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Abstract
The increasing use of transparent ceramics in laser systems presents a challenge; their low damage threshold has become a significant impediment to the development of powerful laser systems. Consequently, it is imperative to undertake research into the damage sustained by these materials. Micropores, the most common structural defects in transparent ceramics, inevitably remain within the material during its preparation process. However, the relationship between the density and size of these micropores and their impact on nanosecond laser damage threshold and damage evolution remains unclear. In this study, we utilize the annealing process to effectively manage the density and size of micropores, establishing a correlation between micropores in relation to damage thresholds. This study confirms for the first time that micropores significantly contribute to laser damage, comparing and analyzing the damage morphology characteristics of both front and rear surfaces of transparent ceramics. It also presents, potential mechanisms that may contribute to these differences in damage. This paper offers guidance for controlling micropores during the preparation and processing of transparent ceramics with high laser damage thresholds. The findings are expected to further improve the anti-nanosecond laser damage capabilities of transparent ceramics.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Lasers have been a topic of study since their discovery in the 1960s. In 1995, sintering of Nd:YAG transparent ceramic by Ikesu et al. introduced the possibility of transparent ceramic as a novel type of laser gain medium with significant potential [1]. Over the past two decades, extensive research has been conducted on the preparation and investigation of transparent ceramic, yielding many intriguing and important findings [2–6] and leading to an increase in the use of transparent ceramics in laser systems [7–11]. LuAG (Lu3Al5O12) transparent ceramic is viewed as a compelling alternative to YAG (Y3Al5O12) transparent ceramic due to its superior thermal properties and enhanced accommodation of lanthanide elements [12]. Previous work successfully demonstrated high energy nanosecond pulse amplified Nd:LuAG lasers [13,14], however, the lower damage threshold of transparent ceramic has emerged as a significant obstacle for strong laser systems aiming for higher quality lasing. As strong laser technology continues to advance towards higher power and larger output energy, there is an increased demand for higher anti-damage requirements for transparent ceramic. Consequently, it appears timely to initiate research into transparent ceramic damage.
Researchers have begun to explore the damage characteristics of laser-induced transparent ceramics, including the correlation between damage depth, area, laser energy density, and damage threshold. This research can be utilized to predict the damage of transparent ceramic materials under varying laser parameters, thereby providing a reference for engineering applications [11,15,16]. However, due to limitations in structural defect control and damage detection methods, the origin of nanosecond laser damage in transparent ceramics remains unclear. Furthermore, the damage characteristics and mechanisms of both front and back surfaces of transparent ceramics have not been thoroughly discussed. Therefore, it is imperative to investigate the damage response caused by structural defects in transparent ceramics under intense laser irradiation, to reveal its nanosecond laser damage process and mechanism.
Micropores represent the most common structural defects in transparent ceramics and are inevitably left in the body during the preparation process [17,18]. The relationship between the density and size factor of these micropores and the laser damage threshold, as well as damage evolution, is not yet fully understood. Prior to this research, there have been no studies reported in literature that explore the relationship between the density and size factor of micropores in transparent ceramics and nanosecond laser damage. Consequently, it is necessary to examine the behavior of ceramic damage and propagation caused by micropores defects under nanosecond laser irradiation. This research could provide a theoretical basis for understanding the origin and evolution mechanism of laser damage and facilitate the improvement of transparent ceramics’ resistance to laser damage. Studies have shown that the density and size of micropores are significantly influenced by the annealing process, without affecting the intrinsic properties of transparent ceramics [19,20]. Therefore, using a post-annealing process may be able to control the density and size of micropores enabling accurate study of their role as potential damage sources.
