Transport and dynamics of optically excited electrons in metals

Time-resolved two-photon photoemission, based on the equal-pulse correlation technique, is used to measure the energy relaxation and the transport of the photoexcited carriers in thin Ag and Au films. The energy-dependent relaxation time shows a significant thickness dependence in the Ag film, whereas for Au a much smaller effect is observed. These experimental observations are compared with a theoretical model based on the Boltzmann equation, which includes secondary (Auger) electrons and transport. A good agreement between experimental and theoretical results is found for Au. However, in our calculations, we did not find any significant change in the thickness dependence in the case of Ag. In order to explain the strong effect in Ag, we discuss the possibility of surface excitations. Received: 15 May 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Dynamics of optically excited quasiparticles in YBa2Cu3O7

We present a detailed investigation of the dynamics of laser-excited quasiparticles in YBa₂Cu₃O₇ thin films below the critical temperature. Reflectivity transients at low temperature trace the generation and recombination behavior of quasiparticles. The quasiparticle cascading and recombination rates are determined by comparison with a detailed nonlinear model of the quasiparticle dynamics based on extended Rothwarf-Taylor equations.     

Excitons in highly optically excited gallium phosphide

A study has been made of luminescence in weakly (10¹⁵-10¹⁶ cm^-3) and heavily (10¹⁸-10¹⁹ cm^-3) N-doped GaP crystals induced by 1.78, 2.34 and 3.56 eV photons from Q-switched ruby or neodymium lasers with a KDP crystal for second harmonic generation. The results which were obtained at excitations up to 10²⁰ cm^-3 electron-hole pairs are interpreted as the transitions: single bound excitons, bound excitonic molecules, free excitons in weakly-doped GaP, as well as Auger processes and the formation of a new excitonic state similar to a solid metal with high density of single excitons and excitonic molecules bound to isoelectronic traps in heavily-doped GaP.     

Kinetic effects in an optically excited gas

A resonance transition in a one-dimensional layer of gas contained between transparent parallel plates and optically excited by external radiation has been treated using kinetic theory. A perturbation method has been used to obtain the “first scattering” results for the velocity distribution and number density of excited atoms and the intensity of radiation at any point in the gas. Two special cases are discussed in detail: broad band excitation with inhomogeneous broadening and monochromatic excitation with homogeneous broadening. The effects of particle streaming and wall quenching are shown to produce boundary layer behavior in the excited level density which scales with the particle mean free path. In addition, line reversal of the radiation reemited from the gas is shown to occur and to be a direct result of particle streaming. Numerical and asymptotic results are presented which show these effects. These results should be pertinent to many laboratory and industrial devices in which the particle and photon mean free paths are comparable and to diagnostic techniques which use resonance fluorescence to infer excited level densities.     

Transient optically detected ESR in the excited state of ruby

The dynamics of the(² E) state of Cr³⁺ in Al₂O₃ have been investigated. After excitation by a short laser pulse (<10ns) the relative intensity of the optically detected ESR signal (ODMR) as a function of the delay timet _d after the excitation is measured. A remarkable increase was found upon lengtheningt _d, while the line width remained constant. This increasing population difference of the Zeeman levels of(² E) can be explained by different resonant-reabsorption induced effective lifetimes. The experimental data could be reproduced by use of a phenomenological model.     

Localization of optically excited states by self-trapping

We discuss the localization of the surface exciton at the Si(111)- (2x1) surface due to self-trapping, which leads to a characteristic temperature-dependent linewidth of the optical response and to a significant Stokes shift of the luminescence. Self-trapping results in this case from a structural relaxation in the excited state, caused by the interplay between electronic and geometric degrees of freedom. The most significant contribution to this effect comes from one single geometric deformation mode which is driven by the internal electronic charge transfer in the self-trapped exciton.     

Quasiparticle relaxation in optically excited high-Q superconducting resonators

The quasiparticle relaxation time in superconducting films has been measured as a function of temperature using the response of the complex conductivity to photon flux. For tantalum and aluminum, chosen for their difference in electron-phonon coupling strength, we find that at high temperatures the relaxation time increases with decreasing temperature, as expected for electron-phonon interaction. At low temperatures we find in both superconducting materials a saturation of the relaxation time, suggesting the presence of a second relaxation channel not due to electron-phonon interaction.     

Existence of the transverse relaxation time in optically excited bulk semiconductors

Two basic types of depolarization mechanisms, carrier-carrier (CC) and carrier-phonon (CP) scattering, are investigated in optically excited bulk semiconductors (3D), in which the existence of the transverse relaxation time is proven based on the vector property of the interband transition matrix elements. The dephasing rates for both CC and CP scattering are determined to be equal to one half of the total scattering-rate-integrals weighted by the factors (1-\cos\chi), where \chi are the scattering angles. Analytical expressions of the polarization dephasing due to CC scattering are established by using an uncertainty broadening approach, and analytical ones due to both the polar optical-phonon and non-polar deformation potential scattering (including inter-valley scattering) are also presented by using the sharp spectral functions in the dephasing rate calculations. These formulas, which reveal the trivial role of the Coulomb screening effect in the depolarization processes, are used to explain the experimental results at hand and provide a clear physical picture that is difficult to extract from numerical treatments.     

