Maskless fabrication of concave microlens arrays on silica glasses by a femtosecond-laser-enhanced local wet etching method

Controllable Fabrication of Highly Ordered Spherical Microcavity Arrays by Replica Molding of In Situ Self-Emulsified Droplets

Ordered spherical hollow micro- and nanostructures hold great appeal in the fields of cell biology and optics. However, it is extremely challenging for standard lithography techniques to achieve spherical micro-/nanocavities. In this paper, we describe a simple, cost-effective, and scalable approach to fabricate highly ordered spherical microcavity arrays by replica molding of in situ self-emulsified droplets. The in situ self-emulsion involves a two-step process: discontinuous dewetting-induced liquid partition and interfacial tension-driven liquid spherical transformation. Subsequent replica molding of the droplets creates spherical microcavity arrays. The shapes and sizes of the microcavities can be easily modulated by varying the compositions of the droplet templates or utilizing an osmotically driven water permeation. To demonstrate the utility of this method, we employed it to create a spherical microwell array for the mass production of embryoid bodies with high viability and minimal loss. In addition, we also demonstrated the optical functions of the generated spherical microcavities by using them as microlenses. We believe that our proposed method will open exciting avenues in fields ranging from regenerative medicine and microchemistry to optical applications.

Miniature bioinspired artificial compound eyes: microfabrication technologies, photodetection and applications

As an outstanding visual system for insects and crustaceans to cope with the challenges of survival, compound eye has many unique advantages, such as wide field of view, rapid response, infinite depth of field, low aberration and fast motion capture. However, the complex composition of their optical systems also presents significant challenges for manufacturing. With the continuous development of advanced materials, complex 3D manufacturing technologies and flexible electronic detectors, various ingenious and sophisticated compound eye imaging systems have been developed. This paper provides a comprehensive review on the microfabrication technologies, photoelectric detection and functional applications of miniature artificial compound eyes. Firstly, a brief introduction to the types and structural composition of compound eyes in the natural world is provided. Secondly, the 3D forming manufacturing techniques for miniature compound eyes are discussed. Subsequently, some photodetection technologies for miniature curved compound eye imaging are introduced. Lastly, with reference to the existing prototypes of functional applications for miniature compound eyes, the future development of compound eyes is prospected.

Single femtosecond pulse writing of a bifocal lens

In this Letter, a method for the fabrication of bifocal lenses is presented by combining surface ablation and bulk modification in a single laser exposure followed by the wet etching processing step. The intensity of a single femtosecond laser pulse was modulated axially into two foci with a designed computer-generated hologram (CGH). Such pulse simultaneously induced an ablation region on the surface and a modified volume inside the fused silica. After etching in hydrofluoric acid (HF), the two exposed regions evolved into a bifocal lens. The area ratio (diameter) of the two lenses can be flexibly adjusted via control of the pulse energy distribution through the CGH. Besides, bifocal lenses with a center offset as well as convex lenses were obtained by a replication technique. This method simplifies the fabrication of micro-optical elements and opens a highly efficient and simple pathway for complex optical surfaces and integrated imaging systems.

An Ultraviolet-Lithography-Assisted Sintering Method for Glass Microlens Array Fabrication

Glass microlens arrays (MLAs) have tremendous prospects in the fields of optical communication, sensing and high-sensitivity imaging for their excellent optical properties, high mechanical robustness and physicochemical stability. So far, glass MLAs are primarily fabricated using femtosecond laser modification assisted etching, in which the preparation procedure is time-consuming, with each concave-shaped microlens being processed using a femtosecond laser point by point. In this paper, a new method is proposed for implementing large-scale glass MLAs using glass particle sintering with the assistance of ultraviolet (UV) lithography. The glass particles are dispersed into the photoresist at first, and then immobilized as large-scaled micropillar arrays on quartz glass substrate using UV lithographing. Subsequently, the solidified photoresist is debinded and the glass particles are melted by means of sintering. By controlling the sintering conditions, the convex microlens will be self-assembled, attributed to the surface tension of the molten glass particles. Finally, MLAs with different focal lengths (0.12 to 0.2 mm) are successfully fabricated by utilizing different lithography masks. Meanwhile, we also present the optimization of the sintering parameter for eliminating the bubbles in the microlenses. The main factors that affect the focal length of the microlens and the image performance of the MLAs have been studied in detail.

Microheater-Integrated Microlens Array for Robust Rapid Fog Removal

In extreme environments, fog formation on a microlens array (MLA) surface results in a device failure. One reliable solution for fog removal is to heat the surface using a microheater. However, due to the surface interference, the combination of these two microdevices remains elusive. In this study, we introduce lift-off and electroless plating into femtosecond laser processing to fabricate a microheater integrated MLA (μH-MLA) on the same substrate with high light transmittance, durability, and fog removal efficiency. Laser-induced micro-nano grooves enable the microheater to be tightly coupled with the MLA and have high heating performance, thus maintaining a stable performance for over 24 h during continuous operation as well as under long time ultrasonic vibration and mechanical friction. With a rapid response time (τ0.5) of 17 s and a high working temperature of 188 °C, the μH-MLA removed fog that covers the entire face within 14 s. Finally, we prove the use of this fabrication method in large areas and curved surface environments. This study provides a flexible, stable, and economical method to integrate micro-optical and microelectrical devices.

