Ponderomotive acceleration of photoelectrons in surface-plasmon-assisted multiphoton photoelectric emission

Photoelectrons 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.     

Nonlinear photoelectric emission from metals

The nonlinear photoeffect of gold was investigated with a sub-picosecond laser at a photon energy of 2.57 eV. Absorption of a minimum of two photons is required to produce photoelectrons. The measured electron spectra display a broad continuum resulting from absorption of additional photons. The intensity dependence was examined by means of the pump-probe technique and yields the order of non-linearity for the investigated process.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Stimulated multiphoton emission from exciton Bose condensate

The coherent recombination of several (N) Bose-condensed excitons with simultaneous creation of N photons is considered. Due to the momentum conservation law, the total momentum of created photons is zero because of the zero momentum of excitons in Bose condensate. This requirement, in conjunction with the fact that the photon wavenumbers are fixed and equal to approximately E _g /c (E _g is the semiconductor gap and c is the speed of light), determines the mutual orientation of the wavevectors of emitted photons. This can be seen from the photon angular correlation in the experiments with several appropriately oriented detectors operating in the time-coincidence mode. It is shown that, if these processes are induced by N? 1 external laser beams (each with wavevector k _i ), then a unidirectional radiation with recoil wavevector k=?∑_i k _i should emerge from the exciton system. The intensities of coherent three-and four-exciton recombination are estimated for the exciton system in Cu₂O.     

Field emission from submicron-grained tungsten

Tungsten specimens subjected to intense plastic strains up to the true logarithmic deformation of e=7 were studied. Transmission electron microscopy revealed a decrease in the mean size of crystal grains down to 100 nm. The field ion-and field electron-emission studies revealed considerable distinctions between the energy distributions for electrons in submicron-grained tungsten and in a coarse-grained metal.     

Backward enhanced emission from multiphoton processes in aerosols

We have investigated, both theoretically and experimentally, multiphoton-induced processes in aerosol particles using femtosecond laser pulses. More specifically, we have demonstrated that both multiphoton (1, 2 and 3 photon)-induced fluorescence (MPEF) and laser-induced breakdown (LIB) emissions are strongly enhanced in the backward direction. The backward enhancement increases from 1.8 to 35 (emission ratio between the backward direction and 90°) with increasing non-linear process order n. Application to non-linear lidar of biological aerosols is discussed. Received: 24 April 2002 / Revised version: 3 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-472/431507, E-mail: wolf@lasim.univ-lyon1.fr     

Anomalous multiphoton photoelectric effect in ultrashort time scales

In 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.     

Irradiance dependence of photofield emission from tungsten

Measurements of the irradiance dependence of photofield emission from tungsten have been extended to 1.0 GW m⁻². The photoyield (the photocurrent per unit irradiance) from the (111) crystal plane is found to be almost independent of irradiance. The (211) and (310) crystal planes are found to exhibit small nonlinearities which are attributed to changes in the work function of the emitting surface caused by laser heating. Even at the highest irradiance, no measurable current of photoelectrons arising from multiphoton excitation is observed.     

Thermally assisted spin Hall effect

The spin polarized charge transport is systematically analyzed as a thermally driven stochastic process. The approach is based on Kramers' equation describing the semiclassical motion under the inclusion of stochastic and damping forces. Due to the relativistic spin-orbit coupling the damping experiences a relativistic correction leading to an additional contribution within the spin Hall conductivity. A further contribution to the conductivity is originated from the averaged underlying crystal potential, the mean value of which depends significantly on the electric field. We derive an exact expression for the electrical conductivity. All corrections are estimated in lowest order of a relativistic approach and in the linear response regime.     

Thermally assisted adiabatic quantum computation

We study the effect of a thermal environment on adiabatic quantum computation using the Bloch-Redfield formalism. We show that in certain cases the environment can enhance the performance in two different ways: (i) by introducing a time scale for thermal mixing near the anticrossing that is smaller than the adiabatic time scale, and (ii) by relaxation after the anticrossing. The former can enhance the scaling of computation when the environment is super-Ohmic, while the latter can only provide a prefactor enhancement. We apply our method to the case of adiabatic Grover search and show that performance better than classical is possible with a super-Ohmic environment, with no a priori knowledge of the energy spectrum.     

Thermally assisted current-driven domain-wall motion

Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive Langevin equations that describe the nonzero-temperature dynamics of a rigid domain wall. We derive an expression for the average drift velocity of the domain wall r(dw) as a function of the applied current, and find qualitative agreement with recent magnetic semiconductor experiments. Our model implies that at any nonzero-temperature r(dw) initially varies linearly with current, even in the absence of nonadiabatic spin torques.     

