Convex silica microlens arrays via femtosecond laser writing

Polymeric microlens array formed on a discontinuous wetting surface using a self-assembly technique

In this paper, we demonstrate a facile way to prepare polymeric microlens arrays (MLAs) based on a discontinuous wetting surface using a self-assembly technique. A patterned hydrophobic-octadecyltrichlorosilane (OTS) surface was prepared by U V/O 3 irradiation through a shadow mask. The area exposed to U V/O 3 irradiation turned highly hydrophilic, whereas the area protected by the mask remained highly hydrophobic, generating the patterned OTS surface. The surface energy of the OTS/glass surface changed from 23 to 72.8 mN/m after 17 min of U V/O 3 treatment. The scribing of the optical glue-NOA 81 onto the microhole array enabled one to obtain the MLAs due to the generation of the NOA 81 droplet array via the surface tension. After UV light curing, the cured NOA 81 droplet array with uniform dimensions within a large area exhibited excellent MLA characteristics. Moreover, the method developed in this study is simple in operation, low-cost, and requires neither a clean room nor expensive equipment.

Fabrication of Large-Area Silicon Spherical Microlens Arrays by Thermal Reflow and ICP Etching

In this paper, we proposed an efficient and high-precision process for fabricating large-area microlens arrays using thermal reflow combined with ICP etching. When the temperature rises above the glass transition temperature, the polymer cylinder will reflow into a smooth hemisphere due to the surface tension effect. The dimensional differences generated after reflow can be corrected using etching selectivity in the following ICP etching process, which transfers the microstructure on the photoresist to the substrate. The volume variation before and after reflow, as well as the effect of etching selectivity using process parameters, such as RF power and gas flow, were explored. Due to the surface tension effect and the simultaneous molding of all microlens units, machining a 3.84 × 3.84 mm2 silicon microlens array required only 3 min of reflow and 15 min of ICP etching with an extremely low average surface roughness Sa of 1.2 nm.

Arbitrary fabrication of complex lithium niobate three-dimensional microstructures for second harmonic generation enhancement

Lithium niobate (LN) crystal plays important roles in future integrated photonics, but it is still a great challenge to efficiently fabricate three-dimensional micro-/nanostructures on it. Here, a femtosecond laser direct writing-assisted liquid back-etching technology (FsLDW-LBE) is proposed to achieve the three-dimensional (3D) microfabrication of lithium niobate (LN) with high surface quality (Ra = 0.422 nm). Various 3D structures, such as snowflakes, graphic arrays, criss-cross arrays, and helix arrays, have been successfully fabricated on the surface of LN crystals. As an example, a microcone array was fabricated on LN crystals, which showed a strong second harmonic signal enhancement with up to 12 times bigger than the flat lithium niobate. The results indicate that the method provides a new approach for the microfabrication of lithium niobate crystals for nonlinear optics.

Superhydrophobic Multifocal Microlens Array with Depth-of-Field Detection for a Humid Environment

Microlens array (MLA) has been widely applied in augmented reality and optical imaging. When used in a humid environment or medical endoscopy, MLA needs to be both superhydrophobic and multifocal. However, it is not easy to achieve both superhydrophobic and multifocal function by integrating superhydrophobic and multifocal structures on the same surface by means of a simple, efficient, and precise method. In this paper, the superhydrophobic multifocal MLA with superhydrophobic properties and multifocal functions is successfully designed for preparation based on a method of 3D lithography and soft lithography. The 3D lithography can further help the preparation of a multifocal MLA with varying apertures and a multistep superhydrophobic structure with a round dome. The superhydrophobic multifocal MLA with periods 50 and 120 μm has perfect superhydrophobic property. The water droplet can slide and bounce off the surface at a roll angle of less than 12.9° with both multifocal and integrated imaging function, as well as up to 397 μm depth-of-field (DOF) detection range; this greatly exceeds the conventional MLA. The perfect superhydrophobic and optical property can be achieved in an extremely humid environment. The superhydrophobic multifocal MLA proposed in this paper has a promising prospect for actual practices.

