Optical properties calculated for multidielectric ... - OSA Publishing

Optical properties calculated for multidielectric quarter-wave coatings on microspheres P. Voarino, C. Deumié, C. Amra Institut Fresnel Marseille, UMR CNRSTIC EGIM site Nord, Université d'Aix-Marseille Domaine universitaire de St Jérôme 13397 Marseille Cedex 20, France Tél: 33 491 28 83 28 Fax: 33 491 28 80 67 [email protected]

Abstract: Optical properties of overcoated microspheres are calculated and compared to those of planar multilayers, in regard to the sphere diameter. The classical criteria for in-situ optical monitoring is analyzed to control the growth of films on the spheres. Most coatings are multi-dielectric quarterwave stacks used in the thin film community. ©2004 Optical Society of America OCIS codes: (290.4020) Mie theory; (310.1620) Coatings

References and links 1.

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Deumié C., Voarino P., Amra C., “Overcoated microspheres for the spectral control of diffuse light,” Appl. Opt. 41, 3299-3305 (2002). C. Deumié, P.Voarino, C. Amra, “Interferential powders for the spectral control of light scattering,” Advances in Optical Thin Films 2003, Proceedings of SPIE 5250. H.A. Macleod, Thin Film Optical Filters, (London Adam Hilger, 1986). H.C. van de Hulst, Light Scattering by Small Particles, (Dover Publications, Inc. New York, 1981). J.A. Stratton, Electromagnetic theory,( Mc Graw-Hill Book Company, New York, 1941). C.F. Bohren and D.R. Hufman, Absoption and scattering of light by small particules, (Wiley-Interscience, New York, 1983). G. Burlak, S. Koshevaya, J. Sanchez- Mondragon, V. Grimalsky, “E/lectromagnetic oscillations in a multilayer spherical stack,” Opt. Commun. 180, 49-58 (2000). F. Onofri, G. Gréhan, G. Gouesbet, “Electromagnetic scattering from a multilayered sphere located in an arbitrary beam,” Appl. Opt. 34, 7113-7124 (1995). C. Amra, “From light scattering to the microstructure of thin film multilayer,” Appl. Opt. 32, 5481-5491 (1993). C. Amra, “ Light scattering from multilayer optics. Part A: investigation tools,” J. Opt. Soc. Am. 11, 197210 (1994). C. Amra, “Light scattering from multilayer optics. Part B: application to experiment,” J. Opt. Soc. Am. 11, 211-226 (1994). W.J. Wiscombe, “Improved Mie scattering algorithms,” Appl. Opt. 19, 1505-1509 (1980). A.V. Tikhonarov, M.K. Trubetsov, and G.W. DeBell, “Application of the needle optimization technique to the design of optical coatings,” Appl. Opt. 35, 5493-5508 (1996).

1. Introduction Optical interference coatings have always been used to control light and waves, and the community of thin films has today reached a high level of expertise for all design, production and characterization techniques of these micro-components. However most of these filters are produced on plane substrates, and are hence designed for the control of specular light which is reflected and transmitted according to the Snell/Descartes relationships. For some applications of critical interest including cosmetics and paints (colour effects), furtivity (high reflectance dielectric particles) and biophotonics (particles with high radiation pressure), as well as optical communications (end coating of optical fibers)…, a demand exists to overcoat #4834 - $15.00 US

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Received 2 August 2004; revised 8 September 2004; accepted 9 September 2004

20 September 2004 / Vol. 12, No. 19 / OPTICS EXPRESS 4476

microspheres in order to reach specific properties similar or not to those of interferential multilayers. Such overcoated spheres can be used in an isolated form or in a powder form. Within this framework we have built two specific deposition processes based on electron beam deposition [1,2] and ion beam sputtering, that allow overcoating of microspheres with diameters in the range 3 µm-300 µm. However a key problem lies in the in-situ monitoring process that one should use to control the growth of thin films on microspheres. In this paper we analyze the classical criteria used for quarter-wave stacks on planar substrates, which constitutes the basis of optical monitoring [3]. This criteria is investigated in regard to the sphere diameter. In a more general way we address the design problem of specific stacks to be deposited on the spheres, in order to reach spectral properties similar to planar multilayer mirrors, beam splitters and narrow-band filters... 2. The quarter-wave criteria 2.1 Mie theory for overcoated spheres The electromagnetic model that we use is the well-known Mie theory [4,5,6,7,8] that was extended to concentric multilayers deposited on microspheres. All constants were determined thanks to tangential field continuities at concentric radia. Materials can be dielectric or metallic. As usual the theory does not take account of interactions or multiple reflections between different microspheres. It allows to plot angular and wavelength variations of scattering from a unique overcoated sphere Notice that in the case of a 500 nm radius sphere, Mie theory is valid with an incident wavelength in the visible range. For smaller spheres radii, it was numerically verified that the scattered fields approach the Rayleigh scattering. An example is given in Fig. 1 where the initial radius of the silica substrate sphere is a0 = 500 nm, with an overcoating given by a 7 layer quarterwave mirror of design: M7 = Air/HBHBHBH/Glass where H and B designate high and low index layers with optical thicknesses equal to a quarter-wavelength: nH.eH = nB.eB = λ0/4, with λ0 = 633 nm, where nH and nB are respectively the high and low refractive indices and, eH and eB the mechanical thicknesses of layers. Thin film materials are Ta2O5/SiO2. The angular scattering I(θ,λ) is normalized to the incident flux, as we usually do for Angle Resolved Scattering (ARS) or BRDFcosθ curves to match scattering measurements [9,10,11].

