Recent Advances in Laser-Induced Surface Damage of KH2PO4 Crystal

applied sciences

Review Recent Advances in Laser-Induced Surface Damage of KH2PO4 Crystal

Mingjun Chen *, Wenyu Ding, Jian Cheng, Hao Yang and Qi Liu

School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] (W.D.); [email protected] (J.C.); [email protected] (H.Y.); [email protected] (Q.L.) * Correspondence: [email protected]; Tel.: +86-0451-86403252

Received: 22 August 2020; Accepted: 21 September 2020; Published: 23 September 2020

Abstract: 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.

Keywords: laser-induced damage; energy deposition; initial defect; multi-physics coupling

1. Introduction Inertial confinement fusion (ICF) has gradually become one of the most promising clean energy sources that people are striving for [1,2]. Lawrence Livermore National Laboratory (LLNL) built the largest large-scale laser fusion device: National Ignition Facility (NIF) [3,4]. The Limeil Laboratory of France and the China Academy of Engineering Physics (CAEP) [5] also built laser fusion ignition devices successively. Nature published two consecutive articles in 2014 [6] and 2019 [7] to report new advances in inertial confined fusion. In the driven laser system engineering for ICF devices, large quantity, caliber, and special performance of optical components are required [8]. To achieve favorable absorption of the laser by the target fuel, the ultraviolet (UV) light with shorter wavelength is preferred. The most effective way to obtain high power UV light is to convert the laser from fundamental frequency to multi-frequency and triple-frequency light through large diameter KH2PO4 (KDP) crystals. As a result, the laser damage resistance of the optical crystal materials in the devices determines the transmission quality of the whole optical path, seriously restricting the quality of the output laser, which is an urgent technical problem in the engineering application [9,10]. KDP crystals are used as electro-optical switches and frequency conversion elements at all levels of bridging optical paths in laser-driven fusion engineering due to the wide spectral transparency and high nonlinear conversion efficiency [11]. Laser-induced damage (LID) [12] will occur inside or on the surface of materials when the KDP crystals are exposed to the sufficiently high laser intensity

Appl. Sci. 2020, 10, 6642; doi:10.3390/app10196642 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 6642 2 of 32 that exceeds the extreme value that the materials can withstand. Yoreo et al. [13] summarized two kinds of crystal damage depending on the location of damage: bulk damage and surface damage. The bulk damage is relatively evenly distributed inside the crystal. The porosity and the deposited impurities during the growth of crystal will absorb the sufficient laser energy and explode to form the central melted and fractured zone several to tens of micrometers in diameter [14–16]. In contrast, the surface damage is usually initiated by the near surface impurities, the defects, such as particles or scratches, remaining during the processing and surface contamination [17,18]. In response to these varied damages with different laser intensity, the laser induced damage threshold (LIDT) is introduced to show the initiation of laser induced damage [19,20]. At present, a series of studies on the damage mechanism and the damage threshold of fused silica and other optical elements in ICF devices under laser irradiation have been carried out. The bulk damage of KDP crystal materials has been mostly studied by Ogorodnikov et al. [21,22]. However, the impurity particles inside the crystal have been avoided gradually due to the improvement of the growth technology of KDP crystal materials. The intrinsic damage mechanism of the surface damage of crystal materials should be considered significantly with the advent of ultra-short and short pulse regime. In addition, the actual laser induced damage threshold (LIDT) of the KDP crystal used in the current project is about 12–15 J/cm2 (1 ns) under the laser irradiation, which is still much smaller than the theoretical value of 147–200 J/cm2 (1–3 ns). The reason is that the mechanism of laser-induced surface damage of materials has not been fully interpreted [23]. Because of the transparent properties of the KDP crystal material, part of the energy of the incident laser is reflected or scattered, the other part of the energy is absorbed by the material, and the rest continues to propagate forward through transmission. At this time, the nonlinear absorption of the laser will cause physical phenomena such as temperature rise, melting, vaporization, thermal compression shock, plasma expansion, and wave shock, which are extremely complex process [24]. It is of great significance to understand the mechanism of laser induced damage for improving the laser induced damage threshold of optical crystal materials. To determine the degree of damage in KDP crystal materials caused by the laser irradiation, LIDT is usually the criterion that is used to improve the anti-laser damage ability of materials. Keldysh and Sparks et al. [25,26] proposed the ionization rate model. Jing et al. [27] integrated the whole ionization process and used the evolution process of free electron density to characterize the trend of laser-induced damage under the influence of incident laser intensity indirectly. The light intensity modulation phenomenon of the incident laser inside the material was introduced to characterize the change of damage threshold by Feit [28]. In addition, the finite-difference time-domain (FDTD) algorithm was also applied to KDP crystals to determine the light intensity enhancement factor (LIEF) inside the material to show the trend of damage [29,30]. However, both the exploration of free electrons and LIEF in an electric field can only give the single trend of damage. The damage trend reflected by LIDT cannot be intuitively characterized. A simulation technique that can intuitively characterize the damage threshold needs to be further explored. To understand the principles of laser-induced damage of KDP crystals deeply, the surface damage caused by the irradiation of laser on KDP crystal materials will be reviewed in detail from three stages of energy deposition, damage initiation, and damage forming. The energy deposition regime of KDP crystals in the early stage of laser irradiation was analyzed, which will be helpful to establish the evaluation criteria of the anti-laser damage ability of KDP crystal elements, so as to delay the service life of the project. In addition, combined with different forms of defects, the experiments and theoretical simulation of laser induced damage mechanism were sorted and reviewed. The initial energy deposition and the damage caused in the middle and late stages were discussed to optimize the processing parameters of KDP crystal materials. The damage threshold prediction method has also been sought. Appl. Sci. 2020, 10, 6642 3 of 32

2. Mechanism of Pulsed Laser-Induced Surface Damage With the continuous upgrading of high-power laser systems, the interaction mechanism between

Appl.laser Sci. and 2020 crystal, 10, x FOR materials PEER REVIEW is of interest. The interaction between laser and crystal materials is closely3 of 32 related to laser parameters such as pulse width, wavelength, and spot area. FigureFigure 11[ [31,32]31,32] shows shows the the physical phenomenon phenomenon on on optical optical materials materials induced induced by by lasers lasers with with differentdiﬀerent powerpower densities. densities. As As a transparenta transparent optical opti material,cal material, KDP crystalKDP hascrysta a relativelyl has a relatively low absorption low absorptioncoeﬃcient. coefficient. When the laser When is irradiatedthe laser is on irradiated the ideal on KDP the crystal, ideal KDP most crystal, of the energymost of of the the energy laser willof the be lasertransmitted, will be transmitted, and only a small and only part a of small the laser part energy of the laser will beenergy absorbed. will be absorbed. ItIt isis necessary necessary to considerto consider how how to determine to determine the occurrence the occurrence of damage of exactly,i.e., damage theexactly, determination i.e., the determinationof the damage initiatedof the damage for understanding initiated for the understanding LIDT of KDP crystal.the LIDT It is of generally KDP crystal. believed It is that generally the LID believedis initiated that when the LID any permanentis initiated when change any occurs permanen on thet surfacechange ofoccurs the material. on the surface At present, of the the material. critical Atfree present, electron the density critical is free used electron as the criteriondensity is of used damage as the judgment criterion toof determinedamage judgment the LIDT. to Thedetermine critical thefree LIDT. electron The density criticaln freecr of electron the material density can n becr of expressed the material as Equation can be expr (1)essed [33]: as Equation (1) [33]:

εωm*22 = ε00mee∗ω nncrcr = (1)(1) ee22

wherewhere ee isis the the electronic electronic charge, charge,ε0 representsε0 represents the dielectricthe dielectric constant constant of electron of electron space, mspace,e is the m electrone is the electronreduced reduced mass, and mass,ω is and the angularω is the frequencyangular frequency of incident of incident laser. laser.

FigureFigure 1. 1. TheThe physical physical phenomenon phenomenon of of the the interaction interaction of of laser laser and and optical optical crystal crystal materials materials at at different diﬀerent lightlight intensity intensity levels. levels. 2.1. Near Surface Impurity Thermal Absorption Damage 2.1. Near Surface Impurity Thermal Absorption Damage Unlike the manifestations of bulk damage, near-surface impurities will cause surface damage. Unlike the manifestations of bulk damage, near-surface impurities will cause surface damage. KDP crystals are easily doped with various metal cation impurities in the near surface during the KDP crystals are easily doped with various metal cation impurities in the near surface during the growth process. Miki et al. [34] studied the KDP crystals and found that the impurity particles growth process. Miki et al. [34] studied the KDP crystals and found that the impurity particles contained in the near surface of crystal would scatter or converge the incident laser, resulting in the contained in the near surface of crystal would scatter or converge the incident laser, resulting in the enhancement of the local light ﬁeld of the element. In addition, the researchers [35–37] have found that enhancement of the local light field of the element. In addition, the researchers [35–37] have found the incident energy of the laser was strongly absorbed by the impurity ions. Generally, the laser energy that the incident energy of the laser was strongly absorbed by the impurity ions. Generally, the laser absorbed by impurities is several orders of magnitude higher than the intrinsic absorption of crystal energy absorbed by impurities is several orders of magnitude higher than the intrinsic absorption of materials. Therefore, the strong absorption of the laser by impurities in the crystals will cause the rise crystal materials. Therefore, the strong absorption of the laser by impurities in the crystals will cause of local temperature, which will further lead to the melting and cracking of the material surface. the rise of local temperature, which will further lead to the melting and cracking of the material Koldunov and Manenkov et al. [31,38,39] assumed that the impurities inside the optical materials surface. were in the shape of small balls with the radius of r. Based on this assumption, the thermal explosion Koldunov and Manenkov et al. [31,38,39] assumed that the impurities inside the optical materials were in the shape of small balls with the radius of r. Based on this assumption, the thermal explosion (TE) model was constructed to explain the absorption of radiation by impurities in the materials. The heat conduction equation can be described to show the radiation absorption of impurities [40,41]. Appl. Sci. 2020, 10, 6642 4 of 32

(TE) model was constructed to explain the absorption of radiation by impurities in the materials. Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 32 The heat conduction equation can be described to show the radiation absorption of impurities [40,41].

∂∂∂∂1 ∂ ∂TT （(ρρcTcT)）==+((),()r 22k ) + QQ()( TT, IgIg( tt)) (2)(2) ∂∂∂∂ttrrr2 ∂r ∂r

wherewhere ρρ,, cc,, and and kk areare the the density, density, thermal capacity and and thermal thermal conductivity conductivity of of impurities impurities and and host host materials,materials, respectively. respectively. TT isis the the temperature temperature of of impurities, impurities, QQ isis the the power power density density of of the the thermal thermal sources,sources, and and I is thethe laserlaser intensityintensity and andg (gt()t is) is the the temporal temporal shape shape of theof the laser laser pulse. pulse. It is It clear is clear that thatQ(T, QIg((T,t)) Ig is(t associated)) is associated with with absorption absorption of radiation of radiation by impurities. by impurities.

 TT− ! ()= ξ T T0 0 QQTIgt(T,,()Ig(t)) = QIQ ()exp(I) expξ − (3)(3) TT0 0 where ξ is the material parameter, and T is the initial temperature. where ξ is the material parameter, and T00 is the initial temperature. ChenChen et et al. al. [35] [35] explored explored the the influence inﬂuence of of the the ne nearar surface surface impurities impurities in in the the KDP KDP crystal crystal on on its its surroundingsurrounding temperaturetemperature and and stress, stress, as shownas shown in Figure in Figure2, which 2, wellwhich explained well explained the strong the absorption strong absorptionof energy of of impurities. energy of impurities.

