Fabrication Of Micro-optical Elements Research Articles

Single femtosecond pulse writing of a bifocal lens.

In this Letter, a method for the fabrication of bifocal lenses is presented by combining surface ablation and bulk modification in a single laser exposure followed by the wet etching processing step. The intensity of a single femtosecond laser pulse was modulated axially into two foci with a designed computer-generated hologram (CGH). Such pulse simultaneously induced an ablation region on the surface and a modified volume inside the fused silica. After etching in hydrofluoric acid (HF), the two exposed regions evolved into a bifocal lens. The area ratio (diameter) of the two lenses can be flexibly adjusted via control of the pulse energy distribution through the CGH. Besides, bifocal lenses with a center offset as well as convex lenses were obtained by a replication technique. This method simplifies the fabrication of micro-optical elements and opens a highly efficient and simple pathway for complex optical surfaces and integrated imaging systems.

Benzylidene Cyclopentanone Derivative Photoinitiator for Two-Photon Photopolymerization-Photochemistry and 3D Structures Fabrication for X-ray Application.

Micron- and submicron-scale 3D structure realization nowadays is possible due to the two-photon photopolymerization (TPP) direct laser writing photolithography (DLW photolithography) method. However, the achievement of lithographic features with dimensions less than 100 nm is in demand for the fabrication of micro-optical elements with high curvature values, including X-ray microlenses. Spectroscopic and photochemical study of a photoinitiator (PI) based on a methyl methacrylate derivative of 2,5-bis(4-(dimethylamino)benzylidene) cyclopentanone was performed. Enhanced intersystem crossing in the methyl methacrylate derivative results in increased radical generation for the subsequent initiation of polymerization. A comprehensive study of the new photocompositions was performed, with particular emphasis on photochemical constants, the degree of photopolymerization, and topology. The optimal parameters for the fabrication of mechanically stable structures were determined in this research. The threshold dose parameters for lithography (radiation power of 5 mW at a speed of 180 µm/s) when trying to reach saturation values with a conversion degree of (35 ± 1) % were defined, as well as parameters for sub-100 nm feature fabrication. Moreover, the 45 nm feature size for elements was reached. Fabrication of X-ray lens microstructures was also demonstrated.

High laser induced damage threshold photoresists for nano-imprint and 3D multi-photon lithography

Abstract Optics manufacturing technology is predicted to play a major role in the future production of integrated photonic circuits. One of the major drawbacks in the realization of photonic circuits is the damage of optical materials by intense laser pulses. Here, we report on the preparation of a series of organic–inorganic hybrid photoresists that exhibit enhanced laser-induced damage threshold. These photoresists showed to be candidates for the fabrication of micro-optical elements (MOEs) using three-dimensional multiphoton lithography. Moreover, they demonstrate pattern ability by nanoimprint lithography, making them suitable for future mass production of MOEs.

Large-Scale and Localized Laser Crystallization of Optically Thick Amorphous Silicon Films by Near-IR Femtosecond Pulses.

Amorphous silicon (-Si) film present an inexpensive and promising material for optoelectronic and nanophotonic applications. Its basic optical and optoelectronic properties are known to be improved via phase transition from amorphous to polycrystalline phase. Infrared femtosecond laser radiation can be considered to be a promising nondestructive and facile way to drive uniform in-depth and lateral crystallization of -Si films that are typically opaque in UV-visible spectral range. However, so far only a few studies reported on use of near-IR radiation for laser-induced crystallization of -Si providing less information regarding optical properties of the resultant polycrystalline Si films demonstrating rather high surface roughness. The present work demonstrates efficient and gentle single-pass crystallization of -Si films induced by their direct irradiation with near-IR femtosecond laser pulses coming at sub-MHz repetition rate. Comprehensive analysis of morphology and composition of laser-annealed films by atomic-force microscopy, optical, micro-Raman and energy-dispersive X-ray spectroscopy, as well as numerical modeling of optical spectra, confirmed efficient crystallization of -Si and high-quality of the obtained films. Moreover, we highlight localized laser-induced crystallization of -Si as a promising way for optical information encryption, anti-counterfeiting and fabrication of micro-optical elements.