Polarisation Control in Arrays of Microlenses and Gratings: Performance in Visible-IR Spectral Ranges

Microlens arrays (MLAs) which are increasingly popular micro-optical elements in compact integrated optical systems were fabricated using a femtosecond direct laser write (fs-DLW) technique in the low-shrinkage SZ2080^TM photoresist. High-fidelity definition of 3D surfaces on IR transparent CaF₂ substrates allowed to achieve ∼50% transmittance in the chemical fingerprinting spectral region 2-5 μm wavelengths since MLAs were only ∼10 μm high corresponding to the numerical aperture of 0.3 (the lens height is comparable with the IR wavelength). To combine diffractive and refractive capabilities in miniaturised optical setup, a graphene oxide (GO) grating acting as a linear polariser was also fabricated by fs-DLW by ablation of a 1 μm-thick GO thin film. Such an ultra-thin GO polariser can be integrated with the fabricated MLA to add dispersion control at the focal plane. Pairs of MLAs and GO polarisers were characterised throughout the visible-IR spectral window and numerical modelling was used to simulate their performance. A good match between the experimental results of MLA focusing and simulations was achieved.

Femtosecond laser hybrid processing strategy of transparent hard and brittle materials

With high hardness, high thermal stability, chemical inertness and excellent optoelectronic properties, transparent hard and brittle materials have drawn significant attentions in frontier domains such as aerospace, photoelectric detection, and high-intensity lasers. Femtosecond laser processing technology demonstrates great potential for transparent hard and brittle materials processing due to its outstanding advantages such as non-contact, true 3D processing and programmable design. However, high-energy laser ablation usually causes severe damage to the surface of the materials, resulting in low processing accuracy, low processing efficiency and poor surface quality. Femtosecond laser hybrid processing strategies have been proven to be an effective solution to solve the above problems. This mini-review summarizes the fundamentals and research progress of femtosecond laser hybrid processing strategies of transparent hard and brittle materials in recent years. Moreover, the challenges and application prospects of these techniques are discussed.

Rapid fabrication of large-area concave microlens arrays on silica glasses by femtosecond laser bursts

An efficient and flexible method using femtosecond laser bursts assisted by wet etching is presented to fabricate large-area high-quality microlens arrays (MLAs) on a silica glass surface. In this method, femtosecond laser bursts can ablate micro craters on silica glass in a fast, single-step process by controlling the electron density and a high-speed scanning galvanometer, and the influence mechanism of the number of pulses within a burst on the accuracy and quality of micro craters is analyzed in detail. The experimental results show that the preparation efficiency of micro craters is significantly improved to approximately 32,700 per second. By subsequent acid etching, concave microlenses with controllable dimensions, shapes, and alignments are easily obtained. A large area close-packed hexagonal concave MLA is successfully fabricated by using this method and shows high surface quality and uniformity, which excellently demonstrates the feasibility and flexibility of rapidly fabricating MLAs in the burst regime.

Stable Nonwetting Artificial Compound Eye with Low Adhesion

Microlens arrays (MLAs) are the key components of miniaturized optical systems. To meet the stringent requirements for their application in humid environments, achieving waterproof properties in these objects is an urgent task. It is noteworthy that conventional methods of microlens production usually sacrifice optical performance for stable superhydrophobicity by increasing the surface roughness of the microlens. In this paper, a large area artificial compound eye (ACE) is efficiently fabricated by combining photolithography and inkjet printing. The added micropillars separated the outside droplet from the microlens, and the water droplet was afterward suspended on the top of micropillars. Furthermore, the micropillars enabled superhydrophobicity (at a contact angle above 150°) and low surface adhesion (at a sliding angle of ∼2.8°) of the microlens without affecting its optical performance. Furthermore, when released from the height of 1 and 2 cm, the droplets were fully detached from the surface without sticking. The surface of the ACE was shown to have relatively stable nonwettability due to a small spacing between the micropillars. This means that tuning the morphology and spacing between micropillars allows one to noticeably improve the surface nonwettability stability. Finally, the performance of the fabricated optical system was demonstrated in a water washing experiment. Therefore, the findings of present study may open up the prospects for significant advancement in superhydrophobicity of the optical systems without affecting their imaging performance for real outdoor applications.

Hybrid laser precision engineering of transparent hard materials: challenges, solutions and applications

Laser has been demonstrated to be a mature and versatile tool that presents great flexibility and applicability for the precision engineering of a wide range of materials over other established micromachining techniques. Past decades have witnessed its rapid development and extensive applications ranging from scientific researches to industrial manufacturing. Transparent hard materials remain several major technical challenges for conventional laser processing techniques due to their high hardness, great brittleness, and low optical absorption. A variety of hybrid laser processing technologies, such as laser-induced plasma-assisted ablation, laser-induced backside wet etching, and etching assisted laser micromachining, have been developed to overcome these barriers by introducing additional medium assistance or combining different process steps. This article reviews the basic principles and characteristics of these hybrid technologies. How these technologies are used to precisely process transparent hard materials and their recent advancements are introduced. These hybrid technologies show remarkable benefits in terms of efficiency, accuracy, and quality for the fabrication of microstructures and functional devices on the surface of or inside the transparent hard substrates, thus enabling widespread applications in the fields of microelectronics, bio-medicine, photonics, and microfluidics. A summary and outlook of the hybrid laser technologies are also highlighted.