Deep focus light-field camera for handheld 3D intraoral scanning using crosstalk-free solid immersion microlens arrays

3D in vivo imaging techniques facilitate disease tracking and treatment, but bulky configurations and motion artifacts limit practical clinical applications. Compact light-field cameras with microlens arrays offer a feasible option for rapid volumetric imaging, yet their utilization in clinical practice necessitates an increased depth-of-field for handheld operation. Here, we report deep focus light-field camera (DF-LFC) with crosstalk-free solid immersion microlens arrays (siMLAs), allowing large depth-of-field and high-resolution imaging for handheld 3D intraoral scanning. The siMLAs consist of thin PDMS-coated microlens arrays and a metal-insulator-metal absorber to extend the focal length with low optical crosstalk and specular reflection. The experimental results show that the immersion of MLAs in PDMS increases the focal length by a factor of 2.7 and the transmittance by 5.6%-27%. Unlike conventional MLAs, the siMLAs exhibit exceptionally high f-numbers up to f/6, resulting in a large depth-of-field for light-field imaging. The siMLAs were fully integrated into an intraoral scanner to reconstruct a 3D dental phantom with a distance measurement error of 82 ± 41 μm during handheld operation. The DF-LFC offers a new direction not only for digital dental impressions with high accuracy, simplified workflow, reduced waste, and digital compatibility but also for assorted clinical endoscopy and microscopy.

Large-Area and Rapid Fabrication of a Microlens Array on a Flexible Substrate for an Integral Imaging 3D Display

A curved integral imaging three-dimensional (3D) display attracts a lot of interest due to its enhanced 3D sense of immersion and wider viewing angle. In this paper, a microlens array (MLA) based on a flexible poly(ethylene terephthalate) (PET) substrate was achieved by a straightforward, rapid, and low-cost technique. The reactive ion etching (RIE) process treated PET/CYTOP covered with a flexible mask to generate a hydrophilic-hydrophobic patterned surface. The well-designed arrays of confined adhesive droplets with a controlled geometry on a hydrophilic-hydrophobic patterned surface were formed using the blade-coating method. A flexible MLA with a diameter of 820 μm, a size of 5.3 cm × 5.1 cm, and a radius of curvature of 25 cm was fabricated and combined with a curved two-dimensional (2D) monitor to realize a lateral viewing range of 6.4 cm at a viewing distance of 25 cm, which is 4 times larger than with flat integral imaging 3D display system. The flexible MLA has the advantages of a controllable lens profile and large pitch, and it can be manufactured on a large scale. In addition, it provides a large viewing angle for the reconstructed 3D image.

Flexible and efficient fabrication of a terahertz absorber by single-step laser direct writing

Laser direct writing (LDW) is a promising candidate for the fabrication of all-dielectric THz absorbers for its high flexibility and material compatibility. However, multi-step processing or multi-layer materials are required to compensate for the nonideal features of LDW to realize good absorption performance. To further explore the potential of LDW in flexible and cost-effective THz absorber fabrication, in this work, we demonstrate a design method of THz absorbers fully considering and utilizing the characteristics of laser processing. Specifically, we first numerically analyze that by properly combining basic structures processed by single-step LDW, good and adjustable absorption performance can be achieved on a single-layer substrate. Then we experimentally fabricate THz absorbers by processing periodic composite structures, which are combined by grooves and circular holes, on single-layer doped silicon using LDW. Experimental results show that our method can fabricate THz absorbers at a speed of 3.3 mm²/min with an absorptivity above 90% over a broadband of 1.8-3 THz. Our method provides a promising solution for the flexible and efficient fabrication of all-dielectric broadband THz absorbers.

Artificial Hyper Compound Eyes Enable Variable-Focus Imaging on both Curved and Flat Surfaces

The artificial compound eye (ACE) with zoom imaging requires complex power sources. Meanwhile, its curved substrate makes it difficult for the ACE to realize the zoom imaging on flat surfaces. To realize a wide field of view and a zoom function on both curved and flat surfaces simultaneously, a novel ACE is proposed, which is a bionic design inspired by an ancient creature, trilobite. Compared with a dragonfly, photosensitive units of a trilobite's compound eye are composed of ommatidia with different focal lengths. By learning from this concept, an artificial hyper compound eye (AHCE) was fabricated. Its basic components are five microlenses with different curvatures, and they are capable of being treated as five ommatidia with different focal lengths. Five ommatidia form a photosensitive unit to realize a zoom function. AHCE is capable of variable-focus imaging on curved surfaces. With the information share function, we found that the AHCE not only images on curved surfaces but also has a zoom-imaging function on flat surfaces. The results confirm that the AHCE demonstrates an advanced imaging capability, a variable-focus imaging function on both curved and flat surfaces, which may open new opportunities in developing advanced micro-optical devices.

