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

Smallest cyclically covering subspaces of F-q(n), and lower bounds in Isbell's conjecture

For a prime power q and a positive integer n, we say a subspace U of Fqn is cyclically covering if the union of the cyclic shifts of U is equal to Fqn. We investigate the problem of determining the minimum possible dimension of a cyclically covering subspace of Fqn. (This is a natural generalisation of a problem posed in 1991 by the first author.) We prove several upper and lower bounds, and for each fixed q, we answer the question completely for infinitely many values of n (which take the form of certain geometric series). Our results imply lower bounds for a well-known conjecture of Isbell, and a generalisation theoreof, supplementing lower bounds due to Spiga. We also consider the analogous problem for general representations of groups. We use arguments from combinatorics, representation theory and finite field theory.PostprintPeer reviewe

On Nori's Fundamental Group Scheme

We determine the quotient category which is the representation category of the kernel of the homomorphism from Nori's fundamental group scheme to its \'etale and local parts. Pierre Deligne pointed out an error in the first version of this article. We profoundly thank him, in particular for sending us his enlightning example reproduced in Remark 2.4 2).Comment: 29 page

Can we predict the duration of an interglacial?

Differences in the duration of interglacials have long been apparent in palaeoclimate records of the Late and Middle Pleistocene. However, a systematic evaluation of such differences has been hampered by the lack of a metric that can be applied consistently through time and by difficulties in separating the local from the global component in various proxies. This, in turn, means that a theoretical framework with predictive power for interglacial duration has remained elusive. Here we propose that the interval between the terminal oscillation of the bipolar seesaw and three thousand years (kyr) before its first major reactivation provides an estimate that approximates the length of the sea-level highstand, a measure of interglacial duration. We apply this concept to interglacials of the last 800 kyr by using a recently-constructed record of interhemispheric variability. The onset of interglacials occurs within 2 kyr of the boreal summer insolation maximum/precession minimum and is consistent with the canonical view of Milankovitch forcing pacing the broad timing of interglacials. Glacial inception always takes place when obliquity is decreasing and never after the obliquity minimum. The phasing of precession and obliquity appears to influence the persistence of interglacial conditions over one or two insolation peaks, leading to shorter (~ 13 kyr) and longer (~ 28 kyr) interglacials. Glacial inception occurs approximately 10 kyr after peak interglacial conditions in temperature and CO2, representing a characteristic timescale of interglacial decline. Second-order differences in duration may be a function of stochasticity in the climate system, or small variations in background climate state and the magnitude of feedbacks and mechanisms contributing to glacial inception, and as such, difficult to predict. On the other hand, the broad duration of an interglacial may be determined by the phasing of astronomical parameters and the history of insolation, rather than the instantaneous forcing strength at inception

Electrical and structural characterisation of plasma-polymerized TEOS thin films as humidity sensors

In this study, we used plasma polymerization of TEOS to deposit thin water molecule sensitive layers on two intredigitated aluminum electrodes evaporated on glass substrate. Electrical and structural analyses of the deposited sensitive layers have been evaluated through current-impedance responses and FTIR spectroscopy. The elaborated humidity resistive sensor exhibited a detectable response to relative humidity (RH) percentages ranging from 20 to 95%. The films showed good sensitivity to water molecule due to the presence of hydroxyl groups OH. These groups provide the adsorption sites for water and play an important role to the humidity sensor properties. The low impedance, good sensitivity as characterized by a linear change in impedance from 106 to 103 Ω over RH interval of 20–80% and low observed hysteresis of about 4%, make the elaborated layer a promising candidate for humidity sensors development.In this study, we used plasma polymerization of TEOS to deposit thin water molecule sensitive layers on two intredigitated aluminum electrodes evaporated on glass substrate. Electrical and structural analyses of the deposited sensitive layers have been evaluated through current-impedance responses and FTIR spectroscopy. The elaborated humidity resistive sensor exhibited a detectable response to relative humidity (RH) percentages ranging from 20 to 95%. The films showed good sensitivity to water molecule due to the presence of hydroxyl groups OH. These groups provide the adsorption sites for water and play an important role to the humidity sensor properties. The low impedance, good sensitivity as characterized by a linear change in impedance from 106 to 103 Ω over RH interval of 20–80% and low observed hysteresis of about 4%, make the elaborated layer a promising candidate for humidity sensors development

Very low R/sub ON/ measured on 4H-SiC accu-MOSFET high power device

International audienceThis paper describes the I-V characteristics obtained from a 4H-SiC current limiting device. Some specific aspects of the specific on-resistance are discussed in simulation with the DESSIS ISE software. The device behaviors place it in the field of the best Implanted Channel MOSFET (IC-MOSFET) obtained in the literature. The best on-resistance measured is 13 mWcm 2 and the saturation current density reaches 900 Acm-2

Essai de fabrication au laboratoire de canules en matières plastiques pour fistules permanentes des réservoirs digestifs

Brunaud M., Raynaud Paul. Essai de fabrication au laboratoire de canules en matières plastiques pour fistules permanentes des réservoirs digestifs. In: Bulletin de l'Académie Vétérinaire de France tome 109 n°8, 1956. pp. 373-383

Fistulisation permanente du rumen du mouton par la mise en place en une seule opération d’une canule en chlorure de polyvinyle

Brunaud M., Raynaud Paul. Fistulisation permanente du rumen du mouton par la mise en place en une seule opération d’une canule en chlorure de polyvinyle. In: Bulletin de l'Académie Vétérinaire de France tome 109 n°8, 1956. pp. 385-392

Simulation of electron energy loss spectra with the turboEELS and thermo-pw codes

For some materials like noble metals, electron energy loss spectra have a complex structure that makes them difficult to analyze without the help of ab initio calculations. Various theoretical approaches can be used for this purpose, among which the time-dependent density functional perturbation theory (TDDFPT) which has been widely used to study plasmons in a number of bulk and surface systems. In the present paper we present a comparison of the results and performance of two different numerical implementations of TDDFPT: the Sternheimer and Liouville-Lanczos methods. The former approach is implemented in the thermo-pw module and the latter one in the turboEELS code of the QUANTUM ESPRESSO package for electronic structure calculations. In the present paper a comparison is made for bulk bismuth, a semimetal, taking into account spin-orbit coupling, as well as for bulk gold, a noble metal. We show that for these two examples, both codes gives identical results and the turboEELS code has a better performance than the thermo-pw code, and point out in which cases the usage of thermo-pw alone or of both codes can be advantageous