Fabrication of large area resin microlens arrays using gas-assisted ultraviolet embossing

Asymmetrical interface design for unidirectional light extraction from spectrum conversion films

In this study, we propose a micro-sized photonic structure that extracts 89% of the intrinsic trapped photons from the spectrum conversion film into free space using the Monte-Carlo ray-tracing method. Furthermore, the spectrum of the spectral-shifting film can be accurately simulated based on a mean free path concept, providing the estimation of its overall performance including the external quantum efficiency and the self-absorption efficiency. The simulations show that the spectrum conversion film with micro-structures shows a two-fold increase in the total external quantum efficiency and a four-fold increase in the external quantum efficiency in the forward viewing direction compared to the planar spectrum conversion films without micro-structures.

Fabrication of Double-Sided Micro-Lens Array Using UV Injection Molding

This paper reports the fabrication of double-sided micro-lens arrays using a UV injection molding process. The apparatus for UV injection molding is designed and implemented. Because the double-sided micro-lens array is fabricated using a UV-curable resin, the molds must be transparent. An alignment system is also required to align the cavities. In preparing the transparent molds, the cavities for the concave micro-lens and the alignment marks are first machined on the aluminum block. Using electroforming and hot embossing, transparent polycarbonate (PC) molds that have cavities for the MLAs and the alignment marks are fabricated. The PC molds are mounted on the UV injection molding apparatus using a pneumatic clamp. A real-time optical alignment system that comprises a CCD and an X-Y table is used to align the marks in the upper and lower molds. After alignment, the UV-curable resin is injected into the molds using a pneumatic dispenser. When the resin is cured with UV light, a double-sided micro-lens array is fabricated. The cycle time is 45 s. The respective degrees of replication for a convex and a concave micro-lens array are 99.74 % and 99.00 %. The respective standard deviation values for the diameter and the height are 1.3 μm, and 1.5 μm. The optical properties of the double-sided micro-lens array are measured. The average effective focal length is 1.686 mm, with a standard deviation of 0.007 mm, which demonstrates good formability and uniformity. Using the fabricated micro-lens array, the 1.2 mm diameter of the original light source is reduced to a 50 μm spot diameter. The images of an “A” pattern are complete and clear. This study demonstrates that a UV injection molding process that uses transparent PC molds can be used to fabricate a micro-lens array and other double-sided microstructures.

Light extraction enhancement and directional control of scintillator by using microlens arrays

The total internal reflection restricts light extraction efficiency of scintillator, leading to reduced detection efficiency and signal-to-noise ratio in the field of scintillator-based radiation detection system. This research presents the method of applying microlens arrays to improve the light extraction efficiency as well as achieve directional control of emission for scintillators. For BGO (Bi₄Ge₃O₁₂) scintillator covered with PMMA (polymethyl-methacrylate) hemispherical microlens array, the 2.59-fold in particular angle (θ_em = 45°) and overall 1.94-fold angle-integrated enhancement ratios have been obtained. Furthermore, we analyze and optimize some parameters of microlens arrays such as the packing arrangement, duty ratio, size, refractive index, and shape. As a result, when the refractive index of microlens is slightly larger than that of scintillator, a maximum 6.23-fold angle-integrated enhancements can be achieved. It can be concluded that the microlens array covered on scintillator has considerable value for practical applications on radiation detection.

Simple method based on intensity measurements for characterization of aberrations from micro-optical components

We report a simple method, based on intensity measurements, for the characterization of the wavefront and aberrations produced by micro-optical focusing elements. This method employs the setup presented earlier in [Opt. Express 22, 13202 (2014)] for measurements of the 3D point spread function, on which a basic phase-retrieval algorithm is applied. This combination allows for retrieval of the wavefront generated by the micro-optical element and, in addition, quantification of the optical aberrations through the wavefront decomposition with Zernike polynomials. The optical setup requires only an in-motion imaging system. The technique, adapted for the optimization of micro-optical component fabrication, is demonstrated by characterizing a planoconvex microlens.

A simple method for quality evaluation of micro-optical components based on 3D IPSF measurement

This paper presents a simple method based on the measurement of the 3D intensity point spread function for the quality evaluation of high numerical aperture micro-optical components. The different slices of the focal volume are imaged thanks to a microscope objective and a standard camera. Depending on the optical architecture, it allows characterizing both transmissive and reflective components, for which either the imaging part or the component itself are moved along the optical axis, respectively. This method can be used to measure focal length, Strehl ratio, resolution and overall wavefront RMS and to estimate optical aberrations. The measurement setup and its implementation are detailed and its advantages are demonstrated with micro-ball lenses and micro-mirrors. This intuitive method is adapted for optimization of micro-optical components fabrication processes, especially because heavy equipments and/or data analysis are not required.