Considering this, the objective of this study is to ascertain the correlation between density and size factors of micropores in transparent ceramics and damage thresholds. It also aims to summarize the nanosecond laser damage characteristics on both front and back surfaces of transparent ceramics, while elucidating the impact mechanism of micropores as a source of damage. Ultimately, this study offers an effectively guide for the fabrication of high-quality transparent ceramics, thereby facilitating their application in high-power laser systems. Initially, through the application of a post-annealing process, we successfully controlled the density and size of micropores, thereby establishing a correlation between micropores density, size factors, optical transmittance, and laser damage thresholds. We innovatively demonstrate that micropores, as structural defects, are a significant source of damage that triggers laser damage. Subsequently, for transparent ceramics with varying micropores parameters, we compared and analyzed the surface damage morphology of both incident and back surface following nanosecond laser damage. Based on our experimental results and theoretical analysis, we summarized possible mechanisms that lead to differences in damage on the front and back surfaces.
2 Experiment
2.1 Sample preparation
High-purity powders of Al2O3 (99.99%, Aladdin), Lu2O3 (99.99%, Alfa Aesar), and Nd2O3 (99.99%, AlfaAesar) were utilized as raw materials, with MgO (99.99%, Aladdin) employed as a sintering additive. In addition, Ethanol (99.99%, Qiangsheng Company, Jiangsu) was used as an aid in ball milling. The raw materials were uniformly mixed using ball milling for a duration of 12 hours, subsequently dried for 2 hours at a temperature of 70°C. The mixture was then sieved and subjected to dry pressing at 5 MPa, followed by cold isotactic pressing at 210 MPa.
The Nd:LuAG green bodies were initially pre-sintered in a tungsten mesh-heated furnace at a temperature of 1670 °C under a vacuum with a pressure less than 10<συπ>−3 Pa for a duration of 5 hours. This process was executed to ensure the removal of pores within the grain and the creation of closed pores surrounding the grain boundaries. Following this, the pre-sintered Nd:LuAG ceramics underwent post-sintering via hot isostatic pressing (HIP) at 1750 °C in an argon atmosphere with a pressure of 200 MPa for 5 hours. The objective of this step was to eliminate the closed pores surrounding the grain boundaries.
2.2 Annealing treatment
In order to efficiently carry out laser damage related tests and experiments, the large size of Nd:LuAG transparent ceramics was precisely cut to obtain a size of 35 mm*18 mm*3 mm for each sample. After cutting, it was subjected to a laser grade polishing process. The purpose is to effectively avoid the impact of surface defects on LIDT testing. After polishing, the surface of transparent ceramics is smooth, without surface defects such as pits or protrusions. As detected by a laser confocal microscope, the surface roughness Ra is 5 nm, as shown in Fig. 1(a). The surface roughness of the annealed sample was also tested, and there was no significant change in the Ra value. Therefore, it effectively ensures the uniform and consistent surface roughness of transparent ceramics. The sample before annealing is shown in Fig. 1(b). The transmittance is close to the highest record in existing reports (T = 84%) [21]. There are two implementation schemes for air pressure free annealing, A and B. Scheme A: The annealing temperature is fixed at 1560 °C, and the annealing heat preservation time is set from 8 to 14 h, with a time gradient of 2 h. The transparent ceramic after annealing is shown in Fig. 1(c). The purpose of extending the insulation time is to provide conditions for the expansion of micro pores, and the obtained transparent ceramics contain larger micro pore sizes. Scheme B: The annealing heat preservation time is fixed at 10 h, and the temperature of the annealed air is between 1540°C and 1620°C, with a temperature gradient set at 20 °C. The purpose of increasing the annealing temperature is to provide conditions for increasing the density of micro pores, and the density of micro pores contained in the obtained transparent ceramics increases with the increase of annealing temperature. After annealing, there is a significant change in transparency, as shown in Fig. 1(d), which shows the transparent ceramic after annealing. The samples shown in Fig. 1(c) and (d) are not unique samples under each annealing condition. In fact, 5 transparent ceramic samples were tested simultaneously under each annealing condition, and the same experimental results were obtained through five repeated experiments. The above interesting results provide an effective method for exploring laser damage.
