Spatial Density Profiles Research Articles

Mapping Nanoscale Hotspots with Single-Molecule Emitters Assembled into Plasmonic Nanocavities Using DNA Origami.

Fabricating nanocavities in which optically active single quantum emitters are precisely positioned is crucial for building nanophotonic devices. Here we show that self-assembly based on robust DNA-origami constructs can precisely position single molecules laterally within sub-5 nm gaps between plasmonic substrates that support intense optical confinement. By placing single-molecules at the center of a nanocavity, we show modification of the plasmon cavity resonance before and after bleaching the chromophore and obtain enhancements of ≥4 × 103 with high quantum yield (≥50%). By varying the lateral position of the molecule in the gap, we directly map the spatial profile of the local density of optical states with a resolution of ±1.5 nm. Our approach introduces a straightforward noninvasive way to measure and quantify confined optical modes on the nanoscale.

High contrast periodic plasma pattern formation during the laser-induced breakdown in transparent dielectric

Based on a simple 1D initial-time model, we have carried out the numerical simulation for the spatio-temporal evolution of femtosecond laser pulse induced breakdown in transparent dielectric (fused silica) at the nonlinear stage of the plasma resonance ionization instability. The instability develops from very small seed perturbations of the medium permittivity and results in, because of the strong mutual enhancement of the electric field and plasma density perturbations in the plasma resonance region, the formation of the subwavelength periodic plasma-field structure consisting of the overcritical plasma layers perpendicular to the laser polarization. The calculation of the time-course and spatial profiles of the plasma density, field amplitude, and energy deposition density in the medium during one breakdown pulse has allowed us to establish the main possible scenarios of the process considered and to found the laser intensity range where this process can underlie the nanograting modification of the medium by repeated pulses.

Effects of direct current discharge on the spatial distribution of cylindrical inductively-coupled plasma at different gas pressures

Stable operations of single direct current (DC) discharge, single radio frequency (RF) discharge and DC + RF hybrid discharge are achieved in a specially-designed DC enhanced inductively-coupled plasma (DCE-ICP) source. Their plasma characteristics, such as electron density, electron temperature and the electron density spatial distribution profiles are investigated and compared experimentally at different gas pressures. It is found that under the condition of single RF discharge, the electron density distribution profiles show a ‘convex’ shape and ‘saddle’ shape at gas pressures of 3 mTorr and 150 mTorr respectively. This result can be attributed to the transition of electron kinetics from nonlocal to local kinetics with an increase in gas pressure. Moreover, in the operation of DC + RF hybrid discharge at different gas pressures, the DC discharge has different effects on plasma uniformity. The plasma uniformity can be improved by modulating DC power at a high pressure of 150 mTorr where local electron kinetics is dominant, whereas plasma uniformity deteriorates at a low pressure of 3 mTorr where nonlocal electron kinetics prevails. This phenomenon, as analyzed, is due to the obvious nonlinear enhancement effect of electron density at the chamber center, and the inherent radial distribution difference in the electron density with single RF discharge at different gas pressures.

Enhanced control of the ionization rate in radio-frequency plasmas with structured electrodes via tailored voltage waveforms

Radio-frequency capacitively coupled plasmas that incorporate structured electrodes enable increases in the electron density within spatially localized regions through the hollow cathode effect (HCE). This enables enhanced control over the spatial profile of the plasma density, which is useful for several applications including materials processing, lighting and spacecraft propulsion. However, asymmetries in the powered and grounded electrode areas inherent to the hollow cathode geometry lead to the formation of a time averaged dc self-bias voltage at the powered electrode. This bias alters the energy and flux of secondary electrons leaving the surface of the cathode and consequentially can moderate the increased localized ionization afforded by the hollow cathode discharge. In this work, two-dimensional fluid-kinetic simulations are used to demonstrate control of the dc self-bias voltage in a dual-frequency driven (13.56, 27.12 MHz), hollow cathode enhanced, capacitively coupled argon plasma over the 66.6–200 Pa (0.5–1.5 Torr) pressure range. By varying the phase offset of the 27.12 MHz voltage waveform, the dc self-bias voltage varies by 10%–15% over an applied peak-to-peak voltage range of 600–1000 V, with lower voltages showing higher modulation. Resulting ionization rates due to secondary electrons within the hollow cathode cavity vary by a factor of 3 at constant voltage amplitude, demonstrating the ability to control plasma properties relevant for maintaining and enhancing the HCE.

