Microhole Array Research Articles

Study of a GaN-Based Light-Emitting Diode with a Specific Hybrid Structure

A specific hybrid structure, including 45° sidewalls, a shallower (0.25 μm in depth) microhole array, and a 3-D like backside reflector, is used to fabricate GaN-based light-emitting diodes (LEDs). The 3-D like backside reflector is formed by the deposition of 100 nm-diameter SiO2 nanospheres (NSs) and an Al layer on the sapphire substrate. Based on the use of 45° sidewall and microhole array, the reduced total internal reflection (TIR) and Fresnel reflection and the increased scattering probability and the opportunity of photons to find escape cones can be expected. In addition, the use of 3-D like backside reflector can effectively reflect scatter, and redirect the downward photons emitted from multiple quantum well (MQW) region into arbitrary directions. Thus, the light extraction performance can be substantially improved. In experiment, under an injection current of 400 mA, the studied Device D with this hybrid structure shows enhancements of 47.4%, 47.7%, 33.1%, and 47.8% in light output power (LOP), external quantum efficiency (EQE), luminous flux, and wall-plug efficiency (WPE), respectively, as compared to a conventional LED. Moreover, the related electrical properties are not degraded for the studied Device D. Therefore, this hybrid structure shows a promise for high-performance GaN-based LED applications.

Multi-plane confocal microscopy with multiplexed volume holographic gratings [Invited

A volume holographic (VHG) grating-based multi-plane differential confocal microscopy (DCM) is proposed for axial scan-free imaging. Also, we briefly reviewed our previous works on volume holographic-based confocal imaging. We show that without degrading imaging performance, it is possible to simultaneously obtain two depth-resolved optically sectioned images with improved axial resolution using multi-plane DCM. The performance of our multi-plane DCM was evaluated by measuring the surface profile of a silicon micro-hole array with depths separation around 10 µm. The axial sensitivity of the system is around 25 nm. Our system has the advantages of multi-plane imaging with high axial sensitivity and high optical sectioning ability. Our method can be used for reflective surface profiling and multi-plane fluorescence imaging. The present methods may find important applications in surface metrology for label-free biological samples, as well as industrial applications.

Tunable liquid crystal microlens array with negative and positive optical powers based on a self-assembled polymer convex array

ABSTRACT In this report, an LCMLA with a negative-positive tunable focal length is designed using a self-assembled highly uniform polymer convex array. The polymer convex array was prepared by scribing UV-sensitive glue onto the indium tin oxide (ITO) substrate modified with a microhole array patterned hydrophobic layer. This substrate is assembled with another ITO substrate coated with a homogeneous polyimide alignment layer and rubbed in one direction, then filled with an LC material to form a sandwich structure. The refractive index of the selected polymer is kept between ne and no of the LC materials. By rotating the polarisation direction of the incident light for 90°, the focal length of the LCMLA can be switched from negative to positive. Otherwise, by varying the electric field, the continuous negative-to-positive tuning of the focal length of the LCMLA is implemented. For instance, an LCMLA with the aperture of 200 μm and a fill factor of 82.2%, the cell gap is 20 μm. Changing the voltage from 0 to 0.7 Vrms allows the focal length to be adjusted from −4 mm to ∞; in turn, varying the voltage from 1 to 10 Vrms results in the focal length from 6.0 to 4.5 mm.

Femtosecond laser drilled micro-hole arrays in thick and dense 2D nanomaterial electrodes toward high volumetric capacity and rate performance

