Investigation of the electronic and physical properties of defect structures responsible for laser-induced damage in DKDP crystals

Improving fourth harmonic generation performance by elevating the operation temperature of ADP crystal

In current inertial confinement fusion (ICF) facilities, potassium dihydrogen phosphate (KH2PO4, KDP) type crystals are the only nonlinear optical (NLO) materials that can satisfy the aperture requirement of the ICF laser driver. Ammonium dihydrogen phosphate (NH4H2PO4, ADP) crystal is a typical isomer of KDP crystal, with a large nonlinear optical coefficient, high ultraviolet transmittance, and large growth sizes, which is an important deep ultraviolet (UV) NLO material. In this paper, we investigated the effect of ADP temperature on its fourth-harmonic-generation (FHG) performance. When the temperature of the ADP crystal was elevated to 48.9 °C, the 90° phase-matched FHG of the 1064 nm laser was realized. Compared with the 79° phase-matched FHG at room temperature (23.0 °C), the output energy at 266 nm, conversion efficiency, angular acceptance, and laser-induced damage threshold (LIDT) increased 113%, 71%, 623%, 19.6%, respectively. It shows that elevating ADP temperature is an efficient method to improve its deep UV frequency conversion properties, which may also be available to other NLO crystals. This discovery provides a very valuable technology for the future development of UV, deep UV lasers in ICF facilities.

Unveiling sub-bandgap energy-level structures on machined optical surfaces based on weak photo-luminescence

Sub-bandgap defect energy levels (SDELs) introduced by the point defects located in surface defect areas are considered the main factors in decreasing laser-induced damage thresholds (LIDTs). The suppression of SDELs could greatly increase LIDTs. However, no available method could detect SDELs, limiting the characterization and suppression of SDELs. Herein, a self-designed photo-luminescence detection system is developed to explore the weak transient-steady photo-luminescence properties of machined surfaces. Based on the excitation laser wavelength dependence of photo-luminescence properties, a sub-bandgap energy-level structure (SELS) containing SDELs is unveiled for the first time. Based on the developed mathematical model for predicting LIDTs, the feasibility of the detection method was verified. In summary, this work provides a novel approach to characterize SDELs on machined surfaces. This work could construct electronic structures and explore the transition behaviors of electrons, which is vital to laser-induced damage. Besides, this work could predict the LIDTs of the machined surfaces based on their PL properties, which provides convenience for evaluating the LIDTs of various optical elements in industrial production. Moreover, this work provides a convenient method for raising the LIDTs of various optical elements through monitoring and suppressing the SDELs on machined surfaces.

Formation mechanism of DKDP surface in single point diamond fly-cutting process and the resulting degradation of laser-induced damage performance

Laser damage performance of DKDP (KD_2xH_2(1-x)PO₄) crystal is largely determined by the surface microstructures generated in the manufacturing process, more specifically, single point diamond fly-cutting process. However, because of the lack of knowledge about the formation mechanism and damage performance of the microstructures, laser induced damage of DKDP crystal remains a key issue limiting the output energy of the high power laser systems. In this paper, the influence of fly-cutting parameters on the generation of DKDP surface and the underlying material deformation mechanism have been investigated. Except for cracks, two kinds of new microstructures, namely micro grains and ripples, have been found on the processed DKDP surfaces. GIXRD, nano-indentation and nano-scratch test results prove that the micro grains are generated by the slip motion of the crystal, while the simulation results show that the cracks are induced by the tensile stress formed behind the cutting edge. Moreover, the formation of micro grains can facilitate the plastic chip flow through the mechanism of grain boundary sliding, which will further lead to a periodic fluctuation of the chip separation point and the formation of micro ripples. Finally, laser damage test results demonstrate that cracks will degrade the damage performance of DKDP surface significantly, while the formation of micro grains and micro ripples has little impact. The results of this study can deepen the understanding of the formation mechanism of the DKDP surface during the cutting process and provide guidance to improve the laser-induced damage performance of the crystal.

