Refractive Microlens Research Articles

The impact of anodic mesh density and halogen anions on the optical characteristics of gradient refractive index microlens arrays fabricated in chalcohalide glass via microthermal poling

Gradient refractive index microlens arrays (GRIN MLAs), covering the visible to mid-infrared wavelength range, are imprinted in GeS2-Ga2S3-CsX (X = Cl, Br, and I) chalcohalide glasses by an efficient and economical microthermal poling process. This study investigates the impact of anodic mesh number (ranging from 400 to 2000 mesh) and halogen anions on the optical properties, including focal length, resolution, and aberration. Findings reveal that as the mesh number decreases, the focal length and aberration of GRIN MLAs increase. Optimal resolution is achieved with the 500 mesh sample, which also shows the best phase difference performance. The migration depth of Cs+ ions, influenced by halogen anions (from Cl to I), results in contrasting optical properties between lower mesh (400–750) and higher mesh (1000–2000) samples. Notably, the Cl-500 GRIN MLAs demonstrate a focal length of 615.0 μm, a maximum resolution of 6.35 lp/mm, and minimal aberration of −0.0591. These changes are attributed to the predominant transverse and longitudinal migration of Cs+ ions in mesh square areas. This research highlights the relationship between microthermal poling conditions, ion migration, and optical performance, providing valuable insights for the application of GRIN MLAs in advanced optical systems.

Microlens Hollow-Core Fiber Probes for Operando Raman Spectroscopy.

We introduce a flexible microscale all-fiber-optic Raman probe which can be embedded into devices to enable operando in situ spectroscopy. The facile-constructed probe is composed of a nested antiresonant nodeless hollow-core fiber combined with an integrated high refractive index barium titanate microlens. Pump laser 785 nm excitation and near-infrared collection are independently characterized, demonstrating an excitation spot of full-width-half-maximum 1.1 μm. Since this is much smaller than the effective collection area, it has the greatest influence on the collected Raman scattering. Our characterization scheme provides a suitable protocol for testing the efficacy of these fiber probes using various combinations of fiber types and microspheres. Raman measurements on a surface-enhanced Raman spectroscopy sample and a copper battery electrode demonstrate the viability of the fiber probe as an alternative to bulk optic Raman microscopes, giving comparable collection to a 10 objective, thus paving the way for operando Raman studies in applications such as lithium battery monitoring.

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

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

Challenges and prospects for multi-chip microlens imprints on front-side illuminated SPAD imagers.

The overall sensitivity of frontside-illuminated, silicon single-photon avalanche diode (SPAD) arrays has often suffered from fill factor limitations. The fill factor loss can however be recovered by employing microlenses, whereby the challenges specific to SPAD arrays are represented by large pixel pitch (> 10 µm), low native fill factor (as low as ∼10%), and large size (up to 10 mm). In this work we report on the implementation of refractive microlenses by means of photoresist masters, used to fabricate molds for imprints of UV curable hybrid polymers deposited on SPAD arrays. Replications were successfully carried out for the first time, to the best of our knowledge, at wafer reticle level on different designs in the same technology and on single large SPAD arrays with very thin residual layers (∼10 µm), as needed for better efficiency at higher numerical aperture (NA > 0.25). In general, concentration factors within 15-20% of the simulation results were obtained for the smaller arrays (32×32 and 512×1), achieving for example an effective fill factor of 75.6-83.2% for a 28.5 µm pixel pitch with a native fill factor of 28%. A concentration factor up to 4.2 was measured on large 512×512 arrays with a pixel pitch of 16.38 µm and a native fill factor of 10.5%, whereas improved simulation tools could give a better estimate of the actual concentration factor. Spectral measurements were also carried out, resulting in good and uniform transmission in the visible and NIR.

Commented review on refractive microlenses and microlens arrays metrology

The fabrication of high-quality microlenses is possible only when it is assisted by efficient and accurate metrology. For this reason, developing expertise in measurement procedures is crucial for the micro-optics manufacturer. We review and comment on this topic by first discussing what features of microlenses must be characterized and controlled. We then review the existing techniques and instruments that can be employed for this task. Finally, we detail the limitations of these different methods, we compare them, and we propose practical guidelines to setup effective metrology methodology. We believe that this paper can be an introduction to the topic but also serves as a reference document for the more experienced reader.

Back-side illuminated optical stack optimized with a high refractive index micro-lens array for CMOS image sensors

We simulate a simplified optical stack model with Lumerical FDTD over wide ranges of optical parameters: wavelength, 550 nm to 1.4 μm, and pixel array pitch, 1 to 10 μm. By increasing the lens and the planarization layer refractive indices from the usual 1.5 to 2, we study the improved lens focalization abilities, in terms of spot width, power, and depth below the micro-lens. We show an NIR wavelength range benefit from slightly increasing the optical stack index to 1.7 while visible wavelengths have less interest. Indeed, increasing the lens and planarization layer index enables thinner back-side illuminated optical stacks for higher quantum efficiencies and angular response improvement.

