Micro-optical Elements Research Articles

High-resolution grayscale patterning using extreme ultraviolet interference lithography

Grayscale patterning is technologically relevant in the fabrication of micro-optics elements, microfluidics, micro-electromechanical devices, to name a few. So far, the state-of-the-art is limited to micrometric scale, which is of interest for optical applications in the visible spectrum. In this work, we used extreme ultraviolet light and an interference lithography method to demonstrate the feasibility of grayscale patterning with high lateral and vertical resolution. A double exposure was carried out on poly(methyl methacrylate) photoresist using periodic lines/spaces of two different pitches (100nm and 50nm). The resulting morphology of photoresist after development, analyzed by scanning electron microscopy and atomic force microscopy, showed that a dense one-sided blaze profile consisting of three grayscale levels was obtained. The equivalent groove density of the blazed grating was 10,000lines/mm, which is a remarkable achievement of significant interest for applications such as high resolution and high efficiency diffraction optics. Furthermore, combinations of overlay from non-multiple pitches (100nm and 80nm) was accomplished and unconventional structures with nested trenches with total period of 400nm were obtained.

Wave-optical modeling beyond the thin-element-approximation.

The optical design and analysis of modern micro-optical elements with high index contrasts and large numerical apertures is still challenging, as fast and accurate wave-optical simulations beyond the thin-element-approximation are required. We introduce a modified formulation of the wave-propagation-method and assess its performance in comparison to different beam-propagation-methods with respect to accuracy, required sampling densities, and computational performance. For typical micro-optical components, the wave-propagation-method is found to be considerably faster and more accurate at even lower sampling densities compared to the different beam-propagation-methods. This enables realistic wave-optical simulations beyond the thin-element-approximation for micro-optical components. As an example, the modified wave-propagation-method is applied for in-line holographic measurements of strongly diffracting objects. From a direct comparison of experimental results and corresponding simulations, the geometric parameters of a test object could be retrieved with high accuracy.

Microlens-aided focusing of linearly and azimuthally polarized laser light.

We have investigated a four-sector transmission polarization converter (4-SPC) for a wavelength of 633 nm, that enables the conversion of a linearly polarized incident beam into a mixture of linearly and azimuthally polarized beams. It was numerically shown that by placing a Fresnel zone plate of focal length 532 nm immediately after the 4-SPC, the incident light can be focused into an oblong subwavelength focal spot whose size is smaller than the diffraction limit (with width and breadth, respectively, measuring FWHM = 0.28λ and FWHM = 0.45λ, where λ is the incident wavelength and FWHM stands for full-width at half maximum of the intensity). After passing through the 4-SPC, light propagates in free space over a distance of 300 μm before being focused by a Fresnel zone plate (ZP), resulting in focal spot measuring 0.42λ and 0.81λ. The focal spot was measured by a near-field microscope SNOM, and the transverse E-field component of the focal spot was calculated to be 0.42λ and 0.59λ. This numerical result was verified experimentally, giving a focal spot of smaller and larger size, respectively, measuring 0.46λ and 0.57λ. To our knowledge, this is the first implementation of polarization conversion and subwavelength focusing of light using a pair of transmission micro-optic elements.

Photomask displacement technology for continuous profile generation by mask aligner lithography

Mask aligner lithography is one of the most widely used technologies for micro-optical elements fabrication. It offers a high throughput with high-yield processing. With different resolution enhancement technologies shadow printing is a mature alternative to the more expensive projection or electron-beam lithography. We are presenting a novel mask aligner tool that allows shifting the photomask with high accuracy between sequential exposure shots. It offers possibilities such as double patterning or gray tone lithography by applying different light doses at different locations. Within this publication, we show the first results for high resolution blazed grating structures generated in photoresist by multiple exposures using a conventional binary photomask.

Continuous Sensing Photonic Lab-on-a-Chip Platform Based on Cross-Linked Enzyme Crystals.

