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Abstract
Al2O3 coating is chemically unstable in hot water and transforms into a porous structure with a broadband anti-reflection (AR) property. We investigate the influences of treatment time on the AR property and structure morphology of the Al2O3 coating deposited by electron beam evaporation. Al2O3 coating treated for 7 minutes is found to possess the best AR property with an average reflectance of approximately 0.3% in the wavelength range of 350 nm to 1100 nm. The genetic algorithm simulation shows the treated Al2O3 coating possesses a graded-refractive index profile. The scattering calculation shows a large scattering in the short wavelength range. Moreover, we investigate the laser-resistance of the treated Al2O3 coating, and it shows the potential for application in laser systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Broadband anti-reflection (AR) coatings can reduce reflectance and prevent the loss of light and play an important role in various applications, such as solar cells [1], optoelectronic devices [2], and laser systems [3]. Many methods for preparing broadband AR coatings have been studied, including multilayer AR coatings [4], glancing angle deposition (GLAD) [5,6], and biomimetic photonic nanostructure [7,8]. However, multilayer AR coatings usually suffer from the strong dependence on the wavelength and incident angle of the incident light [9], and there is a trade-off between bandwidth and overall reflectance [10]. Due to the self-shadowing effect, GLAD can deposit nano-porous films [11,12], which are usually used as low refractive index materials in multilayer AR coatings [5,13]. Lord Rayleigh proposes the use of the graded-refractive index (GRIN) layers to achieve broadband AR coatings [14]. For the GRIN AR design, a layer with a graded refractive index or multiple layers with different refractive indices is required. Biomimetic photonic nanostructures, such as the eyes of certain butterflies and moths, are natural GRIN structures [15,16]. These natural structures can be replicated by designing patterned sub-wavelength structures [17–19] or using biological sources as templates [20]. However, these methods are limited by factors such as complex fabrication processes, high costs, and difficulty in large-scale implementation [10]. Kauppinen et al. [21] study a new type of AR coating that benefits from the chemical instability of Al2O3 coating in hot water [22]. They investigate the effect of water treatment temperature on the morphology and transmittance of Al2O3 coatings deposited on Corning microscope slides by atomic layer deposition (ALD) [21]. It is found that the ALD-grown Al2O3 coating transforms into a porous grass-like structure after being treated in 90 ℃ deionized water for 30 minutes. The grass-like structure coating has an average transmittance of 99.0% in the range of 350 nm to 800 nm. This new method of preparing AR coatings has the advantage of broadband anti-reflection performance, simple fabrication process, and batch processing [21]. There is a report shows that this approach also has potential for use on crystalline Al2O3 [23]. It is necessary to do further work to study the application of this method in other applications, such as lasers.
In this work, the Al2O3 coating is prepared by electron beam evaporation because it is favorable in high-power laser applications. The effect of 90 ℃ deionized water treatment time on the AR property, morphology, surface roughness and coating thickness of the Al2O3 coating is systematically investigated. The iterative genetic algorithm simulation method is used to obtain the refractive index profile. The scattering calculation is used to evaluate the optical loss. The laser-induced damage threshold (LIDT) of the AR coating is tested to evaluate its application in laser systems.
2. Experiments
2.1 Sample preparation
The Al2O3 coating with a design thickness of 44 nm is deposited on silicon and fused silica substrates by electron beam evaporation. The silicon substrate is used to characterize the thickness of the coating, while the single-sided polished and double-sided polished fused silica substrates are used to investigate other properties of the coating. In order to eliminate the surface defects of the substrate, the fused silica substrate for LIDT measurement is cleaned by the following steps: first, etching in a mixture solvent of ∼1% HF and ∼15% NH4F for 8 minutes; second, ultrasonic cleaning in deionized water for 5 minutes; third, drying with an oven lamp. The fused silica substrates for other measurements are cleaned by using the last two steps of the above cleaning procedure. The silicon substrate is cleaned with ethanol. The deposition chamber is heated to 200 ℃ and kept for 60 minutes before the deposition. The deposition rate and oxygen pressure of Al2O3 are 0.13 nm/s and 1.3 × 10−4 mbar, respectively. Two series of samples are prepared: single-sided coated samples (on silicon substrate, single-sided polished and double-sided polished fused silica substrates) and double-sided coated samples (on double-sided polished fused silica substrate).