Two dimensional (2D) nanomaterials have great application potential in developing the energy storage systems with high volumetric energy and power densities. However, due to the sluggish ion diffusion, achieving both high volumetric capacities and rate performance in thick and dense 2D nanomaterial electrodes is still challenging. Here, we report a femtosecond laser drilling method to introduce appropriate micro-hole arrays into the thick and dense electrodes for facilitated ion transport. These micro-hole arrays shorten ion transport paths and thus significantly reduce the ion transport resistance. More importantly, constructing micro-hole arrays with hole spacing higher than 40 μm rarely sacrifices the density of the electrode. The effectiveness of micro-hole arrays to improve capacities and rate performance of the thick and dense electrodes is evidenced by two model cases: a TiO2-based electrode constructed with the upper-layer TiO2-coated graphene hybrids and under-layer graphene (dTiO2-G/G), and a graphene electrode. As presented, the micro-hole array with a hole spacing of 40 μm decreases the ionic resistance of the dTiO2-G/G electrode, and consequently improves the volumetric capacity at 10C from 15.8 to 97 mA h cm−3. Such micro-hole array also improves the volumetric capacity and rate performance of the graphene electrode.

Self-assembly of superstructures at all scales

Summary Controllable assembly of molecular and nanoscale building blocks into uniform superstructures up to bulk dimensions remains a key challenge in the next phase of nanotechnology development. Here, we report the self-assembly of superstructures at all scales by taking advantage of the partial miscibility of water and 1-butanol to generate transient aqueous emulsion droplets that can encapsulate the target materials and introduce them into template holes. Further diffusion of water into 1-butanol depletes the emulsion droplets, assembling the building blocks into one well-defined superstructure in each hole. Superstructuring of various types and shapes of nanoparticles, biomolecules, and inorganic compounds could be achieved without the need for surfactants and chemical modifications. The versatility, scalability, low-cost, and accurate positioning are crucial advantages for the future development of advanced precision manufacturing.

Investigation on hole diameter non-uniformity of hole arrays by ultrasonic vibration-assisted EDM

Ultrasonic vibration-assisted electrical discharge machining (EDM) as an alternative advanced processing technology has been widely used to machine micro-hole arrays. However, there exists a concentration of debris particles in the discharge gap, which affects the machining accuracy of micro-hole arrays by ultrasonic vibration-assisted EDM with tool electrode arrays. In this study, a series of experiments of ultrasonic vibration-assisted EDM of micro-hole arrays with tool electrode arrays were performed, and the experimental results demonstrate that the hole diameter in the central holes of hole arrays is a little bigger than those in the peripheral holes of hole arrays. To illustrate the mechanism of hole diameter non-uniformity of micro-hole arrays by ultrasonic vibration-assisted EDM with tool electrode arrays, a three-dimensional gap flow field model for ultrasonic vibration-assisted EDM of micro-hole arrays was proposed based on computational fluid mechanics. In the flow field model, the laws of flow field velocity and debris particles distribution were simulated numerically under the effect of ultrasonic vibration. To investigate the rules of flow field velocity and debris particles distribution during ultrasonic vibration-assisted EDM of micro-hole arrays, the flow field model of 3 × 3 micro-hole arrays, 5 × 5 micro-hole arrays, and 10 × 10 micro-hole arrays were considered. The numerical simulation results show that in the flow field, the concentration degree of debris particles in the peripheral holes of micro-hole arrays is lower than those of the central holes of micro-hole arrays. The non-uniformity distribution of debris particles of the central holes and the peripheral holes of micro-hole arrays strength the local electric field and leads to non-uniformity of discharge breakdown probability, which causes the hole diameter non-uniformity of micro-hole arrays by ultrasonic vibration-assisted EDM. The experimental and numerical simulation results reveal the mechanism of hole diameter non-uniformity of micro-hole arrays by ultrasonic vibration-assisted EDM with tool electrode arrays and suggest that decreasing the concentration of debris particles is the effective way to improve the performance of EDM of micro-hole arrays.

Design of Continuous Transport of the Droplet by the Contact-Boiling Regime.

