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Showing papers on "Free electron model published in 2002"


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the transient evolution of the distribution function of the electron gas in a metal during and after irradiation with a sub-picosecond laser pulse of moderate intensity.
Abstract: Irradiation of a metal with an ultrashort laser pulse leads to a disturbance of the free-electron gas out of thermal equilibrium. We investigate theoretically the transient evolution of the distribution function of the electron gas in a metal during and after irradiation with a subpicosecond laser pulse of moderate intensity. We consider absorption by inverse bremsstrahlung, electron-electron thermalization, and electron-phonon coupling. Each interaction process is described by a full Boltzmann collision integral without using any relaxation-time approach. Our model is free of phenomenological parameters. We solve numerically a system of time- and energy-dependent integro-differential equations. For the case of irradiation of aluminum, the results show the transient excitation and relaxation of the free-electron gas as well as the energy exchange between electrons and phonons. We find that laser absorption by free electrons in a metal is well described by a plasmalike absorption term. We obtain a good agreement of calculated absorption characteristics with values experimentally found. For laser excitations near damage threshold, we find that the energy exchange between electrons and lattice can be described with the two-temperature model, in spite of the nonequilibrium distribution function of the electron gas. In contrast, the nonequilibrium distribution leads at low excitations to a delayed cooling of the electron gas. The cooling time of laser-heated electron gas depends thus on excitation parameters and may be longer than the characteristic relaxation time of a Fermi-distributed electron gas depending on internal energy only. We propose a definition of the thermalization time as the time after which the collective behavior of laser-excited electrons equals the thermalized limit.

417 citations


Journal ArticleDOI
TL;DR: In this paper, free and trapped charge carriers in polycrystalline TiO2 following band gap irradiation are characterized by diffuse reflectance IR spectroscopy (DRIFTS), indicating the presence of free conduction band electrons coupled with acoustic phonons in the lattice.
Abstract: Free and trapped charge carriers in polycrystalline TiO2 following band gap irradiation are characterized by diffuse reflectance IR spectroscopy (DRIFTS). A spectrum-wide absorption signal proportional to λ1.7 (λ = wavelength/μm) indicates the presence of free conduction band electrons coupled with acoustic phonons in the lattice. Free electrons appear to decay according to saturation kinetics. The fitted parameters indicate a limited number of trapping states. The concentration of these states appears to be diminished by sequential UV treatments. The free carrier decay lifetime is lengthened as the samples are dehydrated, which suggests an excited-state relaxation event during electron trapping. Photogenerated free electrons are comparable to conduction band electrons injected from surface-bound chromophores, and the lifetime of these electrons can be extended across several orders of magnitude. A broad IR absorption peak centered at 3380 cm-1 is attributed to an electronic transition from an occupied su...

192 citations


Journal ArticleDOI
25 Jul 2002-Nature
TL;DR: This experiment uses an electron field emitter to coherently illuminate two detectors, and finds anticorrelations in the arrival times of the free electrons, which represents the fermionic twin of the Hanbury Brown–Twiss effect for photons.
Abstract: Fluctuations in the counting rate of photons originating from uncorrelated point sources become, within the coherently illuminated area, slightly enhanced compared to a random sequence of classical particles. This phenomenon, known in astronomy as the Hanbury Brown-Twiss effect, is a consequence of quantum interference between two indistinguishable photons and Bose Einstein statistics. The latter require that the composite bosonic wavefunction is a symmetric superposition of the two possible paths. For fermions, the corresponding two-particle wavefunction is antisymmetric: this excludes overlapping wave trains, which are forbidden by the Pauli exclusion principle. Here we use an electron field emitter to coherently illuminate two detectors, and find anticorrelations in the arrival times of the free electrons. The particle beam has low degeneracy (about 10(-4) electrons per cell in phase space); as such, our experiment represents the fermionic twin of the Hanbury Brown-Twiss effect for photons.