Fast fabrication of optical vortex generators by femtosecond laser ablation

Fast fabrication of micro-optical elements for generation of optical vortex beams based on the q-plate design is demonstrated by femtosecond (fs) laser ablation of gold film on glass. Q-plates with diameter of ∼0.5 mm were made in ∼1 min using galvanometric scanners with writing speed of 5 mm/s. Period of gratings of 0.8μm and groove width of 250 nm were achieved using fs-laser ablation at λ=343 nm wavelength. Phase and intensity analysis of optical vortex generators was carried out at 633 nm wavelength and confirmed the designed performance. Efficiency of spin–orbital conversion of the q-plates made by ablation of 50-nm-thick film of gold was ∼3%. Such gratings can withstand thermal annealing up to 800 °C. They can be used as optical vortex generators using post-selection of polarisation.

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.

Fabrication of micro-optical elements on curved substrates by electrostatic induced lithography

This article reports the fabrication and characterization of polymeric micro-optical elements on curved substrates using electrostatic induced lithography.

Fabrication and characterization of microlenses made of tellurite and heavy metal oxide glass developed with hot embossing technology

In this paper we describe the work on application of hot embossing process to fabrication of micro-optical elements transmitting in the visible, near infrared and mid-infrared part of spectrum, using multi-component soft glasses as starting materials. Description of used hot embossing (HE) process is provided, followed by discussion of rheological and viscosity properties of several glass types considered for demonstrator fabrication experiment. Demonstrator microlenses were fabricated from lead-bismuth-gallium oxide (PBG08) and tellurite glasses (TWPN/I/6), with transmission windows extending from the visible up to the mid-infrared (up to around \(6.5\,\upmu {m}\)). Fabricated elements, which included diffractive and refractive lenses and lens arrays, were examined in context of homogeneity (quality of pattern replication) and obtained optical properties (\( {M}^{2}\), focal length, resolution). A demonstrator microscope objective was constructed using refractive plano-concave, plano-convex and biconvex lenses fabricated with the HE setup and its optical characteristics were provided.

Optimization of hot embossing method for development of soft glass microcomponents for infrared optics

Abstract Hot embossing is an attractive alternative to other micro-replication methods, since its cost effectiveness makes it suitable for mass-fabrication of wide range of elements. In this work, hot embossing is presented in context of fabrication of micro-optical elements made of soft glasses for near and mid infrared spectral region. Particular attention is given to the selection of suitable materials from which the mold is prepared, in order to minimize sticking and degradation of the embossing stamps. Refractive lens and diffractive gratings are successfully developed with the proposed approach.

Functional epoxy polymer for direct nano-imprinting of micro-optical elements

This work presents a functional resist for the direct fabrication of micro-optical elements with UV-enhanced substrate conformal imprint lithography (UV-SCIL). This functional resist is based on a UV-curable epoxy polymer where TiO2 nanoparticles are enclosed. Adding these particles to the polymer raises the refractive index of the functional resist with raising particle content from 1.52 up to 1.64 at a wavelength of 633nm. Therefore, the refractive index can be matched for many micro-optical applications. Here, multilevel Fresnel lenses from a 200mm silica substrate are transferred into the functional resist by UV-SCIL with nanometer resolution and high structure fidelity.

