Mechanically tunable aspheric lenses via additive manufacture of hanging elastomeric droplets for microscopic applications

Abstract

Mechanically deformable lenses with dynamically tunable focal lengths have been developed in this work. The fabricated five types of aspheric polydimethylsiloxane (PDMS) lenses presented here have an initial focal length of 7.0, 7.8, 9.0, 10.0 and 10.2 mm. Incorporating two modes of operation in biconvex and concave–convex configurations, the focal lengths can be tuned dynamically as 5.2–10.2, 5.5–9.9, 6.6–11.9, 6.1–13.5 and 6.6–13.5 mm respectively. Additive manufacturing was utilized to fabricate these five types of aspheric lenses (APLs) via sequential layering of PDMS materials. Complex structures with three-dimensional features and shorter focal lengths can be successfully produced by repeatedly depositing, inverting and curing controlled PDMS volume onto previously cured PDMS droplets. From our experiments, we empirically found a direct dependence of the focal length of the lenses with the amount (volume) of deposited PDMS droplets. This new mouldless, low-cost, and flexible lens fabrication method is able to transform an ordinary commercial smartphone camera into a low-cost portable microscope. A few microscopic features can be readily visualized, such as wrinkles of ladybird pupa and printed circuit board. The fabrication technique by successively applying hanging droplet and facile mechanical focal-length-tuning set-up can be easily adopted in the development of high-performance optical lenses.     

Pattern-integrated interference lithography: single-exposure fabrication of photonic-crystal structures

Multibeam interference represents an approach for producing one-, two-, and three-dimensional periodic optical-intensity distributions with submicrometer features and periodicities. Accordingly, interference lithography (IL) has been used in a wide variety of applications, typically requiring additional lithographic steps to modify the periodic interference pattern and create integrated functional elements. In the present work, pattern-integrated interference lithography (PIIL) is introduced. PIIL is the integration of superposed pattern imaging with IL. Then a pattern-integrated interference exposure system (PIIES) is presented that implements PIIL by incorporating a projection imaging capability in a novel three-beam interference configuration. The purpose of this system is to fabricate, in a single-exposure step, a two-dimensional periodic photonic-crystal lattice with nonperiodic functional elements integrated into the periodic pattern. The design of the basic system is presented along with a model that simulates the resulting optical-intensity distribution at the system sample plane where the three beams simultaneously interfere and integrate a superposed image of the projected mask pattern. Appropriate performance metrics are defined in order to quantify the characteristics of the resulting photonic-crystal structure. These intensity and lattice-vector metrics differ markedly from the metrics used to evaluate traditional photolithographic imaging systems. Simulation and experimental results are presented that demonstrate the fabrication of example photonic-crystal structures in a single-exposure step. Example well-defined photonic-crystal structures exhibiting favorable intensity and lattice-vector metrics demonstrate the potential of PIIL for fabricating dense integrated optical circuits.     

Triply periodic bicontinuous structures through interference lithography: a level-set approach

Interference lithography holds the promise of fabricating large-area, defect-free photonic structures on the sub-micrometer scale both rapidly and cheaply. There is a need for a procedure to establish a connection between the structures that are formed and the parameters of the interfering beams. There is also a need to produce self-supporting three-dimensional bicontinuous structures. A generic technique correlating parameters of the interfering beams with the symmetry elements present in the resultant structures by a level-set approach is developed. A particular space group is ensured by equating terms of the intensity equation to a representative level surface of the desired space group. Single- and multiple-exposure techniques are discussed. The beam parameters for certain cubic bicontinuous structures relevant to photonic crystals, viz.,the diamond(D), the simple cubic (P), and the chiral gyroid (G) are derived by utilizing either linear or elliptically polarized light.     

Fabrication of three-dimensional photonic crystals by interference lithography with low light absorption

We report on the fabrication of polymer templates of photonic crystals by means of holographic (or interference) lithography. The grating is written in a SU-8 photoresist using a He-Cd laser of wavelength 442 nm. The use of the wavelength found within the photoresist low absorption band enables fabricating structures that are uniform in depth. Parameters of the photoresist exposure and development for obtaining a porous structure corresponding to an orthorhombic lattice are determined.     

