Two-dimensional electron gas at noble-metal surfaces

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 defects, e.g. impurity atoms, adatoms, or step edges, leading to quantum-interference patterns in the local density of states around these defects. We have used the quantum-interference phenomena to quantitatively measure the electron phase-relaxation length and to probe long-range adsorbate interactions. Received: 12 October 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

Phonons and electrons at metal surfaces

Some aspects of the coupling between phonons and electrons and the interaction between electrons at metal surfaces are reviewed. Surface science techniques as diverse as electron energy loss spectroscopy, angle-resolved photoelectron spectroscopy, and scanning tunnelling microscopy are employed to study these interactions. Electron–phonon and electron–electron coupling are discussed in terms of renormalized phonon dispersion relations, and increased decay rates of electronic excitations. PACS 63.20.Kr; 68.35.Ja; 68.37.Ef; 68.43.Pq; 72.10.Fk; 72.15.Qm; 73.20.At     

High-resolution magnetic measurements at surfaces with spin polarized electrons

Observing the spin polarization of emitted electrons reveals surface magnetic information. In particular, high resolving power is achieved in different respects: 1) Magnetic micrography with a lateral resolution of 50 nm in a scanning electron microscope; 2) Non-destructive magnetic depth profiling in the 5–50 Å range with secondary electron emission; 3) Element specific chemical resolution using Auger electron emission; 4) Time-resolved magnetization measurements with pulsed-laser photoemission in less than 10 ns. The state-of-the-art of these techniques is illustrated with specific examples of surface magnetism.     

Theory of nonlinear optical response of gauge invariant currents to linear and nonlinear couplings

When a gauge field interacts with a quantum condensed matter system, at first order of the gauge field it couples to the current operator of the electrons. Higher orders of the gauge field couple to electrons through other operators such as the stress tensor, etc. On the other hand, when one performs a measurement on a quantum system, not only the current operator, but also stress tensor operator of the electrons, etc. are hidden in the measurement, as they contribute to the gauge invariant current. We formulate a general problem of nonlinear optical response of the gauge invariant currents in presence of nonlinear couplings. We show that the new couplings along with new responses arising from field current have a very simple structure which can be formulated as time ordered multi-particle correlation functions. We also obtain their Lehman representation and thereby show that one need not use non-equilibrium formulations to deal with them. These new correlation functions suggest that in nonlinear optical response many new processes are possible. The experimental detection of the new terms in the current operator, and application corresponding multi-photon processes needs further theoretical and experimental investigations.     

Dynamics of electrons and holes at surfaces

We present ab initio calculation results for electron-phonon (e-ph) contribution to hole lifetime broadening of the surface state on Al(0 0 1). We show that e-ph coupling in this state is significantly stronger than in bulk Al at the Fermi level. It makes the e-ph decay channel very important in the formation of the hole decay in the surface state at . We also present the results for e-e lifetime broadening in a quantum-well state in 1 ML K/Cu(1 1 1). We show that this contribution is not negligible and is much larger than that in a surface state on Ag(1 1 1).     

Controlled observation of a nonequilibrium Ising-Bloch transition in a nonlinear optical cavity

We report the controlled observation of the nonequilibrium Ising-Bloch transition in a broad area nonlinear optical cavity (a quasi-1D single longitudinal-mode photorefractive oscillator in a degenerate four-wave mixing configuration). Our experimental technique allows for the controlled injection of the domain walls. We use cavity detuning as control parameter and find that both Ising and Bloch walls can exist for the same detuning values within a certain interval of detunings; i.e., the Ising-Bloch transition is hysteretic in our case. A complex Ginzburg-Landau model is used for supporting the observations.     

Scattering of hot electrons by adatoms at metal surfaces

The scattering of electrons in image-potential states by Cu adatoms on Cu(001) surfaces has been investigated by means of time- and angle-resolved two-photon photoemission. Several interband and intraband-scattering mechanisms have been identified and their contributions to the total decay of the states determined quantitatively. The adsorbates mainly cause quasielastic scattering processes. Inelastic processes in contrast are due to interactions with electrons in the substrate and are not significantly increased by Cu adatoms. Quasielastic scattering into bulk bands contributes significantly to the depopulation of surface states.     

