Surveying the Directional Transport of Water Droplets upon Impact on Metallic Topographic Gradients.

Drop impact phenomena on raw, polished, and topography-altered gradient surfaces are investigated and presented. The main aim of this study is to demonstrate that in using a one-step industrial patterning process, it is possible to obtain metal topographical wetting gradients that can produce various desired outcomes after droplet impact. The findings could be applied to improving wind or steam turbine blades. The ranges of Weber (We) and Reynolds (Re) numbers in the study are 3-300 and 650-6500, respectively. It is demonstrated that for a fixed We, the droplet transport outcomes change from bouncing-off to side-flipping to deposition depending on the impact location and the gradient strength. The effect of We in combination with the gradient strength was also considered to demonstrate droplet behavior similar to that observed on a uniform water repellent surface and on biphilic systems. In addition, full bouncing-off and directional control have been demonstrated. For the condition We = 95 ± 3, it was possible to achieve a maximum droplet recoil height of ∼6 mm and a side motion of almost 8 mm. A combination of different outcomes (e.g., splashing on one side of a droplet and passive horizontal translation on another) was observed on the studied gradients at We > 200 due to different wetting regimes across the droplet's three-phase line.

Development of a Coating-Less Aluminum Superhydrophobic Gradient for Spontaneous Water Droplet Motion Using One-Step Laser-Ablation.

Nature shows various approaches to create superhydrophobicity, such as the lotus leaf, where the superhydrophobic (SHPB) surface arising from its hierarchical surface consists of random microscale bumps with superimposed nanoscale hairs. Some natural systems, such as the hydrophilic silk of some spider's webs, even allow the passive transport of water droplets from one part of a surface to another by creating gradients in surface tension and Laplace pressure. We look to combine both ideas and replicate the superb water repellence of the lotus leaf and the surface tension gradient-driven motion of the spider silk to form an all-metal, coating-less surface that promotes spontaneous droplet motion. We present the design, fabrication, and investigation of such superhydrophobic gradient surfaces on aluminum, which are aimed at spontaneous water droplet movement for improved surface water management. One surface demonstrates a droplet travel distance of almost 2 mm for a 11 µL droplet volume. We also present surfaces that map the theoretical ranges of the surface tension gradient surfaces tested here.

Study of Micro- and Nanopatterned Aluminum Surfaces Using Different Microfabrication Processes for Water Management.

Superhydrophobic surfaces demonstrate extreme water-repellence, promoting drop-wise over film-wise condensation, increasing liquid mobility, and reducing thermal resistance for heat-exchanger applications. Introducing topographic structures can lead to modified surface free energy, as inspired by natural systems like the lotus leaf, potentially allowing coating-free ice- and frost-free surfaces under certain conditions. This work presents a study of coating-free aluminum micro/nanopatterns fabricated using micromilling or laser-etching techniques and the resultant wetting properties. Our review and experiments clarify the roles of line-edge-roughness and microstructural geometry from each microfabrication technique, which manifests in technique-specific nano- to midmicro-scale roughness, producing a hierarchical structure in both cases. For micromilling, line-edge-roughness consists of jagged burrs of 1-8 µm thickness with 10-25 µm periodicity along the microlines with constantly changing height on the order of 1-20 µm. These effects simultaneously raise the water contact angle from 52° (unprocessed aluminum) up to 136° but with strong edge pinning effects. On the other hand, laser-etched surfaces exhibit line-edge-roughness with a microstructure of 3-20 µm width and 5-10 µm in height superimposed with evenly spread spikes of 50-250 nm. This results in a high contact angle (>150°) coupled with a low contact angle hysteresis (<15°), promoting superhydrophobicity on a coating-free aluminum surface. It is also shown that for certain cases, line-edge-roughness is more important for the resultant wetting properties than the structure geometry.

Survey of Micro/Nanofabricated Chemical, Topographical, and Compound Passive Wetting Gradient Surfaces.

