Open Access
Add to BibSonomy
Abstract
Broader spectra, lower reflectivity and higher reliability are the performance requirements for broadband antireflective (BBAR) films. In this work, a BBAR film structure was proposed, which maintains extremely low reflectivity, ultra-wide spectra, low polarization sensitivity and practical reliability. The BBAR film consists of a dense multilayer interference stack on the bottom and a nano-grass-like alumina (NGLA) layer with a gradient low refractive index distribution on the top. The film was deposited by atomic layer deposition, while the NGLA layer was formed by means of a hot water bath on Al2O3 layer. The top NGLA layer has extremely high porosity and ultra-low refractive index, along with extremely fragile structure. To surmount the fragility of NGLA layer, a sub-nano layer of SiO2 was grown by atomic layer deposition to solidify its structure and also to adjust the refractive index with different thicknesses of SiO2. Finally, in the wide wavelength range of 400-1100 nm, the average transmittance of the double-sided coated fused quartz reaches 99.2%. The absorption, light scattering, reliability and polarization characteristics of BBAR films were investigated. An optimized BBAR film with low polarization-sensitivity and improved reliability was realized, which should be potentially promising for application in optical systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Antireflective (AR) films are one of the most common types of optical films, which are used to increase the energy transfer efficiency and reduce stray light of the optical system. They are widely used in optical components, optoelectronic devices, instruments and systems. With the development of optical and optoelectronic technologies, the need of broadband AR (BBAR) films is increasingly required. Normally, for BBAR films, wider spectral range or incident angle range will lead to higher average residual reflectance. However, the broadband and wide-angle characteristics of the interference stack can be effectively improved by using a material with sufficiently low refractive index as the outermost layer [1]. Therefore, reducing the effective refractive index of the outermost layer has been one of the focuses of research. The ultimate challenge for AR films is to achieve continuous tunability of the film refractive index between those of air and dense materials. At present, there are many structures to realize AR function, including glancing angle deposition graded-index structures [2], bionic photonic structures [3] and multilayer films [4]. Due to the self-shadowing effect and limited adatom diffusion, glancing angle deposition can be adopted to deposit nanoporous films [5], which are commonly used as low-index materials in multilayer AR films [6]. It is also an effective method to artificially reduce the effective refractive index of materials by introducing nanoscale pores into materials [7–9]. The increase in the porosity of materials will lower the density of light-matter interactions, thereby reducing the effective refractive index [10–12]. Therefore, nanoporous materials have been extensively studied in the preparation of AR films on glass and other substrates [13–16]. Since layers with gradient-index can provide broadband and omnidirectional properties of AR films [17], nanoporous AR films are usually fabricated with gradient-index profiles [10–12]. The ultra-broadband AR films usually consist of refractive-index-graded structures [18]. In biomimicry, the eyes of some butterflies and moths include natural graded-index structures [19,20]. Such biomimetic photonic nanostructures have been realized by designing patterned subwavelength structures [21] or directly using biological sources as templates for replication [22]. Although the performance of AR thin films designed with these structures seem well, the fabrication methods are limited by drawbacks such as complex manufacturing processes, high cost, and difficulty in large-scale implementation. The features of precise thickness control, high densities, and conformal growth of the films prepared by atomic layer deposition (ALD) bring more possibilities for optical thin films [23–27], which offer the advantage of producing highly conformal optical films on complex shaped surfaces [28]. It was found that Al2O3 films are chemically unstable in hot water, causing the change in morphology of Al2O3 after long-term hot water bath [29–31]. Kauppinen et al. investigated a new AR film derived from the chemical instability of Al2O3 films in hot water [32], achieving a gradient refractive index structure as nano-grass-like alumina (NGLA) in a low-cost and simple way, and AR films can be prepared with superior performance in this method, such as Reuna et al. demonstrated a AR film coated on GaInP solar-cells, which reflectance reaches 3.3% in the wavelength range of 400-2000nm [29]. Yin et al. demonstrated a double-side AR film coated on fused-silica, which reflectance reaches 0.3% in the wavelength range of 350–800 nm [33]. Wang et al. demonstrated a double-side AR film coated on BK7 glass, which reflectance reaches 0.4% in the wavelength range of 400–1100 nm at angles of incidence of 6° [34]. However, there are still problems such as unstable refractive index, narrow bandwidth of AR film, unstable structure and poor reliability, which obstruct the application of such films.
