Anomalous Multiphoton Photoelectric Effect in Ultrashort Time Scales

International audienceIn a multiphoton photoelectric process, an electron needs to absorb a given number of photons to escape the surface of a metal. It is shown for the first time that this number is not a constant depending only on the characteristics of the metal and light, but varies with the interaction duration in ultrashort time scales. The phenomenon occurs when electromagnetic energy is transferred, via ultrafast excitation of electron collective modes, to conduction electrons in a duration less than the electron energy damping time. It manifests itself through a dramatic increase of electron production. A basic hypothesis of the photoelectric process is that the photoemissive properties of matter remain unaltered during the interaction with light. Light-metal coupling is tacitly assumed as a perturbation of the electron population that remains in equilibrium during the interaction. Now, it has recently been shown that transient nonequilibrium electron states can exist in ultrashort time scales, in particular , when electromagnetic energy is transferred from a laser pulse to conduction electrons in a lapse of time shorter than the electron-phonon energy transfer duration [1– 4]. In this Letter, we address the basic question of whether the photoemissive properties of a metal can be modified through ultrafast energy transfer and nonequilib-rium electron heating. In a metallic electron gas, transient density disturbances can result in electron collective oscillation modes in the volume and near the surface. Under certain conditions, these so-called surface plasmon (polariton) modes can be excited by light [5,6]. In the case of thin metal films, the surface plasmon modes on the two surfaces can be coupled [7–9] and energy can be transferred from one surface plasmon mode to the other [10]. Collective electron oscillations can exist as well at the interface [11] between two perfect metals due to symmetry breaking at the metal-metal interface. Furthermore, interface and surface plas-mon modes can be coupled [12] in a bilayer metal system made of a metal M 1 (of electron density n 1) covered by a thin metallic layer M 2 (of electron density n 2 < n 1). If the overlayer metal M 2 is thin enough, the field of the surface plasmon can tunnel through the M 2 bulk and excite electron density fluctuations at the interface between the two metals (see Fig. 1). If the metal overlayer is too thick, the field of the surface plasmon must tunnel through too large a distance to excite the density fluctuations between the two metals. Conversely, if it is too thin, the surface plasmon amplitude is damped because of increasing coupling between the two opposite faces of the overlayer. There exists therefore an optimum thickness of the overlayer for which the amplitude of the induced interface plasmon is maximum. An interesting consequence of the interface or surface plasmon coupling effect is that the electron population in the metal overlayer can be in transient nonequilibrium energy states through ultrafast energy transfer from the coupled interface and surface plasmons. Actually, the conduction electrons near the surface and the metal-metal interface experience an effective nonlinear low-frequency force, the so-called ponderomotive force [13,14], resulting from the strongly inhomogeneous high-frequency field of the plasmons, and are accelerated toward regions of decreasing field amplitude. The ponderomotive force plays the role of an applied electrostatic force that transfers electromagnetic energy in a coherent way to an electron population, in contrast with stochastic energy transfer via thermal heating. The maximum energy that can be transferred to a free electron with initial energy E 0 through ponderomotive acceleration in a strong oscillating electri

Ponderomotive Acceleration of Photoelectrons in Surface-Plasmon-Assisted Multiphoton Photoelectric Emission