Superhydrophobic Artificial Compound Eye with High Transparency

Natural compound eyes have inspired the development of self-cleaning, waterproof, and antifog optical devices. However, the traditional methods generally sacrifice the transparency of optical units to introduce hydrophobicity, which significantly limits the practical applications of state-of-the-art hydrophobic technologies. This work aims to fabricate a microimaging system by combining photolithography, inkjet printing, and chemical growth. Herein, an artificial compound eye (ACE) is endowed with stable superhydrophobicity and high transparency without affecting its optical performance. The obtained ACE system possesses good static and dynamic dewetting properties along with excellent optical performance. Its static contact angle exceeds 160°, whereas the sliding angle and contact angle hysteresis values are ∼5.5° and ∼3.8°, respectively. Furthermore, the contact time is found to be 11.97 s for the Weber number of 12. The droplet undergoes a reversible process during compressing and stretching, and the ACE exhibits no adhesion under a pressure load of 4 mN. This proves that the introduction of nonwetting nanohairs on the sidewalls of the microcone arrays significantly improves the dynamic dewetting of the system. More importantly, the properly designed position of nanohairs ensures that the optical performance of ACE is maintained at a level of ∼95% compared to that of the bare glass. The superhydrophobic ACE exhibits low adhesion and great transparency. This rationally designed ACE may provide useful guidelines for fabrication of superhydrophobic optical devices with high transparency and enable potential applications in military, medical, and some outdoor activity fields.

Artificial Compound Eye Systems and Their Application: A Review

The natural compound eye system has many outstanding properties, such as a more compact size, wider-angle view, better capacity to detect moving objects, and higher sensitivity to light intensity, compared to that of a single-aperture vision system. Thanks to the development of micro- and nano-fabrication techniques, many artificial compound eye imaging systems have been studied and fabricated to inherit fascinating optical features of the natural compound eye. This paper provides a review of artificial compound eye imaging systems. This review begins by introducing the principle of the natural compound eye, and then, the analysis of two types of artificial compound eye systems. We equally present the applications of the artificial compound eye imaging systems. Finally, we suggest our outlooks about the artificial compound eye imaging system.

Microlenses arrays: Fabrication, materials, and applications

Microlenses have become an indispensable optical element in many optical systems. The advancement of technology has led to a wider variety of microlenses fabrication methods, but these methods suffer from, more or less, some limitations. In this article, we review the manufacturing technology of microlenses from the direct and indirect perspectives. First, we present several fabrication methods and their advantages and disadvantages are discussed. Then, we discuss the commonly used materials for fabricating microlenses and the applications of microlenses in various fields. Finally, we point out the prospects for the future development of microlenses and their fabrication methods.

Fast and Accurate Light Field View Synthesis by Optimizing Input View Selection

There is a trade-off between spatial resolution and angular resolution limits in light field applications; various targeted algorithms have been proposed to enhance angular resolution while ensuring high spatial resolution simultaneously, which is also called view synthesis. Among them, depth estimation-based methods can use only four corner views to reconstruct a novel view at an arbitrary location. However, depth estimation is a time-consuming process, and the quality of the reconstructed novel view is not only related to the number of the input views, but also the location of the input views. In this paper, we explore the relationship between different input view selections with the angular super-resolution reconstruction results. Different numbers and positions of input views are selected to compare the speed of super-resolution reconstruction and the quality of novel views. Experimental results show that the speed of the algorithm decreases with the increase of the input views for each novel view, and the quality of the novel view decreases with the increase of the distance from the input views. After comparison using two input views in the same line to reconstruct the novel views between them, fast and accurate light field view synthesis is achieved.

Fabrication of microlenses with continuously variable numerical aperture through a temporally shaped femtosecond laser

We developed a novel method for fabricating microlenses and microlens arrays by controlling numerical aperture (NA) through temporally shaped femtosecond laser on fused silica. The modification area was controlled through the pulse delay of temporally shaped femtosecond laser. The final radius and sag height were obtained through subsequent hydrofluoric acid etching. Electron density was controlled by the temporally shaped femtosecond laser, and the maximum NA value (0.65) of a microlens was obtained in the relevant studies with femtosecond laser fabrication. Furthermore, the NA can be continuously adjusted from 0.1 to 0.65 by this method. Compared with the traditional methods, this method exhibited high flexibility and yielded microlenses with various NAs and microlens arrays to meet the different demands for microlens applications.

Designable Ultratransparent and Superhydrophobic Surface of Embedded Artificial Compound Eye with Extremely Low Adhesion

Real-world implementation of artificial compound eye (ACE) has been limited by its poor transparency and high requirement for the stable Cassie state. In general, the improvement of surface dewetting performance sacrifices the transparency of ACE. Herein, ACE was obtained by an integrated manufacturing technology that combined photolithography, microprinting, and chemical growth. Through skillful manipulation of the fabrication process, dewetting hairs were fabricated on the top of micropillars and around the microlens. The combination of nanohairs and micropillars resulted in outstanding superhydrophobicity (∼170°), pristine lotus effect with low sliding angle (∼1°), and contact angle hysteresis (∼2°). Moreover, the surface showed almost no adhesion under a preload of 4 mN, exhibiting excellent stable Cassie state and antiadhesion performance. Furthermore, dynamic impact showed that the impacting droplet was quickly detached from the surface (contact time ∼14.1 ms) without sticking for We = 60. The designed transparency resulted in high performance of optical unit (∼99%, bare glass for comparison). Moreover, ACE exhibited better focusing and imaging capability under larger aperture diameter than microlens without nanohairs. We envision that this research presents a significant advancement in imparting superhydrophobicity and transparency to a so-far inapplicable family of optical devices for many practical outdoor applications.

