Periodic Pattern Of Holes Research Articles

Transparent laser-structured glasses with superhydrophilic properties for anti-fogging applications

Ultrashort pulse laser structuring enables direct modification of glass surfaces to generate superhydrophilic properties for anti-fogging applications. This approach makes coatings dispensable and the generated surfaces remain thermally, mechanically, and chemically resistant. However, the laser-generated structures usually cause scattering, which decreases transmission and may disturb the vision through the modified glass in the dry state. The aim of this study was to find a laser-processing strategy to achieve superhydrophilic, anti-fogging properties on glass surfaces with maximum transmission and minimal visual perception of the generated structure. For this purpose, we used an ultrashort-pulsed laser to generate periodic patterns of rippled circles or rough holes with varying pitch. The water contact angle and transmission of the structured glasses were measured as a function of the structured area. It was found that a periodic pattern of holes, which covers less than 1% of the surface, is already sufficient to reach the superhydrophilic state (contact angle < 5°) and provides nearly the same transmission as pristine glass. Pictures of objects imaged through dry, structured glasses, which were placed close to the lens or object, showed in both cases only a minimal decrease of contrast. If this minor drawback can be accepted, this direct laser structuring approach could be an interesting alternative to coating-based techniques and leaves even room to apply additional coatings for the fabrication of multi-functional special glasses.

Effective conditions for the reflection of an acoustic wave by low-porosity perforated plates

AbstractThis paper describes an investigation of the acoustic properties of a rigid plate with a periodic pattern of holes, in a compressible, ideal, inviscid fluid in the absence of mean flow. Leppington &amp; Levine (J. Fluid Mech., vol. 61, 1973, pp. 109–127) obtained an approximation of the reflection and transmission coefficients of a plane wave incident on an infinitely thin plate with a rectangular array of perforations, assuming that a characteristic size of the perforations is negligible relative to that of the unit cell of the grating, itself assumed to be negligible relative to the wavelength. One part of the present study is of methodological interest. It establishes that it is possible to extend their approach to thick plates with a skew grating of perforations, thus confirming recent results of Bendali et al. (SIAM J. Appl. Math., vol. 73 (1), 2013, pp. 438–459), but in a much simpler way without using complex matched asymptotic expansions of the full wave or to a grating of multipoles. As is well-known, effective compliances for the plate can then be derived from the corresponding approximations of the reflection and transmission coefficients. These compliances are expressed in terms of the Rayleigh conductivity of an isolated perforation. Consequently, in one other part of the present study, the methodology recently introduced by Laurens et al. (ESAIM, Math. Model. Numer. Anal., vol. 47 (6), 2013, pp. 1691–1712) to obtain sharp bounds for the Rayleigh conductivity has been extended to include the case for which the openings of the perforations on the upper and lower sides of the plate are elliptical in shape. This not only enables the determination of these bounds and of the associated reflection and transmission coefficients for actual plates with tilted perforations but also yields single expressions covering all usual cases of perforations: straight or tilted with a circular or an elliptical cross-section.

Applicability of a binary amplitude mask for creating correctors of higher-order ocular aberrations in a photoresistive layer

Ocular aberrations can be corrected with wavefront correctors created in a photoresist layer. The simplest type of the mask used in optical lithography is a binary amplitude mask. It is known that such a mask has a periodic hole pattern. The purpose of this research was to assess applicability of a binary amplitude mask for creating ocular wavefront correctors. The photoresist was applied to the substrate by using the dip-coating method. The photoresist layer was illuminated through a mask printed on a transparent film by using a laser printer. The surface of the wavefront correctors was evaluated by aberrometry, scanning electron microscopy and profilometry method. The dip-coating method can be used to apply an uniform photoresist layer on the substrate. Despite rapid variations in the surface depth the required shape of the wavefronts can still be obtained. Because of strong light scattering the wavefront correctors manufactured by using a binary amplitude mask aren't suited for studying visual functions. However, the wavefront correctors manufactured by such a type of the mask may be used for calibration of aberrometers.

1D Koch fractal electromagnetic bandgap microstrip structures with r /a ratios higher than 0.5

A 1D Koch fractal electromagnetic bandgap (KFEBG) microstrip structure is proposed.It is conceived by replacing the conventional holes etched in the ground plane of a microstrip line by level-1 Koch fractal cell geometries, which have been obtained from a hexagonal shape. In the case of the conventional 1D electromagnetic bandgap (EBG) microstrip structure with periodic hole pattern, the design is limited to r/a ratios lower than 0.45, whereas the proposed pattern allows achieving r/a ratios higher than 0.5. It is shown that the conventional EBG and KFEBG microstrip structures behave as a stopband filter as r/a 0.5 can be useful for the design of low-pass filters. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:646–649, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25763

Design of superperiodic photonic-crystal light-emitting plates with highly directive luminance characteristics

A “superperiodic” photonic-crystal (PhC) light-emitting diode (LED) design exhibiting high-luminance patterns is proposed for applications that require highly directive vertical light extraction. The superperiodic design employs a periodic arrangement of defective missing holes on an otherwise perfect periodic pattern of holes for the basic two-dimensional PhC structure. Avoiding the fragile semiconductor membrane structure that is popular in current PhC laser designs, the design utilizes a thick epitaxy structure with electromagnetic density-of-mode singularity. This singularity is created by the defect mode in a three-dimensional frequency versus wave vector relation within the band matched with the gain spectrum of the LED that is placed on top of the base reflector plate material. The superperiodic structure of this design is shown to provide an effective means for sharpening the vertical beam in the far field, based on discrete-Fourier transformation theory. Subsequent simulations based on the finite-difference time-domain analysis verify the effectiveness of the design principle on GaN epitaxy structures with various bottom layers.

Efficiency improvement of near-ultraviolet InGaN LEDs using patterned sapphire substrates

The use of conventional and patterned sapphire substrates (PSSs) to fabricate InGaN-based near-ultraviolet (410 nm) light-emitting diodes (LEDs) was demonstrated. The PSS was prepared using a periodic hole pattern (diameter: 3 /spl mu/m; spacing: 3 /spl mu/m) on the (0001) sapphire with different etching depths. From transmission-electron-microscopy and etch-pit-density studies, the PSS with an optimum pattern depth (D/sub h/=1.5 /spl mu/m) was confirmed to be an efficient way to reduce the thread dislocations in the GaN microstructure. It was found that the output power increased from 8.6 to 10.4 mW, corresponding to about 29% increases in the external quantum efficiency. However, the internal quantum efficiency (@ 20 mA) was about 36% and 38% for the conventional and PSS LEDs, respectively. The achieved improvement of the output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using the PSS. Finally, better long-time reliability of the PSS LED performance was observed.

Self-assembling nanoparticles into holographic nanopatterns

We demonstrate a method to self-assemble metal nanoparticles into two-dimensional lattices. Monodisperse cobalt nanoparticles were synthesized within inverse micelles of polystyrene-block-poly(2-vinylpyridine) copolymer in toluene. A periodic hole pattern of photoresist (PR) was fabricated on a GaAs substrate by holographic lithography. The nanoparticles as prepared above were self-assembled onto the PR nanopatterns by dip or spin casting. They were selectively positioned in the holes due to the capillary forces related to the pattern geometry. Our study reveals that self-assembled nanoparticles in two-dimensional lattices can be obtained with a controllable number of particles per lattice point.