Lensless light focusing with the centric marine ... - OSA Publishing

Lensless light focusing with the centric marine diatom Coscinodiscus walesii Luca De Stefano1, Ilaria Rea1, 2, Ivo Rendina1, Mario De Stefano3, and Luigi Moretti4 1

National Council of Research-Institute for Microelectronics and Microsystems-Department of Naples, Via P. Castellino 111, 80131 Naples, Italy 2 Physical Science Department, University of Naples “Federico II”, via Cinthia, Monte S. Angelo, 80126 Naples, Italy 3 Environmental Science Department, 2nd University of Naples, via A. Vivaldi 43, 81100, Caserta, Italy 4 DIMET, University “Mediterranea” of Reggio Calabria, Località Feo di Vito, 89060 Reggio Calabria, Italy [email protected]

Abstract: In this work, we report on the light focusing ability exploited by the microshell of a marine organism: the Coscinodiscus wailesii diatom. A 100 μm spot size of a red laser beam is narrowed up to less than 10 μm at a distance of 104 μm after the transmission through the regular geometry of the diatom structure, which thus acts as a microlens. Numerical simulations of the electromagnetic field propagation show a good qualitative agreement with the experimental results. The focusing effect is due to the superposition of the waves scattered by the holes present on the surface of the diatom valve. Very interesting applications in micro-optic devices are feasible due to the morphological and biological characteristic of these unicellular organisms. ©2007 Optical Society of America OCIS codes: (160.1435) Biomaterials, (170.0170) Medical optics and biotechnology, (170.1420) Biology, (220.3630) Lenses.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Enabling Technology for MEMS and Nanodevices, Volume Editors: H. Baltes, O. Brand, G. K. Fedder, C. Nierold, J. G. Korvink, O. Tabata, (Wiley-VCH, Weinheim, 2004). V.C. Sundar, A.D. Yablon, J.L. Grazul, M. Ilan, J. Aizenberg, “Fibre-optical features of a glass sponge,” Nature 424, 899-900 (2003). P. Vukusic and J.R. Sambles, “Photonic structures in biology,” Nature 424, 852-5 (2003). F.E. Round, R.M. Crawford, D.G. Mann, The diatoms. Biology & morphology of the genera, (Cambridge University Press, Cambridge, 1990). M. Sumper, “A phase separation model for the nanopatterning of diatom biosilica,” Science 295, 2430-33 (1990). M. Sumper and E. Brunner, “Learning from diatoms: nature’s tools for the production of nanostructured silica,”Adv. Funct. Mater. 16, 17-26 (2006). G. Josten, H.P. Weber, W. Luethy, “Lensless focusing with an array of phase-adjusted optical fibers,” Appl. Opt. 28, 5133-37 (1989). M. De Stefano, L. De Stefano, “Nanostructures in diatom frustules: functional morphology of valvocopulae in Cocconeidacean monoraphid taxa,” Journal of Nanoscience and Nanotechnology 5, 1524, (2005). J.D. Joannopoulus, R.D. Meade, J.N. Winn, Photonic crystals. Molding the flow of light. (Princeton University Press, Princeton, NJ, 1995). Results are the subject of a different paper. S.S. Hong, B.K.P. Horn, D.M. Freeman, M.S. Mermelstein, “Lensless focusing with subwavelength resolution by direct synthesis of the angular spectrum,” Appl. Phys. Lett. 88, 261107 (2006). P. Xi, C. Zhou, E. Dai, and L. Liu, “Generation of near-field hexagonal array illumination with a phase grating”, Opt. Lett. 27, 228, (2002). F.M. Huang, N. Zheludev, Y. Chen, F.J. Garcia de Abajo, “Focusing of light by a nanohole array”, Appl. Phys. Lett. 90, 091119 (2007) Principles of Optics, 6th Edition, M. Born and E. Wolf, (Pergamon Press, 1980). G.R. Wein, “A video technique for the quantitative analysis of the Poisson spot and other diffraction patterns,” Am. J. Phys. 67, 236 (1999). R.L. Lucke, “Rayleigh–Sommerfeld diffraction and Poisson’s spot,” Eur. J. Phys. 27, 193 (2006).

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Received 22 Aug 2007; revised 29 Oct 2007; accepted 7 Nov 2007; published 18 Dec 2007

24 December 2007 / Vol. 15, No. 26 / OPTICS EXPRESS 18082

17. 18. 19. 20. 21. 22. 23.

R. Scarmozzino, A. Gopinath, R. Pregla, S. Helfert, “Numerical Techniques for Modeling Guided-Wave Photonic Devices,” IEEE J. Sel. Top. Quantum Electron. 6, 150 (2000). G.R. Hadley, “Wide-angle beam propagation using Pade approximant operators,” Opt. Lett. 17, 1426, (1992). T. Fuhrmann, S. Landwehr, M. El Rharbi-Kucki, M. Sumper, “Diatoms as living photonic crystals,” Appl. Phys. B 78, 257-260 (2004). H. Ottevaere, H. Thienpont, “Comparative study of glass and plastic refractive microlenses and their fabrication techniques,” Proceedings of the Symposium IEEE/LEOS Benelux Chapter: 218-221 (2002). M.R. Weatherspoon, M.A. Allan, E. Hunt, Y. Cai, K.H. Sandhage, “Sol-gel synthesis on self-replicating single-cell scaffolds: applying complex chemistries to nature’s 3-D nanostructured templates,” Chem. Commun. 651-53 (2005). C.E. Hamm, R. Merkel, O. Springer, P. Jurkojc, C. Maier, K. Prechtel, V. Smetacek, “Architecture and material properties of diatom shells provide effective mechanical protection,” Nature 421, 841-843 (2003). B. Stager, M.T. Gale, M. Rossi, “Replicated micro-optics for automotive applications,” Proceedings of SPIE 5663, 238-245 (2005).

