Deuterated Potassium Dihydrogen Phosphate Crystals Research Articles

Impact of longitudinal phase-matching variations on three-wave nonlinear interactions

A general study of three-wave nonlinear mixing in the presence of longitudinal variations in phase-matching conditions is presented. The efficiency of second-harmonic generation and optical parametric amplification is quantified using a normalized set of equations and a polynomial description of the wave-vector mismatch as a function of the longitudinal coordinate. These modeling results are used to estimate the impact of spatial variations in wave-vector mismatch experimentally obtained for five partially deuterated potassium dihydrogen phosphate crystals. The longitudinal inhomogeneities in the properties of crystals of similar quality are not expected to have a significant impact on their use for second-harmonic generation and optical parametric amplification, but the efficiency of nonlinear processes in crystals with larger variations could decrease.

Ultraviolet laser-induced damage characteristics of 70% deuterated potassium dihydrogen phosphate crystals

Bulk damage of deuterated potassium dihydrogen phosphate (DKDP, KD2PO4) crystal can be induced by intense laser irradiation, severely restricting the output of high- power laser energy and the usage of DKDP crystals in high-power laser systems. In this paper, laser-induced damage threshold (LIDT) and damage growth characteristics of DKDP crystal under 355 nm laser irradiation were systematically studied. The bulk laser-induced damage (LID) density of the crystal increased exponentially with the increase of the laser irradiation fluence. LID closely relates to the growth defects called precursors. Laser conditioning could effectively enhance the LID resistance by modifying the precursors. The LID density of DKDP crystals decreases after laser conditioning, and the increasing conditioning fluence can reduce the LID density. Damage growth after the initial LID by 1-on-1 measurement is different from that by R-on-1 measurement since the process of R-on-1 measurement leading to initial LID is equal to the laser conditioning procedure. The study in this paper provides a reference for improving the application of DKDP crystal in high-power laser systems.

Off-line sub-nanosecond laser conditioning on large aperture deuterated potassium dihydrogen phosphate crystal

The large aperture deuterated potassium dihydrogen phosphate (DKDP) is an important frequency conversion crystal in a large power laser device. There are many defects inside the DKDP bulk material, including the varying element impurities and electronic defects. Comparing with the defect-free material, these bulk defects can easily absorb incident laser energy and pose the risks of initiating damage sites when exposed to high-energy lasers. Beside bulk defects, there are surface defects originating from the DKDP machining process, including cracks, scratches and protuberances. These surface defects affect the damage performance of DKDP crystal by increasing light absorption and weakening local mechanical strength. Due to the defects from both bulk and surface, the actual damage threshold of DKDP crystal is much lower than the expected theoretical value. The lack of its laser damage resistance seriously restricts the laser output power. In this study, the off-line sub-nanosecond laser conditioning technology is proposed to effectively improve the laser damage performance of large aperture DKDP crystal. Its principle is to irradiate DKDP with a mild laser fluence in advance. Although the laser pretreatment cannot directly eliminate the impurities, dislocations, grain boundaries or other macro structural defects in crystals, it indeed changes the distribution and density of intrinsic point defects on a micro-scale. It suggests that the complicated reactions of electron-hole, atom-vacancy and the intrinsic point defect annihilation caused by the microstructural transformation of crystal materials under laser conditioning are the possible reasons for reducing absorption and improving the damage resistance. In this experiment, the result of the damage to high-power laser device shows that the mean surface damage density of DKDP crystal at 9 J/cm<sup>2</sup> decreases from 5.02 pp/cm<sup>2</sup> to 0.55 pp/cm<sup>2</sup> after sub-nanosecond laser conditioning. The laser conditioning can remove the protuberance defects on the surface, thus reducing the surface damage density. In addition, the damage size probably decreases after laser conditioning. There is a leftward shift in the damage size curve after laser conditioning, and its peak decreases from 25 μm to 18 μm–20 μm. In addition, due to the removal effect of laser conditioning on defects, the spatial distribution of damage points after sub-nanosecond laser irradiation turns more uniform. This study provides a foundation for the applications of off-line sub-nanosecond laser conditioning technology in high-power laser facility.

Investigation of piezoelectric ringing effects in deuterated potassium dihydrogen phosphate crystals

The piezoelectric ringing phenomenon for the Pockels cell with DKDP crystal is analyzed. The dependencies of contrast ratio of Pockels cell with the DKDP crystal on high-voltage pulse duration in the range from 100 ns up to 5 μs and on the pulse frequency in range from 50 Hz to 250 kHz are obtained and analyzed. Measurements that map the piezoelectric ringing amplitude distribution in the aperture of the DKDP crystal of the Pockels cell are performed as well.

Frequency-doubled FDML-MOPA laser in the visible.

