Microlens Structure 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.

Conversion of a Gaussian-distributed circular beam to a flat-top-distributed square beam in laser shock processing based on a micro-lens array structure

A novel beam homogenisation system based on a micro-lens array and beam expansion structure is proposed to convert a Gaussian-distributed circular beam to a flat-top-distributed square beam of size 2–5 mm and a homogeneity degree exceeding 95%. The novel micro-lens array based on a cylindrical lens element can potentially be widely applied in practice because of the absence of the preparative limitations of size and focal length. By employing this beam homogenisation system, laser shock processing experiments were performed on 7050 titanium alloy. The laser shock processing parameters were optimised. When the laser repetition frequency was 1 Hz, pulse width was 15 ns, and laser energy was 20 J, a residual stress of > 400 MPa was achieved on the titanium alloy. The experimental results demonstrate the ability of this beam-homogenisation system to perform laser shock processing.

Optimization of the Cryogenic Light-Emitting Diodes for High-Performance Broadband Terahertz Upconversion Imaging

High-performance terahertz (THz) imaging devices have drawn wide attention due to their significant application in a variety of application fields. Recently, the upconversion device based on the integrated homo-junction interfacial workfunction internal photoemission detector and light-emitting diode (HIWIP-LED) has emerged as a promising candidate for broadband THz upconversion pixelless imaging device. In this paper, systematical investigations on the cryogenic-temperature performances of the LED part in HIWIP-LED devices, including electroluminescence (EL) spectra and the EL efficiency, have been carried out by elaborating the radiative recombination mechanism in the quantum well, internal quantum efficiency, and the light extraction efficiency (LEE) both experimentally and theoretically. On this basis, we have further studied the operation mode of the HIWIP-LED and concluded that the LEE could directly determine the upconversion efficiency. A numerical simulation has been performed to optimize the LEE. Numerical results show that the device with a micro-lens geometry structure could significantly improve the LEE of the LED thereby increasing the upconversion efficiency. An optimal upconversion efficiency value of 0.12 W/W and a minimum noise equivalent power (NEP) of 14 pW/Hz1/2 are achieved using the micro-lens structure together with anti-reflection coating. This work gives a precise description of cryogenic LED performance in the HIWIP-LED device and provides an optimization method for the broadband HIWIP-LED THz upconversion pixelless imaging device.

Pneumatically Actuated Thin Glass Microlens for On-Chip Multi-Magnification Observations

This paper presents a self-contained micro-optical system that is magnification-controlled by adjusting the positions of the microlens in the device via pneumatic air pressure. Unlike conventional dynamic microlenses made from a liquid or polydimethylsiloxane (PDMS) that change their shapes via external actuation, this system combines a fixed-curvature glass microlens, an inflatable PDMS layer, and the external pneumatic air pressure supply as an actuator. This device showed several advantages, including stable inflation, firm structure, and light weight; it achieved a larger displacement using the glass microlens structure than has been reported before. This fixed-curvature microlens was made from 120 µm-thick flat thin glass slides, and the system magnification was manipulated by the deflection of a 100 µm-thick PDMS layer to alter the distance from the microlens to the microfluidic channel. The system magnification power was proportional to the air pressure applied to the device, and with a 2.5 mbar air pressure supply, a 2.2X magnification was achieved. This optical system is ideal for combining with high resolving power microscopy for various short working distance observation tasks, and it is especially beneficial for various chip-based analyses.

Light distribution control of layer-structured PDLC fabricated by using micro lens structure and anisotropically diffused UV light.

We have established a method to control the polymer aggregation structure of the PDLCs by using a micro-lens structure and irradiating them with uni-directionally diffused UV light. The micro-lens structure on the surface of the substrate produced an uneven illuminance distribution of UV light in the LC-monomer mixture, and the monomer was polymerized along the direction of UV light irradiation at positions where the UV illuminance was high and formed a layered structure. The proposed PDLCs could control the light diffusion distribution by the internal polymer aggregation structure and had a high diffusion efficiency without polarization dependency in the haze value.

Hybrid Computational Near-Eye Light Field Display

We propose a hybrid computational near-eye light field display based on micro-lens arrays (MLAs) and structure parameters prioritization method (SPPM) that can render correct accommodation and expand the depth of field (DOF). The proposed display consists of two MLAs and two micro-displays. Two sets of depth-sliced images are superimposed to reconstruct the object's light field. The system's ray resolution and the spectra difference between the proposed light field and the conventional one are discussed. The DOF of the proposed system is extended without the requiring of high refresh rate display. Furthermore, SPPM is proposed to extend the spatial frequency of the light field. A prototype is demonstrated and the experimental result shows that the depth field of the reconstructed images is significantly increased compared with conventional near-eye light field display based on single MLA.