Fabrication and evaluation of negative axicons for ultrashort pulsed laser applications

We report on the fabrication and evaluation of a sharp tip negative axicon paving the way for applications in high-power ultrashort pulsed laser systems. The negative axicon is manufactured by applying a two-step all laser-based process chain consisting of ultrashort pulsed laser ablation and CO₂ laser polishing finishing the component in less than 5 minutes. The finalized negative axicon reveals a surface roughness of 18 nm, fulfilling optical quality. Two measurement setups, including the ultrashort pulsed laser itself, are used to evaluate the formation of Bessel beams in detail. By applying a focusing lens behind the negative axicon, well-developed Bessel beams are generated while their lengths depend on the distance between the negative axicon and the lens. Furthermore, the diameter of the Bessel beams increase strongly with the propagation distance. By adding a second focusing lens, Bessel beams are generated at its focal position, being almost invariant of its position. Hence, the typical Bessel beam intensity distribution is observed over an entire moving range of this second lens of 300 mm. While these Bessel beams show superior quality in terms of sharp peaks with homogeneous concentric rings, only minor deviations in intensity and diameter are observed over the moving range.

Manufacturing of a microlens array mold by a two-step method combining microindentation and precision polishing

A novel two-step method for manufacturing microlens array molds by combining microindentation and precision polishing is proposed. Compared with conventional manufacturing methods, such as single-point diamond turning, this two-step method, as an alternative method, presents great advantages on cost and flexibility on spherical microlens array mold fabrication. Various curvatures of radii and arrangements for microlens array molds can be fabricated in the same way. In this paper, a hexagonal microlens array with 1.58 mm curvature radius was demonstrated to prove the feasibility of the proposed method. First, a large number of precise steel balls were organized in hexagonal arrangement and pressed into the mold's surface to generate multiple microdimples. Second, the pileups around the microdimples were removed from the mold surface by precision polishing. The geometrical accuracy and surface quality were investigated by an optical surface profiler. The measurement indicated that, compared with the initial surface, the surface inside the dimple had significantly higher hardness and better surface quality than that of the steel balls. Then the microlens array on the mold was further replicated to poly(methyl methacrylate) substrates by a precision compression molding process. The experimental results showed that the fabricated mold and the polymer replicas have high fidelity, great uniformity, and good surface roughness. The proposed two-step, low-cost mold fabrication method can produce highly uniform microlens arrays and is therefore suitable for high-volume fabrication of precise optical elements such as integrated light-emitting diodes and other similar micro-optics.

Naked-Eye 3D Display Based on Microlens Array Using Combined Micro-Nano Imprint and UV Offset Printing Methods

An optical film integrating microlens array (MLAs) and 3D micro-graphics is an important way to achieve the naked-eye 3D display effect. The 3D micro-graphics is traditionally generated by the micro-nano imprint technology based on precision engraving mold, which leads to high production cost and low production efficiency, and thus restricts the rapid response to production tasks and large-scale popularization and application. In this study, a process scheme for large-scale printing of 3D micro-graphics using UV offset printing based on presensitized (PS) plate was proposed, matching with the MLAs fabricated by micro-nano imprint process to achieve naked-eye 3D display effect. We used the laser confocal microscope to systematically measure and analyze the geometric and optical performance of the fabricated MLAs in terms of height, curvature radius, center distance, spacing, focal length, and numerical aperture, and evaluated the influence of the publishing resolution of the PS plate on the display effect of 3D micro-graphics. The printing quality and display effect of 3D micro-graphics were further improved by adjusting process parameters such as printing speed and printing pressure. The results of the current study demonstrate that the combined application of micro-nano imprint technology based on precision mold and UV offset printing technology based on PS plate can achieve an excellent naked-eye 3D display effect in 360° all angles, which is efficient, cost-saving, and highly flexible.

Biologically inspired artificial eyes and photonics

Natural visual systems have inspired scientists and engineers to mimic their intriguing features for the development of advanced photonic devices that can provide better solutions than conventional ones. Among various kinds of natural eyes, researchers have had intensive interest in mammal eyes and compound eyes due to their advantages in optical properties such as focal length tunability, high-resolution imaging, light intensity modulation, wide field of view, high light sensitivity, and efficient light management. A variety of different approaches in the broad field of science and technology have been tried and succeeded to duplicate the functions of natural eyes and develop bioinspired photonic devices for various applications. In this review, we present a comprehensive overview of bioinspired artificial eyes and photonic devices that mimic functions of natural eyes. After we briefly introduce visual systems in nature, we discuss optical components inspired by the mammal eyes, including tunable lenses actuated with different mechanisms, curved image sensors with low aberration, and light intensity modulators. Next, compound eye inspired photonic devices are presented, such as microlenses and micromirror arrays, imaging sensor arrays on curved surfaces, self-written waveguides with microlens arrays, and antireflective nanostructures (ARS). Subsequently, compound eyes with focal length tunability, photosensitivity enhancers, and polarization imaging sensors are described.