FigureFigure 2. 2. EffectEffect of of near near surface surface impurities impurities in inKDP KDP crys crystaltal on ontemperature temperature andand stress stress of materials. of materials. (a) a Distribution( ) Distribution of temperature of temperature field field around around impuriti impuritieses under under laser laser irradiatio irradiationn with with different different energy energy densities. The enlarged view shows the three-dimensional temperature field at I = 6 J/cm2, a = 0.5 µm; densities. The enlarged view shows the three-dimensional temperature field at I=6 J/cm2, a=0.5 μm; (b)Temperature variation curve of impurity with its dimension; (c) Temperature variation curve from (b)Temperature variation curve of impurity with its dimension; (c) Temperature variation curve from the center to the edge of Fe impurity; (d) The contract curve of maximum temperature and thermal the center to the edge of Fe impurity; (d) The contract curve of maximum temperature and thermal stress of different types of impurities [35]. stress of different types of impurities [35]. It can be seen that the temperature near the impurity is directly related to the energy density of It can be seen that the temperature near the impurity is directly related to the energy density of the incident laser. The greater the incident laser energy density, the more energy the impurity absorbs, the incident laser. The greater the incident laser energy density, the more energy the impurity resulting in a sharper temperature rise of impurity. The temperature rise of impurities is approximately absorbs, resulting in a sharper temperature rise of impurity. The temperature rise of impurities is proportional to the energy density of the incident laser. approximately proportional to the energy density of the incident laser. In addition, the properties of impurities also result in energy absorption. Chen and Mu et al. [42] In addition, the properties of impurities also result in energy absorption. Chen and Mu et al. [42] indicated that the size and density of impurities have an effect on the energy absorption of impurities. indicated that the size and density of impurities have an effect on the energy absorption of impurities. Chen et al. proposed that there was a critical impurity size that made the impurities absorb the most Chen et al. proposed that there was a critical impurity size that made the impurities absorb the most energy and had the highest temperature. Once the impurity radius a is less than or greater than this energy and had the highest temperature. Once the impurity radius a is less than or greater than this critical value, the energy absorbed by the impurities after laser irradiation will be very small, and the critical value, the energy absorbed by the impurities after laser irradiation will be very small, and the temperature near the impurity will be very low, which is not easy to cause damage. The type of temperature near the impurity will be very low, which is not easy to cause damage. The type of impurity inside the KDP crystal also absorbs energy differently. The absorption of energy by different impurity inside the KDP crystal also absorbs energy differently. The absorption of energy by different types of impurities will lead to the differences in temperature rise and thermal stress, as shown in Figure 2d. Bercegol and Carr et al. [43,44] established the models containing near surface impurities, and the damage process of submicron-level impurities in optical materials induced by laser was Appl. Sci. 2020, 10, 6642 5 of 32 types of impurities will lead to the differences in temperature rise and thermal stress, as shown in Figure2d. Bercegol and Carr et al. [43,44] established the models containing near surface impurities, and the damageAppl. Sci. 2020 process, 10, x FOR of submicron-level PEER REVIEW impurities in optical materials induced by laser was simulated5 of by32 hydrodynamic (HD) simulations, as shown in Figure3. In contrast to the bulk surface, the impurities simulated by hydrodynamic (HD) simulations, as shown in Figure 3. In contrast to the bulk surface, near the surface will form craters on the surface and cause considerable damage. the impurities near the surface will form craters on the surface and cause considerable damage.

Figure 3.3. ContrastContrast between between the the hydrodynamic hydrodynamic simulation simulati ofon laser-induced of laser-induced damage damage of optical of materials optical materialscontaining containing impurities impurities and the experimental and the experimental of laser-induced of laser-induced damage. (a) Cross-sectionaldamage. (a) Cross-sectional view of AFM (Atomicview of ForceAFM Microscope)(Atomic Force measurement Microscope) of damagemeasurem pitent by laserof damage irradiation; pit (byb) HESIONElaser irradiation; simulation (b) HESIONEof damage simulation pit by laser of irradiation; damage pit (c by) Illustration laser irradiation; of the fracture(c) Illustration wave generatedof the fracture by the wave irradiation generated of byimpurities the irradiation at different of impurities times following at different the arrival times of thefollowing laser pulse; the (arrivald) HD simulationof the laser of pulse; the formation (d) HD simulationof impurity absorptionof the formation front (AF) of whileimpurity the laserabsorp wastion on front and the (AF) temperature while the profile; laser (wase) SEM on imageand the of temperaturethe damage sites profile; produced e SEM by image absorption of the frontdamage initiator sites pulsesproduced followed by absorption by various front growth initiator feet [pulses43,44]. followed by various growth feet [43,44]. The simulation results are consistent with the experimental data, which reflects the mechanism of impurityThe simulation thermal results absorption are consistent damage well. with Koldunov the experimental et al. [45 data,] analyzed which thereflects thermal the mechanism conduction modelof impurity and constructedthermal absorption a nonlinear damage heating well. model Koldun ofov impurities. et al. [45] Itanalyzed was found the thatthermal the relationshipconduction betweenmodel and the constructed LIDT and the a nonlinear pulse width heating under model different of impurities. laser pulse It shapes was found (rectangular that the and relationship gaussian) canbetween be expressed the LIDT by and the the following pulse width formula, under especially different in laser the casepulse of shapes longer (rectangular pulse laser irradiation.and gaussian) can be expressed by the following formula, especially in the case of longer pulse laser irradiation. rect Ith I (τp) = I (4) I rect ()τ = 1 exp(th τp/2τ) p −−− −ττ (4) 1exp(/2)p gauss h 2i I (τp) = Ith exp (τ/τp) (5) IIgauss ()τττ= exp(/)2 pth p (5) where the τp is pulse width and τ is heat relaxation time in the medium. It is worth noting that there is τp τ. I is the threshold intensity. On the basis of the analysis of above models, it is determined that where≥ theth τp is pulse width and τ is heat relaxation time in the medium. It is worth noting that there theis τp thermal ≥ τ. Ith is explosion the threshold model intensity. caused by On impurities the basis is of reasonable the analysis only of when above the models, laser irradiation it is determined time is longthat the enough. thermal explosion model caused by impurities is reasonable only when the laser irradiation time is long enough.

2.2. Ionization Damage

2.2.1. Photoionization With the improvement of the processing technology of KDP crystal materials, the impurities in KDP crystal materials can be avoided gradually. The study of intrinsic damage gradually became clearer. As a transparent crystal material, KDP crystals without any impurities have relatively low absorption of incident laser energy. Therefore, the impurity-based TE model is no longer applicable [46]. For the laser-induced damage without impurities, Carr et al. [47–49] proposed that the laser damage of transparent optical crystal materials was caused by the change of energy level in multiphoton absorption, and a locally dense plasma area will be formed in the process of damage. In addition, the birth of high-power density ultrashort pulse laser made rapid ignition become the key for laser driven fusion engineering. The appearance of ultra-short pulse laser was helpful to understand the interaction between laser and transparent optical materials. The intrinsic damage of KDP crystal materials under laser irradiation has gradually become the key point of research. Photoionization refers to the process in which the electrons in the equilibrium state inside the material absorb photons directly and are excited to the excitation state when the laser interacts with optical crystal materials [50]. Actually, the excitation condition for electrons is to ensure that the energy of the incident photon is equal to the band gap between the excited state and the ground state. In general, the electron absorbs only one photon from ground state to excited state. However, if the energy of a single photon is less than this bandgap, the electron cannot be excited [51]. Keldysh et al. [25] noted the laser intensity and frequency, and proposed a comprehensive theory of photoionization. It is possible that an electron will absorb two or more photons and transition if the width of this band gap is equivalent to the total energy of two or more photons under the intense electric field. This kind of mechanism is called multiphoton ionization regime. Nowadays, the laser electric field has been able to reach a certain level, far exceeding the perturbation limit of crystal materials [52,53]. This situation means that the external reinforcement field will break the Coulomb well of the bound electrons and excite the free electrons through short tunnel ionization. As a result, there are two different regimes of photoionization, the multiphoton ionization (MPI) and the tunneling ionization (TI). Keldysh used the following formula to describe the photoionization rate based on the understanding of photoionization. ! " # 2ω ωme∗ K(γ1) E(γ1) WPI(I(t)) = Q(r, x) exp π x + 1 − (6) 9π h¯ √γ1 − h i E(γ2) where h¯ is the reduced Planck constant. me represents effective electronic mass. ω is the angular frequency of the incident laser. K(γ), E(γ) represent the first and second complete elliptic integral, respectively. γ is the adiabatic parameter of the solid material, also known as the Keldysh parameter, which can be shown as " #1/2 ν mecnε0Eg γ = (7) e I where ν is the laser frequency, me, e, and n are the reduced mass, electronic charge, and the index of refraction of materials, respectively. Eg is the band gap, which is 7.7 eV. Figure4[ 29,52,53] shows the photoionization rate as a function of laser intensity for 800 nm laser in KDP. Appl. Sci. 2020, 10, 6642 7 of 32 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 32

Figure 4. Correlations between incident laser intensity I and PI rate WPI with the consideration of full Figure 4. Correlations between incident laser intensity I and PI rate WPI with the consideration of full Keldysh’s ionization, individual multiphoton ionization (MPI) and tunneling ionization (TI) ionizations Keldysh’s ionization, individual multiphoton ionization (MPI) and tunneling ionization (TI) under 800 nm lasers irradiation [29]. ionizations under 800 nm lasers irradiation [29]. The effect of multiphoton ionization and tunneling ionization on photoionization can be accurately The effect of multiphoton ionization and tunneling ionization on photoionization can be characterized by the Keldysh coefficient γ, as shown in Figure4. accurately characterized by the Keldysh coefficient γ, as shown in Figure 4. The process of photoionization is quite different under the strong field and the weak field. It can The process of photoionization is quite different under the strong field and the weak field. It can be seen that the photoionization is a mix between tunneling and multiphoton ionization when the be seen that the photoionization is a mix between tunneling and multiphoton ionization when the Keldysh coefficient is around 1.5. With the increase of laser intensity, the multiphoton ionization Keldysh coefficient is around 1.5. With the increase of laser intensity, the multiphoton ionization coefficient will increase rapidly. In general, there is a linear relationship between the photoionization coefficient will increase rapidly. In general, there is a linear relationship between the photoionization rate and the laser intensity, which is W I. rate and the laser intensity, which is W∝ ∝ I. 2.2.2. Avalanche Ionization 2.2.2. Avalanche Ionization Actually, the laser will interact with the lattice, electron, and phonon in the crystal material when the incidentActually, laser the irradiates laser will the interact crystal with material. the lattice, As a result,electron, the and crystal phonon material in the will crystal absorb material the incident when laser.the incident There are laser two irradiates types of electrons the crystal in thematerial. materials: As bounda result, electrons the crystal and freematerial electrons. will absorb Generally, the theincident ionization laser. potential There are of two the types bound of electron electrons is muchin the largermaterials: than bound the photon electrons energy and of free the electrons. incident laser,Generally, so the the bound ionization electron potential cannot generally of the bound absorb el photons.ectron is However,much larger the than normal the impurity-free photon energy KDP of crystalsthe incident are dielectric laser, so materials the bound that electron do not have cannot free generally electrons inabsorb the body. photons. Therefore, However, the source the normal of free electronsimpurity-free is very KDP important crystals during are di laserelectric irradiation. materials It that is clear do thatnot thehave KDP free crystal electrons materials in the will body. not absorbTherefore, the laserthe source energy of greatly. free electrons is very important during laser irradiation. It is clear that the KDPBased crystal on materials the understanding will not absorb of the the mechanism laser energy of photoionization,greatly. it can be found that the free electronsBased generated on the understanding by photoionization of the mechanism in the early of stage photoionization, will be used asit can seed be electrons found that to the absorb free photonelectrons energy generated by inverse by photoionization bremsstrahlung in under the early the subsequent stage will laserbe used irradiation. as seed Whenelectrons the absorbedto absorb energyphoton reachesenergy by a certaininverse standard, bremsstrahlung the seed under electrons the subsequent will collide laser with irradiation. the bound When electrons the absorbed in the valenceenergy bandreaches (VB) a insidecertain the standard, material, the causing seed itelectr transitionons will to the collide conduction with the band bound (CB) [electrons54]. These in new the freevalence electrons band with(VB) lowerinside energy the material, will be causing generated, it transition which will to continuethe conduction to absorb band energy (CB) and [54]. collide These withnew thefree atoms. electrons With with the lower increase energy of the will density be generated, of free electrons which inwill KDP continue crystal to materials, absorb energy the crystal and materialscollide with with the dielectric atoms. With properties the increase will graduallyof the density become of free the electrons metal-like in KDP materials. crystal Furthermore,materials, the withcrystal the subsequentmaterials with irradiation dielectric of laser, properties the energy will is continuouslygradually become absorbed, the andmetal-like the density materials. of free electronsFurthermore, inside with the the materials subsequent will increaseirradiation rapidly. of laser, Researchers the energy have is continuously successively absorbed, constructed and and the optimizeddensity of the free corresponding electrons inside models the formaterials avalanche will ionization increase torapidly. determine Researchers the density have of free successively electrons generatedconstructed by and avalanche optimized ionization. the corresponding models for avalanche ionization to determine the density of free electrons generated by avalanche ionization. Sparks [26,55,56] criticized the classical avalanche model with the imperfection of calculating the avalanche ionization rate and built the Sparks model for the avalanche ionization. Appl. Sci. 2020, 10, 6642 8 of 32

Sparks [26,55,56] criticized the classical avalanche model with the imperfection of calculating the avalanche ionization rate and built the Sparks model for the avalanche ionization.

2 2 0.693e E τK 0.693¯hωp WAI Sparks = (8) − ( + 2 2 ) − Egτ me∗Eg 1 ω τK L where τK represents the electro-acoustic relaxation time in the case of large angle scattering, 15 τK = 1.36 10− s. τL is the electro-acoustic relaxation time when scattering at large and small × 16 angles, and τ = 8.77 10 s. E is the radiation electric ﬁeld amplitude. ωp is the average phonon L × − frequency, there is h¯ωp = 0.025 eV. The model assumes that the collision frequency and collision energy loss are constant during collisions between electrons. Sparks indicated that only when the average electron energy is greater than the energy required for electron impact ionization, can electrons reach the ionization limit, triggering avalanche ionization. Drude et al. constructed the avalanche ionization model based on the heating phenomenon of free electrons in the conduction band [57].

