Research on Thermal Effect and Laser-Induced Damage Threshold of 10.6 µm Antireflection Coatings Deposited on Diamond and ZnSe Substrates

Abstract

In this study, ZnS/YbF₃-10.6 µm antireflection (AR) coatings were fabricated on CVD single-crystal diamond and ZnSe substrates. The spectral characteristics of the coatings and their performance under continuous wave laser radiation at 10.6 µm were systematically investigated. The fabricated AR coatings exhibited excellent spectral properties in the target wavelength range. Both theoretical calculations and experimental results indicated that, at the same power density, the 10.6 µm AR coatings on diamond substrates exhibited a lower temperature rise compared to those deposited on ZnSe substrates. Due to its high thermal conductivity, the diamond substrate is expected to exhibit reduced thermally induced surface distortion. The laser-induced damage threshold (LIDT) test results indicate that the AR coating deposited on the ZnSe substrate exhibits a damage threshold of 11,890 W/cm², whereas the AR coating on the diamond substrate achieves a threshold of 15,287 W/cm², representing a 28.5% improvement over the ZnSe substrate. Additionally, graphite formation occurs on the diamond substrate under high power density. These findings provide both theoretical and experimental support for the potential application of diamond materials in high-power laser systems.

1. Introduction

The 10.6 µm CO₂ laser system has become indispensable in advanced applications including laser processing [1], laser dielectric acceleration [2,3], extreme ultraviolet lithography [4], laser surface modification [5,6], and light–matter interactions [7]. As a critical transmission component, the output window’s optical-thermal and LIDT characteristics directly affect the system’s operational stability and beam propagation quality [8]. Zinc selenide (ZnSe) is the prevalent window material due to its superior transmittance at 10.6 µm [9]. However, the low thermal conductivity of ZnSe synergistically combines with the inherent absorption of its AR infrared coating, leading to localized heat accumulation. This heat accumulation can induce thermal lensing distortion and interfacial stress concentration, ultimately resulting in wavefront degradation and mechanical failure [10,11]. Moreover, Yelisseyev and Gurbatov et al. employed AR nanostructures to achieve high transmittance, which holds promise for mitigating thermal effects caused by film absorption [12,13,14]. However, considering the harsh working conditions at 10.6 μm, the long-term stability and durability of such nanostructures remain uncertain and warrant further investigation. Therefore, to improve the operational stability and output power of laser systems, it is imperative to explore window materials that exhibit both high transmittance and superior thermal conductivity.

Diamond is an exceptionally promising material for optical windows, offering a broad transparent range from ultraviolet (226 nm) to far-infrared (500 µm), with a phonon absorption region only between 2.5 and 6 µm [15,16]. Additionally, diamond exhibits a wide bandgap (5.5 eV) [17], outstanding mechanical hardness, excellent chemical resistance, and high thermal conductivity at room temperature, making diamond material suitable for various optical applications. In 1974, Douglas-Hamilton [18] demonstrated the potential of natural diamond as a special window material for the 10.6 µm CO₂ laser system. However, the high cost and limited size of natural diamond constrain its application in high-power laser systems. Fortunately, the advancement of chemical vapor deposition (CVD) technology has enabled the fabrication of diamond with physical properties comparable to those of natural diamond. This development expands the potential applications of diamond in high-power optical components, such as beamsplitters [19], mirrors [20], lenses [21], and diffractive optics [22,23]. Compared to ZnSe and Ge, CVD diamond has high thermal conductivity and hardness and is regarded as a very promising material in the field of high-power CO₂ lasers [9,24]. For example, under a pulse of CO₂ with a pulse width of 100 ns, the damage threshold of Ge is 1.7 J/cm², that of ZnSe is 2.8 J/cm², and that of CVD diamond is 8 J/cm² [25]. The damage threshold of CVD diamond is significantly higher than the other materials under the same conditions, and a higher damage threshold can support a stronger power and thus realize a higher energy output. Pulse laser-induced damage originates from dielectric breakdown caused by multiphoton ionization and avalanche ionization. In contrast, continuous-wave (CW) laser damage results from the gradual increase in local temperature under prolonged irradiation; once the accumulated heat exceeds the material’s thermal diffusion capacity, melting damage occurs [26].

