Aspheric Microlens Research Articles

An aspherical microlens assembly for deep brain fluorescence microendoscopy

Fluorescence microendoscopy is becoming a standard technique in neuroscience for visualizing neuronal activity in the deep brain. Gradient refractive index (GRIN) lenses are increasingly used for fluorescence microendoscopy; however, they inherently suffer from strong aberrations and distortion. Aspherical lenses change their radius of curvature with distance from the optical axis and can effectively eliminate spherical aberrations. The use of these lenses has not been fully explored in deep brain fluorescence microendoscopy due to technical difficulties in manufacturing miniature aspherical lenses. In this study, we fabricated a novel microendoscope lens assembly comprised two nested pairs of aspherical microlenses made by precision glass molding. This assembly, which was 0.6 mm in diameter and 7.06 mm in length, was assembled in a stainless steel tube of 0.7 mm outer diameter. This assembly exhibited marked improvements in monochromatic and chromatic aberrations compared with a conventional GRIN lens, and is useful for deep brain fluorescence microendoscopy, as demonstrated by two-photon microendoscopic calcium imaging of R-CaMP1.07-labeled mouse hippocampal CA1 neurons. Our aspherical-lens-based approach offers a non-GRIN-lens alternative for fabrication of microendoscopic lenses.

Tolerancing the surface form of aspheric microlenses manufactured by wafer-level optics techniques

Tolerancing is an important step toward the fabrication of high-quality and cost-effective lens surfaces. It is critical for wafer-level optics, when up to tens of thousands microlenses are fabricated in parallel and whose surfaces cannot be formed individually. However, approaches developed for macro-optics cannot be directly transposed for microlenses because of differences in fabrication and testing techniques. In particular, microlens surfaces are usually limited to conical surfaces. Here, we study the connection between the microlens optical performance and the form of its surface, suggesting surface form representations suited for tolerancing purposes. Then, we compare them with common representations for tolerancing real optical systems. Measured surface forms of microlenses are also provided to make the tolerancing procedure realistic. In addition, we propose term definitions for micro-optics, complements to typical terms for macro-optics, to ease the communication between optical designers and manufacturers. Based on the results presented in this paper, guidelines are proposed for tolerancing microlenses. We suggest applying them as a first step toward a more effective and comprehensive tolerancing procedure.

Ultra-long-working-distance spectroscopy of single nanostructures with aspherical solid immersion microlenses

In light science and applications, equally important roles are played by efficient light emitters/detectors and by the optical elements responsible for light extraction and delivery. The latter should be simple, cost effective, broadband, versatile and compatible with other components of widely desired micro-optical systems. Ideally, they should also operate without high-numerical-aperture optics. Here, we demonstrate that all these requirements can be met with elliptical microlenses 3D printed on top of light emitters. Importantly, the microlenses we propose readily form the collected light into an ultra-low divergence beam (half-angle divergence below 1°) perfectly suited for ultra-long-working-distance optical measurements (600 mm with a 1-inch collection lens), which are not accessible to date with other spectroscopic techniques. Our microlenses can be fabricated on a wide variety of samples, including semiconductor quantum dots and fragile van der Waals heterostructures made of novel two-dimensional materials, such as monolayer and few-layer transition metal dichalcogenides.

Improvement in light out-coupling efficiency of OLED by using high fill factor parabola curve microlens arrays

Aspheric microlens arrays (MLAs) are introduced on glass substrate to improve the light out-coupling efficiency (LOCE) of organic light-emitting diode (OLED). Geometric ray tracing was employed to simulate the improved effects and optimize the structure parameters of the MLAs pattern. The simulation results show that when the parameters of MLAs as follows, the curved profile is parabola, the base shape is hexagon, the fill factor is 1, the aspect ratio is 0.6, and the material refractive index should be match to that of the substrate of the OLED, which can improve LOCE maximum.Poly-dimethyl-siloxane (PDMS) parabola curve MLAs films ware fabricated by maskless lithography technique and template replication. The experimental results show that the MLAs can improve the LOCE and the trends of the experimental results agree with the simulation results. A maximum increase of 49 % in the coupling efficiency was achieved.

