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.     

Capillary‐Assisted Fabrication of Biconcave Polymeric Microlenses from Microfluidic Ternary Emulsion Droplets

In this study, a simple capillary‐based approach for producing biconcave polymeric microlenses with uniform size and shape from ternary emulsion droplets is presented. Monodisperse ternary emulsion droplets (0.6–4.0 nL) are produced which contain a photocurable segment of an acrylate monomer and two non‐curable segments of silicone oil (SO) by using a microfluidic sheath‐flowing droplet generator on a glass chip. The curvature radius of the interfaces separating the droplet segments, as well as the droplet size, and production rate can be flexibly varied by changing the flow conditions of the organic and aqueous phases. Subsequently, off‐chip suspension photopolymerization yields non‐spherical polymeric microparticles with two spherical concave surfaces templated by two SO segments at random positions. By ultraviolet light irradiation of ternary droplets with two SO segments trapped by the interior wall of a cylindrical microcapillary (internal diameter: 130 μm), biconcave microlenses can be produced with two spherical concave surfaces with a common lens axis. The produced lenses are suitable for use as optical diverging lenses.     

Design of microlenses with long focal depth based on the general focal length function

A general focal length function is proposed to design microlenses with long extended focal depth and high lateral resolution. The focal performance of the designed microlenses, including the actual focal depth, the focal spot size, and the diffraction efficiency, is calculated by rigorous electromagnetic theory and the boundary-element method for several f-numbers. In contrast to conventional microlenses, the numerical results indicate that the designed microlenses can exhibit long extended focal depth and good focal performance. It is expected that the long focal length function will be widely used to design microlenses with long focal depth characteristics.     

Evaluation of microlens properties in the presence of high spherical aberration

Microlenses can be generated with various fabrication technologies. Some of these technologies cause large spherical aberrations in the resulting microlenses. We describe an algorithm based on Rayleigh's quarter-wave criterion, which allows the evaluation of lens parameters for those microlenses. Specifically, we investigate numerical aperture, focal length, and space-bandwidth product with respect to applications in optical microsystems. We apply our algorithm to different types of microlenses, three gradient-index lenses, and one surface-relief lens. The experimental results demonstrate that our algorithm provides a helpful characterization method for microlenses with large aberrations.     

Fabrication of polymer-based reflowed microlenses on optical fibre with control of focal length using differential coating technique

Thermal reflow of polymer to generate spherical profile has been used to fabricate microlenses in this paper. A simple model based on the volume conservation (before and after reflow) and geometrical consideration of lens profile, shows that the focal length of the microlens produced by reflow technique is a function of the initial geometry of microcylinders, i.e. diameter and thickness. This relationship of focal length with diameter and thickness is used as a basis to control focal length. A simple spin coating technique on dual surface is used to achieve differential thickness, to control the focal length of microlenses produced on the same substrate. A biomedical application of such combination of microlenses is endoscopy where the lenses of varying diameter and equal focal length are needed on top of optical fibre bundles to provide independent function of illumination and imaging. This paper incorporates the differential thickness technique to show a micro fabrication process to produce the polymer reflowed microlenses, with a control of focal length based on thickness. The design also helps to integrate these microlenses on top an optical fibre with accurate alignment.     

Silicone microlenses and interference gratings

Interference gratings, plano-convex microlenses, and spherical microlenses have been made in silicone. Lenses were fabricated by the melting method. Two substrates have been tried: glass and Teflon. The latter substrate lets us fabricate low-f-number lenses. We made spherical microlenses by placing pieces of silicone near a thermal source and studied resolution of the lenses by investigating the images they gave of a test chart. We made low-spatial-frequency gratings by recording interference patterns and studied parameters involved in the recording. A study of the profile of the gratings and lenses was done with a mechanical surface analyzer.     

Low f‐Number Diffraction‐Limited Pancharatnam–Berry Microlenses Enabled by Plasmonic Photopatterning of Liquid Crystal Polymers

Microlenses are desired by a wide range of industrial applications while it is always challenging to make them with diffraction‐limited quality. Here, it is shown that high‐quality microlenses based on Pancharatnam–Berry (PB) phases can be made with liquid crystal polymers by using a plasmonic photopatterning technique. Based on the generalized Snell's law for the PB phases, PB microlenses with a range of focal lengths and f‐numbers are designed and fabricated and their point‐spread functions and ability to image micrometer‐sized particles are carefully characterized. The results show that these PB microlenses with f‐number down to 2 are all diffraction‐limited. The capability of arraying these PB microlenses with 100% filling factor with a step‐and‐flash approach is further demonstrated.     