International audiencePhotoelectrons emitted from a gold target via a surface-plasmon-assisted multiphoton photoelectric process under a femtosecond laser pulse of moderate intensity are much more energetic than in an ordinary photoeffect without electron collective excitation. The phenomenon is interpreted in terms of time-dependent ponderomotive acceleration of the particles by the resonant field localized at the metal surface. The amplitude of the plasmon resonance may be directly estimated by means of the electron energy spectra. The development of powerful lasers more than three decades ago has allowed the investigation of the generalization of the classical photoelectric emission from metals to processes involving the absorption of several photons [1]. In recent years, the advent of laser pulses of ultra-short duration has favored studies in the femtosecond time regime [2]. These investigations can lead to the creation of new high-current ultrafast electron sources. Experimental studies have revealed that the electron emission rate can be greatly enhanced by the excitation of collective electron modes of the metal, the so-called surface plasmons [3,4]. The increase of the photoelectric signal can be qualitatively explained in terms of an assisted photoelectric effect where the energy of femtosecond light pulses is stored by the surface plasmon, creating a hot-electron population that does not have enough time to transfer its energy to the crystal lattice. While the presence of a surface-plasmon excitation is efficient in increasing the production of photoelectrons, an important open question is how the energy of the emitted electrons in such a " surface-plasmon-assisted " photoelectric process may differ from the energy predicted by the familiar photoelectric equation generalized to multiphoton processes. In this Letter, we show that the photoelectron energy is strongly affected by the surface-plasmon field, the modification from the classical values depending on the characteristics of the plasmon resonance. This fact may be easily understood by considering a simple analysis of the photo-electron behavior in the inhomogeneous high-frequency electric field surrounding the metal surface. The analysis involves simple classical concepts such as the notion of time-dependent ponderomotive effects, which have been successfully used in the context of multiphoton ionization of atoms in high-intensity lasers [5]. Consider an electron released from the metal surface after having absorbed a required number n of photons from the laser beam to overcome the work function W of the metal. While traveling in the vacuum dressed by the high-frequency field E sp of the surface plasmon, the total energy of the electron consists of the sum of its kinetic energy § n (given by the Einstein multiphoton photoelectric equation § n ෇ n ¯ hv 2 W) and its quiver energy U sp ෇ e 2

Electron acceleration by surface plasma waves in the interaction between femtosecond laser pulses and sharp-edged overdense plasmas

International audienceThe relativistic acceleration of electrons by the field of surface plasma waves created in the interaction between ultrashort high-intensity laser pulses with sharp-edged overdense plasmas has been investigated. It is shown that the initial phase of the wave experienced by the electrons play a leading part by yielding a well-defined peaked structure in the energy distribution function. This study suggests that resonant excitation of surface plasma waves could result in quasi-monokinetic energetic electron bunches. When the space charge field becomes too strong, this mechanism can evolve toward a true absorption process of the surface wave energy via an enhanced ''vacuum heating'' mechanism generalized to the case of surface plasma waves

Strongly enhanced laser absorption and electron acceleration via resonant excitation of surface plasma waves

International audienceTwo-dimensional (2D) particle-in-cell numerical simulations of the interaction between a high-intensity short-pulse p-polarized laser beam and an overdense plasma are presented. It is shown that, under appropriate physical conditions, a surface plasma wave can be resonantly excited by a short-pulse laser wave, leading to strong relativistic electron acceleration together with a dramatic increase, up to 70%, of light absorption by the plasma. Purely 2D effects contribute to enhancement of electron acceleration. It is also found that the angular distribution of the hot electrons is drastically affected by the surface wave. The subsequent ion dynamics is shown to be significantly modified by the surface plasma wave excitation

Consensus on circulatory shock and hemodynamic monitoring. Task force of the European Society of Intensive Care Medicine.

OBJECTIVE: Circulatory shock is a life-threatening syndrome resulting in multiorgan failure and a high mortality rate. The aim of this consensus is to provide support to the bedside clinician regarding the diagnosis, management and monitoring of shock. METHODS: The European Society of Intensive Care Medicine invited 12 experts to form a Task Force to update a previous consensus (Antonelli et al.: Intensive Care Med 33:575-590, 2007). The same five questions addressed in the earlier consensus were used as the outline for the literature search and review, with the aim of the Task Force to produce statements based on the available literature and evidence. These questions were: (1) What are the epidemiologic and pathophysiologic features of shock in the intensive care unit ? (2) Should we monitor preload and fluid responsiveness in shock ? (3) How and when should we monitor stroke volume or cardiac output in shock ? (4) What markers of the regional and microcirculation can be monitored, and how can cellular function be assessed in shock ? (5) What is the evidence for using hemodynamic monitoring to direct therapy in shock ? Four types of statements were used: definition, recommendation, best practice and statement of fact. RESULTS: Forty-four statements were made. The main new statements include: (1) statements on individualizing blood pressure targets; (2) statements on the assessment and prediction of fluid responsiveness; (3) statements on the use of echocardiography and hemodynamic monitoring. CONCLUSIONS: This consensus provides 44 statements that can be used at the bedside to diagnose, treat and monitor patients with shock

