Spherical Microlenses Research Articles

Patterned microsphere-lens projection lithography using an electrohydrodynamic-jet-printing-assisted assembly

Microlens arrays have been widely used in the fields of micro-optics and micro- and nanofabrication. Traditional preparation methods utilize commercial photoresists and thermosetting materials, thereby restricting the optical properties of microlenses. In recent years, significant advancements have been achieved in near-field super-resolution imaging by utilizing microspheres and forming arrays of microsphere lenses via self-assembly. However, self-assembly approaches lack flexibility in terms of pattern selection. This study proposes a method that utilizes electrohydrodynamic jet (E-jet) printing to code ultraviolet (UV)-curable adhesives and assist in the assembly of patterned microsphere-lens arrays. Simulation results demonstrate that the UV-curable adhesive has little impact on the optical properties of the microsphere lens. Moreover, the microsphere lens exhibits a superior imaging resolution compared with traditional microlenses. A projection-lithography system is developed to achieve an accurate alignment between the focal plane of the microsphere lenses and the plane of the photoresist, facilitating the fabrication of patterned nanostructures. The lithographic nanostructures have a minimum feature size of 850 nm. This method enables the fabrication of arrays of microsphere lenses with arbitrary patterns and presents an inexpensive and simple strategy for fabricating micro- and nanostructure arrays with submicrometer features.

Microsphere lens array embedded microfluidic chip for SERS detection with simultaneous enhancement of sensitivity and stability

Surface enhanced Raman spectroscopy (SERS) utilizes the fingerprint features of molecular vibrations to identify and detect substances. However, in traditional single focus excitation scenarios, its signal collection efficiency of the objective is restricted. Furthermore, the uneven distribution of samples on the SERS substrate would result in poor signal stability, while the excitation power is limited to avoid sample damage. SERS detection system always requires precise adjustment of focal length and spot size, making it difficult for point-of-care testing applications. Here, we proposed a SERS microfluidic chip with barium titanate microspheres array (BTMA) embedded using vacuum self-assembled hot-pressing method for SERS detection with simultaneous enhancement of sensitivity and stability. Due to photonic nano-jets and directional antenna effects, high index microspheres are perfect micro-lens for effective light focusing and signal collecting. The BTMA can not only disperse excitation beam into an array of focal points covering the target uniformly with very low signal fluctuation, but enlarge the power threshold for higher signal intensity. We conducted a proof-of-principle experiment on chip for the detection of bacteria with immuno-magnetic tags and immuno-SERS tags. Together with magnetic and ultrasonic operations, the target bacteria in the flow were evenly congregated on the focal plane of BTMA. It demonstrated a limit of detection of 5 cells/mL, excellent signal reproducibility (error∼4.84%), and excellent position tolerance of 500 μm in X–Y plane (error∼5.375%). It can be seen that BTMA-SERS microfluidic chip can effectively solve the contradiction between sensitivity and stability in SERS detection.

Responsive Liquid Crystal Network Microstructures with Customized Shapes and Predetermined Morphing for Adaptive Soft Micro-Optics.

Stimuli-responsive materials have garnered substantial interest in recent years, particularly liquid crystal networks (LCNs) with sophisticatedly designed structures and morphing capabilities. Extensive efforts have been devoted to LCN structural designs spanning from two-dimensional (2D) to three-dimensional (3D) configurations and their intricate morphing behaviors through designed alignment. However, achieving microscale structures and large-area preparation necessitates the development of novel techniques capable of facilely fabricating LCN microstructures with precise control over both overall shape and alignment, enabling a 3D-to-3D shape change. Herein, a simple and cost-effective in-cell soft lithography (ICSL) technique is proposed to create LCN microstructures with customized shapes and predesigned morphing. The ICSL technique involves two sequential steps: fabricating the desired microstructure as the template by using the photopolymerization-induced phase separation (PIPS) method and reproducing the LCN microstructures through templating. Meanwhile, surface anchoring is employed to design and achieve molecular alignment, accommodating different deformation modes. With the proposed ICSL technique, cylindrical and spherical microlens arrays (CMLAs and SMLAs) have been successfully fabricated with stimulus-driven polarization-dependent focusing effects. This technique offers distinct advantages including high customizability, large-area production, and cost-effectiveness, which pave a new avenue for extensive applications in different fields, exemplified by adaptive soft micro-optics and photonics.