While numerous experimental studies have demonstrated the significant advantages of diamond as an optical substrate material, its performance at the 10.6 µm CW lasers remains incompletely explored. Diamond exhibits significantly higher thermal conductivity than ZnSe; however, its optical absorption coefficient is also greater. Therefore, further investigation is required to elucidate the performance differences between these materials under CW laser irradiation. In this article, high refractive index material ZnS and low refractive index material YbF₃ were used to design and fabricate 10.6 µm AR coatings on CVD diamond substrates and ZnSe substrates. The influence of diamond and ZnSe substrates on the thermal effects and damage performance of anti-reflective (AR) coatings in the 10.6 µm wavelength range was systematically investigated. The results show that the transmittance of the AR coatings on both substrates exceeds 98% at 10.6 µm. At the same laser power density, the 10.6 µm AR coating deposited on the diamond substrate exhibits a lower temperature rise. Moreover, the AR coating deposited on the diamond substrate exhibited a 28.5% higher LIDT (15,287 W/cm²) compared to that on the ZnSe substrate (11,890 W/cm²), highlighting its considerable potential as an output window material for high-power laser systems. The experimental methods are described in Section 2. Section 3 reports the influence of substrates on the performance of AR coatings, particularly the thermal effects under CW laser irradiation. A summary of our findings is presented in Section 4.

2. Materials and Methods

2.1. Substrates

The refractive index and extinction coefficients of CVD diamond and ZnSe substrates in the infrared wavelength range are described using the Sellmeier dispersion model [27]:

$$n^2(\lambda )=A_0+{\displaystyle \frac{A_1{\lambda }^2}{{\lambda }^2-A_2}}+{\displaystyle \frac{A_3{\lambda }^2}{{\lambda }^2-A_4}}$$

(1)

where A₀, A₁, A₂, A₃, and A₄ are the Sellmeier dispersion coefficients of the refractive index, which are determined by the material, and λ represents the incident wavelength. The Sellmeier dispersion coefficients for CVD diamond and ZnSe substrates are presented in

Table 1. Sellmeier dispersion coefficients for diamond and ZnSe substrates.

Substrate	A0	A1	A2	A3	A4
ZnSe	5.957772	0.0136324	0.16445938	0.5302597	1292.685333
Diamond	5.512878	0.06116683	33.71128963	0.01220433	342.7813797

. Figure 1a and Figure 1b show the optical constants of CVD diamond and ZnSe, respectively. The extinction coefficient of diamond is greater than that of ZnSe. At 10 µm, the refractive index of CVD diamond is 2.375, which is in good agreement with the previously reported value of (2.377 ± 0.0005) [28] for CVD diamond, and the accepted value of 2.3756 for type IIa natural diamond. The infrared refractive index of CVD diamond is approximately 2.37, while that of ZnSe is approximately 2.40. These values are close, indicating that the same AR coatings can be applied to both CVD diamond and ZnSe substrates without affecting spectral efficiency.

2.2. Design

We selected ZnS and YbF₃ as thin film materials. Since ZnS and YbF₃ are transparent in the range of 1–12 µm, their refractive indices are described using the Sellmeier dispersion model (Equation (1)). The extinction coefficient, on the other hand, is described using a nonparametric method. For the nonparametric model, the discrepancy function is expressed as follows [29]:

$$D{F}^2={\displaystyle \sum _{j=1}^L[S(n({\lambda }_j}),k({\lambda }_j),d,{\lambda }_j)-\stackrel{\wedge }{S}({\lambda }_j){]}^2+{\alpha }_1{\displaystyle \sum _{j=1}^L[n^{″}}({\lambda }_j){]}^2+{\alpha }_2{\displaystyle \sum _{j=1}^L[k^{″}}({\lambda }_j){]}^2$$

(2)

where $S(n,k,d,\lambda )$ is the theoretical spectral properties of the film; $\stackrel{\wedge }{S}(\lambda )$ is the experimentally measured spectral properties of the film; j = 1, … L, is the wavelength grid in the relevant spectral range; $n^{″}({\lambda }_j)$ and $k^{″}({\lambda }_j)$ denote the second-order derivatives of the refractive index, and extinction coefficient are denoted with the second and third terms in Equation (2) specifying the additional requirements for the smoothing of the refractive index and the extinction coefficient as a function of wavelength; and the parameters ${\alpha }_1$ and ${\alpha }_2$ are weight factors balancing the smoothness and fitting demands. The fitting results were determined by minimizing the error value between the measured and calculated values.

