Fresnel Microlenses Research Articles

Spatial light assisted femtosecond laser direct writing of a bionic superhydrophobic Fresnel microlens arrays

High quality imaging and self-cleaning function are two significant factors for Fresnel Microlens Arrays (FMLAs) to operate properly in outdoor environment. Bionic superhydrophobic FMLAs are explored for outdoor uses due to their ability allowing free rolling down of water droplets taking away dust particles. Current photolithographic method of fabricating FMLAs involves multi-processing steps, which increase the complexity of the microlens array structure as well as reduces the optical imaging capability of the lenses. In this study, we report a method for creating bionic superhydrophobic FMLAs by integrating the Nepenthes Alata crescent structures with FMLAs fabricated by spatial light assisted femtosecond laser direct writing, i.e. Two-Photon Polymerization (TPP). The fabricated FMLAs were further modified with Perfluorooctyltrimethoxysilane (F13) to enhance hydrophobicity and surface uniformity. Results show that the optimized bionic FMLAs exhibit superhydrophobicity in different liquid media and excellent self-cleaning ability. The water contact angle (CA) reached 156.3°, a low rolling angle of 4.7°, and surface adhesion force of 25.5 μN. The FMLAs designed have excellent imaging characteristics and focusing ability. This research provides insights into the fabrication and performance optimization of bionic superhydrophobic FMLAs, offering potential applications for outdoor optical systems.

Fabrication of high-performance lens arrays for micro-concentrator photovoltaics using ultraviolet imprinting

Micro-concentrator photovoltaics (micro-CPV) is a cutting-edge CPV approach aimed at increasing the efficiency and reducing the cost and carbon footprint of solar electricity by downscaling concentrator solar cells and optics. The reduced size of micro-CPV provides several advantages over conventional CPV, including shorter optical paths and lower temperature and resistive losses in the cell, resulting in higher electrical efficiencies. This may increase the energy yield per area compared to conventional CPV or silicon modules. Cost reduction is achieved through material savings and the use of continuous manufacturing methods enabled by the tiny size of cells and optics, such as roll-to-roll (R2R) and roll-to-plate (R2P) ultraviolet (UV) imprinting for optics production. However, adapting these processes to large-area arrays of Fresnel micro-lenses with no wasted areas and high efficiency remains a challenge. In this study, we present a comprehensive methodology for the development of micro-CPV optics with full area coverage—from design and mastering to up-scaling, tooling, and replication. The methodology involves designing a non-rotationally symmetric elementary insert tailored to ultraviolet imprinting. Crucially, multiple inserts are originated via precision machining and recombined to form a single array master mold without wasted areas. The master is then replicated into a flexible working stamp for UV imprinting of Fresnel lens arrays, utilizing different UV curable materials. The functional characterization of the lenses demonstrates an optical efficiency of 80% at 178X under collimated white light, representing the highest effective concentration achieved using UV-imprinted Fresnel lenses. Furthermore, initial reliability tests confirm the absence of degradation during thermal cycling or outdoor exposure. This methodology paves the way for continuous high-throughput manufacturing of micro-lens arrays using R2R or R2P methods, presenting a significant step forward in micro-CPV.

Flexible fabrication of Fresnel micro-lens array by off-spindle-axis diamond turning and precision glass molding

Fresnel micro-lens arrays with close-to-wavelength features have significant advantages in minimization of optical systems. However, in the fabrication of a Fresnel micro-lens array by conventional diamond turning, the discontinuous features of the surface profile cause interference between the tool flank face and the finished surface. In this manuscript, a novel off-spindle-axis diamond turning method is proposed to fabricate Fresnel micro-lens array mold insert and a toolpath generation algorithm is developed. The generation of each lenslet is treated as an independent turning operation, and the whole fabrication of a Fresnel micro-lens array is completed by a succession of repetitive turning operations. With this approach, the interference between the tool and the finished surface is avoided, and the form accuracy and surface integrity are improved for each lenslet. Then, the Fresnel micro-lens array mold insert is further utilized in precision glass molding to replicate the micro-lens structures onto transparent polymer substrates. The experimental results indicate that both the diamond-turned and molded Fresnel micro-lens arrays are achieved with homogeneous quality. As an application, a compact imaging system based on the molded Fresnel micro-lens array is demonstrated. The proposed machining method in this study can be employed in the fabrication of Fresnel micro-lens arrays and other micro-optics with discontinuous profiles with high accuracy and machining efficiency.