183 citations


Journal ArticleDOI
TL;DR: The results show that the temporal coherence of light pulses is transferred to free electron wave packets, thus opening the door to a whole variety of exciting experiments.
Abstract: Interferences of free electron wave packets generated by a pair of identical, time-delayed, femtosecond laser pulses which ionize excited atomic potassium have been observed. Two different schemes are investigated: threshold electrons produced by one-photon ionization with parallel laser polarization and above threshold ionization electrons produced by a two-photon transition with crossed laser polarization. Our results show that the temporal coherence of light pulses is transferred to free electron wave packets, thus opening the door to a whole variety of exciting experiments.

138 citations


Journal ArticleDOI
TL;DR: In this article, optical transmittance spectra of In2O3 : Sn (ITO) films were simulated with a computer program based on dielectric modelling, and the parameters of these excitations were evaluated as a function of the carrier density.
Abstract: Optical transmittance spectra of In2O3 : Sn (ITO) films were simulated with a computer program based on dielectric modelling The films were prepared by radiofrequency sputtering under various oxygen fluxes such that the carrier density varies from 3×1019 to 15×1021 cm-3 The dielectric function used is the sum of three types of electronic excitations: intraband transitions of free electrons (Drude model), band gap transitions, and interband transitions into the upper half of the conduction band The parameters of these excitations are evaluated as a function of the carrier density The damping in the Drude term was modelled frequency-dependent to account for the low extinction coefficient observed in the visible spectral range The parameters resulting from the optical measurements were compared with those from the electrical measurements Both the optical mobility and carrier density are found to be higher than those of the respective electric parameters These discrepancies are attributed to a pronounced microstructure with badly conducting grain boundaries The refractive index at 550 nm decreases linearly with increasing electron concentration This is due both to the shift of the plasma edge and the Burstein-Moss shift of the band edge All band gap transitions go up to the Fermi level

130 citations


Journal ArticleDOI
TL;DR: In this paper, the behavior of the metallic state of lightly electron-doped SrTiO3 has been studied using angle-integrated ultraviolet photoemission spectroscopy (UPS) and angle-resolved photo-emission (ARPES).

94 citations


Journal ArticleDOI
TL;DR: In this article, a detailed calculation of the free electron concentration and conduction and valence band edges of AlGaN/GaN heterojunction field effect transistors is presented.
Abstract: A detailed calculation of the free electron concentration and conduction and valence band edges of AlGaN/GaN heterojunction field-effect transistors is presented. The model is based on a self-consistent solution of the Schrodinger, Poisson, and charge balance equations and includes the effect of exchange correlation on the Coulomb interaction. It also includes surface acceptor and donor states populated according to Fermi–Dirac statistics. The piezoelectric and spontaneous polarization discontinuities across the material interfaces are rigorously taken into account. The influence of the polarization discontinuity on the magnitude of the charge in the two-dimensional electron gas is investigated. From charge conservation, it is shown that the polarization discontinuity does not behave as dopants in the same manner as substitutional impurities. Any free electrons within the structure must originate from some other source, either from the surface through surface donors, or from the bulk through unintentional...

81 citations


Journal ArticleDOI
TL;DR: In this paper, a femtosecond breakdown model is combined with the classical rate equations to determine both time and position-dependent electron density during femto-cond optical breakdown in water.
Abstract: Optical breakdown by ultrashort laser pulses in dielectrics presents an efficient method to deposit laser energy into materials that otherwise exhibit minimal absorption at low laser intensities. During optical breakdown, a high density of free electrons is formed in the material, which dominates energy absorption, and, in turn, the material removal rate during ultrafast laser-material processing. Classical models assume a spatially uniform electron population and constant laser intensity in the focal region, which results in time-dependent expressions only, i.e., the rate equations, to predict electron evolution induced by nanosecond and picosecond pulses. For femtosecond pulses, however, the small spatial extent of the pulse requires that the pulse propagation be considered, which results in an inhomogeneous plasma and localized electron formation during optical breakdown. In this work, a femtosecond breakdown model is combined with the classical rate equations to determine both time- and position-dependent electron density during femtosecond optical breakdown in water. The model exhibits good agreement when compared with experimental results. For other transparent or moderately absorbing dielectric media, the model also shows promise for determining the time- and position-dependent electron evolution induced by ultrashort laser pulses. Another interesting result is that the maximum electron density formed during femtosecond-laser-induced optical breakdown may exceed the conventional limit imposed by the plasma frequency.