Conformal polishing of a glass microlens array through oxyhydrogen flame-induced viscoelasticity flow at nano/microscale

Glass microlens arrays with small unit sizes and high integration capacities are widely utilized in various optical systems, but the femtosecond laser milling produces microscale residual steps and nanoscale rough topography on microlens surface. Accordingly, an oxyhydrogen flame polishing is proposed after femtosecond laser milling, aiming to efficiently fabricate the ultrasmooth surface without damaging shape accuracy by triggering a viscoelasticity flow at nano/microscale. Through simultaneous modelling of the polishing process, a large thermal power results in a high viscoelasticity flow velocity for nanoscale topography smoothing, followed by microscale step melting. Then, a single-point polishing experiment is conducted to determine the critical transition temperatures for nanoscale smoothing and microscale melting. It is further experimentally applied to achieve an ultrasmooth surface of cylindrical microlens array with its roughness less than 0.2 nm and no hydroxyl introduction. This process also effectively repairs subsurface damage and reduces the form error to ±0.5 μm, improving the surface quality and shape accuracy. Similar results are also observed for the polishing of spherical microlenses. In conclusion, the method of combining femtosecond laser milling and oxyhydrogen flame polishing holds promise for high-efficiency and high-quality fabrication of glass components with micro features.

Fabrication of Large-Area Silicon Spherical Microlens Arrays by Thermal Reflow and ICP Etching.

In this paper, we proposed an efficient and high-precision process for fabricating large-area microlens arrays using thermal reflow combined with ICP etching. When the temperature rises above the glass transition temperature, the polymer cylinder will reflow into a smooth hemisphere due to the surface tension effect. The dimensional differences generated after reflow can be corrected using etching selectivity in the following ICP etching process, which transfers the microstructure on the photoresist to the substrate. The volume variation before and after reflow, as well as the effect of etching selectivity using process parameters, such as RF power and gas flow, were explored. Due to the surface tension effect and the simultaneous molding of all microlens units, machining a 3.84 × 3.84 mm2 silicon microlens array required only 3 min of reflow and 15 min of ICP etching with an extremely low average surface roughness Sa of 1.2 nm.

Influence of refractive index contrast between microsphere lens and immersion medium on imaging resolution

A microsphere lens could couple the near-field evanescent wave and convert it into propagation wave, which helps microscope to break the diffraction limit and improve its imaging resolution. However, the imaging conditions for microsphere lens is still not clear, and also the conversion process between evanescent wave and propagation wave. Here, we deduced the imaging conditions of microsphere lens through the refractive index contrast between microsphere and immersion medium, and simulated the coupling and scattering process of evanescent wave by microsphere lens. At last, we demonstrated the microsphere assisted super-resolution imaging under different immersion conditions. The experimental results verified that, only within the range predicted by theoretical deduction, the evanescent wave could be converted and the imaging resolution of objective lens could be improved. Our work predicts the super-resolution imaging condition of microsphere lens, and explains its mechanism with evanescent wave conversion, which could guild the design of microsphere lens under different immersion conditions and extend its application in various fields.

Normal-Stressed Electromagnetic Triaxial Fast Tool Servo for Microcutting

Short strokes and complicated structures commonly degrade the operating performance of current piezo-actuated triaxial fast tool servos (FTSs). To overcome those deficiencies, we present a novel normal-stressed electromagnetic triaxial FTS for micro-cutting, which achieves a compact structure and a high resonant frequency with triaxial motions in hundreds of micrometers. The magnetic FTS has a single armature to provide the triaxial decoupled and non-contact driving forces, as well as a symmetric 3-DOF corrugated compliant bearing to support and guide the decoupled outputs. With the parameters jointly optimized by an analytical and finite element model, the prototype showed a high degree of agreement with the design target for both static and dynamic performances. A linear active disturbance rejection control is implemented for the triaxial FTS, and an iterative learning scheme is further employed for the triaxial FTS to achieve fast and accurate trajectory tracking. The developed triaxial FTS is comprehensively demonstrated by fabricating a hexagonal spherical micro-lens array with each lenslet generated within 0.24&#x00A0;s. The practical error for the spatial trajectory tracking was within <inline-formula><tex-math notation="LaTeX">$ \pm$</tex-math></inline-formula>30&#x00A0;nm, and good surface quality with a form error of 63.66&#x00A0;nm and surface roughness of <i>Sa</i>=2.05&#x00A0;nm is achieved.

