Highly corrected submicrometer grid patterning on curved surfaces

A compact holographic projector system was built and tested. This projection system offers a practical approach for making a highly corrected mesh or grid pattern on curved surfaces. The pattern can range in size from multimicrometer to submicrometer dimensions and be recorded in either positive or negative photoresist. Standing-wave interference patterns in the form of a diverging close-packed lattice of either hexagonal or square rodlike intensity maxima extending outward from a point or a locus of points are produced by multiple-beam holography that involves the combination of a holographic diffraction grating and a hypercomatic focusing objective.     

Electrowetting-driven variable-focus microlens on flexible surfaces

We demonstrate a flexible, electrowetting-driven, variable-focus liquid microlens. The microlens is fabricated using a soft polymer polydimethylsiloxane. The lens can be smoothly wrapped onto a curved surface. A low-temperature fabrication process was developed to reduce the stress on and to avoid any damage to the polymer. The focal length of the microlens varies between -15.0?mm to +28.0?mm, depending on the applied voltage. The resolving power of the microlens is 25.39 line pairs per mm using a 1951 United States Air Force resolution chart. The typical response time of the lens is around 50?ms.     

Patterned close-packed nanoparticle arrays with controllable dimensions and precise locations

Patterned close-packed nanoparticle arrays are fabricated using lithography and self-assembly. Microcontact printing is used to selectively transfer ordered nanoparticle monolayers, which are self-assembled at the air/water interface, onto relief structures, which are defined lithographically. The morphology and position of the nanoparticle arrays are determined by the relief structures, while the internal order of the arrays is achieved through the self-assembly process and is maintained during the transfer.     

Performance and optimum design analysis of convective fin arrays attached to flat and curved primary surfaces

A model has been developed analytically to carry out the performance and optimum design analysis of four fin arrays, namely, longitudinal rectangular fin array (LRFA), annular rectangular fin array (ARFA), longitudinal trapezoidal fin array (LTRA) and annular trapezoidal fin array (ATFA) under convective cooling conditions. The performance parameters such as fin efficiency, fin effectiveness and augmentation factor are evaluated for a wide range of design variables. It has been observed that the conduction through the supporting structure and the convection from the interfin spacing have a pronounced effect on the performance of a fin array. The optimum fin dimensions in a fin assembly have been determined by consideration of the constant total height of the fin assembly and interfin spacing. From the results, it can be highlighted that the optimum fin dimensions in fin arrays differ from that of the individual fins.     

Focal-plane pixel-energy redistribution and concentration by use of microlens arrays

A physical-optics calculation was performed to study the effects of a microlens array placed over a focal-plane detector array. In certain conditions the light is further concentrated to a spot size that is smaller than the point-spread function of the receiver optics. It is also shown that the microlens refocuses a sinc-squared point-spread function to a shape that is more uniform as well as narrower. Numerical examples were made for the far IR.     

Scattering measurements on curved rough surfaces

Abstract

A theoretical model is developed to examine light scattered from a curved optical surface with a defined surface form. The contribution of the surface form to scattering patterns is separated from that of the surface roughness. Experiments are carried out on a curved surface. Measurements are analysed and compared with theoretical calculations. The power spectrum density function and the autocorrelation function of the surface roughness are recovered from the measured scattering pattern.     

Substrate-free growth, characterization and growth mechanism of ZnO nanorod close-packed arrays

ZnO nanorod close-packed arrays are successfully fabricated in a substrate-free manner by a citric acid assisted annealing process at a low growth temperature of 400?°C. Each nanorod of ZnO nanorod close-packed arrays grows along the [0001] direction and is single crystalline with an average diameter of 50?nm, and an average length of 0.5?μm. The aspect ratio is 10. The ZnO nanorod close-packed arrays show a strong exciton absorption peak at 372?nm in UV-visible absorption spectra, exhibiting a blue-shift relative to the bulk exciton absorption (387?nm). Finally, a new growth mechanism is proposed for the substrate-free preparation of ZnO nanorod close-packed arrays by a citric acid assisted annealing process.     

Survey of structural and electronic properties of C60 on close-packed metal surfaces

The adsorption of buckminsterfullerene (C₆₀) on metal surfaces has been investigated extensively for its unique geometric and electronic properties. The two-dimensional systems formed on surfaces allow studying in detail the interplay between bonding and electronic structures. Recent studies reveal that C₆₀ adsorption induces reconstruction of even the less-reactive close-packed metal surfaces. First-principles computations enable access to this important issue by providing not only detailed atomic structure but also electronic properties of the substrate–adsorbate interaction, which can be compared with various experimental techniques to determine and understand the interface structures. This review discusses in detail the ordered phases of C₆₀ monolayers on metal surfaces and the surface reconstruction induced by C₆₀ adsorption, with an emphasis on the different types of reconstruction resulting on close-packed metal surfaces. We show that the symmetry matching between C₆₀ molecules and metal surfaces determines the local adsorption configurations, while the size matching between C₆₀ molecules and the metal surface lattice determines the supercell sizes and shapes; importantly and uniquely for C₆₀, the number of surface metal atoms within one supercell determines the different types of reconstruction that can occur. The atomic structure at the molecule–metal interface is of crucial importance for the monolayer’s electronic and transport properties: these will also be discussed for the well-defined adsorption structures, especially from the perspective of tuning the electronic structure via C₆₀–metal interface reconstruction and via relative inter-C₆₀ orientations.     