Cross-scale additive direct-writing fabrication of micro/nano lens arrays by electrohydrodynamic jet printing

High-quality micro/nanolens arrays (M/NLAs) are becoming irreplaceable components of various compact and miniaturized optical systems and functional devices. There is urgent requirement for a low-cost, high-efficiency, and high-precision technique to manufacture high-quality M/NLAs to meet their diverse and personalized applications. In this paper, we report the one-step maskless fabrication of M/NLAs via electrohydrodynamic jet (E-jet) printing. In order to get the best morphological parameters of M/NLAs, we adopted the stable cone-jet printing mode with optimized parameters instead of the micro dripping mode. The optical parameters of M/NLAs were analyzed and optimized, and they were influenced by the E-jet printing parameters, the wettability of the substrate, and the viscosity of the UV-curable adhesive. Thus, diverse and customized M/NLAs were obtained. Herein, we realized the fabrication of nanolens with a minimum diameter of 120 nm, and NLAs with different parameters were printed on a silicon substrate, a cantilever of atomic force microscopy probe, and single-layer graphene.

Fabrication of Artificial Compound Eye with Controllable Field of View and Improved Imaging

Many arthropods have compound eyes, which are made up of numerous separate visual units (microlenses) or ommatidia. These natural compound eyes have exceptional optical properties such as wide field of view (FOV), low aberration, and fast motion tracking capability. In this paper, a large-scale artificial compound eye (ACE) is fabricated efficiently using a combination of inkjet printing and air-assisted deformation processes. Both size and geometry of the microlens are controlled via superposed drops on the substrate. The simulation results show that the light intensity of the ACE follows a systematic distribution for tilted incident light, which represents a significant improvement, compared to planar distributed microlenses. We then manufacture ACEs with different heights and diameters, and their FOVs are compared with the theoretically predicted results. The measured FOV was 50°-140°. The acceptance angles for the different ACEs are determined, and their relationship with the ratio of height to radius (H/r) of the microlens is investigated in more detail. Furthermore, the imaging properties of the microlenses with different angles of incidences are studied, which suggest a FOV up to 140° and an acceptance angle of about 50°. The microlens captures images even at an angle of incidence of about 60°. The corresponding distortion in both the x and y directions is also investigated. Our findings provide guidelines for the development and fabrication of ACEs with large FOVs and acceptance angles, which may find applications in military, robotics, medical imaging, and astronomy.

CO2 laser thermal reflow shaped convex glass microlens array after Bessel picosecond laser inscribing and hydrofluoric acid processing

In this paper, a convex micro-glass lens array fabrication process that utilizes ${{\rm CO}_2}$CO₂ laser thermal reflow in the Bessel picosecond laser inscribing and hydrofluoric acid processed micro-glass pillars array is presented. The Bessel picosecond laser permits high tolerance and precise micro-pillar fabrication. In the thermal reshape process, the ${{\rm CO}_2}$CO₂ laser power, relative defocus length, and scanning velocity are three crucial parameters to the microlens array's focal length. By using this method, microlens arrays with focal length ranging from several tens of micrometers to several hundred micrometers can be created. This research provides another way to fabricate convex micro-glass lens arrays with several hundred micrometers focal length in good utility.

Artificial compound eye-tipped optical fiber for wide field illumination

In this Letter, we present a novel, to the best of our knowledge, component with beam delivering and wide field beam homogenizing functions by grafting an artificial compound eye (ACE) micro-structure onto the polymer optical fiber (POF) end face. The 3D ACE mold is fabricated by femtosecond laser-assisted micro machining, and the ACE micro-structure is transferred onto the end face through high accuracy nano-imprinting. The resultant POF end face integrates over 400 spherical micro-lenses, enabling a 40% enhancement in both the acceptance angle and the effective numerical aperture. Meanwhile, the integrated ommatidia array serves as an outstanding beam homogenizer, shaping the output beam into quasi flat-top distribution, which demonstrates promise in wide field homogeneous illumination, by reflection and transmission imaging experiments in both visible and near infrared bands.

Underwater superoleophobic and anti-oil microlens array prepared by combing femtosecond laser wet etching and direct writing techniques

As an important micro-optical device, microlens array (MLA) also has broad applications in aqueous environment apart from atmosphere, such as bioscience research, ocean exploration, and microfluidic systems. However, the surface of the normal MLA is easily polluted by oil contaminations when the MLA is practically applied in a water medium, leading to the loss of its optical imaging ability. Herein, we fabricated a functional MLA with underwater anti-oil and self-cleaning abilities by combining the femtosecond laser wet etching (FLWE) and the femtosecond laser direct writing (FLDW) techniques. The as-prepared close-packed MLA is composed of 10000 single microlenses with the aperture diameter of 50 µm. The surface of each microlens is further textured with micro/nanoparticles. Clear and uniform images could be captured by using the resultant MLA in water, demonstrating great underwater imaging ability. The modulation transfer function value is larger than 0.6 at 55 lp/mm. In addition, the micro/nanostructures endow the as-fabricated MLA surface with underwater superoleophobicity and oil-repellent performance. Various oils can be repelled by the resultant MLA in water. Underwater 1,2-dichloroethane oil droplet on the textured MLA has a contact angle of 158.0 ± 0.5° and a sliding angle of 2.0 ± 0.2°. The underwater superoleophobic MLA also has good mechanical durability. The anti-oil and self-cleaning functions will broaden the applications of the MLA in ocean exploration, bioscience research, microfluidic system, and many underwater MLA-based systems.