In this work, a novel structure of BBAR film, consisting of a bottom multilayer stack and a top NGLA layer with gradient refractive index, was designed and fabricated by ALD method. A protective ultrathin layer was deposited on the whole surface of NGLA by ALD to achieve enhanced stability, which benefits the cleaning of optics in practical application. In addition, the regulation of the refractive index of NGLA with different thicknesses of protective layer was investigated. The polarization sensitivity characteristics over wide incident angle range, along with the reliability with different friction distance were investigated carefully to meet practical application requirements.
2. Materials and methods
The film materials HfO2 (symbol H), Al2O3 (symbol M) and SiO2 (symbol L) used in this work were all deposited by thermal ALD (TFS500, Beneq Oy, Finland), with the precursors of the metal elements tetrakis(dimethylamino)hafnium (TDMAH), trimethylaluminum (TMA) and bis(tert-butylamino) silane (BTBAS), respectively (all provided by Nanjing ai mou yuan Scientific equipment Co., Ltd, China, purity of metal element >99.99%). The oxidant is O3. The ALD process parameters are listed in Table 1. Taking Al2O3 as an example, the ALD process is as follows: the source temperature of the TMA precursor is kept at 20 °C to achieve a stable vapor pressure, and the reaction chamber is kept at 300 °C constantly for at least 30 minutes before starting the deposition. The whole ALD process sequence of Al2O3 layer in one cycle is as follows: TMA pulse (0.2 s), purge with nitrogen (3 s), O3 pulse (3 s) and purge with nitrogen (3 s).
Table 1. Parameters of ALD process for HfO2, Al2O3 and SiO2 deposition.
To explore the structure of NGLA, samples of single-layer Al2O3 were prepared on single-sided polished silicon wafers, and characterized by Ellipsometry (V-VASE, J. A. Woollam, USA) after a hot water bath (deionized water, resistivity ∼0.5 MΩ*cm). The thickness of the natural oxide layer on the silicon wafer was calibrated as 2.27 nm by a variable angle spectroscopic ellipsometer (VASE). The processing method of the Al2O3 samples is as follows: after deionized water (DIW) in the test tubes was heated to 90 °C, the samples were placed in heated water for one hour, then taken out and drip-washed with alcohol to remove the surface moisture, after that the samples were placed on a high-temperature aluminum plate (120 °C) for rapid drying.
Spectroscopic ellipsometry (SE) was used to determine the thicknesses and the dispersion formulas of single-layer samples, with spectral range of 0.8-5.2 eV (wavelength range 238-1550 nm) per 0.1 eV at three incident angles of 65°, 70° and 75°. The refractive indices of the three materials were obtained by modeling fitting which aims to get lower Mean Square Error (MSE) value. For each material, the thickness was divided by the total cycle number of ALD to obtain the growth per cycle (GPC) in ALD. The transmission spectra of the AR films were measured by a commercial spectrophotometer (Lambda 900, PerkinElmer, USA) with wavelength range 200-1200 nm per 2 nm and spectral resolution 2 nm at normal incidence. The data acquisition speed is better than 3 data per second and the accuracy is about 0.1%. The surface and cross-sectional microstructures of the SEM (scanning electron microscope) images of thin film samples presented in this work were obtained by FESEM (Hitachi, FESEM-4800-1) with a resolution of several nanometers. Cleavage method was used to fabricate the cross section for SEM images.
The abrasion resistance of the film samples was measured using the tribological method. The specific experimental treatment was as follows: the back side of the sample of size 15 mm * 15 mm was glued on the top surface of an aluminum alloy cylinder with a total mass of about 80 g. Then the cylinder was inverted with the coating side of the sample contacting with a multilayer dust-free cloth well moistened with ethanol. A force was applied in the horizontal direction to make the sample rub against the dust-free cloth with a contact area of 2.25 cm2. After rubbing for different distances, the transmission spectra or VASE measurement of the samples were performed to evaluate the degree of wear and the cleanable capacity.