Surface wetting gradients are desirable due to their ability to passively transport liquid droplets without the aid of gravity. Such surfaces can be prepared through topographical or chemical methods or a compound approach involving both methods. By altering the surface free energy across a surface, a droplet that contacts such a surface will experience an actuation force toward the hydrophilic region. Such transport properties make these surfaces attractive for a range of applications from thermal management to microfluidics to the investigation of biomolecular interactions. This paper reviews passive wetting gradients that have been demonstrated over the last three decades, focusing on the types of surfaces that have been developed to date along with the materials that have been used. The corresponding wetting ranges and physical lengths over which droplet mobility has been achieved on these various types of gradient surfaces are compared to guide future developments.

Bandgap control in two-dimensional semiconductors via coherent doping of plasmonic hot electrons.

Bandgap control is of central importance for semiconductor technologies. The traditional means of control is to dope the lattice chemically, electrically or optically with charge carriers. Here, we demonstrate a widely tunable bandgap (renormalisation up to 550 meV at room-temperature) in two-dimensional (2D) semiconductors by coherently doping the lattice with plasmonic hot electrons. In particular, we integrate tungsten-disulfide (WS2) monolayers into a self-assembled plasmonic crystal, which enables coherent coupling between semiconductor excitons and plasmon resonances. Accompanying this process, the plasmon-induced hot electrons can repeatedly fill the WS2 conduction band, leading to population inversion and a significant reconstruction in band structures and exciton relaxations. Our findings provide an effective measure to engineer optical responses of 2D semiconductors, allowing flexibilities in design and optimisation of photonic and optoelectronic devices.

Uncoupling Molecular Testing for SARS-CoV-2 From International Supply Chains.

The rapid global rise of COVID-19 from late 2019 caught major manufacturers of RT-qPCR reagents by surprise and threw into sharp focus the heavy reliance of molecular diagnostic providers on a handful of reagent suppliers. In addition, lockdown and transport bans, necessarily imposed to contain disease spread, put pressure on global supply lines with freight volumes severely restricted. These issues were acutely felt in New Zealand, an island nation located at the end of most supply lines. This led New Zealand scientists to pose the hypothetical question: in a doomsday scenario where access to COVID-19 RT-qPCR reagents became unavailable, would New Zealand possess the expertise and infrastructure to make its own reagents onshore? In this work we describe a review of New Zealand's COVID-19 test requirements, bring together local experts and resources to make all reagents for the RT-qPCR process, and create a COVID-19 diagnostic assay referred to as HomeBrew (HB) RT-qPCR from onshore synthesized components. This one-step RT-qPCR assay was evaluated using clinical samples and shown to be comparable to a commercial COVID-19 assay. Through this work we show New Zealand has both the expertise and, with sufficient lead time and forward planning, infrastructure capacity to meet reagent supply challenges if they were ever to emerge.

Revealing Strong Plasmon-Exciton Coupling between Nanogap Resonators and Two-Dimensional Semiconductors at Ambient Conditions.

Strong coupling of two-dimensional semiconductor excitons with plasmonic resonators enables control of light-matter interaction at the subwavelength scale. Here we develop such strong coupling in plasmonic nanogap resonators, which allows modification of exciton strength by altering electromagnetic environments in nearby semiconductor monolayers. Using this system, we not only demonstrate a large vacuum Rabi splitting up to 163 meV and splitting features in photoluminescence spectra but also reveal that the effective exciton number contributing to the coupling can be reduced down to the single-digit level (N<10), which is 2 orders lower than that of previous systems, close to single-exciton based strong coupling. In addition, we prove that the strong coupling process is not affected by the large exciton coherence size that was previously believed to be detrimental to the formation of plasmon-exciton interaction. We provide a deeper understanding of strong coupling in two-dimensional semiconductors, paving the way for room-temperature quantum optics applications.

