Polydimethylsiloxane Microlens Arrays Research Articles

Enhanced light extraction for red organic light emitting diodes via PDMS convex lens based on breath figure method

To inhibit the light loss of substrate mode at the substrate/air interface in organic light emitting diodes (OLEDs), regular porous polystyrene (PS) template was fabricated via the “Breath figure” method. The structure of porous mold was optimized by adjusting the PS solution concentration from 10 to 60 mg/ml and relative humidity from 60% to 80%. The optimal porous PS mold was achieved at 50 mg/ml PS solution under 70% relative humidity. Then, elastic micro-lens arrays based on transparent elastomer polydimethylsiloxane (PDMS) were obtained via a two-step transferring process from the porous PS mold. The projection experiment exhibits excellent focusing effect of the PDMS convex lens relative to the planar PDMS film. These PDMS micro-lens films were in situ fabricated onto the external face of the red OLED device to extract substrate mode light. By using the PDMS micro-lens array with a height of 1.2 μm and a diameter of 6.7 μm, the OLED device demonstrates an enhancement of 32.7%, 35.6%, 46.5%, and 27.6% for maximum luminance, current efficiency, power efficiency, and external quantum efficiency, respectively, without changing its color purity. The device with PDMS micro-lens exhibits a higher intensity than an ideal Lambertian emission pattern, especially at viewing angles from 30° to 60°. It suggests that the introduction of external micro-lens structure significantly improves the light outcoupling efficiency.

Fabrication of a light screen-aperture integrated flexible thin film micro-lens array for a biomimetic superposition compound eye.

Micro-lens array, an artificial compound eye vision system, provides a wide field of view and multi-perspective view. However, it has not been adopted as a computer vision application due to its limited visible range and high optical interference. In this research, a novel fabrication method for the flexible polydimethylsiloxane micro-lens array with a polytetrafluoroethylene light screen-aperture integrated layer was established by the simple protrusion method. The integrated layer provided longer visible range by one meter while maintaining the wide field-of-view of 100 °. The resulting images were used for obtaining depth information of a target as an example and for analyzing the rectangular and hexagonal arrangements of the micro-lenses for the future applications. With the improved visual range, wide field-of-view and flexibility, the fabricated micro-lens array can be applied to the small and curved CMOS image sensors in the future.

Integration of Great Water Repellence and Imaging Performance on a Superhydrophobic PDMS Microlens Array by Femtosecond Laser Microfabrication

Microlens arrays (MLAs), as the important optical devices, are easily polluted by the water droplets or power‐like‐contaminates in the air. Endowing the artificial MLAs with anti‐water and self‐cleaning abilities remains great challenge. In this paper, the authors report a novel method for fabricating superhydrophobic polydimethylsiloxane (PDMS) MLAs by the combination of femtosecond laser wet etching and femtosecond laser direct writing methods. The resultant surface is composed of a convex MLA and the surrounding rough micro/nanoscale hierarchical structures. Water droplet on the surface of the as‐prepared MLA shows a contact angle of 162° and can easily roll away when the substrate is slightly tilted 0.5°. In addition to their excellent imaging performance, such ultralow adhesive and ultrahigh superhydrophobicity also endows the as‐prepared MLA with excellent anti‐water ability as well as the self‐cleaning function relative to the normal MLA. The authors believe that the anti‐water and self‐cleaning MLAs will potentially have many important applications in solar cells, medical endoscopes, and other optical systems that are often used in the humid environment or outdoors.

Optical performance of fluorescent collectors integrated with microlens arrays

Fluorescent collectors have been considered for being used as a light concentrator in photovoltaic applications. It is able to replace the expensive area of solar cells in order to minimize the energy cost. In this study, fluorescent collectors integrated with microlens arrays were developed in order to improve the collection efficiency. Fluorescent collectors were prepared by casting an epoxy resin solution containing Rhodamine 6G dye in the polydimethylsiloxane master mold. Polydimethylsiloxane-based microlens arrays with actual diameters of 804.06–1008.02 µm were fabricated using a replica molding technique. The microlens arrays were then integrated with the fluorescent collectors with various dye concentrations from 2.5 × 10−4 to 4.5 × 10−4 Molar. Experimental results were demonstrated that the implementation of polydimethylsiloxane microlens arrays on the top of fluorescent collectors was improved for both light absorption and fluorescent intensity.