3. Results and discussion
3.1 Single layer investigation
The film thicknesses of 100-500 cycles ALD Al2O3 are about 10-50 nm, so the GPC of ALD Al2O3 is about 0.1 nm/cycle. Although NGLA formed by heated DIW treatment is a structure with a graded refractive index [32], an over-complicated structure-model may bring false solutions in the analysis of ellipsometric parameters, which cannot reflect the real structural parameters. Therefore, to make the fitting results converge reliably and be suitable for the design of optical films, the NGLA film is divided into upper and lower layers for fitting. Figure 1(a) is the structural model of NGLA on Si wafer for ellipsometric analysis. As shown in Fig. 1(b), the fitting data obtained by using the structural model and experimental data from VASE measurement have good consistency. Figure 1(c) and (d) show that the refractive index of NGLA near the substrate is higher, and that of the layer near the air is smaller. As the ALD period number increases, the thickness of the formed NGLA increases, as does its refractive index. Taking the sample of 300 cycles as an example, the refractive index and thickness are 1.64 and 30 nm before heated DIW treatment, and transform to been 1.15 and 131 nm for layer1, 1.06 and 145 nm for layer2 after heated DIW treatment. The overall thickness reaches 276 nm, which is 9 times the thickness value before heated DIW treatment, and correspondingly, its refractive index is also greatly reduced due to the high porosity of NGLA. For NGLA with 100 cycles, the refractive index even reaches below 1.02 for layer2. Therefore, there is wide refractive index adjustment range of NGLA layer, and the measured values can be substituted into the software to design an ideal AR coating system.
Fig. 1. Details of ellipsometric analysis, (a) NGLA ellipsometric structure model, (b) fitting data and experimental data of ellipsometric parameter Delta, (c) refractive index of wavelength 620 nm and thickness of layer1, (d) refractive index of wavelength 620 nm and thickness of layer2.
The microstructure of NGLA samples was characterized by SEM to explore the structural differences that result in different properties of thin films, as shown in Fig. 2. The NGLA film presents a “grass-like” structure in the cross-section view and a “petal-like” structure in the surface view, and there are a large number of pores in the entire structure. The porosity increases from the bottom to the top of the film, which induce lowered refractive index of the material. In the ellipsometric analysis, the entire thickness of the NGLA of 100, 250, and 500 cycles are 182, 249, and 347 nm, respectively. In Fig. 2(a-c) of the SEM images, the thicknesses of the structures displayed by SEM are greater than those from SE. The same results were found by other research groups [32]. The ellipsometric fit results in the effective thickness and refractive index. Effective thickness is less than physical thickness due to high porosity. Figure 2(d-f) show that the distribution of grass-like structures of 100-cycle NGLA are relatively sparse, while the grass-like structures of 500-cycle NGLA are densely distributed and larger in size, which is consistent with the SE fitting results in Fig. 1.
Fig. 2. SEM images of NGLA of different ALD cycles on silicon substrates, (a)(d) 100 cycles, (b)(e) 250 cycles, (c)(f) 500 cycles, (a-c) cross-sectional views, (d-f) surface views.
Because of the fragility of the overall structure of NGLA, it is easy to be damaged. To improve the practicability, the NGLA structure was solidified by ALD SiO2. Utilizing the conformity characteristics of ALD, the NGLA was covered with a SiO2 “shell” and bonded to the substrate more strongly. Figure 3 shows that the protective films of SiO2 with different thicknesses of 0.1-10 nm were added on NGLA samples on silicon substrates. Figure 3(a)(b) show that as the thickness of SiO2 increases, the “petal"-shaped nano flake of the film gradually thickens, and the additional SiO2 does not destroy the original grass-like structure. ALD offers films to grow conformally with extreme topography, which protects the original structure. Figure 3(c) shows that the original pores will be filled when the overcoated film is too thick. Figure 3(d) shows the NGLA refractive index after deposition of SiO2 with different deposition periods. It is proved that this method can be used to fabricate films with controllable low refractive index between that of air and bulk materials. In addition, the refractive index hardly changes with 0.1 nm or 0.5 nm SiO2, but increases after 10 nm SiO2 deposition. The SiO2 layer can well protect the film and retain the grass-like structure of the graded refractive index, and the reliability test results are presented in a separate section 3.3. Therefore, SiO2 layer was added on NGLA film in the subsequent experiments.
Fig. 3. SEM images of 250 cycles NGLA on a silicon substrate with different thicknesses of SiO2 protective film, (a) 0.1 nm, (b) 0.5 nm, (c) 10 nm, (d) the refractive index changing at 2.0 eV (620 nm wavelength) after adding SiO2.
