Porous anodic alumina with low refractive index for broadband graded-index antireflection coatings Junwu Chen, 1 Biao Wang, 1, * Yi Yang, 1 Yuanyuan Shi, 1 Gaojie Xu, 1,2 and Ping Cui 1 1 Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China 2 e-mail: *Corresponding author: Received 2 July 2012; revised 29 August 2012; accepted 3 September 2012; posted 4 September 2012 (Doc. ID 171666); published 27 Septemb
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  Porous anodic alumina with low refractive index forbroadband graded-index antireflection coatings Junwu Chen, 1 Biao Wang, 1, * Yi Yang, 1 Yuanyuan Shi, 1 Gaojie Xu, 1,2 and Ping Cui 1 1 Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China 2 e-mail:*Corresponding author: 2 July 2012; revised 29 August 2012; accepted 3 September 2012;posted 4 September 2012 (Doc. ID 171666); published 27 September 2012 Materials with very low refractive index are essential to prepare broadband graded-index antireflection(AR) coatings. However, the availability of such materials is very limited. In this study, large-area( 4  cm×  4  cm) low refractive index porous anodic alumina (PAA) coatings on glass substrate were pre-pared successfully by electron-beam evaporation, electrochemical oxidation, and chemical etching method. The nanopore size of PAA film is smaller than 40 nm, and the refractive index of PAA filmis  n  1 . 08 . Besides, five-layered graded-index broadband PAA coatings with refractive indices following the Gaussianprofile were also prepared to noticeablyeliminate the reflectance of glass over a broadbandwavelength, and the lowest reflectivity is 0.64% at the wavelength of 534 nm at normal incidence. ThePAA AR coatings having an omnidirectional nature are likely to have practical applications in photo- voltaic cells and optical devices. © 2012 Optical Society of America OCIS codes:  220.4241, 310.1210, 310.4165, 310.6860. 1. Introduction It is well known that reflections will inevitably ap-pear when light propagates across an interface of two mediums with different refractive indices. Thegraded-index antireflection (AR) coatings whose re-fractive indices gradually change from one mediumto the other medium can effectively suppress the re-flection and increase the light transmission of theoptical devices [1,2]. One of the common ambient mediums is air, whose refractive index is 1.0. How-ever,optical materials withvery low refractiveindices(low- n ) that closely match the refractive index of airare difficult to obtain. Particularly for dense materi-als, refractive indices of 1.10 or 1.20 do not exist.Therefore, porous nanomaterials with low refractiveindex are often used for preparing graded-index AR coatings.Different methods of making graded-index ARcoatings have been reported, including lithographyand wet etching [3], sol-gel process [4], integrated nanoisland coating arrays on nano-conical-frustumarrays [5], improved metal-induced chemical etching [6], and oblique-angle deposition [7,8]. At present, the lowest refractive index  n  1 . 05  was reportedby Xi and Schubert  et al.  [7] using oblique-angle de-position, and their study showed that oblique-angledeposition has good control over the refractive indexprofile when the substrates are flat. The preparationof graded-index AR coatings on uneven surfaces israrely reported.Porous anodic alumina (PAA) formed by electro-chemical oxidation has been investigated for manyyears [9] and been widely used as templates forfabrication of nanowires or nanotubes [10 –  12]. Thenanopores in PAA are independent and can be en-larged easily in acid solution, so the refractive in-dices of PAA films can be controllably varied and a very low refractive index can be reached. In thisstudy, large-area ( 4  cm ×  4  cm) low- n  PAA coatingson glass substrate were prepared by combinationof electron-beam evaporation, electrochemical oxida-tion, and chemical etching methods. Furthermore, 1559-128X/12/286839-05$15.00/0 © 2012 Optical Society of America 1 October 2012 / Vol. 51, No. 28 / APPLIED OPTICS 6839  five-layered graded-index AR coatings with refrac-tive indices following a Gaussian profile were suc-cessfully prepared on flat and patterned glasses,respectively. 2. Experimental Details Ultra clear glass substrates (the thickness is 2 mmand the size is  4  cm ×  4  cm) were cleaned by ultraso-nication in detergent, acetone, deionized water, andethanol successively. Pure Al (99.99%) films withthickness 850 nm were deposited onto glass byelectron-beam evaporation in a vacuum of   10 − 5 Pa(MUE-ECO-EB, ULVAC Technologies. Inc.). Thenthe coated glasses were anodized at 20 V in 0.3 Moxalic solutions at5°C. Next, the anodizedfilms wereimmersed in 5 wt. % H 3 PO 4  solutions at 30°C for apredetermined time to adjust the pore diameter. Fi-nally, the samples were annealed in the air at 550°Cfor 5 h.The wavelength dependence reflectivity wasmeasured by a spectrophotometry (Perkin Elmer,Lambda 950) at incident angle of 8°, and the incidentangle dependence reflectivity was measured byellipsometry (M-1500DI, J. A. Woollam). The surfaceand the cross-section scanning electron microscopy(SEM) images were characterized by field-emissionscanning electron microscopy (FE-SEM, HITACHIS-4800) operated at 5 kV. 3. Results and Discussion Figure 1 shows a surface SEM image of the low- n PAA on glass acid etched at 30°C for 28 min. FromFig.1we canseethatthenanoporediameter isabout40 nm and the thickness of pore wall is less than10 nm. The refractive index of PAA film measuredby ellipsometry is  n  1 . 