Microlens Research Articles

Switchable liquid crystal lenticular microlens arrays based on photopolymerization-induced phase separation for 2D/3D autostereoscopic displays.

Conventionally, the fabrication of liquid crystal lenticular microlens arrays (LCLMLAs) is complicated and costly. Here, we demonstrate a one-step fabrication technique for LCLMLAs, which is prepared through the photopolymerization-induced phase separation in the LC/polymer composite. The LCLMLAs possess both polarization-dependent and electrically tunable focusing properties. Furthermore, we construct a 14-view 2D/3D switchable autostereoscopic display prototype based on a 2D LCD panel and the prepared LCLMLA, which has a viewing angle of 14° and a crosstalk of 46.2% at the optimal viewing zone. The proposed LCLMLAs have the merits of simple fabrication, large-scale production, and low cost.

Investigation on the machinability of polycrystalline ZnSe by elliptical vibration diamond cutting.

Polycrystalline zinc selenide is widely used in advanced optical systems due to its superior optical properties. However, the soft and brittle properties bring a challenge for high-quality surface processing. In recent years, elliptical vibration cutting has been proven as a promising method for machining brittle materials. In the present research, a series of grooving and planning experiments were carried out to investigate the machinability of zinc selenide with elliptical vibration cutting. The removal mechanism was analyzed from fracture characteristics, chip morphology, and phase transformation. The results show that elliptical vibration cutting is effective in suppressing cleavage-induced craters. Reducing the nominal cutting speed is beneficial to inhibit the spring back-induced tearing of grains. A 94-time increase in the critical depth of cut was achieved by vibration trajectory optimization compared to ordinary cutting. Moreover, the influence mechanism of feed on the evolution of surface morphology was revealed. Finally, a zinc selenide microlens array was successfully fabricated. The performance was evaluated by geometric parameter measurements and a multiple imaging test. The findings provide a prospective method for ductile regime machining of zinc selenide.

Experimental validation and mathematical simulation for laser protection performance of light field imaging

Photoelectric imaging systems typically employ a focal plane detector structure, rendering them vulnerable to laser damage. Laser damage can severely impair or even completely deprive the information acquisition capability of photoelectric imaging systems. A laser damage protection method based on a microlens array light field imaging system is proposed to prevent photoelectric imaging systems from laser damage. The technique utilizes the light field modulation effect of the microlens array to homogenize the spot energy, thereby reducing the maximum single-pixel receiving power at the image sensor. The method’s effectiveness has been verified through numerical simulations and experimental validation. First, the laser transmission theoretical model of light field imaging is proposed. Then an experimental setup is established, and measurements are conducted to capture the spot profiles and intensity distributions on the imaging plane across various defocus distances. Finally, the impact of the propagation distance on the maximum single-pixel receiving power and suppression ratio of the light field imaging system is experimentally measured. The simulation and experimental results indicate that, with the proposed method, the energy suppression ratio can easily reach two orders of magnitude, significantly reducing the probability of laser damage in photoelectric imaging systems.

LinoSPAD2: an FPGA-based, hardware-reconfigurable 512×1 single-photon camera system.

We report on LinoSPAD2, a single-photon camera system, comprising a 512×1 single-photon avalanche diode (SPAD) front-end and one or two FPGA-based back-ends. Digital signals generated by the SPADs are processed by the FPGA in real time, whereas the FPGA offers full reconfigurability at a very high level of granularity both in time and space domains. The LinoSPAD2 camera system can process 512 SPADs simultaneously through 256 channels, duplicated on each FPGA-based back-end, with a bank of 64 time-to-digital converters (TDCs) operating at 133 MSa/s, whereas each TDC has a time resolution of 20 ps (LSB). To the best of our knowledge, LinoSPAD2 is the first fully reconfigurable SPAD camera system of large format. The SPAD front-end features a pitch of 26.2 μm, a native fill factor of 25.1%, and a microlens array achieving 2.3× concentration factor. At room temperature, the median dark count rate (DCR) is 80 cps at 7 V excess bias, the peak photon detection probability (PDP) is 53% at 520 nm wavelength, and the single-photon timing resolution (SPTR) is 50 ps FWHM. The instrument response function (IRF) is around 100 ps FWHM at system level. The LinoSPAD2 camera system is suitable for numerous applications, including LiDAR imaging, heralded spectroscopy, compressive Raman sensing, and other computational imaging techniques.