Shape control of multivalent 3D colloidal particles via interference lithography

We present a new route for the fabrication of highly nonspherical complex multivalent submicron particles. This technique exploits the ability of holographic interference lithography to control geometrical elements such as symmetry and volume fraction in 3D lattices on the submicron scale. Colloidal particles with prescribed complex concave shapes are obtained by cleaving low volume fraction connected structures fabricated by interference lithography. Controlling which Wyckoff sites in the space group of the parent structure are connected assures specific "valencies" of the particles. Two types of particles, 2D "4-valent" and 3D "6-valent" particles are fabricated via this technique. In addition to being able to control multivalent particle shape, this technique has the potential to provide tight control over size, yield, and dispersity.     

Large-area pattern transfer of metallic nanostructures on glass substrates via interference lithography

In this paper, we report a simple and effective nanofabrication method for the pattern transfer of metallic nanostructures over a large surface area on a glass substrate. Photoresist (PR) nano-patterns, defined by laser interference lithography, are used as template structures where a metal film of controlled thickness is directly deposited and then transferred onto a glass substrate by the sacrificial etching of the PR inter-layer. The laser interference lithography, capable of creating periodic nano-patterns with good control of their dimensions and shapes over a relatively large area, allows the wafer-scale pattern transfer of metallic nanostructures in a very convenient way. By using the approach, we have successfully fabricated on a glass substrate uniform arrays of hole, grating, and pillar patterns of Ti, Al, and Au in varying pattern periodicities (200 nm-1 μm) over a surface area of up to several cm(2) with little mechanical crack and delamination. Such robust metallic nanostructures defined well on a transparent glass substrate with large pattern coverage will lead to advanced scientific and engineering applications such as microfluidics and nanophotonics.     

Spectral interference Mirau microscope with an acousto-optic tunable filter for three-dimensional surface profilometry

A nonmechanical scanning Mirau-type spectral interference microscope has been developed for the measurement of three-dimensional surface profiles of discontinuous objects. An acousto-optic tunable filter (AOTF) is used as a high-resolution spectral filter, which scans the optical frequency of the broadband light emitted from a superluminescent diode. To generate spectral fringes that make full use of the limited coherence length of the filtered light we unbalanced the Mirau interferometric system by positioning the reference mirror nearly halfway between the top and the bottom of the step height. When the frequency of the broadband light source is scanned by an AOTF, the interference fringes move in opposite directions on the top and the bottom of the object. To uniquely determine the sign of the fringe movement over the large area of the object, we developed a three-dimensional Fourier-transform technique, and from the detected sign of the fringe movement and phase information, we determined the three-dimensional step height. Experimental results of the measurement of 100-microm step height are presented. The main advantages of the proposed system are that it provides nonmechanical scanning and a large measurement range without ambiguity in the sign of the phase.     

Quantitative sizing of nano/microparticles with a tunable elastomeric pore sensor

The use of a "size-tunable" polyurethane resistive pulse sensor for quantitative sizing of nano- and microparticles is presented. A linear relationship, as first suggested by Maxwell, between particle volume and change in electric resistance across the pore was observed. Particle sizes were quantified for a given size-tunable membrane, by first creating a linear calibration curve to a series of monodisperse carboxylated polystyrene particles of various diameters and then applying this curve to calculate the size of "unknown" nanoparticles. The diameters of a selection of synthetic and biological particles, being PMMA and nonfunctionalized polystyrene particles, along with biological nanoparticles (adenovirus) were calculated using this methodology. Calculated particle diameters and coefficients of variation were shown to be in good agreement with both transmission electron microscopy and dynamic light scattering results.     

Patterning of various silicon structures via polymer lithography and catalytic chemical etching

We demonstrate a facile fabrication of a rich variety of silicon patterns with different length scales by combining polymer lithography and a metal-assisted chemical etching method. Several types of polymer patterns were fabricated on silicon substrates, and silver layers were deposited on the patterned silicon surfaces and used to etch the silicon beneath. Various silicon patterns including topographic lines, concentric rings, and square arrays were created at a micro-?and nanoscale after etching the silicon and subsequent removal of the patterned polymer masks. Alternatively, the arrays of sub-30?nm silicon nanowires were produced by a chemical etching of the silicon wafer which was covered with highly ordered polystyrene-block-polyvinylpyridine (PS-b-PVP) micellar films. In addition, silicon nanohole arrays were also generated by etching with hexagonally packed silver nanoparticles that were prepared using PS-b-PVP block copolymer templates.     