Fabrication of High Precision Silicon Spherical Microlens Arrays by Hot Embossing Process

In this paper, a high-precision, low-cost, batch processing nanoimprint method is proposed to process a spherical microlens array (MLA). The nanoimprint mold with high surface precision and low surface roughness was fabricated by single-point diamond turning. The anti-sticking treatment of the mold was carried out by perfluorooctyl phosphoric acid (PFOPA) liquid deposition. Through the orthogonal experiment of hot embossing with the treated mold and subsequent inductively coupled plasma (ICP) etching, the microstructure of MLA was transferred to the silicon substrate, with a root mean square error of 17.7 nm and a roughness of 12.1 nm Sa. The average fitted radius of the microlens array units is 406.145 µm, which is 1.54% different from the design radius.

Phase-Type Fresnel Zone Plate with Multi-Wavelength Imaging Embedded in Fluoroaluminate Glass Fabricated via Ultraviolet Femtosecond Laser Lithography

Herein, we report a novel optical glass material, fluoroaluminate (AlF₃) glass, with excellent optical transmittance from ultraviolet to infrared wavelength ranges, which provides more options for application in optical devices. Based on its performance, the phase-type Fresnel zone plate (FZP) by ultraviolet femtosecond (fs) laser-inscribed lithography is achieved, which induces the refractive index change by fs-laser tailoring. The realization of ultraviolet fs-laser fabrication inside glass can benefit from the excellent optical performance of the AlF₃ glass. Compared with traditional surface-etching micro-optical elements, the phase-type FZP based on AlF₃ glass exhibits a clear and well-defined geometry and presents perfect environmental suitability without surface roughness problems. Additionally, optical focusing and multi-wavelength imaging can be easily obtained. Phase-type FZP embedded in AlF₃ glass has great potential applications in the imaging and focusing in glass-integrated photonics, especially for the ultraviolet wavelength range.

Plastic deformation of synthetic quartz nanopillars by nanoindentation for multi-scale and multi-level security artefact metrics

Individual authentication using artefact metrics has received increasing attention, as greater importance has been placed on the security of individual information. These artefact metrics must satisfy the requirements of individuality, measurement stability, durability, and clone resistance, in addition to possessing unique physical features. In this study, we proposed that nanostructures of synthetic quartz (SQ) deposited on an SQ plate may provide sophisticated artefact metrics if morphological changes could be intentionally introduced into the SQ nanostructures at certain positions. We fabricated SQ nanopillars using a mass-production method (ultraviolet nanoimprint lithography) and investigated their mechanical deformation using nanoindentation with a spheroid diamond tip through a loading and unloading cycle. The SQ nanopillars with an aspect ratio of 1 (i.e., diameters D of 100 and 200 nm with corresponding heights H of 100 and 200 nm, respectively) could be plastically deformed without collapsing within a specified pillar-array format at programmed positions. The plastically deformed SQ nanopillar arrays demonstrated multi-scale (sub-millimetre, micrometre, and nanometre) and multi-level (shape, area, diameter, and height) individuality authentication and clone resistance. Because SQ is physically and chemically stable and durable, individuality authentication can be a highly reliable tool on Earth and in space.

Optimization of selective laser etching (SLE) for glass micromechanical structure fabrication

In this work, we show how femtosecond (fs) laser-based selective glass etching (SLE) can be used to expand capabilities in fabricating 3D structures out of a single piece of glass. First, an investigation of the etching process is performed, taking into account various laser parameters and scanning strategies. These results provide critical insights into the optimization of the process allowing to increase manufacturing throughput. Afterward, various complex 3D glass structures such as microfluidic elements embedded inside the volume of glass or channel systems with integrated functional elements are produced. A single helix spring of 1 mm diameter is also made, showing the possibility to compress it by 50%. Finally, 3D structuring capabilities are used to produce an assembly-free movable ball-joint-based chain and magnet-actuated Geneva mechanism. Due to minimized friction caused by low (down to 200 nm RMS) surface roughness of SLE-produced structures, the Geneva mechanism was shown to be capable of rotating up to 2000 RPM.