66 citations


Journal ArticleDOI
TL;DR: The results of a numerical simulation to the Schrödinger equation are consistent with the experimental data and point out several applications in atom optics that could be realized in electron optics.
Abstract: Bragg scattering has been observed for free electrons using a standing wave of light. Both the rocking curve and the angular electron distribution have been measured. The results of a numerical simulation to the Schrodinger equation are consistent with our experimental data. Unlike the diffraction regime which uses thin crystals, the Bragg regime requires the use of thick crystals. We point out several applications in atom optics that could be realized in electron optics.

66 citations


Journal ArticleDOI
TL;DR: In this article, a three-dimensional test particle simulation model that solves the relativistic Newton-Lorentz equations of motion in analytically specified laser fields is studied.
Abstract: The interaction of free electrons with intense laser beams in vacuum is studied using a three-dimensional test particle simulation model that solves the relativistic Newton–Lorentz equations of motion in analytically specified laser fields. Recently, a group of solutions was found for very intense laser fields that show interesting and unusual characteristics. In particular, it was found that an electron can be captured within the high-intensity laser region, rather than expelled from it, and the captured electron can be accelerated to GeV energies with acceleration gradients on the order of tens of GeV/cm. This phenomenon is termed the capture and acceleration scenario (CAS) and is studied in detail in this article. The accelerated GeV electron bunch is a macropulse, with duration equal to or less than that of the laser pulse, which is composed of many micropulses that are periodic at the laser frequency. The energy spectrum of the CAS electron bunch is presented. The dependence of the energy exchange in...

63 citations


Journal ArticleDOI
TL;DR: In this article, high harmonics generated due to the scattering of relativistic electrons from high intensity laser light is studied in an Nd:glass laser system with a peak intensity of 2×1018 W cm−2 in underdense plasma.
Abstract: High harmonics generated due to the scattering of relativistic electrons from high intensity laser light is studied. The experiments are carried out with an Nd:Glass laser system with a peak intensity of 2×1018 W cm−2 in underdense plasma. It is shown that, at high intensities, when the normalized electric field approaches unity, in addition to the conventional atomic harmonics from bound electrons there is significant contribution to the harmonic spectrum from free electrons. The characteristic signatures of this are found to be the emission of even order harmonics, linear dependence on the electron density, significant amount of harmonics even with circular polarization and a much smaller spatial region over which these harmonics are produced as compared to the atomic case. Imaging of the harmonic beam shows that it is emitted in a narrow cone with a divergence of 2 to 3 degrees.

Journal ArticleDOI
TL;DR: In this article, a density functional study of zigzag carbon nanotubes containing potassium atoms inside is presented. And the effect of potassium doping is not simple charge transfer. But the nearly free electron state of nanotube couples with the $\mathrm{K} 4s$ orbital.
Abstract: We present a density functional study of zigzag carbon nanotubes containing potassium atoms inside. We find that the effect of potassium doping is not simple charge transfer. The nearly free electron state of nanotube couples with the $\mathrm{K} 4s$ orbital. The state comes downward as the tube is thicker and crosses the Fermi level when the diameter is as large as $0.8--1\mathrm{nm}$. It is distributed near the center of the tube and extends to the tube direction, which suggests high conductivity via this state.

