The influence of SiO ₂ half wave coating on laser damage of HfO ₂/SiO ₂ high reflective film

Abstract: This study aims to investigate the effect of SiO ₂ half wave coating on the laser damage characteristics of HfO ₂/SiO ₂ high reflective films. HfO ₂/SiO ₂ high reflective film samples with different SiO ₂ half wave coverage states were prepared by electron beam evaporation technology. Laser irradiation experiments were conducted on the samples using a 1064nm pulsed laser, and characterization techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the damage morphology, roughness changes, and differences in damage thresholds of the film layers. The research results indicate that the reasonable introduction of SiO ₂ half wave coating can effectively improve the anti laser damage performance of high reflection films, providing important reference for the design and preparation of high-performance reflection films in high-power laser systems.

1. Introduction
In high-power laser systems, the reflective film serves as a key optical component, and its performance directly affects the output energy, beam quality, and stability of the system. HfO ₂/SiO ₂ high reflective films are widely used in high-power laser applications at 1064nm wavelength due to their excellent optical properties and relatively high resistance to laser damage. However, with the continuous development of laser technology, higher requirements have been put forward for the anti laser damage performance of reflective films. Laser damage can lead to a decrease in reflectivity, deterioration of optical performance, and even detachment of the film layer, seriously affecting the normal operation of the laser system.

Research has shown that factors such as the microstructure, impurity content, defect distribution, and interfacial characteristics between film layers can significantly affect their resistance to laser damage. Introducing a cover layer is an effective means of improving the performance of the film layer. SiO ₂ materials have the advantages of high chemical stability, good optical uniformity, and low absorption at a wavelength of 1064nm, and are often used as cover layer materials. This study systematically investigates the effect of SiO ₂ half wave coating prepared on the surface of HfO ₂/SiO ₂ high reflection film on the laser damage performance of high reflection film, in order to provide theoretical and experimental basis for the optimization design of reflection film in high-power laser systems.

2、 Experimental section
2.1 Sample Preparation
Using K9 glass with a surface roughness of less than 1nm as the substrate, HfO ₂/SiO ₂ high reflective film was prepared on the substrate using electron beam evaporation technology. HfO ₂, as a high refractive index material, has a refractive index of approximately 2.0-2.1 at 1064nm; SiO ₂, as a low refractive index material, has a refractive index of approximately 1.45 at this wavelength. Based on the theory of multilayer interference, a film system with (HL) sH as the structure was designed, where H represents the HfO ₂ film layer, L represents the SiO ₂ film layer, and s is the period number. After optimization calculation, it was determined that when s=15, the target reflectivity of over 99.5% can be achieved at a wavelength of 1064nm.

During the preparation process, the vacuum chamber is first pumped to 8 Pa pressure using a mechanical pump and ROOT pump, and then lowered to below 1.0 × 10 ⁻ ³ Pa using a condenser pump. The substrate is heated by a quartz heater at the bottom of the vacuum chamber. When heated above 200 ℃, the temperature drift does not exceed 8 ℃. However, due to the radiation heat generated by the electron beam heating source material, when heated from room temperature to this temperature range, the air pressure in the vacuum chamber will rise by nearly 2 orders of magnitude due to exhaust gas. Therefore, it is necessary to wait for 5-6 hours until the air pressure drops back to below 1.0 × 10 ⁻ ³ Pa before coating. The HfO ₂ source material is provided by Beijing Nonferrous Metals Research Institute, containing 2% -3% ZrO ₂ and particle size of 1.5-4.0mm. Before deposition, it is pre melted layer by layer to remove gas and form a flat pre melted surface to ensure stable deposition rate. The SiO ₂ source material is provided by Optron Corporation in Japan, with a purity of 99.99% and particle size of 2-3mm. The geometric thickness of the film layer is monitored by a quartz crystal oscillator controller, the optical thickness is controlled by an OMS3000 optical control system, the deposition rate is monitored by a quartz crystal oscillator, and the electron gun power is adjusted to maintain stability through a Leycom control system.