A novel and rapid fabrication method for a high fill factor hexagonal microlens array using thermal reflow and repeating spin coating

This paper presents an innovative process for the fabrication of a high fill factor microlens array using thermal reflow and repeating spin coating. The repeated spin coating method creates a hexagonal microlens array after the thermal reflow processes as an alternative to the expensive gray-mask and longtime electroplating method. The first, the round base and hexagonally arranged array, precisely define the bottom shape of the liquid photoresist, and a tall hemispherical microlens array with a small radius of curvature is fabricated during the photoresist thermal reflow process. The second, the surplus photoresist, is then rotated at high velocity using a spinner. The micro-plano-convex microlens molds are then finished. The polydimethylsiloxane (PDMS) microlens array is fabricated using a replica molding process. The experimental results show that a high fill factor hexagonal PDMS microlens array is successfully fabricated. The fabricated microlens array has good surface roughness. The proposed fabrication method facilitates mass production and gives increased luminance efficiency and luminance power efficiency to produce a high fill factor microlens array that is suitable for use in the light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs).

Fabrication of microlens array and bifocal microlens using the methods of laser ablation and solvent reflow

Based on the techniques of laser microdrilling and solvent reflow, this study reports on a straightforward approach for fabricating plastic microlens arrays (MLAs). First, we use the ArF excimer laser to drill microholes on a polymethylmethacrylate plate for defining the lens number, initial depth, and diameter. The propylene glycol monomethyl ether acetate solvent is then employed to regulate the surface profile that leads to a resulting negative (concave) MLA. The corresponding positive (convex), polydimethyl-siloxane MLA is obtained by the soft-replica-molding technique. Through varying the pattern size and period on the mask and the light intensity for laser drilling and regulating the solvent in the reflow process, we exhibit the feasibility of making MLAs with various sizes and shapes. By modifying the laser ablation step to drill two microholes with different diameters and depths at two levels, we fabricate a bifocal microlens. The obtained microlenses have excellent surface and optical properties: surface roughness down to several nanometers and focal lengths varying from hundreds to thousands of micrometers. This approach is flexible for constructing microlenses with various sizes and shapes and can fabricate MLAs with a high fill factor.

Fabrication of polydimethylsiloxane microlens arrays on a plastic film by proton beam writing

This paper reports the fabrication of polydimethylsiloxane (PDMS) microlens arrays (MLAs) on a 5–13 μm-thick PDMS layer on polyethylene terephthalate (PET) films using proton beam writing (PBW). The PBW is performed with a focused 1.3 μm beam with an energy of 1.0 MeV. The sensitivity of the PDMS layer on an indium tin oxide (ITO)-coated PET film is improved compared to that on a bare PET film. The arrays of the PDMS microlenses with diameters of 20 μm and a height of 4.5 μm are fabricated by changing the proton beam fluence. The ITO layer on the PET film provides substrate conductivity for PDMS and the transparency required for flexible optofluidic devices. The patterning of 35 × 35 MLAs is demonstrated in an area of 1.0 × 1.0 mm2 within 16 min. The PDMS MLAs can be transferred to a Ni mold by electroplating and are then duplicated by ultraviolet imprint lithography on PET films.

Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique

This paper presents a facile and effective method to fabricate microlens array in polydimethylsiloxane (PDMS). The microlens array model is fabricated in photoresist via digital maskless grayscale lithography technique and the replica molding technique is used to fabricate PDMS microlens array. A convex PDMS microlens array with rectangular aperture and concave PDMS microlens array with hexagonal aperture are fabricated. The morphological characteristics of the microlens arrays are measured by microscope and 3D profiler. The results indicate that the profiles of the PDMS microlens arrays are clear and distinct. This method provides a simple and low-cost approach to prepare large area, concave or convex with arbitrary shape microlens array, which has potential application in many optoelectronic devices.