Dissociative attachment of electrons to excited molecules

The development and the first results from an experiment to carry out dissociative attachment to excited molecules are discussed. A brief summary of the relevance and status of such measurements are given. Apart from measuring the absolute cross sections from excited and state selected SO₂ molecule, we have been able to characterize the negative ion resonances using the excited state dissociative attachment. In addition, the state specificity of the electron attachment process has been used to derive information on the excited neutral state itself which has not been possible using optical spectroscopy. The applicability of this technique to other species are also discussed.     

Phase conjugation at the surfaces of optically excited CuI films

The possibility of realizing an optical phase conjugation in an excited semiconductor medium is shown theoretically and experimentally. A phase conjugation is revealed for the photon energy equal to half the energy of the radiative recombination of excitons in CuI films pumped by a nitrogen laser at room temperature. The dependences of the phase-conjugation signal intensity on its spectral composition are investigated. The quadratic interaction of light and exciton electromagnetic oscillations in the semiconductor medium is suggested as an explanation of this effect.     

Nuclear spin nanomagnet in an optically excited quantum dot

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei shifts the optical transition energy close to resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of the quantum dot electron. As a result the optically selected single quantum dot represents a tiny magnet with the ferromagnetic ordering of nuclear spins-the nuclear spin nanomagnet.     

Spin relaxation of electrons excited in p-doped semiconductor heterostructures

We present a calculation of the spin-relaxation time of photoexcited electrons in p-doped quantum wells of GaAs with the spin-flip mechanism due to the electron–hole exchange interaction. We have observed shorter spin-relaxation times for electrons close to the conduction-band edge when including the spin mixing of the valence-hole states. This spin mixing allows exchange spin-relaxation channels which are energy forbidden in the case of pure-spin holes.     

Collisions of electrons with molecules in excited vibrational-rotational states

Results are presented of calculations of cross sections for scattering of electrons by diatomic molecules in specific excited vibrational-rotational states. The calculations were made using an approximation based on a quantum theory of scattering in a system of several bodies which can be applied to calculations of direct reactions and reactions involving the formation of an intermediate transition complex. Results of calculations of cross sections for collisions of electrons with hydrogen, nitrogen, lithium, sodium, and hydrogen halide molecules are compared with existing experimental data and the results of calculations made by other authors.     

Conformational defects in a conducting polymer

Electrically conducting organic polymers represent serious candidates for some of the electronic materials of tomorrow. The role of certain charge-carrying self-localized states, such as solitons, polarons and bipolarons, in determining the nature of the optical and electronic transport properties of these conjugated polymers is well established. These self-localized states are commonly referred to as ‘defects'. Recently, the list of possible ‘defects’ in these conducting polymers has expanded to include real defects which degrade the electrical properties, but lead to other interesting and potentially useful properties, such as colour changes.     

Ultrafast relaxation of optically excited nonequilibrium electron-hole distributions in germanium

New measurements of the nonlinear, nonequilibrium optical (1.06μm) properties of the germanium solid-state plasma are presented. Single pulse transmission has been measured as a function of incident pulse energy at sample temperatures of 105 K and 297 K. In addition the relative transmission of a probe pulse as a function of time delay after an excitation pulse has been measured for three different excitation pulse energies and two temperatures. A model which explains the observed behavior of germanium under intense radiation is briefly, qualitatively described, and theorectical curves are plotted with the experimental data.     

Quasi-one-dimensional transport in conducting polymer nanowires

Recent advances in the synthesis of nanowires based on conducting conjugated polymers are described. The results of recent experimental studies of the electrical properties of polymer nanowires are critically analyzed. The applicability of various theoretical models of tunneling in one-dimensional conductors (variable-range hopping conductivity, Luttinger liquid, Wigner crystal, etc.) to the interpretation of the experimental data on the electronic transport in polymer nanosystems is discussed. A phenomenological model is proposed describing the mechanism of transport in polymer nanowires with allowance for the quasi-one-dimensional nature of molecules of conjugated polymers. The first applications of polymer nanowires in nanoelectronics and prospects for the future are discussed.     

Change in the states of optically excited Rb atoms near sapphire surface

The changes in the states of excited Rb atoms approaching a single-crystal sapphire surface have been investigated by methods of laser-excitation spectroscopy and luminescence of Rb vapor in a cell with sapphire windows, the gap between which was varied from 250 to 500 nm. Upon resonant excitation of Rb atoms by two semiconductor lasers with powers of 20 and 40 mW, luminescence from optically excited 5D _3/2 and 5D _5/2 states and optically unexcited 6P _1/2 and 6P _3/2 states is observed. It is established that the luminescence intensity from unexcited states is only a few times lower than that from excited states, with allowance for the fact that excited atoms are rapidly and almost completely quenched on the sapphire surface. The found anomalously strong luminescence from optically unexcited 6P _J states is explained by their nonradiative occupation near the sapphire surface from optically excited 5P _J states, in which atoms fail to reach the sapphire surface because of the repulsion from it. This repulsion is due to the polarization interaction between sapphire and the atoms in the 5P _J states near the surface. Nonradiative transition from the 5P _J state to the 6P _J ^?1 state is accompanied by excitation of two optical phonons in sapphire.