Fig. 1. (a) The surface roughness of the ceramic post-laser level polishing was examined. (b) The sample before annealing. (c) Samples annealed according to Scheme A. (d) Samples annealed according to Scheme B.
2.3 Damage experiments
The annealed Nd:LuAG transparent ceramics were placed on the test platform shown in Fig. 2(a) at a wavelength of 1064 nm a pulse length (full width at half maximum, FWHM) of about 10 ns and an output frequency of 10 Hz. The pulse laser, generated by a pump laser (specifically, the Spectra Physics Q-switched Nd: YAG nanosecond pulse laser), is directed through a series of components including a half-wave plate, polarizer, and lens. The final focus is on the surface of the Nd:LuAG transparent ceramics, resulting in laser damage to the said surface. The delay between the pump pulse and the charge coupled device (CCD) was realized by a digital delay generator (DG645). Before testing the samples, each incident laser energy emitted was measured by an Ophir calorimeter. The beam profile was meticulously measured utilizing a Spiricon beam analyzer, as depicted in inset 2(b). Furthermore, the spatial distribution of the laser beam closely aligns with the Gaussian distribution diagram 2(c). (Laser-induced Damage Threshold) LIDT of the samples was tested rigorously according to international standard ISO 11254 by 1-on-1 method until back surface damage was observed on the imaging CCD [22]. At least 10 sites were irradiated under each fluence level in the test. The surface morphology of each irradiated site can be observed using an optical microscope. If irreversible damage is observed, it is considered a point of injury. The damage probability was calculated based on the percentage of damage spots. Damage probability diagrams under different fluence levels were plotted. The zero-probability laser damage threshold is the intersection point of the line fit and the x-axis, which is regarded as the damage probability.
Fig. 2. Platform used for transparent ceramic damage threshold test. (a) Optical path diagram of damage threshold test platform. (b) Laser beam profile. (c) Spatial distribution of laser beam.
2.4 Testing and characterization
The online transmittance spectra of Nd:LuAG transparent ceramics were investigated using an ultraviolet-visible-near infrared spectrophotometer (UV-3600, Shimadzu, Japan). The surface grain micro-morphology of the transparent ceramic was observed before and after being impacted by a nanosecond pulsed laser, using a scanning electron microscope (SU8220, HITACHI, Japan). The elemental distribution of the laser exit face of the transparent ceramic was characterized by combining EDS energy spectrum analysis. A confocal microscope (CLSM) ZEISS-LSM 900 was utilized to test the surface roughness and micropores information of the sample in vivo (micropore rate, volume of each micropores, three-dimensional spatial coordinates, etc.). The phase structure of the laser exit face of the transparent ceramic was characterized using a Bruker-D8 Advance (Smartlab, Germany) X-Ray diffractometer.
3. Results and discussion
In this study, we measured the damage thresholds of Nd:LuAG transparent ceramics with varying micropores parameters. We established a relationship between micropores density, average size, and laser damage initiation, thereby elucidating the role of micropores in laser damage. We also demonstrated the difference in damage morphology between the front and back surfaces of the material. Through theoretical analysis, we clarified the potential mechanism causing this difference.
3.1 Laser damage is influenced by the size of the micropores
In recent years, many researchers have used Confocal Laser Scanning Microscopy (CLSM) to precisely obtain key information on micropores in transparent ceramics (such as spatial proportion, size, and location), making it an important tool for exploring micropores in current research [22–24]. The transparent ceramics after Annealing Scheme A treatment were characterized by CLSM, and the results are shown in Fig. 3. The distribution of micropore sizes within the transparent ceramics adhered to the log-normal distribution rules. This finding aligns with previous literature, which typically posits that micropores conform to a log-normal distribution [16]. The function of this size distribution is as follows:
Fig. 3. Size and density distribution information of micropores in transparent ceramics after air annealing at 1560°C for 8 to 14 hours.