3.2 Multilayer design and characterization
After confirming the controllability of the refractive index and thickness of the film and the reliability of the structure with the additional ALD protective film, we carried out the design and fabrication of multi-layer BBAR film. The BBAR film consisting of 10 layers was designed in commercial software Film Wizard. In the design process, 260-cycle ALD Al2O3 was selected to form the outermost layer with precise thickness 25 nm, which would be used to form a NGLA layer after heated DIW treatment, and the final BBAR film structure would be formed after subsequent ALD process of protective film. The optical constants in the wavelength range of 0.8-5.2 eV (238.4-1549.8 nm) of 260-cycle NGLA film were substituted into the optimization process of the 10-layer BBAR film design. The film structure used in the ALD process is Sub | 20.00Lp 11.28 Hp 60.30Lp 28.20 Hp 40.45Lp 45.12 Hp 39.34Lp 28.20 Hp 112.17Lp 25Mp | Air, where p represents the physical thickness and the value before the symbol represents the physical thickness with unit nm. The dispersion curves of the three materials are shown in Fig. 4(a). For the H layers of BBAR film, 1 nm Al2O3 interlayer was inserted per ∼5 nm to improve the optical performance of HfO2.
Fig. 4. (a) Dispersion curves of Al2O3, SiO2 and HfO2 materials, (b) cross-sectional SEM image of BBAR with silicon as substrate, (c) transmission spectra of BBAR film on fused quartz at normal incidence. (d) transmission spectrum of optimized BBAR on fused quartz at normal incidence.
As Fig. 4(b) shown, the layers boundaries of BBAR films are obvious, and the interfaces are quite parallel and smooth. The deposited film thicknesses are basically consistent with the design values, which approves the accurate coating control of ALD. Compared with the theoretical value of NGLA thickness of 258 nm from single layer investigation, the total thickness of NGLA top layer measured by SEM is about 400 nm. Due to the extremely high porosity of the outermost part of NGLA layer with the hair flocculent structure, the EMA model consistent of 95% air and 5% Al2O3 inferred that the refractive index of the outermost part was very close to that of air. The effective thickness of NGLA layer was indicated between the two red dotted lines, less than the physical thickness.
As Fig. 4(c) shown, the transmittance of the uncoated fused quartz substrate is about 93.2%. In the designing, the average transmittance of the double-side BBAR coated fused quartz was above 99.0%. Figure 4(c) indicates that the BBAR with NGLA layer has a higher and wider transmission spectrum than the traditional multilayer dense BBAR film without NGLA layer. However, the transmittance decreases significantly at 400 nm, which is considered due to the scattering and absorption of the NGLA layer. It can be seen from Fig. 2 that more ALD cycles of Al2O3 induces larger “petal” size of NGLA layer, leading to greater light scattering. Therefore, NGLA made of 150-cycle Al2O3 was adopted for optimization of design. The improved design for coating is Sub | 20.00Lp 13.38 Hp 51.70Lp 46.83 Hp 22.31Lp 86.90 Hp 24.10Lp 40.14 Hp 103.12Lp 13.10Mp | Air. Figure 4(d) shows the experimental results of the improved design. The transmittance has increased and the average transmittance of fused quartz-based double-side BBAR films reached 99.2% in the wavelength range of 400–1100 nm.
The large surface roughness of the NGLA film causes light scattering. To investigate the light scattering and absorption properties of BBAR separately, the total transmittance and total reflectance of the samples were measured using an integrating sphere. The integrating sphere consists of foamed polytetrafluoroethylene (PTEF). Figure 5(a)(b) are the spectra of BBAR film with 260-cycle and 150-cycle NGLA layer. The average total transmittance of 260-cycle double-side BBAR films in the wavelength range of 400-1100 nm is 99.4%, but the average specular transmittance is only 99.0%. As a comparison, the average total transmittance of the 150-cycle double-side BBAR films is 99.3%, but the average specular transmittance is 99.2%, which shows smaller light scattering than the 260-cycle one, as Fig. 5(c) shown. This is due to the larger “petal” size of 260-cycle NGLA, which enhances the scattering. In addition, it can be seen from Fig. 5(a)(b) that the total reflectance of the double-side BBAR films deposited with 150-cycle Al2O3 is relatively high with a peak value 0.82% at 770 nm and an average value 0.50% among 400–1100 nm. On the contrary, there are peak value 0.25% at 745 nm and average value 0.25% for the one with 260-cycle. The BBAR film of 260-cycle has better theoretical design resulting with an overall lower actual reflectance, but due to higher light scattering and absorption of thicker NGLA layer, as Fig. 5(c)(d) shown, it turns out to lower transmittance.
