phosphate (DKDP) crystal are investigated in this paper for fourth harmonic generation (FHG) of Nd:YAG crystal and Nd:glass lasers, which include exterior angle deviation, deuterium content and PM temperature. DKDP crystals of different deuterium content were grown from aqueous solutions and cut for type-I non-critical phase-matching with the direction at 90° to the crystal Z axis (θ = 90°) and at 45° to the crystal X axis (Ф = 45°). The samples are measured for FHG at 1064 nm and 1053 nm, respectively. The deuterium content of DKDP crystal that supports non-critical phase-matching for FHG experiments is confirmed by measuring the relationship of deuterium content and phase-matching angle. Additionally, the non-critical phase matching temperature is also examined.

Effect of supersaturation on hillock of directional Growth of KDP crystals

KDP single crystals were grown in aqueous solution by using “point seeds” with a defined crystallographic direction of 59° to the Z axis. When hillock slopes on the (100) face of KDP crystals were measured within the supersaturation (σ) range of 0 < σ ≤ 0.06, the slope of hillocks with hollow cores depended nonlinearly on supersaturation. Below σ = 0.02, the hillock slope depended on supersaturation, but when σ was ≥ 0.02, the hillock slope increased more gradually and was less dependent on supersaturation. Hollow funnel-shaped growth dislocation on the (100) face of KDP crystals was observed at σ = 0.04, characterized by large holes with micro-steps and step bunching inside, the formation of which were analyzed. The result verified that the reversed growth appears to occur within hollow channels found on growth hillocks.

Effect of raw material and growth method on optical properties of DKDP crystal

Three kinds of KH2PO4 raw material are used to grow deuterated potassium dihydrogen phosphate (DKDP) crystals by traditional and rapid growth methods, respectively. The growth habit dependence on the purity of raw material is described and analyzed. The optical properties including transmission spectra and laser-induced damage threshold of these crystals have been measured. It is found that the growth method affects the optical properties of crystal more obviously than the raw material with the mass content of main metal ions below 1 ppm. Moreover, the morphology of the core in the observed damage sites indicates that an explosion process probably occurs during laser-induced breakdown.

Refractive indices in the whole transmission range of partially deuterated KDP crystals

Refractive indices of partially deuterated potassium dihydrogen phosphate (DKDP) crystals with 55%, 70% and 80% deuterium contents were measured by auto-collimation method at 293 K between 0.254 to 1.529 μm. Dependence of refractive indices of DKDP on deuterium content show different trend in the infrared region as in uv-visible region. Dependence of n2 (the square of refractive index) on the mole fraction of deuterium shows a difference between pure KDP and partially deuterated KDP. The Sellmeier equations were obtained by the least square method. The non-critical phase matching angles calculated from the fitted formula were in good agreement with laser experiment results, by which the reliability of these Sellmeier equations was confirmed.

Growth and electro‐elastic properties of K(H1−xDx)2PO4 single crystals

A series of K(H1−xDx)2PO4 crystals were grown from deuterated aqueous solution by using a point-seed rapid-growth method. The elastic, piezoelectric and dielectric constants of these crystals were determined using the resonance technique and impedance analysis, respectively. The elastic compliance constant s11 measured in this experiment is larger than that in previous experiments, and the reliability of the results presented herein has been verified. These results show that, with increasing content of deuterium, s11, s44, s66, d36, e36 and k36 are gradually increased, whereas s12, s13, s33, d14, e14 and k14 have no demonstrable variation. The reasons for the composition dependence of these constants were analyzed based on the variation of the crystal structures resolved by single-crystal X-ray diffraction.

New annealing method to improve KD2PO4 crystal quality: learning from high temperature phase transition

The existence of a high temperature phase transition in KD2PO4 was confirmed by thermal analysis, dielectric analysis and Raman spectroscopy. The results showed that KD2PO4 presented only one high temperature phase transition at about Ttr = 430 K, which is much lower than the thermal decomposition temperature. The absence of Raman shift attributable to the dehydration product indicated that the HTPT had nothing to do with thermal decomposition. The structural change during the HTPT was discussed. The strong similarity between the HT-phase and tetragonal phase decreases the temperature of the HTPT in the KD2PO4 crystal. The hydrogen–deuterium exchange can be suppressed under a protective ambient environment, which helps improve the annealing temperature. A new annealing method was developed by using silicone oil as a protective ambient environment under higher temperature up to 403 K, thus improving the quality of KD2PO4 crystals.

Study on the properties of the DKDP crystal with different deuterium content

In this paper, DKDP crystals with deuterium content of 55%, 65% and 70% were grown by the traditional temperature-reduction method. The property including lattice constant, transmittance spectra, Raman spectra, conoscopic image, and laser damage threshold (LDT) were measured. The results showed that with increasing the degrees of deuteration for DKDP crystals, the value of lattice parameter a (b) increases and the transmittance also increases in the range of 350-1800 nm while the laser damage threshold (LDT) decrease. For DKDP crystals, the spectra of Raman scattering split into two parts and the peak intensity of the totally symmetric PO4 vibration decrease compared with that of KDP crystals. The homogeneity of the DKDP crystals has not obvious difference.

Study of the degree of deuteration on the microhardness of deuterated potassium dihydrogen phosphate crystals

A series of deuterated potassium dihydrogen phosphate (DKDP) crystals with different degrees of deuteration are grown from aqueous solution by the point-seed technique. The microhardness of (100), (001) and so-called ‘tripler’ faces for these DKDP crystals were measured. Initially the hardness number of (001) face for each crystal increases with the increase of the applied load until it reaches 25 g. With further increase in load, the hardness number decreases gradually. The hardness numbers decline with the increase in deuterium content. These composition dependences are expected since the bond strength is weakened by the substitution of deuterium for hydrogen. The hydrogen bond is considered to play the key role in effecting the crystal’s hardness. The visible hardness anisotropy of the different faces is attributed to the inhomogeneous distribution of the oxygen–hydrogen bond on these faces.

Study on fourth harmonic generation of DKDP crystal

This paper shows DKDP crystals of different deuterium content are cut for type I non-critical phase-matching with the direction at 90° to the crystal Z axis (θ=90°) and at 45° to the crystal X axis (Φ=45o), and are measured at the FHG experiments with fundamental frequency wavelength 1064nm and 1053nm, respectively. The optimum deuterium content of DKDP crystal, that can achieve non-critical phase-matching at FHG experiments, is confirmed by measuring the relationship of deuterium content and phase-matching angle.

Effect of Ba in KDP crystal on the wavelength dependence of laser-induced damage

Ba impurity in potassium dihydrogen phosphate (KDP) is studied with the first-principle simulation method. The relaxed configurations and density of the states of KDP crystal with Ba impurity are calculated. We find that Ba can generate a K vacancy and an interstitial O-H unit for charge compensation. The band gap of KDP crystal narrowed down to about 3.9 eV, which is consistent with the experimental data from previously reported studies and indicates that Ba may be a source of low-damage threshold.