2.2 Deionized water treatment
The Al2O3 coating samples are treated in deionized water inside a digital thermal bath with a built-in temperature control function. Before treating the sample, the deionized water is heated to 90 ℃ and kept for one hour. Different treatment time, between 3 minutes to 60 minutes, is used. After treatment in the deionized water bath, the samples are flushed with deionized water for 2 minutes and then dried with an oven lamp for 60 minutes.
2.3 Characterization
A UV-NIR spectrometer (Perkin Elmer Lambda 1050) is used to measure the reflectance spectra, transmittance spectra, and scattering. The Root-mean-square (RMS) surface roughness is characterized by an atomic force microscope (AFM, Veeco Dimension-3100) in tapping mode with a scan area of 5 μm × 5 μm. The AFM needle (NanoSensors, PPP-NCHR) has a tip radius of <10 nm and a tip height of 12.5 μm. The scattering at 632.8 nm is measured by a home-made system based on a total integrated scattering method [24]. The coating absorption at 1064 nm is measured by a home-made system based on surface thermal lensing technique [25] using a Gaussian-shape 1ω Nd: YAG laser with 4.2 W pumping power and 80 Hz chopping frequency, and the effective spot area on the sample surface is about 0.015 mm2. An optical microscope (OLYMPUS IX83) is used to observe the surface quality with a magnification of 100. The 1-on-1 LIDT is measured according to ISO 21254 using a Gaussian-shape 1ω Nd: YAG laser (1064 nm, 12 ns). The LIDT test is carried out under normal incidence, and the effective spot area on the sample surface is about 0.072 mm2. The damage is evaluated by comparison of the tested areas before and after laser irradiation with the help of an in-situ camera and the damage is considered to occur when the coating surface changes irreversibly after laser irradiation [26]. A scanning electron microscope (SEM, Carl Zeiss AURIGA CrossBeam) is used to obtain the cross-sectional morphology of the sample and the surface morphology of the laser-induced damage site. Before SEM observation, a 20 nm-thick Cr coating is deposited on the sample surface by ion beam sputtering (Quorum Q150 T ES).
2.4 Genetic algorithm simulation
By using the iterative genetic algorithm simulation method, the distribution of refractive index with thickness is inverted according to the measured reflectance spectrum [27]. The merit function used in the simulation is defined as the variance (RV) between the measured and simulated reflectance in the wavelength range of 350-800 nm, which is determined by
2.5 Scattering calculation
The substrate roughness is much lower than the coating roughness, and the scattering is basically affected by the coating roughness. Therefore, the scattering of the treated coating is defined as surface scattering ($S_s^{}$), composed of the reflection scattering ($S_s^r$) and transmission scattering ($S_s^t$), which can be given by Eq. (2–4) [29] when the roughness is much lower than the wavelength of the incident light:
3. Results and discussion
3.1 Anti-reflection performance and microstructure morphology
The reflectance spectra of a single-sided polished fused silica substrate, an as-deposited Al2O3 coating, and Al2O3 coatings treated with different time are shown in Fig. 1. The Al2O3 samples for reflectance measurement are deposited on single-sided polished fused silica substrates. When the treatment time is less than 3 minutes, no obvious changes are observed in the measured spectra. As the treatment time is increased to 4 minutes, the average reflectance (Ra) in the wavelength range of 350 nm to 1100 nm quickly drops to 0.4% from 5.3% of the untreated sample. As the treatment time is further increased to 7 minutes, the Ra reaches about 0.3%, which is a minimum value among all treatment time in this work. For a treatment time longer than 7 minutes, the Ra value increases as the treatment time increases.