Fig. 5. Comparison of optical properties of two BBAR thin films designed based on NGLA of different ALD cycles with quartz as substrate at normal incidence, (a)(b) the total transmission, specular transmission, total reflection, absorption spectrum of BBAR films with 260-cycle, 150-cycle NGLA, respectively, (c) diffuse transmission spectra, (d) absorption spectra.
3.3 Polarization and reliability characteristics
Thin-film polarization dependence and reliability are critical for optical systems in practical applications [35]. Typically, optical elements or systems have different polarization responses to light irradiation. Linear polarization sensitivity (LPS) is generally used to characterize the response inconsistency of the system to linear polarization light, and is defined as Eq. (1):
where, Imax and Imin represent the maximum and minimum response of the system to linear polarized light, respectively. For optics coated with AR films, since maximum and minimum transmittance normally is that of p-polarized and s-polarized light, LPS is defined to facilitate the analysis of ellipsometric measurement as Eq. (2): where, TP and TS represent the transmittance of p-light and s-light, respectively, and φ is the ellipsometric parameter directly obtained by ellipsometry.Figure 6(a)(b) show the LPS of the BBAR film with NGLA of 260-cycle and the conventional BBAR film without NGLA. At the angles of incidence (AOI) 30 deg, 50 deg, and 70 deg, the average LPS of BBAR films with NGLA in the 400-1000 nm band range are 0.06%, 0.52%, and 3.80%, respectively, while the LPS of conventional BBAR films without NGLA are 1.83%, 6.42%, and 18.25%, respectively. The LPS of BBAR film with NGLA is smaller in the AR band, as Fig. 6(c)(d) shown. The average LPS of BBAR films with and without NGLA in AOI 30-70 deg at wavelength 620 nm is 1.06% and 7.85%. The average LPS BBAR film with NGLA in AOI 30-70 deg and wavelength 400-1000 nm is 0.78%, 6.02% lower than that of the BBAR film without NGLA 6.80%.
Fig. 6. LPS of double-sided BBAR films with and without NGLA, (a)(b) different AOI in the wavelength range of 400-1000 nm, (c) LPS at the wavelength of 620 nm, (d) average LPS in the wavelength range of 400-1000 nm.
Reliability evaluation of the samples was performed by ellipsometry on the samples of 250-cycle NGLA on Si wafers with different protective film after different reliability test. In this experiment, 0, 0.1, 0.5, and 10 nm of ALD SiO2 were deposited on the NGLA samples, where 0 nm means no protective film. The vertical pressure used in the friction experiment is about 0.8 N, and the frictional length (FL) of 0, 0.12, 0.82, 4.82, 12.82, 22.82 m are adopted respectively. After friction testing, ellipsometry was performed on the samples, and the ellipsometric fitting was carried out with the structural model mentioned above, and the refractive indices at 620 nm wavelength and thicknesses (d1, n1, d2, n2) of layer1 and layer2 were obtained respectively, as shown in Table 2.
