Hydrogel-actuated tunable-focus microlens arrays mimicking compound eyes

We demonstrate enhanced light extraction in organic light-emitting devices (OLEDs) by using microlens arrays fabricated by a soft lithography technique. A large-area and close-packed polystyrene (PS) monolayer was formed on the SiO2 substrate using a convective-capillary assembly method, and a polydimethylsiloxane polymer was used to obtain a concave template, from which microlens arrays were fabricated from a photopolymerizable transparent optical adhesive. The microlens contact angle and array fill factor both depend on the size of PS microspheres, and nearly close-packed, hemispherical microlens arrays with microlens contact angle of (85 ± 5 deg) and array fill factor of (85 ± 3)% were obtained with 100-μm PS microspheres. The enhancement in the light-extraction efficiency in OLEDs when such fabricated microlens arrays were attached to the light-emitting surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large-area (12 × 12 mm) fluorescent OLED with a near close-packed hemispherical microlens array, a maximum enhancement of (70 ± 7)% in the light-extraction efficiency was achieved.

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.

We analyze photometric observations of stars, which experienced microlensing events at the considered time, in order to compare the efficiency of detecting exoplanets in observations performed at thirteen different telescopes and with several approaches to the selection of observable events. In constructing an algorithm of the optimal selection of targets for these observations and in comparing the detection efficiencies for several telescopes, we considered models of the night-sky brightness that satisfy the data of infrared observations carried out in 2011 with the Optical Gravitational Lensing Experiment (OGLE) telescope and the RoboNet telescopes (FTS, FTN, and LT) used to search for planets with the microlensing method. The considered models of the night-sky brightness can be used for various observations (not only microlensing events). The time intervals, during which microlensing events can be observed, were determined with accounting for the positions of the Sun and the Moon and the other constraints on the telescope pointing. Our algorithm allows us to determine the already known microlensing events that are accessible for observation with a particular telescope and to select targets, for which the probability of detecting an exoplanet is maximal. The events, which would maximize the probability of detecting exoplanets, were selected for observations. The probability of detecting an exoplanet is usually proportional to the mirror diameter of a telescope. Telescopes with a wider field of view, such as the OGLE, are more effective in finding new microlensing events. To observe different microlensing events, it is usually better to use different nearby telescopes. However, all such telescopes are often better to use for observing the same event in those relatively short time intervals that correspond to the peak brightness of the event.

A new method, named step simulation, is proposed to fabricate refractive 128/spl times/128 element Si microlens (MLS) arrays for hybrid integration with PtSi IR Focal Plane Arrays (FPAs). By the method, the focal length of rectangle-based multilevel Si MLS arrays is over 1 millimeter, much longer than typical focal length of MLS array fabricated by conventional binary optics Fresnel technique. Si MLS arrays and IR FPAs are coupled with infrared glue. The IR responsivity of the hybrid device is improved significantly.

An artificial neural network and genetic algorithm were used to achieve a high quality microlens array fabrication using a UV proximity printing process in this study. The UV proximity printing process can precisely control the geometric profile of the microlens array in the fabrication process without thermal reflow. The major objective in using the robust design is to reduce the variations in the focal length of the microlens array, allowing improved focus and enhanced illumination brightness. The artificial neural network was used first to characterize the nonlinear relationship between the manufacturing parameters and the properties derived from experimental data. The manufacturing parameters which affect microlens array uniformity include: (1) geometrical ratio, (2) printing gap, (3) spin coating revolution speed, (4) exposure time and (5) developing time. It is very important to control these parameters to decrease the sensitivity to noise. The L18 orthogonal array was used as the learning data for the artificial neural network to construct a system model that could predict the focal length for arbitrary setting of parameters. Then, the genetic algorithm was applied to obtain the robust setting of parameters. The results showed that the microlens array quality could be significantly improved in comparison with the original design.