Fabrication of ridge waveguide structure from photosensitive TiO2/ormosil hybrid films by using an ultraviolet soft imprint technique

Photosensitive TiO2/organically modified silane hybrid films were prepared by combining a low-temperature sol–gel process with a spin-coating technique. Optical properties and photochemical activities of the as-prepared hybrid sol–gel films under different UV irradiation time were characterized and monitored by prism coupling technique, UV–visible spectroscopy, and Fourier transform infrared spectroscopy. Surface morphology of the hybrid films was also observed by an atomic force microscopy. Advantages for fabrication of ridge waveguide structure based on the photosensitive hybrid films were demonstrated by an ultraviolet soft imprint technique. Effects of imprint force, imprint time, and UV irradiation time on high replication fidelity of the ridge waveguide structure were also investigated. An altitude replication fidelity of 99.7% can be obtained when the imprint force of 2MPa, imprint time of 30min and UV irradiation time of 45min were chosen. Scanning electron microscopy and surface profiler were used to characterize the morphological and surface profile properties of the as fabricated ridge waveguide structure. Results indicate that the as-prepared photosensitive hybrid materials have great applicability for the fabrication of micro-optical elements and advantage as the imprint layer under the ultraviolet soft imprint technique.

Photo-induced optical changes in GexAs40Se60–xthin films

AbstractGe-As-Se chalcogenide thin films show a wide range of photosensitivity, which is utilized for the fabrication of micro-optical elements for integrated optics. The photosensitivity of GexAs40Se60–x(x=0, 15) chalcogenide thin films for UV light was presented. For that purpose, the bulk samples of GexAs40Se60–x(x=0, 15) chalcogenide glasses were prepared using conventional melt quenching technique, and thin films were prepared using thermal evaporation technique. These thin films were exposed to UV light for two hours. Amorphous natures of bulk samples and thin films were verified by XRD and chemical compositions were verified by EDX measurements. The thicknesses of the thin films were measured using a thickness profilometer. Linear optical analysis of these thin films was done using transmission spectra in wavelength range of 300–900 nm. Optical bandgap was determined by first peak of transmission derivative as well as extrapolation of Tauc's plot. R2 analysis was done using R software to ensure that the material is indirect bandgap material. It is observed that two hours UV exposure causes photo-darkening along with photo-expansion in As40Se60 thin films, while photo-bleaching and photo-densification for Ge15As40Se45 thin films. However, the amounts of photo-induced optical changes for Ge15As40Se45 thin films are smaller than those for As40Se60 thin films. The changes in optical absorption, bandgap and thickness are understood based on the bonding rearrangement caused by UV exposure.

Etching blazed grating into quartz substrate with selectivity 1:1 against photoresist by inductively coupled plasma technology

Inductively coupled plasma (ICP) etching technology is becoming widely used in the fabrication of micro-optical elements. Because of its advantages such as relatively high etching rate and anisotropic etching, ICP technology is receiving more attention. Selectivity 1:1 of the substrate against photoresist is very convenient, because we can easily get the same structures which were designed on the photoresist. Through extensive experiments we have achieved an optimized etching condition under which the transfer process generates fairly smooth etched structures with selectivity 1:1. The first order diffraction efficiency of the etched blazed gratings is 83.5% which is a little bit lower than the theoretical efficiency 88.9%. The diffraction of etched blazed gratings matches the theoretical results well.

Improving the imaging quality of MOEs in DMD-based maskless lithography

Digital-micromirror device (DMD)-based maskless lithography technique has been applied to fabricate microoptical elements (MOEs). Due to the binary pulse-width modulation in DMD, however, some ruled corrugations will appear on the aerial image of MOEs when a laser light is used as exposure source. In this paper, the forming mechanism of the corrugations is explored and simulated annealing algorithm is adopted to remove the corrugations. The experimental results demonstrate that DMD-based maskless lithography technique is an effective tool for the fabrication of MOEs.

Surface patterning on amorphous chalcogenide nanomultilayers

The surface patterning on amorphous chalcogenide nanomultilayers was successfully performed employing laser-irradiation or electron-beam recording. Significant changes of thickness reaching up to 10% of initial values investigated by AFM are accompanied by modifications of optical transmittance and refractive index. Observed phenomena can be used for the one-step fabrication of micro-optical elements.