Table 2. Reliability test of NGLA with SiO2 protective film of different thicknesses.a
In order to ensure that the fitted film structure can show regular changes after different FL the material parameters of layer1 were set constant before layer2 collapsed. However, for NGLA of 0 nm and 0.1 nm SiO2, both layer1 and layer2 were damaged during the friction process, so the material parameters of the two layers were set as variables. Figure 7 shows the experimental results of the ellipsometric parameter Psi for different thicknesses of SiO2 ALD with FLs of 0, 0.12, 0.82, and 22.82 m. For FL 0 m, the refractive indices and thicknesses of NGLA deposited with 0, 0.1 and 0.5 nm SiO2 are numerically close. The refractive index of NGLA changed significantly after 10 nm SiO2 ALD, as Table 2 and Fig. 3(d) shown. Since ALD is a conformal deposition method, the ALD SiO2 will fills in the pores of the NGLA. Although ALD SiO2 of less than 0.5 nm are not sufficient to change the overall optical properties of the film, 10 nm SiO2 protective film will fill up the pore structure of NGLA, especially the bottom part, resulting in an evident increase in the overall refractive index and optical thickness of the NGLA film. As the optical thickness of the film increases, the interference effect becomes stronger, resulting in the redshift of interference peaks and more peaks, as shown in Fig. 7(a). For FL 0.12 m, the curve of ellipsometric parameter Psi of the unprotected NGLA sample has changed greatly, as shown in Fig. 7(b), indicating that its surface structure has been destroyed. For FL 0.82 m, the unprotected NGLA structure was completely destroyed, showing the same Psi curve with the silicon substrate, and the Psi curve of 0.1 nm SiO2 changes drastically, as shown in Fig. 7(c). For FL 22.82 m, the NGLA with 0.1 nm SiO2 protective film was also completely destroyed. However, the Psi curve of the 0.5 nm SiO2 protective film hardly changes, as shown in Fig. 7(d), indicating that the 0.5 nm SiO2 protective film can protect the NGLA film well.
Fig. 7. Experimental results of ellipsometric parameter Psi in reliability tests after different FL at the AOI 70 deg, (a-d) FL = 0, 0.12, 0.82, 22.82 m, respectively.
Furthermore, we examined the reliability of the BBAR films with 150-cycle NGLA and 0.5 nm SiO2, using the same frictional pattern. Reflectance measurements were performed on the BBAR film on a black glass substrate to eliminate reflected light from the backside of the substrate. As shown in Fig. 8(a), after friction experiments of different FL, the residual reflectance spectra vary very little and are in the acceptable range for practical application, which means the overall structure hardly changes. Before the friction test of the BBAR film, the average specular reflectance in the wavelength range of 400-1100 nm is 0.24%. After rubbing for 5.00 and 21.00 m, both the average reflectance become 0.25%, which proves that its reliability is excellent. Considering that NGLA produces light scattering, the total reflectance of BBAR film after rubbing for 21.00 m was measured using integrating sphere for estimating diffuse reflectance, as shown in Fig. 8(b), and no evident additional light scattering was observed.
Fig. 8. Reflection spectra of BBAR after frictional test, (a) specular reflection spectra of BBAR (150-cycle) thin film with different FL, at the AOI 20 deg. (b) total reflection spectrum at the AOI 8 deg.
4. Conclusions
A BBAR film using NGLA as top layer with excellent characteristics of high transmittance, low polarization sensitivity and improved reliability was realized. The average transmittance of the double-side coated BBAR films on fused quartz reached 99.20% under normal incidence in the wavelength range of 400-1000 nm. The average LPS of 30-70 deg is 0.78%, and the LPS of 30 deg is as low as 0.06%. The ALD SiO2 layer of sub-nanometer thickness could well protect the grass-like structure of NGLA, which was adopted as the top layer of the proposed BBAR film. With the SiO2 protective film, the proposed BBAR film can be used and wiped clean in the environment of application. In addition, the light scattering characteristics of NGLA were studied, showing that BBAR film with thicker NGLA had lower reflection, but produced higher scattering and absorption. Due to the large scattering, the actual transmittance decreased for thicker NGLA. The scattering generated by the grass-like porous structure is the main reason for the further reduction of the reflectivity of the anti-reflection film. Also, a means of manipulating the low refractive index between dense material and air was proposed and certified, which could play a key role in the design of novel anti-reflection films and other applications. The BBAR films with protected NGLA layers could be very useful for a variety of optical devices, such as liquid crystal displays, camera lenses and remote sensing instruments.
Funding
National Natural Science Foundation of China (61805267, 62275053, 62275256); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2019241, 2020244, 2023248); Projects funded by the central government to guide local Scientific and Technological Development (YDZX20213100003011); National Key Research and Development Program of China (2021YFB3701504); Science and Technology Commission of Shanghai Municipality (18ZR1445400).
Disclosures
The authors declare no conflicts of interest.
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.