PIC/MCC analysis of a photoresonance plasma sustained in a sodium vapor

A parallel 1d3v Particle in Cell/Monte Carlo Collision (PIC/MCC) code was derived and applied for the investigation of the formation of photoplasma in sodium vapor. The effects of particle weighting and the Courant number on the computed plasma properties were examined, and the convergence of the numerical method with respect to these parameters was demonstrated. Simulations were carried out for the stepwise spatial profile of the resonant sodium atoms density. The basic plasma parameters (such as eedf, iedf, atomic and molecular ion and electron densities, and electric field and potential) were computed. The results of the PIC/MCC simulations were compared to those obtained from the fluid model. Simulations revealed a strong spatial non-uniformity in the electron density and the electric potential over the computational domain that provides evidence in favour of photovoltaic conversion of light energy into electrical energy.

Electrical 2π phase control of infrared light in a 350-nm footprint using graphene plasmons

Phase velocity of graphene plasmons is electrically controlled in a set-up enabling tuning of the phase between 0 and 2π. Modulating the amplitude and phase of light is at the heart of many applications such as wavefront shaping1, transformation optics2,3, phased arrays4, modulators5 and sensors6. Performing this task with high efficiency and small footprint is a formidable challenge7,8. Metasurfaces5,9 and plasmonics10 are promising, but metals exhibit weak electro-optic effects. Two-dimensional materials, such as graphene, have shown great performance as modulators with small drive voltages11,12. Here, we show a graphene plasmonic phase modulator that is capable of tuning the phase between 0 and 2π in situ. The device length of 350 nm is more than 30 times shorter than the 10.6 μm free-space wavelength. The modulation is achieved by spatially controlling the plasmon phase velocity in a device where the spatial carrier density profile is tunable. We provide a scattering theory for plasmons propagating through spatial density profiles. This work constitutes a first step towards two-dimensional transformation optics3 for ultracompact modulators7 and biosensing13.

Large-scale atomistic calculations of clusters in intense x-ray pulses

We present the methodology of our recently developed Monte Carlo/molecular-dynamics method for studying the fundamental ultrafast dynamics induced by high-fluence, high-intensity x-ray free electron laser (XFEL) pulses in clusters. The quantum nature of the initiating ionization process is accounted for by a Monte Carlo method to calculate probabilities of electronic transitions, including photo absorption, inner-shell relaxation, photon scattering, electron collision and recombination dynamics, and thus track the transient electronic configurations (EC) explicitly. The freed electrons, atoms and ions are followed by classical particle trajectories using a molecular dynamics algorithm. Our calculations reveal the role of electron–ion recombination processes that lead to the development of nonuniform spatial charge density profiles in x-ray excited clusters over femtosecond timescales at XFEL intensities exceeding 1020 W cm−2. The Ar cluster fluorescence spectrum is found to be very different from the Ar atom spectrum, in which recombination processes enable additional pathways to reach the required EC for fluorescence transitions. In the high-intensity limit, recombination dynamics can play an important role in the calculated scattering response even for a 2 fs pulse. We demonstrate that our numerical codes and algorithms can make efficient use of the computational power of massively parallel supercomputers to investigate the intense-field dynamics in systems with increasing complexity and size at the ultrafast timescale and in nonlinear x-ray interaction regimes. In particular, picosecond trajectories of XFEL clusters with attosecond time resolution containing millions of particles can be efficiently computed on upwards of 262 144 processes.

Spatial profiles of interelectrode electron density in direct current superposed dual-frequency capacitively coupled plasmas

We present experimentally determined spatial profiles of the interelectrode electron density (ne) in dual-frequency capacitively coupled plasmas in which the negative direct current (dc) bias voltage (Vdc) is superposed; in the experiment, 13 MHz (Plow) was applied to the lower electrode and 60 MHz (Phigh) to the upper electrode. The bulk ne increased substantially with increases in the external power, Phigh, Plow, and with increases in Vdc. When Plow was insufficient, the bulk ne decreased as the Vdc bias increased. The bulk ne increased due to its dependence on Vdc, especially for |Vdc| > 500 V. This may correspond to the sheath voltages (Vs) of the lower electrode. The ne values in front of the upper electrode were coupled with the Vdc: the Vdc dependence first decreased and then increased. The dc currents (Idc) of the upper electrode were collected when a large Plow was applied. The value of Idc at the threshold value of Vdc ≈ Vs (e.g. −500 V) increased with an increase in ne. When |Vdc| exceeded the threshold, the spatial ne profile and the Idc dependence were changed relative to the electrical characteristics of the dc superposition; this led to a change in the location of the maximum ne, the width of the area of ne depletion in front of the electrodes, and a transition in the electron heating modes.