Fabrication of hexagonal compound eye microlens array using DMD-based lithography with dose modulation.

In this study, a method is proposed for the rapid fabrication of a hexagonal compound eye microlens array (HCE-MLA) using maskless lithography technique based on digital micromirror device (DMD), in which a hexahedron array is lithographed, and subsequently, a microlens structure is lithographed layer by layer upon the hexahedron. Owing to the high filling rate of the hexagon and the aspheric surface of the paraboloid profile of the microlens, after the distribution of the required exposure dose of HCE-MLA was calculated based on dose modulation, a series of circular pattern with different radius could be obtained by equal-arc-mean slicing strategy that can adapt to the variable curvature of the target contour and improve its reconstruction precision. Then, after a hexagonal photoresist island was fabricated on the substrate, and the dose accumulated over multiple exposures, the required exposure dose profiles were reconstructed on the hexagonal photoresist island. Finally, a high-quality HCE-MLA with a smooth surface was fabricated via thermal reflow. The fabrication of the designed HCE-MLA using this method was demonstrated in experiments. The results indicate that the actual profile of HCE-MLA shows good agreement with the design profile and that simultaneously increasing the number of the slicing layers can improve the quality of the profile. The research indicated that this method is feasible and convenient for the fabrication of high-quality HCE-MLA.

Kaleidoscopic imaging patterns of complex structures fabricated by laser-induced deformation.

Complex surface structures have stimulated a great deal of interests due to many potential applications in surface devices. However, in the fabrication of complex surface micro-/nanostructures, there are always great challenges in precise design, or good controllability, or low cost, or high throughput. Here, we present a route for the accurate design and highly controllable fabrication of surface quasi-three-dimensional (quasi-3D) structures based on a thermal deformation of simple two-dimensional laser-induced patterns. A complex quasi-3D structure, coaxially nested convex–concave microlens array, as an example, demonstrates our capability of design and fabrication of surface elements with this method. Moreover, by using only one relief mask with the convex–concave microlens structure, we have gotten hundreds of target patterns at different imaging planes, offering a cost-effective solution for mass production in lithography and imprinting, and portending a paradigm in quasi-3D manufacturing.

The micro-optic photovoltaic behavior of solar cell along with microlens curved glass substrate

A hybrid of microlens structure and curved surface may produce high value-added micro-optic performance. Hence, the microlens array is proposed on macro curved glass substrate of thin film solar cell. The objective is to understand how the micro-optic behavior of microlens curved array influences indoor power conversion efficiency and outdoor electricity generation. First, the absorptivities of visible light and infrared light were analyzed in connection with the curved microlens sizes; then the microlens curved glass substrate was fabricated by a Computer Numerical Control (CNC) micro-grinding with micro diamond wheel V-tip; finally, its photovoltaic properties and electricity generation were measured, respectively. It is shown that the microlens curved surface may strongly absorb and scatter light to solar cell. It increases the absorptivity of visible light against plane surface, but it decreases the one of infrared light against microlens surface. When it is applied to solar cell, it enhances the power conversion efficiency by 3.4–10.6% under oblique illumination. When it is applied to solar device, it increases the electricity generation of daylight by 119–106% against microlens surface and by 260–419% against traditional plane surface, respectively. The surprising finding is that it produces much larger electricity generation during cloudy day than during sunny day, but traditional plane surface does not so.

Study on weak-light photovoltaic characteristics of solar cell with a microgroove lens array on glass substrate

Microgroove lens with 500-800 µm in depth is proposed on the glass substrate of thin-film solar cell. The objective is to improve photovoltaic characteristics under weak-light illumination. First, the micro-optical characteristics were theoretically studied in connection with micro-lens structure; then a diamond micro-grinding was employed to fabricate microgroove lens; finally, indoor conversion efficiency and outdoor electrical generation were measured. It is shown that the 800-µm-depth microgroove lens is able to absorb and scatter the inclined light to solar cell for improving weak-light conversion efficiency. It enhances the electricity generation by 118% and 185% in cloudy and overcast days, respectively.

