Rotationally tunable varifocal 3D metalens.

Varifocal optics have a variety of applications in imaging systems. Metasurfaces offer control of the phase, transmission, and polarization of light using subwavelength engineered structures. However, conventional metasurface designs lack dynamic wavefront shaping which limits their application. In this work, we design and fabricate 3D doublet metalenses with a tunable focal length. The phase control of light is obtained through the mutual rotation of the singlet structures. Inspired by Moiré lenses, the proposed structure consists of two all-dielectric metasurfaces. The singlets have reverse-phase profiles resulting in the cancellation of the phase shift in the nominal position. In this design, we show that the mutual rotation of the elements produces different wavefronts with quadratic radial dependence. Thus, an input plane wave is converted to spherical wavefronts whose focal length depends on the rotation. We use a combination of a nanopillar and a phase plate as the unit cell structure working at a wavelength of 1500 nm. Our design holds promise for a range of applications such as zoom lenses, microscopy, and augmented reality.

Switchable X-Ray Orbital Angular Momentum from an Artificial Spin Ice.

Artificial spin ices (ASI) have been widely investigated as magnetic metamaterials with exotic properties governed by their geometries. In parallel, interest in x-ray photon orbital angular momentum (OAM) has been rapidly growing. Here we show that a square ASI with a patterned topological defect, a double edge dislocation, imparts OAM to scattered x rays. Unlike single dislocations, a double dislocation does not introduce magnetic frustration, and the ASI equilibrates to its antiferromagnetic (AFM) ground state. The topological charge of the defect differs with respect to the structural and magnetic order; thus, x-ray diffraction from the ASI produces photons with even and odd OAM quantum numbers at the structural and AFM Bragg conditions, respectively. The magnetic transitions of the ASI allow the AFM OAM beams to be switched on and off by modest variations of temperature and applied magnetic field. These results demonstrate ASIs can serve as metasurfaces for reconfigurable x-ray optics that could enable selective probes of electronic and magnetic properties.

Hybrid metasurfaces for simultaneous focusing and filtering.

This work presents the design and fabrication of polymeric, structural optical filters that simultaneously focus light. These filters represent a novel, to the best of our knowledge, design at the boundary between diffractive optics and metasurfaces that may provide significant advantages for both digital and hyperspectral imaging. Filters for visible and near-infrared wavelengths were designed using finite-difference time-domain (FDTD) simulations. Prototype filters were fabricated using two-photon lithography, a form of nanoscale 3D printing, and have geometries suitable to replication by molding. The experimentally measured spectral transmission and focused spot size of each filter show excellent agreement with simulation.

Limiting regimes for electron-beam induced deposition of copper from aqueous solutions containing surfactants.

Focused electron beam induced deposition of pure materials from aqueous solutions has been of interest in recent years. However, controlling the liquid film in partial vacuum is challenging. Here we modify the substrate to increase control over the liquid layer in order to conduct a parametric study of copper deposition in an environmental scanning electron microscope. We identified the transition from electron to mass-transport limited deposition as well as two additional regimes characterized by aggregated and high-aspect ratio deposits. We observe a high deposition efficiency of 1-10 copper atoms per primary electron that is consistent with a radiation chemical model of the deposition process.

Real-time dose control for electron-beam lithography.

Shot-to-shot, or pixel-to-pixel, dose variation during electron-beam lithography is a significant practical and fundamental problem. Dose variations associated with charging, electron source instability, optical system drift, and ultimately shot noise in the e-beam itself conspire to critical dimension variability, line width/edge roughness, and limited throughput. It would be an important improvement to e-beam based patterning technology if real-time feedback control of electron-dose were provided so that pattern quality and throughput would be improved beyond the shot noise limit. In this paper, we demonstrate control of e-beam dose based on the measurement of electron arrival at the sample where patterns are written, rather than from the source or another point in the electron optical column. Our results serve as the first steps towards real-time dose control and eventually overcoming the shot noise.

The radiation chemistry of focused electron-beam induced etching of copper in liquids.

Well-controlled, focused electron-beam induced etching of copper thin films has been successfully conducted on bulk substrates in an environmental scanning electron microscope by controlling liquid-film thickness with an in situ correlative interferometry system. Knowledge of the liquid-film thickness enables a hybrid Monte Carlo/continuum model of the radiation chemistry to accurately predict the copper etch rate using only electron scattering cross-sections, radical yields, and reaction rates from previous studies. Etch rates depended strongly on the thickness of the liquid film and simulations confirmed that this was a result of increased oxidizing radical generation. Etch rates also depended strongly, but non-linearly, on electron beam current, and simulations showed that this effect arises through the dose-rate dependence of reactions of radical species.

Altering the radiation chemistry of electron-beam lithography with a reactive gas: a study of Teflon AF patterning under water vapor.