Spatial profiles of electron density, electron temperature, average ionic charge, and EUV emission of laser-produced Sn plasmas for EUV lithography

Spatial profiles of the electron density (ne), electron temperature (Te), and average ionic charge (Z) of laser-produced Sn plasmas for EUV lithography, whose conversion efficiency (CE) is sufficiently high for practical use, were measured using a collective Thomson scattering (TS) technique. For plasma production, Sn droplets of 26 µm diameter were used as a fuel. First, a picosecond-pulsed laser was used to expand a Sn target. Next, a CO2 laser was used to generate plasmas. By changing the injection timing of the picosecond and CO2 lasers, three different types of plasmas were generated. The CEs of the three types of plasmas differed, and ranged from 2.8 to 4.0%. Regarding the different plasma conditions, the spatial profiles of ne, Te, and Z clearly differed. However, under all plasma conditions, intense EUV was only observed at a sufficiently high Te (> 25 eV) and in an adequate ne range [1024–(2 × 1025) m−3]. These plasma parameters lie in the efficient-EUV light source range, as predicted by simulations.

Density profiles around A+B→C reaction-diffusion fronts in partially miscible systems: A general classification.

Various spatial density profiles can develop in partially miscible stratifications when a phase A dissolves with a finite solubility into a host phase containing a dissolved reactant B. We investigate theoretically the impact of an A+B→C reaction on such density profiles in the host phase and classify them in a parameter space spanned by the ratios of relative contributions to density and diffusion coefficients of the chemical species. While the density profile is either monotonically increasing or decreasing in the nonreactive case, reactions combined with differential diffusivity can create eight different types of density profiles featuring up to two extrema in density, at the reaction front or below it. We use this framework to predict various possible hydrodynamic instability scenarios inducing buoyancy-driven convection around such reaction fronts when they propagate parallel to the gravity field.

Excited atoms in the free-burning Ar arc: treatment of the resonance radiation

The collisional–radiative model with an emphasis on the accurate treatment of the resonance radiation transport is developed and applied to the free-burning Ar arc plasma. This model allows for analysis of the influence of resonance radiation on the spatial density profiles of the atoms in different excited states. The comparison of the radial density profiles obtained using an effective transition probability approximation with the results of the accurate solution demonstrates the distinct impact of transport on the profiles and absolute densities of the excited atoms, especially in the arc fringes. The departures from the Saha–Boltzmann equilibrium distributions, caused by different radiative transitions, are analyzed. For the case of the DC arc, the local thermodynamic equilibrium (LTE) state holds close to the arc axis, while strong deviations from the equilibrium state on the periphery occur. In the intermediate radial positions the conditions of partial LTE are fulfilled.

Time-dependent study of a black-hole laser in a flowing atomic condensate

We numerically study the temporal evolution of a black-hole laser configuration displaying a pair of black and white hole horizons in a flowing atomic condensate. This configuration is initially prepared starting from a homogeneous flow via a suitable space-dependent change of the interaction constant and the evolution is then followed up to long times. Depending on the values of the system parameters, the system typically either converges to the lowest energy solution by evaporating away the horizons or displays a continuous and periodic coherent emission of solitons. By making a physical comparison with optical laser devices, we identify the latter regime of continuous emission of solitons as the proper black-hole laser effect. We include some movies of the temporal evolution of the spatial density and velocity profiles in the most significant cases.

Experimental measurements of Stark widths for Mn I lines in long laser spark

We report the experimental Stark widths of Mn I lines belonging to multiplets z6P°→a6S and z6D°→a6D in long spark induced by laser. We have used aluminum alloy containing 80ppm of manganese as a target to avoid strong self-absorption of Mn I lines. Its absence was proved by the comparison of observed intensities with relative strengths of lines within multiplets. Electron density of plasma estimated by Mg I (5172.68Å) and Al II (2816.18Å) lines was within the range of (4–30)×1016cm−3. The shortest possible gate allowed the observation of symmetric atomic and ionic lines. The spatial profiles of plasma temperature and electron density along the axis of long spark demonstrated that both values were lower than for spherical plasma. Measured Stark widths of resonance multiplet z6P°→a6S decrease from 0.075Å for its first component to 0.055Å for the last one, while Stark widths of components of multiplet z6D°→a6D increase from 0.095Å to 0.125Å. No Stark shifting was observed for the studied multiplets.