08 , which confirms the por-ous nanostructure observed in Fig. 1. Furthermore,the nanopore size is much smaller than the wave-length ofvisiblelight,so MieandRayleighscattering can be neglected over the visible spectrum. Next,we will show that the low- n  PAA film was used inpreparing graded-index AR coatings and virtuallyeliminates Fresnel reflection.Figure 2(a) shows three different graded-indexprofiles with linear, quintic, and Gaussian profilesthat have index matching with air and a glass sub-strate with refractive index of   n s  1 . 5 . Figures 2(b)and 2(c) show the calculated reflectance for thesegraded-index profiles depending on spectral and an-gular, respectively. All of the three profiles have goodperformance with low reflectivity for both transverseelectric (TE) and transverse magnetic (TM) polariza-tions over a broad wavelength [Fig. 2(b)] and angle-of-incidence range [Fig. 2(c)]. Moreover, it is easy tofind out that the Gaussian index profile has the bestperformance, with  R <  0 . 1%  for the spectrum from300 to 1500 nm and the angle from 0° to 60°.Therefore, the Gaussian index profile is the optimumprofile, which is in consistent with the previousconclusion reached by Chen [13].Figure 3 shows the preparation processes of graded-index PAA films designed following theGaussian index profile. First, pure Al (99.99%) filmwas evaporated on the glass using electron-beamevaporation. Then the Al film was anodized in oxalic Fig. 1. Surface SEM image of the low- n  PAA coating.Fig. 2. (Color online) Different index profiles for graded-indexcoating. (a) Linear, quintic index, and Gaussian profiles for a sub-strate ofglass with n s   1 . 5 . (b) Calculatedwavelength-dependentreflectance for linear, quintic index, and Gaussian profiles at inci-dent angle of 8°. (c) Calculated angular-dependent reflectance forlinear, quintic index, and Gaussian profiles at wavelength 600 nm.The calculation is approximated by Essential Macleod using 100layers of equal thickness. 6840 APPLIED OPTICS / Vol. 51, No. 28 / 1 October 2012  acid to form the first PAA layer and immersed intoH 3 PO 4  solution to enlarge the nanopores in the firstPAA layer. After careful cleaning with deionizedwater, the sample was anodized again to form thesecond PAA layer and immersed in acid solutionagain to enlarge the nanopores both in the first andsecond PAA layers. By repeating the anodization andacid etching process for five times, graded-index PAA films composed of five layers were finally completed.By adjusting the time of anodization and acid etch-ing,wecan control the refractive indexand thicknessof each PAA layer easily. The parameters of prepara-tion processes are list in Table 1.Figure 4 is the cross-sectional SEM picture of thePAAfilm prepared basedonthe aboveprocesses.Therefractiveindexofeachlayermeasuredby ellipsome-try and the thickness determined according to SEM,are listed in Table 1. Because the five-layer structurewas too complex to estimate the refractive indices of the layers precisely, we prepared five single-layersamples and then measured the refractive indicesof the layers individually. The measured refractiveindex of the top layer is 1.08, which is very low andclose to the index of air. The bottom layer is 1.47,which closely matches the index of glass. Figure 4clearly shows that the PAA film is composed of fivelayers and the nanopore diameter of each layer de-creases gradually from top to bottom. Furthermore,the thickness and refractive index of each layer iswell matched with the Gaussian profile, as shownin Fig. 4. Therefore, the PAA nanostructure matchestherefractiveindicesofairandthesubstrateandhasexcellent AR characteristic.More importantly, our method could prepare the AR films not only on flat substrates but also onpatterned substrates. Figure 5 is a photograph of graded-index PAA coating on two different sub-strates, the left is flat substrate while the right ispatterned substrate (the shape of the pattern is in- verted pyramid and the depth of the pattern is about0.3 mm), showing clearly both samples possessed very good transparency. In particular, the graded-index PAA coatings on both substrates are almostneutral colors, which means the interference colors Fig. 3. Schematic illustration of the preparation processes of graded-index PAA films consist of five layers. Table 1. Time of Electrochemical Oxidation, Time of Chemical Etching,Measured Thickness, and Measured Refractive Index of the AR Film Graded-IndexLayerNumber AnodizationTime(min)Etching Time(min)MeasuredThickness(nm)MeasuredRefractiveIndex1 25 5 504 1.082 6 5 149 1.183 5.5 7 133 1.334 5 11 108 1.415 5 0 103 1.47Fig. 4. Cross-sectional SEM image of graded-index coating with a Gaussian index profile. The coating consists of five porousanodicaluminalayerswhoseporosityincreasesprogressivelyfromsubstrate to air.Fig. 5. (Color online) Optical photograph of graded-index PAA coating on the flat (left) and patterned (right) surfaces of glass.The inset is the high magnification image of the boxed area.The rule is in centimeters. 