Tunable Valley Polarized Plasmon-Exciton Polaritons in Two-Dimensional Semiconductors.

Monolayers of transition-metal dicalcogenides have emerged as two-dimensional semiconductors with direct bandgaps at degenerate but inequivalent electronic "valleys", supporting distinct excitons that can be selectively excited by polarized light. These valley-addressable excitons, when strongly coupled with optical resonances, lead to the formation of half-light half-matter quasiparticles, known as polaritons. Here we report self-assembled plasmonic crystals that support tungsten disulfide monolayers, in which the strong coupling of semiconductor excitons and plasmon lattice modes results in a Rabi splitting of ∼160 meV in transmission spectra as well as valley-polarized photoluminescence at room temperature. More importantly we find that one can flexibly tune the degree of valley polarization by changing either the emission angle or the excitation angle of the pump beam. Our results provide a platform that allows the detection, control, and processing of optical spin and valley information at the nanoscale under ambient conditions.

Herpin effective media resonant underlayers and resonant overlayer designs for ultra-high NA interference lithography.

Dielectric waveguide resonant underlayers are employed in ultra-high NA interference photolithography to effectively double the depth of field. Generally a single high refractive index waveguiding layer is employed. Here multilayer Herpin effective medium methods are explored to develop equivalent multilayer waveguiding layers. Herpin equivalent resonant underlayers are shown to be suitable replacements provided at least one layer within the Herpin trilayer supports propagating fields. In addition, a method of increasing the intensity incident upon the photoresist using resonant overlayers is also developed. This method is shown to greatly enhance the intensity within the photoresist making the use of thicker, safer, non-absorbing, low refractive index matching liquids potentially suitable for large-scale applications.

Mode Modification of Plasmonic Gap Resonances Induced by Strong Coupling with Molecular Excitons.

Plasmonic cavities can be used to control the atom-photon coupling process at the nanoscale, since they provide an ultrahigh density of optical states in an exceptionally small mode volume. Here we demonstrate strong coupling between molecular excitons and plasmonic resonances (so-called plexcitonic coupling) in a film-coupled nanocube cavity, which can induce profound and significant spectral and spatial modifications to the plasmonic gap modes. Within the spectral span of a single gap mode in the nanocube-film cavity with a 3 nm wide gap, the introduction of narrow-band J-aggregate dye molecules not only enables an anticrossing behavior in the spectral response but also splits the single spatial mode into two distinct modes that are easily identified by their far-field scattering profiles. Simulation results confirm the experimental findings, and the sensitivity of the plexcitonic coupling is explored using digital control of the gap spacing. Our work opens up a new perspective to study the strong coupling process, greatly extending the functionality of nanophotonic systems, with the potential to be applied in cavity quantum electrodynamic systems.

Manipulating light absorption in dye-doped dielectric films on reflecting surfaces.

We experimentally and numerically developed a tunable absorbing nanoscale thin-film system, comprising of dye molecules doped dielectric coatings on reflecting surfaces, the absorption behaviors of which can be flexibly tuned by adjusting the system parameters, i.e. the coating thickness and the doping concentration of dye molecules. Specifically, with appropriate system parameters, our absorbing thin-film system exhibits very directional and polarization dependent absorption properties, which can be significantly altered if applied with different parameters. Calculations demonstrate the unique absorption behaviors are a result of coupling between molecular absorption and Fabry-Perot resonances in the thin-film cavity. In addition, we theoretically show that both the spectral and directional range of the absorption in the thin-film system can be intentionally regulated by doping dyes with different absorption band and setting proper excitation conditions of Fabry-Perot resonances.

Tunable SERS using gold nanoaggregates on an elastomeric substrate.