An Epoxy-Based Micro-Structure Surface for Improving Anti-Reflection Efficiency of a Solar Cell Module

This paper proposes an epoxy-based anti-reflective micron structure layer for solar cell modules. A Solidworks software is used to design the micro-structure layers with different size and shapes (inverted pyramid and micro-lens). Then, An optical simulation software, Tracepro, is used to simulate the anti-reflection efficiency under the standard lighting source of AM1.5G. The difference between the flat layer and micro-structured layer has been analyzed to obtain the best micro-structure layer for solar cell modules. From the simulated results, the inverted pyramid textured layer that each pyramid’s width is 10μm and height is 5μm can improve the flux increment rate up to 13% compared with the flat layer at normal incidence. However, the best efficiency for micro-lens structure layer increases about 10% with radius of 25μm and height of 18.75μm and 25μm at 15o incidence. In addition, the thinner the Epoxy layer is, the better the anti-reflection efficiency is. Therefore, the proposed Epoxy-based micro-structure can improve the solar module for obtaining higher efficiency and best qualities.

Improving Light Outcoupling Efficiency for OLEDs with Microlens Array Fabricated on Transparent Substrate

Low light outcoupling efficiency restricts the wide application of organic light‐emitting diodes in solid state light market although the internal quantum efficiency of the device could reach near to 100%. In order to improve the output efficiency, different kinds of microlens array on the substrate emission surface were designed and simulated using light tracing method. Simulation results indicate that the microlens array on the substrate could efficiently improve the light output efficiency and an enhancement of 1.8 could be obtained with optimized microlens structure design. The microlens array with semicircle shape using polymer material was fabricated on glass substrate by a facile approach. Finally, the organic device with microlens array substrate was manufactured and the light output of the device with surface microlens structure could increase to 1.64 times comparing with the device without microlens.

Ommatidia structure based on double layers of liquid crystal microlens array

We present a new liquid crystal (LC) microlens structure having ommatidia function, which consists of the overlapped patterned electrode layers. Each electrode layer has a circular electrode array with a different size. Two electrode layers are deposited on the surface of a glass substrate and insulated by thin SiO₂ coating, which act as controlling electrodes. A plane electrode layer deposited on the surface of another glass substrate acts as the base electrode. Two glass substrates are made into a LC cell filled with nematic LC material. When a voltage signal is applied to the controlling electrode and base electrode, each circular unit of the array can focus along the optical axis and has good focusing character, and it behaves as a common microconvex lens. The whole circular array achieves ommatidia optical characters because of many microconvex lenses imaging to the objects simultaneously. Two circular arrays can both image to the objects and have different fields of view, which have extraordinary ommatidia effects. The common optical properties of the LC microlens are also demonstrated experimentally.

Polarization-independent adaptive lens with two different blue-phase liquid-crystal layers

An adaptive microlens structure is proposed using two polymer-stabilized blue-phase liquid-crystal layers whose Kerr constant is largely mismatched. This device exhibits several favorable features, such as polarization independence, simple structure, and good parabolic phase profile. Its applications for 2D/3D switchable displays and other photonic devices are emphasized.

Asymmetric focusing microlens array fabricated using off-axis lithography

This research aims to make a microlens with a given inclined angle and applied to the light signal processing system. With two circular pattern masks with different diameters, a circular photoresist column structure is firstly made with the photolithography, whose first layer would be got after the small sized circular mask is taken for exposure and development. It is finalized through hard bake and then sputtered with a layer of metal (Cu), and then remained by taking the lift-off method. In the second part, the large sized circular mask is used. And the large sized circular pattern is shifted toward one direction to cover the first layer of metal pattern for the second exposure by taking the counterpoint exposure method to obtain the photoresist column structure. During the process of thermal reflow, there is no change in surface morphology of the first layer of metal pattern since it has been fixed. However, in the second layer of photoresist which goes through the deviation counterpoint exposure, the photoresist column is converted to rubbery state when its temperature is increased to the glass transition temperature (Tg) during the thermal reflow. Then the asymmetric microlens structure can be formed by shifting the arc vertex of microlens toward one direction with the fact that the copper coating surface is superior to silicon substrate surface in hydrophobicity. In this research, a 55° asymmetric microlens array can be manufactured by properly controlling the copper pattern size and the offset of two centers with the above method.