The improvement of bonding metal layers for high resolution micro-LED display application

In this article, a resolution of 15 × 30 blue flip-chip micro-LED array with a pixel size of 20 × 35 μm2 was fabricated. The micro-LED array was hybridized with the silicon backplane via flip-chip bonding technology and liftoff process. The Au/In/Au multilayers could provide lower electrical resistivity and reduce cracks and voids at the film interface, leading to a micro-LED display that had uniform brightness and high reliability. The blue micro-LED display exhibits excellent optical and electrical characteristics, including a low turn-on voltage of 2.8 V and a brightness reach of 1.78 × 107 cd/m2 (nits) at 5 V. Additionally, there appears in the emission spectrum at a voltage of 3 V a blue shift of a peak wavelength from 455.51 to 453.96 nm. Further, the average shear strength of the micro-LED bonded by Au/In/Au multilayers increased to 2.24 g. The presented method has tremendous potential in high resolution micro-LED displays for augmented and virtual reality devices.

Single-crystalline GaN microdisk arrays grown on graphene for flexible micro-LED application

We report the growth of single-crystalline GaN microdisk arrays on graphene and their application in flexible light-emitting diodes (LEDs). Graphene layers were directly grown on c-sapphire substrates using chemical vapor deposition and employed as substrates for GaN growth. Position-controlled GaN microdisks were laterally overgrown on the graphene layers with a micro-patterned SiO2 mask using metal–organic vapor-phase epitaxy. The as-grown GaN microdisks exhibited excellent single crystallinity with a uniform in-plane orientation. Furthermore, we fabricated flexible micro-LEDs by achieving heteroepitaxial growth of n-GaN, In x Ga1−x N/GaN multiple quantum wells, and p-GaN layers on graphene-coated sapphire substrates. The GaN micro-LED arrays were successfully transferred onto bendable substrates and displayed strong blue light emission under room illumination, demonstrating their potential for integration into flexible optoelectronic devices.

Enhancing microstructure and device performance of InGaN quantum dot micro-LEDs through substrate off-cut angle modulation

InGaN quantum dots (QDs) are regarded as a compelling candidate material for the fabrication of high-quality GaN-based micro-LEDs. In this work, to study the impact of a substrate structure on InGaN QDs and QD-based micro-LEDs, GaN-on-sapphire substrates with off-cut angles toward the a-axis of 0.2°, 0.4°, and 0.7° were utilized as templates for the fabrication of InGaN QDs and InGaN QDs-based micro-LEDs. Experimental results show that GaN template with 0.4° off-cut angle exhibits the narrowest terrace width and enables InGaN QDs to be higher and more uniform. The InGaN QD sample grown on 0.4° substrate has a very small wavelength shift of 2.5 nm with temperature increasing and owns the longest photoluminescence peak wavelength implying the highest In content. Furthermore, electroluminescence (EL) spectra demonstrate that QD-based micro-LED array has excellent wavelength stability under various injection currents, and the stability can be improved further on a GaN template with narrower terraces. The results indicate that altering the terrace width of GaN template is a feasible scheme for improving the properties of GaN-based micro-LEDs.

Superhydrophobic Multifocal Microlens Array with Depth-of-Field Detection for a Humid Environment.

Microlens array (MLA) has been widely applied in augmented reality and optical imaging. When used in a humid environment or medical endoscopy, MLA needs to be both superhydrophobic and multifocal. However, it is not easy to achieve both superhydrophobic and multifocal function by integrating superhydrophobic and multifocal structures on the same surface by means of a simple, efficient, and precise method. In this paper, the superhydrophobic multifocal MLA with superhydrophobic properties and multifocal functions is successfully designed for preparation based on a method of 3D lithography and soft lithography. The 3D lithography can further help the preparation of a multifocal MLA with varying apertures and a multistep superhydrophobic structure with a round dome. The superhydrophobic multifocal MLA with periods 50 and 120 μm has perfect superhydrophobic property. The water droplet can slide and bounce off the surface at a roll angle of less than 12.9° with both multifocal and integrated imaging function, as well as up to 397 μm depth-of-field (DOF) detection range; this greatly exceeds the conventional MLA. The perfect superhydrophobic and optical property can be achieved in an extremely humid environment. The superhydrophobic multifocal MLA proposed in this paper has a promising prospect for actual practices.

