Microhole Array Research Articles

Patterning the surface structure of transparent hard-brittle material β-Ga2O3 by ultrashort pulse laser

Femtosecond laser direct writing technology offers a powerful and advantageous tool for precise three-dimensional (3D) micro/nanofabrication of transparent hard and brittle materials. In this paper, micro-craters, micro/nano-rectangular columns, and microchannels structures with different spatial characteristics are achieved after irradiation of crystalline (100) β-Ga2O3 via an optical-fiber femtosecond laser (515/1030 nm, 285 fs, 100 kHz) with different processing parameters in air ambient. Laser scanning confocal microscope, atomic force microscopy, and Raman spectroscopy are employed to characterize the roughness and constitutive property of the surface structure under tightly focused laser conditions. The influences of different pulse numbers and different pulse energies on the morphology of the pits were investigated. Through precise control and optimization of processing parameters, the formation of cracks and chipping can be effectively avoided. Crack-free ablation holes with smooth edges were fabricated under low-energy (E = 22 nJ) single-pulse conditions. Furthermore, the effects of different laser processing parameters (single-pulse scanning interval, pulse number and scanning repetition times) on the microchannel morphology were also explored. From this work, high-precision micro-hole arrays, micro-nano rectangular column arrays, and microchannels were successfully fabricated on (100) β-Ga2O3 substrates. Due to the interaction of single-pulse femtosecond laser with β-Ga2O3 materials, this fast fabrication method of 3D submicron structures has potential applications in sensing, photonics, or electronics.

A Simple Approach to Achieving Ultrasmall III-Nitride Microlight-Emitting Diodes with Red Emission.

The microdisplays for augmented reality and virtual reality require ultrasmall micro light-emitting-diodes (μLEDs) with a dimension of ≤5 μm. Furthermore, the microdisplays also need three kinds of such μLEDs each emitting red, green, and blue emission. Currently, in addition to a great challenge for achieving ultrasmall μLEDs mainly based on III-nitride semiconductors, another fundamental barrier is due to an extreme difficulty in growing III-nitride-based red LEDs. So far, there has not been any effective approach to obtain high indium content InGaN as an active region required for a red LED while maintaining high optical performance. In this paper, we have demonstrated a selective epitaxy growth approach using a template featuring microhole arrays. This allows us to not only obtain the natural formation of ultrasmall μLEDs but also achieve InGaN with enhanced indium content at an elevated growth temperature, at which it is impossible to obtain InGaN-based red LEDs on a standard planar surface. By means of this approach, we have demonstrated red μLEDs (at an emission wavelength of 642 nm) with a dimension of 2 μm, exhibiting a high luminance of 3.5 × 107 cd/m2 and a peak external quantum efficiency of 1.75% measured in a wafer form (i.e., without any packaging to enhance an extraction efficiency). In contrast, an LED grown under identical growth conditions but on a standard planar surface shows green emission at 538 nm. This highlights that our approach provides a simple solution that can address the two major challenges mentioned above.

Temperature-induced enhancement of tensile strength of perforated Ti–6Al–4V sheet revealed through mechanical-thermal coupling quasi-static tension

In order to investigate the effect of temperature on the tensile strength of perforated Ti–6Al–4V (TC4) alloy, the tensile mechanical properties of TC4 alloy with various micro-hole array distributions were obtained at a temperature in a range from 20 °C to 500 °C by using a novel mechanical-thermal coupling tensile test instrument developed by ourselves. The experimental results showed that the tensile strength of all types of TC4 alloy specimens decreased with the increase in temperature, and the elastic modulus of most types of TC4 alloy specimens decreased with the increase in temperature. The tensile strength of the perforated specimen iii was 14.8% lower than that of the solid specimen and the elastic modulus of perforated specimen ii was 5.5% lower than that of the solid specimen at room temperature (RT), and all types of the perforated specimens continued to exhibit relatively lower tensile strengths and elastic modulus at 180 °C and 340 °C. Abnormally, the tensile strength of the perforated specimen iii was 2.1% higher than that of the solid specimen and the elastic modulus of perforated specimen ii was 23% higher than that of the solid specimen at 500 °C. With the increase of temperature from 20 °C to 500 °C, the fracture mode of the solid specimen gradually changed from equiaxed dimple to shear dimple. Regarding the perforated specimens, the cracks and dimples adjacent to the holes gradually decreased as a function of temperature. The existence of prefabricated holes in the specimens dissipated part of the thermal stress, maintained the integrity of the micro-hole structure, and made the nearby section relatively flat, which induced a relatively higher tensile strength and structural stiffness at an elevated temperature. The relatively lower stress in the gauge section of the perforated specimens obtained through finite element analysis also indicated the temperature-induced enhancement of tensile strength.