Integration of refractive micro-optical elements with differential-pair optical-thyristor arrays

We demonstrate a refractive micr-optical system by using ion-exchange microlenses and microprisms, which are combined to generate a superposition of two shifted images. The microlenses, fabricated with field-assisted Ag-Na exchange, achieve diffraction-limited imaging with a single-lens system and with a double-lens system for a field of 800 μm × 800 μm. Furthermore, we demonstrate cascading of two separate differential-pair optical-thyristor arrays by transcribing the information of a source array onto a second destination array.     

Dynamical optical microelements on dye-sensitized gels

We discuss the optical recording and relaxation of low spatial frequency gratings and negative microlenses by a dyed polyacrylamide gel. An analysis of the grating diffraction efficiency and the focal distance of microlenses is shown. A study of the evolution of the surface modulation of both types of elements with an interference microscope is also included.     

Testing of refractive silicon microlenses by use of a lateral shearing interferometer in transmission

Wave aberrations of refractive photoresist microlenses and silicon microlenses were measured with a lateral shearing interferometer. Because of the silicon elements, a near-infrared working wavelength (lambda = 1.32 mum) was used. The wave front was evaluated by a phase step technique with four steps. Integration of the phase distributions was performed with a computationally efficient Fourier transformation algorithm. The influence of the detector vidicon nonlinearity on the measured wave front was calculated. The defocusing behavior of the interferometer was investigated by fitting the measured wave fronts with the help of Zernike circle polynomials. It is shown that the reproducibility can be kept below lambda/100 rms. Examples for the measured wave fronts of plano-convex silicon microlenses are discussed in detail.     

Realization and optimization of planar refracting microlenses by Ag-Na ion-exchange techniques

We report the fabrication of planar microlenses with numerical apertures (N.A.'s) of 0.2 by field-assisted Ag-Na ion exchange in glass. To measure the N.A. of microlenses, different definitions can be used. We discuss the issue of measuring the N.A. and suggest an additional definition based on diffraction-limited performance. According to a simple model, the N.A. of a spherical lens is limited by the maximum index difference. Owing to this model, the N.A. for Ag-Na ion exchange is limited to a value of ~0.1. From measurements of microlenses, fabricated by field-assisted ion exchange, we obtained N.A.'s as high as 0.2, providing for diffraction-limited performance within the whole aperture.     

Confocal microscopy using variable-focal-length microlenses and an optical fiber bundle

The use of variable-focal-length (VFL) microlenses can provide a way to axially scan the foci across a sample by electronic control. We demonstrate an approach to coupling VFL microlenses individually to a fiber bundle as a way to create a high-throughput aperture array with a controllable aperture pattern. It would potentially be applied in real-time confocal imaging in vivo for biological specimens. The VFL microlenses that we used consist of a liquid-crystal film sandwiched between a pair of conductive substrates for which one has a hole-patterned electrode. One obtains the variation of the focal length by changing the applied voltage. The fiber bundle has been characterized by coupling with both coherent and incoherent light sources. We further demonstrate the use of a VFL microlens array in combination with the fiber bundle to build up a confocal system. The axial response of the confocal system has been measured without mechanical movement of the sample or the objective, and the FWHM is estimated to be approximately 16 microm, with asymmetric sidelobes.     

Replicated, high-aspect-ratio micro-optical components fabricated from inorganic solgel materials

A replication process for the fabrication of refractive microlenses from a purely inorganic solgel material based on tetraethoxysilane is presented. The geometrical dimensions and optical properties of the inorganic microlenses are characterized and compared with those of microlenses replicated in a hybrid xerogel containing organic additives. By a reduced solvent content in the sol composition, together with modifications in the replication process, it was possible to obtain inorganic xerogel lenses with exceptionally high sagittal height values of as much as 28 microm. Compared with the hybrid xerogel, the inorganic xerogel has the advantage of an absorption coefficient that is five times lower in the visible spectral range and exhibits optical transparency in the near-ultraviolet range for wavelengths down to 200 nm.     

