Main Physical Effect Research Articles

Giant Ultrafast All-Optical Modulation Based on Exceptional Points in Exciton-Polariton Perovskite Metasurfaces.

Ultrafast all-optical modulation with optically resonant nanostructures is an essential technology for high-speed signal processing on a compact optical chip. Key challenges that exist in this field are relatively low and slow modulations in the visible range as well as the use of expensive materials. Here we develop an ultrafast all-optical modulator based on MAPbBr3 perovskite metasurface supporting exciton-polariton states with exceptional points. The additional angular and spectral filtering of the modulated light transmitted through the designed metasurface allows us to achieve 2500% optical signal modulation with the shortest modulation time of 440 fs at the pump fluence of ∼40 μJ/cm2. Such a value of the modulation depth is record-high among the existing modulators in the visible range, while the main physical effect behind it is polariton condensation. Scalable and cheap metasurface fabrication via nanoimprint lithography along with the simplicity of perovskite synthesis and deposition make the developed approach promising for real-life applications.

Sparse holographic imaging for an integrated augmented reality near-eye display.

Diffraction is the main physical effect involved in the imaging process of holographic displays. In the application of near-eye displays, it generates physical limits that constrain the field of view of the devices. In this contribution, we evaluate experimentally an alternative approach for a holographic display based mainly on refraction. This unconventional imaging process, based on sparse aperture imaging, could lead to integrated near-eye displays through retinal projection, with a larger field of view. We introduce for this evaluation an in-house holographic printer that allows the recording of holographic pixel distributions at a microscopic scale. We show how these microholograms can encode angular information that overcomes the diffraction limit and could alleviate the space bandwidth constraint usually associated with conventional display design.

Enchained growth and cluster dislocation: A possible mechanism for microbiota homeostasis.

Immunoglobulin A is a class of antibodies produced by the adaptive immune system and secreted into the gut lumen to fight pathogenic bacteria. We recently demonstrated that the main physical effect of these antibodies is to enchain daughter bacteria, i.e. to cross-link bacteria into clusters as they divide, preventing them from interacting with epithelial cells, thus protecting the host. These links between bacteria may break over time. We study several models using analytical and numerical calculations. We obtain the resulting distribution of chain sizes, that we compare with experimental data. We study the rate of increase in the number of free bacteria as a function of the replication rate of bacteria. Our models show robustly that at higher replication rates, bacteria replicate before the link between daughter bacteria breaks, leading to growing cluster sizes. On the contrary at low growth rates two daughter bacteria have a high probability to break apart. Thus the gut could produce IgA against all the bacteria it has encountered, but the most affected bacteria would be the fast replicating ones, that are more likely to destabilize the microbiota. Linking the effect of the immune effectors (here the clustering) with a property directly relevant to the potential bacterial pathogeneicity (here the replication rate) could avoid to make complex decisions about which bacteria to produce effectors against.

Nonlinear neoclassical two-fluid theory of response of tokamak plasma to resonant error-field

A nonlinear, neoclassical, two-fluid theory of the interaction of a single-helicity magnetic island chain with a resonant error-field in a quasi-cylindrical, low-β, tokamak plasma is presented. In particular, the analysis of Fitzpatrick [Phys. Plasmas 25, 042503 (2018)] is generalized to take explicit time dependence into account. Aside from the ability to more accurately treat time-varying problems, the main physical effect that is introduced into the theory by the incorporation of explicit time dependence is ion inertia. The formalism developed in the paper is used to analyze two time-varying problems. First, the interaction of a pre-existing magnetic island chain with a resonant error-field. Second, an error-field-maintained magnetic island chain. The latter problem is of direct relevance to experiments in which deliberately applied, multi-harmonic, resonant magnetic perturbations are used to suppress edge localized modes (ELMs) in tokamak plasmas. Indeed, the predictions of the theory are strikingly similar to data recently obtained from ELM suppression experiments in the DIII-D tokamak [R. Nazikian et al., Nucl. Fusion 58, 106010 (2018)].

Energy concentration and explicit Sommerfeld radiation condition for the electromagnetic Helmholtz equation

We study the electromagnetic Helmholtz equation(∇+ib(x))2u(x)+n(x)u(x)=f(x),x∈Rd, with the magnetic vector potential b(x) and n(x) a variable index of refraction that does not necessarily converge to a constant at infinity, but can have an angular dependence like n(x)→n∞(x|x|) as |x|→∞. We prove an explicit Sommerfeld radiation condition∫Rd|∇bu−in∞1/2x|x|u|2dx1+|x|<+∞ for solutions obtained from the limiting absorption principle and we also give a new energy estimate∫Rd|∇ωn∞(x|x|)|2|u|21+|x|dx<+∞, which explains the main physical effect of the angular dependence of n at infinity and deduces that the energy concentrates in the directions given by the critical points of the potential.