Evolution of the point defects involved under the action of mechanical forces on mechanically machined fused silica surfaces

Point defects with different species are concentrated on most mechanically machined fused silica optical surfaces with surface defects, which would sharply decrease the laser damage resistance under intense laser irradiation. Various point defects have distinct roles in affecting the laser damage resistance. Especially, the proportions of various point defects have not been identified, posing the challenge in relating the intrinsic quantitative relationship among various point defects. To fully reveal the comprehensive effect of various point defects, it is necessary to systematically explore the origins, evolution laws and especially the quantitative relationship among point defects. Herein, seven types of point defects are determined. The unbonded electrons in point defects are found to tend to be ionized to induce laser damage and there is a definite quantitative relationship between the proportions of oxygen-deficient point defects and that of peroxide point defects. The conclusions are further verified based on the photoluminescence (PL) emission spectra and the properties (e.g., reaction rule and structural feature) of the point defects. On basis of the fitted Gaussian components and electronic-transition theory, the quantitative relationship between PL and the proportions of various point defects is constructed for the first time. E'-Center accounts for the highest proportion among them. This work is beneficial for fully revealing the comprehensive action mechanisms of various point defects and providing new insights in elucidating the defect-induced laser damage mechanisms of optical components under intense laser irradiation from the atomic scale.

Progress on deuterated potassium dihydrogen phosphate (DKDP) crystals for high power laser system application

In this review, we introduce the progress in the growth of large-aperture DKDP crystals and some aspects of crystal quality including determination of deuterium content, homogeneity of deuterium distribution, residual strains, nonlinear absorption, and laser-induced damage resistance for its application in high power laser system. Large-aperture high-quality DKDP crystal with deuteration level of 70% has been successfully grown by the traditional method, which can fabricate the large single-crystal optics with the size exceeding 400 mm. Neutron diffraction technique is an efficient method to research the deuterium content and 3D residual strains in single crystals. More efforts have been paid in the processes of purity of raw materials, continuous filtration technology, thermal annealing and laser conditioning for increasing the laser-induced damage threshold (LIDT) and these processes enable the currently grown crystals to meet the specifications of the laser system for inertial confinement fusion (ICF), although the laser damage mechanism and laser conditioning mechanism are still not well understood. The advancements on growth of large-aperture high-quality DKDP crystal would support the development of ICF in China.

Novel abrasive-free jet polishing for Bulk single-crystal KDP with a low viscosity microemulsion

In present work, the abrasive-free jet polishing (AFJP) of bulk single-crystal KDP was first fulfilled, when using a newly-designed low-viscosity microemulsion as the AFJP fluid. The novel AFJP fluid shows a typical water-in-oil structure, in which the water cores uniformly distribute in the BmimPF6 IL, with a particle size of about 20-25 nm. What's more, the AFJP fluid is a controllable and selective non-abrasive jet fluid that the shape of the removal function is regular and smooth, presenting a similar Gaussian function, meanwhile, the dispersion coefficient of the removal rate is only 1.9%. Finally, the surface quality of the bulk single-crystal KDP is further improved by AFJP, meanwhile, the subsurface damage is first obviously mitigated.

Laser-Induced Damage Threshold of Single Crystal ZnGeP2 at 2.1 µm: The Effect of Crystal Lattice Quality at Various Pulse Widths and Repetition Rates

The ZnGeP2 crystal is a material of choice for powerful mid-IR optical parametric oscillators and amplifiers. In this paper, we present the experimental analysis of the optical damage threshold of ZnGeP2 nonlinear crystals induced by a repetitively-pulsed Ho3+:YAG laser at 2091 nm. Two types of ZnGeP2 crystals grown under different conditions were examined using the laser and holographic techniques. The laser-induced damage threshold (LIDT) determined by the pulse fluence or peak intensity was studied as a function of the pulse repetition rate (PRR) and laser exposure duration. The main crystal structure factor for a higher LIDT was found to be a reduced dislocation density of crystal lattice. The ZnGeP2 nonlinear crystals characterized by the high structural perfection with low density of dislocations and free from twinning and stacking faults were measured to have a 3.5 J/cm2 pulse fluence damage threshold and 10.5 MW/cm2 peak intensity damage threshold at 12 kHz PRR; at 40 kHz PRR the pulse fluence damage threshold increased to over 6 J/cm2, but the peak intensity damage threshold dropped to 5.5 MW/cm2.

Optimizing sub-nanosecond laser conditioning of DKDP crystals by varying the temporal shape of the pulse

We propose a strategy to optimize the laser conditioning of DKDP crystals by varying the temporal shape of sub-nanosecond pulses. Four sub-ns temporally shaped pulses with nearly the same full width at half maxima of ∼600 ps but different rising-falling statuses were designed to conduct laser-induced damage (LID) and laser conditioning experiments on DKDP crystals. The shape of the pulse substantially influences the damage pinpoints size and LID threshold (LIDT) of the crystals in the sub-nanosecond range. After sub-nanosecond laser conditioning, the ns R-on-1 LIDT showed that slow-rising fast-falling pulse (R400-F200 and High-foot pulses) conditioning achieved a 14%-20% LIDT enhancement than the traditional Gaussian pulse (R300-F300 pulse). The 8-ns laser damage morphologies after slow-rising fast-falling pulse conditioning showed cracks, whereas those after fast-rising slow-falling pulse (R200-F400 pulse) conditioning were pinpoint core, as usual. These results suggest that the rising front plays an important role in the LID and laser conditioning of the DKDP crystals. A pulse with a slower rising front is beneficial for thermal modification, thereby leading to better LID properties. This strategy greatly expands and enriches the manipulation methods to improve the LIDT of DKDP crystals, and sheds light on understanding the laser damage mechanisms.