Following an air-annealing process at 1560 °C for durations of 8–14 hours, the micropores within transparent ceramics were retained at approximately 0.1%. However, a progressive increase in the average diameter of these micropores was observed, reaching 0.95 µm after 8 hours (Fig. 3(a)), 1.08 µm after 10 hours (Fig. 3(b)), 1.16 µm after 12 hours (Fig. 3(c)), and 1.19 µm after 14 hours (Fig. 3(d)). Meanwhile, the maximum radius of micropores also changed significantly, from 2.3 µm@8 h to 2.9 µm@14 h. After CLSM testing, the three-dimensional spatial information and size statistics of the micropores can be found in Supplement 1. The experimental results proved that appropriately extending the annealing and heat preservation time would increase the size of micropores, which is consistent with the law reported in literature [18].
The transmittance of all samples was determined by an UV-Vis-NIR spectrophotometer, and the transmittance spectra of transparent ceramics with different holding times after being annealed at the same temperature (1560 °C) are shown in Fig. 4. The transparent ceramic without annealing has a transmittance of 84.2%@1064 nm, which is close to the theoretical transmittance of LuAG transparent ceramics. The sample after air-annealed for 8 h has a transmittance reduced to 82.7%@1064 nm, and as the holding time continues to extend, the transmittance will continue to decrease until the transmittance decreases to 78.7%@1064 nm after 14 h of holding, which is related to the expansion of micropores caused by the extension of annealing time [17,25]. CLSM test results show that in annealing scheme A, the smaller the transmittance of transparent ceramics, the larger the average size of micro-pores, and the results are shown in Fig. 5(a). Using a 1-on-1 test method to test surface damage threshold values, it is found that with the increase of average size of micropores, damage threshold values show a clear downward trend. In samples without air-annealing treatment, the average size of micropores is 0.9µm, and the damage threshold value is 4.5J/cm2. After samples were annealed at 1560 °C for 14 h, the average size of micropores increased to 1.19µm, and the damage threshold decreased to 2.9J/cm2. The results prove that the size of micropores will seriously affect the damage threshold values of transparent ceramics, as shown in Fig. 5(b).
Fig. 4. Presents the transmittance spectra after heat preservation for a duration of 8–14 hours, during which the sample was subjected to air annealing at a temperature of 1560°C.
Fig. 5. Illustrates the relationship between the average size of micropores and corresponding damage thresholds in transparent ceramics, as a function of transmittance and average size, over varying annealing holding times. The figure is divided into two parts: (a) depicts the correlation between transmittance and average size of micropores within transparent ceramics, (b) presents the damage thresholds for transparent ceramics with different average sizes of micropores.
The morphology of the front (Fig. 6(a)-(e)) and back surfaces (Fig. 6(f)-(j)) after damage of Nd:LuAG transparent ceramics irradiated by a nanosecond pulse laser with an energy density of 4.5J/cm2 is shown in Fig. 6. When the transparent ceramics were not annealed by scheme A or B, the damage threshold was high. The nanosecond laser did not cause irreversible damage, and the grains on the front and back surfaces of the ceramics remained flat. With the extension of the annealing heat preservation time, micropores have enough time to expand and make the average size become larger and larger, at this time, material damage will be observed on both the incident and back surface of transparent ceramics after nanosecond laser irradiation, but the damage morphology characteristics are different, showing obvious differences. The specific damage morphology is manifested as follows: the main manifestation of the front surface is laser ablation and microcrack, which may be caused by micropores in transparent ceramics absorbing laser energy and causing thermal explosion [26]. To observe and understand the local damage morphology and characteristics of the front and rear surfaces, we enlarged the representative key areas in the left image and placed them on the right side, such as a1 and a2. It is not difficult to see from the damage morphology that the larger the average size of micro pores in transparent ceramic samples, the larger the laser ablation area and the more significant the damage pits after laser damage. The main manifestation on the back is grain resolidified, almost simultaneous occurrence of laser ablation damage on the front surface. The white dashed range represents the morphology of the grains after resolidified, at which point the grain boundaries have disappeared. The area of resolidified also increases with the increase of average micropore size. It is interesting that micropores are often observed in the resolidified area. Previous studies on laser damage in fused quartz glass have demonstrated that surface damage is primarily induced by bubbles [27]. This paper seeks to clarify that micropores also significantly contribute to the formation of surface damage. Obviously, the damage on the front surface is more serious, which will cause greater trouble for the long-term efficient use of transparent ceramics. Whether resolidified crystal grains on the back surface will have a significant impact on phase structure and element distribution is discussed in detail in “3.3 Damage mechanism”.
