Structural changes across thermodynamic maxima in supercooled liquid tellurium: A water-like scenario.

Liquid polymorphism is an intriguing phenomenon that has been found in a few single-component systems, the most famous being water. By supercooling liquid Te to more than 130 K below its melting point and performing simultaneous small-angle and wide-angle X-ray scattering measurements, we observe clear maxima in its thermodynamic response functions around 615 K, suggesting the possible existence of liquid polymorphism. A close look at the underlying structural evolution shows the development of intermediate-range order upon cooling, most strongly around the thermodynamic maxima, which we attribute to bond-orientational ordering. The striking similarities between our results and those of water, despite the lack of hydrogen-bonding and tetrahedrality in Te, indicate that water-like anomalies may be a general phenomenon among liquid systems with competing bond- and density-ordering.

Universal Two-Component Dynamics in Supercritical Fluids.

Despite the technological importance of supercritical fluids, controversy remains about the details of their microscopic dynamics. In this work, we study four supercritical fluid systems─water, Si, Te, and Lennard-Jones fluid─via classical molecular dynamics simulations. A universal two-component behavior is observed in the intermolecular dynamics of these systems, and the changing ratio between the two components leads to a crossover from liquidlike to gaslike dynamics, most rapidly around the Widom line. We find evidence to connect the liquidlike component dominating at lower temperatures with intermolecular bonding and the component prominent at higher temperatures with free-particle, gaslike dynamics. The ratio between the components can be used to describe important properties of the fluid, such as its self-diffusion coefficient, in the transition region. Our results provide an insight into the fundamental mechanism controlling the dynamics of supercritical fluids and highlight the role of spatiotemporally inhomogeneous dynamics even in thermodynamic states where no large-scale fluctuations exist in the fluid.

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.

High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the European X-Ray Free-Electron Laser.

We introduce a setup to measure high-resolution inelastic x-ray scattering at the High Energy Density scientific instrument at the European X-Ray Free-Electron Laser (XFEL). The setup uses the Si (533) reflection in a channel-cut monochromator and three spherical diced analyzer crystals in near-backscattering geometry to reach a high spectral resolution. An energy resolution of 44 meV is demonstrated for the experimental setup, close to the theoretically achievable minimum resolution. The analyzer crystals and detector are mounted on a curved-rail system, allowing quick and reliable changes in scattering angle without breaking vacuum. The entire setup is designed for operation at 10 Hz, the same repetition rate as the high-power lasers available at the instrument and the fundamental repetition rate of the European XFEL. Among other measurements, it is envisioned that this setup will allow studies of the dynamics of highly transient laser generated states of matter.

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.

An approach for the measurement of the bulk temperature of single crystal diamond using an X-ray free electron laser.

We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within [Formula: see text] for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.

A hybrid achromatic metalens.

Metalenses, ultra-thin optical elements that focus light using subwavelength structures, have been the subject of a number of recent investigations. Compared to their refractive counterparts, metalenses offer reduced size and weight, and new functionality such as polarization control. However, metalenses that correct chromatic aberration also suffer from markedly reduced focusing efficiency. Here we introduce a Hybrid Achromatic Metalens (HAML) that overcomes this trade-off and offers improved focusing efficiency over a broad wavelength range from 1000-1800 nm. HAMLs can be designed by combining recursive ray-tracing and simulated phase libraries rather than computationally intensive global search algorithms. Moreover, HAMLs can be fabricated in low-refractive index materials using multi-photon lithography for customization or using molding for mass production. HAMLs demonstrated diffraction limited performance for numerical apertures of 0.27, 0.11, and 0.06, with average focusing efficiencies greater than 60% and maximum efficiencies up to 80%. A more complex design, the air-spaced HAML, introduces a gap between elements to enable even larger diameters and numerical apertures.

Nanoscale focused electron beam induced etching of nickel using a liquid reactant.