Hot embossing of aspherical Fresnel microlenses: design, process, and characterization

Hot embossing is a technique used to fabricate high-precision and high-quality polymeric components that combines low costs with high aspect ratio fidelity replication. In this study, we manufactured two aspherical Fresnel molds employing the single-point diamond turning process on an electrolytic copper workpiece, one with 10 μm constant height 30 zones and the other with 250 μm constant width 40 zones. The micromachined mold reproduced PMMA convex-plane lens optical quality replicas through the micro hot embossing technique. We used a scanning electron microscope (SEM), spectrophotometry, and non-contact optical profilometer to evaluate the replication fidelity qualitatively: the lens mold and the fine three-dimensional microstructures on the PMMA substrate surfaces. The results of the surface finish of the diamond machined mold sample are in the range of 4.92 nm (areal average surface roughness Sa) and 6.04 nm (areal root-mean-squared roughness Sq), respectively, and the values for the replicas being 4.73 nm and 5.94 nm, respectively. The results demonstrated that the geometry form accuracy obtained of the microfeatures was at the submicron level with little viscoelastic recovery. The surface roughness in the nanometer level got successfully replicated.

Rapid Fabrication of Continuous Surface Fresnel Microlens Array by Femtosecond Laser Focal Field Engineering.

Femtosecond laser direct writing through two-photon polymerization has been widely used in precision fabrication of three-dimensional microstructures but is usually time consuming. In this article, we report the rapid fabrication of continuous surface Fresnel lens array through femtosecond laser three-dimensional focal field engineering. Each Fresnel lens is formed by continuous two-photon polymerization of the two-dimensional slices of the whole structure with one-dimensional scan of the corresponding two-dimensional engineered intensity distribution. Moreover, we anneal the lens array to improve its focusing and imaging performance.

Design and fabrication of micro-Fresnel lenses for thermoelectric conversion of near-infrared solar light

Design and fabrication of micro-Fresnel lenses for thermoelectric conversion of near-infrared solar light

Single crystal diamond blazed diffraction gratings and Fresnel microlens arrays with improved optical performance by high-resolution 3D laser lithography and pattern transfer by dry etching

We present a method to fabricate high precision 3D micro-optical components in single crystal diamond, and we demonstrate and experimentally characterize Fresnel microlens arrays and blazed diffraction gratings. The fabrication process is based on high-resolution 3D dip-in laser lithography and pattern transfer by chlorine-based reactive ion etching with a low diamond:photoresist selectivity (1:15) to obtain smooth etched surfaces with Ra < 2 nm. The fabricated microlenses have a focal length that deviates by 4% from the target value, while the blazed diffraction gratings have a maximum relative efficiency of 66% in the first order.

Compact compound-eye imaging module based on the phase diffractive microlens array for biometric fingerprint capturing.

A compound-eye imaging system based on the phase diffractive microlens array as a compact observation module is proposed. As compared with the refractive microlens in common compound-eye imaging systems, the diffractive microlens is a flat imaging optics featuring high relative aperture, thin component thickness and compatibility with lithography techniques. As an application, a compact fingerprint imaging module was demonstrated using this compound-eye imaging system. The phase Fresnel microlens array with continuous trough morphology was fabricated via the self-developed gray-scale laser direct write equipment. An image reconstruction method is proposed by extracting the effective image information of each Fresnel microlens, removing the complex signal separator layer from the compound-eye imaging system. The illumination optics is further planarized through the waveguide backlighting and the waveguide functions as the touch panel for fingerprint recording. The novel compound-eye imaging device length was only restricted by the focal length of the microlens with a low limit of 4.12f. The applicability of this novel compound-eye imaging system was further demonstrated by recording the human fingerprint texture, paving ways for various applications as a compact imaging system.

Fluorescent sensing with Fresnel microlenses for optofluidic systems

The concept of fluorescent sensing in a microchannel equipped with focusing light Fresnel lenses has been demonstrated. The concept employs a line or array of Fresnel lenses generating a line or array of focused light spots within a microfluidic channel, to increase the sensitivity of fluorescent signal detection in the system. We have presented efficient methods of master mold fabrication based on the lithography method and focused ion beam milling. The flexible microchannel was fabricated by an imprint process with new thiolene-epoxy resin with a good ability to replicate even submicron-size features. For final imprinted lenses, the measured background to peak signal level shows more than nine times the increase in brightness at the center of the focal spot for the green part of the spectrum (532 nm). The effectiveness of the microlenses in fluorescent-marked Escherichia coli bacteria was confirmed in a basic fluoroscope experiment, showing the increase of the sensitivity of the detection by the order of magnitude.

Tailored-waveguide based photonic chip for manipulating an array of single neutral atoms.

We propose a tailored-waveguide based photonic chip with the functions of trapping, coherently manipulating, detecting and individually addressing an array of single neutral atoms. Such photonic chip consists of an array of independent functional units spaced by a few micrometers, each of which is comprised of one silica-on-silicon optical waveguide and one phase Fresnel microlens etched in the middle of the output interface of the optical waveguide. We fabricated a number of photonic chips with 7 functional units and measured optical characteristics of these chips. We further propose feasible schemes to realize the functions of such photonic chip. The photonic chip is stable, scalable and can be combined with other integrated devices, such as atom chips, and can be used in the future hybrid quantum system and photonic quantum devices.