Table 2. Sellmeier dispersion coefficients of ZnS and YbF₃ films.

Material	A0	A1	A2	A3	A4
ZnS	4.463858	0.1933777	2.5811114	0.1934265	2.580743
YbF3	1.051848	0.2770681	0.3349626	0.5022135	6.775195

states the Sellmeier model parameters of the ZnS and YbF₃ films. Figure 2a and Figure 2b show the experimental and fitted transmittance spectra of ZnS and YbF₃ films, respectively. Obviously, the fitted curves agree well with the experimental transmittance spectra, which means that the models are sufficiently accurate in calculating the optical constants. Figure 2c and Figure 2d describe the refractive indices and extinction coefficients of ZnS and YbF₃ films, respectively, showing distinctive refractive indices and low absorption in the long-wave infrared band.

Based on the above data, AR coatings operating in the 10.6 µm wavelength range were designed using ZnS and YbF₃ as the thin film materials. Since the lattice constant of ZnS is close to that of ZnSe, ZnS tends to crystallize on the surface of the ZnSe substrate, which affects the spectral performance [30]. Therefore, YbF₃ was employed as an interfacial layer to enhance the structural compatibility between the film and the substrate. The final structure designed by Optilayer software (OptiLayer 15.12) [31] is shown in Figure 3. The total thickness of the coating is 1.5 µm with nine layers, which makes the structure highly feasible for fabrication. Due to the similar refractive indices of diamond and ZnSe at 10.6 µm, the same film stack structure is fully applicable to both substrates.

2.3. Fabrication

The 10.6 µm AR coatings were fabricated on diamond substrates (Φ25.4 × 1 mm) and ZnSe substrates (Φ25.4 × 1 mm) using a Leybold ARES 1110 (Leybold, Germany) vacuum deposition system. Double-sided coating was applied. ZnS films were deposited by thermal evaporation using a molybdenum boat, while YbF₃ films were deposited by ion-assisted thermal evaporation. Before film deposition, the substrates were cleaned using an ion beam from an APS source to enhance the adhesion between the films and the substrates. The ion source voltage and current were set to 120 V and 40 mA, respectively. The deposition process was carried out under a vacuum of 8 × 10⁻⁶ mbar, with the substrate temperature maintained at 150 °C. The deposition rate and film thickness were monitored using a quartz crystal oscillator. The deposition rates for ZnS and YbF₃ were 1.0 nm/s and 0.5 nm/s, respectively. The deposition temperature was carefully selected based on the intrinsic properties of the thin film materials. Elevated temperatures can enhance the film’s packing density, thereby reducing optical absorption. However, in the case of ZnS, excessively high temperatures may lead to sulfur loss, resulting in a deviation from the stoichiometric ratio and increased absorption. According to our literature review [32], 150 °C is considered an optimal temperature that effectively balances these competing factors. The application of an ion source during the deposition of YbF₃ was motivated by its material characteristics, as ion-assisted deposition can reduce moisture absorption and improve film quality.

2.4. Characterization

The 9–12 µm transmission spectra of the samples were measured using a Fourier-transform infrared spectrometer (INVENIOS, Bruker) with a resolution of 2 cm⁻¹, at an incident angle of 0°. The crystal phase structure was determined via X-ray diffraction analysis using a Rigaku XRD instrument from Tokyo, Japan. Cu Kα (λ = 0.15405 nm) and Ni were employed as the target material and filter, respectively. The tube voltage and current were set at 40 kV and 100 mA, while the scanning angle ranged from 10° to 70° at a speed of 2°/min. Additionally, the surface morphology of the nanocomposites was investigated through atomic force microscopy (AFM) using a Bruker instrument in Madison, WI, USA at different scales. The scan line was comprised of 512 points, with a scanning area of 2 μm × 2 μm. Additionally, the temperature rise and LIDT measurements were carried out using a CO₂ laser system independently developed by Huazhong University of Science and Technology [33]. This laser is capable of delivering up to 1000 W of continuous-wave output. After being focused through a transmission optical path, the Gaussian beam had a 1/e² diameter of 1.5 mm at the focal plane. The beam quality factor was M² ≤ 1.15. For each type of substrate, three independent samples were tested under identical conditions. The reported LIDT values represent the average results of the three measurements. During the temperature rise test, a thermocouple was attached to the sample surface to record the temperature. The thermocouple was positioned approximately (2 ± 0.5) mm from the center of the laser spot. The thermocouple was connected to a computer for real-time temperature monitoring. Three points were measured for each sample, with the thermocouple’s distance from the laser spot kept consistent at different positions. The average value of the temperature at each time point was used as the sample’s temperature. For the damage threshold test, the laser power step size was 10 W, with a laser irradiation time of 60 s. After irradiation, the surface morphology of the samples was observed using a Leica microscope and a Focused Ion Beam Scanning Electron Microscope (FIB-SEM) (Cross Beam 350).

