Short-wavelength Modes Research Articles

The cosmological constant emerging from a symmetry invariant

The cosmological constant is normally introduced as an additional term entering the Einstein–Hilbert (EH) action. In this letter, we demonstrate that, instead, it appears naturally from the standard EH action as an invariant term emerging from spacetime symmetries. We then demonstrate that the same constraint emerging from this invariant suppresses the short wavelength modes and it favors the long wavelength ones. In this way, inside the proposed formulation, the observed value for the vacuum energy density is obtained naturally from the zero-point quantum fluctuations.

Reconstructing the long-wavelength matter density fluctuation modes from the scalar-type clustering fossils

Revealing the large-scale structure from the 21cm intensity mapping surveys is only possible after the foreground cleaning. However, most current cleaning techniques relying on the smoothness of the foreground spectrum lead to a severe side effect of removing the large-scale structure signal along the line of sight. On the other hand, the clustering fossil, a coherent variation of the small-scale clustering over large scales, allows us to recover the long-wavelength density modes from the off-diagonal correlation between short-wavelength modes. In this paper, we revisit the reconstruction based on the short-wavelength matter density modes in real space and scrutinize the requirements for an unbiased and optimal clustering-fossil estimator. We show that (A) the estimator is unbiased only when using an accurate bispectrum model for the long-short-short mode coupling and (B) including the connected four-point correlation functions is essential for characterizing the noise power spectrum of the estimated long mode. For matter in real space, the clustering fossil estimator based upon the leading-order bispectrum yields an unbiased estimation of the long-wavelength (k ≲ 0.01 [h/Mpc]) modes with the cross-correlation coefficient of 0.7 at redshifts z = 0 to 3.

Localized phase contrast imaging at the Wendelstein 7-X stellarator

In its basic form, phase contrast imaging (PCI) provides line-integrated measurements of electron density fluctuations in plasmas. As turbulent fluctuations in magnetically confined plasmas have wave vectors almost perpendicular to the background magnetic field, the signals scattered by fluctuations from different parts of the PCI line-of-sight (LoS) are spatially separated in focal planes of the plasma. This allows localized PCI measurements by placing a mask in such a plane, to only permit signals from specific parts of the LoS to reach the PCI detectors. The present paper describes modeling and design of localization masks for the PCI system at the Wendelstein 7-X (W7-X) stellarator as well as the first results obtained using the masks in the recent long-pulse W7-X experimental campaign. During this project, we have extended the theory describing the mask response within the Fraunhofer diffraction model. As a novel development, we show from first principles that the mask response is determined by the fraction of power of the scattered beam spots that passes the mask. These insights have been used to select the W7-X mask design, consisting of a circular cutout, allowing the unscattered beam spot to pass the mask, with wedges covering a fixed angular range outside the central cutout. In the recent W7-X experimental campaign, the masks have verified the location of the main turbulence features observed by the PCI system and provided new information about the location of short-wavelength magnetohydrodynamic modes.

Infrared bands of neutral gas-phase carbon clusters in a broad spectral range.

The identification of species in the interstellar medium requires precise and molecule-specific spectroscopic information in the laboratory framework, in broad spectral ranges and under conditions relevant to interstellar environments. In this work, we measure the gas-phase infrared spectra of neutral carbon clusters, CN (N = 6-11), in a molecular beam. The CN distribution is formed by photofragmentation of C60 molecules, concurrently showing a top-down formation mechanism. A broad spectral range in the infrared between 500-3200 cm-1 (20-3.125 μm) is investigated. We observe strong bands between 5 and 6 μm, in conjunction with novel features in the 3 μm region. Density functional theory calculations reveal that these short wavelength modes correspond to combination bands with significant infrared intensity. Moreover, we identify the N ≤ 10 clusters as linear, while C11 adopts a ring configuration, placing the linear-to-ring transition at N = 11 under our molecular beam conditions. The linearity of C10 is discussed based on the formation pathway from larger clusters in energetic conditions. Given the vast and very precise infrared information already been released from the James Webb Space Telescope mission, this infrared spectroscopic data set in conjunction with information on formation mechanisms is of major relevance for identifying neutral carbon clusters in astronomical environments.