Variable-pressure electron-beam lithography (VP-EBL) employs an ambient gas at subatmospheric pressure to reduce charging of insulating films and substrates during electron exposure. In this work, VP-EBL proves to be an efficient method for patterning a widely used, but challenging to process, fluoropolymer, Teflon AF. However, rather than solely mitigating charging, the ambient gas is found to alter the radiation chemistry of the exposure process. Specifically, irradiating Teflon AF under water vapor increases the dissolution rate of the exposed regions in non-fluorinated solvents and enables complete patterning in a positive tone process. When compared to conventional e-beam resists, the contrast (≈4), clearing dose (<700 µC cm-2), and resolution (≈40 nm half-pitch) of Teflon AF are adequate. However, these figures of merit are quite remarkable when the process is considered as a means for directly patterning a functional material with extremely low surface energy, dielectric constant, and refractive index. Intriguingly, VP-EBL of Teflon AF under water vapor also exhibits non-reciprocity, through dose-rate dependence, and exhibits anomalous proximity effects. Thus, the influence of the ambient gas on radiation chemistry must be considered for VP-EBL, and some of the resulting effects may offer significant benefits for patterning both functional and lithographic materials.

Rhodopseudomonas palustris-based conversion of organic acids to hydrogen using plasmonic nanoparticles and near-infrared light.

The simultaneous elimination of organic waste and the production of clean fuels will have an immense impact on both the society and the industrial manufacturing sector. The enhanced understanding of the interface between nanoparticles and photo-responsive bacteria will further advance the knowledge of their interactions with biological systems. Although literature shows the production of gases by photobacteria, herein, we demonstrated the integration of photonics, biology, and nanostructured plasmonic materials for hydrogen production with a lower greenhouse CO2 gas content at quantified light energy intensity and wavelength. Phototrophic purple non-sulfur bacteria were able to generate hydrogen as a byproduct of nitrogen fixation using the energy absorbed from visible and near-IR (NIR) light. This type of biological hydrogen production has suffered from low efficiency of converting light energy into hydrogen in part due to light sources that do not exploit the organisms' capacity for NIR absorption. We used NIR light sources and optically resonant gold-silica core-shell nanoparticles to increase the light utilization of the bacteria to convert waste organic acids such as acetic and maleic acids to hydrogen. The batch growth studies for the small cultures (40 mL) of Rhodopseudomonas palustris demonstrated >2.5-fold increase in hydrogen production when grown under an NIR source (167 ± 18 µmol H2) compared to that for a broad-band light source (60 ± 6 µmol H2) at equal light intensity (130 W m-2). The addition of the mPEG-coated optically resonant gold-silica core-shell nanoparticles in the solution further improved the hydrogen production from 167 ± 18 to 398 ± 108 µmol H2 at 130 W m-2. The average hydrogen production rate with the nanoparticles was 127 ± 35 µmol L-1 h-1 at 130 W m-2.

Focused electron beam induced deposition of copper with high resolution and purity from aqueous solutions.

Electron-beam induced deposition of high-purity copper nanostructures is desirable for nanoscale rapid prototyping, interconnection of chemically synthesized structures, and integrated circuit editing. However, metalorganic, gas-phase precursors for copper introduce high levels of carbon contamination. Here we demonstrate electron beam induced deposition of high-purity copper nanostructures from aqueous solutions of copper sulfate. The addition of sulfuric acid eliminates oxygen contamination from the deposit and produces a deposit with ∼95 at% copper. The addition of sodium dodecyl sulfate (SDS), Triton X-100, or polyethylene glycole (PEG) improves pattern resolution and controls deposit morphology but leads to slightly reduced purity. High resolution nested lines with a 100 nm pitch are obtained from CuSO4-H2SO4-SDS-H2O. Higher aspect ratios (∼1:1) with reduced line edge roughness and unintended deposition are obtained from CuSO4-H2SO4-PEG-H2O. Evidence for radiation-chemical deposition mechanisms was observed, including deposition efficiency as high as 1.4 primary electrons/Cu atom.

Relative Contribution of Lateral Packing Density to Albumin Adsorption on Monolayers.

The effect of functional group density on protein adsorption is systematically studied to support ongoing efforts in molecular imprinting of surfaces and bulk materials. In these applications, functional commodity chemicals are molded to complement the shape and chemistry of the target molecule. Here, we study the relationship between bovine serum albumin adsorption and ligand density for carboxylate, alcohol, and alkyl terminal groups. Control surfaces consisting of densely packed self-assembled monolayers (SAMs) are contrasted with low-density SAMs formed through thiol-yne chemistry. Direct comparison consistently yielded greater protein adsorption on low-density SAMs than conventional pure component SAMs of the same functional group. Critically, the carboxylate and alcohol low-density SAMS are more hydrophobic than their analogous dense SAMs. Mixed functional group, dense SAMs were formed with alkyl diluents to match the hydrophobicity of the low-density SAMs. Once hydrophobicity is matched, the dense carboxylate and alcohol SAMs have higher adsorption than the low-density SAMs. We conclude (1) surface charge and hydrophobicity trends dominate over surface density contributions; (2) when hydrophobicity is matched, greater adsorption occurs on dense hydrophilic groups than on lower density hydrophilic groups; (3) when hydrophobicity is matched, greater adsorption occurs on lower density hydrophobic groups than on higher density hydrophobic groups.