Wavelength-swept lasers enable high-speed measurements in absorption spectroscopy, Raman spectroscopy, nonlinear Raman hyperspectral microscopy, rapid confocal microscopy, short impulse generation, and most importantly for high-speed optical coherence tomography, with speeds up to video-rate volumetric imaging. Recently, we introduced a pulsed wavelength-swept laser based on the Fourier domain mode-locked (FDML) laser principle combined with a master-oscillator power amplifier (MOPA) architecture. The high peak powers reached with this laser enabled rapid two-photon microscopy and two-photon fluorescence lifetime microscopy and high-speed light detection and ranging measurements. Here, we present the extension of this laser into the visible wavelength range by frequency doubling the output from 1064 nm to 532 nm via second harmonic generation in a deuterated potassium dihydrogen phosphate crystal. The result is a wavelength-swept laser source around 532 nm that outputs a pulse train of distinct, almost monochromatic wavelengths at an 88 MHz pulse repetition rate and 342 kHz sweep repetition rate. This swept-source laser in the visible can open up new research applications in spectroscopy, metrology, sensing, and high-speed imaging.

Large aperture and non-critical phase-matched fourth harmonic generation of Nd:Glass lasers

A fourth harmonic generation (FHG) scheme in focusing beams is proposed and demonstrated for large aperture Nd:glass laser facilities. By placing the focusing lens before the FHG crystal, the problem of ultraviolet damage can be overcome, largely without affecting FHG conversion efficiency owing to the large angular acceptance of the non-critical phase matching technique. A numerical simulation of the FHG process indicates that angular acceptance can be appropriately increased by lowering the working temperature and jointing the two adjacent compensating angles, so that FHG in focusing beams with relatively small F numbers becomes feasible. With a 170 mm × 170 mm × 7 mm and 65% deuterated potassium dihydrogen phosphate crystal mounted in a high-precision, temperature-controlled system, high-efficiency FHG has been demonstrated in the focusing beam with a full beam convergence angle of 36 mrad. When driven with a 223 J, second harmonic radiation (2ω), 1 ns flat-top pulse with a beam area of 130 cm2, corresponding to 1.7 GW/cm2 2ω input intensity, 182 J of fourth harmonic radiation (4ω) were generated.

Habit control of deuterated potassium dihydrogen phosphate crystal for laser applications

In this study we investigate the habit of partially deuterated potassium dihydrogen phosphate (DKDP) crystals in the presence of Al3+ ions. We have grown single DKDP crystals in (50wt% and 80wt%) partially deuterated solutions and in solutions doped with Al3+ ions (2 ppm) by the point-seed rapid growth technique at controlled supercooling (ΔT=10°C). The growth length of each crystal face was measured and the aspect ratio was calculated. We found that crystals grown in partially deuterated solutions are similar in aspect ratio, while, crystals grown in deuterated solutions doped with Al3+ ions showed a relative change in aspect ratio, the crystal increased in size in the pyramidal direction (vertical axis direction). Crystal characteristics were also analyzed by X-ray diffraction, FTIR and Raman spectroscopy. We have speculated that the relative habit modification is due to a probably adsorption and inclusions of Al3+ ions in the prismatic section of the crystal.

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.

Transient material properties during defect-assisted laser breakdown in deuterated potassium dihydrogen phosphate crystals

We investigate theoretically the transition from solid dielectric materials to warm solid density plasma during laser-induced breakdown in DKDP crystals (KD2PO4). Evidence taken from the experimentally measured wavelength dependence of the breakdown threshold suggests that the material excitation mechanisms mainly consist of a sequence of one-photon absorptions between short-lived vibronic defect states spanning the band gap with a quasi-continuum of states. The transition between excitation paths involving different number of photons yields information about the role of temperature in determining the width of the transition and corresponding threshold conduction band density prior to initiation of breakdown. This physical system is well adapted to study a plasma warming up at solid density leading to the so-called warm dense matter.

Raman Spectra of Deuterated Potassium Dihydrogen Phosphate Crystals with Different Degrees of Deuteration

The polarized Raman spectra of deuterated potassium dihydrogen phosphate crystals with different deuterium concentrations are measured. With the increasing deuterium concentration, the Raman peaks which are assigned as the internal vibrations of the (H/D)2PO−4 anion shift to lower wavenumbers. This red-shift contributes to the decrease in the bonding force of the P—O bond as a result of the substitution of deuterium for hydrogen. Moreover, the intensity of the strongest peak of these crystals decreases first, and reaches the minimum value while the mole percentage of the deuterium content in the crystal is about 74%. After that, the intensity increases with the increase of the deuterium concentration in the crystal.

Defect population variability in deuterated potassium di-hydrogen phosphate crystals

Bulk laser damage variability in deuterated potassium dihydrogen phosphate (DKDP) crystals is well known and makes online conditioning of multiple-beam laser systems difficult to optimize. By using an empirical model, called Absorption Distribution Model (ADM), we were able to map the damage variability of the crystals (boule to boule as well as within same boule) in terms of defect population variations. The defect population variation was found to coalesce into two distinct groupings that can be identified by the defect population in the late growth region of the boule. This result allows us to optimize the conditioning protocol for an arbitrary number of beams with crystals of differing damage quality.