Effect of hot-electron preheating on the multimode bubble-front growth of the ablative Rayleigh–Taylor instability

Hot electrons (HEs) generated via parametric instabilities at high laser intensities are a critical concern of laser-driven inertial confinement fusion (ICF), which can significantly impact the ICF performance by preheating the target. In this paper, the effects of HE preheating with moderate HE energy on the evolution of two-dimensional multimode ablative Rayleigh–Taylor instability (ARTI) up to the self-similar growth stage are studied through numerical simulations with a multigroup diffusion model. It is found that HE preheating stabilizes the linear growth of multimode ARTI and delays the onset of the self-similar growth regime. This time delay is more significant for the short-wavelength mode ARTI and higher energy HE cases. It is also shown that the variation of self-similar growth coefficients under HE preheating is not very significant. The delay to the onset of the nonlinear stage of multimode ARTI by HE preheating with moderate energy may be beneficial to ICF implosions.

Random lasing in dye-doped electrospun PMMA fibers with different emission modes

Random lasers (RLs) have significant applications in micro-nano light emitters, micro-detection, and speckle-free imaging due to their novel properties in low spatial coherence, small size and multi-emission directions. A flexible, affordable approach for creating new scattering materials has recently been suggested for electrospun fibers, which are practical materials for RLs. In this article, we employ the electrospun PMMA fibers as scatters and the DCJTB as gain materials to generate an RL system. The sample shows obvious incoherent random lasing properties. A decrease of the lasing threshold by 32 % is realized when we increase the electrospun voltage from 12 kV to 15 kV due to the generation of a small mean diameter size and concentrated diameter distribution. The lasing threshold of the DCJTB/Au NPs-doped PMMA sample is further reduced to 19.8 μJ/mm2 by incorporating Au NPs into the electrospun dye-doped fibers. This leads to a lasing threshold reduction of 40 % from the electrospun sample without doping Au NPs and 52 % from the spin-coated film sample, respectively. It indicates that the electrospun method is an efficient and practical approach to make RLs. Furthermore, using single pulse laser stimulation, we investigated several different lasing modes, including spontaneous emission, short wavelength domain, mode competition, and single mode. The actions of photon scattering are analyzed to explain the dramatic lasing process. Due to the photon scattering optical path, there are mode competition and single mode emission regions under different pump energy. The light spot quality of RL shows low spatial coherence with the useful application of speckle-free imaging. This research provides a new method of electrospun to produce low-threshold and high-quality RLs and potentially opens the door for expanding their use in optics and optoelectronic equipment.

Effects of ablation velocity on ablative Rayleigh–Taylor instability

The influence of the ablation velocity Va on the evolution of single-mode ablative Rayleigh–Taylor instability from the linear to the deeply nonlinear phases is investigated via two-dimensional numerical simulations. Linear growth rates from simulations agree well with the asymptotic theory except for larger discrepancies in the intermediate Froude number regime. The weakly nonlinear growth behavior of the bubble amplitude is found dependent on a critical perturbation wavenumber in a broad Froude number regime. For a linearly stable mode, its nonlinear excitation threshold is higher for larger Va and thus harder to be exceeded. For short-wavelength modes taking significant ablation effects, the bubble penetration velocity is found to reaccelerate after the first saturation and eventually saturate at a larger value with larger Va, due to stronger vortex-acceleration effects and more significant increase in g.

Confronting the Chaplygin gas with data: Background and perturbed cosmic dynamics

In this paper, we undertake a unified study of background dynamics and cosmological perturbations in the presence of the Chaplygin gas (CG). This is done by first constraining the background cosmological parameters of different Chaplygin gas models with SNeIa and [Formula: see text] data for detailed statistical analysis of the CG models. Based on the statistical criteria we followed, none of the models has substantial observational support, but we show that the so-called “original” and “extended/generalised” Chaplygin gas models have some observational support and less observational support, respectively, whereas the “modified” and “modified generalised” Chaplygin gas models miss out on the category less observational support, but cannot be ruled out. The so-called “generalised” cosmic Chaplygin gas model, on the other hand, falls under the no observational support category of the statistical criterion and can be ruled out. The models which are statistically accepted are considered for perturbation level in both theoretical and observational aspects. We also apply the [Formula: see text] covariant formalism of perturbation theory and derive the evolution equations of the fluctuations in the matter density contrast of the matter–Chaplygin gas system for the models with some or less statistical support. The solutions to these coupled systems of equations are then computed in both short-wavelength and long-wavelength modes. Then we feed these observationally restricted parameters into the analysis of cosmological perturbations to address the growth of density contrast through redshift. Using the most recent linear growth of the data [Formula: see text], CG models are considered to study the linear growth of the structure.