Refractive sapphire microlenses fabricated by chlorine-based inductively coupled plasma etching

We have fabricated refractive sapphire microlenses and characterized their properties for what we believe to be the first time. We use thermally reflown photoresist lenslet patterns as a mask for chlorine-based dry etch of sapphire. Pattern transfer to the mechanically hard and chemically inert sapphire substrate is made possible by an inductively coupled plasma etch system that supplies a high-density plasma gas. Processed sapphire microlenses exhibit properties close to the ideal and operate nearly in the diffraction limit.     

Relief gratings and microlenses fabricated with silicone

Divergent microlenses and low-spatial-frequency interference gratings have been fabricated in low-cost silicone layers. The size of the microlenses ranges from approximately 1 mm to 100 microm while spatial frequencies of interference gratings range from 4 to 18 l/mm. The fabrication method involves the recording of spatial distributions of mid-IR light. At recording time silicone layers are in a gel state. Then layers are cured by heat. The final silicone layers are transparent and rigid.     

Reflowed solgel spherical microlens for high-efficiency optical coupling between a laser diode and a single-mode fiber

To improve the coupling efficiency between a laser diode and a single-mode fiber, we propose a two-microlens coupling scheme that uses two solgel spherical microlenses for high coupling efficiency. The conventional reflow technique was employed and extended to the inorganic-organic hybrid SiO2/ZrO2 solgel material to form the microlenses. Preliminary results show that the coupling efficiency was increased to--1.28 dB (74.5%) by the proposed scheme, compared with a coupling efficiency of--10.13 dB (9.7%) by the butt-joint method. The proposed fabrication technique demonstrates that use of a reflowed solgel spherical microlens is a cost-effective mass-production approach to application of micro-optical elements in optical communication.     

Preparation of plastic spherical microlenses by use of a fluoropolymer stencil and oil-bath heating

A new method for fabricating plastic spherical microlenses was developed, which allowed self-alignment of lenses and self-organized formation of a spherical shape. First a low-surface-energy fluoropolymer thin film was deposited and patterned as a stencil. Then photosensitive phenol resin was patterned on it as the lens material. Finally the resin was annealed in an oil bath to form a sphere. The molten phenol resin spontaneously formed a sphere and positioned itself in the center of the fluoropolymer ring pattern as a result of the difference of surface free energy and the equivalently zero-gravity condition in the oil bath. When a light-emitting-diode printer head was loaded with spherical microlenses, its optical output increased by 1 order of magnitude.     

Modulation transfer function measurements of microjetted microlenses

We measured the modulation transfer function for microjetted microlenses with diameters ranging between 109 and 400 mum and with focal lengths ranging between 135 and 540 mum. We found that single-drop 109-mum-diameter microlenses perform close to their theoretical cutoff frequency. However, the larger lenses made with multiple droplets have a cutoff frequency that is 35% of the theoretical value. We interpret this as an illustration of the rapid increase in spherical aberration as the diameter of a lens increases.     

Design of PDMS microlenses bonded to a lab-CD chip for ELISA applications

A novel device comprising polydimethyl-siloxane (PDMS) microlenses bonded to a microfluidic compact disk (CD) is proposed for enzyme-linked immunosorbent assay (ELISA) applications. The PDMS microlenses were fabricated using a simple soft replica molding method and were bonded to the microfluidic CD using oxygen plasma treatment. A commercial software tool (ZEMAX) has been used to analyze the focal length of the microlens. A laser-induced fluorescence bio-detection system, consisting of the integrated microfluidic CD/PDMS microlenses and an optical detection module, was constructed and used to examine the enzymatic reaction of 3-(4-hydroxy) phenly propionic acid. The experimental results show that the PDMS microlens focusing effect yields a significant improvement in the intensity of the detected fluorescence signals. As a result, the proposed device represents an ideal solution for ELISAs and other high-sensitivity bio-detection applications.     

Polyvinylidene fluoride—piezoelectric polymer for integrated infrared optics applications

The suitability of polyvinylidene fluoride films for IR integrated optics applications was demonstrated. Polyvinylidene fluoride is a piezoelectric polymer, which demonstrates high transparency in the middle-IR and has several transparent windows in the far-IR. The fabrication of cylindrical microlenses and microlens arrays by CO₂ laser irradiation of polyvinylidene fluoride substrates has been demonstrated. Pseudo-spherical microlenses were also fabricated by direct laser writing. Strong piezoelectric properties, high chemical resistance, stability to UV radiation and high continuous-use temperature make PVDF suitable for various IR integrated optics applications.