Microfluidics or lab-on-a-chip technology offer clear advantages over conventional systems such as a dramatic reduction of reagent consumption or a shorter analysis time, which are translated into cost-effective systems. In this work, we present a photonic enzymatic lab-on-a-chip reactor based on cross-linked enzyme crystals (CLECs), able to work in continuous flow, as a highly sensitive, robust, reusable, and stable platform for continuous sensing with superior performance as compared to the state of the art. The microreactor is designed to facilitate the in situ crystallization and crystal cross-linking generating enzymatically active material that can be stored for months/years. Thus, and by means of monolithically integrated micro-optics elements, continuous enzymatic reactions can be spectrophotometrically monitored. Lipase, an enzyme with industrial significance for catalyzed transesterification, hydrolysis, and esterification reactions, is used to demonstrate the potential of the microplatforms as both a continuous biosensor and a microreactor for the synthesis of high value compounds.

Effects of 3D microlens transfer into fused silica substrate by CF4/O2 dry etching

Nowadays, 3D microoptical elements find a variety of applications from light emitting diodes and household appliances to precise medical endoscopes. Such elements, fabricated in a fused silica substrate by combining 3D e-beam patterning and dry etching, can be used as a mold for the high throughput replication in polymeric materials by UV nanoimprint technique. Flexible and precise control of 3D shape in the resist layer can be achieved by e-beam patterning, but it is also very important to know peculiarities of 3D pattern transfer from resist layer into the fused silica substrate. This paper reports on the effects of PMMA 3D microlens pattern transfer into fused silica substrate by CF4/O2 dry etching. It is demonstrated that etching rate ratio between PMMA and fused silica changes during plasma treatment. Thus, the resulting shape of transferred 3D profile is different from the shape in PMMA and this variation must be assessed during the design phase.

Hybrid curved nano-structured micro-optical elements.

Tailoring the spatial degree of freedom of light is an essential step towards the realization of advanced optical manipulation tools. A topical challenge consists of device miniaturization for improved performance and enhanced functionality at the micron scale. We demonstrate a novel approach that combines the additive three-dimensional (3D) structuring capability of laser polymerization and the subtractive subwavelength resolution patterning of focused ion beam lithography. As a case in point hybrid (dielectric/metallic) micro-optical elements that deliver a well-defined topological shaping of light are produced. Here we report on hybrid 3D binary spiral zone plates with unit and double topological charge. Their optical performances are compared to corresponding 2D counterparts both numerically and experimentally. Cooperative refractive capabilities without compromising topological beam shaping are shown. Realization of advanced designs where the dielectric architecture itself is endowed with singular properties is also discussed.

Sub-micrometre accurate free-form optics by three-dimensional printing on single-mode fibres.

Micro-optics are widely used in numerous applications, such as beam shaping, collimation, focusing and imaging. We use femtosecond 3D printing to manufacture free-form micro-optical elements. Our method gives sub-micrometre accuracy so that direct manufacturing even on single-mode fibres is possible. We demonstrate the potential of our method by writing different collimation optics, toric lenses, free-form surfaces with polynomials of up to 10th order for intensity beam shaping, as well as chiral photonic crystals for circular polarization filtering, all aligned onto the core of the single-mode fibres. We determine the accuracy of our optics by analysing the output patterns as well as interferometrically characterizing the surfaces. We find excellent agreement with numerical calculations. 3D printing of microoptics can achieve sufficient performance that will allow for rapid prototyping and production of beam-shaping and imaging devices.

Photonic Lab-on-a-Chip: Integration of Optical Spectroscopy in Microfluidic Systems.

The integration of micro-optical elements with microfluidics leads to the highly promising photonic lab-on-a-chip analytical systems (PhLoCs). In this work, we re-examine the main principles which are underneath the on-chip spectrophotometric detection, approaching the PhLoC concept to a nonexpert audience.