Inverse cascade based on nonlinear Schrödinger equation analysis with nonlinear feedback control

This paper focuses on the wave inverse cascade instability analysis with self-regulating feedback control for a fixed external potential field and a highly localized finite-amplitude initial pulse. The wave inverse cascade instability analysis is carried out by solving the corresponding two-dimensional generalized nonlinear Schrödinger equation. The wave field firstly suffers from the modulation instability, followed by collapse into turbulence containing the shortest-wavelength modes in the system. This is followed by inverse cascade of the shortest wavelength modes back to the longer-wavelength ones, until a statistical stationary turbulent state is reached. It is found that the inverse cascade is limited to the shorter-wavelength modes with the wavenumber $\left |k\right |\geq 100$ . This shows that the viscous damping $p_i$ acts like a control switch to the inverse cascade, and the feedback control can also regulate the intensity of the inverse cascade mode.

Minimal decoherence from inflation

We compute the rate with which super-Hubble cosmological fluctuations are decohered during inflation, by their gravitational interactions with unobserved shorter-wavelength scalar and tensor modes. We do so using Open Effective Field Theory methods, that remain under control at the late times of observational interest, contrary to perturbative calculations. Our result is minimal in the sense that it only incorporates the self-interactions predicted by General Relativity in single-clock models (additional interaction channels should only speed up decoherence). We find that decoherence is both suppressed by the first slow-roll parameter and by the energy density during inflation in Planckian units, but that it is enhanced by the volume comprised within the scale of interest, in Hubble units. This implies that, for the scales probed in the Cosmic Microwave Background, decoherence is effective as soon as inflation proceeds above ∼ 5 × 109 GeV. Alternatively, if inflation proceeds at GUT scale decoherence is incomplete only for the scales crossing out the Hubble radius in the last ∼ 13 e-folds of inflation. We also compute how short-wavelength scalar modes decohere primordial tensor perturbations, finding a faster rate unsuppressed by slow-roll parameters. Identifying the parametric dependence of decoherence, and the rate at which it proceeds, helps suggest ways to look for quantum effects.

Curvature effect on the instabilities of lid-driven flow in a toroidal square cavity

The effect of curvature on the three-dimensional instabilities of lid-driven flow in a toroidal square cavity is numerically studied using the linear stability analysis based on the spectral element method. The critical stability curves are determined, and four types of instability modes are predicted. For small curvatures (δ ≤ 0.2), the most dangerous mode is a steady short-wavelength mode (λc ≈ 0.4), which is consistent with that in the straight square cavity. In a narrow range of 0.22 ≤ δ ≤ 0.28, the critical mode is oscillatory and of medium wavelength (0.84 < λc < 1.07). For δ ≥ 0.29, two types of steady long-wavelength modes (λc > 1.26) become the critical mode. The disturbance energy analyses show that all the instabilities are caused by the centrifugal instability mechanism.

On erosion transition from the incubation stage to the accumulation stage in liquid impingement erosion

The evolution of surface profiles during erosion caused by the continuous impact of a liquid jet was studied using a water-jet apparatus. In this study, we focused on the erosion transition from the incubation stage to the accumulation stage during liquid impingement erosion, which is important for water-jet cleaning and aging management for nuclear power plants. By computing the local erosion rate distribution and surface roughness parameters, the formations of isolated pits and asperities were investigated. The structure of erosion caused by liquid impingement was extracted by using a dynamic mode decomposition technique. The surface profile changes were primarily caused by the formation of isolated pits and asperities during the incubation stage. The roughness parameters remained unchanged during the incubation stage, with barely measurable erosion depth, whereas the skewness of the surface profile began to increase during this stage. The initial erosion with a small-scale roughness gradually evolved into a large-scale crater. Dynamic mode decomposition analysis showed the presence of a short-wavelength (mode A) surface depression during the incubation stage, followed by a long-wavelength (mode B) erosion crater caused by the merging of mode A surface depressions during the accumulation stage.

Towards Bright Single-Photon Emission in Elliptical Micropillars.

In recent years, single-photon sources (SPSs) based on the emission of a single semiconductor quantum dot (QD) have been actively developed. While the purity and indistinguishability of single photons are already close to ideal values, the high brightness of SPSs remains a challenge. The widely used resonant excitation with cross-polarization filtering usually leads to at least a two-fold reduction in the single-photon counts rate, since single-photon emission is usually unpolarized, or its polarization state is close to that of the exciting laser. One of the solutions is the use of polarization-selective microcavities, which allows one to redirect most of the QD emission to a specific polarization determined by the optical mode of the microcavity. In the present work, elliptical micropillars with distributed Bragg reflectors are investigated theoretically and experimentally as a promising design of such polarization-selective microcavities. The impact of ellipticity, ellipse area and verticality of the side walls on the splitting of the optical fundamental mode is investigated. The study of the near-field pattern allows us to detect the presence of higher-order optical modes, which are classified theoretically. The possibility of obtaining strongly polarized single-photon QD radiation associated with the short-wavelength fundamental cavity mode is shown.

