Functional Biomimetic Microlens Arrays with Integrated Pores

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 arrays. 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 biological , 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 elements that guide and concentrate light onto photosensitive tissue , 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 migration 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 transmission 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 devices that can mimic the unusual design of the optical elements of the brittlestar and their consequent outstanding optical 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 …