Localized multiphoton emission of femtosecond electron pulses from metal nanotips

Intense multiphoton electron emission is observed from sharp (approximately 20 nm radius) metallic tips illuminated with weak 100-pJ, 7-fs light pulses. Local field enhancement, evidenced by concurrent nonlinear light generation, confines the emission to the tip apex. Electrons are emitted from a highly excited nonequilibrium carrier distribution, resulting in a marked change of the absolute electron flux and its dependence on optical power with the tip bias voltage. The strong optical nonlinearity of the electron emission allows us to image the local optical field near a metallic nanostructure with a spatial resolution of a few tens of nanometers in a novel tip-enhanced electron emission microscope.     

Field emission energy distribution from adsorbate covered tungsten

We discuss the effect of various classes of adsorbates on the surface states and resonances known to produce structure in the field emission energy distribution from (100) tungsten. We show that it is the overlayer geometry which is of importance in determining whether or not the resonances and concomittant FEED structure persist in the presence of adsorbates. In particular, a c(2 × 2) krypton overlayer destroys the structure while a (1 × 1) gold overlayer does not.     

Thermal field emission and thermal photofield emission from tungsten (110)

The total energy distibution in field emission from the (110) surface of tungsten has been measured at temperatures in the range from 300 to 1400 K, which are sufficiently high to cause significant emission from thermally populated electronic states. Above the Fermi level, a striking departure from the prediction of free electron theory is observed. A similar anomaly is observed in the total energy distribution in photofield emission from thermally populated initial states on tungsten (110). In s-polarized light at normal incidence the anomaly vanishes when the polarization vector is parallel to the [001] direction. The anomalous behaviour is attributed to emission from the p-like energy band that is responsible for the ellipsoidal sheet of the Fermi surface centred at the symmetry point N. A comparison between the enhancement factors measured in sand p-polarized light yields no evidence for surface resonances close to the upper limit of the p-like energy band.     

Thermally assisted writing for perpendicular MRAM

Spin valves composed of TbCo/CoFe/Cu/CoFe/TbFeCo were fabricated with perpendicular magnetization and GMR ratios of 4.5%. The (TbCo/CoFe) layers and (CoFe/TbFeCo) layers are referred to the free and the pinned layers, respectively. The compositions of two layers were chosen to have a lower Curie temperature (130 °C) but higher coercivity (13.2 kOe) of the free layer at room temperature than those of the pinned layer; therefore, the free layer is quite stable at room temperature but its magnetization can be easily switched at a relatively low temperature. Spin valves were patterned into 100-μm-wide cells and their coercivity was reduced with increasing writing current due to the temperature rise by current-heating. When the current density of the writing current was increased to 2.1×10⁶ A/cm², the required switching field for the free layer was only 10 Oe.     

Linear intensity dependence of photo-induced field emission from tungsten

As a first step in an investigation of the polarization dependence of photo-induced field emission from tungsten, the dependence of the photocurrent on the intensity of illumination has been studied. The beam from a CW krypton ion laser operating at selected wavelengths in the visible region of the spectrum was focussed onto the field emitter, and the emission current in a narrow energy range (chosen to minimize tip heating effects) was measured as a function of intensity. The apparatus was designed to ensure that the position of the focal spot and the intensity of illumination could be adjusted without mutual influence. The photocurrent from each crystal plane was found to be linearly proportional to the luminous intensity over the range of intensity investigated (0–35 MW m^?2). It is concluded that in tungsten there exists a low intensity regime where the external current in photo-induced field emission is caused predominantly by one-photon excitations.     

Space and surface-state effects in multiphoton emission

We investigate multiphoton emission of electrons from solid surfaces in the presence of nonperturbative monochromatic laser fields. We study the dependence of transmission probabilities on surface states, the penetration depth of laser fields in solids and the size of the laser focus in vacuum. To this end we present a one-space-dimensional model which permits to study these problems and develop a numerically stable algorithm for solving the corresponding time-dependent Schrödinger equation.     

Simultaneous measurements of second-harmonic generation and two-photon photoelectric emission from Au

Received: 1 October 1998 / Accepted: 30 October 1998     

Thermally assisted hopping transport in disordered systems

The response to an external field of localized electrons coupled to phonons is investigated. The low frequency (ω<T) linear response function is shown to obey a kinetic equation. The transition probabilities (including multiphonon contributions) can be expressed in terms of the dynamical correlation functions(k, ?) of the phonons. The low temperature d.c. conductivity in three dimensions obeys a law σ(0)=σ₀ · exp(? (T ₀/T)^1/4). By a combined variational and “nearest neighbor” approximation upper limits for the exponential as well as the pre-exponential factor are obtained. In two dimensions the 1/4 in the exponent has to be replaced by 1/3. The one-dimensional case requires separate considerations which do not simply lead to an exponent 1/2. An expression for the thermopower in the hopping regime is derived and evaluated.