Joule-heat-driven directional transport of liquid droplets has comprehensive engineering applications in various water and thermal management, cooling systems, and self-cleaning. Generally, the driving force for the transport of liquid droplets was always observed at an extremely high Leidenfrost temperature, which limits the potential application between liquid boiling and Leidenfrost points. In this work, we design a new strategy to directionally drive the transport of droplets by blockading the vapor cushion at a temperature much lower than the Leidenfrost point. On the surface of the microhole arrays, we observed the continuous rebound behavior of ethanol droplets at Ts = 110 °C. Employing the thermal multiphase lattice Boltzmann model, the continuous rebound behavior was reproduced, verifying that the driving force was provided by the blockaded vapor pressure in microholes. By cooperating with the Laplace pressure difference, we directionally transport ethanol and water droplets on the horizontal asymmetrical concentric microridge surface. The horizontal velocity of water is 11.25 cm/s at Ts = 180 °C, similar to the traditional ratchets at the Leidenfrost point. The design of microtextures enriches the fundamental understanding of how to drive droplets at far below the Leidenfrost point and pushes the application in nongravity-driven self-cleaning and cooling systems.

Fabrication of microholes array on titanium foil by a femtosecond laser and a surface’s wettability switching

In this study, an effective method is proposed for controlling a titanium foil surface’s wettability. A microholes array series is fabricated on the surface of titanium foil by a femtosecond laser under different laser energy and pulse number. The changes of the titanium surface’s morphology are characterized. When placed in a darkroom with high-temperature treatment and immersed in alcohol under UV irradiation, respectively, the femtosecond laser treated surfaces display switchable wettability. It is demonstrated that the changing between Ti−OH and Ti−O prompts the transformation between superhydrophilic and superhydrophobic. Compared with existing reports, the switchable wetting cycle is shortened to 1.5 h. The functional surfaces with switchable wettability have potential applications in oil–water separation and water mist collection.

Preparation of Ni micropillar arrays with high aspect ratios using anodic porous alumina template and their application to molds for imprinting.

Anodic porous alumina templates with controlled microscale geometrical structures were prepared by a process combining mask formation and subsequent selective etching of the alumina layer. In this process, the anisotropic etching of anodic porous alumina allows the preparation of anodic porous alumina with microhole array patterns having high aspect ratios. The electrodeposition of Ni using the obtained alumina templates generated an array of Ni micropillars with high aspect ratios. The height of Ni micropillars could also be controlled by adjusting the thickness of the anodic porous alumina. The obtained Ni micropillar array with a high aspect ratio was applied as a mold for imprinting. The ordered microstructures of TiO2 with high aspect ratios were prepared by imprinting using the Ni mold.

Precision EDM of Micron-Scale Diameter Hole Array Using in-Process Wire Electro-Discharge Grinding High-Aspect-Ratio Microelectrodes

Micro-electrical discharge machining (micro-EDM) is a good candidate for processing micro-hole arrays, which are critical features of micro-electro-mechanical systems (MEMS), diesel injector nozzles, inkjet printheads and turbine blades, etc. In this study, the wire vibration of the wire electro-discharge grinding (WEDG) system has been analyzed theoretically, and, accordingly, an improved WEDG method was developed to fabricate micron-scale diameter and high-aspect-ratio microelectrodes for the in-process micro-EDM of hole array with hole diameter smaller than 20 μm. The improved method has a new feature of a positioning device to address the wire vibration problem, and thus to enhance microelectrodes fabrication precision. Using this method, 14 μm diameter microelectrodes with less than 0.4 μm deviation and an aspect ratio of 142, which is the largest aspect ratio ever reported in the literature, were successfully fabricated. These microelectrodes were then used to in-process micro-EDM of hole array in stainless steel. The effects of applied voltage, current and pulse frequency on hole dimensional accuracy and microelectrode wear were investigated. The optimal processing parameters were selected using response–surface experiments. To improve machining accuracy, an in-process touch-measurement compensation strategy was applied to reduce the cumulative compensation error of the micro-EDM process. Using such a system, micro-hole array (2 × 80) with average entrance diameter 18.91 μm and average exit diameter 17.65 μm were produced in 50 μm thickness stainless steel sheets, and standard deviations of hole entrance and exit sides of 0.44 and 0.38 μm, respectively, were achieved.

Ultrasound-Assisted Through-Mask Electrochemical Machining of Hole Arrays in ODS Superalloy.