We propose an experimental method consisting of a standard Fizeau interferometer with transmission sphere (TS) for the determination of the focal length of microlens array (MLA) by spherical wavefronts. The TS is axially translated to produce a spherical wavefront of different curvatures with respect to the MLA. The align mode provision of the interferometer helps to monitor the tilt of the MLA with respect to the spherical wavefront. The focal length is determined from the measured distance of adjacent image spots for various spherical wavefronts at the focal plane of the MLA. Error analysis and experimental demonstration with an off-the-shelf MLA are addressed here.

Compound eye-inspired imaging devices can find vast applications due to their remarkable imaging characteristics, such as extremely large field of view angle, low aberrations, high acuity to motion, and infinite depth of field. Recently, researchers have successfully developed a digital camera that resembles the structure and functions of apposition compound eyes of arthropod, by combining an elastic array of microlenses with a stretchable array of photodetectors in their planar form and then transforming into a hemispherical shape. Designing an elastomeric microlens array that can be mechanically stretched to very large extent without deteriorating the optical performance is critical to this development. In this study, mechanics and optics of the stretchable microlens array, in which each hemispherical microlens sits on top of a supporting post connected to a base membrane, are studied. The results show that proper designs of the microlenses, supporting posts and base membrane are critically important to meet both mechanical and optical requirements simultaneously. This study can have important implications in not only the design of artificial compound eye cameras, but also other developments that require stretchable optical elements.

To compare the different retinal blood flow velocities (BFVs) acquired with different fields of view (FOVs) using the retinal function imager (RFI), twenty eyes of twenty healthy subjects were enrolled in the study. Retinal microvessel BFV in the macula was acquired with both a wide FOV (35 degrees, 7.3 × 7.3 mm2) and a commonly used small FOV (20 degrees, 4.3 × 4.3 mm2). The 35-degree FOV was trimmed to be equivalent to the 20-degree FOV to compare the BFVs of the similar FOVs using different settings. With the 35-degree FOV, both retinal arteriolar and venular BFVs were significantly greater than the 20-degree FOV (P < 0.001). When the 20-degree FOV was compared to the trimmed equivalent 20-degree FOV acquired using the 35-degree FOV, significant BFV differences were found in both the arterioles (P = 0.029) and venules (P < 0.001). This is the first study to compare retinal blood flow velocities acquired with different FOVs using RFI. The conversion factor from 35 degrees to 20 degrees is 0.95 for arteriolar BFV and 0.92 for venular BFV, which may be used for comparing BFVs acquired with different FOVs.

Context.The sensitivity of a radio interferometer is a key figure of merit (FoM) for a radio telescope. The sensitivity of a single polarized interferometer is typically given as an antenna effective area over a system temperature,Ae/Tsys, assuming an unpolarized source. For a dual-polarized polarimetric interferometer intended to observe sources of unknown polarization, the state of polarization must not be assumed a priori. Furthermore, in contrast to the narrow field of view (FoV) of dish-based interferometers, the sensitivity of a polarimetric low-frequency radio interferometer warrants a careful review because of the very wide FoV of the dual-polarized antennas. A revision of this key FoM is particularly needed in the context of the Low-Frequency Square Kilometre Array (SKA-Low) where the sensitivity requirements are currently stated usingAe/Tsysfor a single-polarized antenna system, which produces ambiguity for off-zenith angles.Aims.This paper aims to derive an expression for the sensitivity of a polarimetric radio interferometer that is valid for all-sky observations of arbitrarily polarized sources, with neither a restriction on FoV nor with any a priori assumption regarding the polarization state of the source. We verify the resulting formula with an all-sky observation using the Murchison Widefield Array telescope.Methods.The sensitivity expression was developed from first principles by applying the concept of system equivalent flux density (SEFD) to a polarimetric radio interferometer (not by computingAe/Tsys). The SEFD was calculated from the standard deviation of the noisy flux density estimate for a target source due to system noise.Results.The SEFD for a polarimetric radio interferometer is generally not 1/√2 of a single-polarized interferometer as is often assumed for narrow FoV. This assumption can lead to significant errors for a dual-polarized dipole based system, which is common in low-frequency radio astronomy: up to ∼15% for a zenith angle with a coverage of 45° and up to ∼45% for 60° coverage. The worst case errors occur in the diagonal planes of the dipole for very wide FoV. This is demonstrated through theory, simulation, and observations. Furthermore, using the resulting formulation, the calculation of the off-zenith sensitivity is straightforward and unambiguous.Conclusions.For wide FoV observations pertinent to a low-frequency radio interferometer such as SKA-Low, the narrow FoV and the single-polarized sensitivity expressions are not correct and should be replaced by the formula derived in this paper.