2 p σ 3.2e πε0 me∗ n(t) WAI Drude = I(t) = h · iI(t) (9) − Eg 2 2 2 2 √2cn0me∗ n (t) + 0.08me∗(πε0 ωEg) where σ represents the absorption cross section of the photon, and τc is the time of the electron collision. Thornber et al. [58] proposed an avalanche ionization model based on the semi-empirical formula, which can be applied to all electrical intensities.

vseE EI WAI Thornber = exp( ) (10) − Eg −E(1 + E/EP) + EKT

7 where e is the electronic charge. vs 2 10 cm/s, which represents the saturation drift velocity. E , E , ≈ × I P and EKT are, respectively, the electric fields that required to overcome ionization scattering, photon scattering, and thermal scattering. Thornber assumed that there is WAI Thornber E √I if the laser p − ∝ ∝ intensity satisfies this condition: E > EpEI. However, the theoretical and experimental values of the predicted laser induced damage threshold by this model are quite different [59]. Stuart et al. [33] believed that the avalanche ionization rate is linearly related to the laser intensity. Stuart et al. [60] constructed the reasonable models for the avalanche ionization process of the specific materials under laser irradiation. For the heating problem of electrons in the conduction band, the dependence of electron energy on conductivity is considered in this model based on the Drude model. WAI Stuart = α I(t) (11) − · where α is the avalanche ionization coefficient. This model proved that there is no connection between the laser-induced damage threshold and pulse width under short pulses. The decrease of LIDT is mainly affected by ionization [61]. Cheng [29] summarized these avalanche ionization models and the relationship between avalanche ionization rate and electric intensity is shown in Figure5. The free electron density n in the valence band under avalanche ionization alters with time can be shown in Equation (12): dn = W n (12) dt AI· Appl. Sci. 2020, 10, 6642 9 of 32 Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 32

Figure 5. Dependence of avalanche ionization rate WAI on electric intensity E calculated by AI models Figure 5. Dependence of avalanche ionization rate WAI on electric intensity E calculated by AI models of Stuart, Sparks, and Thornber of KDP [29]. of Stuart, Sparks, and Thornber of KDP [29]. 3. Exploration of the Initial Energy Deposition The free electron density n in the valence band under avalanche ionization alters with time can The initial deposition stages of energy in the material determines the damage that the KDP crystal be shown in Equation (12): can withstand, while the final release stage of energy determines the morphology of damaged sites. The initial energy deposition of KDPdn crystal material is different under the laser irradiation with =⋅Wn )(12) different pulse widths, which will leaddt to theAI diversification of the damage morphology in later stage. It is of great significance to study the main factors that reduce the damage threshold of KDP crystal 3.surface Exploration for preventing of the Initial the occurrence Energy Deposition of damage. Therefore, it is very important to study the initial energy deposition inside the material under laser irradiation. TheSince initial the high-power deposition short-pulsestages of energy lasers arein th usede material in the ignition determines devices the of damage ICF mostly, that thethe energy KDP crystaldeposition can withstand, and damage while morphology the final underrelease high-power stage of energy short-pulse determines laser werethe morphology summarized of particularly. damaged sites. The initial energy deposition of KDP crystal material is different under the laser irradiation with different3.1. The Formationpulse widths, and Evolutionwhich will of lead Plasma to the diversification of the damage morphology in later stage. It is of great significance to study the main factors that reduce the damage threshold of KDP crystal surface3.1.1. Irradiation for preventing of Ultra-Short the occurrence Pulse Laserof damage. Therefore, it is very important to study the initial energyStuart deposition et al. [inside61] found the material that when under the laser pulse irradiation. period is greater than 100 ps, the damage of the dielectricSince materialthe high-power is mainly short-pulse caused by lasers the traditional are used in heat the ignition transfer, devices while the of damageICF mostly, of the the material energy depositionoriginates fromand thedamage formation morphology of laser-induced under plasmahigh-power when short-pulse the pulse period laser is were very shortsummarized (<10 ps). particularly.As a result, the formation of plasma formation plays an important role in the initial energy deposition under the irradiation of short (picosecond to a couple of nanoseconds timescale) and ultra-short pulses 3.1.(femtosecond The Formation timescale) and Evolution [62–64 ].of Plasma Based on the understanding of ionization, it can be found that the photoionization will be induced 3.1.1. Irradiation of Ultra-Short Pulse Laser inside the material when high power ultra-short laser pulses irradiate transparent dielectric materials. TheStuart formed et free al. electrons[61] found will that be when acted asthe seed pulse electrons period tois absorbgreater thethan incident 100 ps, laser the energydamage through of the dielectricvarious absorption material is mechanismsmainly caused in by a verythe traditio smallnal range heat and transfer, a very while short the time, damage which of will the material generate originatesavalanche from ionization the formation and make of the laser-induced electrons in the plasma action when area reach the pulse extremely period high is very temperature short (<10 quickly. ps). As a result,Then, the formation material will of plasma be directly formation excited plays from an the important solid state role to the in the form initial of high-density energy deposition plasma. underThe high-density the irradiation plasma of willshort transfer (picosecond their kinetic to a co energyuple toof thenanoseconds lattice over timescale) a picosecond and timescale ultra-short and pulsesperform (femtosecond energy deposition timescale) within [62–64]. the skin depth of the material through the reverse bremsstrahlung regimeBased under on thethe subsequentunderstanding laser of irradiation. ionization, During it can thisbe period,found that the electronthe photoionization density will continuewill be inducedto increase inside until the the material plasma when frequency high power approaches ultra-short the laser frequency pulses ofirradiate the incident transparent laser dielectric radiation, materials.i.e., the critical The formed plasma free density electrons value will is reached. be acted as seed electrons to absorb the incident laser energy through various absorption mechanisms in a very small range and a very short time, which will generate avalanche ionization and make the electrons in the action area reach extremely high temperature quickly. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 32

Then, the material will be directly excited from the solid state to the form of high-density plasma. The high-density plasma will transfer their kinetic energy to the lattice over a picosecond timescale and perform energy deposition within the skin depth of the material through the reverse bremsstrahlung regime under the subsequent laser irradiation. During this period, the electron density will continue to increase until the plasma frequency approaches the frequency of the incident laserAppl. radiation, Sci. 2020, i.e.,10, 6642 the critical plasma density value is reached. 10 of 32 Jing et al. [27] established a rate equation to describe the evolution of free electron density ne(t) in the dielectricJing et al. medium [27] established KDP under a rate the equation irradiation to describe of laser the evolutionpulse below of free 10ps. electron density ne(t) in the dielectric medium KDP under the irradiation of laser pulse below 10ps. ∂nt() e =+WItW(()) (()) ItntWntt ⋅− () ((),) (13) ∂n ∂(t) PI AV e rel e = ( ( )) + ( ( )) ( ) ( ( ) ) WPI I t W I t ne t W n t , t (13) ∂t AV · − rel where Wrel(n(t),t) is the plasma energy decay term that associated with the diffusion and recombinationwhere Wrel(n(t),t) of electrons.is the plasma energy decay term that associated with the diﬀusion and recombination ofFigure electrons. 6a shows the formation of plasma under the irradiation of ultra-short laser pulses. Based on the criterionFigure6a of shows critical the formationelectron ofdensity, plasma underthe growth the irradiation of electron of ultra-short density laser predicted pulses. Basedby different on the criterion of critical electron density, the growth of electron density predicted by diﬀerent ionization ionization models under the irradiation of femtosecond laser was analyzed in references [65] and models under the irradiation of femtosecond laser was analyzed in references [65,66]. [66].

FigureFigure 6. The 6. Theprocess process of energy of energy deposition deposition and and evolut evolutionionunder under femtosecond femtosecond laser laser irradiation. irradiation. (a) The (a) The electron density ne(t) curve of KDP materials under the femtosecond laser irradiation based on Equation e electron density n (t) curve of KDP materials2 under the femtosecond laser irradiation based on (13), λ = 800 nm, τ = 55 fs, I0 = 68 TW/cm . Region I: photoionization, and Region II: avalanche Equation (13), λ = 800 nm, τ= 55 fs, I0 = 68 TW/cm2. Region I: photoionization, and Region II: ionization and formation of plasma [65]; (b) Deposition energy density distributions in dielectric avalanchematerials ionization predicted and by nonlinear formation absorption of plasma model, [65];τ = 50(b fs,) Deposition pulse energies energy and laser density peak intensity distributions of b1 in dielectric100 nJ, materials 2.8 1013 predictedW cm 2, b2 by250 nonlinear nJ, 7 10 13absorptionW cm 2, and model,b3 1000 τ = nJ,50 28fs, pulse1013 W energiescm 2, respectively; and laser peak × · − × · − × · − intensity(c) Inﬂuence of b1 100 of nJ, plasma 2.8 × formation 1013 Wꞏcm on-2, laserb2 250 irradiation nJ, 7 × 10 with13 Wc1ꞏcmthe-2, and transmission b3 1000 ofnJ, high-intensity28 × 1013 Wꞏcm-2, respectively;femtosecond (c) pulseInfluence laser of irradiation plasma asformation a function on of laser focal irradiation position, with and c2c1the the bulk transmission absorption asof ahigh- intensityfunction femtosecond of the laser pulsepulse energy laser irradiation [67]. as a function of laser focal position, and c2 the bulk absorption as a function of the laser pulse energy [67]. In addition, there is a sharper drop in transmission at the near-front surface of dielectric materials in Figure6c, and the greater the light intensity, the weaker the transmission performance and the In addition, there is a sharper drop in transmission at the near-front surface of dielectric stronger the absorption performance. Combined with the deposition energy density shown in Figure6b, materialsit is clear in Figure that the 6c, energy and depositionthe greater density the light inside intensity, the material the graduallyweaker the increases transmission with the performance increase and ofthe laser stronger intensity the dueabsorption to the formation performance. of plasma. Comb Kandalained with et al. the [68 deposition] showed that energy there density is the high shown in Figureconcentration 6b, it is clear near thethat focal the energy point and deposition the end of density the laser inside source the due material to the high gradually laser intensity increases and with the reﬂection of light by the plasma, respectively. Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 32 the increase of laser intensity due to the formation of plasma. Kandala et al. [68] showed that there is the high concentration near the focal point and the end of the laser source due to the high laser intensity and the reflection of light by the plasma, respectively.

3.1.2. IrradiationAppl. Sci. 2020 of, 10 Short, 6642 Pulse Laser 11 of 32 Although the formation of plasma under short-pulse lasers occurs as in the fs case, a significant difference3.1.2. is that Irradiation energy of Shortdeposition Pulse Laser is time dependent. The absorption of energy by seed electrons occurs in theAlthough later period the formation of short ofpulse plasma laser under irradi short-pulseation, so lasers the occursformation as in theof fsplasma case, a significantcan be controlled by avalanchedifference ionization, is that energy which deposition is well isconfirmed time dependent. in Figure The absorption7a. Actually, of energy a longer by seed time electrons contributes to occurs in the later period of short pulse laser irradiation, so the formation of plasma can be controlled effective plasma heating. Liu et al. [69] observed the transient dynamics reaction of early plasma by avalanche ionization, which is well confirmed in Figure7a. Actually, a longer time contributes formationto eofffective KDP plasmasurface heating. during Liu nanosecond et al. [69] observed laser thewith transient the time dynamics resolved reaction pump of early and plasma probe (TRPP) technology,formation as shown of KDP in surface Figure during 7c. The nanosecond schematic laser diagram with the of time time resolved resolved pump pump and probe and (TRPP)probe (TRPP) technologytechnology, is shown as shown in Figure in Figure 8. The7c. The plasma schematic is gradually diagram of generated time resolved due pump to andionization, probe (TRPP) as shown in the darktechnology region of isFigure shown 7(c2). in Figure Due8. The to the plasma abso isrption gradually of generatedlaser energy due to by ionization, the plasma, as shown a temperature in differencethe will dark generate region of Figurebetween7(c2). the Due air to theand absorption the material, of laser which energy byresults the plasma, in the a formation temperature of shock difference will generate between the air and the material, which results in the formation of shock wave in waveair. It in is air. clear It is that clear with that with the theexpansion expansion of of the the plasma, the the shock shock wave wave becomes becomes more and more more and more significant,significant, which which also also reflects reflects thethe deposition deposition process process of early energy.of early The plasmaenergy. region The continuously plasma region continuouslygrows grows along with along the increasewith the of increase the pump of pulse. the pump pulse.

