Diameter Of Microlens Research Articles

Femtosecond laser writing of infrared microlens arrays on chalcogenide glass

Sulfur-based glasses have gradually developed into a new generation of infrared optical materials with high transmittance in the mid-and long-infrared wavelengths. As2Se3, as a representative chalcogenide glass in the mid-infrared band, is commonly prepared for some infrared optical devices. In this paper, we prepare infrared microlens arrays with an area of 0.64 mm2 by femtosecond laser technology on As2Se3. In terms of morphology, the surface of the prepared microlens array has excellent morphology and smoothness. The uniform IR microlens array achieves high transparency of up to 60 % in the mid-infrared region. And there is no significant change in transmittance before and after laser processing, which has outstanding application prospects. In terms of the size of the lens, we can control the diameter and depth of microlens by adjusting the laser power and the number of pulses. In infrared imaging, the lateral magnification of the lens can be adjusted by controlling the focal length of the lens so that the size and clarity of the letter “F” can be controlled. The cross-sectional intensity distribution of the focusing spots was calculated, which demonstrates the superior uniformity of our microlenses.

Microfabrication of Microlens by Timed-Development-and-Thermal-Reflow (TDTR) Process for Projection Lithography.

This paper presents a microlens fabrication process using the timed-development-and-thermal-reflow process, which can fabricate various types of aperture geometry with a parabolic profile on a single substrate in the same batch of the process. By controlling the development time of the uncrosslinked negative photoresist, a state of partial development of the photoresist is achieved, called the timed development process. The thermal reflow process is followed after the timed development, which allows the photoresist to regain its liquid state to form a smooth meniscus trench surrounded by a crosslinked photoresist sidewall. Microlens with larger aperture size forms deeper trench with constant development time. With constant aperture size, longer developing time shows deeper meniscus trench. The depth of the meniscus trench is modeled in the relationship of the development time and aperture size. Other characteristics for the microlens including the radius of curvature, focal length, and the parabolic surface profile are modeled in the relationship of the microlens thickness and diameter. Microlens with circular, square, and hexagonal bases have been successfully fabricated and demonstrated where each geometry of the lens-bases shows different fill factors of the lens arrays. To test the fabricated lenses, a miniaturized projection lithography scheme was proposed. A centimeter-scale photomask pattern was photo-reduced using the fabricated microlens array with a ratio of 133, where the smallest linewidth was measured as 2.6 µm.

Surface curvature measurement of microlenses using a white-light interference microscope and fast geometric fit algorithm

We present a technique for measuring the microlens radius of curvature (ROC). The technique is based on the principle of three-dimensional (3-D) surface topography using a white light interference microscope. By measuring 3-D point clouds of the surface combined with fitting the measured data to an ideal sphere, the radius of the surface is determined. We take advantage of the fast speed of the maximum intensity method and the anti-interference capability of the fitting method of fringe analysis techniques in white-light interferometry to measure the 3-D topography of the microlens array surface. To measure each microlens’s ROC, we use the fast geometric fit algorithm for sphere where the input data are the spherical surface point cloud. We have built a white light interference microscope and experimentally measured the geometric parameter and the surface curvature of some microlens of a commercial microlens array. Our measurement results of the ROC, the surface height, and the diameter of microlens showed a good agreement with the results obtained using the Alpha-Step D-500 stylus profiler and the values reported by the manufacturer, indicating the applicability of this technique.

Control of diameter and numerical aperture of microlens by a single ultra-short laser pulse.

We demonstrate a versatile method for fast and flexible fabrication of either one or an array of microlenses. Multi-foci axial intensity distribution generated by a phase pattern displayed on a spatial light modulator irradiates silica, causing ablation and its internal modification. The following wet etching step defines the diameter r, while the radius of curvature R (hence, the focal length f) is maintained the same. As a result, the numerical aperture NA=r/f changes from 0.2 to 0.4 for the same pulse energy (but different number of multi-foci) during ablation. An isotropic wet etching of silica becomes highly anisotropic for the initial stages of etching following the irradiated pattern. Subsequent evolution of the shape is governed by an isotropic silica etch and forms a spherical lens. This method can be extended to other materials and geometries of micro-optical elements.

