Ultraviolet-cured polymer microlens arrays

A method for fabricating microlens arrays that uses the contraction effect of UV-curable photopolymers is presented. Lenses with diameters ranging from 0.2 to 2 mm that were made under different conditions are optically evaluated. The optimum conditions for producing low f-number lenses are discussed.     

Single-step fabrication of refractive microlens arrays

Arrays of submillimeter microlenses are made from droplets of UV-curable optical adhesive dispensed from a pressurized syringe under computer control. Measurements of the focal length uniformity, the minimum focused spot size, and the spherical aberration are presented. An excellent lens diameter and focal length uniformity are achieved over 100 element arrays.     

Patterning flood illumination with microlens arrays

We describe a convenient lithographic technique that can produce simple, repetitive micropatterns over large areas (several square centimeters). The technique uses an illuminated array of micrometer-scale lenses to generate an array of optical patterns in an image plane located within micrometer distances from the lens array. A layer of photoresist, placed in the image plane, records the patterns. Microlenses with different sizes, profiles, composition, and indices of refraction produce corresponding patterns in exposed and developed photoresist. Both spherical and nonspherical microlenses were examined. Several types of optical element containing arrays of microlenses were fabricated and used to demonstrate that this technique can generate uniform micropatterns over large areas (>4 cm2) in a single exposure. The smallest features produced had dimensions of approximately 100 nm.     

Sequential shrink photolithography for plastic microlens arrays

Endeavoring to push the boundaries of microfabrication with shrinkable polymers, we have developed a sequential shrink photolithography process. We demonstrate the utility of this approach by rapidly fabricating plastic microlens arrays. First, we create a mask out of the children's toy Shrinky Dinks by simply printing dots using a standard desktop printer. Upon retraction of this pre-stressed thermoplastic sheet, the dots shrink to a fraction of their original size, which we then lithographically transfer onto photoresist-coated commodity shrink wrap film. This shrink film reduces in area by 95% when briefly heated, creating smooth convex photoresist bumps down to 30 μm. Taken together, this sequential shrink process provides a complete process to create microlenses, with an almost 99% reduction in area from the original pattern size. Finally, with a lithography molding step, we emboss these bumps into optical grade plastics such as cyclic olefin copolymer for functional microlens arrays.     

Polymer-based flexible microlens arrays with hermaphroditic focusing properties

Polymer microlens arrays with hermaphroditic focusing behaviors are demonstrated. Each microlens in an arrays exhibits either converging or diverging focus, depending on the polarization direction of the incident light. A polymer film with patterned microlens arrays is flexible, lightweight, and ultrathin (approximately 50 microm). Details of the lens structure, device fabrication, and lens performance are described.     

Focal-plane pixel-energy redistribution and concentration by use of microlens arrays

A physical-optics calculation was performed to study the effects of a microlens array placed over a focal-plane detector array. In certain conditions the light is further concentrated to a spot size that is smaller than the point-spread function of the receiver optics. It is also shown that the microlens refocuses a sinc-squared point-spread function to a shape that is more uniform as well as narrower. Numerical examples were made for the far IR.     

Conical microlens arrays that flatten optical-irradiance profiles of nonuniform sources

One can create conical microlens arrays on a glass plate by simply spin coating spin-on-glass film on the glass, patterning the photoresist, and then etching the glass in a HF solution. These arrays can be used to obtain more-uniform irradiance profiles from nonuniform incoherent sources.     

Implementation of field lens arrays in beam-deflecting microlens array telescopes

Laterally displaceable microlens array telescopes allow for variable and fast beam deflection. The generation of spurious light usually leads to a reduction of transfer efficiency with increasing displacement. We present the introduction of an array of field lenses on the back side of a recollimating microlens array that results in a reduced deflection angle dependency of transfer efficiency. A paraxial matrix formalism is used to prove the theoretical elimination of spurious light by use of a field lens array. The fabrication of well-aligned double-sided lens arrays by UV replication is discussed. Measurements of transfer efficiency with and without the use of field lens arrays are compared with the results of numerical wave-optic simulations.     

