Millimeter-Sized Lens Array on Polymer Surface

A shallow lens array with hexagonally arranged millimeter-sized features is fabricated on thin, transparent and flexible polymer layer. A rapid, single-step inexpensive technique is demonstrated for the fabrication using soft- imprint lithography method. The patterned polymer layer is planar on one side and has concave dimples on the other side. The individual features have a size of 2 mm and the array shows efficient focusing characteristics with focal length of 4.891 mm and numerical aperture of nearly 0.205. The optical property of the patterned surface is studied with a pixel-level analysis of the recorded spot size, and is presented as the validation of the lensing nature of the surface. The extremely simple method for making the millimeter lens array has yielded a thin surface which possesses the focusing effect with a short focal length and diverges at larger distances. It can serve as an ideal diverging lens for the small-sized light sources such as LED and OLED, as well as a portable lens for viewing small objects in inexpensive microscope designs.

I. Introduction

A lens array is generally termed as a two-dimensional arrangement of small lenses with diameters between micrometers and a millimeter, and is known as millimeter or micro lens array (MLA) [1]–[3]. Micro lens arrays are more common though millimeter-sized features as lens arrays are also gaining popularity. MLA are widely used in optical interconnections and couplings, imaging, photocopying, beam shaping, optical switching networks, enhancing the efficiency of the LED and OLED and many more applications [2]–[3]. Various manufacturing techniques are developed and are being attempted to fabricate lens arrays. Some reported techniques are based on thermal reflow of photoresist, microdroplet jetting, direct laser writing, hot embossing, laser printing and additive manufacturing [2]–[4]. Regardless of enormous progress in the fabrication techniques of lens array, still some limitations exist for these techniques including high overall cost, multiple and complex fabrication steps, use of glass for high transparency in visible range, high time consumption in fabrication, lack of fabrication flexibility and lack of reproducibility with equal precision in multiple steps.