Proceedings ArticleDOI
TL;DR: In this paper, the authors used a rate equation model considering multiphoton ionization and avalanche ionization to numerically simulate the temporal evolution of the free electron density during the laser pulse for a given irradiance, and to calculate the irradiance dependence of free-electron density and volumetric energy density reached at the end of a laser pulse.
Abstract: The irradiance threshold for femtosecond optical breakdown in aqueous media is approximately equals 1.0x1013W cm-2. At the breakdown threshold, a plasma with a free electron density of about 1021cm-3 is generated, and the energy density in the breakdown region is sufficiently high to cause the formation of a bubble which can be experimentally observed. We found previously that plasmas with a free electron density <1021cm-3 are formed also in a fairly large irradiance range below the breakdown threshold. The present study investigates the chemical, thermal, and thermomechanical effects produced by these low-density plasmas. We use a rate equation model considering multiphoton ionization and produced by these low-density plasmas. We use a rate equation model considering multiphoton ionization and avalanche ionization to numerically simulate the temporal evolution of the free electron density during the laser pulse for a given irradiance, and to calculate the irradiance dependence of the free-electron density and volumetric energy density reached at the end of the laser pulse. The value of the energy density created by each laser pulse is then used to calculate the temperature distribution in the focal region after application of a single laser pulse and of series of pulses. The results of the temperature calculations yield, finally, the starting point for calculations of the thermoelastic stresses that are generated during the formation of the low-density plasmas. We found that, particularly for short wavelengths, a large 'tuning range' exists for the creation of spatially extremely confined chemical, thermal and mechanical effects via free electron generation through nonlinear absorption. Photochemical effects dominate at the lower end of this irradiance range, whereas at the upper end they are mixed with thermal effects and modified by thermoelastic stresses. Above the breakdown threshold, the spatial confinement is partly destroyed by cavitation bubble formation, and the laser-induced effects become more disruptive. Our simulations revealed that the highly localized ablation of intracellular structures and intranuclear chromosome dissection recently demonstrated by other researchers are probably mediated by free-electron- induced chemical bond breaking and not related to heating or thermoelastic stresses. We conclude that low density plasmas below the optical breakdown threshold can be a versatile tool for the manipulation of transparent biological media and other transparent materials. (enabling, e.g., the generation of optical waveguides in bulk glass). Low density plasmas may, however, also be a potential hazard in multiphoton microscopy and higher harmonic imaging.

Journal ArticleDOI
TL;DR: In this paper, variable-temperature longitudinal optical (LO) phonon-assisted luminescence spectra of free excitons and free electrons in heteroepitaxial GaN thin films have been conducted.
Abstract: Investigation of variable-temperature longitudinal optical (LO) phonon-assisted luminescence spectra of free excitons and free electrons in heteroepitaxial GaN thin films has been conducted. It is found that thermal broadening of the LO phonon-assisted photoluminescence peaks is much slower than those of the peaks of their parents so that the first-order LO peaks of the free exciton transition and the band-to-band transition can be well resolved even at room temperature, leading to a direct determination of the band A free exciton binding energy as 25.4±0.9 meV. At the same time, we demonstrate that the simple hydrogenlike model still is a good approximation to describe the energy level structure of free excitons in GaN.

Journal ArticleDOI
TL;DR: In this article, electron holography was used to directly profile the potential and charge distribution across a GaN/In0.18Ga0.82N/GaN quantum well structure, and present an analysis of the fine-scale potential variations at the quantum well.
Abstract: Strong internal electric fields (of the order of MV/cm) influence the electronic properties and light emission process of InGaN quantum wells. We have used electron holography to directly profile the potential and charge distribution across a GaN/In0.18Ga0.82N/GaN quantum well structure, and present here an analysis of the fine-scale potential variations at the quantum well. The potential profiles show a drop of 0.6±0.2 V across, and an internal electric field of −2.2±0.6 MV/cm in the quantum well. An analysis based on Poisson’s equation suggests that the field is caused by electronic charges with a peak density of 8×1020 cm−3, corresponding to a sheet charge density of 0.027 C/m2. Free electron and hole densities of the order of 1020 cm−3 are confined separately in the quantum well. These free carriers screen only part of the electric field due to the polarization effect.