Fabrication of Micro-Patterned Chip with Controlled Thickness for High-Throughput Cryogenic Electron Microscopy.

A major limitation for the efficient and high-throughput structure analysis of biomolecules using cryogenic electron microscopy (cryo-EM) is the difficulty of preparing cryo-EM samples with controlled ice thickness at the nanoscale. The silicon (Si)-based chip, which has a regular array of micro-holes with graphene oxide (GO) window patterned on a thickness-controlled silicon nitride (SixNy) film, has been developed by applying microelectromechanical system (MEMS) techniques. UV photolithography, chemical vapor deposition, wet and dry etching of the thin film, and drop-casting of 2D nanosheet materials were used for mass-production of the micro-patterned chips with GO windows. The depth of the micro-holes is regulated to control the ice thickness on-demand, depending on the size of the specimen for cryo-EM analysis. The favorable affinity of GO toward biomolecules concentrates the biomolecules of interest within the micro-hole during cryo-EM sample preparation. The micro-patterned chip with GO windows enables high-throughput cryo-EM imaging of various biological molecules, as well as inorganic nanomaterials.

Boundary effect of the glass microlens array in partial-filling hot embossing

Partial-filling hot embossing (PFHE) is a flexible and rapid method to fabricate the glass microlens array (GMLA) with high efficiency and mass production. However, the sidewall of the microhole mold strongly affects the deformation modes and optical performance of the GMLA during PFHE. The microholes of a mold fabricated by electrical discharging machining (EDM) and focused ion beam (FIB) were employed for hot glass embossing to reveal the boundary effect of the GMLA in PEHE. The detailed characterizations were implemented for 3 × 3 microhole arrays by white-light interfering profilometer and scanning electron microscope to highlight the anomaly in the sidewall topography of different mold cavities. Also, the three-dimensional topography and optical quality of the GMLA were measured by a white-light interfering profilometer, parallel light system, and ultraviolet spectrophotometer. Experimental results indicated that the sidewall of the mold cavity significantly affected the filling modes, appearance, and optical quality of the GMLA. The phenomenon of sidewall slip occurs at the edge of the GMLA in the FIB-mold. The sidewall quality of the EDM-mold strongly affects deformation modes and light transmittance of the GMLA. The work provides a better understanding of fabricating the GMLA in PFHE.

Tungsten carbide molds for precision glass molding process: Mechanism of high-temperature degradation

Tungsten carbide (WC) is a well-established material used in glass molding field due to its low thermal expansion and high compressive strength at elevated temperatures. During glass molding process, the WC mold has to withstand thousands of thermo-mechanical coupled interactions in the aerobic environment. Understanding the degradation mechanism of WC substrate during precision glass molding is very important for the design and preparation of protective coatings and the optimization of glass molding process. In this study, surface failure analysis of WC substrate was carried out using barrel compression test and ball-to-disk friction test. The microstructures, element compositions, surface morphology of the WC substrates were measured by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and white light interferometer (WLI). Damaging phenomena such oxidation, interdiffusion, spallation, glass sticking, and surface roughening on the WC substrate were observed. The experimental results showed that the WC substrate degraded very rapidly at above 600 °Cand air environment. The fundamental degradable reason of WC substrate is the oxidation, which directly leads to the decrease of the material strength, the aggravation of the wear, occurring of the glass sticking, and the increase of surface roughness. Furthermore, the WC mold microhole array without protective coatings can be used to fabricate the glass microlens array in a suitable glass molding environment. This work provides engineering guidance for the fabrication of glass optics during glass molding process.

Laser direct patterning induced the tunable optical properties of indium tin oxide micro-hole arrays films

Here we introduce a facile method to fabricate patterned indium tin oxide (ITO) thin films via selective laser ablation at ambient conditions. By scanning the ITO thin films with focused Nd: YAG pulsed laser, the ITO thin films were selective ablated and patterned without using any conventional chemical etching or photolithography steps. Then we investigated the effects of scanning rate for the structure, morphology and optical properties of patterned ITO thin film. These results indicate that the epsilon-near-zero (ENZ) wavelength of ITO thin films can be tuned from 1100 nm to 1340 nm by adjusting the period of the micro-hole array in microstructure. The nonlinear absorption response of patterned ITO films was about 2.85 time than of the as-deposited ITO thin film. Additionally, the results of the Finite-Difference Time-Domain (FDTD) simulation are in good agreement with those of the experiments.