Ultrashort pulsed laser backside ablation of fused silica

We report on the fabrication of rectangular microchannels with vertical sidewalls in fused silica by laser backside ablation. A 515 nm femtosecond laser is focused by an objective with a NA of 0.5 through the sample on the glass/air interface, allowing processing from the backside into the bulk material. Experimental investigations reveal a logarithmically increasing depth of the channels with an increasing number of scans, while keeping the focal position fixed. A certain number of scans has to be applied to generate rectangular shaped channels while their depth can be controlled by the applied fluence from 2.64 µm to 13.46 µm and a corresponding ablation roughness R_a between 0.20 µm and 0.33 µm. The channel width can be set directly via the number of parallel ablated lines demonstrated in a range from 10 µm to 50 µm. By adjusting the focal position after each scan the channel depth can be extended to 49.77 µm while maintaining a rectangular channel geometry. Finally, concentric rings are ablated to demonstrate the flexibility of the direct writing process.

Sequential Photodamage Driven by Chaotic Systems in NiO Thin Films and Fluorescent Human Cells

A laser ablation process assisted by the feedback of a sensor with chaotic electronic modulation is reported. A synchronous bistable logic circuit was analyzed for switching optical signals in a laser-processing technique. The output of a T-type flip-flop configuration was employed in the photodamage of NiO films. Multiphotonic effects involved in the ablation threshold were evaluated by a vectorial two-wave mixing method. A photoinduced thermal phenomenon was identified as the main physical mechanism responsible for the nonlinearity of index under nanosecond irradiation at 532 nm wavelength. Comparative experiments for destroying highly transparent human cells were carried out. Potential applications for developing hierarchical functions yielding laser-induced controlled explosions with immediate applications for biomedical photothermal processes can be contemplated.

Stepped laser-ablation fabrication of concave micromirrors in rectangular optical waveguides for low loss vertical coupling

In this report, we present a stepped laser-ablation method for the fabrication of concave micromirrors in rectangular optical waveguides. The numerically simulated vertical coupling loss of the reflection of the concave micromirror can be reduced to 1.53 dB. The processing parameters of the utilized excimer laser, such as the step number, width, and depth, were optimized to fabricate the concave micromirrors. After the thermal reflow process, the measured curve of the circular concave micromirrors obtained using a 3D optical profiler agreed well with a standard circle with a surface roughness of 39.56 nm. Furthermore, vertical coupling for 62.5 µm MMF revealed that the loss of the circular concave micromirror coated with a 50 nm thick Au film is as low as 1.83 dB, corresponding to a high coupling efficiency of 65.61%. This new, convenient, and efficient fabrication technology for the fabrication of concave micromirrors can be applied to vertical coupling for optical printed circuit board (OPCB) interconnection technology.

Ablation in Externally Applied Electric and Magnetic Fields

To harness light-matter interactions at the nano-/micro-scale, better tools for control must be developed. Here, it is shown that by applying an external electric and/or magnetic field, ablation of Si and glass under ultra-short (sub-1 ps) laser pulse irradiation can be controlled via the Lorentz force F=eE+e[v×B], where v is velocity of charge e, E is the applied electrical bias and B is the magnetic flux density. The external electric E-field was applied during laser ablation using suspended micro-electrodes above a glass substrate with an air gap for the incident laser beam. The counter-facing Al-electrodes on Si surface were used to study debris formation patterns on Si. Debris was deposited preferentially towards the negative electrode in the case of glass and Si ablation. Also, an external magnetic field was applied during laser ablation of Si in different geometries and is shown to affect ripple formation. Chemical analysis of ablated areas with and without a magnetic field showed strong chemical differences, revealed by synchrotron near-edge X-ray absorption fine structure (NEXAFS) measurements. Harnessing the vectorial nature of the Lorentz force widens application potential of surface modifications and debris formation in external E-/B-fields, with potential applications in mass and charge spectroscopes.