Two-dimensional chemically tunable patterns with cellular structures fabricated via thermal pressing method

A novel and versatile soft lithography method, i.e. thermal pressing method has been established to create colloid arrays by using multilevel inks. Patterned poly(dimethylsiloxane) stamp containing silicone dioxide microparticles was pressed into a polycaprolactone (PCL) film at the temperature around the T_m of PCL. Subsequent removal of the colloids left cavity arrays. By initially incorporating chitosan, albumin or CdTe quantum dots into the silicone dioxide microparticles, removal of the ordered SiO₂ microspheres would then release these substances which were stably embedded into the PCL matrices or suspended in the interiors of the cellular structures. By coating the SiO₂ microspheres with multilayers previously, thin covers on the cellular structures could be obtained after removal of the templates.     

Fabrication of moth eye structures via charged nanoparticle lithography with size and density control

We have developed and demonstrated an effective method for fabricating the moth eye structures formed by charged nanoparticle lithography (cNPL). We attached high-density gold nanoparticles (Au NPs) to GaN without aggregation by making the surface of Au NPs negatively charged and GaN positively charged. Au NPs were effectively used as an etching mask, and the moth eye structures of GaN were formed by inductively coupled plasma reactive ion etching. The moth eye structures with various sizes and densities were fabricated by the cNPL, and the moth eye structures reduced the reflectance. The cNPL, as a simple technique with size and density control, will provide high potential to optical device applications.     

Nanoimprinting lithography of a two-layer phase mask for three-dimensional photonic structure holographic fabrications via single exposure

We report a combined holographic and nanoimprinting lithography technique to produce three-dimensional woodpile photonic crystal templates through only one single exposure. The interference lithography process uses an integratable diffractive optical element for large throughout 3D pattern manufacturing. The diffractive optical element consists of two layers of phase grating separated by an intermediate layer, fabricated by repeated nanoimprinting lithography, followed by an SU8 photoresist bonding technique. Grating periods, relative orientation, diffraction angle, and efficiency, as well as layer to layer phase delay, are well designed during manufacturing. By thermally optimizing the thickness of the intermediate layer, this paper demonstrates the fabrication of interconnected 3D photonic structures with arbitrary symmetry through a single laser exposure. The two-layer phase mask approach enables a CMOS-compatible monolithic integration of 3D photonic structures with other integrated optical elements and waveguides.     

Generation of tunable chain of three-dimensional optical bottle beams via focused multi-ring hollow Gaussian beam

We report here the generation of a chain of three-dimensional (3-D) optical bottle beams by focusing a π-phase shifted multi-ring hollow Gaussian beam (HGB) using a lens with spherical aberration. The rings of the HGB of suitable radial (k(r)) and axial (k(z)) wave vectors are generated using a double-negative axicon chemically etched in the optical fiber tips. Moving the lens position with respect to the fiber tip results in variation of the semi-angle of the cones of wave vectors of the HGBs and their diameter, using which we demonstrate tunability in the size and the periodicity of the 3-D optical bottle beams over a wide range, from micrometers to millimeters. The propagation characteristics of the beams resulting from focusing of single- and multi-ring HGBs and resulting in a quasi-non-diffracting beam and a chain of 3-D optical bottle beams, respectively, are simulated using only the input beam parameters and are found to agree well with experimental results.     

YIG/SRR结构的磁可调太赫兹左手材料

利用钇铁石榴石(YIG)在外加磁场作用下,等效磁导率为负值和电谐振环结构可以实现负介电常数的性质,组合设计出了一种新型磁可调左手材料.通过理论分析与计算表明,该结构在太赫兹频段范围内实现了左手通带,且左手通带可以随着外加磁场的增大而产生蓝移,达到了磁可调的目的.该结构具有通带性能良好、损耗低、制作方便等优点,在未来太赫兹波功能器件设计中具有广泛的应用前景.     

Exploring network structures via blockmodeling of the shortest path length matrix

Among the various methods of network analyses, blockmodeling is one of the key approaches to analyzing network structures. It can be applied not only to position analysis of the individual nodes, but also to cohesive subgroup discovery of the whole network. In this paper, we explore network structures via blockmodeling of the shortest path length matrix. First of all, we dichotomize the shortest path length matrix with different thresholds to generate reachability matrices within different steps. Then, through blockmodeling of the reachability matrix within n steps, we can analyze the network based on the cohesive subgroup, n-clique. Furthermore, through blockmodeling of the maximum reachability matrix, we can fit the network properly into the bowtie structure.     