Journal ArticleDOI
TL;DR: In this article, the authors considered the use of 2D Bragg structures formed from doubly-corrugated waveguide sections of coaxial geometry for providing spatially coherent radiation from an annular electron beam.
Abstract: The use of two-dimensional (2D) distributed feedback is considered as a method of providing spatially coherent radiation from an oversized annular electron beam. To realize the feedback mechanism, 2D Bragg structures formed from doubly-corrugated waveguide sections of coaxial geometry are suggested. The properties of two types of coaxial cavities formed using such structures are compared: a single-section 2D Bragg cavity and a two-mirror cavity. The eigenmodes of both cavities are found and their high selectivity over both azimuthal and longitudinal indices was demonstrated. Time-domain analyses of the excitation of the cavities by an annular electron beam were carried out. The influence of the cavity parameters on the oscillation regime is analyzed and discussed. It was shown that for a specific set of 2D Bragg cavity parameters it is possible to obtain a regime of steady-state oscillations when the transverse size of the beam exceeds the wavelength by a few orders of magnitude, while outside this parameter space multimode oscillation takes place. The design of a 2D Bragg free-electron maser oscillator based on a high-current accelerator at the University of Strathclyde is discussed. (C) 2002 American Institute of Physics.

Journal ArticleDOI
TL;DR: In this paper, semi-empirical intermediate-neglect-of-differential-overlap (INDO) calculations for free electron polarons, single-triplet excitons and the excitonic phase in BaTiO3 perovskite crystal are presented.
Abstract: As an extension of our previous study on the electron polarons and excitons in KNbO3 and KTaO3 [1, 2], we present here results of semiempirical intermediate-neglect-of-differential-overlap (INDO) calculations for free electron polarons, single-triplet excitons and the excitonic phase in BaTiO3 perovskite crystal. Our INDO calculations confirm the existence of self-trapped electrons in BaTiO3. The corresponding lattice relaxation energy is 0.24 eV and the optical absorption energy 0.69 eV. An electron in the ground state occupies the t2g orbital of the Ti 3+ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn–Teller modes when four nearest equatorial O atoms are displaced by 1.53% a0 outwards in the x–y plane and another two nearest oxygens shift 1.1% inwards, along the z-axis. Our INDO calculations show that creation of charge-transfer vibronic exciton (CTVE) in BaTiO3 crystal is accompanied by a strong lattice distortion; the relevant energy gain due to CTVE formation is 2.2 eV. Moreover, our INDO calculations predict the existence of a new crystalline phase—that of CTVEs in BaTiO3 where strongly correlated CTVEs are located in each unit cell of a crystal. (Some figures in this article are in colour only in the electronic version)

Journal ArticleDOI
TL;DR: In this paper, the authors used the quantum interference phenomena to quantitatively measure the elec- tron phase-relaxation length and to probe long-range adsor-bate interactions.
Abstract: The electrons of the surface states on the (111) surfaces of the noble metals Au, Ag, and Cu form a quasi- two-dimensional (2D) free electron gas which is confined to the first few atomic layers at the crystal surface. They are scattered by the potential associated with surface de- fects, e.g. impurity atoms, adatoms, or step edges, lead- ing to quantum-interference patterns in the local density of states around these defects. We have used the quantum- interference phenomena to quantitatively measure the elec- tron phase-relaxation length and to probe long-range adsor- bate interactions.