Predicting laser-induced bulk damage and conditioning for deuterated potassium dihydrogen phosphate crystals using an absorption distribution model

We present an empirical model that describes the experimentally observed laser-induced bulk damage and conditioning behavior in deuterated potassium dihydrogen phosphate (DKDP) crystals. The model expands on an existing nanoabsorber precursor model and the multistep absorption mechanism to include two populations of absorbing defects, one with linear absorption and another with nonlinear absorption. We show that this model connects previously uncorrelated small-beam damage initiation probability data to large-beam damage density measurements over a range of nanosecond pulse widths. In addition, this work predicts the damage behavior of laser-conditioned DKDP.

Electro-Optic Characteristics of a Cooled Deuterated Potassium Dihydrogen Phosphate Crystal

The half-wave voltage of a deuterated potassium dihydrogen phosphate (DKDP) crystal was measured between 231 K and room temperature. It was found to decrease linearly with crystal temperature at a rate of 55.1 V/K. The lowest voltage of 511 V was observed at 231 K, which is one eighth of that at room temperature. The cooled DKDP crystal was discussed for use as an electro-optic modulator as it provides various advantages such as the reduced voltage of the power supply needed and the lower B-integral.

Raman scattering from deuterated potassium dihydrogen phosphate crystals

The normal vibration modes of deuterated potassium dihydrogen phosphate (DKDP) crystals were analyzed basing on the space group theory. The lattice vibration spectra of DKDP, which arise mainly from internal vibrations of P–O tetrahedrons, were recorded by a micro-Raman spectrophotometer system. The four characteristic internal vibration modes of the D 2PO 4 anion in KD 2PO 4 are assigned as 880 cm −1 ( υ 1 , asymmetrical P(OD) 2 stretching vibration), 356 cm −1 ( υ 2 , in-plane P(OD) 2 bending vibration), 512/540 cm −1 ( υ 3 , PO 2 bending vibration) and 967 cm −1 ( υ 4 , in-plane PO 2 symmetrical stretching vibration). Excellent agreement has been obtained between the theoretical studies and experimental investigations.

Formation of scatter particles in KDP (DKDP) crystals

Scatter particles in potassium dihydrogen phosphate crystal and deuterated potassium dihydrogen phosphate crystals have been observed and described by using transmission electron microscopy. The components were analyzed by electron probe microanalyzer. The formation mechanism of the scatter particles is proposed and discussed.

Ultraviolet-induced transient absorption in potassium dihydrogen phosphate and its influence on frequency conversion

The optical properties of an ultraviolet (266-nm) -induced transient absorption in normal and deuterated potassium dihydrogen phosphate crystals are discussed. Our main contribution has been to analyze this effect quantitatively, assess its effect on ultraviolet (UV) frequency-conversion applications, and examine sample variability. As observed previously [ Chem. Phys. Lett.207, 540 ( 1993)], the transient absorption is broad (~200–700 nm) and is suggested to arise from an induced defect state resulting from the promotion of an electron into the conduction band by two-photon absorption. At room temperature we find that the defect absorption decays over a wide variety of time scales, ranging from fractions of a second to several days, depending on the particular sample under study. The decay time does not appear to be correlated with deuteration or chemical impurities. The temperature-dependent decay rate has an Arrhenius behavior with a single activation energy of 0.51 ± 0.04 eV for all samples studied, and the decay rate appears to be mediated by a diffusional process as previously postulated. We determined the defect absorption cross section at its spectral peak to be 6 ± 3 × 10−18 cm2. Based on these and other measurements, we have numerically modeled previously published experimental data on fourth-harmonic frequency conversion of a Nd:YLF laser and found good agreement when the effects of transient absorption are included.

High-power wavelength converters with feedback

We have shown theoretically and experimentally that very high conversion efficiencies can be obtained in high-peak-power wavelength converters utilizing small amounts of feedback, at fluences similar to the no-feedback case. Maximum efficiencies are obtained with radially varying reflectivity profiles. We have demonstrated the concept, using second-harmonic generation in a deuterated potassium dihydrogen phosphate crystal in which 70% energy conversion was obtained, in agreement with simulation results. Energy conversion efficiencies of 56% were also obtained in a beta-barium borate optical parametric oscillator.

Tangentially phase matched second-harmonic generation in various crystals

Tangential phase matching is studied for second-harmonic generation. It occurs at any wavelength for certain values of noncollinearity in phase matching when it provides an increase in angular tolerance between the interacting beams. It is verified experimentally for barium borate and deuterated potassium dihydrogen phosphate crystals.