Integrated optical components often require materials with regularly repeating 2-D and 3-D structures with features below the micrometer size range which are difficult to fabricate by standard technologies1. On the other hand, biological organisms can exhibit ordered geometries and complex photonic structures which sometimes overcome the best available man-made products2-3. Diatoms are microalgae with a peculiar cell wall made of amorphous hydrated silica valves, reciprocally interconnected like a Petri dish in a structure called “frustule”. The valve surfaces exhibit specie-specific patterns of regular arrays of holes, called “areolae”. The diameter of the areolae can range from a few hundreds of nanometers to a few microns and can be circular, polygonal or elongate4. The forming of the frustules and their patterns can be described by the self-organised phase separation model5-6. Despite of the high level of knowledge on the genesis and morphology of diatom frustules, their functions are not completely understood. Herewith we are showing that the silica frustule of a centric marine diatom, Coscinodiscus walesii, has unsuspected optical properties: we found that the diatom valve can focus an incoming laser light in a small spot of few microns. Our experimental results and numerical calculations indicate that this phenomenon is due to a coherent superposition of the light scattered by the areolae7.

Fig. 1a

Fig. 1b

Fig. 1. Morphology and ultrastructure of Coscinodiscus walesii frustule. a, The valve showing the radial symmetric arrays of areolae arranged in a hexagonal pattern. b, Details of the fine structure of the valve with morphometric measurements of the areolae diameter, interareolae spaces, and thickness (where not reported, bar scale is 1 μm).

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When observed by scanning electron microscopy, the valves of the C. walesii strain we studied range between 150 μm and 200 μm (Fig. 1a) and have an average thickness of approx. 700 nm (Fig. 1b, down). The valve surface exhibit almost perfect hexagonal patterns of areolae, with a lattice constant (nearest neighbour center-to-center) of 2600-2700 nm, as it can be seen in Fig. 1b (top). Before scanning electron microscopy and optical characterisation, the diatoms have been cleaned by their organic matter by washing them in a strong acid solution, and then deposited on a glass slide. A detailed description of the cleaning process can be found in ref. 8. The areolae arrays of C. walesii are arranged on the diatom surface with a radial symmetry which is very similar to the one of some man-made optical devices, such as photonic crystal fibres or phase-locked arrays of optical fibres or lasers: the uncommon features of light propagation through these regular structures don’t only depend on the interaction with the matter but also on the spatial order of the periodic lattice7, 9. We have therefore investigated the light transmission characteristics of a single valve, by using the experimental set-up shown in Fig. 2. We have selected the central section of a diode laser beam (@λ=785 nm, elliptical spot size of approximately 2 mm) by a 100 μm pinhole placed at 1 cm from the glass slide to fit the valve dimension. The transmitted signal is collected by a 20x objective, with a numerical aperture of 0.49, and recorded by a CCD camera (Leica DFC300 FX).

Pinhole

Glass Slide

Diatom 20X

Laser

CCD Camera

Z Fig. 2. The experimental set-up used to investigate the light transmission characteristic of the diatom valve. Distances are not in scale.

We have verified that the optical setup (pin-hole, glass slide and objective) without the diatom does not change the features of the laser light: the beam profile divergence and its intensity change less than 5 % over a distance of 250 μm from the focal plane of the diatom. The measurement starts when the diatom surface is in the objective focal plane; then, we have registered the transmitted light spot image by moving the objective up to 200 μm by steps of 4 μm. We have found, quite surprisingly, that the valve acts as a microlens: the laser beam is highly focused at an output distance from the valve surface ranging from 100 μm to 110 μm; then the light beam diverges. The beam is confined in 8.1 μm (value of the full width at half maximum) in its narrowest point, resulting in a spot size about 12 times smaller than the pinhole diameter (see Fig. 3(a) and 3(b)). The light focusing occurs at the centre of the diatom valve in correspondence of the uniform zone, free of areolae, which is about 15 μm in size. Even if it’s possible to demonstrate that the diatom valve could act as a guiding structure and hence support a guided mode in this defect10, we attribute this focusing effect to a coherent superposition of the unfocused wave fronts coming from the approx. 600 areolae of C. walesii valve which are quasi-regularly disposed on the diatom surface11: the light scattered by the holes on the diatom surface interferes constructively only at a fixed distance, which also depends on the holes spatial disposition, determining a well-defined spotlight. This #86785 - $15.00 USD

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lensing effect is in a manner similar to the Talbot effect that can be observed in periodic and quasi-periodic diffraction arrays12, 13. The difference between the two systems is that while there exists one degenerate Talbot distance (equal to 31/2 tx/λ2 for the perfect hexagonal arrays, where tx is the lattice constant) for periodic gratings, in the quasi-periodic case, such as for the diatoms, the self-imaging distance changes for different orders of diffraction and wavelengths. The result is that the reconstruction of a non perfectly regular arrays of holes is a complex process which is the sum of a large number of reconstructions at different heights from the array surface giving to the existence of a well defined focus at a well defined distance. In this view, it is also important to take into account the influence of the numerical aperture of the objective which corresponds to a gathering light angular semi-cone of 30°. In the Fraunhofer regime (N=a2/λz