Journal ArticleDOI
TL;DR: In this article, the problem of the on-and off-center D 0 and D -S-states in semiconductor heterostructures was reduced to the similar ones in an isotropic effective space with variable fractional dimension starting from the variational principle.
Abstract: We reduce the problems of the on- and off-center D 0 and D - S-states in semiconductor heterostructures to the similar ones in an isotropic effective space with variable fractional dimension starting from the variational principle. The dimension of this space is defined as a scaling parameter that relates the radii of a set of spherical boxes to the charge densities within induced by the free electron ground state in the heterostructure. Explicit expressions for the effective space dimensionality in a quantum well (QW), quantum-well wire (QWW) and a quantum dot (QD) are found by using this definition. To solve the wave equations for the free election ground state in the heterostructure and for the hydrogen-like atom S-states in the fractional-dimensional space, we use the numerical trigonometric sweep method. The three-parameter Hylleraas trial function is used to solve the similar problem for a negative-hydrogen-like ion in the effective space. Ground state binding energies for off-center neutral and negatively charged donors in QWs and spherical QDs are calculated. Our results are in a good agreement with those of the variational and Monte Carlo methods. In addition, novel results for the D - binding energy as a function of the cylindrical GaAs/Ga 0.7 Al 0.3 As QWW radius and the magnetic field intensity are presented. It is found that the D - binding energy in the wire increases from 0.055 Ry* up to about 1.230 Ry* as the radius decreases to 30 A. It is also shown that the magnetic field produces a considerable enhancement of negative-donor binding energy in QWW only for radii greater than 100 A.

Journal ArticleDOI
TL;DR: In this article, the radiative recombination of a free electron into excited states of bare, high-Z ions and the subsequent photon decay are studied in the framework of density matrix theory.
Abstract: The radiative recombination of a free electron into excited states of bare, high-Z ions and the subsequent photon decay are studied in the framework of the density matrix theory. Emphasis is placed, in particular, on the angular correlation between the recombination and the decay photons. The general expression for the photon–photon angular correlation function is derived, based on Dirac's equation as appropriate for high-Z ions. Computations for the dependence of the photon–photon correlation function on the nuclear charge and the projectile energies are carried out for the capture into the 2p3/2 level and the subsequent Lyman-α1 (2p3/2 → 1s1/2) radiation.

Journal ArticleDOI
TL;DR: In this paper, the authors developed a suitable theoretical framework for studying plasmons in nanotube bundles, which can also be applied to more sophisticated models for carbon nanotubes.
Abstract: We develop a suitable theoretical framework for studying plasmons in nanotube bundles. In the plane perpendicular to the tubes, the nanotubes form a two-dimensional lattice. We use a simple model of free electron gas confined to arrays of cylindrical surfaces, however the theoretical framework can also be applied to more sophisticated models for carbon nanotubes. Both intrasubband (classical) and intersubband (quantum) plasmons in such nanotube bundle systems are studied. Analytical solutions have been obtained for the case where only one subband is occupied, while numerical solutions have been obtained for the case of many occupied subbands. Intertube Coulomb coupling is found to play an intricate role, as it can both harden and soften plasmon modes in the same nanotube bundle. Intertube Coulomb coupling is also responsible for sizable plasmon dispersions in the transverse plane. All plasmons are found to be undamped by the corresponding single particle type electron-hole pair excitations. The classical plasmon exhibits three-dimensional character in the long wavelength limit, and one-dimensional character in the short wavelength limit. For quantum plasmons, the plasmon energy can be significantly larger than the corresponding single particle excitation energy between subbands. This feature is similar to quantum plasmons in semiconductor quantum wire systems.

Journal ArticleDOI
TL;DR: The photoluminescence of single InAs/GaAs self-assembled quantum dots for a range of excitation powers, excitation energies and sample temperatures 4 K30 K was studied in this paper.
Abstract: We study the photoluminescence of single InAs/GaAs self-assembled quantum dots for a range of excitation powers, excitation energies and sample temperatures 4 K30 K), this effect vanishes due to the essential decrease of the steady-state free electron concentration in the GaAs barrier as a result of thermally excited free holes appearing in the GaAs barrier valence band which provides an effective recombination channel for the free electrons. These experimental observations could be used as an effective tool to create and study charged excitons in quantum dots.

