Flat polymeric microlens array

A polymer-based flat microlens array is demonstrated. Within each pixel of the array, the polymer presents a circular central-symmetric inhomogeneous orientation. Pixel with such a structure behaves a lens-like character. A suitable amount of liquid crystal mixed in the polymer host can improve not only the lens flexibility but also the lens performance. Moreover, the polymer-based microlens array has the advantages of real planar surface, ultra-thin thickness, and can be designed with any aperture size. 2005 Elsevier B.V. All rights reserved. Microlens array is an important optical element for information processing, optoelectronics, optical communications, and three-dimensional (3D) displays. Several methods for fabricating microlens and microlens array using glass [1], polymer [2,3], and liquid crystal (LC) [4– 13] have been demonstrated. Among them, the solidified polymeric lens is particularly attractive because of its flexibility, simple fabrication process, and low cost. For example, polymer can be easily molded to any lens shape in its glassy state. Unlike glass materials, a solidified polymer is flexible but not fragile. Different from LC lenses, a solidified polymer can stand alone and does not require any substrates to support. Therefore, the polymeric lens system can be light-weight and ultra-thin. Integrating with a 2D display device, such as a liquid crystal display, the flexible microlens array hold promise for improving the viewing angle [14] and creating 3D effect [15,16]. Similar to a glass lens, a conventional polymer lens belongs to the surface-relief lens category. A lens with surface-relief structure can provide high focal power, however, to decrease the thickness of the polymer lens and the size of the lens aperture is rather difficult and the associated cost is high. A more serious problem is that a polymer microlens cannot be placed inside the LC device due to its surfacerelief structure. Usually, the lens system is placed outside 0030-4018/$ see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.12.021 * Corresponding author. Tel.: +1 407 823 4763; fax: +1 407 823 6880. E-mail address: [email protected] (S.-T. Wu). of the LC device. Thus, the lens surface has to be protected from being damaged. In a previous publication [13], we reported a planar lens which was fabricated by using nanosized polymer-dispersed liquid crystals to fill the sags of the lens instead of air. Essentially, the fabrication method of this planar lens is similar to that of the surface-relief lens. As a result, the above-mentioned problems for the surface-relief lens still exist. In this paper, we demonstrate a polymer-based microlens which can focus light due to its central-symmetric inhomogeneous gradient index distribution, rather than surface-relief structure. In this microlens, polymers present central-symmetric inhomogeneous orientation similar to that of the LC directors in a LC lens. To prepare a demo, we used patterned hole-electrode to fabricate microlens arrays. Depending upon the size and shape of the hole electrode and the LC cell gap, various planar microlens arrays can be fabricated easily. Fig. 1 illustrates the fabrication procedures of the proposed polymer lens. The diacrylate monomer we selected exhibits a nematic phase. Therefore, the UV-curable LC diacrylate monomers can be treated with homogeneous alignment, as shown in Fig. 1(a). This is an essential part of the material system. The indium–tin-oxide (ITO) electrode on the upper substrate was etched with circular holes. When a voltage is applied across the LC cell, as Fig. 1(b) depicts, a central-symmetric inhomogeneous electric field is generated near the holes. This electric field induces the Fig. 2. Interference fringe patterns observed under a polarized optical microscope: (a) V = 0, (b) V = 15 Vrms and cured using a UV light, and (c) V = 0. V a