Fabrication of polydimethylsiloxane microlens array on spherical surface using multi-replication process

In comparison to traditional planar optical devices, compound-eye structured optical elements can reduce the number of components and their volume when being applied to wide-field imaging and sensing systems. However, the fabrication process for microstructures on a curvilinear surface has many difficulties since traditional fabrication techniques are planar. In this paper, we present a cost-effective method to fabricate microlenses on a spherical surface. Microlenses, of which the fill factor was about 78%, were formed using the thermal reflow technique, followed by multiple replication processes to transfer the microlenses from the planar substrate onto a spherical surface. We produced a curved mold with concave microlenses, and it allowed this method to be replicable. A polydimethylsiloxane elastomer was employed as the material for both the microlenses and the mold. The radius of curvature of the spherical surface was approximately 6.1 mm. The variation of the microlenses was analyzed, demonstrating high uniformity. The imaging performance of the microlenses is also presented. The curved microlens arrays were combined with image sensors, and sub-images of objects at various distances are shown. The experimental results demonstrate a high potential for curved microlens arrays being applied to compact mobile camera lenses.

Shape-controllable polymeric microlens array duplicated by electrostatic force deformed template

Here, we introduced a new method to fabricate a shape-controllable microlens array (MLA). A conductive membrane was deformed by electrostatic force through applied voltage on its underneath electrodes, and polymer primer was poured in this concave template to form an MLA after curing and a demolding process. Through changing the initial contour of the template as well as the location and bias of the underneath electrodes, the so-called molding by electrostatic force deformed template (MEFDT) method has the ability to realize a shape-controllable microsized MLA in a flexible, easy-to-design, and low-cost way. Polydimethylsiloxane MLAs were fabricated to verify this MEFDT method, and the fabricated MLAs had a surface roughness less than 2 nm that showed acceptable optical smoothness, and their focal lengths were demonstrated to be adjustable by applying different voltage on their affiliated templates.

Flexible PDMS micro-lens array with programmable focus gradient fabricated by dielectrophoresis force

Micro-lens arrays with programmable focus gradient were fabricated using polydimethylsiloxane (PDMS) droplets on a polyethylene (PE) membrane which isolates PDMS from concentric circular electrodes. The copper electrodes were patterned on a flexible print circuit so that gradient voltages could be applied on the droplets to generate dielectrophoresis forces of gradient magnitudes. The radii of curvature of the droplets changed from 0.9 to 1.6mm when the applied bias voltage increased from 0 to 350V. After curing the PDMS lenses, the PE substrate was peeled off to form a flexible PDMS micro-lens array. An 8×8 lens-array with focus gradient has been demonstrated.

Fabrication of polydimethylsiloxane microlens arrays by surface tension induced liquid separation

This Letter presents a novel fabrication method to build polymer microlens arrays. The key mechanism of the fabrication process is surface tension, which induces liquid separation phenomenon when a post is immersed and detached from a bottom liquid mixture. The surface tension also acts on the formation of lens surface at liquid-air interface resulting in an excellent surface roughness. The surface profile of lenses is well fitted with an ideal lens profile. The other feature of the presented fabrication method is self-positioning of the lens into the post. The range of focal lengths was between 110.4 and 282.0--m, which were determined by three parameters such as a post radius, gap and bottom liquid thickness. In summary, the STILS method can be a powerful tool to create microlens arrays with a smooth surface and a high scalability for the future applications in optoelectronics or microfluidics.

Self-patterning of a polydimethylsiloxane microlens array on functionalized substrates and characterization by digital holography

Microlens arrays are realized through a self-arrangement process of thin liquid polymeric polydimethylsiloxane (PDMS) film on a functionalized polar dielectric crystal substrate. The self-arrangement process is named the pyro-electro-wetting mechanism. The substrate, a LiNbO3 (LN) z-cut wafer, has been micro-engineered with periodically poled ferroelectric domains, with the aim to provide an appropriate wettability patterning induced by a thermal stimulus. Different experimental procedures have been explored demonstrating that arrays of thousands of microlenses, having a diameter size of 100 µm and focal lengths ranging between 300 and 1100 µm, can be fabricated. Furthermore, a microscope interference method based on digital holography is adopted for microlens characterization.