Micro-hole arrays have found wide applications in aerospace, precision instruments, and biomedicine. Among various methods of their production, including mechanical, laser, and electrical discharge, electrochemical machining (ECM) is considered the most lucrative due to its wide processing range, high surface quality, and excellent productivity. In particular, ultrasound-assisted through-mask ECM exhibits an enhanced machining precision due to ultrasonic cavitation, which promotes the removal of the electrolytic products and bubbles. In this study, the equation of cavitation bubble oscillation was derived and numerically solved to study the influence of six different parameters on the ultrasonic cavitation and electrolysis process, and their optimal values were determined. The feasibility of the proposed ultrasound-assisted through-mask ECM technology with the optimized parameters was experimentally corroborated by the fabrication of a high-quality hole array in an oxide dispersion strengthened (ODS) MA956 superalloy.

Enhanced light extraction of light-emitting diodes with micro patterns by femtosecond laser micromachining for visible light communication.

A significant enhancement of light extraction of light-emitting diodes (LEDs) with micro patterns has been experimentally investigated. The micro patterns on the surface of a polymer layer are fabricated by a femtosecond laser Bessel beam for obtaining microhole arrays with large depth, resulting in the reduction of photon loss by total internal reflection (TIR) at the surface of the LED. The light output power of the LED is apparently increased by introducing the array patterns without influencing its current-voltage (I-V) characteristics. Moreover, the electroluminescence spectra of a multi-color LED and its angular radiation profiles with orthogonal and hexagonal patterns also have been explored. In addition, the optical field distributions of the micro patterns simulated by the finite difference time domain method have expressed the modulation effect of the array depth. Finally, the patterned LED as a transmitter is embedded in the visible light communication system for evaluating the transmission signal quality.

Coaxial hole array fabricated by ultrafast femtosecond-laser processing with spatially multiplexed vortex beams for surface enhanced infrared absorption

Here, we combined shaping and multiplexing of the laser beam to achieve ultrafast femtosecond-laser patterning of Au films with coaxial and circular hole arrays at a printing rate of 106 elements per second. Fabrication quality of the developed parallel laser-printing approach was found to be enough to replicate coaxial hole arrays with a resonant transmission over 90% in the mid-IR spectral range resulted from coupling between localized electromagnetic mode supported by coaxial unit cell and the lattice-type SPPs. Coupling of the surface plasmons localized within the coaxial holes to the vibration modes of the deposited analyte, widely used antihistamine drag Diphendramine, was found to provide around 800-fold enhancement of the amplitude of characteristic IR bands allowing reliable SEIRA-based fingerprint identification at trace concentrations. Our findings are consistent with the calculated amplitude of the electromagnetic hot spots suggesting their predominant contribution to the observed SEIRA effect. The ability to tune in a facile way the hole geometry and arrangement in the process of fabrication allows to tailor the spectral position of the resonance spanning practically relevant spectral range from 4 to 10 μm. This makes the coaxial microhole arrays produced with ultrafast direct laser printing promising for IR filtering and sensing.

Water-assisted femtosecond laser drilling of 4H-SiC to eliminate cracks and surface material shedding

This study adopted femtosecond laser with a wavelength of 515 nm to drill high-aspect-ratio micro through holes on a 500-μm thickness single-crystal SI-type 4H-SiC wafer. Firstly, through holes with a high aspect ratio of 20 were fabricated in air. However, the heat affect zone (HAZ), cracks, and surface material shedding around entrances and exits of the holes are inevitable in air even after chemical corrosion post-processing. In order to remove these defects, the water-assisted femtosecond laser drilling of 4H-SiC was investigated. The high-quality through holes free of cracks, surface material shedding, and HAZ were obtained under the action of internal scour and heat diffusion of water. Besides, the water layer thickness and the laser repetition frequency have a great influence on the processing quality and efficiency of the micro-holes. Finally, high-quality high-ratio-rate through micro-hole arrays on 4H-SiC were fabricated with the optimal process parameters, which is significant for the development of SiC electronic devices and the high-quality micro-fabrication of other hard and brittle materials.