Fig. 6. The surface damage morphology of transparent ceramics with different average size of micropores irradiated by a 4.5 J/cm2 nanosecond pulsed laser.
To facilitate comprehension and induction, the micropores is treated as a standard spherical cavity, with the gas within the cavity considered an ideal gas, as illustrated in Fig. 7. Under strong laser irradiation (1064 nm/10 Hz/10 ns), the micropores serve as both scattering and absorption sources. As a scattering source, scattering effects occur, affecting the distribution and transmission of strong lasers in electromagnetic fields, and affecting near-field effects. Under the subsequent strong laser irradiation, the micropores as the absorption source will absorb laser energy, triggering the effect of energy deposition. The molecules inside the micropores undergo thermal diffusion, and the pressure inside the micropores increases, leading to an expansion in the volume of micropores. However, due to the presence of surrounding ceramic grains, the increase in micropores volume is constrained, resulting in a minimal change in radius Δr, compared to the initial radius r1. Consequently, the internal pressure within the micropores experiences a significant upsurge, which precipitates stress concentration around the micropores and thereby facilitates the generation and propagation of micro-cracks. When these micro-cracks attain a certain length and density, they trigger material damage and failure. It is obvious that the larger the size or volume of the micropores themselves, the larger the volume of expansion after laser irradiation, and the greater the pressure on the surrounding area. Therefore, they are more likely to cause irreversible damage to transparent ceramic materials, which is consistent with the results shown in Fig. 6.
3.2 Laser damage is influenced by the density of micropores
The ceramic samples treated with annealing scheme B were also tested by CLSM, and the density, size, spatial location and other information of micropores in the unit volume (300 µm × 300 µm × 200 µm) are shown in Fig. 8. The transparent ceramic without annealing had a micropores content as low as 0.01%, with 60 pores and a total volume of 313.13µm3. After annealed at 1540°C for 10 h, the micropores content increased significantly, and the number of pores increased to 1013 and the total volume increased to 1589.78µm3, and the microporosity rose to 0.07%. The results show that as the annealing temperature increases, the density and number of micropores and the total pore volume of micropores continue to increase. Until annealed at an annealing temperature of 1620°C for 10 h, the number of micropores increased to 8106, and the total volume reached 10741.07µm3, and the microporosity was 0.71%. The size distribution of micropores still conformed to the log-normal distribution rule in Eq. (2). After CLSM testing, the three-dimensional spatial information of the micropores, as well as the corresponding volume and density statistics, are displayed in Supplement 1.
Fig. 8. Volume and density information of micropores after 10 hours of air annealing from 1540 to 1620 °C.
The transmittance of all samples was determined by a UV-Vis-NIR spectrophotometer, and the results showed that after air-annealing at temperatures between 1540°C and 1620°C for 10 h, the transmittance of transparent ceramics decreased from 84.2%@1064 nm without annealing to 74%@1064 nm (Fig. 9). This is in good agreement with the density results of micropores shown in Fig. 8, where increasing annealing temperature eventually leads to an increase in micropores and a decrease in transmission [20].
Fig. 9. Shows the transmittance spectra after air-annealing at temperatures ranging from 1560°C to 1620°C.
