Hot Electron Dynamics Research Articles

Phonon Contribution to the Lifetime of Surface State Quasiparticles Confined in Nanopyramids

We present a scanning tunneling microscopy investigation of the dynamics of hot electrons and holes in Ag pyramidal nanostructures. The geometry of the nanostructure leads to a strong reduction of the decay mechanism into the bulk states and then to a large reflection coefficient of the surface electronic waves. Therefore, in contrast to quantum corrals and adatom islands which show a dominant lossy scattering contribution to the linewidth, the narrow observed structures in the differential conductivity spectra reveal the expected linewidth minimum at the Fermi energy. The electron-phonon contribution to the lifetime is shown to be dominant, in agreement with previous photoemission measurements.

Femtosecond surface plasmon interferometry

We demonstrate femtosecond plasmonic interferometry with a novel geometry. The plasmonic microinterferometer consists of a tilted slit-groove pair. This arrangement allows for (i) interferometric measurements at a single wavelength with a single microinterferometer and (ii) unambiguous discrimination between changes in real and imaginary parts of the metal dielectric function. The performance is demonstrated by monitoring the sub-picosecond dynamics of hot electrons in gold.

Time and angle resolved photoemission spectroscopy using femtosecond visible and high-harmonic light

The angle resolved photoelectron spectroscopy (ARPES) has emerged as a leading technique in identifying static key properties of complex systems such as the electronic band structure of adsorbed molecules, ultrathin quantum-well films or high temperature superconductors. We efficiently combined ARPES by using a two-dimensional analyzer for parallel energy (E) and momentum (k||) detection with femtosecond time-resolved spectroscopies. Using time and angle resolved two photon photoemission (2PPE) with visible light pulses, the hot electron dynamics in complex electronic structures are directly accessible by means of angle resolved hot electron lifetime mapping. Furthermore, femtosecond ARPES spectra recorded with high harmonic generation (HHG) light pulses are presented, showing the potential of this technique for future investigations of surface dynamics and photo-induced phase transition processes.

Hot-electron dynamics in thin films of sodium-doped perylene-3,4,9,10-tetracarboxylic dianhydride

Time-resolved two-photon photoemission of the doping process of perylene-tetracarboxylicdianhydride (PTCDA) thin films with sodium gives insight into the dynamics of hot electrons in this system. Interaction with Na results in a charge transfer from Na to PTCDA and increases the lifetime of optically excited electrons. A similar increase in lifetime can be observed for ultrathin PTCDA layers on Ag(111). Both results are discussed in the one-electron picture, treated in ``classical'' photoemission theory, and in the many-electron picture as commonly considered for strongly correlated systems.

Theory of ultrafast intraband relaxation in carbon nanotubes

AbstractUltrafast intraband carrier relaxation in carbon nanotubes can be described by Carbon Nanotube Bloch Equations (CNBE) treating the Coulomb interaction beyond the Hartree‐Fock level to include carrier‐carrier scattering. We obtain equations of motion for carrier densities and two‐particle carrier correlations that are solved numerically. The complex non‐Markovian relaxation dynamics of hot electrons is calculated for various carbon nanotubes of different diameter and chirality. All calculated nanotubes show an ultrafast relaxation to equilibrium within approximately 100 femtoseconds. The observed relaxation is only weakly dependent on chirality, but depends on the diameter of the nanotube. With increasing diameter of the CNT the relaxation is slowed down, because of the weaker Coulomb interaction in larger diameter CNT. Additionally, we find oscillations of the electron plasma that are due to memory effects in the electron equilibration. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hot and Cold Electron Dynamics Following High-Intensity Laser Matter Interaction

The characteristics of fast electrons laser accelerated from solids and expanding into a vacuum from the rear target surface have been measured via optical probe reflectometry. This allows access to the time- and space-resolved dynamics of the fast electron density and temperature and of the energy partition into bulk (cold) electrons. In particular, it is found that the density of the hot electrons on the target rear surface is bell shaped, and that their mean energy at the same location is radially homogeneous and decreases with the target thickness.

Capture dynamics of hot electrons on quantum dots in RTDs studied by noise measurement

We investigate the noise in quantum dot resonant tunnelling diodes (QDRTDs), where the quantum dots (QDs) placed in the collector experience electric fields that vary in a wide range. The trapping/detrapping of electrons on the QDs dominated the measured electrical noise. The model that we derived for the noise explains the experimental data well. The QD capture cross-section is one to two orders of magnitude smaller than the physical size of the QDs due to the reduced probability of capturing a hot electron on the QD. The model is a powerful tool to design the noise characteristics of QDRTD single photon-detectors.

Modelling the influence of nonthermal electron dynamics in thin and ultrathin gold films

The effect of both nonthermal and thermal hot electron populations on the ultrafast transient reflectivity of thin and ultrathin gold films has been considered. The hot electron dynamics are calculated based on the recent model of Carpene [E. Carpene, Physical Review B 74 (2006) 024301], modified for film thickness phenomena. The effect of laser fluence, film thickness, probe wavelength and metal band structure are all considered. The consequences of the model for calculating rates of hot electron attachment to adsorbates are discussed. Finally the predictions are compared with experimental data.