Direct Gaugino Mediation

We describe renormalizable supersymmetric four-dimensional theories which lead to gaugino mediation and various generalizations thereof. Even though these models are strongly coupled, we can demonstrate the parametric suppression of soft scalar masses via Seiberg duality. We show that our models have a parameter which continuously interpolates between suppressed soft scalar masses and their conventional gauge mediated contribution. The main physical effect which we utilize is the general relation between massive deformations in one frame and the Higgs mechanism in the dual frame. Some compelling and relatively unexplored phenomenological scenarios arise naturally in this framework. We offer preliminary comments on various aspects of the phenomenology and outline several of the outstanding open problems.

Influence of secondary emission yield on the saturation properties of multipactor discharges between two parallel metal plates

A detailed numerical simulation analysis is made of the saturation stage of multipactor discharges between two infinite parallel metal plates in vacuum exposed to a rf voltage. The main physical effect causing saturation of the multipactor discharge is increased space charge, as the electron density becomes large. It is found that the properties of the saturation stage depend crucially on the value of the secondary emission yield of the metal surfaces. Below a certain threshold value, the discharge has a two-sided character, but at this threshold the discharge makes an abrupt transition into two decoupled single-sided multipactor discharges containing significantly increased electron densities. The result of the numerical simulations gives a good picture of the saturation properties of the multipactor discharge and is also supplemented by an approximate analytical investigation that highlights and explains the characteristic properties of the observed saturation behavior.

Physical colors in cultural heritage: Surface plasmons in glass

Abstract Gold ruby glass and lustre ceramics are the almost unique examples of physical colors in the cultural heritage. The main physical effect at the origin of their color is the excitation of surface plasmon modes in metal nanoparticles. Moreover, in lustre, interference effects due a multilayer structure add a bright iridescence. The principle of plasmons is recalled in detail and applied to Gold ruby glass. The case of luster ceramics is treated in more detail due to the complexity of the effects involved: plasmon, scattering, interference between specular reflected light beams and also between scattered beams. To cite this article: J. Lafait et al., C. R. Physique 10 (2009).

Frequency shift in the moving periodic structure

The interaction between the electromagnetic wave and the moving periodic structure is analyzed. The main physical effect revealed by theoretical analysis is that the frequency shift of the scattered or transmitted mode is proportional to the speed and the mode order m over the period a. The frequency shift is independent of the incidence direction and the carrier frequency. The physical effect is verified by an experiment. The analysis released here is fundamental in speed measurement and may have certain significance in interdisciplinary fields.

Control of Gold Surface Diffusion on Si Nanowires

Silicon nanowires (NW) were grown by the vapor-liquid-solid mechanism using gold as the catalyst and silane as the precursor. Gold from the catalyst particle can diffuse over the wire sidewalls, resulting in gold clusters decorating the wire sidewalls. The presence or absence of gold clusters was observed either by high angle annular darkfield scanning transmission electron microscopy images or by scanning electron microscopy. We find that the gold surface diffusion can be controlled by two growth parameters, the silane partial pressure and the growth temperature, and that the wire diameter also affects gold diffusion. Gold clusters are not present on the NW side walls for high silane partial pressure, low temperature, and small NW diameters. The absence or presence of gold on the NW sidewall has an effect on the sidewall morphology. Different models are qualitatively discussed. The main physical effect governing gold diffusion seems to be the adsorption of silane on the NW sidewalls.

Time‐Distance Helioseismology: Sensitivity off‐mode Travel Times to Flows

Time-distance helioseismology has shown that f-mode travel times contain information about horizontal flows in the Sun. The purpose of this study is to provide a simple interpretation of these travel times. We study the interaction of surface-gravity waves with horizontal flows in an incompressible, plane-parallel solar atmosphere. We show that for uniform flows less than roughly 250 m s$^{-1}$, the travel-time shifts are linear in the flow amplitude. For stronger flows, perturbation theory up to third order is needed to model waveforms. The case of small-amplitude spatially-varying flows is treated using the first-order Born approximation. We derive two-dimensional Fr\'{e}chet kernels that give the sensitivity of travel-time shifts to local flows. We show that the effect of flows on travel times depends on wave damping and on the direction from which the observations are made. The main physical effect is the advection of the waves by the flow rather than the advection of wave sources or the effect of flows on wave damping. We compare the two-dimensional sensitivity kernels with simplified three-dimensional kernels that only account for wave advection and assume a vertical line of sight. We find that the three-dimensional f-mode kernels approximately separate in the horizontal and vertical coordinates, with the horizontal variations given by the simplified two-dimensional kernels. This consistency between quite different models gives us confidence in the usefulness of these kernels for interpreting quiet-Sun observations.