Figure 7. The process of energy deposition and evolution of dielectric under short pulse laser irradiation. Figure 7. The process of energy deposition and evolution of dielectric under short pulse laser (a) Curve of the total electron density (Nt) and multiphoton produced electron density (Nm) with irradiation.incident (a) (fluenceCurve 10.1of the J/cm total2) and electron transmitted density (fluence (N 9.7t J)/ cmand2) laser multiphoton pulse, τ = 100 produc ps [70];ed (b) Evolutionelectron density 2 2 (Nm) withof incidenttheat of KDP (fluence as a function 10.1 ofJ/cm the fluences) and transmittedF1ω and F3ω [(fluence71]; and ( c9.7) The J/cm transient) laser images pulse, of τ plasma = 100 ps [70]; produced by early ionization with the delay between pump and probe pulse of 2.8 ns to 0.8 ns, the area (b) Evolution of theat of KDP as a function of the fluences F1ω and F3ω [71]; −and (c) The transient images of plasmaA onproduced the left side by of early the red ionization dotted line with is air, the and Bdelay on the between right side pump is KDP [and69]. probe pulse of –2.8 ns to 0.8 ns, the area A on the left side of the red dotted line is air, and B on the right side is KDP [69]. Appl. Sci. 2020, 10, 6642 12 of 32 Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 32 Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 32

Figure 8. Schematic diagram of time Resolved Pump and Probe (TRPP) technology [69]. FigureFigure 8. 8. Schematic Schematic diagram diagram of of time time Resolved Resolved Pump Pump and and Probe Probe (TRPP) (TRPP) technology technology [69]. [69]. After the ionization is completed, the plasma formed in the early stage will slowly absorb most of AfterAfter the the ionization ionization is is completed, completed, the the plasma plasma formed formed in in the the early early stage stage will will slowly slowly absorb absorb most most the energy of the incident laser through inverse bremsstrahlung under the irradiation of the subsequent ofof thethe energyenergy ofof thethe incidentincident laserlaser throughthrough ininverseverse bremsstrahlungbremsstrahlung underunder thethe irradiationirradiation ofof thethe short pulse laser (ps–ns), which makes the plasma in the KDP crystal reach high temperature with a subsequentsubsequent shortshort pulsepulse laserlaser (ps–ns),(ps–ns), whichwhich mamakeskes thethe plasmaplasma inin thethe KDPKDP crystalcrystal reachreach highhigh stable form. As shown in Figure7b, the 1 ω (1053 nm) radiation is more efficient than the 3ω (351 nm) temperaturetemperature with with a a stable stable form. form. As As shown shown in in Figure Figure 7b, 7b, the the 1 1ωω (1053 (1053 nm) nm) radiation radiation is is more more efficient efficient for the plasma heating. The thermal conduction and hydrodynamics will also allow energy to be thanthan the the 3 3ωω (351 (351 nm) nm) for for the the plasma plasma heating. heating. The The thermal thermal conduction conduction and and hydrodynamics hydrodynamics will will also also transferred away from the focus in the duration of input laser pulse [72]. The characteristics of the allowallow energyenergy toto bebe transferredtransferred awayaway fromfrom thethe focusfocus inin thethe durationduration ofof inputinput laserlaser pulsepulse [72].[72]. TheThe surrounding material such as absorption will be influenced by the transferred energy. Part of the characteristicscharacteristics ofof thethe surroundingsurrounding materialmaterial such such as as absorptionabsorption willwill be be influencedinfluenced byby thethe transferredtransferred absorbed and deposited laser energy will be transferred from electron to lattice of material gradually energy.energy. Part Part of of the the absorbed absorbed and and deposited deposited laser laser ener energygy will will be be transferred transferred from from electron electron to to lattice lattice of of through the electron–phonon coupling technology in the picosecond timescale. materialmaterial gradually gradually through through the the electron–phonon electron–phonon co couplingupling technology technology in in the the picosecond picosecond timescale. timescale. 3.2. Effects of Surface Defects on Initial Energy Deposition of KDP Crystal 3.2.3.2. Effects Effects of of Surface Surface Defects Defects on on Initial Initial Energy Energy Deposition Deposition of of KDP KDP Crystal Crystal DueDue toto thethe characteristiccharacteristic ofof KDPKDP crystalcrystal materials,materials, micro-nano-scalemicro-nano-scale defectsdefects willwill bebe introducedintroduced into theDue surface to the characteristic when the single-point of KDP crystal diamond material ultra-precisions, micro-nano-scale fly-cutting defects (SPDF) will is carriedbe introduced out on intointo thethe surfacesurface whenwhen thethe single-pointsingle-point diamonddiamond ultrultra-precisiona-precision fly-cuttingfly-cutting (SPDF)(SPDF) isis carriedcarried outout onon thethe surfacesurface ofof material.material. VariousVarious typestypes ofof defectsdefects willwill existexist onon thethe surfacesurface ofof KDPKDP crystalcrystal duringduring thethe theflying surface cutting. of material. As a typical Various soft andtypes brittle of defects material, will KDP existcrystal on the issurface more proneof KDP to crystal produce during brittle the or flyingflying cutting. cutting. As As a a typical typical soft soft and and brittle brittle material material, ,KDP KDP crystal crystal is is more more prone prone to to produce produce brittle brittle or or plasticplastic defectsdefects during during processing processing [73 [73,74].,74]. Figure Figure9 shows 9 shows the the topography topography of KDP of KDP chip chip generated generated in the in plasticfinishing defects stage. during processing [73,74]. Figure 9 shows the topography of KDP chip generated in thethe finishing finishing stage. stage.

Figure 9. Topography of the machined zone and chip in groove cutting test [74]. FigureFigure 9. TopographyTopography of the machined zone an andd chip in groove cutting test [[74].74]. Appl. Sci. 2020, 10, 6642 13 of 32

Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 32 Based on the characteristics of KDP crystal materials, Wang et al. [20] further summarized all the Based on the characteristics of KDP crystal materials, Wang et al. [20] further summarized all defects that appeared on the surface of eight KDP components during processing and classiﬁed them the defects that appeared on the surface of eight KDP components during processing and classified into ﬁve categories, as shown in Figure 10. them into five categories, as shown in Figure 10.

Figure 10. 10. OpticalOptical microscopy microscopy images images of of the the five ﬁve kinds kinds of ofKDP KDP surface surface defects. defects. (a) Fracture (a) Fracture pits; pits; (b) Convex(b) Convex scratches; scratches; (c) Surface (c) Surface protuberances; protuberances; (d) (Cracks;d) Cracks; and and (e) Plastic (e) Plastic scratches scratches [20]. [20 ].

µ Figure 10a10a shows thethe fracturefracture pitspits defectsdefects usually usually with with the the widths widths of of 20–200 20–200 mμm and and the the depths depths of µ of2–6 2–6m. μm. Figure Figure 10b,c 10b,c present present diff erentdifferent typical typical forms forms of protrusion of protrusion damage. damage. The cracks The cracks that are that shown are µ shownin Figure in 10 Figured with 10d the lengthswith the of 20–120lengths mof are20–120 also aμ kindm are of defectalso a thatkind occurs of defect more that frequently. occurs Duemore to frequently.the characteristics Due to ofthe KDP, characteristics the typical of plastic KDP, scratchthe typical defects plastic are scratch also often defects present are duringalso often processing, present duringas shown processing, in Figure 10ase. shown Wang etin al. Figure used the10e. fluorescent Wang et emissional. used the detection fluorescent technology emission to characterize detection technologythe impact ofto KDPcharacterize crystals the on impact energy of absorption KDP crysta underls on these energy five absorption defects [20 under,75] and these found five thatdefects the [20,75]fluorescence and found intensity that of the the fluorescence defect-free area intensity is significantly of the defect-free smaller than area that is significantly of the defect area.smaller Among than thatthe aboveof the defects, defect area. the fluorescence Among the intensity above defects, of plastic the scratches fluorescence is greatly intensity weaker of plastic than the scratches other four is greatlydefects. weaker The surface than defectsthe other in Figurefour defects. 10a–d have The asurface stronger defects absorption in Figure capacity 10a–d than have the a defect-free stronger absorptionKDP surface, capacity and there than will the bedefect-free more energy KDP deposition surface, and near there the will defects. be more energy deposition near the defects.Based on the investigation of the influence of surface defects on the LID of KDP crystal materials, ChenBased et al. on [76 the] showedinvestigation that theof the light influence intensity of surf is seriouslyace defects a ffonected the LID by surfaceof KDP crystal defects, materials, and the Chencharacteristics et al. [76] of showed the light that field the modulation light intensity are is strongly seriously dependent affected onby thesurface geometry defects, of and defects. the characteristicsThe modulation of of the the light light field field modulation inside the material are strongly will cause dependent an uneven on the distribution geometry of of light defects. intensity, The modulationwhich in turn of a theffects light the field difference inside in the initial material energy will deposition cause an insideuneven the distribution material. Therefore, of light intensity, it is very whichimportant in turn to consider affects the the difference effect of defects in initial in the energy energy deposition deposition inside stage the of inducedmaterial. damage. Therefore, it is very Duchateauimportant to et consider al. [77] adopted the effect a new of defects mechanism in the based energy on deposition thermally driven stage of secondary induced pointdamage. defect formationDuchateau to simulate et al. the[77] process adopted of energya new absorptionmechanism in based precursor on thermally defects and driven surrounding secondary materials. point defectThe formation existence to ofsimulate precursor the defectsprocess aofffects energy the absorption in of precursor energy by defects electrons, and which surrounding in turn materials.leads to a rapid increase in lattice temperature. As shown in Figure 11a, the temperature exhibits nonlinearThe existence behavior of as precursor a function defects of temporal affects and the spatial absorption (T and of S) energy scales. Theby electrons, energy deposition which in takesturn leadsplace to inside a rapid the materialincrease beforein lattice the temperature. irradiation time As shown of 0.5 ns. in AtFigure the same11a, the time, temperature the plasma exhibits formed nonlinearinside the behavior material expandsas a function at an of almost temporal constant and spatial rate, which (T and makes S) scales. the volumeThe energy of energy deposition deposition takes placegradually inside increase, the material resulting before in a significantthe irradiation increase time in of temperature. 0.5 ns. At the After same 0.5 time, ns, the the thermal plasma explosion formed insidebehavior the ismaterial initiated, expands and the at subsequent an almost constant damage rate, is caused. which makes the volume of energy deposition graduallyThe e ffincrease,ect of precursor resulting defects in a significant on the initial increase energy in deposition temperature. is well After verified. 0.5 ns, Therefore, the thermal it is explosionnecessary behavior to further is explore initiated, the and influence the subsequent of defect morphologydamage is caused. and size parameters on LIDT, and to findThe out theeffect most of threateningprecursor defects surface on defects. the initial Then, energy the methods deposition for suppressingis well verified. or repairing Therefore, surface it is necessarydefects during to further the subsequentexplore the influence processing of candefect be morphology proposed to and improve size parameters the LID resistance on LIDT, of and KDP to findcrystals. out the Some most researchers threatening summarized surface defects. the various Then, defectsthe methods that introduced for suppressing by SPDF or repairing technique surface on the defectssurface during of KDP the crystal, subsequent and further processing explored can the be eff ectsproposed of diff erentto im defectsprove the on energyLID resistance deposition of usingKDP crystals.simulation Some and researchers experimental summarize methods.d the various defects that introduced by SPDF technique on the surface of KDP crystal, and further explored the effects of different defects on energy deposition using simulation and experimental methods. Appl. Sci. 2020, 10, 6642 14 of 32 Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 32

Figure 11.11. TheThe temporal temporal and and spatial spatial (T and(T and S) evolution S) evolutio of then predictedof the predicted lattice temperature, lattice temperature, the electronic the electronicdensities, anddensities, the absorbed and the powerabsorbed density power of density the precursor. of the precursor. (a) Lattice (a temperature) Lattice temperature with a1 temporal with a1 temporalscale, a2 spatialscale, a2 scale, spatial and scale,a3 the and physical a3 the quantities physical quantities depend both depend on space both andon space time; (andb) Electronic time; (b) Electronicdensities with densitiesb1 temporal with b1 scale temporal and b2 scalespatial and scale; b2 spatial and ( cscale;) Absorbed and (c) power Absorbed density power with densityc1 temporal with c1scale temporal and c2 scalespatial and scale. c2 spatial The position scale. The of precursor position of precursora1, b1, and ofc1 a1is, b1 in, the and center, c1 is in and the the center, actuation and theduration actuation of a2 duration, b2, and ofc2 a2is, 500b2, psand [77 c2]. is 500 ps [77].

Cheng et al. [18,78] [18,78] conducted the detailed invest investigationsigations on on surface cracks (Figure 12a)12a) and scratches (Figure 1212b)b) byby establishingestablishing the the simplified simplified defect defect models. models. The The surface surface scratches scratches produced produced by byflying flying cutting cutting were were simplified simplified into circular into circular and triangular and triangular to explore to the explore light intensification the light intensification distribution, distribution,as shown in Figureas shown 12(a2,b2). in Figure At present,12(a2,b2). the At light pres intensityent, the enhancementlight intensity factor enhancement (LIEF) was factor introduced (LIEF) wasinternationally introduced tointernationally characterize the to characterize intensity of distortion the intensity caused of distortion by defect caused inside theby defect materials. inside the materials. LIEF = Imax/I0 (14) = LIEF Imax/ I 0 (14) where Imax is the largest light intensity inside the KDP crystal with various defects, and I0 is the light whereintensity Imax inside is the the largest KDP light crystal intensity with ideal inside surface. the KDP A series crystal of ionization with various effects defects, such as and photoionization I0 is the light intensityand avalanche inside ionization the KDP of thecrystal material with will ideal be moresurface. likely A to series occur whenof ionization the LIEF reacheseffects asuch certain as photoionizationcritical value. The and electron avalanche density ionization will also of increase the materi significantly,al will be more resulting likely in to more occur energy when deposition the LIEF reachesinside the a certain material. critical The value. finite-di Theff erenceelectron time-domain density will (FDTD)also increase was usedsignificantly, by Cheng resulting to describe in more the energylight intensification deposition inside distribution the mate nearrial. the The defects finite-difference to reflect the time-domain difference in (FDTD) the initial was energy used by deposited Cheng toof describe different the types light of intensification defects. FDTD distribution algorithm near can maturelythe defects deal to reflect with non-uniformthe difference and in the complex initial energyshapes deposited of target radiation of different and types electromagnetic of defects. FD scattering.TD algorithm In addition, can maturely it is very deal convenientwith non-uniform to give anda demonstration complex shapes process of target of the radiation advancement and ofelectromagnetic each electromagnetic scattering. field In with addition, time. it The is lightvery convenientintensification to give can bea demonstration solved and visualized. process of Visualization the advancement can clearly of each simulate electromagnetic the distortion field caused with time.by light The intensification light intensification with the can influence be solved of defects,and visualized. and FDTD Visualization will not be limitedcan clearly by the simulate size of the distortionsimulated caused object, whichby light brings intensification convenience with to the the influence analysis [ 30of ,defects,62]. and FDTD will not be limited by the size of the simulated object, which brings convenience to the analysis [30,62]. Appl. Sci. 2020, 10, 6642 15 of 32 Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 32

FigureFigure 12. 12. ExplorationExploration of of the the influence influence of ofdefects defects on onenergy energy deposition. deposition. (a) Cracks; (a) Cracks; (b) Scratches; (b) Scratches; and (andc) Plastic (c) Plastic scratches scratches with withbrittle brittle cracks. cracks. a1 anda1 b1and areb1 theare topography the topography images images of defects; of defects; a2 anda2 andb2 areb2 theare simplified the simplified defect defect models; models; and a3 and anda3 b3and areb3 theare intensity the intensity distribution distribution with simplified with simplified defect model defect simulatedmodel simulated by finite-difference by finite-diff time-domainerence time-domain (FDTD) (FDTD) [18,78,79]. [18 ,78,79].