Random spherical microlens array fabricated by elliptical vibration diamond cutting and molding.

Microlens arrays (MLAs) are widely used in homogenized laser beams due to excellent optical properties. However, the interference effect generated in traditional MLA (tMLA) homogenization will reduce the quality of the homogenized spot. Hence, the random MLA (rMLA) was proposed to reduce the interference effect in the homogenization process. To achieve mass production of these high-quality optical homogenization components, the rMLA with randomness in both period and sag height was proposed first. Subsequently, MLA molds were ultra-precision machined on S316 molding steel by elliptical vibration diamond cutting. Furthermore, the rMLA components were precisely fabricated by applying molding technology. Finally, Zemax simulation and homogenization experiments were carried out to verify the advantage of the designed rMLA.

Study on the tool setting for microlens array machining via swing cutting

Ultra-precision cutting is widely applied in the fabrication of micro-spherical lens arrays (MSLAs). However, due to the geometric limitations of the cutting tool and the degrees of freedom of the machine tool, the traditional cutting strategy cannot be employed to fabricate high-quality microscale lenses with high efficiency. In this paper, the novel tool swing cutting (TSC) method is presented to efficiently fabricate MSLAs, and its tool setting technique is introduced. First, the principle of TSC is analyzed, and the relationships between the tool parameters and machining structure are determined. In the proposed method, the tool is precisely located in the rotating center of the B-axis via charge-coupled device (CCD) camera tool setting and trial cutting. Finally, an MSLA is experimentally machined based on the proposed TSC method, and the results reveal the achievement of good machining quality with a form error of <354 nm and a surface roughness of <3.74 nm.

Imaging properties of microsphere superlenses with varying background refractive indices under inclined illumination.

Microsphere lenses can overcome the optical diffraction limit and can be used to observe features smaller than 200 nm under white light. Inclined illumination benefits from the second refraction of evanescent waves in the microsphere cavity, prohibiting the influence of background noise and improving the imaging resolution and quality of the microsphere superlens. Currently, there is a consensus that microspheres immersed in a liquid environment can improve imaging quality. Microsphere imaging under inclined illumination is performed using barium titanate microspheres immersed in an aqueous environment. However, the background medium of a microlens varies depending on its diverse applications. In this study, the effects of continuously changing background media on the imaging properties of microsphere lens under inclined illumination are investigated. The experimental results demonstrate that the axial position of the microsphere photonic nanojet changes with respect to the background medium. Consequently, owing to the refractive index of the background medium, the imaging magnification and the position of the virtual image change. Using a sucrose solution and polydimethylsiloxane with the same refractive index, we demonstrate that the imaging performance of microspheres is related to the refractive index rather than the background medium type. This study helps associate microsphere superlenses with a more universal application spectrum.

Molded high curvature core-aligned micro-lenses for single-mode fibers.

A polymer-based fiber micro-lens molding fabrication technique with, to our knowledge, unprecedented performances is presented along with its advantages and applications. This technique is a fast and affordable tool to achieve a wide variety of possible spherical and aspherical micro-lens sizes and curvatures. The alignment of the micro-lens mold with the fiber core receiving the lens is done optically, which allows high precision. Using the proposed technique, different micro-lenses are fabricated. Then the output beams of two different micro-lenses on single-mode fibers are characterized. Fiber micro-lenses with curvature as small as 5 µm are achieved. This shows how low curvature micro-lenses can achieve collimation and how high curvature ones on single-mode fibers lead to high focusing (FWHM=λ) that is much smaller than what most conventional commercial techniques can reach. Given that this technique imposes no stress and causes no damage to the fiber receiving the micro-lens, it presents a significant potential for compatibility with non-silica-based and micro-structured fibers such as photonic crystal and quasi-crystal fibers.