Millimeter-Scale Zn(3-ptz)2 Metal-Organic Framework Single Crystals: Self-Assembly Mechanism and Growth Kinetics

The solvothermal synthesis of metal-organic frameworks (MOFs) often proceeds through competing crystallization pathways, and only partial control over the crystal nucleation and growth rates is possible. It challenges the use of MOFs as functional devices in free-space optics, where bulk single crystals of millimeter dimensions and high optical quality are needed. We develop a synthetic protocol to control the solvothermal growth of the MOF [Zn(3-ptz)2] n (MIRO-101), to obtain large single crystals with projected surface areas of up to 25 mm2 in 24 h, in a single reaction with in situ ligand formation. No additional cooling and growth steps are necessary. We propose a viable reaction mechanism for the formation of MIRO-101 crystals under acidic conditions, by isolating intermediate crystal structures that directly connect with the target MOF and reversibly interconverting between them. We also study the nucleation and growth kinetics of MIRO-101 using ex situ crystal image analysis. The synthesis parameters that control the size and morphology of our target MOF crystal are discussed. Our work deepens our understanding of MOF growth processes in solution and demonstrates the possibility of building MOF-based devices for future applications in optics.

First-principles studies on optical absorption of [010] screw dislocation in KDP crystals

The stress caused by the [010] dislocation in KDP deforms the crystal structure, introduces extra optical absorption and narrows the band gap.

Recent Advances in Laser-Induced Surface Damage of KH2PO4 Crystal

As a hard and brittle material, KDP crystal is easily damaged by the irradiation of laser in a laser-driven inertial confinement fusion device due to various factors, which will also affect the quality of subsequent incident laser. Thus, the mechanism of laser-induced damage is essentially helpful for increasing the laser-induced damage threshold and the value of optical crystal elements. The intrinsic damage mechanism of crystal materials under laser irradiation of different pulse duration is reviewed in detail. The process from the initiation to finalization of laser-induced damage has been divided into three stages (i.e., energy deposition, damage initiation, and damage forming) to ensure the understanding of laser-induced damage mechanism. It is clear that defects have a great impact on damage under short-pulse laser irradiation. The burst damage accounts for the majority of whole damage morphology, while the melting pit are more likely to appear under high-fluence laser. The three stages of damage are complementary and the multi-physics coupling technology needs to be fully applied to ensure the intuitive prediction of damage thresholds for various initial forms of KDP crystals. The improved laser-induced damage threshold prediction can provide support for improving the resistance of materials to various types of laser-induced damage.

Self-trapped holes (small polarons) in ferroelectric KH2PO4 crystals

Density functional theory is used to establish the ground-state structure of the self-trapped hole (STH) in KH₂PO₄ crystals. The STHs in this nonlinear optical material are free small polarons, a fundamental intrinsic point defect. They are produced with ionizing radiation in the low-temperature orthorhombic structure of KH₂PO₄ and are only stable (i.e. long-lived) below approximately 70 K. A large 129-atom cluster, K₁₉H₄₀P₁₄O₅₆, is constructed to model the STH. The ωB97XD functional with the 6-31+G^* basis set is used and geometry optimization is performed. Our results show that two of the oxygen ions in a PO₄ unit relax toward each other and equally share the hole. These two oxygen ions do not initially have close hydrogen neighbors. This equal sharing of the hole is related to the presence of isolated, slightly distorted, PO₄ units and is significantly different from the small-polaron behavior often observed in other oxide crystals where the hole is localized on only one oxygen ion. The computational results provide a detailed description of the lattice relaxation occurring during formation of the STH. Characteristic spectral features of this defect are a larger hyperfine interaction with one ³¹P nucleus and equal, but smaller, hyperfine interactions with two ¹H nuclei. The computed values for these isotropic and anisotropic hyperfine coupling constants are in excellent agreement with results obtained from electron paramagnetic resonance experiments.

