Shaohua Ji

Non-critical phase-matching fourth harmonic generation of a 1053-nm laser in an ADP crystal

In current inertial confinement fusion (ICF) facilities, KDP and DKDP crystals are the second harmonic generation (SHG) and third harmonic generation (THG) materials for the Nd:glass laser (1053 nm). Based on the trend for the development of short wavelengths for ICF driving lasers, technical solutions for fourth harmonic generation (FHG) will undoubtedly attract more and more attention. In this paper, the rapid growth of an ADP crystal and non-critical phase-matching (NCPM) FHG of a 1053-nm laser using an ADP crystal are reported. The NCPM temperature is 33.7°C. The conversion efficiency from 526 to 263 nm is 70%, and the angular acceptance range is 55.4 mrad; these results are superior to those for the DKDP crystals. This research has shown that ADP crystals will be a competitive candidate in future ICF facilities when the utilisation of high-energy, high-efficiency UV lasers at wavelengths shorter than the present 351 nm is of interest.

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.

Room temperature, high-efficiency, noncritical phase-matching fourth harmonic generation in partially deuterated ADP crystal

High-efficiency frequency conversion into the deep ultraviolet in a partially deuterated ammonium dihydrogen phosphate (ADP) crystal is reported. For a 60% deuterated ADP crystal, the spontaneous Raman scattering intensity decreased by 38% compared with that of a pristine ADP crystal. At room temperature (24°C), noncritical phase-matching fourth harmonic generation of a 1053 nm laser was realized. The angular acceptance was 55.7 mrad, and the external conversion efficiency from 526 to 263 nm reached 85.3%.

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.

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.

Homogeneity of rapid grown DKDP crystal

For rapid grown DKDP crystals, homogeneity should be paid more attention than traditional grown ones because their deuterated levels will be affected more significantly by the large supersaturation, which give harmful impact on optical property as well as the related nonlinear optical applications. In this paper, from the points of deuterated level and transmittance, the homogeneity of a point-seed rapid grown DKDP crystal with sizes of 65 × 65 × 113 mm3 is investigated. The results show that the maximum discrepancy of deuterium content is 5.4%, and the ultra-violet transmittances of the dislocation zone and its surrounding areas are ~30% inferior to other regions.

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.

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.

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 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.