Correction of spherical surface measurements by confocal microscopy

Refractive microlenses are nowadays widely used in optical systems. Characterizing their surface is essential to ensure their quality and to optimize their fabrication process. This is realized by optical surface profilers thanks to their vertical resolution, short measurement time and areal information. However, when measuring non-flat surfaces, errors appear caused by aberrations of the microscope objective used in such systems, which significantly limit the achievable quality of the manufactured spherical surfaces. Approaches have been proposed to tackle these errors, but none of them demonstrated its validity for measurements of high quality microlenses. In this work, we demonstrate that the surface error depends on the surface position within the field of view of the microscope objective and on the surface slope. We then explain how to record the value of this error experimentally: this can be done by measuring a reference ball placed at different positions in the field of view. We finally use a machine learning algorithm to fit the experimental data in order to correct subsequent measurements. We apply this approach to measurements performed by a 20× numerical aperture 0.6 microscope objective of a confocal microscope. The effectiveness of the proposed method is demonstrated by showing that the surface error corresponds to a RMS wavefront error of λ/7 before correction and of λ/50 after correction for glass microlenses used in the visible range. This method thus allows the use of high numerical aperture microscope objectives for an accurate characterization of microlenses. Likewise, the fabrication capability of microlenses in terms of slope and quality is greatly extended, which is especially important for aspheres or freeforms.

Diamond refractive micro-lenses for full-field X-ray imaging and microscopy produced with ion beam lithography.

We demonstrate that ion-beam lithography can be applied to the fabrication of rotationally parabolic refractive diamond X-ray micro-lenses that are of interest to the field of high-resolution X-ray focusing and microscopy. Three single half-lenses with curvature radii of 4.8 µm were produced and stacked to form a compound refractive lens, which provided diffraction-limited focusing of X-ray radiation at the P14 beamline of PETRA-III (DESY). As shown with SEM, the lenses are free of expressed low- and high-frequency shape modulations with a figure error of < 200 nm and surface roughness of 30 nm. Precise micro-manipulation and stacking of individual lenses are demonstrated, which opens up new opportunities for compact X-ray microscopy with nanometer resolution.

折射型红外微透镜阵列器件的发展及制备

折射型红外微透镜阵列器件的发展及制备

Fabrication of refractive silicon microlens array with a large focal number and accurate lens profile

In this paper, we demonstrate the fabrication of refractive silicon microlens array with a large focal number and almost perfect spherical lens shape. Through a modified thermal reflow, photoresist microlens array of a large focal number is fabricated, which is then transferred into the silicon substrate by ion beam milling. To reach an accurate spherical lens profile, we both theoretically and experimentally study the practical factors that harm the pattern transfer fidelity, which mainly include the etching selectivity and faceting effect. Other secondary etching effects, such as the trenching effect and re-deposition effect, are also discussed. Based on these studies, a silicon microlens array with a focal number of 1.35 has been successfully obtained, with the profile error controlled well-below 0.121 μm, less than λ/6 within the whole infrared wavelength band. Besides, the fabricated microlens array exhibits a good uniformity and fine surface smoothness. The fabricated silicon microlens arrays can be applied in minatured infrared and terahertz imaging devices, or used as the master mould for soft lithography.

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.

Quadratic phase modulation and diffraction-limited microfocusing generated by pairs of subwavelength dielectric scatterers

Abstract Diffractive approaches are needed when refractive microlenses reach their focusing limit at the micron-scale in visible light. Previously, we have reported on micron-sized optical lenses based on the diffraction of metallic nanowires. Here, we extend our study to lenses based on pairs of subwavelength dielectric scatterers. Using simulations by two-dimensional finite element method, we demonstrate that focusing holds for pair spacings as small as the wavelength-size. For pairs with distances between inner walls larger than about 1.2λ, the scattered waves generate a quadratic phase modulation on the total propagating field leading to a diffraction-limited focusing i.e. an effective optical lens effect with high numerical aperture. In addition, they have low sidelobe intensities, long depths of focus, and they have a low sensitivity with polarization. For pairs with inner wall distances smaller than about 1.2λ, the focusing phase modulation is accumulated during the propagation through the dielectric pair structure. In this work, we report only on the experimental demonstration for the case of larger wall separation to emphasize on the scattered wave effect on micro-focusing. A pair of parallel polymer lines (cylindrical lens), and a grid of polymer lines (square microlens array) with 2 μm-spacing were fabricated by two-photon induced polymerization. Their focal lengths are comparable to their separating distances, their spot-sizes are 0.37 μm and 0.28 μm at wavelength 530 nm, and their focusing efficiencies are 70% and 60%, respectively.

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.

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.

Refractive micro-lenses and micro-axicons in single-crystal lithium niobate.

The high refractive index of lithium niobate crystal (n = 2.2) and the highly transparent range (300-5000 nm), makes it a perfect material for refractive lenses and other types of micro-optical elements. This material already finds extensive use in waveguides and photonic crystals, however, little work has been done on producing refractive optical components in lithium niobate, presumably due to the challenges associated with its fragility and difficulties in three-dimensional micromachining. In this study, we fabricated high-quality refractive micro-lenses and micro-axicons with low surface roughness (< λvis / 20), with 220 µm diameters and sag heights up to 22 µm in single-crystal LN using focused Xe beam milling. Xe ion beam milling is a flexible and rapid technique allowing realization of complex three-dimensional surface reliefs directly in lithium niobate. We characterized the optical performance of the fabricated elements showing sub-µm focusing capabilities of both the lenses and axicons.