We report on the self-assembly of colloidal gold nanoparticles on a stretchable, elastomeric membrane, and the use of this membrane as a base substrate for far-field confocal Raman measurements. Surface-enhanced Raman scattering (SERS) enhancement for such a substrate was estimated as 10(6) to 10(7). Atomic force microscopy has been used to study the changes in nanoparticle topography when the membrane is stretched. The homogeneous strain defined by average relative motion of nanoparticles is approximately half the macroscopically-applied biaxial strain. The SERS intensity was maximized when the membrane was at rest (i.e. without stretch), and reduced as stretching was increased. Our measurements are consistent with theoretical and numerical SERS enhancements for the interstitial gap between two spheres. The data indicate that the resting gap between the spheres is 11 nm or 16 nm, using two theoretical models. This work represents progress towards particularly facile sample fabrication and in situ tuning techniques for SERS.

A detailed study of resonance-assisted evanescent interference lithography to create high aspect ratio, super-resolved structures.

Higher resolution demands for semiconductor lithography may be fulfilled by higher numerical aperture (NA) systems. However, NAs more than the photoresist refractive index (~1.7) cause surface confinement of the image. In this paper we describe how evanescent wave coupling to effective gain medium surface states beneath the imaging layer can counter this problem. We experimentally demonstrate this at λ = 405 nm using hafnium oxide on SiO2 to enhance the image depth of a 55-nm line and space pattern (numerical aperture of 1.824) from less than 40 nm to more than 90 nm. We provide a design example at λ = 193 nm, where a layer of sapphire on SiO2 counters image decay by an effective-gain-medium resonance phenomena allowing evanescent interferometric lithography to create high aspect ratio structures at NAs of 1.85 (26-nm resolution) and beyond.

Experimental characterization of the transfer function for a Silver-dielectric superlens.

We describe a technique for experimentally determining the spatial-frequency modulation transfer function for near-field super-resolution imaging systems and present such a modulation transfer function for a 20|40|20 nm poly(vinyl alcohol)~(PVA)|Silver|PVA superlens exposed to 365 nm wavelength (i-line) radiation through a 50-nm thick tungsten mask. An extensive spectral characterization is achieved from only two exposures, with transmission coefficients determined for spatial frequencies as high as 13 µm-1, corresponding to λ / 4.75. The resulting transfer function is in good agreement with analytical models that incorporate the effects of mask-superlens interactions.

Controlling the dimensions of amyloid fibrils: toward homogenous components for bionanotechnology.

Amyloid fibrils have been recognized as having potential in a variety of bionanotechnological applications. However, realization of these applications is constrained by a lack of control over morphology and alignment, both crucial for potential end uses. This article focuses on the use of growth and storage conditions to control the length of amyloid fibrils formed from bovine insulin, with length distributions constructed from transmission electron microscopy (TEM) images. Growth temperature, pH, protein concentration, and storage conditions were examined and were seen to offer a range of conditions that favor different length distribution. The use of amyloid fibrils as nanowires is one area where control of fibril dimensions is desirable, for experimental setup and endpoint applications. The conductive properties of fibrils formed from bovine insulin are presented, with these insulin fibrils being shown to have high resistivity in their unmodified state, with current values in the nanoamp range. These low current values can be increased via modification, or the fibrils used in their native state in applications where low current values are desirable. These findings, coupled with the ability to predict and select for various insulin amyloid fibril dimensions, enhances their utility as nanomaterials.

Performance enhancements to absorbance-modulation optical lithography. I. Plasmonic reflector layers.

The ability to improve the transmission and intensity profiles in absorbance-modulation optical lithography [J. Opt. Soc. Am. A 23, 2290-2294 (2006) and Phys. Rev. Lett. 98, 043905 (2007)] through the introduction of a plasmonic metal layer is investigated. In this part of the work, a plasmonic reflector layer (PRL) is placed beneath the photoresist layer. Improvement is expected due to surface plasmons being induced on the plasmonic layer and supporting the transmission of the image deeper into the imaging layer. The introduction of the plasmonic reflector improves the depth of focus markedly, with the image confinement extended up to 60 nm but with a penalty of up to a 50% increase in the minimum full width at half-maximum of the intensity profile. The presented work demonstrates that a PRL can be a valuable tool for near-field lithography.