Effects of temperature on drag reduction in a subsonic turbulent boundary layer via micro-blowing array

A comparative study of two micro-blowing temperature cases has been performed to investigate the characteristics of drag reduction in a subsonic flat-plate flow (where the freestream Mach number is 0.7) by means of Direct Numerical Simulation (DNS). With minute amount of blowing gas injected from a 32 × 32 array of micro-holes arranged in a staggered pattern, the porosity of micro-holes is 23% and the blowing coefficient is 0.125%. The simulation results show that a drag reduction is achieved by micro-blowing, and a lower wall-friction drag can be obtained at a higher blowing temperature. The role of micro-blowing is to redistribute the total kinetic energy in the boundary layer, and the proportion of stream-wise kinetic energy decreases, resulting in the thickened boundary layer. Increasing micro-blowing temperature can accelerate this process and obtain an enhanced drag reduction. Moreover, an explanation of drag reduction by micro-blowing related to the micro-jet vortex clusters is proposed that these micro-jet vortex clusters firmly attached to the wall constitute a stable barrier, which is to prevent the direct contact between the stream-wise vortex and the wall. By Dynamic Mode Decomposition (DMD) from temporal/spatial aspects, it is revealed that small structures in the near-wall region play vital role in the change of turbulent scales. The high-frequency patterns are clearly strengthened, and the low-frequency patterns just maintain but are lifted up.

Single-Step Fabrication of High-Performance Extraordinary Transmission Plasmonic Metasurfaces Employing Ultrafast Lasers.

Plasmonic metasurfaces based on the extraordinary optical transmission (EOT) effect can be designed to efficiently transmit specific spectral bands from the visible to the far-infrared regimes, offering numerous applications in important technological fields such as compact multispectral imaging, biological and chemical sensing, or color displays. However, due to their subwavelength nature, EOT metasurfaces are nowadays fabricated with nano- and micro-lithographic techniques, requiring many processing steps and carrying out in expensive cleanroom environments. In this work, we propose and experimentally demonstrate a novel, single-step process for the rapid fabrication of high-performance mid- and long-wave infrared EOT metasurfaces employing ultrafast direct laser writing. Microhole arrays composing extraordinary transmission metasurfaces were fabricated over an area of 4 mm2 in timescales of units of minutes, employing single pulse ablation of 40 nm thick Au films on dielectric substrates mounted on a high-precision motorized stage. We show how by carefully characterizing the influence of only three key experimental parameters on the processed micro-morphologies (namely, laser pulse energy, scan velocity, and beam shaping slit), we can have on-demand control of the optical characteristics of the extraordinary transmission effect in terms of transmission wavelength, quality factor, and polarization sensitivity of the resonances. To illustrate this concept, a set of EOT metasurfaces having different performances and operating in different spectral regimes has been successfully designed, fabricated, and tested. Comparison between transmittance measurements and numerical simulations has revealed that all the fabricated devices behave as expected, thus demonstrating the high performance, flexibility, and reliability of the proposed fabrication method. We believe that our findings provide the pillars for mass production of EOT metasurfaces with on-demand optical properties and create new research trends toward single-step laser fabrication of metasurfaces with alternative geometries and/or functionalities.

The Electrode Wear and Effective Energy Detection in Micro-EDM Drilling of Micro-hole Array

With the micro-hole array structure more and more used in industrial areas, such as oil nozzle, spinneret and so on, its machining accuracy significantly affects the performance of the product. In the machining of micro-hole array by micro-EDM drilling, unavoidable tool electrode wear is one key factor affecting machining geometric and dimensional accuracy, and the effective energy of the discharge gap directly determines the amount of electrode wear. In present study, the characteristics of electrode wear and gap effective discharge energy were explored to meet the requirements of machining accuracy of micro-hole array machining. The effect of pulse generator parameters and electrode eccentric radius on electrode wear were concerned, and the electrode wear was reduced by optimizing pulse generator parameters and electrode eccentric radius. The geometry evolution rule of electrode was investigated, and the electrode wear model was established based on the electrode wear characteristic. The experimental result show that the electrode tip morphology tended to be stable and the axial wear of the tool electrode could be compensated evenly. In order to realize the accurate calculation of the gap effective discharge energy, a gap effective discharge energy detection system was built based on Lab VIEW. And the variation rule of gap effective discharge energy was obtained by micro-hole array machining experiment. In addition, the relationship between electrode wear and gap effective discharge energy was also studied in this paper. It was found that there was a parabolic relationship between electrode wear and effective energy before electrode wear was stable, and there was a linear relationship between them after electrode wear was stable.