Parametric numerical study of the modulation transfer function in small-pitch InGaAs/InP infrared arrays with refractive microlenses.

We report on the modulation transfer function (MTF) in short-wave infrared indium gallium arsenide (InGaAs) on indium phosphide (InP) planar photodetector arrays. Our two-dimensional numerical method consists of optical simulations using the finite-difference time domain method and drift-diffusion simulations using the finite-element method. This parametric study investigates MTF dependence on pitch, the addition of refractive microlenses, the thickness of the InGaAs absorber, and the doping concentration of the InGaAs absorber. A focus is placed on the connection between the lateral diffusion of photogenerated holes in InGaAs and the MTF. It is found that the MTF of small-pitch arrays exhibit sub-ideal behavior due to pixel cross-talk resulting from a long minority carrier diffusion length. By incorporating monolithic microlenses with the InP substrate, the MTF response is improved for all pitches investigated, particularly for spatial frequencies near the respective cutoff frequencies. We also find a strong dependence of the MTF on the thickness and doping concentration in the absorbing region. Trends in dark current and quantum efficiency are reported.

Fabrication of the glass microlens arrays and the collimating property on nanolaser

The influence of the glass microlens array on the collection efficiency and beam collimating with the ZnO/GaN nanoheterojunction nanolaser is investigated. Ultraviolet random lasing behavior from the ZnO/GaN nanoheterojunction array has been demonstrated with both optical and electrical pumping. The refractive microlens arrays are designed and fabricated on the glass slide using the microfabrication procedures and integrated on the nanolaser. We show that, for the nanolaser with divergent output beams, the introduction of glass microlens arrays reduces the beam divergence and increases transmission. The detected optical power is enhanced by more than 1.8 times for the nanolaser integrated with glass microlens arrays. The drastic increase in the optical collection efficiency is resulted from the laser beam collimating.

Multilayer polymer light guides integrated with refractive microlenses and liquid by micromolding and inkjet printing

Optofluidic behavior has been studied over a decade for the transmission of light through fluidic channels by virtue of versatile fabrication schemes such as (soft-) lithography techniques. One of the crucial factors in the consideration of device design involves the use of a fluidic core filled into the solid channels to effectively control the planar direction of light guiding based on total internal reflection (TIR). However, there are still few studies investigating the optical performance of light transmission associated with an optofluidic design for bifurcation and out-of-plane transmission of light beams. This paper reports a droplet filling method that can significantly improve the controllability over the formation of liquid flow and solid convex lenses utilizing the polydimethylsiloxane (PDMS) layers fabricated by soft-lithography technique. Analytical and experimental and results indicate that the curvatures of hemispherical profiles for the refractive microlenses formed were simply varied by different droplet volumes (single droplet of 1–10pl) placed on the cavity-structured surfaces, thus providing the capability of control over volumes in design. In this way, the individual LGs fabricated from the micromolding and inkjet printing could be applied in many applications, in particular the future lab-on-a-chip (LOC) microsystem and micrototal analysis system (μTAS).

An electrically tunable plenoptic camera using a liquid crystal microlens array.

Plenoptic cameras generally employ a microlens array positioned between the main lens and the image sensor to capture the three-dimensional target radiation in the visible range. Because the focal length of common refractive or diffractive microlenses is fixed, the depth of field (DOF) is limited so as to restrict their imaging capability. In this paper, we propose a new plenoptic camera using a liquid crystal microlens array (LCMLA) with electrically tunable focal length. The developed LCMLA is fabricated by traditional photolithography and standard microelectronic techniques, and then, its focusing performance is experimentally presented. The fabricated LCMLA is directly integrated with an image sensor to construct a prototyped LCMLA-based plenoptic camera for acquiring raw radiation of targets. Our experiments demonstrate that the focused region of the LCMLA-based plenoptic camera can be shifted efficiently through electrically tuning the LCMLA used, which is equivalent to the extension of the DOF.

Rapid fabrication of thermoplastic polymer refractive microlens array using contactless hot embossing technology.

A thermoplastic polymer refractive microlens array has been rapidly fabricated by contactless hot embossing technology through the stainless steel template with micro through-holes array, which has a diameter of 150 µm and a pitch of 185 µm. By optimizing the technical parameters including heating and demoulding temperature, loading pressure, loading and pressure holding time, a series of high quality microlenses arrays of different sags could be obtained. In addition, the sag and the radius of curvature of the microlens are controllable. The geometrical and optical properties of the microlenses are measured and the influence of temperature and pressure duration on the optical properties of the microlenses are analysed. The results show good surface features and optical performances. Unlike previous contactless hot embossing, a low cost and durable stainless steel template was utilized instead of silicon or nickel mold to avoid valuable equipments and complicated fabrication procedure. Besides, the whole contactless hot embossing process was absence of vacuum equipment. We think that the technology could be an attractive high flexibility method for enhancing efficiency and reducing cost.