Energy Concentration and Sommerfeld Condition for Helmholtz Equation with Variable Index at Infinity

We consider the Helmholtz equation with a variable index of refraction n(x), which is not necessarily constant at infinity but can have an angular dependency like $$n(x)\rightarrow n_{\infty} (x/|x|)$$ as $$|x| \rightarrow \infty$$ . Under some appropriate assumptions on this convergence and on n ∞ we prove that the Sommerfeld condition at infinity still holds true under the explicit form $$\int_{{\rm \mathbb{R}}^{d}}\left| \nabla u-in^{1/2}_{\infty}u\frac{x}{\mid x \mid}\right|^{2}\frac{dx}{\mid x \mid}<+\infty.$$ It is a very striking and unexpected feature that the index n ∞ appears in this formula and not the gradient of the phase as established by Saito in [S] and broadly used numerically. This apparent contradiction is clarified by the existence of some extra estimates on the energy decay. In particular we prove that $$\int_{{\rm \mathbb{R}}^{d}}\left|\nabla_{\omega}n_{\infty}\left(\frac{x}{\mid x \mid}\right)\right|^{2}\frac{{\mid u \mid}^2}{\mid x \mid}dx<+\infty.$$ In fact our main contribution is to show that this can be interpreted as a concentration of the energy along the critical lines of n ∞. In other words, the Sommerfeld condition hides the main physical effect arising for a variable n at infinity; energy concentration on lines rather than dispersion in all directions.

Analysis of electronic contributions to the vibrational frequency of CO/Cu 2O(111)

The change in the CO vibrational frequency after adsorption on low-coordinated copper atoms at the Cu 2O(111) surface has been studied theoretically with gradient-corrected density-functional theory (DFT) methods. The surface was modeled with a Cu 28O 14 cluster embedded in point charges and total-ion model potentials. An analysis of the contributions to changes in the CO stretching mode based on the electron density was performed. We show that the proper description of the Cu 3d→CO 2π ∗ back donation, which is responsible for the experimentally observed small red shift, requires the inclusion of electron correlation in a self-consistent way. In addition to this main physical effect, the absolute value of the frequency shift depends on the form of the exchange functional and on the description of electrostatic interactions in the cluster model.

Elastic waves in finely layered sediments: The equivalent medium and generalized O’Doherty‐Anstey formulas

We study the influence of elastic 1-D inhomogeneous random media (e.g., finely layered media with variable density and shear and compressional velocities) on the kinematics and dynamics of the transmitted obliquely incident P‐ and SV‐plane waves. Multiple scattering (resulting in localization and spatial dispersion of the elastic wavefield) is the main physical effect controlling the properties of the wavefield in such media. We analyze the wave propagation assuming the fluctuations of velocities and density to be small (of the order of 20% or smaller). We obtain explicit analytic solutions for the attenuation coefficient and phase velocity of the transmitted waves. These solutions are valid for all frequencies. They agree very well with results of numerical modeling. Our theory shows that fine elastic multilayering is characterized by a frequency‐dependent anisotropy. At typical acquisition frequencies this anisotropy differs significantly from the low‐frequency anisotropy described by the well‐known Backus averaging. The increase of the phase velocity with frequency is quantified. It can partly explain the difference between well‐log‐derived velocities and lower frequency seismic velocities [e.g., vertical seismic profiling (VSP) velocities] in terms of localization. The low‐ and high‐frequency asymptotical results for the phase velocity agree with those of Backus averaging and ray approximation, respectively. The theory describes the angle‐dependent attenuation caused by multiple scattering. The proposed formulas are simple enough to be used in many practical applications as, e.g., in an amplitude variation with offset (AVO) analysis. They can be implemented for taking into account the angle dependence of transmission effects, or they can be used in an inversion for statistical parameters of sediments.