Development of a Non-uniform Heating System for Micro Hot Embossing

In recent years, many replication methods of microstructures on large-area substrates have been reported. The plate-to-plate hot embossing process is a low cost, fast, high throughput and the most possibility of mass production method for replication of micro/nano structures with high precision and high quality. However, micro hot embossing is not practical to use in mass production due to the many difficulties. This paper reports a innovative non-uniform heating hot embossing system for fabrication of large-size, high precise and high brightness light guide plate. Steam is used as heating medium, and water of 12°C is used for cooling. The v-groove patterns in the stamper have been replicated to the light guide plates (LGPs) with an area of 488 mm by 277 mm. The experimental results show that the non-uniform heating system could achieve a transcription rate of over 99%, compared with 95% using conventional uniform heating system. Furthermore, less warpage, the less thickness variation, the higher average luminance and the higher brightness field distribution have been found. The results indicate that non-uniform heating system is likely to be a promising technique for the fabrication of optical elements.

Three dimensional microfluidics with embedded microball lenses for parallel and high throughput multicolor fluorescence detection

We report a 3D microfluidic device with 32 detection channels and 64 sheath flow channels and embedded microball lens array for high throughput multicolor fluorescence detection. A throughput of 358 400 cells/s has been accomplished. This device is realized by utilizing solid immersion micro ball lens arrays for high sensitivity and parallel fluorescence detection. High refractive index micro ball lenses (n = 2.1) are embedded underneath PDMS channels close to cell detection zones in channels. This design permits patterning high N.A. micro ball lenses in a compact fashion for parallel fluorescence detection on a small footprint device. This device also utilizes 3D microfluidic fabrication to address fluid routing issues in two-dimensional parallel sheath focusing and allows simultaneous pumping of 32 sample channels and 64 sheath flow channels with only two inlets.

A facile "air-molding" method for nanofabrication

In this letter, we demonstrate a spherical nanocavities fabrication using an "air-molding" method, which is implemented by modulating the pressure difference across air-liquid interfaces in nanoholes on the mold. The cavities formation is theoretically considered and experimentally verified at macroscale first, and then a series of experiments are performed over a patterned surface with sub-300 nm holes by varying the pressure difference by sending a PDMS prepolymer coated mold into a vacuum chamber with changeable pressure. Results show that the air-molding method for spherical cavities fabrication is feasible not only at macroscale, but also at the nanoscale when introducing a pressure difference across the air-liquid interface. And the cavities shape is easily controlled by modulating the pressure in the vacuum chamber. The spherical cavities in this paper have application potential in the optical field and in micro- and nanofluidics.

Fabrication of spherical microlenses by a combination of isotropic wet etching of silicon and molding techniques

We report a novel process technology of hemispherical shaped microlenses, using isotropic wet etching of silicon in an acid solution to produce the microlenses molds. Governed by process parameters such as temperature and etchant concentration, the isotropic wet etching is controlled to minimize various defects that appear during the molding creation. From the molds, microlenses are fabricated in polymer by conventional replication techniques such as hot embossing and UV-molding. The characterization of molds and measurements of replicated microlenses demonstrate high smoothness of the surfaces, excellent repeatability of mold fabrication and good optical properties. Using the proposed method, a wide range of lens geometries and lens arrays can be achieved.

Hemicylindrical and toroidal liquid microlens formed by pyro-electro-wetting

We found that by opportune functionalization of a polar dielectric substrate, a self-arrangement of hemicylindrical or toroidal-shaped liquid droplets can be obtained. The process takes place when a thermal stimulus is provided to a poled substrate whose surface is covered by an oily substance layer. Liquid droplet self-arrangement is due to the pyroelectric effect, and interferometric characterization of the droplets is also reported. We investigated this open microfluidic system for exploring the possibility to obtain liquid cylindrical microlens with variable focal length. Liquid microtoroidal structures arrays are also realized. They could find application as resonant liquid microcavities for whispering gallery modes.

Tunable liquid microlens arrays in electrode-less configuration and their accurate characterization by interference microscopy

A special class of tunable liquid microlenses is presented here. The microlenses are generated by an electrowetting effect under an electrode-less configuration and they exhibit two different regimes that are named here as separated lens regime (SLR) and wave-like lens regime (WLR). The lens effect is induced by the pyroelectricity of polar dielectric crystals, as was proved in principle in a previous work by the same authors (S. Grilli et al., Opt. Express 16, 8084, 2008). Compared to that work, the improvements to the experimental set-up and procedure allow to reveal the two lens regimes which exhibit different optical properties. A digital holography technique is used to reconstruct the transmitted wavefront during focusing and a focal length variation in the millimetre range is observed. The tunability of such microlenses could be of great interest to the field of micro-optics thanks to the possibility to achieve focus tuning without moving parts and thus favouring the miniaturization of the optical systems.

Large-area and thin light guide plates fabricated using UV-based imprinting

As the demand of larger and thinner flat panel display increasing, conventional methods such as injection molding and hot embossing to fabricate light guide plates (LGPs) become difficult and unsuitable. This study reports a low-cost and high-throughput method to fabricate large-size (320 mm x 240 mm, 15" in diagonal) LGPs by using UV-based imprinting process. With the UV-based imprinting process, a large-size LGP with thickness down to 0.8 mm has been successfully fabricated. The optical property of fabricated LGP has been verified. This study has demonstrated the fabrication of large-size and thin LGPs by using UV-based imprinting process, and the possibility of UV-based imprinting process for fabricating other large thin optical elements.