Ultralow dispersion and broadband gradient refractive index microlens arrays imprinted in chalcohalide glass by microthermal poling

Ultralow dispersion gradient refractive index (GRIN) microlens arrays (MLAs) covering the visible to mid-infrared wavelength range are imprinted in GeS2-Ga2S3-CsI chalcohalide glass by an efficient and economical microthermal poling process. The effect of poling voltage on GRIN microstructure, structural rearrangement, optical properties of the poled glasses, and imaging of the imprinted MLAs have been investigated. Optical focal length of the GRIN MLAs decreases from 353 to 63 μm with increasing the poling voltage from 200 to 1000 V, which is in accordance with imprinting-induced phase difference increase from 0.57λ to 1.19λ. Optimal chromatic aberration (between 520 and 850 nm) as low as 5% is achieved in the GRIN MLAs. Formation mechanism of GRIN MLAs is mainly associated with an enhanced transverse migration of Cs+ ions within mesh square area, which is provided by an improved anode of microthermal poling setup. Thus, an efficient and inexpensive process developed can produce broadband and ultralow dispersion GRIN MLAs for imaging, display, and photovoltaic applications.

Evidence of a sub-solar star in a microlensing event toward the LMC

Gravitational microlensing is known to be an impressive tool for searching dark, small, and compact objects that are missed by the usual astronomical observations. In this paper, by analysing multiple images acquired by DECam, we present the detection and a complete description of the microlensing event LMC J05074558-65574990 which is most likely due to a sub-solar object with mass (0.16±0.10) M⊙, hence in the mass range between a massive brown dwarf and a red dwarf, whose distance is estimated to be 7.8−3.4+4.1×102 pc thanks to the Gaia observation of the source, leading us to consider this lens as one the closest ever detected.

Maskless fabrication of honeycomb random microlens array by a femtosecond laser

A rapid and maskless method is proposed to fabricate a honeycomb random microlens array (rMLA) on silica glasses surface. The micro-crater array is ablated by a femtosecond laser using a point-by-point multiple scanning process. By subsequent acid etching, the rMLAs with different arrangements, diameters and depths can be easily obtained. The relationships between the processing parameters and the morphologies of the rMLAs were analyzed in detail. We designed and successfully fabricated the hexagonal-arranged rMLA and square-arranged rMLA by using this method. The homogenizing experiments have demonstrated that the fabricated rMLAs can break the periodicity of the traditional periodic microlens array (pMLA) and improve the uniformity of the high-coherence laser homogenized spot. It exhibited excellently that two types of the rMLAs had good homogenizing property (more than 86 %) and high energy utilization rate (more than 87 %). The proposed technology might provide an efficient and new method to simply fabricate large-area close-packed rMLAs on silica glass.

Compact OWC Receiver: Micro-Lens and PD Array on Glass Interposer

In this paper, we propose a compact optical wireless receiver based on micro-lens and photodiode array flip-chipped on a glass interposer. With simulation models and experiments, we verify the performance of two optical wireless communication receivers assembled based on this structure with gigabit and high-speed photodiodes. First, co-integrated gigabit photodiodes and micro-lenses indicate that micro-lenses can deliver a 3.5 dB gain of light collection efficiency to the gigabit photodiode array while keeping the field of view larger than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 18 degrees. Besides, a 5.5 Gbps maximum throughput has been verified by this optical wireless communication receiver with discrete multitone modulation. Moreover, co-integrated high-speed photodiodes and micro-lenses prove that micro-lenses realize a 26.6 dB improvement in light collection efficiency, enabling a 10 Gbps error-free connection using as little as -0.5 dBm transmission power. Those results conclude that the co-integration of micro-lenses on the photodiode array can enhance the light collection efficiency of the photodiode array while keeping a larger field of view, providing a high-performance, low-cost solution for the receiver of optical wireless links.