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.

Replication of high refractive index glass microlens array by imprinting in conjunction with laser assisted rapid surface heating for high resolution confocal microscopy imaging

In a multi optical probe confocal imaging system utilizing a microlens arrays as an objective lens, a high numerical aperture is required to improve resolving power. Glass microlens arrays are suitable for high-resolution imaging since they provide outstanding optical properties with a high refractive index. We demonstrated the rapid fabrication of microlens arrays on a high refractive index optical glass substrate via laser assisted thermal imprinting. The optical performance of the fabricated glass microlens arrays were evaluated and compared to that of a polymer microlens. In contrast to the polymer, the real image afforded by, and the calculated resolution of, the imprinted glass microlens arrays were significantly better, at about 0.73 µm compared to the polymer (∼1.56 µm). Our results reveal the considerable potential of direct thermal imprinting as a rapid, single-step, low cost fabrication method for replication of glass microlens array of high dimensional accuracy affording excellent optical performance.

Low-cost high integration IR polymer microlens array

In this Letter, a low-cost refractive convex microlens array device based on infrared a polymer is fabricated by a nanoimprinting technique. The device integrates more than 4000 microlenslets within a footprint of 10  mm×10  mm. The surface quality, spectral transmittance, imaging resolution, and surface damage threshold of the device have been fully characterized. The IR imaging and parallel laser inscription experiments confirm the remarkable optical performance of the fabricated device. Owing to the merits of high optical quality, low fluence lose, and simple fabrication, this device is promising in cutting-edge IR applications, such as IR imaging, laser fabrication, and so on.

Wet-etching-assisted femtosecond laser holographic processing of a sapphire concave microlens array

We report rapid and mask-free fabrication of a sapphire concave microlens array by a combined method of femtosecond laser holographic processing and wet etching. The method features high fabrication efficiency, as crater arrays can be created on sapphire through a parallel processing manner, and the subsequent wet etching facilitates the formation of microlens arrays with a smooth surface. More importantly, the size and spacing of the concave microlenses can be well tuned by varying the distance of craters and etching time. Two types of microlens arrays with a spacing of 25 and 40 μm have been successfully fabricated, both of which showed good imaging performance. This method holds great promise for developing sapphire-based micro-optical components.

Polymer-based diffractive optical elements for rear end automotive applications: design and fabrication process

Advances in illumination technology in the automotive industry are heading toward the use of coherent sources for adaptable and high-resolution head lamps as well as interior and rear-end lights. We present a cost-effective method to fabricate and design laser-based lighting systems for the rear end in automobiles and other vehicles. The design relies on using binary gratings to generate a desired intensity distribution. For cost-effective fabrication, an optical maskless UV lithography system that employs a spatial light modulator (SLM) for projection of the desired structure onto photoresist is introduced. To replicate the structures onto polymers like poly(methyl methacrylate) (PMMA), we use polydimethylsiloxane soft stamps and a hot embossing system. The experimental results show that the proposed design and fabrication process is promising for high-resolution rear-end lights that might be employed to project symbols or information to guide road users in future.

Design and Fabrication of a Three-Dimensional Artificial Compound Eye Using Two-Photon Polymerization

Microlens arrays have been widely used in the fields of micro-optics because of the advantages of their high diffraction efficiency, high fill factor, and wide operating band. However, the microlens array still has problems with its smaller field of view (FOV) and lower utilization of light energy. In this paper, a 3D compound eye system consisting of a microlens array and a pinhole array was designed according to the optical principle of insect compound eye. The artificial compound eye structure was processed in two-photon polymerization processing technology. Ray tracing and optical system simulation of the designed artificial compound eye structure were performed. The results showed that the artificial compound eye structure had a wider FOV and higher light energy utilization than a conventional 2D microlens array. This thesis may lay a theoretical foundation for the structural optimization design of microlens arrays.

Replication of a glass microlens array using a vitreous carbon mold

A low-cost fabrication method for a high-surface-quality glass microlens array (MLA) was proposed using a glass molding technique with a vitreous carbon (VC) mold. A VC mold with a high-surface-quality MLA cavity was fabricated, and the glass MLA with a root mean square surface roughness of 4.59 nm was replicated using the VC mold. To obtain the glass MLA with high replication quality, the effects of molding conditions were examined. The surface quality was not degraded during the proposed VC mold fabrication method and glass molding process. The focused beam spot of the glass molded MLA was analyzed; it showed a diffraction-limited characteristic of the glass molded MLA.

Single-pulse writing of a concave microlens array

This work developed a method of femtosecond laser (fs-laser) parallel processing assisted by wet etching to fabricate 3D micro-optical components. A 2D fs-laser spot array with designed spatial distribution was generated by a spatial light modulator. A single-pulse exposure of the entire array was used for parallel processing. By subsequent wet etching, a close-packed hexagonal arrangement, 3D concave microlens array on a curved surface with a radius of approximately 120 μm was fabricated, each unit lens of which has designable spatial distribution. Characterization of imaging was carried out by a microscope and showed a unique imaging property in multi-planes. This method provides a parallel and efficient technique to fabricate 3D micro-optical devices for applications in optofluidics, optical communication, and integrated optics.