References
1. K. Pfeiffer, L. Ghazaryan, U. Schulz, and A. Szeghalmi, “Wide-angle broadband antireflection coatings prepared by atomic layer deposition,” ACS Appl. Mater. Interfaces 11(24), 21887–21894 (2019). [CrossRef]
2. S. Bruynooghe, D. Tonova, M. Sundermann, T. Koch, and U. Schulz, “Antireflection coatings combining interference multilayers and a nanoporous MgF2 top layer prepared by glancing angle deposition,” Surf. Coat. Technol. 267, 40–44 (2015). [CrossRef]
3. P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the “Moth Eye” principle,” Nature 244(5414), 281–282 (1973). [CrossRef]
4. D. M. Braun and R. L. Jungerman, “Broadband multilayer antireflection coating for semiconductor laser facets,” Opt. Lett. 20(10), 1154–1156 (1995). [CrossRef]
5. K. M. A. Sobahan, Y. J. Park, J. J. Kim, Y. S. Shin, J. B. Kim, and C. K. Hwangbo, “Nanostructured optical thin films fabricated by oblique angle deposition,” Adv. Nat. Sci.: Nanosci. Nanotechnol. 1(4), 045005 (2010). [CrossRef]
6. M. F. Schubert, F. W. Mont, S. Chhajed, D. J. Poxson, and E. F. Schubert, “Design of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials by genetic algorithm,” Opt. Express 16(8), 5290–5298 (2008). [CrossRef]
7. B. E. Yoldas, “Investigations of porous oxides as an antiflective coating for glass surface,” (1980).
8. X. Li, J. Gao, L. Xue, and Y. Han, “Porous polymer films with gradient-refractive-index structure for broadband and omnidirectional antireflection coatings,” Adv. Funct. Mater. 20(2), 259–265 (2010). [CrossRef]
9. S. Chhajed, D. J. Poxson, X. Yan, J. Cho, E. F. Schubert, R. E. Welser, A. K. Sood, and J. K. Kim, “Nanostructured multilayer tailored-refractive-index antireflection coating for glass with broadband and omnidirectional characteristics,” Appl. Phys. Express 4(5), 052503 (2011). [CrossRef]
10. J. Chen, B. Wang, Y. Yi, Y. Shi, and P. Cui, “Porous anodic alumina with low refractive index for broadband graded-index antireflection coatings,” Appl. Opt. 51(28), 6839–6843 (2012). [CrossRef]
11. S. R. Kennedy and M. J. Brett, “Porous broadband antireflection coating by glancing angle deposition,” Appl. Opt. 42(22), 4573–4579 (2003). [CrossRef]
12. F. W. M. D. J. Poxson and M. F. Schubert, “Quantification of porosity and deposition rate of nanoporous films grown by oblique-angle deposition,” Appl. Phys. Lett. 93(10), 1 (2008). [CrossRef]
13. D. Li, F. Huang, and S. Ding, “Sol–gel preparation and characterization of nanoporous ZnO/SiO2 coatings with broadband antireflection properties,” Appl. Surf. Sci. 257(23), 9752–9756 (2011). [CrossRef]
14. M. F. S. David, J. Poxson, W. Frank, E. F. Mont, J. K. Schubert, and Kim, “Broadband omnidirectional antireflection coatings optimized by genetic algorithm,” Opt. Lett. 34(6), 728–730 (2009). [CrossRef]
15. J. Hao, Y. Ke, and Y. Wang, “Antireflective structures via spin casting of polymer latex,” Opt. Lett. 32(5), 575–577 (2007). [CrossRef]
16. J. Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nat. Photonics 1(3), 176–179 (2007). [CrossRef]
17. S. Chhajed, M. F. Schubert, J. K. Kim, and E. F. Schubert, “Nanostructured multilayer graded-index antireflection coating for Si solar cells with broadband and omnidirectional characteristics,” Appl. Phys. Lett. 93(25), 61 (2008). [CrossRef]
18. C. Deng and H. Ki, “Pulsed laser deposition of refractive-index-graded broadband antireflection coatings for silicon solar cells,” Sol. Energy Mater. Sol. Cells 147, 37–45 (2016). [CrossRef]