Numerical stochastic inflation constrained by frozen noise

Stochastic inflation can resolve strong inflationary perturbations, which seed primordial black holes. I present a fast and accurate way to compute these perturbations in typical black hole producing single-field models, treating the short-wavelength Fourier modes beyond the de Sitter approximation. The squeezing and freezing of the modes reduces the problem to one dimension, and the resulting new form of the stochastic equations, dubbed `constrained stochastic inflation,' can be solved efficiently with semi-analytical techniques and numerical importance sampling. In an example case, the perturbation distribution is resolved in seconds deep into its non-Gaussian tail, a speed-up of factor 109 compared to a previous study. Along the way, I comment on the role of the momentum constraint in stochastic inflation.

Short-wavelength axion dark modes of cavity axion polariton

The cavity axion polariton represents a new frontier in the field of topological matter and light-matter interactions. To realize strong axion-photon coupling and low damping of the axion mode, which are two key factors for quantum information applications, we studied the short-wavelength axion modes and the axion-axion interaction theoretically. Our results reveal an unconventional coupling between photon and standing wave axion modes that is in the strong-coupling regime as the number of antinodes is large. When two axion modes are coupled with cavity photons, cavity-mediated axion-axion coupling gives rise to the axion dark mode. Our results suggest that short-wavelength axion dark modes may be potential candidates for quantum manipulation and storage.

Role of hot electrons in mitigating ablative Rayleigh–Taylor instability

In laser-driven inertial confinement fusion (ICF) specifically with high laser intensities, energetic (hot) electrons (HEs) can be generated via laser-plasma instabilities. HEs can significantly impact the target performance by modifying the implosion hydrodynamics. In this paper, the effects of moderate-energy (about 20 to 40 keV) HEs on the evolution of two-dimensional single-mode ablative Rayleigh–Taylor instability (ARTI) are studied through numerical simulations with a multigroup diffusion model in which the HE population is treated as a high-energy group launched from the boundary. With HEs present, it is found that ARTI linear growth rates are reduced even though the acceleration of the implosion shell is enhanced by HEs. The reduction in the linear growth rate is owing to the increase in the ablation velocity and the density scale length, and this stabilization effect is greater in the shorter-wavelength modes and/or higher-energy HE cases. The ARTI linear growth does not get mitigated monotonically as the HE number density increases for a given fixed HE kinetic energy. The HE number density minimizing the ARTI growth rate is found, likely due to the competition of the stabilizing and destabilizing hydrodynamic-parameter variations caused by HEs.

Asymmetric distribution of aminophospholipids stiffens fluid membranes.

Cellular envelopes contain a large number of lipid species that distribute asymmetrically between the two leaflets of the bilayer. For example, mammalian plasma membranes are composed of an outer leaflet enriched in cholinephospholipids, while the majority of the aminophospholipids are confined to the inner leaflet. Membrane asymmetry can theoretically lead to distinct effects regarding its elastic behavior, either due to lipid specific properties (e.g. size, shape), or simply lipid over/under-crowding of a given leaflet. Corresponding experimental data are scarce. This prompted us to determine the structure and bending rigidity of minimal mammalian plasma membrane mimics composed of milk sphingomyelin (MSM), palmitoyl oleoyl phosphatidylcholine (POPC), palmitoyl oleoyl phosphatidylethanolamine (POPE), and palmitoyl oleoyl phosphatidylserine (POPS) using small-angle neutron/X-ray scattering and neutron spin echo spectroscopy. Most strikingly, we observed an anomalous stiffening of freely-floating fluid vesicles (size ∼ 100 nm) in case of inner leaflets composed of POPE/POPS mixtures and outer leaflets enriched in POPC. That is, the bending rigidities of POPCout/(POPE/POPS)in vesicles did not only exceed those of their scrambled (symmetric) analogs, but also those of symmetric vesicles mimicking either their inner and outer leaflets. Addition MSM in the outer leaflet reduced but not abolished this effect, possibly due to hydrocarbon chain interdigitation. To reconcile our findings, we speculate that lipid shape and charge asymmetry may increase the weight of short-wavelength undulatory modes in membranes by inducing a coupling at distances smaller than the membrane thickness.