Fabrication of a Microlens Array with Controlled Curvature by Thermally Curving Photosensitive Gel Film beneath Microholes

A rapid method is developed for fabricating low-cost and high-numerical-aperture photosensitive-gel microlens arrays (MLAs) with well-controlled curvatures. An UV-curable photosensitive-gel film beneath the microholes of a silicon mold can be flexibly deformed by thermally manipulating the surface tension of the photosensitive gel and the pressure difference across the air-photosensitive-gel interface. The concave interface is then solidified through UV curing, forming a MLA with a concave curvature. MLAs with a focal length ranging from 51.4 to 71.9 μm and a numerical aperture (NA) of 0.49 were fabricated. The photocured MLA has high mechanical and thermal strength and is suitable as a master mold for the further production of convex MLAs. The fabricated microlenses have uniform shapes and smooth surfaces. In a demonstration of imaging and focusing performance, clear and uniform images and focused light spots were observed using concave and convex MLAs.

High Numerical Aperture Hexagonal Stacked Ring-Based Bidirectional Flexible Polymer Microlens Array

Flexible imprinted photonic nanostructures that are able to diffract/focus narrow-band light have potential applications in optical lenses, filters, tunable lasers, displays, and biosensing. Nanophotonic structures through holography and roll-to-roll printing may reduce fabrication complexities and expenses and enable mass production. Here, 3D photonic nanostructures of a stacked ring array were imprinted on acrylate polymer (AP) over poly(ethylene terephthalate) (PET) substrate through holography and lift-off processes to create a microlens array (MLA). The surface structure of the array consisted of circular nonostepped pyramids, and repeated patterns were in hexagonal arrangements. Stacked-ring-based MLA (SMLA) on a flexible AP-PET substrate showed efficient bidirectional light focusing and maximum numerical aperture (NA = 0.60) with a reasonable filling factor. The nanostructures produced a well-ordered hexagonally focused diffraction pattern in the far field, and power intensities were measured through angle-resolved experiments. The variation of nanostep dimensions (width and height) and the number of steps resulted in different photonic bandgaps, and the arrays produced distance-dependent narrow-band light focusing. The validation of the SMLA was demonstrated through the text, image, and hologram projection experiments. It is anticipated that imprinted bidirectional SMLA over flexible substrates may find applications in optical systems, displays, and portable sensors.

Fabrication of concave microlens arrays by local fictive temperature modification of fused silica

A simple and convenient means of fabricating concave microlens arrays direct on silica glass by using the local fictive temperature modification of fused silica is presented. This method is based on the fact that an increased fictive temperature results in a much higher HF acid etching rate of fused silica. Combining the abrupt local fictive temperature enhancement by the CO₂ laser pulse and the subsequent etching by the HF acid solution, concave microlens arrays with high fill factors, excellent smoothness, and optical performance are generated on fused silica.

Elevation-azimuth angular selectivity and angle-frequency filtering in asymmetric photonic crystal

Asymmetric photonic crystal (APC) has been proposed and investigated, which only allows the transmission of the incidence at a certain incident plane and angle. Simulation results show that the proposed structure exhibits both azimuth- and elevation- angular selectivity within a broad waveband under p-polarized illumination. Consequently, functional devices such as angle-frequency filter operating in the visible range can be achieved by combining the azimuth-elevation-angular selectivity and frequency selectivity. Our scheme may find potential applications in communication and imaging systems.

Efficient electrothermal actuation of liquid microlens arrays with low voltages

A self-assembled liquid microlens array with controllable focus by electrothermal actuation.

Compact near-eye display system using a superlens-based microlens array magnifier

A new type of very compact optical element for a near-eye display (NED) that uses a pair of microlens arrays (MLAs) is presented. The MLA pair works in conjunction to form a magnifier (collimator). The purpose of this is to aid in the accommodation of the eye on a head-up display that is positioned within several centimeters from the eye; the MLA pair collimates the light rays departing from the display thereby generating a virtual image of the display at optical infinity. By using the MLA pair, we are able to make a collimator that retains a thin profile of about 2 mm in thickness with a system focal length of about 7 mm.

Lens-on-lens microstructures

Microlenses with multiple focal lengths play an important role in three-dimensional imaging and the real-time detection of unconfined or fluctuating targets. In this Letter, we present a novel method of fabricating lens-on-lens microstructures (LLMs) using a two-step femtosecond laser wet etching process. A 3×3 LLM array was made with a diameter of 129.0 μm. The fabricated LLM has two focal lengths, 80.4 and 188.7 μm, showing excellent two-level focusing and imaging abilities. Its size and focal length can be controlled by adjusting laser power and etching time. Its surface roughness remains about 61 nm. This simple and efficient method for large-scale production of LLMs has potential applications in diverse optical systems.

Femtosecond Laser Fabrication of Cavity Microball Lens (CMBL) inside a PMMA Substrate for Super-Wide Angle Imaging

Since microlenses have to date been fabricated primarily by surface manufacturing, they are highly susceptible to surface damage, and their microscale size makes it cumbersome to handle. Thus, cavity lenses are preferred, as they alleviate these difficulties associated with the surface-manufactured microlenses. Here, it is shown that a high repetition femtosecond laser can effectively fabricate cavity microball lenses (CMBLs) inside a polymethyl methacrylate slice. Optimal CMBL fabrication conditions are determined by examining the pertinent parameters, including the laser processing time, the average irradiation power, and the pulse repetition rates. In addition, a heat diffusion modeling is developed to better understand the formation of the spherical cavity and the slightly compressed affected zone surrounding the cavity. A micro-telescope consisting of a microscope objective and a CMBL demonstrates a super-wide field-of-view imaging capability. Finally, detailed optical characterizations of CMBLs are elaborated to account for the refractive index variations of the affected zone. The results presented in the current study demonstrate that a femtosecond laser-fabricated CMBL can be used for robust and super-wide viewing micro imaging applications.