Polydimethylsiloxane Microlens Arrays Fabricated Through Liquid-Phase Photopolymerization and Molding

We report on polydimethylsiloxane (PDMS) microlens arrays fabricated through liquid-phase photopolymerization and molding. The gist of this fabrication process is to form liquid menisci of variable radii of curvature at an array of apertures through pneumatic control, followed by photopolymerization under ultraviolet radiance. The resultant polymerized structures are then transferred to PDMS utilizing two molding steps. By adjusting the pneumatic pressure during the process, a single aperture array can be used to fabricate PDMS microlens arrays with variant focal lengths. The liquid menisci are formed by liquid-air interfaces that are pinned at the top edges of the apertures along hydrophobic-hydrophilic boundaries generated through surface chemical treatments. The microlens arrays are optically characterized. Variant focal lengths from 2.35 to 5.54 mm and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> -numbers from 1.27 to 5.88, dependent on the diameter of apertures and the applied pressure to form the liquid menisci, are achieved with this relatively simple process and match well with the physical model. Owing to the formation from the liquid-air interfaces, the surface roughness of microlenses is measured to be around 25 nm.

Fabrication of 3-D Curved Microstructures by Constrained Gas Expansion and Photopolymerization

This paper describes a novel method of fabricating three-dimensional (3-D) curved microstructures with continuous relief through controlled argon gas expansion into a photocurable resin. A microstructured stamp is placed on top of a nonwetting photopolymerizable liquid resin. The setup is heated, and the argon gas in the blind holes of the stamp expands. The expanded gas displaces the resin at the mouth of the microcavities to form 3-D curved indentations in the liquid resin which is subsequently rapidly solidified by photopolymerization. By changing the duration of the preheating, different curvatures can be produced. Arrays of homogeneous 3-D curved microstructures having different cross-sectional geometries and heights were fabricated using various shapes of the blind holes and preheating times, respectively. As a demonstration of applications, high-quality and uniform polydimethylsiloxane microlens arrays were produced. In addition, thorough investigation was carried out to study the factors influencing the fabricated 3-D curved microstructures. Curved microstructures with diameters as small as 2 microm were demonstrated. A simple model was developed, and such a model allows for predicting the curvatures of indentations with different preheating times. It has been found that the predicted curvatures are in good agreement with experimental data.

Precision patterning of PDMS membranes and applications

This paper presents a method for precision patterning of polydimethylsiloxane (PDMS) membranes based on parylene C lift-off. The process permits the construction of PDMS membranes either with a highly flat, uniform top surface or with a controlled curvature. Effects of varying processing parameters on the geometrical characteristics of the PDMS membranes are described. The paper also demonstrates the application of the PDMS precision patterning method to the construction of PDMS microlens arrays, which require curved top surfaces, and a 3-axis electrostatic positioning stage that uses PDMS membranes with flat surfaces as a bonding material as well as a precisely defined spacer.

Fabrication of PDMS (polydimethylsiloxane) microlens and diffuser using replica molding

This paper reports the fabrication method of PDMS (polydimethylsiloxane) microlens array based on the replica molding process, casting a liquid PDMS into concave microlens molds. Concave microlens molds were fabricated through spin-coating processes suspended a liquid PMMA film on the micro-drilled hole, leading to the formation of the special curvature. Then, PDMS microlens arrays and diffusers were fabricated by replica molding and discussed in this study. The fabricated microlenses have both good surface roughness and high-quality optical properties. Therefore, diffusers were fabricated by two-step replica molding. A high-intensity He–Ne laser beam was uniformly spread through diffusers. Experimental tests demonstrated that all PDMS elements are successfully fabricated by replica molding process and operated during optical measurement.