Nano-proximity direct ion beam writing

AbstractFocused ion beam (FIB) milling with a 10 nm resolution is used to directly write metallic metasurfaces and micro-optical elements capable to create structured light fields. Surface density of fabricated nano-features, their edge steepness as well as ion implantation extension around the cut line depend on the ion beam intensity profile. The FIB beam intensity cross section was evaluated using atomic force microscopy (AFM) scans of milled line arrays on a thin Pt film. Approximation of two Gaussian intensity distributions describes the actual beam profile composed of central high intensity part and peripheral wings. FIB fabrication reaching aspect ratio of 10 in gold film is demonstrated.

Taper array in silica glass for beam splitting

We proposed taper array in silica glass for beam splitting which was fabricated by water-assisted femtosecond laser direct writing technology and the subsequent heat treatment. We divided the array into many fabricating cells which were executed automatically in sequences as specified by the program that contained the information for the three-dimensional stage movements. Each cell could fabricated a rectangular cylinder. The size and distribution of the rectangular cylinder could be controlled by adjusting the position of the fabricating cells. Then the heat treatment should be used to reshape the rectangular cylinders into taper array. The experimental results show that the taper periodic microstructures in silica glass are uniform and smooth, and the tapers can divide the incident light into beam array. The results demonstrated that the combination of the water-assisted femtosecond laser direct writing technology and the heat treatment is accessible and practical for the high quality micro-optical elements. These micro-optical elements will have potential applications in fluorescence detection and beam splitter.

Methods for on-line testing of characteristics of diffractive and conformal optical elements during the manufacturing process

Development of dedicated methods for nondestructive testing of microstructured optical elements is a prerequisite for the wide-scale manufacturing of diffractive, micro-optical and conformal elements. A variety of approaches to the characterization of diverse types of microstructured optical elements were developed at the laboratory of Diffractive Optics at IAE SB RAS. These methods are intended to test wavefront errors, etch depth and diffraction efficiency during and after the manufacturing of elements with binary and multi-level surface profile. The review of the developed techniques and measuring circuits given in the work takes into account specific features of the elements' parameters and application areas. Testing results for the manufactured elements are discussed.

Simple method based on intensity measurements for characterization of aberrations from micro-optical components.

We report a simple method, based on intensity measurements, for the characterization of the wavefront and aberrations produced by micro-optical focusing elements. This method employs the setup presented earlier in [Opt. Express 22, 13202 (2014)] for measurements of the 3D point spread function, on which a basic phase-retrieval algorithm is applied. This combination allows for retrieval of the wavefront generated by the micro-optical element and, in addition, quantification of the optical aberrations through the wavefront decomposition with Zernike polynomials. The optical setup requires only an in-motion imaging system. The technique, adapted for the optimization of micro-optical component fabrication, is demonstrated by characterizing a planoconvex microlens.

Determining the refractive index distribution of optical components by dual-wavelength digital holographic microscopy with a liquid

A method is proposed for measuring the two-dimensional distribution of the refractive index in transparent micro-optical components. The proposed method combines dual-wavelength digital holographic microscopy with liquids to record holograms at different wavelengths and to numerically reconstruct the three-dimensional phase information to obtain the sample’s distribution of the refractive index. A theoretical model is proposed for determining the two-dimensional distribution of the refractive index; an achromatic lens and plate are demonstrated experimentally. Unlike conventional scanning interferometry, the proposed method supports non-scanning measurement and direct observation, independent of the sample’s shape, of the two-dimensional distribution of refractive indices in micro-optical elements.

Custom Designed Fabrication

AbstractThe silver sodium ion exchange process is a versatile and flexible technique for realizing high quality refractive microoptical elements with a high degree of freedom in design. An accurate mask technique allows for implementing a huge variety of optical functions, such as micro lens arrays with 100 % filling factor, custom designed micro lenses with different optical parameters (i. e. focal length, Numerical Aperture, sphericity, etc.) within the same array or beam shaping elements such as vortex elements. The article shows actual results and applications of these elements.