To investigate the effect of SiO ₂ half wave coating, two sets of samples were prepared: one set was HfO ₂/SiO ₂ high reflection film without added SiO ₂ half wave coating (sample A); Another group deposited a SiO ₂ coating layer with a thickness of half wavelength (corresponding to 1064nm wavelength) on the surface of the prepared HfO ₂/SiO ₂ high reflective film using electron beam evaporation technology (sample B).

2.2 Laser damage testing
Laser damage testing was conducted using a high-power pulse laser with a wavelength of 1064nm, a pulse width of 5ns, and a repetition rate of 1Hz. The experiment adopts the 1-on-1 testing method, which applies only one laser pulse to each testing point. By gradually increasing the laser energy density until damage appears on the surface of the film layer, record the laser energy density at this time as the threshold for laser damage resistance. The morphology of the damage is observed through scanning electron microscopy (SEM) to analyze the characteristics and degree of the damage. At the same time, atomic force microscopy (AFM) was used to measure the surface roughness of the damaged area and its surroundings, in order to evaluate the impact of laser damage on the surface quality of the film layer.

3、 Results and Discussion
3.1 Laser damage threshold
The test results showed that the anti laser damage threshold of sample A was 60 J/cm ², while the anti laser damage threshold of sample B was increased to 75 J/cm ². This indicates that the introduction of SiO ₂ half wave coating effectively improves the anti laser damage ability of HfO ₂/SiO ₂ high reflective film. Analysis suggests that the SiO ₂ half wave coating has a low absorption coefficient, which can reduce the absorption of laser energy on the surface of the film layer, lower the temperature rise caused by the absorption of laser energy in the film layer, and thus reduce damage caused by thermal stress. In addition, the presence of SiO ₂ coating can provide a certain buffering effect, slowing down the impact of shock waves generated by laser irradiation on the underlying HfO ₂/SiO ₂ film system, thereby improving the overall laser damage resistance of the film system.

3.2 Damage morphology analysis
The SEM observation results show that after laser damage, sample A exhibits obvious pit like damage on the surface of the film layer, with peeling of the film layer around the damaged area and visible crack propagation in some areas. After sample B was damaged, the size of the pit like damage was relatively small, the peeling phenomenon of the film layer was light, and the crack propagation was not obvious. This further confirms the protective effect of SiO ₂ half wave coating on the film layer. During the laser irradiation process, the SiO ₂ coating layer first bears the laser energy, and its good thermal stability and mechanical properties can effectively suppress further damage and reduce damage to the underlying HfO ₂/SiO ₂ film system.

3.3 Surface roughness variation
The AFM measurement results showed that the root mean square (RMS) surface roughness of the damaged area of sample A increased from the initial 0.5nm to 3.0nm, while the RMS value of the damaged area of sample B increased to 1.5nm. This indicates that the presence of SiO ₂ half wave coating can effectively reduce the impact of laser damage on the surface roughness of the film layer. The increase in surface roughness will lead to enhanced laser scattering, reduce the reflectivity of the film layer, and affect the optical performance. The SiO ₂ coating reduces the degree of damage and maintains the relative flatness of the film surface, which helps to maintain the optical properties of the film after damage.

4、 Conclusion
This study systematically analyzed the effect of SiO ₂ half wave coating on the laser damage performance of HfO ₂/SiO ₂ high reflective film through experimental comparison. The results indicate that introducing a SiO ₂ half wave coating can significantly improve the laser damage resistance threshold of HfO ₂/SiO ₂ high reflective films, reduce the degree of damage, and minimize the impact of damage on the surface roughness of the film layer. This provides important technical ideas for the design and preparation of high-performance reflective films in high-power laser systems. In the future, further in-depth research can be conducted on the influence mechanism of factors such as the thickness, microstructure, and interface bonding characteristics with the underlying film system of SiO ₂ coating on laser damage performance, in order to achieve more precise control of the anti laser damage performance of high reflective films and meet the stringent requirements of the constantly developing high-power laser technology for optical thin film performance.