ItIt can can be be seen seen from from Figure Figure 12a,b12a,b that that all all the the defects defects of of KDP KDP crystal crystal will will produce produce diffraction diffraction enhancedenhanced ripples ripples with with the the irradiation irradiation of of laser. laser. Alth Althoughough the the light light intensity intensity distribution distribution caused caused by by the the triangulartriangular scratch scratch is more complexcomplex andand intenseintense thanthan that that of of the the circular circular scratch scratch due due to to the the existence existence of ofmanufacturing-induced manufacturing-induced front front surface surface scratches, scratches, the LIEFthe LIEF of scratches of scratches is still is smaller still smaller than thatthan of that cracks, of cracks,which iswhich similar is tosimilar reference to reference [20]. As shown[20]. As in shown Figure in12 c,Figure Zhuet 12c, al. [Zhu79] investigated et al. [79] investigated the influence the of influencesurface scratches of surface on thescratches incident on light the incident intensity ligh of KDPt intensity and found of KDP that and once found the brittle that cracksonce the appeared brittle crackson the appeared side of scratches, on the side the of LIEF scratches, would the increase LIEF would significantly, increase which significantly, also validated which thealso hazards validated of thebrittle hazards damage of brittle [19]. damage [19]. ChengCheng and and Yang et al. [[80]80] alsoalso noticednoticed thatthat crackscracks randomly randomly existed existed on on the the front front or or rear rear surfaces surfaces of ofthe the KDP KDP crystal, crystal, which which would would lead to lead different to differen degreest ofdegrees absorption of absorption of light intensity. of light The intensity. enhancement The enhancementof light intensity of light induced intensity by microcracks induced by on microcracks the front surface on the is mainlyfront surface caused is by mainly diffraction caused eff ect,by diffractionstanding wave effect, e ffstandingect and wave interference effect and ripples interfer effect.ence In ripples contrast, effect. the In light contrast, intensity the light enhancement intensity enhancementinduced by the induced rear surface by the crack rear issurface mainly crack due is to ma theinly total due reflection to the total or even reflection double or total even reflection double total(DTR) reflection effect of the(DTR) incident effect laser of atthe the incident crack wall. laser Although at the crack the light wall. intensity Although distribution the light induced intensity by distributionthe micro-cracks induced on the by rearthe micro-cracks surface is relatively on the simple,rear surface a series is relatively of hot spots simple, parallel a series to the of rear hot surface spots parallelwere formed to the betweenrear surface the totalwere reflected formed between light wave the and total incident reflected laser light in thewave sub-surface and incident ofmaterial, laser in thewhich sub-surface will induce of material, a strong phenomenonwhich will induce of light a st intensityrong phenomenon enhancement, of light as indicated intensity in enhancement, Figure 12(a3). asThus, indicated the energy in Figure deposition 12(a3). of Thus, laser the on theenergy rear deposition surface is much of laser more on seriousthe rear than surface the frontis much surface. more seriousBased than on the the front investigation surface. of the energy absorption capacity of different defects to the incident laser,Based Cheng on et the al. investigation [18] further summarized of the energy the absorption trend of LIEF capacity with theof different size parameters defects ofto surfacethe incident crack laser,defects, Cheng and et then al. [18] presented further the summarized degree of energy the trend deposition. of LIEF with the size parameters of surface crack defects,It can and be then seen presented in Figure the 13 thatdegree the of LIEF energy induced deposition. by the micro-crack defects on the surface of the KDPIt crystal can be is seen not in only Figure related 13 that to the the location LIEF induce of thed micro-cracks by the micro-crack and geometric defects structureon the surface parameters, of the KDPbut also crystal the is wavelength not only related of the to incidentthe location laser. of the The micro-cracks LIEF of KDP and rear geometric surface withstructure conical parameters, cracks in butFigure also 13 thec is wavelength more susceptible of the to incident have larger laser. values The inLIEF the of extremes KDP rear under surface the influence with conical of crack cracks depth in Figurethan other 13c is types more ofsusceptible defects. In to contrast,have larger the values LIEF ofin radialthe extremes cracks under and lateral the influence cracks in of Figurecrack depth 13a,b thanwill beother larger types on of the defects. front surfaceIn contrast, than the on LIEF the rearof radial surface. cracks In and general, lateral the cracks calculated in Figure LIEF 13a,b shows will a begradually larger on increasing the front trend surface with than the increaseon the rear of sizesurface. parameters. In general, Therefore, the calculated the number, LIEF geometric shows a gradually increasing trend with the increase of size parameters. Therefore, the number, geometric Appl. Sci. 2020, 10, 6642 16 of 32 Appl. Sci. 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 1616 ofof 3232 parameters,parameters, and and the the positions positions of of defects defects on on the the surf surfaceace of of crystal crystal element element can can be be controlled controlled to to ensure ensure thethethe laserlaser laser damagedamage damage resistanceresistance resistance ofof of KDPKDP KDP crystalcrystal crystal elements.elements. elements.

Figure 13. Inﬂuence of size parameter of surface cracks on light intensity under the irradiation of Figure 13. InfluenceInfluence ofof sizesize parameterparameter ofof surfacesurface crackscracks on light intensity under the irradiation of 1064 1064 nm, 532 nm, and 355 nm lasers. (a) Radial cracks; (b) Lateral cracks; (c) Conical cracks [18]. nm, 532 nm, and 355 nm lasers. (a)) RadialRadial cracks;cracks; ((b)) LateralLateral cracks;cracks; ((cc)) ConicalConical crackscracks [18].[18]. Correspondingly, Cheng et al. [18] used the R-on-1 laser irradiation mode to detect the surface Correspondingly, Cheng et al. [18] used the R-on-1 laserlaser irradiationirradiation modemode toto detectdetect thethe surfacesurface LIDT of KDP crystals with diﬀerent kinds of cracks, as shown in Figure 14. The cracks were established LIDT of KDP crystals with different kinds of cracks, as shown in Figure 14. The cracks were based on the exploration of references [18,81,82]. established based on the explorationion ofof referencesreferences [18,81,82].[18,81,82].

Figure 14. Experimentally tested laser induced damage thresholds (LIDTs) for crack-free and flawed Figure 14. Experimentally tested laser induced damage thresholdsthresholds (LIDTs)(LIDTs) forfor crack-freecrack-free andand flawedflawed KDP surfaces [18]. KDP surfaces [18]. The results of experiments are consistent with the simulation data well. The existence of The results of experiments are consistent with the simulation data well. The existence of brittle brittle cracks will greatly reduce the LIDT, which also verifies the rationality of FDTD algorithm cracks will greatly reduce the LIDT, which also verifiesfies thethe rationalityrationality ofof FDTDFDTD algorithmalgorithm simulationsimulation simulation well. well. To avoid the occurrence of brittle defects and a large amount of initial energy deposition, To avoid the occurrence of brittle defects and a largelarge amountamount ofof initialinitial energyenergy deposition,deposition, WangWang Wang et al. [74] conducted a lot of research on the brittle-plastic transition during the processing, et al. [74] conducted a lot of research on the brittle-plasticttle-plastic transitiontransition duringduring thethe processing,processing, whichwhich which provided a basis for obtaining the surface without brittle damage. provided a basis for obtaining the surface without brittle damage. In the process of processing KDP materials, the first condition that needs to be satisfied is that InIn thethe processprocess ofof processingprocessing KDPKDP materials,materials, thethe fifirstrst conditioncondition thatthat needsneeds toto bebe satisfiedsatisfied isis thatthat the micro-cracks and craters formed in the brittle zone will not extend underneath the final surface, thethe micro-cracksmicro-cracks andand craterscraters formedformed inin thethe brittlebrittle zozone will not extend underneath the final surface, another condition is that the feed rate used in the cutting process should be small enough to ensure Appl. Sci. 2020, 10, 6642 17 of 32 Appl. Sci. 2020, 10, x FOR PEER REVIEW 17 of 32 theanother fracture condition zone isformed that the in feed the rateprevious usedin cutting the cutting can processbe completely should beremoved small enough by the tosubsequent ensure the process,fracture zoneas shown formed in Figure in the 15. previous These cuttingconditions can can be completely be expressed removed as Equation by the (14). subsequent process, as shown in Figure 15. These conditions can be expressed as Equation (14).  <= 2 fx Yhc  22222 f x   Yc < hRxf= −+q( / 2) +q Rxf −− ( / 2)  2 2 2 2 (15)   R (x+ f /2) + R (x f /2) (15) ≤ p − − −  ftgR f c tc gR ≤

where Yc isis the the depth depth of of any any crack crack in in the the brittle brittle domain, domain, h isis the the distance distance between between the the origin origin of of crack crack and the ﬁnalfinal surface,surface, andandx xrepresents represents the the horizontal horizontal distance distance between between the the original original point point of cutting of cutting and andthe originalthe original point point of crack. of crack.R is theR is arc the radius arc radius of tool, of tool,tc is thetc is brittle the brittle plastic plastic transformation transformation depth, depth, and f andrepresents f represents the feed theper feed revolution. per revolution.

FigureFigure 15. 15. FormationFormation of of crack-free crack-free surface surface in in the the processing processing by circular edge cutting tool [74]. [74].

As long as the above conditions are met, the subsurfacesubsurface cracks generated in the brittle cutting zone will not continuecontinue toto propagate,propagate, and and subsequent subsequent processing processing can can also also guarantee guarantee the the appearance appearance of ofnon-destructive non-destructive machining machining surfaces surfaces as as much much as as possible. possible.

4. Simulation Simulation and and Experimental Experimental Exploration Exploration of Laser-Induced Damage Behavior in the Later Later Stage Stage Under the irradiation of subsequent laser with different parameters, the local energy deposited in Under the irradiation of subsequent laser with different parameters, the local energy deposited the early stage will be used as the energy source, which will cause thermal effects, phase changes and in the early stage will be used as the energy source, which will cause thermal effects, phase changes other phenomena in the material. The materials will eventually be damaged. Therefore, the pump and other phenomena in the material. The materials will eventually be damaged. Therefore, the probe method and hydrodynamics simulation techniques are viable solutions that crucial for describing pump probe method and hydrodynamics simulation techniques are viable solutions that crucial for the process between the initial energy deposition stage and the final damage crater formation. describing the process between the initial energy deposition stage and the final damage crater formation.4.1. Exploration of Transient Behavior at the Initiation of Damage