The effect of diamagnetic drift on the boundary conditions in tokamak scrape-off layers and the distribution of plasma fluxes near the target

The standard model for the two-dimensional simulation of the scrape-off layer (SOL) in the divertor is based on the equations for anomalous cross-field particle and energy transport and classical parallel transport. Recently, it has been complemented by the fluid equations of Braginskii to include all classical fluxes in a self-consistent way. The main physical effect of introducing classical fluxes is the appearance of two new terms-an electrical E*B drift and a diamagnetic B* Del P drift, as well as B* Del T contributions to the energy transport. Although the new models provide an adequate consideration of all the classical fluxes inside the plasma, a mistake has typically been made in the literature in formulating the new boundary conditions. The poloidal component of the B* Del Pi drift, existing inside the plasma, is assumed to continue through to the target surface, thus altering the Bohm criterion. Likewise, the poloidal current due to the B* Del (Pi+Pe) term is assumed to reach the surface, changing the surface electric potential drop. In this paper we demonstrate, however, that the diamagnetic fluxes in the whole plasma, including the SOL region, in their major part can be represented by the curl of a vector and, therefore, are self-terminating. We have made a detailed analysis of the distribution of plasma pressure and fluxes at the boundary between the plasma and the surface, encompassing the magnetic presheath and Debye sheath layers. After entering the presheath entrance, the poloidal component of the diamagnetic flux is diverted in the direction along the material surface due to the sharp pressure gradient in the presheath region. This creates boundary drift flows along the surface. Because of that conversion of the poloidal diamagnetic fluxes into boundary fluxes they do not deliver particles and current to the surface and therefore must not be included in the modified boundary conditions. Kinetic analysis of the distribution of the fluxes near the target also reveals an additional ExB drift in the radial direction, which mainly affects the ion motion in the magnetic presheath and Debye sheath layers. This drift is expected to shift the density profile of the plasma near the target in the radial direction a distance of the order of the ion poloidal Larmor radius.

Experimental study of viscosity effects on air‐gun signatures

In computer modeling of air guns the theory is often based on the assumption of an ideal freely oscillating spherical air bubble in an ideal fluid. Theoretical and experimental air‐gun signatures do not match perfectly. Synthetic signatures are under‐damped compared to real signatures. Several physical effects have been proposed as an explanation of this mismatch, among them viscosity effects. The viscosity of the surrounding liquid may contribute to the damping of the bubble oscillation. The numerical value of the viscosity of the surrounding liquid has to be increased considerably beyond the actual value of water to obtain sufficient damping of the synthetic signatures. We therefore performed an experiment to study the effect upon the pressure signature from an air gun when changing the viscosity of the surrounding medium. The motivation for the experiment was to quantify the influence of viscous terms on the output pressure waveform from an air gun. The experiment was carried out in an [Formula: see text] tank. The source was a BOLT 600 B air gun with a firing chamber of [Formula: see text] [Formula: see text]. The gun was placed at 0.5 m depth, and the hydrophone was placed 0.22 m from the gun ports. This configuration was kept constant during the experiment. We observed changes in the output pressure waveform generated by the gun at different liquid viscosities. Sets of five signatures recorded at 12 viscosity values in the range 6–723 centipoise, were analyzed. The effects on the pressure signature when increasing the viscosity of the liquid surrounding the gun are to decrease the primary to bubble ratio and the bubble period; thus, the opposite of what one should expect. We therefore conclude that viscosity is not the main physical effect that explains the damping of an air gun signature.

On an inertial observer's interpretation of the detection of radiation by linearly accelerated particle detectors

The detection of radiation by linearly accelerated particle detectors is discussed from the point of view of an inertial observer. An alternative interpretation to that of Unruh (1976), is presented. It is argued that the main physical effect is the emission of negative energy (as compared with Minkowski space) by the detector. This is shown to be the only important effect for 'macroscopic' detectors.

Weakly nonlinear theory of coherent Langmuir waves. I. Wave wave interaction

A weakly nonlinear analysis of the Vlasov equation is made in the case of small amplitude Langmuir waves. The nonlinear terms are treated as a small perturbation in the framework of the asymptotic theory of Krylov and Bogoliubov. In order to apply this method, the Vlasov system of equations is transformed by taking as new unknown functions a complete system of constants of the motions of the linearized equations. Such a system can be found with the help of essentially Van Kampen’s and Case’s expansion in normal modes. Resonant wave particle interaction is avoided by cutting off the distribution functions in velocity space and by considering waves of phase velocities larger than the cutoff. In this case, the main physical effect is mode coupling and Davidson’s nonlinear system of equations on the wave amplitudes is recovered. The main point is that the derivation is made without neglecting the free streaming portions of the distribution functions. A discussion of the validity of the approximation and of the relevant time scales is presented.