Microlens-assisted microscopy for biology and medicine.

The addition of dielectric transparent microlens in the optical scheme is an effective and at the same time simple and inexpensive way to increase the resolution of a light microscope. For these purposes, spherical and cylindrical microlenses with a diameter of 1-100 μm are usually used. The microlens focuses the light into a narrow beam called a photonic nanojet. An enlarged virtual image is formed, which is captured by the objective of the light microscope. In addition to microscopy, the microlenses are successfully applied to amplify optical signals, increase the trapping force of optical tweezers and are used in microsurgery. This review considers the design and principle of microlens-assisted microscopes. Taking into account the advantages of the super-resolution optical methods for research in life science, the examples of the use of the microlenses in biomedical practice are discussed in detail.

Fabrication of High Precision Silicon Spherical Microlens Arrays by Hot Embossing Process.

In this paper, a high-precision, low-cost, batch processing nanoimprint method is proposed to process a spherical microlens array (MLA). The nanoimprint mold with high surface precision and low surface roughness was fabricated by single-point diamond turning. The anti-sticking treatment of the mold was carried out by perfluorooctyl phosphoric acid (PFOPA) liquid deposition. Through the orthogonal experiment of hot embossing with the treated mold and subsequent inductively coupled plasma (ICP) etching, the microstructure of MLA was transferred to the silicon substrate, with a root mean square error of 17.7 nm and a roughness of 12.1 nm Sa. The average fitted radius of the microlens array units is 406.145 µm, which is 1.54% different from the design radius.

Micro Aspheric Convex Lenses Fabricated by Precise Scraping.

An easy, fast, inexpensive, and simple method utilizing a microshaper with a very small knife nose is used to fabricate microconvex aspherical lenses. The microshaper is mounted on a computer numerical control (CNC) machine. To achieve an accurately designed profile of the lens surface, a cutter-path planning algorithm with compensation for knife interference is developed. Exerting this algorithm in CNC machining, the microconvex aspheric surface is precisely scraped. To verify the precise machining of the cutter path planning algorithm, three aspheric surfaces of conic sections (ellipsoid, paraboloid, and hyperboloid) are successfully fabricated. The profiles scraped by the microshaper agree well and precisely with the designed theoretical conic section curve. Using a simple polishing method to make the machined surface smoother, the roughness is reduced from 143 and 346 nm to 52 and 44 nm for the path line direction and its transverse direction, respectively. The micro-aspherical lenses have moderate machining properties using a simple polishing method. The results show that the designed profiles of micro-aspheric convex lenses can be machined precisely and efficiently by the microshaper with the cutter-path planning algorithm developed in this work. From the image comparison formed by the aspherical and spherical microlenses, the aspherical lenses provide a better image. It is feasible that the designed profile of the micro-aspherical lenses with specific functions could be machined using the cutter-path planning algorithm developed in this work.

Micro Fabry–Perot interferometer based on a microsphere lens for motor rotation speed measurement

In this study, a micro Fabry–Perot interferometer (FPI) based on a microsphere lens for motor rotation speed measurement is proposed and experimentally demonstrated. A 5 μm diameter microsphere is attached to the tip of a tapered single-mode fiber. The FPI can produce photonic nanojets (PNJs) because of the presence of the microsphere. The PNJs can produce small laser spots and enhance the backscattering light from the surface of the object. In this phenomenon, the speckle signal required in measuring motor rotation speed can be easily detected. Experimental results show that the relative errors are less than 0.1%.

