Research on Thermal Effect and Laser-Induced Damage Threshold of 10.6 µm Antireflection Coatings Deposited on Diamond and ZnSe Substrates
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-04-30 |
| Journal | Coatings |
| Authors | Xiong Zi, Xinshang Niu, Hongfei Jiao, Shuai Jiao, Xiaochuan Ji |
| Institutions | Shanghai University, Tongji University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study systematically investigated the performance of 10.6 µm Anti-Reflection (AR) coatings (ZnS/YbF3) deposited on Chemical Vapor Deposition (CVD) diamond versus standard Zinc Selenide (ZnSe) substrates under Continuous Wave (CW) laser irradiation.
- Superior Thermal Management: The AR coating on the diamond substrate exhibited a 36% lower maximum temperature rise (62.5 °C vs. 96.63 °C) compared to the ZnSe substrate under identical 2830 W/cm2 CW laser irradiation.
- Enhanced LIDT: The Laser-Induced Damage Threshold (LIDT) for the diamond-AR coating was 15,287 W/cm2, representing a 28.5% improvement over the ZnSe-AR coating (11,890 W/cm2).
- Thermal Uniformity: Diamond’s exceptional thermal conductivity (2000 W/(m·K)) resulted in highly uniform temperature distribution, reducing the maximum lateral temperature gradient from 18.2 °C (ZnSe) to 0.1 °C (Diamond).
- Spectral Performance: The 9-layer ZnS/YbF3 AR coating achieved excellent spectral properties, with transmission exceeding 98% at 10.6 µm on both substrates.
- Damage Mechanism: Damage on ZnSe was characterized by melting, cracking, and perforation due to thermal runaway. Damage on diamond involved melting, localized graphitization (phase transition), fracture, and perforation.
- Conclusion: CVD diamond is highly promising as an output window material for high-power 10.6 µm laser systems due to its superior thermal stability and higher LIDT, despite its slightly higher optical absorption coefficient compared to ZnSe.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Target Wavelength | 10.6 | µm | CO2 Laser System |
| AR Coating Structure | ZnS/YbF3 (9 layers) | N/A | Total thickness 1.5 µm |
| Transmission (Diamond-AR) | 98.04 | % | At 10.6 µm |
| Transmission (ZnSe-AR) | 98.48 | % | At 10.6 µm |
| Diamond Thermal Conductivity | 2000 | W/(m·K) | Room Temperature |
| ZnSe Thermal Conductivity | 16 | W/(m·K) | Room Temperature |
| Diamond LIDT (CW) | 15,287 | W/cm2 | 60 s irradiation time |
| ZnSe LIDT (CW) | 11,890 | W/cm2 | 60 s irradiation time |
| LIDT Improvement (Diamond) | 28.5 | % | Relative to ZnSe substrate |
| Max Temp Rise (Diamond-AR) | 62.5 | °C | Under 2830 W/cm2 CW laser (200 s) |
| Max Temp Rise (ZnSe-AR) | 96.63 | °C | Under 2830 W/cm2 CW laser (200 s) |
| Temp Rise Reduction (Diamond) | 36 | % | Relative to ZnSe substrate |
| Max Lateral Temp Gradient (Diamond) | 0.1 | °C | Simulated result |
| Max Lateral Temp Gradient (ZnSe) | 18.2 | °C | Simulated result |
| Diamond Extinction Coefficient (10.6 µm) | 2.61 x 10-6 | N/A | Optical absorption |
| ZnSe Extinction Coefficient (10.6 µm) | 6.01 x 10-7 | N/A | Optical absorption |
| Diamond Thermal Expansion | 1.1 x 10-6 | K-1 | Low stress generation |
| ZnSe Thermal Expansion | 7.1 x 10-6 | K-1 | High stress generation |
| RMS Surface Roughness (Diamond-AR) | 2.3 | nm | AFM measurement |
| RMS Surface Roughness (ZnSe-AR) | 2.5 | nm | AFM measurement |
Key Methodologies
Section titled “Key Methodologies”The 10.6 µm AR coatings were fabricated using a Leybold ARES 1110 vacuum deposition system, combining thermal evaporation and ion-assisted deposition (IAD).
-
Substrate Preparation:
- Substrates (Φ25.4 x 1 mm CVD diamond and ZnSe) were cleaned using an ion beam from an APS source prior to deposition to enhance adhesion.
- Ion source parameters: Voltage 120 V, Current 40 mA.
-
Film Deposition Parameters:
- Vacuum pressure: 8 x 10-6 mbar.
- Substrate temperature: Maintained at 150 °C (optimized to balance packing density and sulfur loss in ZnS).
- ZnS Deposition: Thermal evaporation using a molybdenum boat. Rate: 1.0 nm/s.
- YbF3 Deposition: Ion-assisted thermal evaporation (IAD used to reduce moisture absorption). Rate: 0.5 nm/s.
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Optical and Structural Characterization:
- Transmittance: Measured using a Fourier-transform infrared spectrometer (INVENIOS, Bruker) at 0° incidence.
- Surface Morphology: Investigated via Atomic Force Microscopy (AFM) (Bruker instrument) over a 2 µm x 2 µm area.
- Crystallinity: Determined using X-ray diffraction (XRD) (Rigaku instrument, Cu Kα).
- Microstructure: Cross-sectional SEM imaging (FIB-SEM, Cross Beam 350) after Au coating and focused ion beam sectioning.
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Thermal and Damage Testing (CW Laser):
- Laser System: Custom CO2 laser system (Huazhong University of Science and Technology).
- Beam Parameters: Gaussian beam, 1/e2 diameter 1.5 mm, M2 < 1.15.
- Temperature Rise Test: Laser power 50 W (2830 W/cm2 density), irradiation time 200 s. Temperature monitored by a thermocouple (2 ± 0.5 mm from spot center).
- LIDT Test: Laser power step size 10 W, irradiation time 60 s per step. Damage observed via Leica microscope and FIB-SEM.
Commercial Applications
Section titled “Commercial Applications”The development of high-performance, thermally stable AR coatings on CVD diamond substrates is critical for advancing systems that rely on high-power 10.6 µm CO2 lasers.
- High-Power Industrial Laser Processing: Used in cutting, welding, and surface modification where high beam quality and stable output power are essential. Diamond windows minimize thermal lensing distortion, ensuring precise beam focus.
- Directed Energy Systems (Defense/Aerospace): Output windows for high-energy laser weapons require materials with extreme thermal shock resistance and high LIDT to maintain operational stability under intense CW irradiation.
- Laser Dielectric Acceleration: Components requiring high-power transmission windows in advanced particle accelerator research.
- Extreme Ultraviolet (EUV) Lithography: While the paper focuses on 10.6 µm, diamond’s broad transparency and thermal properties make it suitable for managing heat loads in complex optical trains used in EUV generation and delivery systems.
- High-Power Optical Components: Fabrication of robust beamsplitters, mirrors, and lenses where thermal stability prevents catastrophic failure and wavefront degradation.
View Original 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.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
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