Development and discussion of asymmetric magnetic soft mode electromagnetic imprinting process technology

This study proposed an innovative imprinting process technology for the compound casting of a steel shim and poly-dimethylsiloxane solution. The resulting solid magnetic sheet has angular asymmetric changes. Electromagnet-assisted imprinting technology and ultraviolet lamp light curing equipment were used to reproduce a micro-lens structure of different linear gradient molding heights from the original even micro-lens mold by imprinting. Hence, in a single imprinting process, using the same mold to reproduce micro structures of different heights, with controllable and predictable heights of linear gradient, this study developed a simple and low-cost innovative processing technology on the basis of electromagnetic imprinting and compound casting in order to provide an effective alternative for imprinting processes and applications in the broad sense. For result validation, this study used the pressure sensitive soft mode to discuss the distribution of the imprinting force of the asymmetric linear gradient sheet shim using scanning electron microscope, a surface profiler, and other instruments to observe, measure, and analyze the process results.

A study on the application of electromagnetic-field-assisted magnetic soft mold photocuring imprinting technology in micro-structure gradient replication molding

This study proposed a new imprinting process using electromagnetic-field- assisted imprinting technology, a composite PDMS magnetic soft mold, and UV-LED arrays photocuring equipment to change an even microlens mold by imprinting the replication into a gradient microlens structure. This function is similar to the continuous gray scale technology of the semi-conductor industry. However, the structural appearance, as defined by the gray scale photo mask process, is smoother and continuous. As the method only requires one time of imprinting, the process is simple, with lower cost and higher competitiveness, thus providing a more effective alternative for imprinting technology and applications. This study used pressure-sensitive film to explore the imprinting pressure distribution of gradient magnetic soft molding. The imprinting results showed that, magnetic powder is distributed in a linear tilting pattern that is consistent with pressure measurement distribution. Hence, the magnetic powder casting angle control of a precision tilt platform can achieve gradient control of imprinting pressure. Finally, the observations and analysis by instruments, including SEM, a surface profiler, and a Laser Scanning Confocal Microscope, confirmed that the gradient magnetic PDMS film made using a precision tilt platform to control the magnetic powder casting angle, coupled with electromagnetic-field-assisted imprinting and UV-LED arrays light curing technology, can replicate the gradient microlens arrays structure of the same tilt angle through one imprinting.

Fabrication of asperical lensed optical fibers with an electro-static pulling of SU-8 photoresist

A novel method is presented for fabricating lensed optical fibers for enhancing the coupling efficiency between high-power IR laser diodes and gradient-index fibers (GIF). SU-8 photoresist is attached to the fiber tip by means of surface tension forces and a cone-shaped micro-lens structure is then formed using an electrostatic pulling method. It is shown that micro-lenses with various radii of curvature can be easily formed by tuning the intensity of the electric field used in the pulling process. Experimental results show that for a laser diode chip with a central wavelength of 1310 nm, a coupling efficiency of 78% can be obtained using a lensed optical fiber with a radius of curvature of 48 μm. By contrast, the coupling efficiency of a traditional flat-end fiber is just 40%. Overall, the fabrication method proposed in this study provides a rapid and low-cost solution for the mass production of high-quality lensed optical fibers.

P‐176: Effect of Cathode Reflectivity on Light Enhancement of White OLEDs with Microlens Structure

AbstractLight outcoupling enhancement has been found to be dependent on the cathode reflectivity of white organic light emitting diodes (OLEDs) with external microlens structure. It is shown that in this study the Al cathode reflectivity can be a variable with respect to thermal evaporation conditions. The reduction of light enhancement factor is roughly in inverse proportion to that of cathode reflectivity.

Development and Study of Applications Combining Nanopowder Imprint Lithography and Array-Type UV-Curing Technology for Micro-Lens Molding

This study developed creative imprinting technology, combining nano-imprint lithography and array-type UV-curing technology. It used nanopowders as the method to transmit imprint force, and integrated technical features, such as soft lithography, light-curing resistant and gas-assisted imprint technology, in order to study the development of technological processes of micro-lens array manufacturing, and mature the application and technology of nano-imprinting. According to research results, SUS 304 stainless steel sheet with a micro-hole array could be smoothly fabricated into an original micro-lens array mold upon gas-assisted micro-hot embossing. At the same time, a micro-lens array structure with a complementary external form could be precisely remolded and reproduced by PDMS. Complete molding of micro-lens could be effectively achieved by combining imprints of gas-assisted lithography developed in this study, and even UV-NIL. The effective imprinting area and reproducibility of transfer printing could be greatly improved when a micro-lens contacts perfectly with a substrate surface. Moreover, since PDMS soft molds have short remolding times, and are easily feathered during manufacturing, production costs could be effectively reduced through features such as, low surface free energy, resistant to adhering to the mold during imprinting, and collocation of gas-assisted nanopowder imprinting of micro-structural processes.