3. Results and Discussion

3.1. Coatings Performance

Figure 4 illustrates the experimentally obtained spectra of AR coatings deposited on diamond and ZnSe substrates, measured using a Fourier transform infrared spectrometer. Due to the slightly lower refractive index of diamond in comparison to ZnSe, the diamond substrate demonstrates a higher transmittance. The experimental results indicate that the AR coatings on both substrates exhibit a transmittance greater than 98% at 10.6 µm. Specifically, the transmittance of the AR coating deposited on the diamond substrate at 10.6 µm is 98.04%, while the transmittance of the AR coating deposited on the ZnSe substrate at 10.6 µm is 98.48%. As previously mentioned, since the refractive indices of the two substrates are comparable, simulations show that the deviation in transmittance is not due to differences in substrate refractive indices. Instead, it is attributed to the fact that diamond itself has higher absorption than ZnSe, leading to slightly lower spectral performance in the 10.6 μm AR coatings deposited on the diamond substrate compared to those deposited on the ZnSe substrate. Additionally, fitting errors in the material’s optical constants and monitoring errors during the deposition process may also contribute to deviations from the theoretical values [34]. The fabricated 10.6 μm AR coating exhibits excellent spectral properties, achieving higher transmittance compared to those reported in the literature [35].

The surface morphology of the 10.6 μm AR coating was analyzed using atomic force microscopy. Each sample was tested at three distinct surface locations to assess uniformity. For the same sample, the surface morphology was consistent across different locations, with variations in surface roughness within 0.2 nm. The most representative and clearly resolved AFM image was selected to illustrate the surface morphology, and the final surface roughness was calculated as the average of the roughness values from the three locations. The results, as shown in Figure 5, indicate that the surface of the AR coating is smooth, with no noticeable clustering. The root mean square (RMS) surface roughness of the coating was found to be 2.3 nm on the diamond substrate and 2.5 nm on the ZnSe substrate, suggesting that the surface roughness is comparable on both substrates. Since the surface morphology of thin films is closely related to the microstructure of the material, further analysis was conducted using X-ray diffraction to investigate the crystallinity of the coatings on both substrates. The XRD results are shown in Figure 6. For the coating deposited on the diamond substrate, two diffraction peaks were detected at 2θ = 28° and 44°. Comparison with the PDF standard cards revealed that these peaks correspond to the ZnS (111) and diamond (111) planes, respectively. Additionally, a diffraction peak at 2θ = 44° was observed for the pure diamond substrate, which matched the diffraction peak of the coating deposited on the diamond substrate. For the coating deposited on the ZnSe substrate, diffraction peaks were detected at 2θ = 28°, 47°, 56°, and 67°, which coincided with the XRD pattern of ZnSe and corresponded to the ZnSe (111), (220), (311), and (331) planes, respectively. Due to the overlap of the diffraction peaks corresponding to the ZnSe (111) and ZnS (111) planes, it is not possible to directly determine the intensity of the ZnS diffraction peak. However, based on the AFM results, the thin films deposited on both substrates exhibit weak crystallinity. The reduction in the intensity of certain diffraction peaks may be attributed to the presence of the thin film on the substrate surface, which diminishes the penetration depth of the X-rays into the substrate and consequently weakens the diffraction signal. In addition, the thin film may reduce the effective diffracting volume of the substrate, further contributing to the decreased peak intensity.