Simulation of the nanosecond-pulse laser damage of KDP surface by the smoothed particle hydrodynamics method

We present a simulation method to reproduce the damage crater formation and particle ejection phenomena observed in the laser-induced surface damage process of potassium dihydrogen phosphate (KDP) crystals. Based on the smoothed particle hydrodynamics method, which is commonly used for solving shock and blast problems, equivalent explosion simulation models of the laser-induced damage process have been established. Moreover, laser damage experiments combined with time-resolved techniques are performed on KDP surfaces to investigate the impact of laser fluences on the shockwave propagation and the particle ejection speed. We find that the simulation models can predict the laser-induced damage behaviors of the KDP crystal, which verifies the validity of the proposed method.

Investigation of the laser-induced surface damage of KDP crystal by explosion simulation

Under nanosecond pulse irradiation, laser-induced damage of Potassium Dihydrogen Phosphate (KDP) crystal is a multi-physical coupling process which mainly includes energy absorption by precursor defects, temperature and pressure rise in the absorption center, and subsequent micro-explosion event. Till now, related research work mainly focuses on modeling the energy absorption stage and determining the temperature or pressure in the absorption center, but knowledge about the explosion stage is rather limited. In this paper, laser-induced damage of KDP crystal has been investigated through explosion simulation. According to the laser damage test results and morphologies of the damage craters, typical precursor defects inducing KDP surface damage have been determined. Based on the knowledge, equivalent explosion simulation models of the laser damage process have been established to reproduce damage crater formation and shockwave propagation. Finally, laser damage experiments, combined with time resolved techniques, have been utilized to investigate the variation of damage crater size and shockwave speed with laser fluences. Simulation results given by single core explosion models agree well with the experimental results at fluences lower than 60 J/cm², while a multicore explosion model is needed to reliably simulate damage crater formation at higher fluences.

Theoretical modeling and analysis of material removal characteristics for KDP crystal in abrasive-free jet processing

Traditional KDP polishing methods, such as magnetorheological finishing (MRF), ion-beam figuring (IBF), and chemical mechanical polishing (CMP), are limited by either hard-to-remove residual particles, unavoidable heating effect, or applicability that is restricted to large-sized KDP. The abrasive-free jet polishing (AFJP) is regarded as a promising polishing method that can circumvent the above issues. KDP AFJP makes use of a thermodynamically and kinetically stable ionic liquid (IL) microemulsion that contains nanometer range water droplets evenly dispersed in the non-aqueous carrier liquid. The sprayed out nanoscale water droplets can remove material through dissolution. In this paper, the normal impinging of a nanoscale water droplet on the KDP surface is investigated. And then a materials removal model is proposed for water droplets. This model considers two major modes, namely deformation of a water droplet in compressing and deformation restoring of a water droplet in slipping process. Finally, KDP AFJP spot experiments were then conducted to validate the model veracity. The proposed model fits well with the simulation and experimental results which further suggest KDP AFJP's feasibility. This proposed model provides a good explanation for KDP AFJP's removal mechanism.

Influences of surface defects on the laser-induced damage performances of KDP crystal

When potassium dihydrogen phosphate (KDP) crystals are exposed to high-energy laser irradiation, the pre-existing surface defects may act as damage precursors and will reduce the lifespan of the crystal components. Although it has been found that different kinds of surface defects exhibit distinct damage characteristics, the influence of surface defects on the laser-induced damage performance of KDP crystal is not yet clear. In this paper, KDP surface defects have been characterized by multiple measuring methods and classified into five categories according to their structure features. Laser-induced damage tests were then carried out to investigate the laser-induced damage thresholds of different kinds of KDP surface defects as well as the evolution of the morphology of damage sites. The results of the experiment indicate that the damage thresholds of cracks, fracture pits, and surface protuberances are between 6 and 11  J/cm² (355 nm, 3 ns, similarly hereinafter), which are much lower than the thresholds of plastic scratches, discontinuous scratches, and a defect-free KDP surface. In addition, it has been found that fluorescence enhancement is just a necessary condition for reduction of damage thresholds. Finally, reasons for the formation of the most threatening KDP surface defects have been analyzed and corresponding suppression measures have been proposed for increasing the surface damage thresholds of the crystal components.

Effect of annealing on nonlinear optical properties of 70% deuterated DKDP crystals at 355 nm

Thermal annealing decreased the nonlinear absorption and refraction, which could lead to an improvement of the laser-induced damage threshold.