Direct laser writing of Fresnel microlenses with use of the scanning contour ablation method

Modern technological trends require the miniaturization of various devices, and hence the fabrication of micro-scale optical elements. Despite the existence of photolithography methods, which are commonly used for this purpose, there is still a need for developing fast prototyping solutions. In fact, the approach called ‘scanning contour ablation’, a direct laser writing-based method, has been developed for the fast reproducing of spherical micro-structures. In this paper, by introducing beam diameter as a variable and thus by updating the model, an improvement to the corresponding method is shown so that more complex structures like the Fresnel microlens can be reproduced. To confirm the assumptions, fabrication of exemplary microlenses was performed. The results of the study were characterized by a digital optical microscopy and compared to the given profiles.

Fresnel lenses fabricated by femtosecond laser micromachining on polymer one-dimensional photonic crystal

We report the fabrication of micro-Fresnel lenses by femtosecond laser surface ablation on one-dimensional (1-D) polymer photonic crystals. This device is designed to focus the transmitted wavelength (520 nm) of the photonic crystal and filter the wavelengths corresponding to the photonic band-gap region (centered at 630 nm, ranging from 530 to 700 nm). Integration of such devices in a wavelength selective light harvesting and filtering microchip is envisaged.

Microlens array fabricated by a low-cost grayscale lithography maskless system

This work presents the fabrication of a contiguous f/#=f/15 Fresnel microlens array (MLA) by employing a low-cost home-built maskless exposure lithographic system based on a digital light projector technology by using Texas Instruments’ digital micromirror device chip. A continuous diffractive phase relief structure was generated on a photoresist-coated silicon wafer, replicated in polydimethylsiloxane (PDMS) and electrostatically bonded to a glass substrate. The whole exposure time takes 10.8 min to expose a 2.4×2.4 mm MLA, with a resolution of 2.5 μm. This exposure time is relatively short, enabling high throughput or fast prototyping. Optical characterization was carried out using a He-Ne laser source (λ=633 nm ), by evaluating the maximum intensity of each spot generated at the MLA focal plane, Imax, as well as its sharpness by measuring their full width at half maximum (FWHM) intensity values. The resulting FWHM and maximum intensity spot average values were FWHM AVG =20±8% μm and Imax AVG =0.71±7% a.u. , respectively. The quality of replication was evaluated by profile characterization of the resulting mold and replica based on step height measurement along 180 μm. The maximum obtained difference was 32 nm, corresponding to 2.5% of the total mold height or λ/20 . AFM measurements were also carried out to quantify the roughness quality between mold and replica. The resulting RMS roughness was 4.73 nm (λ/130 ) and 6.66 nm (λ/95 ) for mold and replica, respectively. A comparison between theoretical and measured intensity profiles at the MLA focal plane was also carried out. A good correspondence between the results was found. Such an MLA can be applied as a Shack–Hartmann wavefront sensor in optical interconnects and to enhance the efficiency of detector arrays.

Characterization of “Bulk Lithography” Process for Fabrication of Three-Dimensional Microstructures

Various ways of fabricating a three-dimensional (3D) component in a single-layer exposure using spatial variation of exposure dose have been presented in the literature. While some of them are based on dynamic mask process, more recently, a process based on varying intensity of a scanning Gaussian laser beam termed as “bulk lithography” has been proposed. In bulk lithography, the entire varying depth 3D microstructure gets fabricated because of spatial variation of intensity of laser imposed at every point in single layer scan. For the bulk lithography process, this paper first presents experimental characterization of unconstrained depth photopolymerization of resin upon exposure to Gaussian laser beam. Experimental characterization carried out for two resins systems: namely 1,6 hexane diol-diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA), over relatively wider range of Ar+ laser exposure dose and time, show behavior well beyond Beer–Lambert law. A unified empirical model is proposed to represent the nondimensional depth variation with respect to the time and energy of exposure for both resins. Finally, using these models, successful fabrication of several microstructures including micro-Fresnel lens, textured curved surface, otherwise difficult or impossible to fabricate, is demonstrated. Several advantages of the bulk lithography as compared to other similar processes in the literature are highlighted.