1 October 2012 / Vol. 51, No. 28 / APPLIED OPTICS 6841  often with glass coatings have almost been elimi-nated. It is also clear that the patterned glass coatedwith PAA film performs as well as the flat one.Both experimental and theoretical results areshown in Fig. 6. Figure 6(a) shows the wavelength dependence reflectivity of ultra clear glass withPAA coatings on both sides at incident angle of 8°.We can see the shape of measured and calculatedcurves in Fig. 6(a) are similar, having wave peak atthewavelengthabout400nm,havingwavetroughat the wavelength about 500 nm, and starting torise at the wavelength above 900 nm. For each peak in calculated curve, there are two corresponding peaks in the measured curve and this may be causedby the tiny structural difference between the twoPAA coatings on glass. The electrolytic cell used inthis study has only an anode and a cathode, so thePAA coating on the backside has to be fabricatedafter the preparation of PAA coating on the frontof the glass.Inthiscase,thePAA coating on thefrontof glass will experience extra acid solution soak andheat treatment, which will cause slight structuraldifference in the two PAA coatings. The lowest mea-sured reflectivity of the PAA coated glass sample is0.64% while the reflectivity of reference glass with-out coating is 8.73% at the wavelength of 534 nm.Figures 6(b) and 6(c) show measured and calculated results for TM and TE polarization, respectively. Themeasured and calculated results are in good agree-ment and the two curves coincide with each otherespecially below the angle of 60°. The spectral de-pendence and angular dependence of the reflectancecurves demonstrate that our graded-index AR coat-ings based on PAA nanostructure have the broad-band omnidirectional characteristic. 4. Conclusion In conclusion, large-area low- n  PAA coatings onglass substrates were successfully prepared by com-bination of electron-beam evaporation, electrochemi-cal oxidation, and chemical etching methods. Thelowest refractive index is  n  1 . 08 . Furthermore,five-layered graded-index broadband PAA coatingswith refractive indices designed following theGaussian profile were also successfully prepared.The graded-index PAA coatings have a broadbandomnidirectional nature, and the lowest reflectivityof PAA coated glass sample is  R  0 . 64%  at the wa- velength of 534 nm.This work was supported by the National BasicResearch Program of China (2009CB930801), NingboNatural Science Foundation (2010A610158), theNational Natural Science Foundation of China(11204325, 21003145), Zhejiang Provincial NaturalScience Foundation of China (D4080489), the CAS/ SAFEA International Partnership Program for Crea-tiveResearch Teams, Science and Technology Innova-tive Research Team of Zhejiang Province and NingboMunicipality (2009B21005). References 1. J. A. Dobrowolski, D. Poitras, P. Ma, H. Vakil, and M. Acree, “ Toward perfect antireflection coatings: numerical investiga-tion, ”  Appl. Opt.  41 , 3075 –  3083 (2002).2. D. Poitras and J. A. Dobrowolski,  “ Toward perfect antire-flection coatings. 2. Theory, ”  Appl. Opt.  43 , 1286 –  1295(2004).3. B. Päivänranta, T. Saastamoinen, and M. Kuittinen,  “  A wide-angle antireflection surface for the visible spectrum, ”  Nano-technology  20 , 375301 (2009).4. G. M. Wu, J. Wang, J. Shen, T. H. Yang, Q. Y. Zhang, B. Zhou,Z. S. Deng, B. Fan, D. P. Zhou, and F. S. Zhang,  “ Preparationand properties of scratch-resistant nano porous broadband AR silica films derived by a two-step catalytic sol-gel pro-cess, ”  Proc. SPIE  4086 , 807 –  810 (2000).5. H. Park, D. Shin, G. Kang, S. Baek, K. Kim, and W. J. Padilla, “ Broadband optical antireflection enhancement by integrat-ing antireflective nanoislands with silicon nanoconical-frustum arrays, ”  Adv. Mater.  23 , 5796 –  5800 (2011).6. Y. J. Hung, S. L. Lee, K. C. Wu, Y. Tai, and Y. T. Pan,  “  Antire-flective silicon surface with vertical-aligned silicon nanowiresrealized by simple wet chemical etching processes, ”  Opt.Express  19 , 15792 –  15802 (2011).7. 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 materialswithlowrefractiveindexfor broadbandeliminationofFresnelreflection, ”  Nat. Photon.  1 , 176 –  179 (2007).Fig. 6. (Color online) (a) Wavelength dependence reflectivity of glass with PAA coatings on both sides and glass without coating at incident angle of 8°. Incident angle dependence reflectivityof PAA coating and glass without coating for (b) TM and (c) TEpolarization at wavelength of 600 nm. 6842 APPLIED OPTICS / Vol. 51, No. 28 / 1 October 2012
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