19. P. Ar, W. Ra, and Z. Hegedus, “Solar-absorber antireflector on the eye of an Eocene fly (45 Ma),” Proc. R. Soc. London, Ser. B 265(1390), 1 (1998). [CrossRef]
20. P. Vukusic and J. R. Sambles, “Photonic structures in biology,” Nature 424(6950), 852–855 (2003). [CrossRef]
21. S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett. 93(13), 1925 (2008). [CrossRef]
22. G. Zhang, J. Zhang, G. Xie, Z. Liu, and H. Shao, “Cicada wings: a stamp from nature for nanoimprint lithography,” Small 2(12), 1440–1443 (2006). [CrossRef]
23. D. Riihel, M. Ritala, R. Matero, and M. Leskel, “Introducing atomic layer epitaxy for the deposition of optical thin films,” Thin Solid Films 289(1-2), 250–255 (1996). [CrossRef]
24. G. Triani, P. J. Evans, D. R. Mitchell, D. J. Attard, K. Finnie, M. James, T. Hanley, B. Latella, K. Prince, and J. Bartlett, “Atomic layer depostion of TiO2/Al2O3 films for optical applications,” SPIE Proceedings - Optics and Photonics 2005 1, 1 (2005). [CrossRef]
25. J. M. Jarmo Maula, “Atomic layer deposition for industrial optical coatings,” Chin. Opt. Lett. 8(S1), 53–58 (2010). [CrossRef]
26. S. Shestaeva, A. Bingel, P. Munzert, L. Ghazaryan, C. Patzig, A. Tunnermann, and A. Szeghalmi, “Mechanical, structural, and optical properties of PEALD metallic oxides for optical applications,” Appl. Opt. 56(4), C47–C59 (2017). [CrossRef]
27. L. S. Gao, Q. Y. Cai, E. T. Hu, J. Zhou, Y. P. Li, H. H. Luo, B. J. Liu, Y. X. Zheng, and D. Q. Liu, “Double-sided optical coating of strongly curved glass by atomic layer deposition,” Opt. Express 29(9), 13815–13828 (2021). [CrossRef]
28. K. Pfeiffer, U. Schulz, A. Tünnermann, and A. Szeghalmi, “Antireflection coatings for strongly curved glass lenses by atomic layer deposition,” Coatings 7(8), 118 (2017). [CrossRef]
29. J. Reuna, A. Aho, R. Isoaho, M. Raappana, T. Aho, E. Anttola, A. Hietalahti, A. Tukiainen, and M. Guina, “Use of nanostructured alumina thin films in multilayer anti-reflective coatings,” Nanotechnology 32(21), 215602 (2021). [CrossRef]
30. K. Isakov, C. Kauppinen, S. Franssila, and H. Lipsanen, “Superhydrophobic Antireflection Coating on Glass Using Grass-like Alumina and Fluoropolymer,” ACS Appl. Mater. Interfaces 12(44), 49957–49962 (2020). [CrossRef]
31. T. Lertvanithphol, P. Limnonthakul, C. Hom-on, P. Jaroenapibal, C. Chananonnawathorn, S. Limwichean, P. Eiamchai, V. Patthanasettakul, K. Tantiwanichapan, A. Sathukarn, N. Nuntawong, A. Klamchuen, H. Nakajima, P. Songsiriritthigul, and M. Horprathum, “Facile fabrication and optical characterization of nanoflake aluminum oxide film with high broadband and omnidirectional transmittance enhancement,” Opt. Mater. 111, 110567 (2021). [CrossRef]
32. C. Kauppinen, K. Isakov, and M. Sopanen, “Grass-like alumina with low refractive index for scalable, broadband, omnidirectional antireflection coatings on glass using atomic layer deposition,” ACS Appl. Mater. Interfaces 9(17), 15038–15043 (2017). [CrossRef]
33. C. Yin, M. Zhu, T. Zeng, J. Sun, R. Zhang, J. Zhao, L. Wang, and J. Shao, “Al2O3 anti-reflection coatings with graded-refractive index profile for laser applications,” Opt. Mater. Express 11(3), 875 (2021). [CrossRef]
34. H. Wang, C. Yang, Y. Wang, W. Yuan, T. Zheng, X. Chen, Y. Liu, Y. Zhang, and W. Shen, “Wide-angle broadband antireflection coatings with nano-taper hydrated alumina film,” Opt. Express 30(16), 28922–28931 (2022). [CrossRef]
35. Q. Y. Cai, H. H. Luo, Y. X. Zheng, and D. Q. Liu, “Design of non-polarizing cut-off filters based on dielectric-metal-dielectric stacks,” Opt. Express 21(16), 19163–19172 (2013). [CrossRef]