Scanning tunneling microscopy of the nonequilibrium interaction of impurity states at semiconductor surfaces

Interaction of two localized impurity states of Si atoms at a GaAs surface was studied by scanning tunneling microscopy and spectroscopy. The effects of a twofold “switching” on and off of the states of each of the interacting atoms, the tunneling-interaction-induced mutual level pulling of these states, and the level stabilization near E _F were observed. These effects are explained in terms of the extended Anderson model.     

Vertex correction effects on nonlinear optical response

The effects of vertex corrections due to frequency modulation are exactly evaluated on the emission spectra from the two-level electronic system pumped by the laser of arbitrary intensity and the absorption spectra of the weak probe field of this system. Here the frequency modulation is assumed to obey the Gaussian-Markoffian process. The two-time correlation function of the polarizations which describe the emission and absorption spectra is represented as the product of two propagators and a vertex correction. This correction is described in the form of the summation of the products of the matrix continued fractions. This correction is numerically demonstrated to be quite large for the emission spectrum in the case of the slow or strong frequency modulation and weak laser field, and for the absorption spectrum in the case of intermediate modulation and laser intensity.     

Radiation-induced nonlinear optical response of quartz fibers

The intensity of radiation-induced luminescence and transient optical losses in KU-1 (Russia) and K-3 (Japan) quartz glass optical tibers irradiated in a fast pulsed fission reactor (a pulse duration of 80 μs and a neutron flux up to 7 × 10¹⁶ cm^–2 s^–2) has been measured in the visible range. The intensity of the fast luminescence component nonlinearly depends on the neutron flux. The luminescence intensity and the transient optical losses depend on the probe light intensity. Suppression of radiation-induced luminescence is observed at wavelengths that are longer or shorter than the probe light wavelength. Light probing leads to an increase in transient optical losses and a more rapid recovery of transparency. A model of two photon fluxes is proposed to analyze the relationship of the effects of suppression of radiation-induced luminescence and the increase in optical losses upon light probing. The effect of suppression of radiation-induced luminescence can be used to control the optical properties of fibers in radiation fields.     

Surface optical waves at the boundary with a nonlinear Kerr medium

A rigorous solution consistent with a plane wave approximation is given to the boundary problem for Maxwell’s equations for surface optical waves at the boundary with a nonlinear Kerr medium. Exact formulas for the flux intensity (J ₀) and energy density (W ₀) of these waves are derived depending on the parameters of the adjacent media and the propagation constant (ξ). It is shown that these variables as functions of ξ have minima. Thus, J ₀ and W ₀ increase sharply as the propagation constant deviates from the minimum value ξ_min. Their values are greater, the larger the difference between the dielectric constants of the linear and nonlinear media is. An expression for the propagation velocity of a nonlinear surface wave is also obtained.     

Broadband nonlinear optical response of graphdiyne for mid-infrared solid-state lasers

Graphdiyne(GDY), a novel all-carbon nanomaterial, is considered the most easily synthesized and stable carbon allotrope,positioning it as a promising photoelectric material. Herein, we successfully fabricated a high-quality GDY saturable absorber and saturable absorber mirror. Both broadband nonlinear saturable absorption and ultrafast relaxation dynamic properties in midinfrared region of the GDY were investigated. All solid-state diode-pumped short and ultrashort pulsed lasers were realized using the GDY absorber at wavelengths of 2 and 2.8 μm, respectively. The results were then theoretically analyzed. This is the first presentation of ultrashort pulsed lasers in the mid-infrared region with GDY absorbers. These results resolutely confirm that GDY could be an optional broadband SA for all solid-state mid-infrared pulsed lasers, and they evidence its promising applications in mode-locked ultrafast lasers.