Modeling of coating flows on curved surfaces

The equations describing the temporal evolution of a thin, Newtonian, viscous liquid layer are extended to include the effect of substrate curvature. It is demonstrated that, subject to the standard assumptions required for the validity of lubrication theory, the surface curvature is equivalent to an applied time-independent overpressure distribution. Within the mathematical model, a variety of substrate shapes, possessing both inside and outside corners, are shown to be equivalent. Starting with an initially uniform coating layer, the evolving coating profile is calculated for substrates with piecewise constant curvature. Ultimately, surface tension forces drive the solutions to stable minimum-energy configurations. For small time, the surface profile history, for a substrate with a single curvature discontinuity, is given as the self-similar solution to a linear fourth-order diffusive equation. Using a Fourier transform, the solution to the linear problem is found as a convergent infinite series. This Green's function generates the general solution to the linearized problem for arbitrary substrate shapes. Calculated solutions to the non-linear problem are suggestive of coating defects observed in industrial applications.     

Fabrication and characterization of multi-scale microlens arrays with anti-reflection and diffusion properties

In this paper, an effective method for fabricating artificial compound-eye structures is demonstrated. The fabrication of high fill factor microlens arrays (MLAs) with sub-wavelength structures (SWSs) on a polycarbonate (PC) substrate involves nanoimprint and thermo-extrusion techniques by using two different scales of nano/micromolds. In addition, the MLAs with SWSs on the PC substrate would be replicated on a polymethylmethacrylate (PMMA) millimeter concave surface by hot-embossing, forming three-level compound-eye structures. The optical properties of these samples are characterized. The transmittances of two-level PC and three-level PMMA compound structures are increased by 2.5% and 2%, and the uniformities are enhanced by 18% and 24%, respectively.     

Hybrid sample-inverted reflow and soft-lithography technique for fabrication of conicoid microlens arrays

We report a cost-effective fabrication method, with a combination of the sample-inverted reflow technique and the soft-lithography replication method, to fabricate conicoid refractive microlens arrays (MLAs), including hyperboloid, paraboloid, and ellipsoid MLAs in inorganic-organic hybrid SiO2-ZrO2 solgel material. The fabrication procedures involve two basic steps. First, a master of the conicoid MLA was made in photoresist by the sample-inverted reflow technique. Second, we built a negative mold of the master by casting polydimethylsiloxane (PDMS) onto a silicone elastomer against the master, and then the profile was imprinted onto the solgel glass. As a result, the fabricated solgel MLAs have been obtained with excellent smooth profiles, having negligible discrepancies from the profiles of ideal conicoid MLAs.     

Computer-generated holograms of images reconstructed on curved surfaces

When one illuminates a computer-generated hologram by a plane wave, the obtained two-dimensional image is usually displayed on a planar plane. Other possibilities for reconstructing images on arbitrary curved surfaces are discussed herein. As an example, the reconstruction of an image on a virtual spherical surface is demonstrated.     

Optimization and theoretical modeling of polymer microlens arrays fabricated with the hydrophobic effect

High-performance polymer microlens arrays were fabricated by means of withdrawing substrates of patterned wettability from a monomer solution. The f-number (f(#)) of formed microlenses was controlled by adjustment of monomer viscosity and surface tension, substrate dipping angle and withdrawal speed, the array fill factor, and the number of dip coats used. An optimum withdrawal speed was identified at which f(#) was minimized and array uniformity was maximized. At this optimum, arrays of f/3.48 microlenses were fabricated with one dip coat with uniformity of better than Deltaf/f +/- 3.8%. Multiple dip coats allowed for production of f/1.38 lens arrays and uniformity of better than Deltaf/f +/-5.9%. Average f(#)s were reproducible to within 3.5%. A model was developed to describe the fluid-transfer process by which monomer solution assembles on the hydrophilic domains. The model agrees well with experimental trends.     

Amplitude filter and Zernike polynomial expansion method for quality control of microlens arrays

This paper deals with a computer simulation and an experimental realization of an optical setup for automatic quality control of microlens arrays. The method is based on a 4f coherent light correlator setup with an amplitude filter placed in the Fourier plane. The output intensity signal is proportional to the first derivative of the distortion of the input wavefront. An analysis can be carried out with the use of the Zernike polynomial expansion method. It must be carried out separately for each lens, but it allows for a more precise, quantitative assessment of their quality. What is important is that the analysis is computer-based and performed on the basis of the initial single optical measurement.     