Fabrication of Hierarchical Micro/Nano Compound Eyes.

Fabrication of a hierarchical macro-/micro-/nano compound eye is presented in this paper. This bioinspired compound (BIC) eye is obtained by an integrated manufacturing technology that combines (i) nanoimprinting, (ii) picosecond laser swelling, and (iii) air-assisted deformation. The diameter and height of nanopillars, microlens, and macrobase can be controlled precisely by fine-tuning the process parameters. The multifunctional properties of the BIC eye, such as superhydrophobicity, antireflection, and other optical characteristics, are investigated. It is found that the microlens with nanopillars can effectively improve the surface wettability with a contact angle of 152° and contact angle hysteresis of 12°, and enhance transmittance by 2% over the wavelength range of 200-1200 nm. Moreover, the final hierarchical compound eye exhibits the excellent imaging properties and a wide field-of-view of 120° without distortion. These multifunctional properties will enable the widespread application of the compound eye in diverse real-time environmental conditions.

Finite-Difference Time-Domain Simulation of Fractal-Like Microlens Arrays for High Outcoupling Efficiency of Organic Light-Emitting Diodes.

The outcoupling efficiencies (OCEs) of organic light-emitting diodes (OLEDs) were studied for a fractal-like two-dimensional structure consisting of three layers of semicircular microlens on a glass substrate using a finite-difference time-domain (FDTD) method. The OCE with only one semicircular microlens layer was 29.5%, 1.75 times larger than that of the basic OLED. Additional layers with smaller diameters on the first layer did not improve the OCE. The OCE remained constant or slightly decreased with the increase of the number of layers. Two possible origins of this result were suggested; first, the possibility that the escaped light enters the nearby microlens becomes higher with the introduction of an additional protruded layer; second, the Mie scattering effect becomes important with the decrease of the diameter of the semicircular microlens from 20 μm to 0.8 μm. An additional FDTD simulation was performed for the OLED with only one microlens array as a function of the diameter. The OCE decreased approximately monotonously with the decrease of the diameter from 20 μm to 0.2 μm. In particular, the OCE became lower than that of the basic OLED when the diameter decreased from 0.5 μm to 0.2 μm. This is consistent with the observation that smaller fractal-like structures on the large microlens array did not further enhance the OCE.

Analysis of the focusing crosstalk effects of broadband all-dielectric planar metasurface microlens arrays for ultra-compact optical device applications

Microlens arrays have been widely used for different optoelectronic applications. The demand for compact optical devices necessitates the deployment of even smaller microlens arrays; however, as the spacing between individual lenses reduces and the lens diameter approaches the length scale of the incident wavelength of light, diffraction starts playing a critical role and produces a significant impact on the final focusing properties of the optical field. In this paper, we analyze the focusing characteristics of all-dielectric ultra-compact metasurface lens arrays for efficient optical device applications, constructed by kinds of broadband planar lenses composed of subwavelength nano-scatterers. By using the 3D finite-difference time-domain (FDTD) method, focusing and diffraction-based crosstalk effects caused by the changing physical spacing between adjacent metalenses, the diameter of microlenses, the operating wavelength, and the array size are rigorously investigated. Analysis of the achieved results show that a larger spacing, a larger lens size, and a shorter wavelength can lead to a weaker focusing crosstalk effect. Moreover, the crosstalk effect does not have a significant dependence on the array’s overall size. This research study may provide an important technological reference to designing an array of all-dielectric planar metasurface lenses with a well-controlled focusing performance and may pave the way further toward the application of metasurface lens arrays in compact optical sensing, coupling, and detecting system designs.

Effect of various microlens parameters on enhancement of light outcoupling efficiency of organic light emitting diode

Since organic light emitting diode (OLED) is a multilayer device where each layer has different refractive index, total internal reflection (TIR) plays an important role in limiting the efficiency of an OLED. Due to the presence of TIR, a major portion of light is trapped within the device in various wave guiding modes. Of the total light trapped in an OLED, we address only the part that is lost due to wave guiding mode arising from refractive index mismatch at the glass-air interface. Microlens array, to improve luminance, is a method that can be externally applied to the OLEDs without altering its electrical characteristics and is easy to use. Microlens arrays ranging from 10 to 40 μm have been fabricated using an organic elastomeric material polydimethylsiloxane (PDMS) by mold transfer technique. Maximum improvement of 25% in outcoupling efficiency for blue OLED is reported upon using the microlens array with diameter 10 μm. For a given diameter of microlens, out-coupling efficiency of OLED increases as height to diameter (H/D) ratio of microlens array approaches 0.5 (perfect hemisphere). It is also observed that outcoupling efficiency increases with the diameter of microlens for a given H/D ratio. The best luminescence improvement was observed for blue OLED, which can be explained by the higher refractive index of PDMS at lower wavelengths.