Fabrication of GaN nanorods by inductively coupled plasma etching via SiO2 nanosphere lithography

In this study, we present a facile route to fabricate GaN nanorods by employing the nanosphere lithography (NSL) technique. Compared to previous approaches, it was demonstrated that arrays of silica (SiO₂) nanospheres can be effectively used as etching masks for the inductively coupled plasma etching process. By adjusting the etching conditions between SiO₂ nanospheres and GaN substrates, well-defined nanorods, which were as long as a few microns with controllable diameters, were successfully fabricated. This method is much simpler than any other technique currently being used, and can be generally applied to fabricate various types of nanorods.     

Tuning of localized surface plasmon resonance of well-ordered Ag/Au bimetallic nanodot arrays by laser interference lithography and thermal annealing

A novel hybrid approach to fabricate large-area well-ordered Ag/Au bimetallic nanodot arrays and its potential applications for biosensing is investigated. With the combination of laser interference lithography and the thermal annealing technique, Ag/Au bimetallic nanodots about ~50 nm are formed inside periodic nanodisk arrays at a dimension of ~530 nm on quartz substrates. Extinction spectra of the fabricated nanostructures show their localized surface plasmon resonance (LSPR) can be well controlled by Au concentration, which offers a means to flexibly tune the optical properties of the nanodot arrays. To study the sensitivity of the nanodot arrays, resonance wavelength changes per refractive index unit (RIU) are performed in different surrounding environments. This shows a 94% increase in peak shift per refractive index unit (nanometers/RIU) compared to the nanodot arrays formed only by thermal annealing. These results demonstrate a feasible approach to improve LSPR-based biosensor performance.     

Preparation of tricalcium phosphate/calcium pyrophosphate structures via rapid prototyping

Custom made tricalcium phosphate/calcium pyrophosphate bone substitutes with a well-defined architecture were fabricated in this study using 3D powder printing with tricalcium phosphate (TCP) powder and a liquid phase of phosphoric acid. The primary formed matrix of dicalcium phosphate dihydrate (DCPD, brushite) was converted in a second step to calcium pyrophosphate (CPP) by heat treatment in the temperature range 1,100–1,300°C. The structures exhibited compressive strengths between 0.8 MPa and 4 MPa after sintering at 1,100–1,250°C, higher strengths were obtained by increasing the amount of pyrophosphate formed in the matrix due to a post-hardening regime prior sintering as well as by the formation of a glass phase from TCP and calcium pyrophosphate above 1,280°C, which resulted in a strong densification of the samples and compressive strength of >40 MPa.     

Paraffin-based shape-stable phase change materials with graphene/carbon nanotube three-dimensional network structure

A novel kind of paraffin-based shape-stable phase change materials (SSPCMs) was prepared by introducing paraffin into reduced graphene oxide (rGO)/carbon nanotubes (CNTs) aerogel via vacuum-assisted impregnation method. The effects of ratio of rGO to CNTs in 3D network structure on morphology, structure and property of paraffin-based SSPCMs were investigated. The rGO/CNTs 3D network structure with high thermal conductivity, served as thermally conductive skeleton together. In particular, CNTs was used as a secondary heat conductive filler, which could be well dispered in the SSPCMs to conduct heat synergistically. The SSPCMs exhibited high thermal conductivity and excellent shape-stability. And the thermal conductivity of SSPCMs can be regulated by adjusting the ratio of rGO to CNTs in aerogels. These results indicate that 3D rGO/CNTs aerogels have advantages as thermally conductive skeleton, and can endow phase transition materials with stable shape, so as to realize the application of phase change materials in the field of heat dissipation.     

Hybrid finite-element/rigorous coupled wave analysis for scattering from three-dimensional doubly periodic structures

A new hybrid finite-element/rigorous coupled wave analysis formulation is presented for the modeling of electromagnetic wave interactions with doubly periodic structures. The structures under investigation are periodic in two dimensions and have a finite extent in the third dimension. The proposed model can handle structures that have material properties varying arbitrarily in any of the dimensions within the unit cell. Employment of Fourier series expansion and Floquet's theory in one of the periodic dimensions helps to reduce the dimension of the mesh. Results obtained from alternative methods are used to verify the proposed method's validity.