Tight focus of light using micropolarizer and microlens.

Using a binary microlens of diameter 14 μm and focal length 532 nm (NA=0.997) in resist, we focus a 633 nm laser beam into a near-circular focal spot with dimensions (0.35 ± 0.02)λ and (0.38 ± 0.02)λ (λ is incident wavelength) at full width half-maximum intensity. The area of the focal spot is 0.105λ(2). The incident light is a mixture of linearly and radially polarized beams generated by reflecting a linearly polarized Gaussian beam at a 100 μm × 100 μm four-sector subwavelength diffractive optical microelement with a gold coating. The focusing of a linearly polarized laser beam (the other conditions being the same) is found to produce an elliptical focal spot measuring (0.40 ± 0.02)λ and (0.50 ± 0.02)λ. To our knowledge, this is the first implementation of subwavelength focusing of light using a pair of micro-optic elements (a binary microlens and a micropolarizer).

Fast and Transparent Adaptive Lens Based on Plasmonic Heating

In the current trend toward miniaturization, integrated micro-optical elements play a central role in the development of various applications including high-density data storage and imaging. In these operations, fine alignment and focus adjustment are usually performed mechanically, thus, limiting the accuracy, size and speed of devices. Here, we propose a novel reconfigurable microlens based on the engineering of the temperature distribution induced by a patterned plasmonic surface shined with a resonant near-infrared light. The refractive index change generated in a surrounding thermo-optical material acts as an effective lens whose parameters are remotely adjusted by the control near-infrared light. We demonstrate focal distance tunability of tens of microns with subnanometer accuracy along with time-responses down to 200 μs. The applicability of this photothermal lens is proved in the framework of optical microscopy and adaptive optics.

Laser-induced Black-body Heating (LIBBH) as a Method for Glass Surface Modification

The technique for laser-assisted modification of glass surface is suggested. The method is based on the heating of glass by graphite plate irradiated by laser being in direct contact with the glass surface from the backside. Transparency of glass and strong absorption of graphite for an incident laser radiation provide the diversity of applicable lasers wavelength and pulse duration. In our case, the pulsed fiber NIR laser was used. The surface densification of porous glass as well as creation of various micro-optical elements on the quartz glass surface are reported.

Optimization of hybrid polymer materials for 2PP and fabrication of individually designed hybrid microoptical elements thereof

In the last decade, two-photon polymerization (2PP) has gained increasing interest for the production of individually shaped 3D structures. For the successful implementation of 2PP within a production chain for the industrial manufacturing of optical microstructures, advanced material properties of the photo polymer are required. The usable laser dynamic range, shape accuracy as well as surface roughness play a crucial role. In this publication, we present the results of an iterative optimization process aiming at a material applicable for 3D structures written by 2PP. The special focus here lies on the influence of the photo initiator and stabilizer on the usable laser dynamic range and shape fidelity. The application of the optimized photo polymer for the production of a complex prism array, which could be used as master mold, is also shown.

Micro-optics in lighting applications

Abstract The intention of this article is to give a concise overview on current applications of micro-optical components in lighting, including general lighting, automotive lighting, projection, and display backlighting. Regarding the light sources, the focus of this paper is on inorganic light-emitting diodes (LEDs) and the characteristic problems encountered with them. Lasers, laser diodes, and organic light-emitting diodes (OLEDs) are out of scope of this paper. Micro-optical components for current applications of inorganic LEDs may be categorized essentially into three classes: First, components for light shaping, i.e., adjusting the intensity distribution to a desired target; second, components for light homogenization with respect to space and color, and third, large-area micro-optical elements. These large-area elements comprise micro-optical slabs and sheets for guiding, reflection, and refraction of light and are designed without regard to particular details of type, design, arrangement, and layout of the individual light emitters. References are given to textbooks and review articles to guide the interested reader to further and more detailed studies on the problems discussed here.