Summary form only given. Parallel optical interconnect systems are becoming extremely important for high-speed optical data communications. A planar microlens (PML) array is a key component for alignment-free systems. In this study, we investigated its optical characteristics for stacked planar optics, which is aligned on micro-optical bench (MOB).

In this paper, a bidirectional optical module based on a stacked planar optical circuit (SPOC) concept to perform a wavelength division multiplexing (WDM) function is described. The module consists of an optical chip which incorporates planar microlens (PML) arrays and nonpolarizing 1.31/1.55 µm WDM dielectric-multilayer filters. Module design and theoretical estimation of loss tolerance were conducted. From preliminary experiments, the feasibility of SPOC modules was confirmed. A fabricated WDM filter showed a small polarization-dependent loss (PDL) of about 0.48 dB and 0.05 dB for the wavelengths of 1.31 µm and 1.55 µm, respectively. Channel crosstalk and PDL for the SPOC module were characterized by means of discrete filter devices. For the 1.55 µm downstream (1.31 µm downstream/upstream) channel, a crosstalk as low as -40 dB (-26 dB) and a PDL less than 0.23 dB (0.53 dB) were achieved.

Lens effect was obtained in an open microfluidic system by using a thin layer of liquid on a polar electric crystal like LiNbO3. An array of liquid micro-lenses was generated by electrowetting effect in pyroelectric periodically poled crystals. Compared to conventional electrowetting devices, the pyroelectric effect allowed to have an electrode-less and circuit-less configuration. An interferometric technique was used to characterize the curvature of the micro-lenses and the corresponding results are presented and discussed. The preliminary results concerning the imaging capability of the micro-lens array are also reported.

Hyperspectral imaging spectrometers with a wide field of view (FoV) have significant application values. However, enhancing the FoV will increase the volume of the imaging spectrometer and reduce the imaging quality, so a wide-FoV spectrometer system is difficult to design. Based on the theory of off-axis astigmatism, we present a method that includes a "prism box," "partial anastigmatism," and a partial differential equation to solve the parameters of a freeform surface. In this method, a compact wide-FoV imaging spectrometer with a freeform surface is designed. The spectrometer is an Offner structure with two curved prisms as the dispersion elements. The primary mirror and tertiary mirror of the Offner spectrometer are an aspheric surface and a freeform surface, respectively, to correct the off-axis aberration of a wide FoV. The ratio of the slit length to the total length of the spectrometer is close to 0.4. In comparison to conventional spectrometers of the same specifications, the total length of the spectrometer is reduced by 40% and the volume by 70%. The compact imaging spectrometer has potential application in the field of space remote sensing. In addition, the design method of the spectrometer provides a reference for the design of other optical systems with freeform surfaces.

As a result of comparative study on human visual and Free Space Optical (FSO) communication, a new concept of satellites laser interconnect was proposed based on human bionics. For the laser link is difficult to set up directly, a new concept was given out based on the three processes of human visual capturing the objectives: voice assisting to guide, eyes' wide Field Of View (FOV) cursory capturing and eyes' foveal FOV high-resolution imaging. Three similar processes were introduced to spaceborne laser interconnect which were radio assisting to guide, beacons laser wide FOV capturing and communication laser narrow FOV capturing, so that the difficulty of setting up the optical communication link was reduced evidently. Meanwhile, the antenna optical system based on the bionics was given out. Each laser terminal configured two independent optical antenna, through the spaceborne storage and time-domain link switching, complex topology network could be set up. The simulation system was built using Virtual Reality Toolbox, the result showed that the laser terminal could search and set up communication links, which took about only 100 ms from capturing the beacon to setting up communication.

Optical design of a high-resolution spectrometer with a wide field of view