4.1. ExplorationAs shown of in Transient Section3 ofBehavi thisor review, at the duringInitiation the of energyDamage deposition, the plasma gradually formed and further promoted the energy deposition. The deposition energy in the damage area provides necessaryAs shown conditions in Section for subsequent3 of this review, plasma during expansion. the energy Then, deposition, the pressure the plasma will appear. gradually Actually, formed the andcorrespondingly further promoted increased the ofenergy pressure deposition. is the fluid The dynamic deposition effect energy of the energyin the damage deposition area process provides [83]. necessaryIn addition, conditions the for process subsequent from theplasma initial expansion. energy depositionThen, the pres stagesure to will the appear. final damage Actually, crater the correspondinglyformation is quite increased varied under of pressure the subsequent is the fluid irradiation dynamic of effect the ultrashort of the en orergy short deposition pulse laser process [84]. [83]. In general, the plasma formed inside the material will eject with the deposited energy. FerrerIn etaddition, al.[85] obtained the process plasma from emission the initial images energy with didepositionfferent pulse stage energies to the and final pulse damage durations crater by formationusing the lateral is quite imaging varied system. under the It can subsequent be found thatirradiation the energy of the deposition ultrashort inside or short the dielectric pulse laser material [84]. will beIn general, more significant the plasma under formed the irradiationinside the material of high-intensity will eject with ultra-short the deposited pulse laser. energy. In addition,Ferrer et al.the [85] diff obtainedusion range plasma of the emission energy depositedimages with inside different is small, pulse which energies will resultand pulse in a durations high concentration by using theof regional lateral imaging temperature. system. The It can plasma be found with that all the the deposited energy deposition energy will inside eject the from dielectric a small material area of willmaterial be more rapidly. significant It can beunder seen the in Figureirradiation 16a that of high-intensity the emitted plasma ultra-short carries pulse all the laser. energy, In addition, and the thetemperature diffusion onrange the of body the energy of the materialdeposited drops inside suddenly is small, withoutwhich will any result thermal in a conduction,high concentration which of regional temperature. The plasma with all the deposited energy will eject from a small area of material rapidly. It can be seen in Figure 16a that the emitted plasma carries all the energy, and the Appl.Appl. Sci. Sci. 20202020, 10, 10, x, 6642FOR PEER REVIEW 1818 of of 32 32 temperature on the body of the material drops suddenly without any thermal conduction, which also visuallyalso visually reflects reflects the phenomenon the phenomenon that the that plasma the plasma will carry will energy carry energyand eject and from eject the from interior theinterior of the material.of the material. The investigation The investigation of the shock of the shockwave ge wavenerated generated by the by instantaneous the instantaneous expansion expansion and the and emissionthe emission of plasma of plasma have have been been described described in refe in referencerence [86]. [86 The]. The plasma plasma will will expand expand rapidly rapidly after after absorbingabsorbing energy, energy, i.e., i.e., energy energy depo deposition,sition, as as shown shown in in Figure Figure 16b.16b. Due Due to to the the severe severe temperature temperature difference,difference, the outwardoutward released released plasma plasma will will induce induce the stress,the stress, which alsowhich contributes also contributes to the generation to the generationof shock waves. of shock Combined waves. Combined with the with images the detectedimages detected by TRPP by technology TRPP technology in Figure in Figure16, it can16, it be canreasonable be reasonable to infer to thatinfer that that the that shockwave the shockwave in KDP in willKDP be will generated be generated by mechanical by mechanical expansion expansion rather ratherthan thermalthan thermal expansion expansion under under the irradiation the irradiation of ultra-short of ultra-short pulse pulse laser [laser86]. [86].

FigureFigure 16. 16. TheThe shockwave shockwave and and plasma plasma emis emissionsion images images of of dielectric. dielectric. (a ()a )Plasma Plasma emission emission and and the the correspondingcorresponding intensity intensity cross cross sections sections along the z-axis. a1,, a2,, andand a3a3 obtainedobtained forforpulses pulses of of energies energies of of9.6 9.6µ μJ andJ and pulse pulse durations durations of of 100 100 fs, fs, 250 250 fs, fs, and and 550 550 fs, fs, respectively. respectively. In Ina4 a4, the, the pulse pulse duration duration is is 250 250 fs fsand and the the pulse pulse energy energy is 4isµ J;4 (μbJ;) The(b) shadowgraphsThe shadowgraphs of femtosecond of femtosecond laser ablation laser ablation of dielectric of dielectric recorded recordedat six diff aterent six different delay times delay by TRPP,times by where TRPP, S: shockwavewhere S: shockwave front; C: contact front; C: front; contact 1: the front; first stress1: the wavefirst stressfront; wave 2: the front; second 2: the stress second wave stress front; wave and fron F: filament.t; and F: filament. The energy The fluence energy on fluence the target on the surface target is 2 surface40 J/cm is ,40 and J/cm2, laser and pulse laser is 50 pulse fs [85 is, 8650]. fs [85,86].

InIn contrast, contrast, the the irradiation irradiation of of the the short short pulse pulse laser laser ensures ensures the the thermal thermal conduction. conduction. Unlike Unlike the the suddensudden emission emission of of plasma plasma under under the the irradiation irradiation of ofultra-short ultra-short pulse pulse laser, laser, the theemission emission process process of plasmaof plasma under under the irradiation the irradiation of short of short pulse pulse laser laserinvolves involves the parameters the parameters such suchas temperature as temperature and stress.and stress. AtAt the the end end of of the the plasma plasma formation formation process, process, th thee temperature temperature and and pressure pressure in in this this region region are are significantlysigniﬁcantly higher higher than than that that of of the the surroundings. surroundings. When When the the heat heat transferred transferred from from the the center center of of the the laserlaser beam beam to to the the surrounding surrounding area area gradually, gradually, a a non-uniform non-uniform temperature temperature field ﬁeld will will form form inside inside the the crystalcrystal material, material, causing causing a atemperature temperature gradient gradient insi insidede the the material material [87]. [87]. The The existence existence of of temperature temperature gradientgradient inside inside the the crystal crystal will will cause cause compressive compressive stress stress in in the the material. material. The The resulting resulting compressive compressive stressstress also also makes makes the thermalthermal expansionexpansion of of the the irradiation irradiation area area on on the the upper upper side side of the of absorptionthe absorption layer layerlarger larger than thethan expansion the expansion on the loweron the side. lower With side the. increaseWith the of theincrease temperature of the intemperature the irradiation in area,the irradiationthe stress statearea, in the the stress material state will in also theincrease, material which will also will increase, result in thewhich stress will gradients. result in The the resulting stress gradients.pressure gradientsThe resulting will leadpressure to the gradients formation will of lead a blast to the wave formation that propagates of a blast into wave the that background propagates gas. intoIt is the worth background noting that gas. the It pressureis worth gradientnoting that in the the vicinity pressure of gradient the precursor in the defect vicinity will of be the larger precursor due to defectthe large will amountbe larger of due energy to the deposition. large amount of energy deposition. Appl. Sci. 2020, 10, 6642 19 of 32 Appl. Sci. 2020, 10, x FOR PEER REVIEW 19 of 32

AA series series of of processes about the gradual initiation of damage under short-pulse laser irradiation were explored.explored. ManyMany models models were were built built for thefor gradual the gradual initiation initiation of damage. of damage. The equivalent The equivalent explosion explosionsimulation simulation model of the model laser-induced of the laser-induced damage process damage shows process the ejection shows phenomenon the ejection under phenomenon different underexplosion different conditions explosion with conditions the smoothed with the particle smoothed hydrodynamics particle hydrodynamics (SPH) method. (SPH) Yang method. et al. [ 88Yang,89] etestablished al. [88,89] theestablished multi-physics the multi-physics coupling model coupling for the model lateral for crack the lateral defect, crack which defect, further which verified further the verifiedexplosion the process explosion of reference process of [90 reference]. [90]. InIn order toto describedescribe thethe transient transient characteristics characteristics of of the the damage damage initiation initiation accurately, accurately, Wang Wang et al. et [ 90al.] [90]introduced introduced the explosion the explosion damage damage threshold threshol (EDT)d when(EDT) exploringwhen exploring the process the ofprocess damage of initiation.damage initiation.Under the Under irradiation the irradiation of the laser, of the the plasma laser, the explosion plasma will explosion occur, andwill theoccur, substance and the ejection substance will ejectionappear nearwill theappear laser near irradiation the laser point irradiation once the energypoint once reaches the theenergy threshold. reaches This the phenomenon threshold. This can phenomenonlast for several can microseconds last for several until mi thecroseconds damage pituntil is finallythe damage formed. pit Itis canfinally be seen formed. that It the can damage be seen is thatformed the atdamage a certain is formed angle both at a incertain Figure angle 17(a1,a2). both in In Figure addition, 17(a1,a2). with the In increase addition, of with the irradiation the increase time, of thethe irradiation damage pits time, show the the damage trend of pits expanding show the from trend the of section expanding of point from to the cone, section with of di ffpointerent to degrees cone, withof expansion different indegrees both the of horizontalexpansion andin both vertical the horizontal directions, and and vertical eventually directions, form the and bowl-shaped eventually formdamage the pits.bowl-shaped Therefore, damage further pits. exploration Therefore, in termsfurther of exploration damage expansion in terms isof neededdamage to expansion study other is neededsignificant to study phenomena other significant that may aphenomenffect the damagea that may formation. affect the damage formation.

FigureFigure 17. EvolutionEvolution process process of of damage damage initiation. initiation. ( (aa)) Simulated Simulated damage damage crater crater formation formation process process in in thethe laser laser damage damage area; area; ( (b)) Simulated Simulated temperature variation in the laser damage area [[88,90].88,90].

FigureFigure 17b17b shows the temperature variation variation (par (parameterameter T T in in the the model) distribution of of the the explosionexplosion zone zone with with time during nanosecond laser ir irradiation.radiation. It It can can be be seen seen that that the cross-section areaarea of of high-temperature high-temperature region gradually expands. Figure 17(b2)17(b2) shows shows the the phase phase state state of of the the ejected ejected materialmaterial at at different diﬀerent times delay clearly. AsAs shown shown in in Figure Figure 17(b1),17(b1), the the temperature temperature at at the the laser laser irradiation irradiation point point is is the the highest, highest, exceeding exceeding thethe melting melting point point of of KDP KDP crystal, crystal, which which will will cause cause the the material material in in the the area area to be gaseous state. Since Since thethe nanosecond nanosecond laser laser irradiation irradiation ti timeme ensures ensures the the thermal thermal conduction, conduction, the the temperature temperature of of the the central central areaarea will will gradually gradually conduct conduct to to the the vicinity vicinity and and expand, expand, the the thermal thermal damage damage area area will will show show a a growth. growth. Actually, the expansion rate of the high high temperature temperature region region is is particularly particularly severe severe in in the the gaseous gaseous state, state, which will lead to significant signiﬁcant thermal stress near the damage pit. In addition, with reference to Figure 17b, high-temperature gaseous substances were ejecting outward during the initiation of damage. In the case of the increase of time delays, it can be seen that Appl. Sci. 2020, 10, 6642 20 of 32

In addition, with reference to Figure 17b, high-temperature gaseous substances were ejecting Appl. Sci. 2020, 10, x FOR PEER REVIEW 20 of 32 outward during the initiation of damage. In the case of the increase of time delays, it can be seen that in inthe the later later physical physical process process of theof the material material being being irradiated irradiated by laser, by laser, the gaseous the gaseous substance substance in the in earlier the earlierstage wasstage converted was converted into a multi-phaseinto a multi-phase mixture mixtur composede composed of solid, of gaseous,solid, gaseous, and liquid and statesliquid of states KDP. ofThis KDP. phenomenon This phenomenon is consistent is consistent with the substancewith the substance ejection behavior ejection experimentally behavior experimentally that observed that in observedthe laser in damage the laser processes damage reported processes in reported reference in [91 reference]. With the [91]. further With developmentthe further development of the ejection of theprocess, ejection almost process, all thealmost ejected all the particles ejected were particles observed were toobserved exist in to solid exist form. in solid Wang form. et al.Wang indicated et al. indicatedthat the ejection that the of ejection solid particles of solidstarts particles at about starts the at 100about ns the delay, 100 which ns delay, also which means also the initiationmeans the of initiationmechanical of mechanical damage. damage. InIn order toto investigate investigate the the thermal thermal stress stress and mechanicaland mechanical stress causedstress caused by plasma by duringplasma explosion, during explosion,the shock wavethe shock evolution wave process evolution that process showed that with showed TRPP technology with TRPP and technology simulation and means simulation are also meansused to are characterize also used to the characterize energy conversion the energy and conv the processersion and between the process the initial between energy the deposition initial energy stage depositionand the ﬁnal stage damage and the crater final formation damage crater indirectly. formation indirectly. TheThe propagation propagation of of air air shockwave shockwave was was directly directly ex extractedtracted from from the the results results of of FEM FEM (Finite (Finite Element Element Method)Method) simulation simulation [90,92], [90,92], as as shown shown in inFigure Figure 18(a1–a3). 18(a1–a3). It can It canbe seen be seen that thatthe attenuation the attenuation of the of shockwavethe shockwave speed speed follows follows the exponential the exponential law, law,whic whichh has been has been summarized summarized in the in Figure the Figure 18(a4). 18(a4). In addition,In addition, Figure Figure 18(a7) 18(a7) shows shows the the shockwave shockwave velocity velocity well well by by experiments. experiments. It Itis isclear clear that that the the shockwaveshockwave velocityvelocity increaseincrease with with the the increment increment of laser of ﬂuences.laser fluences. The propagation The propagation paths of shockwavepaths of shockwavein Figure 18 in(a5,a6) Figure and 18(a5,a6) Figure 18andb are Figure consistent, 18b are which consistent, also validates which also the correctnessvalidates the of correctness the simulation of theresults simulation in Figure results 18(a1–a3). in Figure 18(a1–a3).

FigureFigure 18. 18. TheThe evolution evolution diagram diagram of of shock shock wave. wave. (a ()a )Air Air shockwave shockwave propagation propagation under under different different 9 9 explosionexplosion conditions. conditions. a1,a1 a2,, a2,andand a3 area3 theare simulated the simulated condition condition that E thatexp = E1eexp−9, =Eexp1e =− 2e, −9E, expand= E2eexp− = , 9 5eand−9, respectively,Eexp = 5e− , respectively,with a4 the influence with a4 the of influenceexplosion of energy explosion on the energy shockwave on the velocity. shockwave a5 velocity.and a6 area5 theand time-resolveda6 are the time-resolved image series image of laser-induced series of laser-induced KDP surface KDP damage surface process damage with process fluences with 20 fluences J/cm2 2 2 and20 J50/cm J/cmand 2, respectively, 50 J/cm , respectively, and a7 the andshockwavea7 the shockwavevelocity [90,93]; velocity (b) The [90, 93propagation]; (b) The propagationof shockwave of inshockwave pump laser in pulse pump duration laser pulse from duration 0.2 ns to from 100 0.2ns [69]. ns to 100 ns [69].