Infrared rectification based on electron field emission in nanoantennas for thermal energy harvesting

ABSTRACT A rectification concept based on field electron emission in a nanoantenna is proposed. The device consists of three parts, i.e., an aspherical micro-lens, a nanoantenna, and an electron collector electrode with proper geometry design to ensure rectified field electron emission. The incident radiation is enhanced in two steps, firstly by micro-lens focusing onto the nanoantenna, and secondly through excitation of the near field in the gap of the nanoantenna to reach a level required for field electron emission from a sharp metallic tip. Furthermore, we show that single nanoantenna cells can be arranged into an array using a bus-bar configuration where the currents of each cell are added up to increase the harvested power. The performance of the nanoantenna in a single cell and in the array is studied by full-wave simulations. Finally, the total efficiency of the proposed device is calculated to demonstrate its superiority over the MIM devices.

Design and Fabrication of Wafer-Level Microlens Array with Moth-Eye Antireflective Nanostructures.

Wafer-level packaging (WLP) based camera module production has attracted widespread industrial interest because it offers high production efficiency and compact modules. However, suppressing the surface Fresnel reflection losses is challenging for wafer-level microlens arrays. Traditional dielectric antireflection (AR) coatings can cause wafer warpage and coating fractures during wafer lens coating and reflow. In this paper, we present the fabrication of a multiscale functional structure-based wafer-level lens array incorporating moth-eye nanostructures for AR effects, hundred-micrometer-level aspherical lenses for camera imaging, and a wafer-level substrate for wafer assembly. The proposed fabrication process includes manufacturing a wafer lens array metal mold using ultraprecise machining, chemically generating a nanopore array layer, and replicating the multiscale wafer lens array using ultraviolet nanoimprint lithography. A 50-mm-diameter wafer lens array is fabricated containing 437 accurate aspherical microlenses with diameters of 1.0 mm; each lens surface possesses nanostructures with an average period of ~120 nm. The microlens quality is sufficient for imaging in terms of profile accuracy and roughness. Compared to lenses without AR nanostructures, the transmittance of the fabricated multiscale lens is increased by ~3% under wavelengths of 400–750 nm. This research provides a foundation for the high-throughput and low-cost industrial application of wafer-level arrays with AR nanostructures.

Off-spindle-axis spiral grinding of aspheric microlens array mold inserts.

A novel approach named off-spindle-axis (OSA) spiral grinding for fabricating aspheric microlens array (AMLA) mold inserts for precision glass molding (PGM) is presented. In OSA spiral grinding, three translational motions of the grinding wheel are synchronized with the rotation of the workpiece to form a local spiral wheel path for individual lens-lets. With this approach, the form accuracy of lens-lets can be compensated within sub-micrometer by means of the on-machine measurement. The determination of wheel path and form error compensation via on-machine measurement are systematically studied. A tungsten carbide mold insert with four convex aspheric lens-lets is fabricated to evaluate the grinding performance. PGM experiments are performed to produce glass AMLA using the ground insert. The experimental results indicate that both the ground and molded AMLA with homogeneous quality are achieved. The form accuracy and surface roughness of both the mold insert and the molded AMLA were less than 0.3 µm in PV and 10 nm in Sa, respectively.

光刻胶热熔法制备的非球面微透镜的设计方法

光刻胶热熔法制备的非球面微透镜的设计方法

Fabrication of high fill-factor aspheric microlens array by dose-modulated lithography and low temperature thermal reflow

A cost-effective fabrication method for high quality and high fill-factor aspheric microlens arrays (MLAs) is developed. In this method, the complex shape of aspheric microlens is pre-modeled via dose modulation in a digital micromirror device (DMD) based maskless projection lithography system. Digital masks for several bottom layers are replaced from circle to hexagon for the purpose of enhancing the fill-factor of MLAs, then a low temperature thermal reflow process is conducted, after which the average surface roughness of microlens is improved to ~ 0.427 nm while the pre-modeled profile keeps unchanged. Experimental results show that the fabricated aspheric MLAs have almost 100% fill-factor, high shape accuracy and high surface quality. The presented method may provide a promising approach for rapidly fabricating high quality and high fill-factor aspheric microlens in a simple and low-cost way.