Highly corrected close-packed microlens arrays and moth-eye structuring on curved surfaces

The fabrication of near-micrometer-sized close-packed coherent microlens arrays on spheric or aspheric surfaces has been accomplished by use of a compact holographic projector system that was developed for producing multimicrometer down to submicrometer grid patterning on curved surfaces. The microlens arrays, which can be utilized as moth-eye relief structures, are formed in a photoimageable bisbenzocyclobutene polymeric resin by a photolytic process involving standing-wave interference patterns from the holographic projector system. Because of absorption, each integral microlenslet of the finished arrays possesses a near-paraboloid contour. The trajectories of the meridional rays from each microlenslet can be optimized to intersect at either a single point or a locus of points.     

Reduction photolithography using microlens arrays: applications in gray scale photolithography

This paper describes the application of reduction photolithography, using arrays of microlenses and gray scale masks, to generate arrays of micropatterns having multilevel and curved features in photoresist. This technique can fabricate, in a single exposure, three-dimensional microstructures (e.g., nonspherical microlens arrays) over areas of approximately 2 x 2 cm2. The simple optical configuration consisted of transparency film (having centimeter-sized features) as gray scale photomasks, an overhead projector as the illumination source, and arrays of microlenses as the size-reducing elements. Arrays of 40- and 100-microm lenses achieved a lateral size reduction of approximately 10(3) and generated patterns of well-defined, multilevel structures; these structures may find use in applications such as diffractive optics.     

Enhanced signal coupling into periodically poled lithium niobate with microlens arrays

The return signal frequency of an eye-safe ladar system is upconverted from the infrared to the visible through sum-frequency generation by incorporation of periodically poled LiNbO3 into the receiver. A quantitative analysis of the angular acceptance and the quantum efficiency is then presented for a single macroscopic receiver optic and a multiaperture microlens array. Comparing both results, a 6x increase in the receiver field of regard and an 18% increase in beam coupling were realized for the microlens design over the macroscopic system.     

Fabrication and characterization of multi-scale microlens arrays with anti-reflection and diffusion properties

In this paper, an effective method for fabricating artificial compound-eye structures is demonstrated. The fabrication of high fill factor microlens arrays (MLAs) with sub-wavelength structures (SWSs) on a polycarbonate (PC) substrate involves nanoimprint and thermo-extrusion techniques by using two different scales of nano/micromolds. In addition, the MLAs with SWSs on the PC substrate would be replicated on a polymethylmethacrylate (PMMA) millimeter concave surface by hot-embossing, forming three-level compound-eye structures. The optical properties of these samples are characterized. The transmittances of two-level PC and three-level PMMA compound structures are increased by 2.5% and 2%, and the uniformities are enhanced by 18% and 24%, respectively.     

Hybrid sample-inverted reflow and soft-lithography technique for fabrication of conicoid microlens arrays

We report a cost-effective fabrication method, with a combination of the sample-inverted reflow technique and the soft-lithography replication method, to fabricate conicoid refractive microlens arrays (MLAs), including hyperboloid, paraboloid, and ellipsoid MLAs in inorganic-organic hybrid SiO2-ZrO2 solgel material. The fabrication procedures involve two basic steps. First, a master of the conicoid MLA was made in photoresist by the sample-inverted reflow technique. Second, we built a negative mold of the master by casting polydimethylsiloxane (PDMS) onto a silicone elastomer against the master, and then the profile was imprinted onto the solgel glass. As a result, the fabricated solgel MLAs have been obtained with excellent smooth profiles, having negligible discrepancies from the profiles of ideal conicoid MLAs.     

Integral imaging with improved depth of field by use of amplitude-modulated microlens arrays

One of the main challenges in three-dimensional integral imaging is its limited depth of field. Such a limitation is imposed by diffraction, among other factors. The easiest way to improve the depth of field is by reducing the numerical aperture of the microlenses. However, such an improvement is obtained at the expense of an important deterioration in the spatial resolution. We propose a technique, which is novel in the context of integral imaging, for improving the depth of field with no deterioration of the spatial resolution. The technique, based on amplitude modulation of the array of phase elements, can substantially improve the figure of merit of the product of depth of the focus and the squared resolution.     