Resonant coupling between surface and interface plasma waves in high-density sharp-edged plasmas produced by ultrafast laser pulses

International audienceWe consider the interaction between an ultrashort laser pulse and a hot high-density sharp-edged laser-created plasma resulting from a microstructured target with a double-step density profile. We demonstrate that an electron plasma wave can be resonantly driven at the density jump between the two plasmas by the field of a laser-excited surface wave. Two different excitation regimes can exist depending on the wavelength and the angle of incidence of the laser. This effect may have interesting experimental consequences in hot electron generation and x-ray emission due to breaking of the resonant plasma wave. S1063-651X9906904-

Resonant coupling between interface plasmons and surface plasmons at the junction between two simple metals

International audienceThe classic problem of the existence of genuine interface plasmons at the junction between two free-electron metals is revisited. By applying Bloch's standard hydrodynamical model for the homogeneous free-electron gas to the case of a bi-metal system, we demonstrate that localized interface plasmons resonantly coupled to surface plasmons do exist. It is shown that the amplitude of the resonant field at the metal-metal interface strongly depends on the layer thickness and disappears for a semi-infinite system. 0 1998 Elsevier Science Ltd. All rights reserved Keywords: A. metals. A. surfaces and interfaces, D. dielectric response, D. tunnelling. The study of the dynamical dielectric response of coated metals has attracted renewed attention [l] because of its interest in topics like growth of oxide layers on metallic surfaces, optical properties of nonuniform metal films and electronic properties of metal-metal junctions. While these questions have been investigated for a rather long time [2], no definite answers have been given, even at the macroscopic level, to the conceptually simple problem of the collective electron behavior at the interface between two different metals. The question of the existence of genuine interface plasmon modes was first considered by Stem and Ferrel [3] for the simplest model of a planar interface between two semi-infinite free-electron metals. In their description , the metals are characterized by (Drude) dielectric constants ~1 = 1-wj,/w2 and ~2 = 1-wi21w2, where wpl and wp2 denote their respective plasma frequencies. The corresponding plasmon fields thus represent collective electron oscillations of angular frequency w = [~o,$ + w;2)11'27 decaying exponentially on both sides of the interface. The Stern-Femel problem was generalized later on by Gadzuk [4] to waves propagating along the interface. In addition to the Stem-Ferrel modes that he recovered in the case of an overlayer of large thickness, he gave a particular solution for the dispersion relation of the coupled modes taking dispersion effects into account and showed that, for a finite overlayer, the surface plasmon and interface plasmon may interfere. However, the existence of the Stem-Ferrel modes in dispersive media still remains controversial. Several authors have reexamined the problem and claimed about the non-existence of these localized interface plasmons when dispersion is taken into account [5]. Besides, the influence of particular choices of boundary conditions [6] has been thoroughly examined and validity of the Bloch hydrodynamical model has been questioned [71. In this paper, we shall reconsider the surface/ interface plasmon coupling problem initiated by Gadzuk [4] with a different approach. Thus, instead of assuming the form of a particular solution, we shall start from the basic equations, using again the hydrodynamical approach (which was the original theoretical framework within which the surface plasmon modes were discovered, indeed) and we shall deduce that localized interface plasmons do exist as the result of coupling with the field of the surface plasmons. We shall then see that the existence of this effect is actually strongly dependent on the layer thickness, so that these stimulated interface plasmons no longer exist for semi-infinite metal 78

Collective surface modes in small spherical metallic systems within the Bloch-Jensen hydrodynamical model

International audienceAn exact generalization of the classical Blach-lensen hydrodynamical model for the homogeneous freeelectron gas is oboined for a two-layer spherical metal system. The dispersion relation of surface plasmons together with the analytical expression of the self-consistent electric field are calculated exactly. It is found that small and large systems exhibit notably different behaviours. In particular, the surface-plasmon frequencies stmngly depend on the layer hihickness. As is the case for the metal-vacuum interface, the electric field is shown to present a sharp maximum at the interface between the two metals. A simple physical interpretation of this effeit is provided. The present two-layer system also gives interesting indications about the part played in collective surface modes by the ion-density profile ne% the surface of small metal clusters, which may be helpful ininterpretingrecent puuling experimental data