Cross-sectional SEM characterization was employed to reveal the multilayer architecture and interfacial features of the deposited thin films. Prior to imaging, the sample was sectioned perpendicular to the substrate using a focused ion beam system, followed by Au coating to enhance conductivity. Figure 7 presents cross-sectional SEM images of the multilayer coatings deposited on ZnSe and diamond substrates. Both samples exhibit a well-defined nine-layer structure with uniform layer thicknesses and sharp interfaces between adjacent layers, indicative of a high degree of deposition precision and repeatability. The multilayer stack demonstrates excellent structural integrity, with no observable defects such as cracks, delamination, or voids. The coating morphology remains consistent regardless of the substrate type, suggesting good adhesion and compatibility with both ZnSe and diamond substrates.

In summary, the AR coatings deposited on both substrates exhibit comparable surface roughness and weak crystallinity. The cross-sectional SEM images further reveal the excellent structural integrity of the coatings, with well-defined interfaces between adjacent layers, indicating high deposition precision and reproducibility.

3.2. Temperature Rise

The thermal effects caused by the temperature rise of optical coatings result in localized heating of the film components. The temperature gradient in the lateral direction leads to surface deformation, while the temperature gradient in the radial direction induces thermal lensing effects. These phenomena significantly degrade the laser beam quality and transmission efficiency.

To systematically compare the impact of ZnSe and diamond substrates on the temperature rise of 10.6 µm AR coatings, temperature rise tests were conducted on the samples in this study. In the experiment, a CW laser beam with a wavelength of 10.6 μm was directed onto the sample surface. The laser output power was set to 50 W, with an incident angle of 0°, a spot diameter of 1.5 mm, a laser power density of 2830 W/cm², and an irradiation time of 200 s. The ambient initial temperature was (18 ± 1) °C. The experimental results of the temperature rise test are shown in Figure 8. Under the conditions of 50 W CW laser radiation for 200 s, the surface temperature of the 10.6 µm AR coating deposited on the diamond substrate reached 62.5 °C, while that of the AR coating deposited on the ZnSe substrate reached 96.63 °C. Additionally, the temperature of the AR coating on the ZnSe substrate increased rapidly within a short period, followed by a slower heating rate. While the AR coating on the diamond substrate also exhibited a rapid initial temperature rise, the subsequent temperature increase tended to stabilize. In comparison, the temperature rise of the AR coating deposited on the diamond substrate was reduced by 36%. This is likely due to the difference in thermal conductivity between the two substrates, with the high thermal conductivity of diamond resulting in a lower temperature rise under CW laser irradiation. Moreover, the transmission spectra measured after the temperature rise test exhibited no deviation from the pre-test spectra, confirming the stability of the optical properties.

To verify the above hypothesis, the thermal effects on the surfaces of AR coatings on different substrates under CW laser irradiation were numerically simulated based on the three-dimensional Fourier heat conduction equation. The heat conduction model used in the numerical simulations is consistent with those reported in the literature [36]. Under high-power CW laser irradiation, the energy is primarily absorbed by the coating layers and the substrate material. The thickness of the AR coating is approximately 1.5 μm, which is negligible compared to the size of the substrate. The heat absorbed by both the coating and the substrate is treated as a surface heat source directly applied to the substrate surface. The laser parameters and simulation conditions were consistent with the experimental setup, and the basic physical parameters of the two substrates are shown in

Table 3. Basic physical parameters of substrates.

Substrate Material	Thermal Expansion (10−6K−1)	Specific Heat (J/(kg−1·K−1))	Density (kg/m3)	Thermal Conductivity Coefficient (W/(m·K))	Young’s Modulus (GPa)	Poisson’s Ratio	10.6 µm Extinction Coefficient
ZnSe	7.1	339	6100	16	67.2	0.28	6.01 × 10−7
Diamond	1.1	515	3520	2000	1000	0.1	2.61 × 10−6

