Functional Biomimetic Microlens Arrays with Integrated Pores** By Shu Yang,* Gang Chen, Mischa Megens , Chaitanya K. Ullal, Yong-Jin Han, Ronen Rapaport, Edwin L. Thomas , and Joanna Aizenberg* Biology provides a multitude of varied new paradigms for the development of adaptive optical networks. [1±7] Here, we present the first example of synthetic, biomimetic microlens arrays with integrated pores, whose appearance and function are strikingly similar to those of their biological prototype, a highly efficient optical element formed by brittlestars. [4] The complex microstructure is created directly by three-beam interference lithography in a single exposure. We show that i) the microlenses have strong focusing ability, and that ii) light-absorbing liquids can be transported in and out of the pores between the lenses, which provides the potential for a wide tunability range of the optical properties of the lens ar- rays. We are interested in learning from natural optical systems, whose hierarchical architecture and hybrid character offer outstanding optical properties and enable multifaceted roles. Recently, we characterized a spectacular example of a biologi- cal, adaptive optical systemÐa close-set, nearly hexagonal array of uniform microlenses formed by the light-sensitive brittlestar, Ophiocoma wendtii (Fig. 1a). [4] The lenses were shown to be involved in photoreception, acting as optical ele- ments that guide and concentrate light onto photosensitive tis- sue, and offering remarkable focusing ability, angular selectiv- ity, and signal enhancement. An interesting design feature of this bio-optical structure is the presence of a pore network surrounding the lenses, which is essential to the diurnal migra- tion of pigment-filled chromatophore cells. [8] Because of the presence of a pore network, the brittlestar microlenses can be considered as an adaptive optical device that exhibits trans- mission tunability with a wide range, achieved by controlled transport of radiation-absorbing intracellular particles. The chromatocyte pigment also allows other functions, including diaphragm action, numerical-aperture tuneability, wavelength selectivity, minimization of the crosstalk between the lenses, and improved angular selectivity. It is highly desirable to have small, complex photonic de- vices that can mimic the unusual design of the optical ele- ments of the brittlestar and their consequent outstanding opti- cal properties, by creating a structure that combines microlens arrays with the surrounding porous microfluidic system. The fabrication of such structures using existing techniquesÐink- jet printing, [9] melting of patterned photoresists, [10] reactive ion etching of silica and silicon, [11] soft-lithography, [12] or self- assembly of monodispersed polymer beads [13] Ðis, however, not straightforward. Most of these techniques only create lenses without pore structures and their optical properties are not tunable. Multibeam interference lithography has been shown to be a fast, simple, and versatile method of creating two-dimensional (2D) and three-dimensional (3D) periodic, defect-free, porous microstructures over a large area. [14±18] The symmetry and the porosity of the resulting structures can be conveniently controlled by the wavevectors and polariza- tions of the interfering beams. [15,19] None of the previous stud- ies, however, have either paid attention to the integration of the two different structures (lenses and pores) into a more COMMUNICATIONS Adv. Mater. 2005, 17, No. 4, February 23 DOI: 10.1002/adma.200401002  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 435 ± [*] Prof. S. Yang Department of Materials Science and Engineering University of Pennsylvania 3231 Walnut Street, Philadelphia, PA 19104 (USA) E-mail: shuyang@seas.upenn.edu Dr. J. Aizenberg, Dr. G. Chen, Dr. Y.-J. Han, Dr. R. Rapaport Bell Laboratories, Lucent Technologies Murray Hill, NJ 07974 (USA) E-mail: jaizenberg@lucent.com Dr. M. Megens Philips Research Laboratories Prof. Holstlaan 4, NL-5656 AA Eindhoven (The Netherlands) C. K. Ullal, Prof. E. L. Thomas Department of Materials Science and Engineering Massachusetts Institute of Technology 77 Massachusetts Ave, Cambridge, MA 02139 (USA) [**] The authors are grateful to A. Hale (OFS) for providing Irgacure 261 (Ciba Specialty Chemicals) in the experiments and helpful discus- sion of photosensitized holography in visible light. We acknowledge M. Li (Cornell) for discussion of microfluidics assemblies. The authors also thank their respective financial sources: Air Force DUR- INT in conjunction with the University of Buffalo (CKU), and ISN ARO(ELT). a b c k 3 k 2 k 1 d Figure 1. Structure of biological and biomimetic porous microlens ar- rays. a) Scanning electron microscopy (SEM) image of the design of a brittlestar lens. Scale bar: 50 lm. b) Calculated light-intensity profile from three-beam interference lithography. Beam wavevectors and polar- izations are described in the Experimental section. c) Corresponding SEM image of a synthetic, biomimetic microlens array with integrated pores. Scale bar: 5 lm. d) Schematic drawing of the beam polarizations used (shown by double-headed arrow; viewed in the (0,0, 1) direction) to realize the biomimetic lens shown in (c).