Journal ArticleDOI
TL;DR: In this paper, the absolute cross sections σe (E) for dissociative electron attachment to the dipolar molecules CH2Br2 and CCl3Br by normalization to known electron attachment rate coefficients were determined.
Abstract: Using the laser photoelectron attachment method with an energy width ≤ 1 meV at low energies (≤ 172 meV) and an electron beam method (energy width about 70 meV) at higher energies (up to 2 eV), we have determined absolute cross sections σe (E) for dissociative electron attachment to the dipolar molecules CH2Br2 and CCl3Br by normalization to known electron attachment rate coefficients. At thresholds for vibrational excitation of the CH2Br2 molecule, the DA cross section exhibits pronounced structure due to coupling of the attachment process with scattering channels; below the ν3 = 1 onset for the symmetric CBr2 stretch vibration, a clear vibrational Feshbach resonance is observed. At low energies the cross sections σe (E) show an energy dependence between E−1/2 and E−1, in essential agreement with predictions of an extended Vogt–Wannier (EVW) capture model which includes the long-range electron–dipole interaction in addition to the polarization force. The determined absolute values are, however, substantially smaller than those predicted by the EVW model. Semiempirical R-matrix calculations have been carried out which show that at low electron energies (E < 0.15 eV) Br− formation from CCl3Br proceeds by s-wave attachment to the anion ground state while a broad peak, observed around 0.6 eV and evolving predominantly into the Cl− channel, is due to an excited anion state. Comparisons are made with cross sections and rate coefficients obtained in previous photoelectron attachment work (TPSA) and in electron beam as well as swarm experiments. Based on our joint experimental results for σe (E), we report the electron temperature dependence of the rate coefficients ke (Te) for free electron attachment involving a Maxwellian electron ensemble and a gas at room temperature (TG = 300 K).

Journal ArticleDOI
TL;DR: In this article, band-gap luminescence from a variety of compound semiconductors has been observed under intense mid-infra-red irradiation by the free electron laser (FEL) at the Institute of Free Electron Laser, Osaka University.
Abstract: Band-gap luminescence from a variety of compound semiconductors has been observed under intense mid-infra-red irradiation by the free electron laser (FEL) at the Institute of Free Electron Laser, Osaka University. The FEL wavelength and power dependences of the FEL-induced luminescence intensity were measured, and the results are compared with those obtained by a full-band Monte Carlo simulation.

Journal ArticleDOI
TL;DR: This work presents the analysis of transmitted currents as a function of the incident energy, based on a model of spin polarization dilution into the first gold layer and ballistic transport close to the vacuum level throughout the sample.
Abstract: An asymmetrical ferromagnetic cobalt bilayer (18 nm Au/0.8 nm Co/2.2 nm Au/1.3 nm Co/1.5 nm Au) operates as a self-calibrated spin polarimeter with a high spin selectivity for free electrons injected at a few eV above the Fermi level. We present the analysis of transmitted currents as a function of the incident energy, based on a model of spin polarization dilution into the first gold layer and ballistic transport close to the vacuum level throughout the sample.

Proceedings ArticleDOI
17 Jun 2002
TL;DR: In this article, the authors investigated the chemical, thermal, and thermomechanical effects produced by low-density plasmas, and their consequences for multiphoton microscopy.
Abstract: Irradiance values employed in multiphoton microscopy are only one order of magnitude below the irradiance threshold for femtosecond optical breakdown in aqueous media (≈ 1.0 × 10 13 W cm -2 ). At the breakdown threshold, a plasma with a free electron density of about 10 21 cm -3 is generated, and the energy density in the breakdown region is sufficiently high to cause the formation of a bubble at the laser focus. We found previously that plasmas with a free electron density <10 21 cm -3 are formed also in a fairly large irradiance range below the breakdown threshold. The present study investigates the chemical, thermal, and thermomechanical effects produced by these low-density plasmas, and their consequences for multiphoton microscopy. We use a rate equation model considering multiphoton ionization and avalanche ionization to numerically simulate the plasma formation. The value of the plasma energy density created by each laser pulse is then used to calculate the temperature distribution in the focal region. The results of the temperature calculations yield, finally, the starting point for calculations of the thermoelastic stresses that are generated during the formation of the low-density plasmas. We found that with femtosecond pulses a large 'tuning range' exists for the creation of spatially extremely confined chemical, thermal and mechanical effects via free electron generation through nonlinear absorption. Photochemical effects dominate at the lower end of this irradiance range, whereas at the upper end they are mixed with thermal effects and modified by thermoelastic stresses. Above the breakdown threshold, the spatial confinement is partly destroyed by cavitation bubble formation, and the laser-induced effects become more disruptive. Our simulations revealed that the highly localized ablation of subcellular structures recently demonstrated by other researchers are probably mediated by free-electron-induced chemical bond breaking and not related to heating or thermoelastic stresses. At the irradiance values employed in multiphoton microscopy (≈1/20 of the breakdown threshold), the model predicts, for λ = 800 nm, an electron density of about 10 11 cm -3 , sufficient to produce free electrons in the focal volume. Multiphoton microscopy may, hence, be accompanied by chemical effects arising from these electrons. We conclude that low density plasmas below the optical breakdown threshold can be a versatile tool for the manipulation of transparent biological media and other transparent materials but may also be a potential hazard in multiphoton microscopy and higher harmonic imaging.