Optimization and theoretical modeling of polymer microlens arrays fabricated with the hydrophobic effect

High-performance polymer microlens arrays were fabricated by means of withdrawing substrates of patterned wettability from a monomer solution. The f-number (f(#)) of formed microlenses was controlled by adjustment of monomer viscosity and surface tension, substrate dipping angle and withdrawal speed, the array fill factor, and the number of dip coats used. An optimum withdrawal speed was identified at which f(#) was minimized and array uniformity was maximized. At this optimum, arrays of f/3.48 microlenses were fabricated with one dip coat with uniformity of better than Deltaf/f +/- 3.8%. Multiple dip coats allowed for production of f/1.38 lens arrays and uniformity of better than Deltaf/f +/-5.9%. Average f(#)s were reproducible to within 3.5%. A model was developed to describe the fluid-transfer process by which monomer solution assembles on the hydrophilic domains. The model agrees well with experimental trends.     

Amplitude filter and Zernike polynomial expansion method for quality control of microlens arrays

This paper deals with a computer simulation and an experimental realization of an optical setup for automatic quality control of microlens arrays. The method is based on a 4f coherent light correlator setup with an amplitude filter placed in the Fourier plane. The output intensity signal is proportional to the first derivative of the distortion of the input wavefront. An analysis can be carried out with the use of the Zernike polynomial expansion method. It must be carried out separately for each lens, but it allows for a more precise, quantitative assessment of their quality. What is important is that the analysis is computer-based and performed on the basis of the initial single optical measurement.     

Enhancing light extraction in top-emitting organic light-emitting devices using molded transparent polymer microlens arrays

The light extraction efficiency in organic light-emitting devices (OLEDs) is enhanced by up to 2.6 times when a close-packed, hemispherical transparent polymer microlens array (MLA) is molded on the light-emitting surface of a top-emitting device. The microlens array helps to extract the waveguided optical emission in the organic layers and the transparent top electrode, and can be manufactured in large area with low cost.     

Fabrication of ellipticity-controlled microlens arrays by controlling the parameters of the multiple-exposure two-beam interference technique

We demonstrate a promising method for fabrication of plastic microlens arrays (MLAs) with a controllable ellipticity and structure, by using the combination of multiple-exposure two-beam interference and plastic replication techniques. Multiple exposures of a two-beam interference pattern with a wavelength of 442?nm into a thick positive photoresist (AZ-4620) were used to form different two-dimensional periodic structures. Thanks to the developing effect of the positive photoresist, fabricated structures consisting of hemielliptical- or hemispherical-shaped concave holes were obtained. By controlling the rotation angle between different exposures, both the shape and structure of the holes varied. By adjusting the dosage ratio between different exposures, the shape of the holes was modified while the structure of the holes was unchanged. The photoresist concave microstructures were then transferred to plastic MLAs by employing replication and embossing techniques. The fabricated MLAs were characterized by a scanning electron microscope and atomic force microscope measurements. We show that the ellipticity of the microlenses can be well controlled from 0 (hemispherical) to 0.96 (hemielliptical) by changing the rotation angle or dosage ratio between the two exposures.     

Design and fabrication of diffractive microlens arrays with continuous relief for parallel laser direct writing

Diffractive microlens arrays with continuous relief are designed, fabricated, and characterized by using Fermat's principle to create an array of spots on the photoresist-coated surface of a substrate for parallel laser direct writing. Experimental results indicate that a diffraction efficiency of 71.4% and a spot size of 1.97 microm (FWHM) can be achieved at normal incidence and a writing laser wavelength of 441.6 nm with an array of F/4 fabricated on fused silica, and the developed array can be used to improve the utilization ratio of writing laser energy.     

Calculation of the average lenslet shape and aberrations of microlens arrays from their far-field intensity distribution

We present a method to obtain the average lenslet shape of microlens arrays and especially their aberrations from the far-field intensity distribution of the whole array. The method is based on the phase-retrieval algorithm introduced by Gerchberg and Saxton [Optik (Stuttgart) 35, 237 (1972)]. We show how to overcome the crucial point of this algorithm, that is finding suitable start parameters to end up with correct results. The procedure is successfully applied to a cylindrical microlens array produced by reflow technique and the result is compared with surface profilometric measurements. The technique is applicable for lenslets having small numerical apertures and fill factors near unity.