Fabrication of embedded microball lens in PMMA with high repetition rate femtosecond fiber laser

Embedded microball lenses with superior optical properties function as convex microball lens (VMBL) and concave microball lens (CMBL) were fabricated inside a PMMA substrate with a high repetition rate femtosecond fiber laser. The VMBL was created by femtosecond laser-induced refractive index change, while the CMBL was fabricated due to the heat accumulation effect of the successive laser pulses irradiation at a high repetition rate. The processing window for both types of the lenses was studied and optimized, and the optical properties were also tested by imaging a remote object with an inverted microscope. In order to obtain the microball lenses with adjustable focal lengths and suppressed optical aberration, a shape control method was thus proposed and examined with experiments and ZEMAX® simulations. Applying the optimized fabrication conditions, two types of the embedded microball lenses arrays were fabricated and then tested with imaging experiments. This technology allows the direct fabrication of microlens inside transparent bulk polymer material which has great application potential in multi-function integrated microfluidic devices.

Fabrication of large-area concave microlens array on silicon by femtosecond laser micromachining

In this Letter, a novel fabrication of large-area concave microlens array (MLA) on silicon is demonstrated by combination of high-speed laser scanning, which would result in single femtosecond laser pulse ablation on surface of silicon, and subsequent wet etching. Microscale concave microlenses with tunable dimensions and accessional aspherical profile are readily obtained on the 1  cm × 1  cm silicon film, which are useful as optical elements for infrared (IR) applications. The aperture diameter and height of the microlens were characterized and the results reveal that they are both proportional to the laser scanning speed. Moreover, the optical property of high-performance silicon MLAs as a reflective homogenizer was investigated for the visible wavelength, and it can be easily extended to IR light.

Flexible, fast, and low-cost production process for polymer based diffractive optics

The generation of diffractive optical elements often requires time and cost consuming production techniques such as photolithography. Especially in research and development, small series of diffractive microstructures are needed and flexible and cost effective fabrication techniques are desirable to enable the fabrication of versatile optical elements on a short time scale. In this work, we introduce a novel process chain for fabrication of diffractive optical elements in various polymers. It is based on a maskless lithography process step, where a computer generated image of the optical element is projected via a digital mirror device and a microscope setup onto a silicon wafer coated with photosensitive resist. In addition, a stitching process allows us to microstructure a large area on the wafer. After development, a soft stamp of the microstructure is made from Polydimethylsiloxane, which is used as a mold for the subsequent hot embossing process, where the final diffractive optical element is replicated into thermoplastic polymer. Experimental results are presented, which demonstrate the applicability of the process.

Using an “underwater superoleophobic pattern” to make a liquid lens array

A new strategy to realize a real liquid lens array without evaporation problems based on a femtosecond laser-induced underwater superoleophobic–oleophobic heterogeneous pattern.

Fabrication of high quality and low cost microlenses on a glass substrate by direct printing technique

The fabrication of high quality and low cost microlenses on a glass substrate using a simple, rapid, and precise direct microplotting technique is shown in this paper. The fabrication method is based on the use of a microplotter system, which is significantly different from the existing inkjet, roll-to-roll printing, and reactive ion etching technology and could work with higher viscosity materials. By optimizing the parameters of voltage, dispense time, and concentration of the polymer solution, high quality microlenses with a diameter of 20 μm could be obtained. The geometrical and optical characteristics of the microlenses are analyzed by measurement of the surface profile and the imaging properties in the near-field and far-field zones as well as the diffraction pattern. We think that the fabricated microlenses could be attractive for enhancing the light extraction efficiency of light emitting diodes.

Large-scale high quality glass microlens arrays fabricated by laser enhanced wet etching

Large-scale high quality microlens arrays (MLAs) play an important role in enhancing the imaging quality of CCD and CMOS as well as the light extraction efficiency of LEDs and OLEDs. To meet the requirement in MLAs' wide application areas, a rapid fabrication method to fabricate large-scale MLAs with high quality, high fill factor and high uniformity is needed, especially on the glass substrate. In this paper, we present a simple and cost-efficient approach to the development of both concave and convex large-scale microlens arrays (MLAs) by using femtosecond laser wet etching method and replication technique. A large-scale high quality square-shaped microlens array with 512 × 512 units was fabricated.The unit size is 20 × 20 μm² on the whole scale of 1 × 1 cm². Its perfect uniformity and optical performance are demonstrated.

Rapid method for fabricating polymeric biconvex parabolic lenslets

Microlenslets as well as microlens arrays have shown tremendous attractions and successes in miniature optical systems in recent decades. However, the fabrication methods for microlenslets and microlenslet arrays are limited. In this Letter, a rapid and low-cost method for fabricating polymeric biconvex lenslets is presented. This newly developed process is simply based on wetting behavior at interface and is able to produce high-quality biconvex lenslets with controllable size and shape. This technology will greatly simplify the production process and reduce the manufacturing costs for micro-optics.