Broadband Metallic Planar Microlenses in an Array: the Focusing Coupling Effect.

The microlens arrays (MLAs) are widely utilized for various applications. However, when the lens size and the spacing between two adjacent microlenses are of the length scale of the working wavelength, the diffraction effect plays a vital role in the final focusing performance. We suggest a kind of broadband metallic planar microlenses, based on which the ultra-compact microlens arrays are also constructed. The focusing coupling effect revealing for such devices is then investigated in detail by using the finite-difference time-domain (FDTD) method, with the emphasis on the changing spacing between adjacent microlenses, the working wavelength, the diameter of microlenses, and the array size. The results show that a larger spacing, a larger lens size, a shorter wavelength, or a smaller array scale can lead to a weaker focusing coupling effect. This research provides an important technological reference to design an array of metallic planar microlenses with the well-controlled focusing performance.

Lens sag and diameter measurement of large-size microlenses using sub-pixel algorithm and optical interferometry

Abstract In this paper, an automatic optical inspection system is designed specifically to measure the diameter and lens sag of large-size microlenses: 1. The proposed algorithm of measuring lens diameter locates the lens center through the Euclidean distance array, and determines the lens edge along an initiated ray using linear interpolation with sub-pixel accuracy. 2. The lens sag is calculated from a single fringe pattern of large-size microlens, in combination with the measured lens diameter. 3. According to the experiment results, the proposed system has advantages of high applicability, rapid processing speed, and good accuracy with the RMS error≤1% of measuring a large-size microlens, but without the requirement of prior training. The system architecture of non-contact measurement would not cause scratches on the lens surface and is inexpensive, thus, which is particularly suitable for the in-line inspection of industry field.

Photoresist thermal reflow 방법을 이용하여 제작한 마이크로렌즈 어레이의 형상 관련 오차 및 이에 대한 보정

Microlens array as basic element of the optical system have been fabricated with various focal length (mainly with long focal length) depending on the purpose of application. In this paper, the microlens arrays were fabricated for observing fluorescent images within sol-gel. Though the fluorescent signal is very low, the microlens array can help obtaining clear images through extracting the fluorescent light from sol-gel. We fabricated microlens arrays with short focal length, which can extract the light using photoresist thermal reflow method. In the experiment, the diameter of microlens decreased after thermal reflow because the solvent within the photoresist was vaporized. Therefore, to compensate the shape error by this reduction, microlens diameter in photomask was altered and spin-coat recipe of photoresist were modified.

Micro lens design for efficiency improvement of red organic light-emitting diode

We have proposed a micro lens to improve the luminance of red organic light-emitting devices (ROLEDs). The micro lenses were applied on the glass/indium tin oxide (ITO)/OLED. The size, thickness and diameter of micro lenses were calculated by using FDTD (finite-difference timedomain) method. Simulations were performed for 5 µm and 10 µm sized. The thickness and the gap of the micro lens were both 1 µm. The material of the micro lenses was silicon dioxide. The highest luminance of an OLED applied with a micro lens was 11,185 cd/m2, at on approval voltage of 14.5 V, The efficiency of the device with a micro lens increased by 3 times compared to that of the device with no micro lens.