Realization of thermally durable close-packed 2D gold nanoparticle arrays using self-assembly and plasma etching

Realization of thermally and chemically durable, ordered gold nanostructures using bottom-up self-assembly techniques are essential for applications in a wide range of areas including catalysis, energy generation, and sensing. Herein, we describe a modular process for realizing uniform arrays of gold nanoparticles, with interparticle spacings of 2?nm and above, by using RF plasma etching to remove ligands from self-assembled arrays of ligand-coated gold nanoparticles. Both nanoscale imaging and macroscale spectroscopic characterization techniques were used to determine the optimal conditions for plasma etching, namely RF power, operating pressure, duration of treatment, and type of gas. We then studied the effect of nanoparticle size, interparticle spacing, and type of substrate on the thermal durability of plasma-treated and untreated nanoparticle arrays. Plasma-treated arrays showed enhanced chemical and thermal durability, on account of the removal of ligands. To illustrate the application potential of the developed process, robust SERS (surface-enhanced Raman scattering) substrates were formed using plasma-treated arrays of silver-coated gold nanoparticles that had a silicon wafer or photopaper as the underlying support. The measured value of the average SERS enhancement factor (2?×?10(5)) was quantitatively reproducible on both silicon and paper substrates. The silicon substrates gave quantitatively reproducible results even after thermal annealing. The paper-based SERS substrate was also used to swab and detect probe molecules deposited on a solid surface.     

Design and fabrication of diffractive microlens arrays with continuous relief for parallel laser direct writing

Diffractive microlens arrays with continuous relief are designed, fabricated, and characterized by using Fermat's principle to create an array of spots on the photoresist-coated surface of a substrate for parallel laser direct writing. Experimental results indicate that a diffraction efficiency of 71.4% and a spot size of 1.97 microm (FWHM) can be achieved at normal incidence and a writing laser wavelength of 441.6 nm with an array of F/4 fabricated on fused silica, and the developed array can be used to improve the utilization ratio of writing laser energy.     

The X-ray measurement of axial stress on cylindrical curved surfaces

A theoretical analysis, which corrects for the geometrical effect of specimen curvature on the angular position of diffracted X-ray peaks recorded using the diffractometer method, has been derived. The analysis has been used to correct the diffraction angles measured using the diffractometer method for the X-ray measurement of longitudinal stress on cylindrically curved surfaces. Residual stress versus depth profiles on a semicircular, machined keyway in an L.P. turbine disc and the residual stress distribution around a shot-peened, L.P. turbine blade, 'fir-tree', notch root, both before and after fatigue loading, have been measured.     

Simulation of synthetic gecko arrays shearing on rough surfaces

To better understand the role of surface roughness and tip geometry in the adhesion of gecko synthetic adhesives, a model is developed that attempts to uncover the relationship between surface feature size and the adhesive terminal feature shape. This model is the first to predict the adhesive behaviour of a plurality of hairs acting in shear on simulated rough surfaces using analytically derived contact models. The models showed that the nanoscale geometry of the tip shape alters the macroscale adhesion of the array of fibres by nearly an order of magnitude, and that on sinusoidal surfaces with amplitudes much larger than the nanoscale features, spatula-shaped features can increase adhesive forces by 2.5 times on smooth surfaces and 10 times on rough surfaces. Interestingly, the summation of the fibres acting in concert shows behaviour much more complex that what could be predicted with the pull-off model of a single fibre. Both the Johnson–Kendall–Roberts and Kendall peel models can explain the experimentally observed frictional adhesion effect previously described in the literature. Similar to experimental results recently reported on the macroscale features of the gecko adhesive system, adhesion drops dramatically when surface roughness exceeds the size and spacing of the adhesive fibrillar features.     

Thermal stability of self-assembled octadecyltrichlorosilane monolayers on planar and curved silica surfaces

Understanding the structural and functional integrity of self-assembled monolayers (SAMs) of alkytrichlorosilane on Si/SiO₂ interface with change in temperature is critical for realizing their utility as antistiction coatings during the fabrication and functioning of microelectromechanical systems. Here we describe the thermal stability of two dimensional (2D) octadecyltrichlorosilane (OTS) monolayers on both n-type Si substrate (planar surface) and silica spheres (curved surface) using results of various surface sensitive spectroscopic techniques like the grazing angle Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Densely packed OTS monolayer on n-type Si surface is found to be thermally stable up to 525 K, while a significant enhancement in the thermal stability is interestingly observed for the case of OTS SAM (up to 625 K) on freshly prepared spherical silica surfaces. Despite this difference in the thermal stability, the results of temperature dependent infrared spectra demonstrate monolayer decomposition in both cases through the involvement of both Si-C and C-C bonds leaving Si-O-Si bond intact.     

Calculation of the average lenslet shape and aberrations of microlens arrays from their far-field intensity distribution

We present a method to obtain the average lenslet shape of microlens arrays and especially their aberrations from the far-field intensity distribution of the whole array. The method is based on the phase-retrieval algorithm introduced by Gerchberg and Saxton [Optik (Stuttgart) 35, 237 (1972)]. We show how to overcome the crucial point of this algorithm, that is finding suitable start parameters to end up with correct results. The procedure is successfully applied to a cylindrical microlens array produced by reflow technique and the result is compared with surface profilometric measurements. The technique is applicable for lenslets having small numerical apertures and fill factors near unity.