Tunable Liquid Microlens Array driven by Pyroelectric effect: full interferometric characterization Lisa Miccio, Simonetta Grilli, Veronica Vespini and Pietro Ferraro Istituto Nazionale di Ottica Applicata del CNR (CNR-INOA) Viale Campi Flegrei 34, Pozzuoli (NA) Italy Tel +390818675003 Fax +390818675118 e-mail lisa.miccio@inoa.it ABSTRACT Liquid lenses with adjustable focal length are of great interest in the field of microfluidic devices. They are, usually, realized by electrowetting effect after electrodes patterning on a hydrofobic substrate. Applications are possible in many fields ranging from commercial products such as digital cameras to biological cell sorting. We realized an open array of liquid lenses with adjustable focal length without electrode patterning. We used a z-cut Lithium Niobate crystal (LN) as substrate and few microliters of an oily substance to obtain the droplets array. The spontaneous polarization of LN crystals is reversed by the electric field poling process, thus enabling the realization of periodically poled LN (PPLN) crystals. The substrate consists of a two-dimensional square array of reversed domains with a period around 200 µm. Each domain presents an hexagonal geometry due to the crystal structure. PPLN is first covered by a thin and homogeneous layer of the above mentioned liquid and therefore its temperature is changed by means of a digitally controlled hot plate. During heating and cooling process there is a rearrangement of the liquid layer until it reaches the final topography. Lenses formation is due to the superficial tension changing at the liquid-solid interface by means of the pyroelectric effect. Such effect allows to create a two-dimensional lens pattern of tunable focal length without electrodes. The temporal evolution of both shape and focal length lenses are quantitatively measured by Digital Holographic Microscopy. Array imaging properties and quantitative analysis of the lenses features and aberrations are presented. 1. INTRODUCTION In last years scientist attention in several research field is in the direction of realizing tunable and adaptive optical elements. Indeed these devices are well suitable in many field such as optical communication, multiplex focusing components or biological applications. One way to accomplish lens with tunable focal length is to employ liquids using their capability to modify own shape by means of surface tension changing, the modification in the shape is followed by a modification of the focal length. Two classes of tunable liquid lenses exists: one based on the elettrowetting effect and another based on the hydrostatic pressure. The two classes comprise many configurations: sometimes immiscible liquids are injected inside a special case made of hydrophobic coating and electrodes. The applied voltage changes the equilibrium among the forces acting on the liquid-liquid and solid-liquid interfaces causing a changing in the curvature of the meniscus at the liquid-liquid interface The applied voltage variation allows to switch between converging and diverging lens with flexible focal lengths [1-8]. In some cases a free standing sessile liquid drop is placed on a plane surface acting as first electrode while the second one is a needle immersed on the top of the drop [7]. Liquid lens based on hydrostatic pressure are accomplished putting liquid in a reservoir having flexible and transparent membranes. By changing the pressure in the liquid volume it is possible to change the shape of the lens [8, 9]. The hydrostatic pressure is responsible for changing the focal power of the microfluidic lens. The focal length is usually retrieved from the knowledge of the liquid depth using simple geometrical lows. In recent papers liquid lens formation based on electro- wetting effect without electrode patterning has been demonstrated [10,11] The substrate is a periodically poled lithium niobate (PPLN) crystal, the liquid employed is an oily substance that changes its topography by means of the pyroelectrical properties of the substrate. This configuration allow to get a microfluidic lens array avoiding the electrode patterning that, sometimes require complex geometry and further fabrication steps. Optical Fabrication, Testing, and Metrology III, edited by Angela Duparré, Roland Geyl Proc. of SPIE Vol. 7102, 710205 · © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.797695 Proc. of SPIE Vol. 7102 710205-1 2008 SPIE Digital Library -- Subscriber Archive Copy