Photo-imageable Sol–Gel Hybrid Materials for Simple Fabrication of Micro-optical Elements

Special attention has been focused on photo-imageable sol-gel hybrid (SGH) materials because of their synergetic effects, such as high photosensitivity and transparency, as well as mechanical and chemical durability resulting from the presence of polymer and silica networks in the hybrid structure. Photo-induced migration, which accompanies photopolymerization and photolocking in these materials, allows for the formation of convex micropatterns with a higher refractive index than the original film through exposure to UV radiation. Controlling the parameters affecting this photo-induced migration can permit modulation of the size and shape of such microstructures for the simple and cost-effective direct photofabrication of micro-optical elements, such as microlenses and microlens arrays.

Laser Induced Backside Wet Etching: Mechanisms and Fabrication of Micro-Optical Elements

A novel method for the fabrication of complex 3 dimensional structures, such as plano-convex microlens arrays in quartz, is presented. This technique is applied for precise micromachining of UV transparent materials, e.g. quartz, by using a conventional XeCl excimer laser and an organic solution which is in contact with the substrate. Strong absorption of the intense laser light results in the formation of the high pressure and temperature jumps at the quartz-liquid interface, which are the key elements in this process. A combination of laser assisted wet etching with the projection of a Diffractive Gray Tone Phase Mask is applied to fabricate diffractive and refractive micro-lens arrays in quartz. A quartz micro-lens array, which consists of 10 × 10 plano-convex microlenses is tested as a beam homogenizer for the quadrupled Nd:YAG laser.

Virtual gray-scale mask for fabrication of micro-optical elements via focused ion beam direct writing

A new fabrication approach, virtual gray-scale mask via focused ion beam (FIB) direct milling for fabrication of micro-optical elements (MOEs), is introduced. The designed MOEs, e.g., micro-diffractive lenses with continuous relief and micro-gratings, are directly milled by the FIB by means of the method of the virtual gray-scale mask with ion energy and ion beam current of 30 keV and 3 nA, respectively. The method has a one-step fabrication process and is free of user computer programming for the one-step direct writing. It is a useful approach for those FIB machines without the user programming function. It will be helpful for integration of MOEs with top-end optical fibers for applications of miniaturization of spectrometers, optical fiber sensors, and micro-optical systems in biochemistry analysis use.

Spatial distribution of irradiation effects on silica glass induced by 15-MeV oxygen ion microbeam

High-purity silica glass was irradiated by a focused 15-MeV O4+ microbeam with diameter of 1μm up to a fluence of 1.0×1014ions/cm2. Spatial distribution of irradiation effects by the O4+ microbeam on silica glass was investigated by optical microscopy, microphotoluminescence (PL)/Raman spectroscopy and atomic force microscopy (AFM). Distribution of refractive index change and defect formation was visualized by optical microscopy and PL mapping, indicating the structural changes of silica glass along the ion track up to the depth of 10μm. In addition, we observed deformed side surface with a groove by AFM along the track suggesting the internal compaction in silica glass. This is accompanied by increased threefold rings of SiO2 network detected by Raman scattering. We also discuss technological implications of these results on the applications of microbeam irradiation effects to the fabrication of microoptical elements.

Fabrication of micro-optical elements in fused silica with laser backside etching(Laser processing)

One of the urgent challenges of current laser material processing is the high quality and precise etching of transparent materials for, e.g., microoptical applications. Indirect laser processing techniques such as LIBWE and LESAL (laser-induced backside wet etching, laser etching at a surface adsorbed layer, respectively) allow the accurate etching of transparent materials like fused silica at low laser fluences, with etch rates in the nanometer scale and a low roughness. In combination with established laser processing techniques, e.g., mask projection, scanning contour mask technique, and direct writing, these etch techniques allow the direct machining of dielectric materials with almost optical quality. The etching of diffractive and refractive topographies for micro prism arrays, free-form surfaces, as well as periodically sub-micron structures with nanometer depth accuracy and a low roughness is demonstrated.