An optimization method of dynamic patterns based on aspheric microlens array

An optimization method is proposed to improve the image quality of dynamic patterns in this paper. In order to obtain parallel emergent beam of high quality, aspheric microlens array is used in the optical system. Besides, an analytical model of the substrate and a single aspheric lens of MLA is presented based on the principle of ray tracing which is used in optical design for virtually tracing rays through an optical system. Moreover, how do the parameters influence the image quality is analyzed and the relationship between the conic constant and the divergence angle of emergent beam is derived. Preliminary optimization results with a second order aspheric lens are compared with the simulation results obtained by ZEMAX. The ray tracing method in this article helps improve the image quality of dynamic patterns and has the potential to be used in designing aspheric surface because it offers a way to improve the parallel rays and optimize the parameters of the optical system.

Heterogeneous microoptical structures with sub-micrometer precision

We demonstrate the fabrication of microoptical elements made from hybrid polymers using two-photon polymerization. To overcome the throughput limitations of two-photon polymerization we use galvanometric mirrors and propose a hatching strategy for rotationally symmetric objects which allows a significant process acceleration to tolerable fabrication times while preserving the surface accuracy of the fabricated elements. Using this strategy and optimized processing parameters we demonstrate the fabrication of an aspheric microlens for diffraction limited focusing which could be fabricated in less than 1.5 min. Thorough topographic characterization with atomic force microscopy and laser scanning microscopy reveal excellent agreement of the fabricated surfaces with the theoretical design with a surface accuracy below 100 nm. Our approach enables the generation of arrays of custom shaped lenses and large arrangements of freeform microoptical elements in a few hours.

In situ fabricated 3D micro-lenses for photonic integrated circuits.

Aspheric astigmatic polymer micro-lenses were fabricated directly onto photonic integrated circuits using two-photon lithography. We observed a 12.6 dB improvement in the free space coupling efficiency between integrated ridge laser pairs with micro-lenses to those without.

Fabrication method of quartz aspheric microlens array for turning mask

In order to solve the two difficult problems of the poor processing controllability and the low surface accuracy of quartz aspheric microlens array processing, a fabrication method of quartz aspheric microlens array for turning mask is proposed. This method mainly uses single point diamond turning technology and reactive ion etching technology, studies the turning and etching properties of the mask material, and optimizes the mask material by experiment. Finally, the fabrication of an aspherical glass microlens array with an area of 5 mm×5 mm was carried out. The experimental results are compared with the expected parameters. The analysis shows that the error root mean square of the quartz glass component is 1.155 nm, and the surface accuracy error is 0.47%.

Fabrication of high quality aspheric microlens array by dose-modulated lithography and surface thermal reflow

A novel fabrication method for high quality aspheric microlens array (MLA) was developed by combining the dose-modulated DMD-based lithography and surface thermal reflow process. In this method, the complex shape of aspheric microlens is pre-modeled via dose modulation in a digital micromirror device (DMD) based maskless projection lithography. And the dose modulation mainly depends on the distribution of exposure dose of photoresist. Then the pre-shaped aspheric microlens is polished by a following non-contact thermal reflow (NCTR) process. Different from the normal process, the reflow process here is investigated to improve the surface quality while keeping the pre-modeled shape unchanged, and thus will avoid the difficulties in generating the aspheric surface during reflow. Fabrication of a designed aspheric MLA with this method was demonstrated in experiments. Results showed that the obtained aspheric MLA was good in both shape accuracy and surface quality. The presented method may be a promising approach in rapidly fabricating high quality aspheric microlens with complex surface.

Monolithic Microlens VCSELs With High Beam Quality

In this paper, we report on high beam quality vertical-cavity surface-emitting lasers (VCSELs) featuring a monolithically integrated aspherical microlens. This lens is fabricated by a one-step diffusion-limited wet etch process. VCSELs with microlenses with different curvature radius are designed, fabricated, and characterized, as well as reference VCSEL devices without the microlens. Significant improvement of beam divergence and single-mode performance are demonstrated in microlens VCSELs. The divergence angles of a conventional VCSEL are ${\text{14.5}}$ °, $16.0$ °, and $17.0$ ° (full width at D4 $\sigma$ ) at 2 mA, 3 mA, and 4 mA, respectively, with a $M^2$ value measured to be 6.9 at 3 mA. Under the same current injection, the divergence angles of a comparable VCSEL with a micro-lens (curvature radius of 31.8 $\mu$ m) are reduced to $4.8$ °, $5.5$ °, and $5.7$ °, respectively. The $M^2$ value of the lensed VCSEL is as low as 1.9 at 3 mA.