The relationship between the transmittance and micropore density of transparent ceramics annealed at different temperatures is shown in Fig. 10(a). The micropores density increases from 0.01% without annealing to 0.71% (1620 °C) step by step, while the transmittance decreases continuously. Under the condition of constant annealing heat preservation time, appropriately increasing the annealing temperature will cause the micropores density to increase, which is consistent with the law reported in the literature [21.22]. Using the 1-on-1 test method to test the surface damage threshold, it is found that as the micropores density increases, the damage threshold decreases step by step, and the sample without air annealing has a microporosity rate of 0.01% and a damage threshold of 4.5J/cm2, and the sample annealed at 1620 °C has a microporosity rate of 0.71% and a damage threshold of 1.4J/cm2. The results prove that the micropores density will also seriously affect the damage threshold of transparent ceramics, as shown in Fig. 10(b).
Fig. 10. Illustrates the correlation between transmittance and micropore density in transparent ceramics subjected to varying annealing holding times, alongside their corresponding damage thresholds. The figure is divided into two parts: (a) depicts the relationship between transmittance and micropore rate in transparent ceramics, while (b) presents the damage thresholds of these ceramics at different micropore rates.
The sample treated with annealing scheme B was irradiated by a pulsed laser with an energy density of 4.5J/cm2 nanosecond. The results are shown in Fig. 11, and similar to the results shown in Fig. 6. The main manifestation of the front surface is still laser ablation collapse and microcrack, and the main manifestation of the back surface is grain resolidified, and the part is shown as a white dashed range in the figure. Nd:LuAG transparent ceramics without high-temperature annealing treatment also did not suffer damage. But as the annealing temperature increases, the micropore density becomes larger and larger, at this time, material damage can be observed on both the incident and back surfaces of the transparent ceramics, and because the incident and back surfaces of the transparent ceramics both show that as the density of micropores increases, more serious damage is caused and a larger damage area is caused, and the damage morphology is shown in Fig. 11.
Fig. 11. Damage morphology of transparent ceramics after irradiation by a 4.5J/cm2 nanosecond pulsed laser with different micropores densities.
Upon irradiation of transparent ceramics by laser, it is observed that an increase in the micropore rate corresponds to a decrease in the damage threshold. This is due to the fact that within a unit volume of transparent ceramics, a greater number of micropores results in a smaller average distance, L, between these micropores (refer to Fig. 12).When transparent ceramics are irradiated by laser, the higher the micropore rate is, the lower damage threshold will be, because in unit volume of transparent ceramics, the more the number of micropores is, the smaller the average distance L of micropores will be (reference Fig. 12). When transparent ceramics are irradiated by laser, as a damage source, micropores will cause local stress concentration in materials due to their existence in materials. When these stresses exceed the ultimate stress of the material, it will lead to the formation and propagation of microcracks, and ultimately accelerate the damage of the material, and the coupling model of inter-micropore compressive stress force is shown in Fig. 12. The stress concentration factor is used to describe the degree of local stress concentration generated by micropores, that is, the size of stress concentration factor is related to the number and distribution of micropores. The stress concentration factor can be calculated by the following formula [28]:
In this context, “K” represents the stress concentration factor, “σmax” signifies the maximum principal stress in proximity to micropores, and “σ0” denotes the average stress at a distance from the micropores. The stress concentration factor increases when the degree of stress concentration is higher near the micropores. As the number of micropores increases, the relative distance between them diminishes, there by intensifying the stress concentration near the micropores. Consequently, an increase in the number of micropores leads to a larger stress concentration factor, which can potentially cause damage to ceramic materials and facilitate the formation and propagation of microcracks.
3.3 Damage mechanism
To investigate whether the crystalline grains of the exit face of the ceramic would have a significant impact on the phase structure and elemental distribution after resolidified, EDS energy spectrum analysis was used to characterize the main four elements (Al, O, Lu, Nd) in Nd:LuAG transparent ceramics. The results showed that both the resolidified area (#1) and the undamaged area (#2) exhibited a uniform distribution of elements (Fig. 13(b)–(e)). Subsequently, the phase structure was characterized using micro-area XRD. Remarkably, upon resolidified, it retained its pure LuAG composition, the diffraction peaks observed in both the resolidified and undamaged sections align with the standard peak (Fig. 13(f)). Within the resolidified zone, the grain boundaries have vanished, with multiple grains coalescing into a singular entity.