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Fig. 1. (a) Definition of angular ranges by transmission (0°-90°) and reflection (90°-180°). (b) Angular and wavelength variations of scattering from a SiO2 sphere of diameter 500 nm, overcoated with a quarterwave mirror of design: M7 = Air/HBHBHBH/Glass. The incident wave is TE polarized. The incident wavelength is the design wavelength of the mirror (633 nm).

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We observe in Fig. 1 numerous oscillations characteristic of the large value of total radius (at = 1257.12 nm) of the overcoated sphere, that also depend on the mirror design. From this angular curve at a given wavelength λ, we are able to extract integrated curves by transmission (ST) or reflection (SR), that is, total scattering in the transmission (0°-90°) and reflection (90°-180°) ranges respectively: ST = ∫θ,φ I(θ,φ) sinθ dθ dφ with 0°< θ < 90° and 0° < φ < 360° SR = ∫θ,φ I(θ,φ) sinθ dθ dφ with 90°< θ < 180° and 0° < φ < 360° Notice here that numerical integration is necessary since the analytical integrated Mie formulae are only valid for the sum S = SR + ST. Furthermore, I(θ,φ) is given by series expressions thanks to Mie theory [5]. The series should converge slowly when the radius of the sphere is larger than the wavelength. Convergence is systematically tested before each numerical result in particular by the Wiscombe criterium [12]. 2.2 The quarter-wave criteria In a general way, the basic criteria to design, control or produce most planar optical coatings is that of quarterwave layers. It is well known that at a particular wavelength λ0, specular reflection (R0) or transmission (T0) from a single transparent layer at normal illumination is a periodic function of the deposited optical thickness. The extrema are reached when the optical thickness (n.e) is a multiple of a quarter-wavelength, that is: n(λ0) ek = k λ0/4 where k is an integer. In the same way, for a given dielectric thickness, variations of R and T versus wavelength λ reveal extrema at specific wavelengths λq with q integer given by: n(λq) e = q λq/4 This result originates from the fact that standard calculation of plane waves within a planar multilayer only involves trigonometric functions with a common period, which constitutes an initial step for most design problems. In the case of concentric multilayers, the problem is strongly different since the size effect is predominant and turns the calculation to be dependent on non periodic Bessel and Hankel functions with parameter pm = 2π nam/λ, where am designates the initial sphere radius or layer thicknesses. This first criteria was studied and is plotted in Fig. 2 at wavelength λ0= 633 nm, for a single high index (nH = 2.25) thin film layer of Ta2O5. Each figure is calculated for a particular diameter of the initial sphere, and represents the variation of total scattering by reflection (SR) or transmission (ST) versus the thin film thickness. When the sphere radius is rather large (a = 15 µm), the variation of SR versus layer thickness (Fig. 2(a)) is quasi-periodic, with an average period given by: EH = 71 nm. This value is quasi-identical to that of a layer deposited on a flat substrate, given by eH = λ0/(2nH) = 70.3 nm. Therefore the quarter-wave criteria is at least valid for high-index spheres of radia greater than 15 µm. When the sphere radius is reduced from 15 µm to 0.5 µm (see Figs. 2(b,c,d), the periodicity progressively vanishes but a pseudo-period can still be detected either on SR or ST.

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Fig. 2. Integrated angular scattering ST and SR at wavelength λ0 = 633 nm calculated for a single SiO2 sphere overcoated with a high index Ta2O5 layer. The abscissa is the thickness of the Ta205 thin film. The initial sphere radius is 15 µm (a), 5 µ m (b), 2.5 µ m (c) and 500 nm (d).