. As observed, the thermal conductivity of diamond is much higher than that of ZnSe, while the optical absorption of ZnSe is lower than that of diamond. Both of these factors influence the temperature rise of the coatings [37]. The numerical simulation results are shown in Figure 9a. After 200 s of laser irradiation, the surface temperature rise of the 10.6 μm AR coating deposited on the diamond substrate was 67.5 °C, while the surface temperature rise of the 10.6 μm AR coating deposited on the ZnSe substrate was 103 °C. To evaluate the robustness of the thermal simulation, a sensitivity analysis was performed by introducing a ±10% variation in both the substrate thermal conductivity and absorption coefficient. The resulting temperature rise of the AR coatings on ZnSe substrates varied by approximately 2% in response to changes in thermal conductivity and by 7% with changes in absorption. For the diamond substrates, the temperature rise varied by 1% and 5%, respectively. These results indicate that the temperature response is more sensitive to variations in absorption, particularly for materials with lower thermal conductivity. Importantly, even with a 10% deviation in these parameters, the findings consistently support the conclusion that AR coatings on diamond substrates exhibit a lower temperature rise under identical laser irradiation conditions. This result is in good agreement with the experimental measurements. The discrepancy between the theoretical and experimental values may stem from slight differences between the actual material physical parameters and the idealized parameters used in the calculations, as well as experimental measurement errors caused by the inability to obtain the actual temperature at the center of the laser irradiated spot due to thermal resistance. Therefore, despite the slightly higher absorption of diamond compared to ZnSe, its exceptionally high thermal conductivity leads to a lower temperature rise under CW laser irradiation.

Figure 9b,c further shows the existence of a temperature gradient on the surface of the AR coatings. The maximum temperature differences in the lateral and longitudinal directions for the AR coating deposited on the ZnSe substrate were 18.2 °C and 8.1 °C, respectively. In contrast, on the diamond substrate, the maximum temperature differences were significantly reduced to 0.1 °C and 0.05 °C, respectively. Figure 10 shows the temperature distribution of the AR coating along the x and z directions. It can be seen that the temperature distribution of the 10.6 µm AR coating deposited on the diamond substrate is more uniform, whereas a significant temperature gradient appears on the ZnSe substrate. This phenomenon is mainly attributed to the superior thermal conductivity of diamond, which allows it to rapidly and effectively diffuse heat. Therefore, selecting substrate materials with high thermal conductivity not only reduces the maximum in-plane temperature rise but also minimizes the temperature gradient and the surface deformation caused by the gradient, thereby suppressing the deterioration of beam quality.

Due to the exceptionally high thermal conductivity of diamond, under the same laser power density, the AR coating deposited on the diamond substrate exhibits a 36% lower temperature under CW laser irradiation compared to the ZnSe substrate. The surface temperature distribution is more uniform, and it shows a lower temperature gradient.

3.3. Laser-Induced Damage Threshold

To evaluate the LIDT of the samples at 10.6 µm, CW laser irradiation with varying power levels was applied for 60 s. Damage points first appeared on the surface of the 10.6 µm AR coating on the ZnSe substrate at an output power of 210 W, whereas they were first observed on the diamond-AR coating at 270 W.

Table 4. The LIDT of AR coatings on different substrates.

Sample	Spot Size	Power (W)	LIDT (W/cm−2)
ZnSe-AR	1.5 mm	210	11,890
Diamond-AR	1.5 mm	270	15,287

summarizes the damage characteristics of the two coatings under CW laser irradiation, with LIDTs of 11,890 W/cm² and 15,287 W/cm², respectively. Figure 11 presents the surface morphology of the samples observed using a microscope and an FIB-SEM after laser irradiation. Analysis of the damage morphology indicates that the damage initiated from the central region and propagated outward [38], primarily due to the higher energy density at the center of the Gaussian beam compared to its periphery. For the ZnSe substrate, Figure 11a demonstrates a molten damage phenomenon, with the substrate being perforated, and cracks observed near the damage point, as shown in Figure 11c. For the diamond substrate, Figure 11b shows that the AR coating deposited on the diamond substrate also exhibits molten damage, accompanied by blackening at the damage point. Additionally, the diamond substrate undergoes fracture and splits into two sections, as depicted in Figure 11d. Figure 12 shows the XRD spectra of the AR coating deposited on the diamond substrate before and after laser damage. Prior to the damage, two diffraction peaks were detected at 2θ = 28° and 44°, corresponding to the ZnS (111) plane and the diamond (111) plane, respectively. After the damage, in addition to the two diffraction peaks at 2θ = 28° and 44°, a small diffraction peak appeared near 2θ = 26.5°, corresponding to the graphite (002) plane. This resembles the diamond graphitization phenomenon at high temperatures reported by Li et al. [39]. These results indicate that graphite was formed on the diamond surface after laser irradiation.

