Investigation and analysis of pin-point damage and damage growth characteristics in KDP and DKDP crystals

In this paper, we report the damage and damage growth in potassium dihydrogen phosphate and its deuterated analog crystals. A time-resolved shadow imaging system was used to investigate the damage behavior in the bulk and on the rear surface. The damage images show differences in the damage sizes of the crystals with different deuterization rates. Theoretical simulations demonstrated that this may be due to differences in the crystallographic defects. The experimental results showed that the development of crystal damage was not only manifested as the expansion of damage on the rear surface of the crystal but also as an increase in pin-point density and size within the crystal. Crystals with higher deuterization rates had higher probability of the increasing of initial damage size, rather than the increasing of pin-point density.

Generation of flattop beams from a distorted optical field by the wavefront shaping technique

Uniform laser beams with controllable patterns are crucial for various applications, including laser processing and inertial confinement fusion. While some methods have been proposed to generate flattop beams, they often require complex optical systems that can become ineffective because of the misalignment of the system or the imperfection of optical elements. To overcome these issues, we utilized feedback-based wavefront shaping (FWS) technology to generate flattop beams with desired patterns from a disordered light. To solve the multi-goal optimization problem, we propose some modifications based on the Non-dominated Sorting Genetic Algorithm II (NSGA2) and successfully generate focal beams with a uniform intensity distribution and controllable beam shape from the disordered light field.

Density Functional Theory Study of the Point Defects on KDP (100) and (101) Surfaces

Surface defects are usually associated with the formation of other forms of expansion defects in crystals, which have an impact on the crystals' growth quality and optical properties. Thereby, the structure, stability, and electronic structure of the hydrogen and oxygen vacancy defects (V_H and V_O) on the (100) and (101) growth surfaces of KDP crystals were studied by using density functional theory. The effects of acidic and alkaline environments on the structure and properties of surface defects were also discussed. It has been found that the considered vacancy defects have different properties on the (100) and (101) surfaces, especially those that have been reported in the bulk KDP crystals. The (100) surface has a strong tolerance for surface V_H and V_O defects, while the V_O defect causes a large lattice relaxation on the (101) surface and introduces a deep defect level in the band gap, which damages the optical properties of KDP crystals. In addition, the results show that the acidic environment is conducive to the repair of the V_H defects on the surface and can eliminate the defect states introduced by the surface V_O defects, which is conducive to improving the quality of the crystal surface and reducing the defect density. Our study opens up a new way to understand the structure and properties of surface defects in KDP crystals, which are different from the bulk phase, and also provides a theoretical basis for experimentally regulating the surface defects in KDP crystals through an acidic environment.

Effect of Pre-Existing Micro-Defects on Cutting Force and Machined Surface Quality Involved in the Ball-End Milling Repairing of Flawed KDP Crystal Surfaces

When serving in extremely high-power laser conditions, KH₂PO₄ (KDP) surfaces are susceptible to incur laser damage points (also known as defects). Using micro-ball end milling cutters to repair and remove the pre-existing damage points on the flawed KDP crystal surface is the most effective method to control the growth of laser damage points on KDP crystal surfaces and prolong their service life. However, there are various forms of micro-defects (such as pits, scratches and brittle fractures) around the laser damage points on KDP crystal surfaces which possess remarkable effects on the micro-milling repair process and consequently deteriorate the repair quality. In this work, combined with nano-indentation experiments, elastic-plastic mechanics and fracture mechanics theory, a constitutive model considering the anisotropic property of KDP crystals and a three-dimensional (3D) finite element model (FEM) were established to simulate the cutting force and surface topography involved in the ball-end milling repairing of flawed KDP crystal surfaces. Besides, the micro-milling experiments were conducted to evaluate the change of cutting force and machined surface quality in the presence of micro-defects with various feed rates. The results show that micro-defects would induce the fluctuation of cutting force and a change of the undeformed cutting thickness (UCT) in the process of repairing the damage points on the crystal surface, which would lead to the brittle-ductile transition (BDT) and affect the machined surface quality. The machined surface quality was found to be deteriorated by the pre-existing micro-defects when the UCT was small (the UCT was less than 375 nm). On the contrary, brittle mode cutting in the local area can be transformed into ductile mode cutting, resulting in an improvement of repaired surface quality that is exhibited by the cutting force and microtopography. This work has great theoretical significance and engineering practical value for the promotion and application of micro-milling repairing technology in the practical manufacturing and operation of KDP optics applied to high-power laser systems.

Determination of intrinsic defects of functional KDP crystals with flawed surfaces and their effect on the optical properties

A practically nonlinear optical material, the functional KH₂PO₄ (KDP) crystal, has an extremely low laser-induced damage threshold (LIDT) due to manufacturing-induced surface defects. The low LIDT induced by these surface defects of KDP crystals hinders their application in laser fusion facilities. Herein, the effect of intrinsic point defects introduced by manufacturing-induced lateral cracks on laser damage was particularly investigated by a combination of spectral detection (i.e., photoluminescence, Raman and Fourier transformed infrared spectra) and first-principles calculation. It was determined that the manufacturing-induced lateral cracks would introduce more hydrogen vacancy and oxygen vacancy intrinsic defects than the ideal surface. These intrinsic defects would form cluster-type defects, lowering the LIDT to 6.600 J cm^-2 by introducing two defect levels of 2.83 eV and 4.89 eV within the band gap of the KDP crystal. To further investigate the effect of intrinsic defects introduced by lateral cracks on laser-induced damage growth, the transformation of charge states of intrinsic defects under laser irradiation was calculated. Combined with the spectral information of the lateral crack-induced laser damage site and the laser damage growth experiments, it was found that the intrinsic defects of lateral cracks have a large amount of evolution, making the crystal prone to severe damage growth. Overall, the avoidance of such lateral cracks with a large number of intrinsic defects is very beneficial for improving the laser damage resistance of KDP crystals.

Evolution of laser-induced bulk damage in KDP crystal

Pinpoint damage is the main type of bulk damage in potassium dihydrogen phosphate (KDP) crystals in high-power lasers. Using time-resolved microimaging, we observed the complete dynamic evolution of pinpoint damage in a KDP crystal. We analyzed changes in the patterns of dark zones formed by decreasing probe transmittance in transient images throughout the process. The mechanical properties of stress waves in KDP crystals were further studied by a depolarized shadowgraph experiment and theoretical simulation. The dynamic evolution of mechanical stress waves was observed, and the correlation between mechanical failure due to stress waves and the static characteristic damage morphology was established.

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.

Study on defect-induced damage behaviors of ADP crystals by 355 nm pulsed laser

High-quality ammonium dihydrogen phosphate (NH₄H₂PO₄, ADP) crystals were grown in Z direction and in defined crystallographic direction (θ=90°, φ=45°) by the rapid growth method, respectively. Defect-induced damage behavior in 355 nm of three types of ADP samples cutting in type-II matching and third harmonic generation direction from the as-grown crystals were investigated, including the initial laser induced damage (LID) characteristics and the physical and chemical properties of defects which serve as the damage precursors. The evaluations of damage behaviors include the "sampling" laser induced damage threshold (LIDT) by 1-on-1 and R-on-1 methods, bulk damage growth and bulk damage morphology. UV-visible transmittance spectrum, ultraviolet absorption spectrum, fluorescence spectrum, positron annihilation spectrum and the online light scattering measurements were carried out to investigate the defect-induced damage behavior in ADP crystals. The study will provide a reference for the investigations on laser induced damage properties of ADP crystals in short wavelength.