Tunable microlens arrays actuated by various thermo-responsive hydrogel structures

We report on liquid-based tunable-focus microlens arrays made of a flexible polydimethylsiloxane (PDMS) polymer. Each microlens in the array is formed through an immiscible liquid–liquid interfacial meniscus. Here deionized water and silicone oil were used. The liquids were constrained in the PDMS structures fabricated through liquid-phase photopolymerization for molding and soft lithography. The microlenses were actuated by thermo-responsive N-isopropylacrylamide (NIPAAm) hydrogel microstructures and could be tuned individually by changing the local temperature. The NIPAAm hydrogels expanded and contracted, absorbing and releasing water, at different temperatures. Thus the pressure across the water–oil interface in the microlenses varied responding to the temperature, tuning their corresponding focal lengths. The microlens diameter was 2.4 mm. The typical microlens focal length was measured to be from 8 to 60 mm depending on the temperature. The microlens response time actuated by different structures and components of the NIPAAm hydrogels were compared. The normalized light intensities of the microlens focused spots were measured, matching well with a Zemax simulation, to study the microlens spherical aberrations. The NIPAAm hydrogel durability was also measured.

Replication and comparison of concave and convex microlens arrays of light guide plate for liquid crystal display in injection molding

AbstractA back light unit (BLU) is a key module of a thin‐film transistor liquid crystal display (TFT‐LCD), frequently utilized in various mobile displays. In this study, we experimentally characterize transcription and optical properties of concave and convex microlens arrays (MLAs) of light guide plate (LGP) fabricated by injection molding with polycarbonate as a LGP substrate material. Nickel mold inserts were manufactured by electroforming on the MLA, which was fabricated by the thermal reflow of photoresist microstructures patterned by UV‐photolithography. For the case of convex microlens, the height of replicated microlens was less than that of the mold insert while maintaining almost the same microlens diameter of the mold insert as the location of the microlens is far from the gate. In contrast, for the concave microlens, the diameter of replicated microlens was larger than that of mold insert, while showing almost the same microlens height as the mold insert. From the optical examination of replicated convex and concave MLAs, it was found that a higher luminance of the LGP was achieved by the concave MLAs compared to the convex MLAs with the same number density of microlenses (about 30% enhancement in this case) due to the utilization of a larger amount of light provided by the light sources. POLYM. ENG. SCI., 50:1696–1704, 2010. © 2010 Society of Plastics Engineers

A high numerical aperture, polymer-based, planar microlens array

We present a novel microfabrication approach for obtaining arrays of planar, polymer-based microlenses of high numerical aperture. The proposed microlenses arrays consist of deformable, elastomeric membranes that are supported by polymer-filled microchambers. Each membrane/microchamber assembly is converted into a solid microlens when the supporting UV-curable polymer is pressurized and cured. By modifying the microlens diameter (40-60 microm) and curing pressure (7.5-30 psi), we demonstrated that it is possible to fabricate microlenses with a wide range of effective focal lengths (100-400 microm) and numerical apertures (0.05-0.3). We obtained a maximum numerical aperture of 0.3 and transverse resolution of 2.8 microm for 60 microm diameter microlenses cured at 30 psi. These values were found to be in agreement with values obtained from opto-mechanical simulations. We envision the use of these high numerical microlenses arrays in optical applications where light collection efficiency is important.

Microlens array fabrication by enhanced thermal reflow process: Towards efficient collection of fluorescence light from microarrays

This paper describes two enhanced resist reflow methods for the fabrication of microlens arrays and demonstrates their use for integrated biomolecular fluorescence detection on printed microarrays. A PDMS (polydimethylsiloxane) microlens array was fabricated by a double soft lithography approach using a photoresist microlens array as master mold. Additionally, by using both a careful control of the surface wettability and thermal treatments, we demonstrate the possibility to extend the resist reflow process in order to tune the diameters of microlens array over a large range by using a unique photomask pattern. We introduce an enhanced reflow on hydrophobic surfaces obtained by fluorosilane treatment and identify a threshold shrinkage temperature (T shrinkage) of 140 °C, above which the diameter of microlenses can be then reduced down to 40% compared with the initial pattern on the photomask. Furthermore, on hydrophilic substrates, achieved by an accurate incomplete development of the photoresist, we demonstrate a nearly perfect linear dependency (1.4 μm/°C) of microlens diameter spreading up to 70% the initial diameter inside a temperature reflow window of 110–140 °C. For both approaches, above a freezing temperature (T freezing) of 170 °C, the microlens profile characteristics are temperature independent. By using high numerical aperture microlens array, we provide a proof of concept for the integration and enhanced light collection of the fluorescent signals collected form a microarray of fluorescent spots thus showing the potential of the concept for biophotonic integration.