Modelling Optical Invisibility in a Lenticular Microlens array with methods of Geometrical Optics and Weber's Contrast

Though many advances have been made in exploring the ‘cloaking’ capabilities of metamaterials and optical systems in a wide range of the light spectrum, e.g. electromagnetic cloaking or paraxial ray optics, the phenomenon of unidirectional cloaking associated with ‘woodpile’ optical structures has not yet been widely discussed in the scope beyond experimental findings or digital integral cloaking; moreover, the relatively low cost of a lenticular array renders the investigation of its concealment properties in the visible regime of great interest for practical purposes. Thus, the following research paper provides an comprehensive theoretical model and analytical criterion for the phenomenon of unidirectional cloaking in the visible regime associated with a lenticular microlens array within the methods of computational ray optics (computing lens’s spherical aberration using governing ray optics equations) and Weber’s contrast (as perceived by the human eye) by considering the concealed object’s virtual image as seen through a lenticular lens. Having shown that the criterion for practical invisibility adheres to our experimental results in determining the boundary case of invisibility, we employed it to explore the effect system parameters have on the array’s cloaking capabilities. We established that the critical parameter for the array’s cloaking capability is the diverging angle of an array’s lenticular lens, the value of which positively correlates with the refraction index and the lens concave segment’s curvature. Therefore, a lenticular array with most prominent cloaking capabilities consists of lenses with the greatest admissible index of refraction and a arc measure for the lens’s concave segment.

Concave microlens arrays with tunable curvature for enhanced photodegradation of organic pollutants in water: A non‐contact approach

AbstractSolar‐driven photodegradation for water treatment faces challenges such as low energy conversion rates, high maintenance costs, and over‐sensitivity to the environment. In this study, we develop reusable concave microlens arrays (MLAs) for more efficient solar photodegradation by optimizing light distribution. Concave MLAs with the base radius of μm are fabricated by imprinting convex MLAs to polydimethylsiloxane elastomers. Concave MLAs possess a non‐contact reactor configuration, preventing MLAs from detaching or being contaminated. By precisely controlling the solvent exchange, concave MLAs are fabricated with well‐defined curvature and adjustable volume on femtoliter scale. The focusing effects of MLAs are examined, and good agreement is presented between experiments and simulations. The photodegradation efficiency of organic pollutants in water is significantly enhanced by 5.1‐fold, attributed to higher intensity at focal points of concave MLAs. Furthermore, enhanced photodegradation by concave MLAs is demonstrated under low light irradiation, applicable to real river water and highly turbid water.image

Development of Shape Prediction Model of Microlens Fabricated via Diffuser-Assisted Photolithography.

The fabrication of microlens arrays (MLAs) using diffuser-assisted photolithography (DPL) has garnered substantial recent interest owing to the exceptional capabilities of DPL in adjusting the size and shape, achieving high fill factors, enhancing productivity, and ensuring excellent reproducibility. The inherent unpredictability of light interactions within the diffuser poses challenges in accurately forecasting the final shape and dimensions of microlenses in the DPL process. Herein, we introduce a comprehensive theoretical model to forecast microlens shapes in response to varying exposure doses within a DPL framework. We establish a robust MLA fabrication method aligned with conventional DPL techniques to enable precise shape modulation. By calibrating the exposure doses meticulously, we generate diverse MLA configurations, each with a distinct shape and size. Subsequently, by utilizing the experimentally acquired data encompassing parameters such as height, radius of curvature, and angles, we develop highly precise theoretical prediction models, achieving R-squared values exceeding 95%. The subsequent validation of our model encompasses the accurate prediction of microlens shapes under specific exposure doses. The verification results exhibit average error rates of approximately 2.328%, 7.45%, and 3.16% for the height, radius of curvature, and contact angle models, respectively, all of which were well below the 10% threshold.

Systematic reanalysis of KMTNet microlensing events, paper I: Updates of the photometry pipeline and a new planet candidate

ABSTRACT In this work, we update and develop algorithms for KMTNet tender-love care (TLC) photometry in order to create a new, mostly automated, TLC pipeline. We then start a project to systematically apply the new TLC pipeline to the historic KMTNet microlensing events, and search for buried planetary signals. We report the discovery of such a planet candidate in the microlensing event MOA-2019-BLG-421/KMT-2019-BLG-2991. The anomalous signal can be explained by either a planet around the lens star or the orbital motion of the source star. For the planetary interpretation, despite many degenerate solutions, the planet is most likely to be a Jovian planet orbiting an M or K dwarf, which is a typical microlensing planet. The discovery proves that the project can indeed increase the sensitivity of historic events and find previously undiscovered signals.