Fig. 1. Reflectance spectra of the treated Al2O3 coatings with a treatment time of (a) 0-7 minutes and (b) 7-60 minutes; (c) The average reflectance of the treated Al2O3 coatings with a treatment time of 0-60 minutes.
The AFM characterized surface morphology, and the SEM characterized cross-sectional morphology of the treated samples are shown in Fig. 2. For a treatment time of 3 minutes, the surface and cross-sectional morphologies of the treated sample do not change significantly compared with the untreated Al2O3 coating. The roughness and coating thickness of the treated sample is close to that of the untreated Al2O3 coating, which is consistent with the measured reflectance spectrum. With the further increase of the treatment time, the morphology of the treated Al2O3 coating changes from a traditional electron beam evaporation deposited coating to a random microstructure. The random microstructure leads to a sharp increase in both the surface roughness and coating thickness. The dense Al2O3 coating tends to transform into hydrolyzed products with treatment time [31]. Al(OH)3, as one of Al2O3 hydrolyzed products, shows a nanorod-like structure when it is made in water and may cause the microstructure of deionized water treated Al2O3 coating [32]. For a treatment time of 7 minutes, the surface roughness and the coating thickness increase from 0.7 nm and 55.5 nm of the untreated Al2O3 coating to 42.4 nm and 440.7 nm after treatment, respectively. When the treatment time is between 8 minutes and 60 minutes, the surface roughness of the treated sample shows little dependence on treatment time. The sample treated with 40 minutes shows a maximum coating thickness, and the coating thickness of the treated sample decreases with the further increase of treatment time.
Fig. 2. (a)-(d) AFM images and (e)-(h) cross-sectional SEM images of the treated Al2O3 coatings with different deionized water treatment time; (i) Surface roughness and coating thickness of the treated Al2O3 coatings as a function of the deionized water treatment time, where the treatment time of 0 minute represents the untreated Al2O3 coating.
3.2 Refractive index profile simulation
Based on the measured reflectance spectrum, the refractive index profile of the random microstructure is simulated through the genetic algorithm. For simulation, the treated Al2O3 coating is divided into 800 sublayers, and the target RV in the wavelength range of 350-800 nm determined by Eq. (1) is set to be less than 10−5. The refractive index profiles along with coating thickness of all the treated Al2O3 samples with a treatment time longer than 4 minutes exhibit parabola-like curves. The refractive index profile of the sample treated for 7 minutes is shown in Fig. 3(a). The simulated reflectance spectrum of the treated sample fit well with the measured one, as shown in Fig. 3(b).
Fig. 3. (a) Simulated refractive index profile along with coating thickness (from the substrate to air), as well as (b) the measured and simulated reflectance spectra of the Al2O3 coating treated for 7 minutes.
3.3 Transmittance performance of the single-sided coated sample
The single-sided coating on the double-sided polished substrate is treated for 7 minutes to study the transmittance performance. The measured reflectance and transmittance are shown in Fig. 4. The Al2O3 coating treated for 7 minutes shows enhanced transmittance in the measured wavelength range of 350 nm to 800 nm. However, the sum of the measured transmittance and reflectance in the short wavelength range of the treated sample is less than 100%, indicating the existence of optical loss. According to SEM images in Fig. 2, the size of the random microstructure in the treated Al2O3 coating is about hundreds of nanometers, which is comparable to the wavelength of the incident light and will result in large scattering loss [33]. According to Eq. (2) to Eq. (6), the total scattering of the treated sample (C-S) is calculated and shown together with measured reflection scattering (M-RS), measured transmission scattering (M-TS), measured reflectance (M-R), and measured transmittance (M-T), as shown in Fig. 4. The scattering loss of the treated sample is unignorable in the short wavelength range.
Fig. 4. Measured reflection scattering (M-RS), measured transmission scattering (M-TS), measured reflectance (M-R), measured transmittance (M-T), calculated reflection scattering (C-RS), calculated transmission scattering (C-TS), and calculated total scattering (C-S) of the treated sample.