A local PDE model of aggregation formation in bacterial colonies

We study pattern formation in a model of cyanobacteria motion recently proposed by Galante, Wisen, Bhaya and Levy. By taking a continuum limit of their model, we derive a novel fourth-order nonlinear parabolic PDE equation that governs the behaviour of the model. This PDE is . We then derive the instability thresholds for the onset of pattern formation. We also compute analytically the spatial profiles of the steady state aggregation density. These profiles are shown to be of the form where the exponent p is related to the parameters of the model. Full numerical simulations give a favorable comparison between the continuum and the underlying discrete system, and show that the aggregation profiles are stable above the critical threshold.

Hard-wall and non-uniform lattice Monte Carlo approaches to one-dimensional Fermi gases in a harmonic trap

We present in detail two variants of the lattice Monte Carlo method aimed at tackling systems in external trapping potentials: a uniform-lattice approach with hard-wall boundary conditions, and a non-uniform Gauss–Hermite lattice approach. Using those two methods, we compute the ground-state energy and spatial density profile for systems of N=4–8 harmonically trapped fermions in one dimension. From the favorable comparison of both energies and density profiles (particularly in regions of low density), we conclude that the trapping potential is properly resolved by the hard-wall basis. Our work paves the way to higher dimensions and finite temperature analyses, as calculations with the hard-wall basis can be accelerated via fast Fourier transforms; the cost of unaccelerated methods is otherwise prohibitive due to the unfavorable scaling with system size. To illustrate this point, we show a brief performance comparison of accelerated versus unaccelerated methods across spatial dimensions.

Serum and retinal responses to three different doses of macular carotenoids over 12 weeks of supplementation

The macular carotenoids lutein (L), zeaxanthin (Z), and mesozeaxanthin (MZ) have been shown to have neuroprotective and visual performance benefits once deposited in retinal tissues. The purpose of this 12-week trial was to determine biweekly the absorption kinetics, efficiency of retinal deposition, and effects on the spatial profile of macular pigment for three levels of L + Z + MZ supplement.This study was a double-blind, placebo-controlled 12-week trial. Twenty-eight healthy subjects, aged 18–25 yrs participated. Subjects were randomly assigned to one of four daily supplementation groups: placebo (safflower oil; n = 5), 7.44 mg total macular carotenoid (n = 7), 13.13 mg total macular carotenoid (n = 8), and 27.03 (n = 8) mg total macular carotenoid. Ratios of the three carotenoids were virtually identical for the three levels of supplement (83% L, 10% Z, 7% MZ). At baseline and every two weeks thereafter over the 12-week study period, a fasting blood draw was conducted and, via heterochromatic flicker photometry, spatial profiles of macular pigment optical density (MPOD) were determined.Compared to placebo, serum concentrations of both L and total Z, for each of the supplement levels, were found to increase significantly from baseline after two weeks of daily ingestion (p < 0.001). Likewise, MPOD increased significantly in all treatment groups (p < 0.001) compared to placebo. Serum responses (L, Z, and L + Z) were linearly related to dose (p < 0.001 for all), but not to retinal response. L: Z serum response ratios decreased exponentially with increases in dose (p = 0.008). The ratio of MPOD change: total serum response was found to be highest for the 13.13 mg level of supplement (p = 0.021), followed by 27.03- and 7.44-mg doses. The very center of the spatial profile of MPOD increased in a fashion commensurate with dose level.Although L serum responses increased with dose, the slope of increase was shallower than for Z. Given the higher levels of L in the supplements, this is suggestive of a compressed response with relatively high doses of L. Although all three doses significantly augmented MPOD, the 13.13 mg/day L + Z supplement level was the most efficient in doing so. The data regarding efficiency may inform recommendations regarding macular carotenoid supplementation for age-related macular degeneration. Lastly (although not statistically significant), the shift toward a more pronounced central peak in the spatial profile of MPOD in all treatment groups suggests that central retinal deposition of Z and MZ was efficient and can be seen after a short period of supplementation, especially with higher (e.g. 27.03 mg) daily doses of macular carotenoids.ISRCTN trial registration number: ISRCTN54990825.