Based on the above simulation of damage initiation process and the detection of TRPP technology under nanosecond laser irradiation, it can be inferred that thermal stress plays an important role in the Appl.process Sci. 2020 of damage, 10, x FOR initiation PEER REVIEW and damage forming. In order to verify the influence that thermal21 stress of 32 plays in damage forming, a series of exploration had been conducted by researchers. CombinedCombined with with the the properties properties of of KDP KDP crystals, crystals, it it can can be be found found that a a non-uniform temperature temperature fieldfield will will form form inside inside the the crystal crystal material material when when the the heat heat transferred transferred from from the center center of of the the laser laser beam beam toto the the area area of of the the surrounding surrounding area area gradually, gradually, ca causingusing a a temperature temperature gradient gradient inside inside the the material. material. The existence existence of of temperature temperature gradient gradient inside inside the the cr crystalystal will will cause cause compressive compressive stress stress in in the the material. material. InIn order toto exploreexplore the the magnitude magnitude and and eff ecteffect of stressof stress more more concisely, concisely, Zeng Zeng et al. [et94 ]al. conducted [94] conducted further furtherinvestigation investigation and found and afound second a second wave wouldwave would be generated be generated inside inside the fused the fused silica silica due todue the to high the highback-pressure back-pressure of the of laser-generated the laser-generated plasma, plasma, as shown as shown in Figure in Figure 19b. 19b. In addition, In addition, Demos Demos et al. et [ 95al.] [95]explored explored and and compared compared the kinetic the kinetic behavior behavior of KDP of KDP and fusedand fused silica silica under under laser laser irradiation. irradiation. It isclear It is clearthat thethat expansion the expansion of shockwaves of shockwaves in air in in air Figure in Figure19a 19a is quite is quite similar similar in both in materialsboth materials at the at same the samedelay delay time, whichtime, which also suggests also suggests that the that initial the energy initial deposited energy deposited in a single in laser-induced a single laser-induced breakdown breakdownevent was roughly event was the roughly same. the same.

FigureFigure 19. TheThe effect eﬀect and distribution of stress. ( (aa)) Comparison Comparison of of the the shockwave shockwave in in fused fused silica silica and and KDP.KDP. The shockwave propagating is denoted by an arrow [94]; [94]; ( (b)) Propagation of of shockwaves in in materials.materials. The The shock shock front front (S), (S), back back-pressure-pressure induced induced shock shock wave wave (B); (B); ( (cc)) Analysis Analysis of of the the stress stress action action onon crystalcrystal materialmaterial under unde laserr laser irradiation irradiation [95]. c1[95].The c1 distribution The distribution of thermal of stressthermal due tostress temperature due to temperaturegradient and gradient expansion; andc2 expansion;The deformation c2 The deformation of crystal materials of crystal under materi tensileals under stress tensile [87]. stress [87].

AlthoughAlthough there there are are relatively few few observations on on the the internal internal response response of of KDP crystals at at present, itit isis reasonable reasonable to to infer infer that that the the back-pressure back-pressure induced induced shockwave shockwave with awith propagation a propagation pattern patternapproximated approximated shown inshown Figure in 19 Figureb will 19b also will occur also inside occur KDP inside crystals KDP crystals based on based the principle on the principle analysis analysisof thermal of stressthermal on stress KDP crystalson KDP and crystals the comparative and the comparative exploration exploration of shockwave of shockwave between fused between silica fusedand KDP. silica However, and KDP. considering However, theconsidering anisotropy the and anis softotropy brittleness and soft of thebrittleness KDP crystal, of the the KDP shape crystal, and thedirectionality shape and of directionality the back-pressure of the shockwave back-pressure generated shockwave inside generated the material inside need the to bematerial further need studied to beand further explored. studied and explored. In later stages, the compressive stress generated in the early stages also makes the thermal expansion of the irradiation area on the upper side of the absorption layer larger than the expansion on the lower side. With the increase of the temperature in the irradiation area, the stress state in the material will increase, and the material is completely in an elastic deformation state. For crystal materials, plastic deformation will occur when the temperature of the irradiated area exceeds the threshold that the material can withstand. Dowden et al. [96,97] proposed that the Appl. Sci. 2020, 10, 6642 22 of 32

Appl. Sci. 2020, 10, x FOR PEER REVIEW 22 of 32 In later stages, the compressive stress generated in the early stages also makes the thermal expansion of the irradiation area on the upper side of the absorption layer larger than the expansion existence of plastic deformation will relieve the stress state. At last, the internal temperature of the on the lower side. With the increase of the temperature in the irradiation area, the stress state in the crystal materialmaterial will will increase, gradually and the cool material down is completely with the in end an elastic of laser deformation irradiation, state. while the generated compressiveFor stress crystal can materials, be relaxed plastic and deformation convert will to occurtensile when stress the temperature [98,99]. KDP of the crystal irradiated materials area will produceexceeds birefringence the threshold during that the laser material irradiation can withstand. due Dowden to its et anisotropic al. [96,97] proposed properties. that the existence Owing to the existenceof of plastic the deformation birefringence will relieve effect, the the stress incident state. At last,laser the will internal be temperaturedecomposed of the into crystal two material beams with will gradually cool down with the end of laser irradiation, while the generated compressive stress can different characteristics. The transmission and focusing characteristics of the incident laser will also be relaxed and convert to tensile stress [98,99]. KDP crystal materials will produce birefringence during be changedlaser accordingly, irradiation due which to its anisotropic will cause properties. an asymme Owingtric to thedistribution existence of of the thermal birefringence stress eff ect,in crystals [100–102].the Therefore, incident laser the will residual be decomposed stress intois easily two beams generated with diff erentduring characteristics. the relaxation The transmission of stress in elastic and plasticand areas. focusing Figure characteristics 19(c2) [87] of the shows incident the laser deformation will also be changedprocess accordingly,of crystal materials which will causeunder tensile stress. an asymmetric distribution of thermal stress in crystals [100–102]. Therefore, the residual stress is easily generated during the relaxation of stress in elastic and plastic areas. Figure 19(c2) [87] shows the In general, based on the investigation of the transient behavior of damage initiation and the deformation process of crystal materials under tensile stress. evolution of Inshock general, wave, based it can on thebe investigationfound that the of therapid transient plasma behavior injection of damage under initiationthe irradiation and the of ultra- short pulseevolution laser ofwill shock cause wave, mechanical it can be found damage. that the However, rapid plasma there injection is a underthermal the irradiationconduction of process under theultra-short irradiation pulse of laser short will causepulse mechanical laser, which damage. will However, lead to there the iscoexistence a thermal conduction of thermal process stress and mechanicalunder stress, the irradiation which has of shortan effect pulse on laser, the which material. will lead to the coexistence of thermal stress and mechanical stress, which has an effect on the material.

4.2. Morphological4.2. Morphological Characteristics Characteristics of Laser-Induced of Laser-Induced Damage Damage in in the the Later Later Stage Stage Based onBased the onexploration the exploration of the of the transient transient behavior behavior at at the the initiation initiation of damage, of damage, the main the damage main damage morphologymorphology under underdifferent diﬀerent pulses pulses can can be be shown shown inin Figure Figure 20 .20.

Figure 20.Figure Typical 20. Typical morphological morphological comparison comparison of of the laserlaser damage damage sites sites on KDP on crystalKDP crystal surfaces surfaces induced inducedby short by and short ultra-short and ultra-short laser laser pulses. pulses. (a) ( aFWHM) FWHM (Full (Full WaveWave at at Half Half Maximum) Maximum)= 6.9 = ns; 6.9 ns; (b) (b) FWHM = 28 fs [29]. FWHM=28 fs [29]. The laser energy deposited inside the crystal material can be transferred to the surrounding Thematerial laser energy area through deposited thermal conductioninside the under crystal the irradiation material of can short be laser. transferred According toto the the analysis surrounding materialof area the thermalthrough conduction thermal in conduction Section 4.1., there under will bethe stress irradiation near the erosion of short pit. Actually,laser. According the brittle to the analysis crystalof the materials thermal have conduction strong fracture in Section toughness 4.1., against there compressive will be stress stress, near but much the erosion smaller against pit. Actually, tensile stress. As a result, the cracks will occur around the molten pit if the tensile stress exceeds the brittlethe crystal fracture toughnessmaterials of have KDP crystals.strong fracture However, KDPtoughness crystal materialsagainst arecompressive anisotropic, whichstress, will but much smaller againstlead to the tensile generation stress. of non-directionalAs a result, the cracks. cracks Therefore, will occur the damage around process the molten of the material pit if the is tensile stress exceedsaccompanied the byfracture melting, cracktoughness propagation of etKDP al., and crystals. the ﬁnal morphologyHowever, of theKDP damage crystal point materials is also are anisotropic,more which complicated, will aslead shown to inthe Figure generation 20a [29]. Theof experimentalnon-directional investigations cracks. ofTherefore, short pulse laserthe damage process ofinduced the material damage haveis accompanied veriﬁed the rationality by melting, of the crack damage propagation mechanism thatet al., discussed and the in final the above morphology sections well. In contrast, the morphology of the damage pit under ultra-short pulse laser irradiation in of the damage point is also more complicated, as shown in Figure 20a [29]. The experimental investigations of short pulse laser induced damage have verified the rationality of the damage mechanism that discussed in the above sections well. In contrast, the morphology of the damage pit under ultra-short pulse laser irradiation in Figure 20b is obviously different from that of short pulse laser irradiation. The laser energy deposited inside the crystalline material is too late for thermal conduction to occur due to the ultra-short pulse period. In addition, the morphology of the damage pit that shown in Figure 20b is a good verification of the correctness of the inference that shockwave is generated by mechanical expansion for KDP under ultrashort pulse irradiation in Section 4.1. As a result, the laser damage only occurred in the local high-temperature plasma area near the focal point of the laser spot, and there was no obvious cracking around the damage point. In the study of nanosecond laser induced damage, Liu et al. [103] found that the incident laser fluence has an effect on the morphology and size of the damage of KDP crystal. Based on the observation of damage Appl. Sci. 2020, 10, 6642 23 of 32

Figure 20b is obviously different from that of short pulse laser irradiation. The laser energy deposited inside the crystalline material is too late for thermal conduction to occur due to the ultra-short pulse period. In addition, the morphology of the damage pit that shown in Figure 20b is a good verification of the correctness of the inference that shockwave is generated by mechanical expansion for KDP under ultrashort pulse irradiation in Section 4.1. As a result, the laser damage only occurred in the local high-temperature plasma area near the focal point of the laser spot, and there was no obvious cracking around the damage point. In the study of nanosecond laser induced damage, Liu et al. [103] Appl. Sci. 2020, 10, x FOR PEER REVIEW 23 of 32 found that the incident laser fluence has an effect on the morphology and size of the damage of KDP crystal. Based on the observation of damage morphology, these damages can be divided into three morphology,typical these types: damages crack, shell, can and be crater, divided as shown into inthree Figure typical 21. types: crack, shell, and crater, as shown in Figure 21.

Figure 21. Microscopic images of the typical damage morphology. (a) Crack, (b) shell, and (c) crater. Figure 21. Microscopic images of the typical damage morphology. (a) Crack, (b) shell, and (c) crater. Λ = 351 nm, pulse duration = 5 ns [102]. Λ = 351 nm, pulse duration = 5 ns [102]. The crack-type damage always generates on the surface without any connection with the thermal 2 Theexplosion crack-type and fracturingdamage inside always the bulkgenerates under laser on ﬂuence the surface smallerthan without 7.5 J/cm any. Liu connection et al. proposed with the that crack damage is caused by the residual stress during processing. Once the laser ﬂuence more thermal explosion and fracturing inside the bulk under laser fluence smaller than 7.5 J/cm2. Liu et al. than 7.5 J/cm2 is used to irradiate KDP crystal material, shell-type damage, and crater-type damage proposedbegin that tocrack appear. damage Among is them, caused the mechanismby the residu of shellal stress damage during is the processing same as that. ofOnce crack the damage, laser fluence more thanwhich 7.5 isJ/cm caused2 is by used the residual to irradiate stress during KDP processing.crystal material, Shell damage shell-type accounts damage, for 80% of and thetotal crater-type damage damage,begin to while appear. crater Among damage is them, mainly the caused mechanism by a large amount of shell of thermal damage absorption is the same and conduction as that of crack damage,due which to the is residualcaused defectsby the inresidual the previous stress processing. during processing. Based on the Shell analysis damage of the accounts morphology for 80% of the total anddamage, composition while ofcrater the damage damage surface, is mainly the morphology caused by characteristicsa large amount of LIDof thermal has been absorption further and supplemented in Figure 22. conduction due to the residual defects in the previous processing. Based on the analysis of the morphology and composition of the damage surface, the morphology characteristics of LID has been further supplemented in Figure 22.