3.4 Transmittance performance of the double-sided coated sample
The double-sided coated sample is treated for 7 minutes, and the measured transmittance is shown in Fig. 5. The treated sample shows broadband AR performance, the average transmittance (in the wavelength range of 350 nm to 1100 nm) is higher than 98.8%, and the maximum transmittance is about 99.6%. Compared with the fused silica substrate, the treated sample exhibits good AR performance, and the reflection of incident light is significantly reduced, as shown in the white area in Fig. 5(b).
Fig. 5. (a) Transmittance spectra of the fused-silica substrate and double-sided coated sample treated for 7 minutes; (b) Photos of the fused-silica substrate (left) and double-sided coated sample treated for 7 minutes (right).
3.5 1-on-1 LIDT test
The single-sided coating on the double-sided polished substrate is treated for 7 minutes to study the laser damage resistance. The 1-on-1 LIDT of the treated Al2O3 coating is compared with that of the untreated Al2O3 coating. As shown in Fig. 6(a), although the treated Al2O3 coating shows an LIDT lower than the untreated Al2O3 coating, the LIDT of 23.9 J/cm2 proves its application potential in laser systems. To understand the damage mechanism of the treated Al2O3 coating, the absorption at 1064 nm is measured. As shown in Fig. 6(b), the overall absorption of the treated sample is slightly higher than that of the untreated sample, and some areas show much higher absorption, which may be due to contamination caused during the heated water treatment process, as shown in Fig. 6(c-d).
Fig. 6. (a) Laser-induced damage probability of the Al2O3 coating treated for 7 minutes and as-deposited Al2O3 coating; (b) Absorption at 1064 nm of different areas on the Al2O3 coating treated for 7 minutes and as-deposited Al2O3 coating; Photos of (c) the Al2O3 coating treated for 7 minutes and (d) as-deposited Al2O3 coating imaged by the optical microscope; Damaged sites of (f-h) the Al2O3 coating treated for 7 minutes and (i-l) the as-deposited Al2O3 coating imaged by SEM.
Damage morphologies of the treated and as-deposited Al2O3 coating are characterized by SEM. After irradiation with a near-LIDT laser fluence, peeling off of the coating (Fig. 6(e)) in the center of the damaged area is observed, as shown in Fig. 6(f). At a higher laser fluence, the peeling-off area becomes larger (Fig. 6(g)). As the laser fluence further increases, laser damage will penetrate into the fused silica substrate and leads to shell-like pits or craters similar to the damage morphology of fused silica (Fig. 6(h)). However, the damage morphologies of the as-deposited Al2O3 coating are all similar to the damage morphology of fused silica (Fig. 6(i-l)).
4. Conclusions
In summary, we have studied the effect of 90 ℃ deionized-water treatment time on the AR performance and microstructure morphology of Al2O3 coatings. It is found that the single-sided coated sample with a treatment time of 7 minutes shows the lowest average reflectance (in the wavelength range of 350 nm to 1100 nm) of approximately 0.3%. When the treatment time exceeds 7 minutes, the average reflectance increases with the increase of treatment time. The refractive index profile, along with coating thickness, is fitted by the genetic algorithm and shows a parabola-like curve. Scattering calculations show that random microstructures will produce large scattering in the short wavelength range. The double-sided coated sample treated for 7 minutes shows broadband AR performance with an average transmittance (in the wavelength range of 350 nm to 1100 nm) higher than 98.8%. The LIDT of the Al2O3 coating treated for 7 minutes is 23.9 J/cm2, showing the potential for application in lasers.
Funding
National Natural Science Foundation of China (61975215, U1831211); Youth Innovation Promotion Association of the Chinese Academy of Sciences; Strategic Priority Research Program of Chinese Academy of Sciences (XDA25020000, XDB16030400).
Acknowledgments
The authors express their appreciation to Yuan’an Zhao and Ziyuan Xu for the LIDT measurement, and Dawei Li for the scattering and absorption measurement.
Disclosures
The authors declare no conflicts of interest.
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