Micro-electrode wear and compensation to ensure the dimensional consistency accuracy of micro-hole array in micro-EDM drilling

The micro-hole array machined by microelectrical discharge machining (micro-EDM) has been widely used in industrial fields, such as inkjet nozzle, oil nozzle, spinneret, and so on. However, it is difficult to guarantee the dimensional consistency accuracy of micro-hole array. In the present study, the characteristic and compensation strategy of microelectrode wear were explored to meet the requirements of dimensional consistency accuracy of micro-hole array machining. The effect of pulse generator parameters on microelectrode wear was studied, and the microelectrode wear was reduced by optimizing pulse generator parameters. The geometry evolution rule of microelectrode was investigated, and the microelectrode wear model was established based on the wear characteristic of microelectrode. By analyzing the wear characteristic of microelectrode, single-hole quantitative compensation strategy was adopted during the machining process. Meanwhile, grouping detection variable compensation strategy was proposed to adjust axial position of microelectrode periodically in order to eliminate the cumulative compensation errors. The array of 256 (16 × 16) micro-holes with an average diameter of 45.28 μm was machined on a 50-μm-thick stainless steel. The dimensional consistency accuracy of micro-hole array was assured by showing a standard deviation of 0.15 μm. In addition, the array of 64 (8 × 8) micro-holes and the array of 25 (5 × 5) micro-holes with an average diameter of less than 20 μm were machined to explore the performance of micro-hole array machining with smaller size, and the dimensional consistency accuracy was shown by the standard deviation of 0.25 and 0.4 μm, respectively.

Coaxial Aperture Arrays Produced by Ultrafast Direct Femtosecond Laser Processing with Spatially Multiplexed Cylindrical Vector Beams

Direct femtosecond laser printing was used to fabricate circular-and coaxial-shaped hole arrays at ultrafast printing rate up to 106 elements per second. To achieve such fast printing rate, we implemented a spatial multiplexing of either a single Gaussian or cylindrical vector beams into linear array of identical laser spots. Being compared to ordinary microholes, the coaxial openings arranged at the same periodicity demonstrate enhanced transmission in the mid-IR spectral range resulted from coupling between localized electromagnetic mode supported by coaxial unit cell and the lattice-type surface plasmon resonance. At optimized geometry of the coaxial openings and their arrangement we demonstrated resonant transmission as high as 92% at wavelengths ranging from 7.5 to 9 μm. This makes the coaxial microhole arrays with tailored spectral properties produced with ultrafast and inexpensive direct laser printing promising for sensing applications based on surface enhanced infrared absorption.

Self-pumping and scalable fog collector with diode-like micro-hole arrays inspired by natural asymmetric wettability

Fog collection has been considered as an effective and sustainable approach for relieving the water deficiency in arid regions. Despite great progresses on bioinspired superwetting surfaces towards fog collection, practical fog-collecting devices face challenges by omnidirectional collection and lower-evaporation storage simultaneously. Inspired by unidirectional liquid movement caused by asymmetric wettability on the holey lotus leaf, a self-revolving collector was developed to efficiently harvest the fog in a wind-driven manner, finally storing it lastingly at lower-evaporation rate. Such a collector provided a superhydrophobic external surface for capturing fogdrops, a diode-liquid micro-hole array for the spontaneous droplet movement, and a hydrophilic internal surface for fast droplet collection and long-term storage. The synergy of these structural features together with composition merits made the self-revolving collector show efficient fog collection and lower-evaporation storage. The fog-laden wind field around the collector as well as the relationship between the self-revolving speed and the fog-laden wind speed were explored theoretically. The corresponding model analyses further supported the 49% enhancement of fog collection rate when the collector whipped by 1 m/s fog-laden wind revolves at 18 rad/s. This work opens an avenue for designing a novel fog collector used in real environment and offers a strategy for optimizing the existing fog collectors towards high efficiency.