Local Avalanche Effect of 4H-SiC p-i-n Ultraviolet Photodiodes With Periodic Micro-Hole Arrays

In this report, 4H-SiC p-i-n photodiodes with various micro-hole arrays are fabricated, then measured and discussed by using photoelectric measurement system and simulation software. Periodic micro-hole arrays etched from p layer to i layer are obtained by using the selective etching technology, which increases the photosensitive area of devices and reduces the ultraviolet light absorption of p layer. The average dark current of devices has an ultralow value of approximate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.0\times 10 ^{-15}$ </tex-math></inline-formula> A in the low reverse bias range of 0–10 V. Furthermore, the device with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 ~\mu \text{m}$ </tex-math></inline-formula> micro-hole exhibits the best performance and its peak responsivity increased by 10.4 % compared to the conventional device. It is significant that the peak responsivity and corresponding external quantum efficiency of devices with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 ~\mu \text{m}$ </tex-math></inline-formula> micro-hole at 40 V bias are 815 % and 819 % higher than those of devices without micro-hole, respectively. This is attribute to the fact that a high electric field is observed around the micro-holes as the reverse bias increased to 40 V, which leads to local avalanche. Thus, the local avalanche photodiodes work at a relatively low bias, which improved the responsivity and quantum efficiency of the device enormously.

Fabrication of Medical Titanium Alloy Microhole Array Substrates Based on High-Speed Polygon Mirror

Fabrication of Medical Titanium Alloy Microhole Array Substrates Based on High-Speed Polygon Mirror

Polyvinyl alcohol microlens array obtained by solvent evaporation from a confined droplet array.

In this study, polyvinyl alcohol (PVA) microlens arrays (MLAs) were prepared, and the dynamics of contact lines and contact angles during confined PVA solution droplet evaporation were investigated by in situ optical microscopy. First, hydrophobic layers patterned with hydrophilic microholes array modified substrates were prepared by photolithography and coating methods. The flowing of PVA solution on the substrates formed droplets in each microhole self-assembly. The substrate was then heated to allow evaporation of the solvent. The results showed the contact line of confined droplets pinned at the junction between the hydrophilic and hydrophobic areas during the whole evaporation process. The apparent contact angle decreased nonlinearly during evaporation. The evaporation of PVA solution droplet in each microhole followed a constant contact radius mode, meaning constant contact area and declined contact angle during evaporation. After complete solvent evaporation, PVA formed a convex shape with convergent lens character in each microhole. In sum, the obtained PVA convex arrays with uniform sizes and good focusing properties would have potential applications in wavefront sensing, infrared focal plane detection or CCD array light accumulation, laser array scanning, laser display, optical fiber coupling, and many other optical systems.

Improved performance of InGaN-based red light-emitting diodes by micro-hole arrays.

This study demonstrates the performance improvements of InGaN-based red light-emitting diodes (LEDs) by fabricating micro-holes in the planar mesa. The peak wavelengths of the micro-hole LEDs (MHLEDs) exhibited a blue-shift of around 3 nm compared to the planar LEDs (PLEDs) at the same current density. The lowest full width at half maximum of MHLEDs was 59 nm, which is slightly less than that of the PLEDs. The light output power and external quantum efficiency of the MHLED with a wavelength of 634 nm at 20 mA were 0.6 mW and 1.5%, which are 8.5% higher than those of the PLED.

3D Microphysiological System‐Inspired Scalable Vascularized Tissue Constructs for Regenerative Medicine

AbstractMicrophysiological systems (MPSs), based on microfabrication technologies and cell culture, can faithfully recapitulate the complex physiology of various tissues. However, 3D tissues formed using MPS have limitations in size and accessibility; their use in regenerative medicine is, therefore, still challenging. Here, an MPS‐inspired scale‐up vascularized engineered tissue construct that can be used in regenerative medicine is designed. Endothelial cell‐laden hydrogels are sandwiched between two through‐hole membranes. The microhole array in the through‐hole membranes enables the molecular transport across the hydrogel layer, allowing long‐term cell culture. Furthermore, the time‐controlled delamination of through‐hole membranes enables the harvesting of cell‐cultured hydrogel constructs without damaging the capillary network. Importantly, when the tissue constructs are implanted in a mouse ischemic model, they protect against necrosis and promoted functional recovery to a greater extent than implanted cells, hydrogels, and simple gel–cell mixtures.