Laser irradiation precipitation from nonlinear optical KH2PO4 crystal

Structural stability of KH₂PO₄ (KDP) crystal under laser irradiation is a great challenge to nonlinear optical devices. Herein, the dependency of structural stability on the propagated laser wavelength was established. Separated precipitations with different morphologies, including irregular fusion, elongated prism, and quasi-equiaxial particles were irradiated from the KDP crystal surface by a laser with different wavelengths, including 0.103, 355, and 785 nm. All the precipitations induced by laser irradiation were identified to be the same KDP phase as its parent phase examined by electron diffraction. Noticeably, the stacked periodicity degree at (001) planes of precipitations becomes irregular somehow. It is believed that our research findings might have new implications and inspirations in constructing KDP devices with better stability.

Laser damage dependence on the size and concentration of precursor defects in KDP crystals: view through differently sized filter pores

We investigate the laser-induced damage performance at 1064 nm of potassium dihydrogen phosphate (KDP) crystals grown using filters of different pore sizes. The aim is to explore a novel method for understanding laser-matter interactions with regard to physical parameters affecting the ability of damage precursors to initiate damage. By reducing the pore size of filters in continuous filtration growth, we can improve laser damage resistance. Furthermore, we develop a model based on a Gaussian distribution of precursor thresholds and heat transfer to obtain a size distribution of the precursor defects. Smaller size and/or lower concentration of precursor defects could lead to better damage resistance.

Mitigation of scattering defect and absorption of DKDP crystals by laser conditioning

The variation of scattering and absorption in DKDP crystals by laser conditioning was investigated by combining light scattering technique and on-site transmittance measurement technique. Laser-induced disappearance of scattering defects was observed, and variation of transmittance was achieved. Using Mie theory, a kind of absorbing defects, aside from scattering defect, was discovered. Moreover, the experimental results demonstrated that the absorption of crystal could be mitigated by laser conditioning.

Suppression of transverse stimulated Raman scattering with laser-induced damage array in a large-aperture potassium dihydrogen phosphate crystal

A laser-induced damage array composed of numerous pinpoints is generated in a large-aperture KDP crystal to suppress the transverse stimulated Raman scattering (TSRS). The 36 cm × 8.5 mm × 7 mm damage array is used to block the propagation of the TSRS photons within the crystal and decrease the TSRS gain length. Then several series of experiments were conducted on a large-aperture laser system to test this method and experimental results show that the amplification of TSRS is significantly suppressed by the laser-induced damage array.

Fast mapping of absorbing defects in optical materials by full-field photothermal reflectance microscopy

We report a technique for rapidly mapping absorbing defects in optical materials, which act as laser-induced damage precursors, based on full-field photothermal reflectance microscopy. An intensity-modulated pump beam heats absorbing defects in the optical sample, creating localized, modulated refractive-index variations around the defects. A probe beam then illuminates the defect sites, and the measured amplitude of the reflectance variation is used to map the distribution of defects in the medium. Measurements show that this method offers a faster defect mapping speed of about 0.03 mm(2) per minute and a detectivity of a few tens of nanometers comparable to that of conventional scanning photothermal deflection microscopy.

Interaction of intense femtosecond laser pulses with KDP and DKDP crystals in the short wavelength regime

We investigate the electronic photo-excitation and relaxation mechanisms involved in the optical breakdown of potassium dihydrogen phosphate crystal (KH2PO4) and its deuterated form. The dynamics and spectroscopic properties of electron-hole pair formation are investigated using time-resolved measurement of the dielectric function, and luminescence spectroscopy. The non-common mechanical and electronic characteristics of these dielectric materials are revealed by the particular structure of ablation craters and also by the complex dynamics observed in the relaxation of excited carriers. This relaxation occurs in two steps, and varies with the initial carrier density and thus with the laser intensity. We show that the defect states play a key role in the excitation pathways, and also determine the relaxation stage. The latter also depends upon the initial amount of energy of the electron-hole pair after photo-excitation. A model based on kinetic equations describing the evolution of the different level populations allows us to successfully interpret and reproduce the experimental data.

Dynamics of transient absorption in bulk DKDP crystals following laser energy deposition

The transient changes in the optical properties of bulk DKDP material arising from its exposure to high temperatures and pressures associated with localized laser energy deposition are investigated. Two methods for initiation of laser-induced breakdown are used, intrinsic, involving relatively large energy deposition brought about by focusing of the laser beam to high intensities, and extrinsic, arising from more localized deposition due to the presence of pre-existing absorbing damage initiating defects. Each method leads to a very different volume of material being affected, which provides for different material thermal relaxation times to help better understand the processes involved.