For the AR coating deposited on the ZnSe substrate, the higher absorption of the thin film material, combined with the lower thermal conductivity of ZnSe, leads to heat accumulation and the onset of melting damage. The melted material becomes a center for energy absorption, continuously absorbing laser energy, and causing heat to propagate into the material, ultimately resulting in thermal runaway and causing perforation of the substrate. Additionally, due to the temperature gradient on the surface and the uneven temperature distribution, the higher thermal expansion coefficient of ZnSe leads to rapid local heating, generating excessive stress that results in cracking.

In contrast, the excellent thermal conductivity of the diamond substrate enables rapid heat dissipation, while its low thermal expansion coefficient minimizes stress-induced damage between the coating and the substrate, preventing the formation of surface cracks. However, as previously mentioned, the higher absorption of the film material and the presence of defects in the film result in localized hotspots under high-power laser irradiation, causing the temperature in those regions to rise sharply. Crystalline materials such as diamond are prone to phase transitions at high temperatures [5,6], and the energy deposition at the center of the laser spot causes the local temperature to exceed the phase transition threshold, leading to graphite formation [39,40]. After graphite formation, the absorption rate increases [41], causing damage to the diamond and resulting in perforation. Furthermore, the mechanical strength of graphite is much lower than that of diamond, making the graphite regions stress concentrators. Despite the relatively uniform temperature rise in diamond, the difference in the thermal expansion coefficients between the graphite region at the laser spot center and the surrounding diamond induces interface shear stress, leading to substrate fracture.

Owing to the superior thermal conductivity and mechanical strength of the diamond substrate, the 10.6 μm AR coating deposited on it demonstrated a higher LIDT, showing an improvement of approximately 28.5% compared to the coating on the ZnSe substrate.

4. Conclusions

In this study, we used ZnS and YbF₃ as film materials to fabricate 10.6 μm AR coatings on diamond and ZnSe substrates. To evaluate the influence of substrate materials on the performance of AR coatings, a systematic study was conducted on the surface thermal effects and laser damage thresholds of the coatings under CW laser irradiation. The following conclusions were drawn:

(1)

Given the similar refractive indices of diamond and ZnSe, the same AR coating system was applied to both substrates. Furthermore, the transmission at 10.6 μm exceeded 98% on both substrates.

(2)

Due to the ultra-high thermal conductivity of diamond, the 10.6 μm AR coating deposited on the diamond substrate experiences a lower temperature rise under the same laser power density, with a 36% reduction in temperature rise compared to the coating deposited on the ZnSe substrate. Furthermore, it exhibits a more uniform temperature distribution, with lower temperature gradients both laterally and radially, which is expected to result in reduced thermally induced surface distortion.

(3)

Under 10.6 μm CW laser irradiation, the laser damage threshold of the AR coating on the ZnSe substrate is 11,890 W/cm², while the threshold for the coating on the diamond substrate is 15,287 W/cm², representing a 28.5% increase. The damage on the ZnSe substrate primarily manifests as interface melting, cracks, and substrate perforation. In contrast, the damage on the diamond substrate is characterized by melting, localized graphitization, fracture, and perforation.

These findings indicate that, compared to AR coatings deposited on ZnSe substrates, those fabricated on diamond substrates not only maintain comparable spectral efficiency but also exhibit reduced thermal rise and a higher LIDT. Although diamond offers certain advantages in enhancing the thermal performance of AR coatings, its high cost and the variability in crystal growth quality present challenges for large-scale application. Defects arising from inconsistencies in diamond growth can also result in instability in its optical properties. Moreover, the long-term stability of AR coatings on diamond substrates under CW laser operation remains to be fully investigated. This study provides both theoretical and experimental support for the potential application of diamond-based materials in high-power laser systems. To comprehensively evaluate the applicability of these coatings, further experimental studies are required to assess their long-term durability and stability under sustained laser irradiation. In addition, the effects of post-deposition treatments—such as annealing and plasma processing—on the optical and structural properties of the AR coatings warrant systematic investigation.