Focusing performance of the closed-boundary cylindrical microlenses analyzed by the boundary element method

In this paper, we investigate the focusing performance of closed-boundary cylindrical microlenses (CBCMs) based on rigorous electromagnetic theory and the boundary element method. The CBCMs with different incident angles, different quantization-level numbers, different microlens diameters, different f-numbers, and different polarizations of incidence are studied. Several focusing performance measures, such as the focal spot size, the diffraction efficiency, the real focal position, and the normalized transmitted power, are presented. It provides very useful information in designing the CBCMs in micro-optical systems.

Wavefront reconstruction with the adaptive Shack–Hartmann sensor

In a Shack–Hartmann sensor the wavefront to be measured is sampled by a microlens array. The beams are directed to a spot in the focal plane. The displacement of the spots is a measure for the local gradient of the wavefront over one microlens. In the adaptive Shack–Hartmann sensor the static microlens array of the conventional sensor is replaced by a dynamic liquid crystal display (LCD). The LCD is used to display an array of Fresnel microlenses. Thereby, the Fresnel microlenses can be adapted to the wavefront and the design parameters like focal length, aperture size, position and number of lenses can be changed very quickly. We present an investigation about the wavefront reconstruction with the adaptive Shack–Hartmann sensor. Because of the large pixelsize of the LCD in comparison to static manufactured holograms the microlens diameter of the adaptive Shack–Hartmann sensor is larger than in a conventional sensor. This requires considerations in the reconstruction algorithm to minimize reconstruction errors. We compare Zernike and B-Spline wavefront reconstruction on the basis of fit accuracy, reconstruction time and the effect of missing measurement data and noise. We come to the conclusion that Zernike reconstruction is better for fitting simple wavefronts while B-Spline reconstruction is the better choice for more complex wavefronts.

A new microlens array fabrication method using UV proximity printing

This paper presents a new microlens array fabrication method that controls the printing gap in the UV lithography process. This method can precisely control the microlens array geometric profile array in the fabrication process without thermal reflow. The proximity printing bends the UV light away from the aperture edges and produces a certain exposure in the photoresist outside the aperture edges due to the diffraction effects. This causes the photoresist bottom in two adjacent patterns to link together after development. The fabricated microlens diameter has the same size as the pitch distance between the two apertures. For example, a 120 µm aperture pitch distance will generate a 120 µm diameter microlens. The proximity gap between the mask and photoresist should be two times the aperture pitch distance or greater to generate an accurate microlens array. The experimental results showed that photoresist microlens arrays could be formed automatically without using the thermal reflow process when the printing gap ranged from 240 µm to 840 µm. A hemispherical microlens array with a 120 µm diameter and various geometric profiles can be fabricated using this method.

Performance of an integrated microoptical system for fluorescence detection in microfluidic systems.

This article presents a new integrated microfluidic/microoptic device designed for basic biochemical analysis. The microfluidic network is wet-etched in a Borofloat 33 (Pyrex) glass wafer and sealed by means of a second wafer. Unlike other similar microfluidic systems, elements of the detection system are realized with the help of microfabrication techniques and directly deposited on both sides of the microchemical chip. The detection system is composed of the combination of refractive circular or elliptical microlens arrays and chromium aperture arrays. The microfluidic channels are 60 microm wide and 25 microm deep. The elliptical microlenses have a major axis of 400 microm and a minor axis of 350 microm. The circular microlens diameters range from 280 microm to 350 microm. The apertures deposited on the outer chip surfaces are etched in a 3000-A-thick chromium layer. The overall thickness of this microchemical system is < 1.6 mm. A limit of detection of 3.3 nM for a Cy5 solution in phosphate buffer (pH 7.4) was demonstrated. The cross-talk signal measured between two adjacent microchannels with 1 mm pitch was < 1:5600, meaning that < or = 1.8 x 10(-4)% of the fluorescence light power emitted from one microchannel filled with a 50 microM Cy5 solution reaches the photodetector at the adjacent microchannel. This performance compares very well with that obtainable in microchemical chips using confocal fluorescence systems, taking differences in parameters, such as excitation power into microchannels, data acquisition rates, and signal filtering into account.