Figure 22. Typical morphology of the laser-induced damage. (a) Damage craters under the irradiation of nanosecond laser (λ = 355 nm) with a1 before irradiation, a2 fluences (17.3 J/cm2), and a3 fluences Appl. Sci. 2020, 10, x FOR PEER REVIEW 23 of 32 morphology, these damages can be divided into three typical types: crack, shell, and crater, as shown in Figure 21.

Figure 21. Microscopic images of the typical damage morphology. (a) Crack, (b) shell, and (c) crater. Λ = 351 nm, pulse duration = 5 ns [102].

The crack-type damage always generates on the surface without any connection with the thermal explosion and fracturing inside the bulk under laser fluence smaller than 7.5 J/cm2. Liu et al. proposed that crack damage is caused by the residual stress during processing. Once the laser fluence more than 7.5 J/cm2 is used to irradiate KDP crystal material, shell-type damage, and crater-type damage begin to appear. Among them, the mechanism of shell damage is the same as that of crack damage, which is caused by the residual stress during processing. Shell damage accounts for 80% of the total damage, while crater damage is mainly caused by a large amount of thermal absorption and conduction due to the residual defects in the previous processing. Based on the analysis of the morphologyAppl. Sci. 2020, 10and, 6642 composition of the damage surface, the morphology characteristics of LID has24 been of 32 further supplemented in Figure 22.

FigureFigure 22. 22. TypicalTypical morphology morphology of of the the laser-induced laser-induced damage. damage. (a (a) )Damage Damage craters craters under under the the irradiation irradiation 2 ofof nanosecond nanosecond laser laser ( (λλ == 355355 nm) nm) with with a1a1 beforebefore irradiation, irradiation, a2a2 fluencesfluences (17.3 (17.3 J/cm J/cm2),), and and a3a3 fluencesfluences (19.5 J/cm2) shows the central core [92]; (b) Damage under λ = 355 nm, and 6.4 ns pulse duration with b1 reflection 3D scan image and b2 transmission 3D scan image [29].

It can be seen in Figure 22 that the damage pit presents a cone-shaped crater with a melted core region and fractured periphery. The cracks stretch outward. It is clear that the laser heating and material structural deformation are coupled to each other and then contribute to the LID of KDP crystal. Hou et al. [104] proposed that the temperature gradient between the thermal affected region and the surrounding normal crystal material will cause a bursting random “clam-shaped” peeling layer with unpredictable directions around the damage point due to the soft, brittle, and the highly anisotropic characteristics of KDP crystal. In addition, the nonlinear absorption characteristics of the initial defects of the KDP crystal also have a great influence on the subsequent damage with the increase of laser fluence. The more energy the material absorbs, the greater the energy of the thermal explosion shock wave. As a result, the damage will be more serious, and the number and size of crater damage will become larger. It is important to observe the initiation of damage in real-time under the irradiation of high fluence.

4.3. Outlook of the Prediction of Damage Threshold The three stages from the initiation to the finalization of LID (i.e., energy deposition, damage initiation, and damage forming) was described clearly in the above sections to understand the mechanism of LID and to predict the LIDT more accurate. Actually, the deposition of energy will cause the initiation of damage once it reaches a certain degree. Based on the analysis of the initial energy deposition under the irradiation of laser, it can be found that all the methods of embodying the peak energy the material can withstand are not intuitive. The electron density curve of KDP materials and the transmission or absorption of the incident laser can only reflect the trend of damage, which is not the great methods to predict the damage threshold. The technology of TRPP in Section 3.1.2. captures the momentary dynamic behavior of damage initiated on KDP materials. However, it is still not enough to improve the anti-laser damage ability of KDP crystal solely rely on the prediction of damage trends. In addition, some surface defects tend to be leaved on the surface of KDP after processing. Under these conditions, accurate and intuitive prediction of damage threshold remains a challenge. To overcome this difficulty, our research group has carried out some relevant research on the prediction of LIDT. In the early stage, Chen et al. [76,78] used the FTDT algorithm to explore the phenomenon of light intensity modulation inside the material under the analysis of the electric field. Based on the exploration of the damage trend, the multi-physics and electric field coupling technology was discovered and applied to the prediction of the LIDT of KDP crystals with different cracks defects in reference [88]. The multi-physics coupling technology reveals the mechanism of damage Appl. Sci. 2020, 10, 6642 25 of 32

Appl.more Sci. comprehensively 2020, 10, x FOR PEER and REVIEW gives the detailed damage thresholds and damage mechanism intuitively,25 of 32 Appl.as shown Sci. 2020 in, 10 Figure, x FOR 23 PEER. REVIEW 25 of 32

Figure 23. Transient dynamic behavior induced by laser damage on the surface of KDP crystal with Figureconical 23. crack Transient defect. (dynamic a) Multi-physics behavior coupling induced simulation by laser damage result with on the a1 cratersurface morphology, of KDP crystal a2 vapor with conicalvelocityconical crack crack distribution, defect. defect. ( (a aand)) Multi-physics Multi-physics a3 temperature coupling coupling distribution simulation simulation of localresult result laser with with damagea1a1 cratercrater area; morphology, morphology, (b) Experimental a2a2 vaporvapor velocityimages captureddistribution, by the and and system a3 temperature in the laser distribution distribution damage micro-region of of local laser with damage b1 material area; ( b ejection) Experimental and b2 imagesplasmaimages capturedexpansion captured by by [89]. the the system system in in the the laser laser damage damage micro-region micro-region with with b1 material ejection and and b2 plasma expansion [89]. [89]. In addition, in order to investigate the LID of KDP crystals more comprehensively, our research groupInIn subsequentlyaddition, addition, in in order order carried to to investigate investigateout the corresponding the the LID LID of of KDP KDP verification crystals crystals moreof more damage comprehensively, comprehensively, mechanism and our research researchdamage groupthreshold subsequently in engineering. carried The out damage the corresponding repair devices verification veriﬁcation [105,106] asof welldamage as repair mechanism strategies and [107,108], damage thresholdwerethreshold explored in in engineering. engineering. with a view The The to achievingdamage damage repair repair the signific devices devicesance [105,106] [105 of, 106predicting] as as well well theas as repair repairdamage strategies strategies threshold. [107,108], [107 Figure,108], were24 demonstrates explored withwith the aa designed viewview toto achieving achievingrepair device the the signiﬁcance significand theance repair of of predicting predictingprocess of the thedamage damage damaged threshold.pits threshold. of our Figureresearch Figure 24 24group.demonstrates demonstrates the the designed designed repair repair device device and and the repairthe repair process process of damaged of damage pitsd of pits our of research our research group. group.

Figure 24. Micro-milling set-up developed for the repairment of damage on KDP crystal. ( a) Overall Figureconfigurationconﬁguration 24. Micro-milling of the 5-axis set-up vertical developed spindle machine for the repairment tool; ( b) The The of repairing repairingdamage onprocess KDP [105]; [crystal.105]; ( (c )( Schematica Schematic) Overall configurationof the generationgeneration of the processprocess 5-axis of verticalof tool tool marks marksspindle in in the machine the micro micro ball-endtool; ball-end (b) millingThe millingrepairing repair repair process process of[105]; KDP of (c opticsKDP) Schematic optics [107]. of[107]. the generation process of tool marks in the micro ball-end milling repair process of KDP optics [107]. For the design of the repair strategies, Chen [105,109] and Liu et al. [110] conducted various measurementsFor the design on LIDT of the to ensurerepair thatstrategies, the repair Chen wo [105,109]rk is effective and Liuin raising et al. [110]the LIDT conducted of KDP variouscrystal. measurements on LIDT to ensure that the repair work is effective in raising the LIDT of KDP crystal. Appl. Sci. 2020, 10, 6642 26 of 32

For the design of the repair strategies, Chen [105,109] and Liu et al. [110] conducted various Appl. Sci. 2020, 10, x FOR PEER REVIEW 26 of 32 measurements on LIDT to ensure that the repair work is eﬀective in raising the LIDT of KDP crystal. Both the experimental and simulation data well demonstrate the reasonableness of the designed Both the experimental and simulation data well demonstrate the reasonableness of the designed restoration strategy, as shown in Figure 25. restoration strategy, as shown in Figure 25.

Figure 25. MorphologiesMorphologies of of the the various various KDP KDP surfaces surfaces before before (a1 (,a1 b1, b1, and, and c1)c1 and) and after after (a2, ( a2b2,, b2and, and c2) 355c2) 355nm-laser nm-laser pulse pulse irradiation. irradiation. (a) Initial (a) Initial crack crackdamaged damaged surfaces; surfaces; (b) Repaired (b) Repaired pit without pit without tool marks; tool (marks;c) Repaired (c) Repaired pit with pit tool with marks. tool marks. a3, b3,a3 ,andb3, andc3 arec3 arethethe light light intensity intensity enhancement enhancement factor factor (LIEF) (LIEF) distribution [18,105]. [18,105]. However, based on the consideration of the different effects of various initial defects of KDP However, based on the consideration of the different effects of various initial defects of KDP materials on energy deposition, a more comprehensive application of multi-physical coupling materials on energy deposition, a more comprehensive application of multi-physical coupling techniques is needed. techniques is needed. 5. Summary and Outlook 5. Summary and Outlook In summary, the emergence of chirped pulse amplification technology has greatly promoted the developmentIn summary, and the application emergence of rapidof chirped ignition pulse in ampl laser-drivenification inertial technology confinement has greatly fusion. promoted However, the development and application of rapid ignition in laser-driven inertial confinement fusion. However, it is necessary to consider the limitations caused by the low LIDT of core optical components such as it is necessary to consider the limitations caused by the low LIDT of core optical components such as KDP crystals when constructing a laser system with high output power density. The impurities in the KDP crystals when constructing a laser system with high output power density. The impurities in process of the formation of crystal materials can be controlled greatly due to the advance of technology. the process of the formation of crystal materials can be controlled greatly due to the advance of However, the occurrence of defects or scratches on surface is inevitable during the application of technology. However, the occurrence of defects or scratches on surface is inevitable during the crystalline optical materials. As a result, it is very necessary to understand the intrinsic damage application of crystalline optical materials. As a result, it is very necessary to understand the intrinsic mechanism of crystal materials under the laser irradiation. Only understanding the mechanism of damage mechanism of crystal materials under the laser irradiation. Only understanding the laser induced damage of the crystal material, can the comprehensive repair of the damaged points be mechanism of laser induced damage of the crystal material, can the comprehensive repair of the proceeded. The accurate mitigation can ensure that there is no residual mechanical or thermal stress to damaged points be proceeded. The accurate mitigation can ensure that there is no residual fail the repair. mechanical or thermal stress to fail the repair. To address this problem, the principles of LID has been reviewed clearly. The mechanism is mainly To address this problem, the principles of LID has been reviewed clearly. The mechanism is summarized as impurity thermal damage and ionization damage. Then, the process from the initiation mainly summarized as impurity thermal damage and ionization damage. Then, the process from the to the finalization of LID has been divided into three stages (i.e., energy deposition, damage initiation, initiation to the finalization of LID has been divided into three stages (i.e., energy deposition, damage and damage forming) to ensure the understanding of LID. It is worth noting that the three stages initiation, and damage forming) to ensure the understanding of LID. It is worth noting that the three stages are complementary to each other. The energy deposition inside the KDP crystal material is serious under high-intensity laser fluence, especially when there are various processing defects remaining on the surface of KDP. The influence of various initial defects on the forming of damage also has great difference. In addition, different degrees of initial energy deposition under the Appl. Sci. 2020, 10, 6642 27 of 32 are complementary to each other. The energy deposition inside the KDP crystal material is serious under high-intensity laser fluence, especially when there are various processing defects remaining on the surface of KDP. The influence of various initial defects on the forming of damage also has great difference. In addition, different degrees of initial energy deposition under the subsequent irradiation with different wavelengths and laser fluence will also have a great impact on damage forming. The damage has various morphology. It is mainly composed of cracks, bursts, and melting pit under short pulse laser irradiation, which also means the thresholds for damage initiation under various types of initial defects are different. Thus, the transient dynamic behavior of multi-physics coupling simulation and some experiments are still needed to further explore to determine the threshold in different situations.

Author Contributions: Conceptualization, M.C. and W.D.; methodology, M.C.; investigation, H.Y. and Q.L.; validation, J.C.; writing—original draft preparation, W.D.; writing—review and editing, M.C., W.D., and J.C.; Funding acquisition, M.C. and J.C. All authors have read and agreed to the published version of the manuscript. Funding: We acknowledge the support of Science Challenge Project [No. TZ2016006-0503-01]; the National Natural Science Foundation of China [Nos. 51775147, 51705105]; the Young Elite Scientists Sponsorship Program by CAST [No. 2018QNRC001]; the China Postdoctoral Science Foundation [Nos. 2017M621260, 2018T110288]; the Heilongjiang Postdoctoral Fund [No. LBH-Z17090]; the Fundamental Research Funds for the Central Universities [No. HIT.NSRIF.2019053]; and the Self-Planned Task Foundation of State Key Laboratory of Robotics and System (HIT) of China [No. SKLRS201718A, SKLRS201803B]. Conﬂicts of Interest: The authors declare that they have no conﬂict of interest.

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