Focused electron beam induced etching of copper in sulfuric acid solutions.

We show here that copper can be locally etched by an electron-beam induced reaction in a liquid. Aqueous sulfuric acid (H2SO4) is utilized as the etchant and all experiments are conducted in an environmental scanning electron microscope. The extent of etch increases with liquid thickness and dose, and etch resolution improves with H2SO4 concentration. This approach shows the feasibility of liquid phase etching for material selectivity and has the potential for circuit editing.

Differentiating surface and bulk interactions using localized surface plasmon resonances of gold nanorods.

We demonstrate a novel localized surface-plasmon resonance sensor that can distinguish surface binding interactions from interfering bulk effects. This is accomplished by utilizing the longitudinal and transverse plasmon modes of gold nanorods. We have investigated, both numerically and experimentally, the effect of change in background refractive index and surface binding on the two resonances of a gold nanorod on an indium tin oxide coated glass substrate.

Effects of a silicon probe on gold nanoparticles on glass under evanescent illumination.

We have numerically investigated the influence of a nanoscale silicon tip in proximity to an illuminated gold nanoparticle. We describe how the position of the high-permittivity tip and the size of the nanoparticle impact the absorption, peak electric field and surface plasmon resonance wavelength under different illumination conditions. We detail the finite element method (FEM) approach we have used, whereby we specify a volume excitation field analytically and calculate the difference between this source field and the total field (i.e., scattered-field formulation). We show that a nanoscale tip can locally enhance the absorption of the particle as well as the peak electric field at length scales far smaller than the wavelength of the incident light.

Ceramic-based microelectrode arrays: recording surface characteristics and topographical analysis.

Amperometric measurements using microelectrode arrays (MEAs) provide spatially and temporally resolved measures of neuromolecules in the central nervous system of rats, mice and non-human primates. Multi-site MEAs can be mass fabricated on ceramic (Al2O3) substrate using photolithographic methods, imparting a high level of precision and reproducibility in a rigid but durable recording device. Although the functional capabilities of MEAs have been previously documented for both anesthetized and freely moving paradigms, the performance enabling intrinsic physical properties of the MEA device have not heretofore been presented. In these studies, spectral analysis confirmed that the MEA recording sites were primarily composed of elemental platinum (Pt°). In keeping with the precision of the photolithographic process, scanning electron microscopy revealed that the Pt recording sites have unique microwell geometries post-fabrication. Atomic force microscopy demonstrated that the recording surfaces have nanoscale irregularities in the form of elevations and depressions, which contribute to increased current per unit area that exceeds previously reported microelectrode designs. The ceramic substrate on the back face of the MEA was characterized by low nanoscale texture and the ceramic sides consisted of an extended network of ridges and cavities. Thus, individual recording sites have a unique Pt° composition and surface profile that has not been previously observed for Pt-based microelectrodes. These features likely impact the physical chemistry of the device, which may influence adhesion of biological molecules and tissue as well as electrochemical recording performance post-implantation. This study is a necessary step towards understanding and extending the performance abilities of MEAs in vivo.

Substrate effects on the electron-beam-induced deposition of platinum from a liquid precursor.

Focused electron-beam-induced deposition using bulk liquid precursors (LP-EBID) is a new nanofabrication technique developed in the last two years as an alternative to conventional EBID, which utilizes cumbersome gaseous precursors. Furthermore, LP-EBID using dilute aqueous precursors has been demonstrated to yield platinum (Pt) nanostructures with as-deposited metal content that is substantially higher than the purity achieved by EBID with currently available gaseous precursors. This advantage of LP-EBID--along with the ease of use, low cost, and relative innocuousness of the liquid precursors--holds promise for its practical applicability in areas such as rapid device prototyping and lithographic mask repair. One of the feasibility benchmarks for the LP-EBID method is the ability to deposit high-fidelity nanostructures on various substrate materials. In this study, we report the first observations of performing LP-EBID on bare and metal-coated silicon-nitride membranes, and compare the resulting Pt deposits to those obtained by LP-EBID on polyimide membranes in terms of nucleation, morphology, size dependence on electron dose, and purity.

Electron-beam-induced deposition of platinum from a liquid precursor.

We demonstrate here the first focused electron-beam-induced deposition (EBID) of nanostructures using a liquid precursor. We have deposited sub-50 nm platinum (Pt) wires and dots from a dilute, aqueous solution of chloroplatinic acid. Existing EBID processes rely on the electron-beam stimulated decomposition of gaseous precursors; as a result, the deposits are highly contaminated (up to 75 at. % carbon or 60 at. % phosphorus for Pt processes). In contrast, we show that deposition of platinum by electron-beam reduction of platinum ions from solution leads to high-purity deposits (approximately 10 at. % chlorine contamination) at rates at least ten times higher than those obtained with other platinum precursors. Liquid-phase EBID offers a new route to deterministic, three-dimensional, nanometer-scale structures composed of multiple materials without complex multistep processing. Thus, it may prove important for prototyping and low-volume production of nanoscale devices and for repair and modification of nanoscale masks and templates used in high-volume production.