自己整列微粒子の形状転写によるマイクロレンズアレイの製作とモールド変形を利用した非球面化の検討

Aspheric micro-lens arrays have been attracting great attention to improve light distribution control. This study proposes a simple and efficient method for fabricating close-packed aspheric micro-lens array. First, silica particles of ϕ10 μm were self-assembled over ϕ10 mm area using sedimentation method. Then the convex profile of the self-assembled particles was replicated to a spherical micro-lens array mold. The mold material was silicone elastomer or polydimethylsiloxane (PDMS). Then, the mold profile was replicated into ultraviolet (UV) curing resin. By applying uniaxial stretching to the mold during the secondary replication, aspheric micro-lens array was fabricated. Deformation of the PDMS mold under uniaxial stretching was analyzed by using finite element analysis (FEA) for rough estimation. Experimental results showed that intended lens profile was obtained though some defects were observed due to the nonuniformity of the particle diameters. Optical characteristic of elliptic micro-lens array was made clear.

Rapid Fabrication of Smooth Micro-Optical Components on Glass by Etching-Assisted Femtosecond Laser Modification.

Femtosecond laser (fs-laser) is unfavorable in applications for the fabrication of micro-optical devices on hard materials owing to the problems of low fabrication efficiency and high surface roughness. Herein, a hybrid method combining fs-laser scanning, subsequent etching, and annealing was proposed to realize micro-optical devices with low roughness on glass. Compared to traditional laser ablation, the fabrication efficiency in this work was improved by one order of magnitude, and the surface roughness was decreased to 15 nm. Using this method, aspherical convex microlenses and spherical concave microlenses that possess excellent focusing and imaging properties are realized on photosensitive glass. The diameter and height of the microlenses were controlled by adjusting the fabrication parameters. These results indicate that the fs-laser-based hybrid method will open new opportunities for fabricating micro-optical components on hard materials.

Label-free non-invasive subwavelength-resolution imaging using yeast cells as biological lenses.

There is a growing interest to use live cells to replace the widely used non-biological microsphere lenses. In this work, we demonstrate the use of yeast cells for such imaging purpose. Using fiber-based optical trapping technique, we trap a chain of three yeast cells and bring them to the vicinity of imaging objects. These yeast cells work as near-field magnifying lenses and simultaneously pick up the sub-diffraction information of the nanoscale objects under each cell and project them into the far-field. The experimental results demonstrated that Blu-ray disc of 100 nm feature can be clearly resolved in a parallel manner by each cell.

Optical Trapping, Sensing, and Imaging by Photonic Nanojets

The optical trapping, sensing, and imaging of nanostructures and biological samples are research hotspots in the fields of biomedicine and nanophotonics. However, because of the diffraction limit of light, traditional optical tweezers and microscopy are difficult to use to trap and observe objects smaller than 200 nm. Near-field scanning probes, metamaterial superlenses, and photonic crystals have been designed to overcome the diffraction limit, and thus are used for nanoscale optical trapping, sensing, and imaging. Additionally, photonic nanojets that are simply generated by dielectric microspheres can break the diffraction limit and enhance optical forces, detection signals, and imaging resolution. In this review, we summarize the current types of microsphere lenses, as well as their principles and applications in nano-optical trapping, signal enhancement, and super-resolution imaging, with particular attention paid to research progress in photonic nanojets for the trapping, sensing, and imaging of biological cells and tissues.

Optical nano-imaging via microsphere compound lenses working in non-contact mode.

Microsphere lens for nano-imaging has been widely studied because of its superior resolving power, real-time imaging characteristic, and wide applicability on diverse samples. However, the further development of the microsphere microscope has been restricted by its limited magnification and small field-of-view. In this paper, the microsphere compound lenses (MCL) which allow enlarged magnification and field-of-view simultaneously in non-contact imaging mode have been demonstrated. A theoretical model involving wave-optics effects is established to guide the design of MCL for different magnifications and imaging configurations, which is more precise compared with common geometric optics theory. Experimentally, using MCL to image the specimen with a tunable magnification from 2.8× to 10.3× is realized. Due to the enlarged magnification, a high-resolution target with 137 nm line width can be resolved by a 10× objective. Besides, the field-of-view of MCL is larger than that of a single microsphere and can be further increased through scanning working manner, which has been demonstrated by imaging a sample with ∼76 nm minimum feature size in a large area. Prospectively, the well-designed MCL will become irreplaceable components to improve the imaging performances of microsphere microscope just like the compound lens in the conventional macroscopic imaging system.