Tight focusing of laser light using microlens array combined with radially polarised light converter

ABSTRACT A facile method to prepare a liquid crystal polarisation conversion microlens array (LCPC MLA) is reported. One-step photolithography was used to form a microhole array patterned fluoropolymer poly (butenyl vinyl ether) (CYTOP) layer on the front and back side of a thin glass substrate. On the front side, LC droplets are radially aligned in the microholes, and a twisted radial arrangement texture is formed by the integration of another glass substrate coated with a parallel alignment layer to create the LCPC array. The MLA is located at the rear side of the thin glass substrate and each microlens centre is completely aligned with the centres of the LC droplets in the LCPC array. The LCPC array can convert linearly polarised light to radially polarised light and the MLA can focus the radially polarised light to a spot array. This tightly focused array has a high intensity and at each focused spot, a large component of the electric field is in the longitudinal direction. The LCPC MLA has a compact structure, good stability, and highly uniformity, and it shows potential for applications such as material processing, phase modulation, polarisation compensation, and polarisation imaging.

Fabrication of an Anti-Reflective Microstructure on ZnS by Femtosecond Laser Bessel Beams.

As an important mid-infrared to far-infrared optical window, ZnS is extremely important to improve spectral transmission performance, especially in the military field. However, on account of the Fresnel reflection at the interface between the air and the high-strength substrate, surface optical loss occurs in the ZnS optical window. In this study, the concave antireflective sub-wavelength structures (ASS) on ZnS have been experimentally investigated to obtain high transmittance in the far-infrared spectral range from 6 μm to 10 μm. We proposed a simple method to fabricate microhole array ASS by femtosecond Bessel beam, which further increased the depth of the microholes and suppressed the thermal effects effectively, including the crack and recast layer of the microhole. The influence of different Gaussian and Bessel beam parameters on the microhole morphology were explored, and three ASS structures with different periods were prepared by the optimized Bessel parameters. Ultimately, the average transmittance of the sample with the ASS microhole array period of 2.6 μm increased by 4.1% in the 6 μm to 10 μm waveband, and the transmittance was increased by 5.7% at wavelength of 7.2 μm.

Ionic Diode and Molecular Pump Phenomena Associated with Caffeic Acid Accumulated into an Intrinsically Microporous Polyamine (PIM‐EA‐TB)

AbstractThe polymer of intrinsic microporosity PIM‐EA‐TB provides a molecularly rigid micropore structure containing tertiary amine sites and is shown here to interact with hydrogen bonding guest molecules such as caffeic acid. Voltammetric data with a PIM‐EA‐TB film on glassy carbon electrodes show that in both acidic solution (pH 2; PIM‐EA‐TB is protonated) and in neutral solution (pH 6; PIM‐EA‐TB is not protonated) caffeic acid is slowly accumulated into the microporous host. Binding constants are estimated and suggested to be linked to hydrogen bonding causing accumulation of caffeic acid. When employing PIM‐EA‐TB as an asymmetric membrane coated onto a 5 μm thick Teflon support film with 10 μm diameter microholes (using either a single microhole or a 10×10 array of microholes), binding of caffeic acid is shown to cause a modulation of the ionic current without affecting the pH‐dependent ionic diode behaviour. Two complementary types of effects of caffeic acid guests are discussed based on blocking anion diffusion pathways and based on removal of positive charges. The caffeic acid transport mechanism/efficiency is investigated in view of selective molecular pumping.

High-Performance Fuel Cells with a Plasma-Etched Polymer Electrolyte Membrane with Microhole Arrays

Interface engineering based on the design and fabrication of micro/nanostructures has received much attention as an effective way to improve the performance of polymer electrolyte membrane (PEM) fu...

Enhanced Light Trapping in Conformal CuO/Si Microholes Array Heterojunction for Self-Powered Broadband Photodetection

In this letter, we demonstrate the fabrication of a conformal CuO/Si microholes array heterojunction through the DC reactive magnetron sputtering from a high-purity Cu target. By using the monolayer graphene as the top electrode, the device served well as a self-powered vis-NIR photodetector, showing a high responsivity of 301.5 mA W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , specific detectivity of 7.96×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> Jones and a fast response speed (rise time 9.9 μ s and fall time 10 μs) upon 530 nm illumination. Compared to its planar counterpart, the responsivity was remarkably enhanced over the broadband region. The underlying reason should be ascribed to the improved light trapping in the microholes via the longitudinal Fabry-Perot (F-P) cavity resonance, according to theoretical simulation by finite-difference time-domain (FDTD) solution. Such an effect gave rise to an enhanced light-matter interaction, leading to the improved photoresponse. This work also opens up an effective way for the on-chip high performance photodetection due to the well compatibility with the current complementary metal-oxide-semiconductor (CMOS) technology.