Performance enhancements to absorbance-modulation optical lithography. II. Plasmonic superlenses.

The ability to improve the transmission and intensity profiles in absorbance-modulation optical lithography (AMOL) [J. Opt. Soc. Am. A 23, 2290 (2006) and Phys. Rev. Lett. 98, 043905 (2007)] through the introduction of a plasmonic metal layer is investigated. In this part of the work, a plasmonic layer is placed between the absorbance-modulation layer and the photoresist layer. Transmission through this layer is possible due to the ability of thin plasmonic layers to act as near-field analogues of negative refraction materials. The superlens performance is best with a thin layer of 10-20 nm, although this causes a full width at half-maximum increase of ~50%. The introduction of the plasmonic layers allows dichroic filtering of the two wavelengths, with a difference of a factor of 10 in the transmitted intensity ratio, reducing undesirable exposure of the resist. The presented work demonstrates that a plasmonic layer can be interfaced with an AMOL system, but that further optimization and material development are needed to allow substantial performance improvements.

Increased process latitude in absorbance-modulated lithography via a plasmonic reflector.

Absorbance-modulated lithography is a relatively new optical patterning method where a thin layer of photochromic molecules is placed between the far-field optics and photoresist. These molecules can be made transparent or opaque by illuminating with wavelengths λ1 or λ2, respectively. By simultaneously illuminating this layer with patterns of both wavelengths it is possible to create an absorption mask capable of subwavelength resolution. This resolution comes at the price of limited contrast and depth-of-focus resulting in poor process latitude. Here it is shown that by using TM polarization for λ1 and integrating a plasmonic reflector process latitude is increased by up to 66%.

An improved transfer-matrix model for optical superlenses.

The use of transfer-matrix analyses for characterizing planar optical superlensing systems is studied here, and the simple model of the planar superlens as an isolated imaging element is shown to be defective in certain situations. These defects arise due to neglected interactions between the superlens and the spatially varying shadow masks that are normally used as scattering objects for imaging, and which are held in near-field proximity to the superlenses. An extended model is proposed that improves the accuracy of the transfer-matrix analysis, without adding significant complexity, by approximating the reflections from the shadow mask by those from a uniform metal layer. Results obtained using both forms of the transfer matrix model are compared to finite element models and two example superlenses, one with a silver monolayer and the other with three silver sublayers, are characterized. The modified transfer matrix model gives much better agreement in both cases.

Patterning, integration and characterisation of polymer optical oxygen sensors for microfluidic devices.

This paper describes a process for the layer-by-layer fabrication and integration of luminescent dye-based optical oxygen sensors into microfluidic devices. Application of oxygen-sensitive platinum(ii) octaethylporphyrin ketone fluorescent dye dissolved in polystyrene onto glass substrates by spin-coating was studied. Soft lithography with polydimethylsiloxane (PDMS) stamps and reactive ion etching in oxygen plasma were used to produce sensor patterns with a minimum feature size of 25 microm. Sensors patterns were integrated into a PDMS microfluidic device by plasma bonding. No degradation of the sensor response as a result of the lithography and pattern-transfer processes was detected. Gaseous and dissolved oxygen (DO) detection was characterised using fluorescence microscopy. The intensity signal ratio of the sensor films was found to increase almost two-fold from 3.6 to 6.8 by reducing film thickness from 1.3 microm to 0.6 microm. Calibration of DO measurement showed linear Stern-Volmer behaviour that was constant for flow rates from 0.5 to 2 mL min(-1). The calibrated sensors were subsequently used to demonstrate laterally resolved detection of oxygen inside a microfluidic channel. The fabrication process provides a novel, easy to use method for the repeatable integration of optical oxygen sensors into cell-culture and lab-on-a-chip devices.