Mach 10 Bow Shock and Gas-Cap Unsteadiness Measurements Using Rayleigh Scattering

This study demonstrates the usefulness of instantaneous and mean density , standard deviation , relative standard deviation , and product standard deviation acquired using a low-sampling-rate instantaneous (10 ns) laser-based method to study effects of high-frequency phenomena on offbody freestream, bow-shock wave, and gas-cap unsteadiness. Spatial correlations of relative flow unsteadiness are acquired using laser Rayleigh scattering performed along a 38.7 mm line containing 200 pixels spanning the freestream, gas cap, near-normal, and oblique shocks created by a multipurpose crew vehicle model in the NASA Langley 31-in. Mach 10 air wind tunnel. A total of 371 instantaneous images are acquired at a fixed stagnation temperature of and five stagnation pressures spanning 2.41–10.0 MPa (350–1454 psi). Maximum occurs at the bow-shock-wave spatial density profile first-derivative maximum for a near-normal and oblique shock. Maximum and occur at the bow-shock-wave spatial density profile second-derivative maximum and minimum, respectively, for both the near-normal and oblique shocks. At , maximum (normal shock) (gas cap), and (gas cap) is (freestream). Additional spatially averaged unsteadiness correlations with flowfield conditions are presented.

On the radial distribution of Galactic cosmic rays

Abstract The spectrum and morphology of the diffuse Galactic γ-ray emission carries valuable information on cosmic ray (CR) propagation. Recent results obtained by analyzing Fermi-LAT data accumulated over 7 yr of observation show a substantial variation of the CR spectrum as a function of the distance from the Galactic Centre. The spatial distribution of the CR density in the outer Galaxy appears to be weakly dependent upon the galactocentric distance, as found in previous studies as well, while the density in the central region of the Galaxy was found to exceed the value measured in the outer Galaxy. At the same time, Fermi-LAT data suggest a gradual spectral softening while moving outwards from the centre of the Galaxy to its outskirts. These findings represent a challenge for standard calculations of CR propagation based on assuming a uniform diffusion coefficient within the Galactic volume. Here, we present a model of non-linear CR propagation in which transport is due to particle scattering and advection off self-generated turbulence. We find that for a realistic distribution of CR sources following the spatial distribution of supernova remnants and the space dependence of the magnetic field on galactocentric distance, both the spatial profile of CR density and the spectral softening can easily be accounted for.

Diagnostics of atmospheric-pressure pulsed-dc discharge with metal and liquid anodes by multiple laser-aided methods

The density and temperature of electrons and key heavy particles were measured in an atmospheric-pressure pulsed-dc helium discharge plasma with a nitrogen molecular impurity generated using system with a liquid or metal anode and a metal cathode. To obtain these parameters, we conducted experiments using several laser-aided methods: Thomson scattering spectroscopy to obtain the spatial profiles of electron density and temperature, Raman scattering spectroscopy to obtain the neutral molecular nitrogen rotational temperature, phase-modulated dispersion interferometry to determine the temporal variation of the electron density, and time-resolved laser absorption spectroscopy to analyze the temporal variation of the helium metastable atom density. The electron density and temperature measured by Thomson scattering varied from 2.4 × 1014 cm−3 and 1.8 eV at the center of the discharge to 0.8 × 1014 cm−3 and 1.5 eV near the outer edge of the plasma in the case of the metal anode, respectively. The electron density obtained with the liquid anode was approximately 20% smaller than that obtained with the metal anode, while the electron temperature was not significantly affected by the anode material. The molecular nitrogen rotational temperatures were 1200 K with the metal anode and 1650 K with the liquid anode at the outer edge of the plasma column. The density of helium metastable atoms decreased by a factor of two when using the liquid anode.

Galactic energetic particles and their radiative yields in clusters

As energetic particles diffuse out of radio and star-forming galaxies (SFGs), their intracluster density builds up to a level that could account for a substantial part or all the emission from a radio halo. We calculate the particle time-dependent, spectro-spatial distributions from a solution of a diffusion equation with radio galaxies as sources of electrons and SFGs as sources of both electrons and protons. Whereas strong radio galaxies are typically found in the cluster (e.g., Coma) core, the fraction of SFGs increases with distance from the cluster center. Scaling particle escape rates from their sources to the reasonably well determined Galactic rates, and for realistic gas density and magnetic field spatial profiles, we find that predicted spectra and spatial profiles of radio emission from primary and secondary electrons are roughly consistent with those deduced from current measurements of the Coma halo (after subtraction of emission from the relic Coma A). Nonthermal x-ray emission is predicted to be mostly by Compton scattering of electrons from radio galaxies off the CMB, whereas $\ensuremath{\gamma}$-ray emission is primarily from the decay of neutral pions produced in interactions of protons from SFGs with protons in intracluster gas.