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Fig. 3. Angular scattering ST and SR at wavelength λ0 = 633 nm calculated for a single SiO2 sphere overcoated with a low index YF3 layer. The abscissa is the thickness of the YF3 thin film. The initial sphere radius is 15 µ m (a), 5 µ m (b), 2,5 µm (c) and 500 nm (d).

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The same study was performed for a low-index (nB = 1.33) YF3 material, as shown in Figs. 3(a,b,c,d). In this case the average pseudo-period can be roughly seen, but for large diameters it is found to be approximately: EB = 119 nm. This value is close to that of the classical case given by: eB = λ0/(2nB) = 118.9 nm. When the radius is decreased, the periodicity is lost as expected. So we conclude in this section that similar criteria can be obtained for the in-situ monitoring of multilayers deposited on flat or concentric substrates. The quarter-wave criteria remains valid for micro-spheres overcoated with high index layers, and for diameters greater than 1µm. The case of low index material deposition appears to be more complex. 3. The case of specific optical functions 3.1 Case of a multilayer mirror This section is now devoted to wavelength properties of the overcoated microspheres. The case of a multidielectric quarterwave mirror is plotted in Fig. 4 for different sphere radia in the range (15 µm-0.5 µm). Materials are those of the preceding section, that are Ta2O5 and YF3. The coating design is again: M7 = Air/HBHBHBH/Glass, with λ0 = 633 nm. All scattering curves SR by reflection are normalized and compared to specular reflection R of the planar multilayer. We observe in Figs. 4(a,b,c) that the spectral ranges of high reflectance can be easily observed, and rather similar to those of planar multilayers, in spite of a wavelength shift and a lower rejection rate. So we conclude that the spectral profiles are similar for concentric and planar multilayer mirrors, provided that the initial sphere radius is greater than a0 = 2.5 µm. When this radius is reduced, resonances occur in the reflectance bandpass, and become predominant for small radia.

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Fig. 4. Comparison between reflection of a planar mirror of design M7, and SR scattering from the same concentric mirror M7. The wavelength range is 400-800 nm. Curves are calculated for different initial sphere radia: 15 µ m (a), 5 µ m (b), 2,5 µm (c) and 500 nm (d).

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3.2 Case of a beam splitter Another kind of design was studied and is plotted in Fig. 5 (planar multilayers) and Fig. 6 (concentric multilayers) for a specific non quarterwave beam-splitter (BS) of design: BS = Air /eH=61,67 nm /eB=106,54 nm /eH=70,07 nm /eB=10,14 nm /eH=45,57 nm /eB=43,23 nm /eH=10,56 nm /Glass with eH and eB the mechanical thicknesses of high and low index materials, respectively. These thicknesses were determined by the needle method [13]. We observe than similar optical properties are emphasized for planar and concentric stacks, provided that the sphere radius is greater than 15 µm. For lower radia, angular scattering by reflection (SR) and transmission (ST) are dissociated.

Fig. 5. Planar reflection and transmission of a beam splitter.

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Fig. 6. ST and SR scattering from the beam splitter of figure 5 but deposited on a microsphere. The wavelength range is 400-800 nm. Curves are calculated for different initial sphere radia: 15 µ m (a), 5 µ m (b), 2,5 µ m (c) and 500 nm (d).

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3.3 Case of a Fabry-Perot filters The last design concerns a Fabry-Perot filter (FP) of design: FP = Air/HBHBH(2B)HBHBH/Glass Results are given in Fig. 7 and show strong alteration of the optical properties, even for large sphere radia. This result can be correlated to the high sensitivity of Fabry-Perot filters.

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Fig. 7. Comparison between planar reflection of a band-pass filter, and SR scattering from the same concentric filter. The wavelength range is 400-800 nm. Curves are calculated for different initial sphere radia: 15 µ m (a), 5 µm (b), 2,5 µm (c) and 500 nm (d).

4. Conclusion Numerical calculation shows that the classical quarter-wave criteria of in situ optical monitoring can still be used to control the growth of films on microspheres. This result is mainly valid for high index thin films deposited on substrate spheres with diameters greater than 1 µm. The case of low-index materials appears more complex, except when the sphere diameter is greater than 30 µm. Notice that such monitoring should require an integrated sphere or a large receiver solid angle, since all calculation is performed for integrated values of scattering by reflection or transmission. Moreover, the in-situ scattering should be measured under the assumption that multiple reflections can be neglected between microspheres. We have also given a range of sphere diameters where similar free space optical properties can be reached for quarter-wave stacks deposited on planar and spherical substrates. In the case of mirrors, the analogy remains valid for sphere diameters down to 1 µm. For beam splitters the sphere diameters should be greater than 30 µm. The analogy disappears for Fabry-Perot filters.

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