Autostereoscopic 3D Measurement Based on Adaptive Focus Volume Aggregation

Autostereoscopic three-dimensional measuring systems are a kind of portable and fast precision metrology instrument. The systems are based on integral imaging that makes use of a micro-lens array before an image sensor to observe measured parts from multiple perspectives. Since autostereoscopic measuring systems can obtain longitudinal and lateral information within single snapshots rapidly, the three-dimensional profiles of the measured parts can be reconstructed by shape from focus. In general, the reconstruction process consists of data acquisition, pre-processing, digital refocusing, focus measures, and depth estimation. The accuracy of depth estimation is determined by the focus volume generated by focus measure operators which could be sensitive to the noise during digital refocusing. Without prior knowledge and surface information, directly estimated depth maps usually contain severe noise and incorrect representation of continuous surfaces. To eliminate the effects of refocusing noise and take advantage of traditional focus measure methods with robustness, an adaptive focus volume aggregation method based on convolutional neural networks is presented to optimize the focus volume for more accurate depth estimation. Since a large amount of data and ground truth are costly to acquire for model convergence, backpropagation is performed for every sample under an unsupervised strategy. The training strategy makes use of a smoothness constraint and an identical distribution constraint that restricts the difference between the distribution of the network output and the distribution of ideal depth estimation. Experimental results show that the proposed adaptive aggregation method significantly reduces the noise during depth estimation and retains more accurate surface profiles. As a result, the autostereoscopic measuring system can directly recover surface profiles from raw data without any prior information.

Parallel Phase-Shifting Digital Holographic Phase Imaging of Micro-Optical Elements with a Polarization Camera

In this paper, we propose a measurement method of micro-optical elements with parallel phase-shifting digital holographic phase imaging. This method can record four phase-shifting holograms with a phase difference of π/2 in a single shot and correct the pixel mismatch error of the polarization camera using a bilinear interpolation algorithm, thereby producing high-resolution four-step phase-shifting holograms. This method reconstructs the real phase information of the object to be measured through a four-step phase-shifting algorithm. The reproduced image eliminates the interference of zero-order images and conjugate images, overcoming the problem that traditional phase-shifting digital holography cannot be measured in real time. A simulation analysis showed that the relative error of this measurement method could reach 0.0051%. The accurate surface topography information of the object was reconstructed from an experimental measurement through a microlens array. Multiple measurements yielded a mean absolute error and a mean relative error for the vertical height of the microlens array down to 5.9500 nm and 0.0461%, respectively.

Exploring the galactic center through gravitational microlensing: Prospects and possibilities with next-generation infrared telescopes

We aim to present a solution to the lack of observational support to our theoretical model of the Supermassive Black Hole (SMBH) and the Galactic Center (GC). To do so, we revisit the abundance of microlensing events produced by the SMBH at the GC as a fixed lens, based on the methods given by Alexander &amp; Loeb 2001 (AL01). By applying updated observational constraints for the distribution of stars within a few arcseconds of the SMBH, we estimate the number of lensing events of distant background sources by the SMBH alone or by it and secondary stellar lenses that lie within the GC. We find our new results to be generally consistent with AL01. We predict that in any snapshot of the central ~1 region taken with a modern detection threshold of 27-28 mag, ~10 microlensed background sources will be amplified for more than 500% in brightness. As more potential microlensing events in the GC are being identified by K-band surveys with a much higher precision than previous speckled observations, we would be able to test our predictions and offer validations on the theoretical models of GC and the SMBH.

Compact object probed by microlensing in the universe: MACHOs, stars, planets?

Nearly 30% of the components in the universe are dark matter, hitherto, astronomers are still uncertain about their properties. This project attempts to constrain whether dark matter is MACHOs or diffused particles via statistics of microlensing events observed toward quasars, which are the brightest objects in the Universe. The identification of isolated microlensing events enables us to study stellar and planetary objects in distant universes that would be otherwise unobservable. During the research two isolated microlensing events toward quasar J1821 is discovered in the first 25 quasars in our sample. The WISE light curves in W1 and W2 bands are almost identical in magnification, consistent with achromatic variability due to microlensing. The crossing times for the two events are about 2 days and 1.3 days, thus the lenses are probably a star or sub-stellar object. By assuming the lenses are in galaxies of the galactic cluster with a redshift of 0.81 lying in our sight of view to the quasar, calculations of the mass of the lenses as a function of its velocity can be made, yielding a planetary mass or less in the relative velocity dominated by the movements of galaxies. If the superluminous motion of jets in a quasar dominates the relative velocity, stellar masses are derived. The data analysis yielded from the microlensing light curves of J1821 suggests microlensing as a method for astronomers to further study stellar and sub-stellar objects in our Universe.