Data Fusion Strategy for Multiscale Surface Measurements

Interdisciplinary research efforts have started focusing on the development of multiscale models and development of designer multiscale surfaces exhibiting specific properties at different scales for a specific purpose. With the rapid evolution of these new engineered surfaces for microelectromechanical systems (MEMS), microfluidics, etc., there is a strong need for developing tools to measure and characterize these surfaces at different scales. In order to obtain all meaningful details of the surface at various required scales, one is left with the only option of measuring the surface using multiple technologies using a combination of instruments. The majority of hardware-based approaches focus on the development of systems housing multiple technologies/capabilities into a single frame. These systems enable the user to obtain different surface maps using various technologies, but the user does not readily have the ability to combine all the obtained data into one single dataset. The effective approach toward multiscale measurement and characterization would be to use the individual measurement tools and finding a method to relate the individual coordinate systems and use an offline virtual tool to unify, manipulate, segment, merge, and retrieve data. Shape primitives and focus-based fusion strategies cannot be used as every data point in the data sets under consideration has to be treated as essentially at optimal focus. A multiscale data fusion strategy results in edge effects on nonplanar and high aspect ratio surfaces. An optimized fusion strategy, the “FWR method,” for the surface metrology domain is proposed where the subimages obtained from discrete wavelet frame (DWF) were separated into three regimes—form, waviness, and roughness—and fusion was not performed on subimages in the form regime. This approach effectively eliminates the edge effects. Individual data-point-level fusion was successfully demonstrated on Fresnel microlens array surface data as a case study of a nondirectional engineered surface with high aspect ratio.

Microlens Array Fabricated by Gray-Scale Lithography Maskless System

This work presents the fabrication of a high fill factor Fresnel microlens array (MLA) by employing a low-cost home-built maskless exposure lithographic system. A phase relief structure was generated on a photoresist-coated silicon wafer, replicated in Polydimethylsiloxane (PDMS) and electrostatically bonded to a glass substrate. Optical characterization was based on the evaluation of the maximum intensity of each spot generated at the MLA focal plane as well as its full width at half maximum (FWHM) intensity values. The resulting FWHM and maximum intensity spot mean values were 50 ± 8%μm and 0.71 ± 7% a.u , respectively. Such a MLA can be applied as Shack-Hartmann wavefront sensors, in optical interconnects and to enhance the efficiency of detector arrays.

A single-pixel wireless contact lens display

We present the design, construction and in vivo rabbit testing of a wirelessly powered contact lens display. The display consists of an antenna, a 500 × 500 µm2 silicon power harvesting and radio integrated circuit, metal interconnects, insulation layers and a 750 × 750 µm2 transparent sapphire chip containing a custom-designed micro-light emitting diode with peak emission at 475 nm, all integrated onto a contact lens. The display can be powered wirelessly from ∼1 m in free space and ∼2 cm in vivo on a rabbit. The display was tested on live, anesthetized rabbits with no observed adverse effect. In order to extend display capabilities, design and fabrication of micro-Fresnel lenses on a contact lens are presented to move toward a multipixel display that can be worn in the form of a contact lens. Contact lenses with integrated micro-Fresnel lenses were also tested on live rabbits and showed no adverse effect.

Designing Fresnel microlenses for focusing astigmatic multi-Gaussian beams by using fractional order Fourier transforms

According to a scalar theory of diffraction, light propagation can be expressed by two-dimensional fractional order Fourier transforms. Since the fractional Fourier transform of a chirp function is a Dirac distribution, focusing a light beam is optically achieved by using a diffractive screen whose transmission function is a two-dimensional chirp function. This property is applied to designing Fresnel microlenses, and the orders of the involved Fourier fractional transforms depend on diffraction distances as well as on emitter and receiver radii of curvature. If the emitter is astigmatic (with two principal radii of curvature), the diffraction phenomenon involves two one-dimensional fractional Fourier transforms whose orders are different. This degree of freedom allows us to design microlenses that can focus astigmatic Gaussian beams, as produced by a line-shaped laser diode source.

Analysis of micro-Fresnel lenses with local grating theory and its comparison with fully electromagnetic methods

We report on a numerical analysis method for diffractive optical elements that consist of features ranging from subwavelength to more than 10lambda. The essence of the method is treating local structures of the optical elements as diffraction gratings. It is shown that the method can provide accuracy of results comparable with fully electromagnetic treatments in much shorter time. The theory and results are explained assuming micro-Fresnel lenses with one-dimensional structures for investigating polarization properties.

SiO_2-based nonplanar structures fabricated using femtosecond laser lithography

SiO2-based hybrid diffractive-refractive microlenses were fabricated by femtosecond laser lithography-assisted micromachining, which is a combined process of nonlinear lithography and plasma etching. The high-aspect-ratio patterns of resist were formed by laser exposure without translating the laser spot. By scanning this rod three-dimensionally, micro-Fresnel lens patterns were written directly inside resists on the convex lenses. Then, the resist patterns were transferred to the underlying lenses by CHF(3) plasma. We obtained SiO2 nonplanar structures with smooth surfaces. This hybridization shifted the focal length of the lens by 216 microm, which was consistent with theoretical value.