Fabrication of high fill-factor aspheric microlens array by digital maskless lithography

A simple and effective method for fabricating high fill-factor aspheric microlens array using digital maskless lithography is reported. The method use the contour planes of the designed profile as binary image, which replace physical mask by DMD display. On the lithography system, the dose accumulated over multiple exposures and the exposure dose reconstructed in wafer, then the designed profile developed. Aspheric microlens arrays with hexagonal base and square base were produced by the method, and they were gapless at each microlens periphery, so the microlens array with fill-factor of 100%.

Improved slicing strategy for digital micromirror device‐based three‐dimensional lithography with a single scan

To improve the profile quality of three-dimensional microstructures, an improved method was proposed in real-time maskless lithography based on digital micromirror device. First, to calculate the spatial distribution of exposure dose for the target profile in design, the relations of the exposure dose and the development depth were investigated. Moreover, a suitable development time was obtained. Second, an improved slicing strategy based on equal-arc algorithm was proposed to reduce the profile error. Different from traditional equal-expose-dose method, this method generated slicing layers with equal arc-length instead of equal-exposure dose. Such an approach has advantages in obtaining a series of suitable section patterns adapting to the curvature variation of the design profile. Finally, an aspheric microlens array was fabricated with this method and the equal-exposure-dose method. Results showed that the resulting microstructure profile by their method was more consistent with the design profile with the same slicing numbers. Moreover, results also showed that the method used can obtain as good profile quality as that of the equal-exposure-dose method with only a half of slicing numbers. This indicated that the proposed method is feasible for achieving good profile quality with slicing efficiency improved remarkably.

Design and fabrication of a biconvex aspherical microlens for maximizing fiber coupling efficiency with an ultraviolet laser diode

This paper introduces a biconvex microlens to maximize the light coupling between an edge emitting laser diode (LD) and an optical fiber. Both convex surfaces have different profiles along their x- and y-axis so that the elliptical light beam from an LD can be fed into the core of a fiber. The optimized lens profiles are first simulated by the ray-tracing software Zemax and then fabricated by an excimer laser micromachining system. Focal spot size around 10μm is achieved experimentally. Light coupling efficiency using this single biconvex microlens is 31.8% and 47.6% for single-mode and multi-mode fiber, respectively.

Beam pen lithography based on focused laser diode beam with single microlens fabricated by excimer laser.

A method is proposed to minimize the focused spot size of an elliptically-diverging laser diode beam by means of a circular aperture and a single plano-convex aspherical microlens. The proposed microlens is fabricated using an excimer laser dragging method and has two different profiles in the x- and y-axis directions. The focused spot size of the beam is examined both numerically and experimentally. The feasibility of the proposed approach for beam pen lithography applications is demonstrated by patterning dotted, straight-line and spiral features on a photo resist layer followed by thin gold layer deposition and metal lift-off. The minimum feature size for dotted pattern is around 2.57 μm, while the minimum line-widths for straight-line and spiral pattern are 3.05 μm and 4.35 μm, respectively. Thus, the technique can be applied to write any arbitrary pattern for high-resolution lithography.

Adjusting light distribution for generating microlens arrays with a controllable profile and fill factor

This paper presents a simple but effective process to fabricate high quality aspheric microlens arrays (MLA) and bifocal microlens arrays (BMLA) by predicting and adjusting the UV light distribution. The profile-controllable BMLA has two different focal lengths simultaneous along the optical axis, measured at f1 = 26 ± 1 μm (the top lens) and f2 = 35 ± 1 μm (the base lens), respectively. The fill-factor and curvature of the obtained profiles have been well optimized by controlling the air gap and exposure time, for instance, the fill factor is topped up to 99.7% from an initial 25.8% (the corresponding aperture diameter is enlarged to 15 μm from an initial 10 μm) and the micro lens height ranges from 2.66 μm to 9.03 μm. Optical tests indicate that MLAs and BMLAs have a uniform shape and a good consistency of the focal length. By adjusting the light distribution, this proposed strategy possesses the ability and flexibility to fabricate profile-controllable MLAs and BMLAs.