Nickel nanostructures have found widespread application as both functional components, e.g. in magnetic systems, and as part of the lithographic pattern transfer process as etch masks, EUV mask absorbers, and imprint templates. Electron-beam induced etching of nickel is highly desirable for the repair and editing of masks and templates with high resolution and without substrate damage. However, there are no known gas-phase reactants that produce volatile nickel products under e-beam irradiation. Here we report the successful local etching of nickel by a focused electron beam in an environmental scanning electron microscope using a liquid reactant, aqueous sulfuric acid. Sulfuric acid did not spontaneously etch nickel under ESEM conditions, but nickel was etched in areas exposed to the electron beam. Etching parameters such as dose, refresh time, and addition of a surfactant were investigated. The extent of the etch increases with dose before terminating at sub-micron feature sizes. The etch resolution improves with the addition of surfactant. This approach enables local nickel patterning with complete film removal but without damaging underlying layers. With further refinement, the process may enable nickel absorber repair and editing and remove a significant obstacle to the use of nickel in EUV lithography.

Two-Component Dynamics and the Liquidlike to Gaslike Crossover in Supercritical Water.

Molecular-scale dynamics in sub- to supercritical water is studied with inelastic x-ray scattering and molecular dynamics simulations. The obtained longitudinal current correlation spectra can be decomposed into two main components: a low-frequency (LF), gaslike component and a high-frequency (HF) component arising from the O-O stretching mode between hydrogen-bonded molecules, reminiscent of the longitudinal acoustic mode in ambient water. With increasing temperature, the hydrogen-bond network diminishes and the spectral weight shifts from HF to LF, leading to a transition from liquid- to gaslike dynamics with rapid changes around the Widom line.

Spontaneous Magnetic Superdomain Wall Fluctuations in an Artificial Antiferromagnet.

Collective dynamics often play an important role in determining the stability of ground states for both naturally occurring materials and metamaterials. We studied the temperature dependent dynamics of antiferromagnetically ordered superdomains in a square artificial spin lattice using soft x-ray photon correlation spectroscopy. We observed an exponential slowing down of superdomain wall motion below the antiferromagnetic onset temperature, similar to the behavior of typical bulk antiferromagnets. Using a continuous time random walk model we show that these superdomain walls undergo low-temperature ballistic and high-temperature diffusive motions.

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.

Intra- and Interspecific Prey Theft in Cicada Killers (Hymenoptera: Apoidea: Sphecius).

We studied prey theft in two cicada killer aggregations: Ruby, Arizona (Sphecius convallis Patton) and Easton, Pennsylvania (Sphecius speciosus Drury). Many prey (Tibicen parallelus Davis [Hemiptera: Cicadidae]) were stolen from S. convallis by kingbirds and Greater Roadrunners at Ruby. Seventy percent of kingbird attacks on provisioning wasps were successful. Using sand-filled trap nests baited with a cicada, we tested the hypothesis that conspecific females might kleptoparasitize by laying an egg on the cicada and closing the nest cell. At Ruby, 45% were so appropriated, and at Easton, 52%. Easton data showed that the longer a nest cell was left open, the higher the rate of kleptoparasitism. Hence, intraspecific kleptoparasitism likely occurs at high rates in both populations. Not needing to dig a burrow, or to hunt, capture, and carry a paralyzed cicada favors intraspecific kleptoparasitism in cicada killers. Low cicada availability and intense avian kleptoparasitism of cicada killers may intensify selection pressure for this behavior at the Arizona site. Pirating cicadas may be the only viable reproductive outlet for females that are small or in environments with few prey. We suggest that provisioned nest cell kleptoparasitism may have evolved in cicada killers as an alternative strategy to standard provisioning, given the dual uncertainties of adult body size and prey availability.

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.

Setup for meV-resolution inelastic X-ray scattering measurements and X-ray diffraction at the Matter in Extreme Conditions endstation at the Linac Coherent Light Source.

We describe a setup for performing inelastic X-ray scattering and X-ray diffraction measurements at the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source. This technique is capable of performing high-, meV-resolution measurements of dynamic ion features in both crystalline and non-crystalline materials. A four-bounce silicon (533) monochromator was used in conjunction with three silicon (533) diced crystal analyzers to provide an energy resolution of ∼50 meV over a range of ∼500 meV in single shot measurements. In addition to the instrument resolution function, we demonstrate the measurement of longitudinal acoustic phonon modes in polycrystalline diamond. Furthermore, this setup may be combined with the high intensity laser drivers available at MEC to create warm dense matter and subsequently measure ion acoustic modes.