Near-field control of gold nanostructure under joint action of surface plasmon polariton and incident light

Localized surface plasmon (LSP) generated by nanostructure subjected to the excitation of surface plasmon polariton (SPP) possesses stronger near-field enhancement and special spectral and dynamic responses, thereby providing a new idea for exploring the interaction between light and matter. Meanwhile, this scheme can also release the signal background noise and structural thermal effect, and improve the performances of plasmonic components and sensing detectors based on LSP. However, the current research in this aspect is still insufficient. In this paper, we investigate the near-field characteristics of a plasmon composite structure composed of plasmon focusing lens and gold nanorod under the excitation of dual-beam by using finite-difference time-domain (FDTD) method. The result shows that the near-field intensity control on the upper surface and in the gap position of the nanorod can be achieved by adjusting the relative time delay between the first light beam (used to excite SPP) and the second light beam (used to excite LSP). Specifically, the maximum adjustment range of the near-field intensity corresponding to 770 nm resonant mode in the gap position is about 23, and the adjustment period is about 2.4 fs. In a resonant mode dominated by SPP at a wavelength of 999 nm, the adjustment range of near-field intensity is as small as 6, and the adjustment period is about 4 fs. On the upper surface of the structure, the adjustment range of the near-field intensities of the two resonant modes (719 nm and 802 nm) are basically the same (about 15), and their adjustment periods are 2.4 fs and 2.8 fs. The achievement of the near field control is attributed to the coherent superposition of SPP-excited LSP with light-excited LSP. In addition, the dephasing time of the coupling field is investigated by using a quasi- normal mode. It is found that the nanorod structure will correspond to different dephasing time under different relative time delay between two excitation light beams. Specifically, for the time delay of 0.72 fs (Δ<i>t</i> = 0.72 fs), the corresponding dephasing times for both modes are the same (6.0 fs). For Δ<i>t</i> = 1.92 fs, the dephasing time of the longer-wavelength mode is 7.1 fs, and the one of the shorter-wavelength mode is 5.8 fs. We attribute the difference in dephasing time to different coupling strengths between the two modes at different delay times. This study may further promote the application of plasmons in the fields of surface-enhanced Raman scattering and plasmon assisted catalysis.

Collective dynamics in liquid Si under high pressure above the melting line minimum

We report an ab initio simulation study of changes in structural and dynamic properties of liquid Si at 7 pressures ranging from 10.2 to 24.3 GPa along the isothermal line 1150 K, which is above the minimum of the melting line. The increase of pressure from 10.2 to 16 GPa causes strong reduction in the tetrahedral ordering of the closest neighbors. The diffusion coefficient shows a linear decay vs drop in atomic volume, that agrees with theoretical prediction for simple liquid metals, thus not showing any feature at the pressures corresponding to the different crystal phase boundaries. The Fourier-spectrum of the velocity autocorrelation function shows two-peak structure at pressures 20 of GPa and higher. These characteristic frequencies correspond well to the peak frequencies of the transverse current spectral function in the second pseudo-Brillouin zone. Two almost flat branches of short-wavelength transverse modes were observed at all the studied pressures. We discuss the pressure evolution of characteristic frequencies in the longitudinal and transverse branches of collective modes.

Periodic necking of misfit hyperelastic filaments embedded in a soft matrix

The necking instability is a precursor to tensile failure and rupture of materials. A quasistatically loaded free-standing uniaxial specimen typically exhibits necking at a single location, thus corresponding to a long wavelength bifurcation mode. If confined to a substrate or embedded in a matrix the same filament can exhibit periodic necking and fragmentation thus creating segments of finite length. While such periodic instabilities have been extensively studied in ductile metal filaments and thin sheets, less is known about necking in hyperelastic materials. Nonetheless, in recent years, there has been a renewed interest in the role of necking in novel materials, for the advancement of fabrication processes and to explain fragmentation phenomena observed in 3D printed active biological matter. In both cases materials are not well described by the existing frameworks that employ J2 deformation theory of plasticity, and existing studies do not account for the significant role of misfit stretches that may emerge in these systems through chemical, or biological contraction. To address these limitations, in this paper we begin by experimentally demonstrating the role of the surrounding matrix on the necking and fragmentation of a compliant filament embedded in a tunable rubber matrix. Using a generalized hyperelastic model with strain softening, our analytical bifurcation analysis explains the experimental observations and is shown to agree with numerical predictions. The analysis reveals three distinct bifurcation modes: the long wavelength necking, thus recovering the Considère criterion; the periodic necking observed in our experiments; and a short wavelength mode that is characterized by localization along the center cord of the filament and is independent of the film-to-matrix stiffness ratio. We find that the softening coefficient and the filament misfit stretch can significantly influence the stability threshold and observed wavelength, respectively. Our results can guide the design and fabrication of novel composite materials and can explain the fragmentation processes observed in active biological materials.