Journal ArticleDOI
TL;DR: In this article, an electron-gun based on a ferroelectric cathode was used in a free-electron maser (FEM) experiment, where the electrons are separated from the cathode surface plasma, and are accelerated in two stages.
Abstract: An electron-gun based on a ferroelectric cathode is studied in a free-electron maser (FEM) experiment. In this gun, the electrons are separated from the cathode surface plasma, and are accelerated in two stages. The electron energyspread is reduced sufficiently for an FEM operation in the microwave regime. A 14 keV, 1–2 A e-beam is obtained in a 0.1–2.1ms pulse width. The pulse repetition frequency attains 3.1 MHz in B50% duty-cycle. This gun is implemented in an FEM oscillator experiment operatingaround 3 GHz. The paper presents experimental results and discusses the applicability of ferroelectric guns in free-electron laser devices. r 2002 Elsevier Science B.V. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, a simulation code on ablations using femto-second lasers was developed to investigate the ablation threshold of various metal samples, including effects of absorption of laser lights by free electrons, that by vibrations of free electrons as fluids, inner shell electrons, effects of thermal reductions from free electrons to lattice ions, ablations by vibration effects and thermal effects.

Journal ArticleDOI
TL;DR: In this paper, the influence of the presence of the X-valley in highly doped n-GaAs on hot free-electron absorption and optical nonlinearities at 10.6μ m wavelength is discussed.
Abstract: A theoretical overview is given about the influence of the presence of the X-valley in highly doped n -GaAs on hot free-electron absorption and optical nonlinearities at 10.6 μ m wavelength. The implications of the extension of the quantum-mechanical model from two to three valleys are discussed. For electron temperatures above 600 K the X-valley presence starts to be observed. We reveal that it is difficult to trace the individual contributions of different X-electron related inter- and intravalley absorption and relaxation phenomena and therefore we suggest to introduce an effective X-valley related deformation potential which is a weighted combination of all the X-valley contributions. We discuss how nonlinear optical experiments can be conducted to determine the LL-intervalley and this effective X-valley deformation potential.

Journal ArticleDOI
TL;DR: In this article, the thickness of aluminum, copper, and gold thin films on silicon substrates was measured with an electron probe microanalyzer using a Monte Carlo simulation of electron scattering.
Abstract: The thicknesses of aluminum, copper, and gold thin films on silicon substrates have been measured with an electron probe microanalyzer. k ratio versus thickness calibration curves are obtained by a Monte Carlo simulation of electron scattering. The simulation results based on two energy loss models are compared. One is the continuous slowing down approximation model and another is the hybrid model for the discrete and the continuous energy loss processes. Inner shell ionizations and free electron excitations are selected out for the discrete process in the hybrid model. In both models the Mott cross section and the Gryzinski cross section are used for elastic collisions and ionizations, respectively. The exact film/substrate boundary condition is considered. The characteristic and continuum fluorescence corrections are also included in the simulation. The simulation results agree well with experimental ones measured with a quartz oscillator. Effects of the introduction of the discrete energy loss process ...