Fig. 13. Elemental distribution and phase structure of the exit face of transparent ceramics. (a) The area within the dark colored is the resolidified area, marked as #1, and the area outside the dotted line is the undamaged area, marked as #2. (b)–(e) Elemental distribution. (f) Phase structure of the resolidified and undamaged parts.
In conjunction with the afore mentioned nanosecond laser damage characteristics of transparent ceramics and considering the damage morphology of both front and back surfaces, we irradiate a transparent ceramic containing micropore defects with a nanosecond laser pulse (1064nm@10ns@10 Hz). The damage evolution mechanism of the front and back surface is delineated in four steps, as illustrated in Fig. 14.
Fig. 14. Surface damage evolution of transparent ceramics under irradiation with nanosecond laser pulses. The front and back surface is delineated in four steps, and the lower part of the figure shows the meanings represented by various icons.
(I): In the absence of a loaded laser beam, micropores—an inherent structural defect of transparent ceramics—are dispersed randomly within the material's body.
(II): Upon the initial loading of the laser beam, both the entrance and exit damage zones of the transparent ceramic exhibit micropores. These micropores absorb the laser's energy, leading to continuous thermal expansion and consequently, the initiation of micro-cracks. As the laser beam traverses this area, it heats up the ceramic grains, resulting in a localized temperature increase. Concurrently, a minimal amount of steam is produced on the surface due to laser irradiation.
(III): When the laser beam is continuously applied to the central section, the vapor at the front surface of the transparent ceramic undergoes decomposition and ionization, leading to the generation of a light plasma and a shock wave. Concurrently, due to the sustained heating from the laser, thermal explosions commence within micropores that attain the conditions necessary for thermal explosion. These micropores subsequently extend and widen. On the back surface of the transparent ceramic, continuous laser loading can cause the ceramic grains in the immediate vicinity of the back surface to reach their melting point. Some of these ceramic grains may melt into a liquid state, potentially causing evaporation.
(IV): When the laser beam is continuously applied to the end section, the thermal explosion of micropores triggers the collapse and vaporization of the front surface grains. Concurrently, plasma and stress waves persist and propagate. At the exit face of transparent ceramics, upon cessation of the laser beam, the molt ceramic grains promptly begin cooling. This rapid cooling causes the liquid fraction to solidify swiftly, leading to resolidified.
4. Conclusion
This study innovatively employs the post-annealing process to effectively control the density and size of micropores, establishing a correlation between the annealing parameters (temperature and duration) and micropores attributes (density and size) in relation to laser damage thresholds. It has been established that micropores structural defects in transparent ceramics significantly contribute to laser damage. The findings indicate an inverse relationship between the density and size of micropores and the damage threshold. By examining the damage morphology and characteristics of both front and back surfaces of transparent ceramics, this paper clarifies the process and mechanism of irreversible damage induced by micropores during nanosecond laser irradiation. This research provides valuable insights into the influence of micropores structural defects on the formation of damage, offering guidance for controlling micropores in the preparation and processing of transparent ceramics with high laser damage thresholds. This study offers insights into the enhanced preparation of high-quality transparent ceramics. By increasing the resistance of these ceramics to nanosecond laser damage, we can achieve superior service performance in commercial laser systems.
Funding
National Key Research and Development Program of China (2022YFB3605700).
Acknowledgment
The authors acknowledge the generous financial support from National Key R&D Program of China (No. 2022YFB3605700) and acknowledge Zeiss for providing confocal laser microscope testing services. The authors would like to thank the referees for their valuable suggestions and comments that have helped improve the paper.
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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