Self-limiting electrospray deposition on polymer templates

Electrospray deposition (ESD) applies a high voltage to liquids flowing through narrow capillaries to produce monodisperse generations of droplets down to hundreds of nanometers in diameter, each carrying a small amount of the delivered solute. This deposition method has been combined with insulated stencil masks for fabricating micropatterns by spraying solutions containing nanoparticles, polymers, or biomaterials. To optimize the fabrication process for micro-coatings, a self-limiting electrospray deposition (SLED) method has recently been developed. Here, we combine SLED with a pre-existing patterned polymer film to study SLED’s fundamental behavior in a bilayer geometry. SLED has been observed when glassy insulating materials are sprayed onto conductive substrates, where a thickness-limited film forms as charge accumulates and repels the arrival of additional charged droplets. In this study, polystyrene (PS), Parylene C, and SU-8 thin films of varying thickness on silicon are utilized as insulated spraying substrates. Polyvinylpyrrolidone (PVP), a thermoplastic polymer is sprayed below its glass transition temperature (Tg) to investigate the SLED behavior on the pre-deposited insulating films. Furthermore, to examine the effects of in-plane confinement on the spray, a microhole array patterned onto the PS thin film by laser dewetting was sprayed with dyed PVP in the SLED mode. This was then extended to an unmasked electrode array showing that masked SLED and laser dewetting could be used to target microscale regions of conventionally-patterned electronics.

Study of GaN-Based Light-Emitting Diode (LED) With a Hybrid Surface Structure

A hybrid surface structure, incorporating an Ag-grid-based aluminum-doped zinc oxide (AZO) transparent conductive layer (TCL), a microhole array, 45°-sawtooth sidewalls, and an SiO2 nanoparticle (NP)/microsphere (MS) passivation layer, is proposed for the characteristic improvement of GaN-based light-emitting diodes (LEDs). In order to optimize the contact behavior between the AZO and p-GaN layers, a 2-D Ag grid is applied on the contact interface. Because series resistance ${R}_{s}$ is reduced by the conductive Ag-grid pattern, a lower forward voltage and a better current spreading ability are obtained. Compared with a conventional GaN LED with an injection current of 200 mA, the proposed device exhibits a forward voltage of 4.01 V, reduced from 4.34 V, and presents 33%, 34.4%, 45.4%, 33.1%, and 45.3% enhancements in the light output power (LOP), luminous flux, luminous efficacy, external quantum efficiency (EQE), and wall-plug efficiency (WPE), respectively. Moreover, a more effective current spreading in the light emission mapping image and a higher intensity in the far-field pattern are achieved. Improvements of both electrical and optical properties verify that the proposed device is promising for practical applications in solid-state lighting.

Effects of Localized Micro-blowing on a Spatially Developing Flat Turbulent Boundary Layer

Direct numerical simulation (DNS) is used to investigate the turbulent flat-plate boundary layer with localized micro-blowing. The 32 × 32 array of micro-holes is arranged in a staggered pattern on the solid wall, located in the developed turbulent region. The porosity of the porous wall is 23%, and the blowing fraction is 0.0015. The Reynolds number based on the inflow velocity is set to be 50,000. The structures of the turbulent boundary layer are carefully analyzed to understand the effects of micro-blowing and its drag reduction mechanism. The DNS results show that the drag reduction is efficient, and the local maximum rate of drag reduction achieves 40%. A low-speed “turbulent spot” near the micro-blowing region thickens the boundary layer. Some turbulent properties, such as the mean velocity profile, stream-wise vorticity and stream-wise velocity fluctuation are lifted up. Particularly, the tilting term of vorticity transport is significantly increased. Meanwhile, the visualization of 3-dimensional vortex displays several concave marks on the surface of the near-wall vortices, which is caused by the micro-jets, leading to more broken vortices and isotropic small scales. This impact travels downstream with a small distance due to the accumulation of the micro-jets, while the uplift effect will gradually disappear. In addition, FIK identity reveals that the spatial development term and mean wall-normal convection term play opposite roles in the contribution to the skin friction drag.