Fabrication of Microlens Arrays with Well‐controlled Curvature by Liquid Trapping and Electrohydrodynamic Deformation in Microholes

The ability to generate curvature-controllable aspheric microlens array (MLA) or micromirror array (MMA) structures allows for a signifi cant reduction of spherical aberration and for a proper numerical aperture in many refractive or refl ective optical elements. [ 1 , 2 ] A variety of strategies have been experimented for fabricating structures with parabolic or other aspheric surfaces for MLA or MMA. Laser ablation, ion-beam milling, and other material microremoving processes have produced optics structures with aspheric surface shapes, [ 3 , 4 ] but are obviously unsuitable for mass application due to the poor cost-effectiveness or the poor surface smoothness which results from the cascaded material removal. Thermal refl ow techniques [ 5–7 ] and the hot embossing method, [ 8 , 9 ] based on interfacial-tension-induced deformation of the photoresist, sol– gel glass, or other thermoplastic polymers, have been used to fabricate MLAs of high surface smoothness economically. But these thermal approaches can only be used to fabricate microlenses with convex surfaces and are faced with a diffi culty in controlling the lens surface geometry or focal length. Recently, an adaptive or tunable focal liquid lens based on electrowetting actuation, [ 10 , 11 ] a liquid-dielectrophoretic (L-DEP) drive, [ 12 ] or the thermoresponsive manipulation of liquid droplets, [ 13 ] has shown an adequate controllability of the lens curvature or focal length by external an electric fi eld or by heating, which changes the geometrical profi le of the lens. However, it can be very hard (if possible) to achieve a high fi ll-factor or a stable curvature for the MLA over a large area, due to the nature of droplet-wise manipulation which causes merging of the droplet array, and also due to electrothermally induced liquid evaporation which causes a volumetric shrinkage of the droplets when working for a long time. In a previous investigation, [ 14 ] we proposed an L-DEP-based process for generating a concave MLA, where a photopolymerizable dielectric liquid component is electrically driven to fi ll a microhole array in a conductive template. Since the L-DEP force exerted on the liquid–air interface in each hole is nonuniform and becomes the largest at the hole’s wall, the top liquid surface