Role of the electric field in surface electron dynamics above the vacuum level

Scanning tunneling spectroscopy (STS) is used to study the dynamics of hot electrons trapped on a Cu(100) surface in field emission resonances (FER) above the vacuum level. Differential conductance maps show isotropic electron interference wave patterns around defects whenever their energy lies within a surface projected band gap. Their Fourier analysis reveals a broad wave vector distribution, interpreted as due to the lateral acceleration of hot electrons in the inhomogeneous tip-induced potential. A line-shape analysis of the characteristic constant-current conductance spectra permits to establish the relation between apparent width of peaks and intrinsic line-width of FERs, as well as the identification of the different broadening mechanisms.

Femtosecond Third-Order Optical Nonlinearity ofAu:Bi2O3 Nanocomposite Films

Ultrafast third-order optical nonlinearities of the as-deposited andannealed Au:Bi2O3 nanocomposite films deposited bymagnetron cosputtering are investigated by using femtosecondtime-resolved optical Kerr effect (OKE) and pump–probe techniques. Thethird-order optical nonlinear susceptibility is estimated to be2.6×10−10 esu and 1.8×10−9 esu at wavelength of800 nm, for the as-deposited and the annealed film, respectively. TheOKE signal of the as-deposited film is nearly temporally symmetricalwith a peak centred at zero delay time, which indicates the dominantcontribution from intraband transition of conduction electrons. For theannealed film, the existence of a decay process in OKE signal impliesthe important contribution of hot electrons. These characteristics arein agreement with the hot electron dynamics observed in pump–probemeasurement.

Hot-electron dynamics and terahertz generation in GaN quantum wells in the streaming transport regime

Hot-electron dynamics and terahertz generation in GaN quantum wells in the streaming transport regime

Simple theory of hot electron dynamics observed by femtosecond ellipsometry

The dynamics of the linear and angular momenta of hot electrons in metals are of key importance for the design and operation of hot electron devices such as spin and tunnel valve transistors. The corresponding relaxation times are expected to lie in the subpicosecond range and must be studied with experimental techniques of adequate (femtosecond) temporal resolution. Here we report a simple theory of the ultrafast ellipsometric response of metals after excitation with femtosecond optical pulses. Although developed in the relaxation time approximation, the theory allows electron linear and angular momentum relaxation times to be extracted.

Ultrafast dynamics and carrier-envelope phase sensitivity of multiphoton photoemission from metal surfaces

Subcycle dynamics of multiphoton-induced photoelectron emission from metal surfaces is analysed using a simple phenomenological model to assess optimum conditions for direct carrier-envelope phase measurement. To gain further insight femtosecond time-resolved measurements were carried out on a polycrystalline gold surface with ultrashort laser pulses to explain the recently found, unexpectedly low carrier-envelope phase dependence of the photoemission process in this particular case. In the higher-order interferometric autocorrelation distribution additional short side wings appeared suggesting that ultrafast dynamics of hot electrons reduce the carrier-envelope phase dependence of the photoemission electron yield produced by few-cycle laser pulses. Other metals can be investigated with this simple and fast method to pave the way towards the construction of a solid-state-based, direct carrier-envelope phase detector.

Optical nonlinearity in glass-embedded silver nanoclusters under ultrafast laser excitation

The nonlinear optical transmission in noble metal clusters excited near the surface plasmon resonance can evolve from saturable absorption to induced absorption on varying the cluster size and applied laser fluence. Here we investigate the role of the laser pulsewidth in effecting this transition. Silver nanoclusters of sizes ranging from 2 to 6 nm, embedded in soda lime glass by the ion-exchange method, are excited with 100 fs and 300 ps laser pulses respectively. From femtosecond pump-probe measurements, the hot electron dynamics is measured in the largest cluster. Results show that thermal effects contribute to the nonlinearity in the picosecond excitation domain. Saturation intensities and effective nonlinear absorption coefficients are numerically evaluated.

Femtosecond Transfer Dynamics of Photogenerated Electrons at a Surface Resonance of Reconstructed InP(100)

Time-dependent two-photon photoemission spectra are used to resolve the femtosecond dynamics of hot electrons at the energetically lowest surface resonance of reconstructed InP(100). Two different cases are studied, where electrons either are lifted into the surface resonance via a direct optical transition or are captured from bulk states. These data are the first of this kind recorded with a time resolution below 70 fs. The microscopic analysis shows that electron-phonon scattering is a major mechanism for electron transfer between surface and bulk states.

Hot electron dynamics and optical nonlinearities in non-uniformly excited indium phosphide

Hot electron transport governed enhancement of light diffraction efficiency on transient grating has been observed in dcbiased semiinsulating InP:Fe crystal at below the energy gap excitation by picosecond laser pulses. Numerical analysis of dcfield-induced changes of free-carrier and electrooptic nonlinearities has been performed and allowed to attribute the observed effect to formation of a high-field domain grating, leading to dominant refractive index modulation by the quadratic electrooptic nonlinearity.

Nonlinear optical measurements of hot carrier induced quadratic electrooptic effect in dc-biased semiconductor

Fast optical nonlinearities which arise in dc-biased GaAs crystal have been investigated numerically and experimentally using the time-resolved picosecond four-wave mixing technique. Hot electron transport induced enhancement of optical nonlinearity as well as oscillatory behaviour of light diffraction efficiency in the subnanosecond time domain has been observed in the region of negative differential resistance. The kinetics of the four-wave mixing is compared with the numerical calculation of refractive index modulation taking into account coexisting free-carrier and quadratic electrooptic nonlinearities. The numerical modelling of hot electron dynamics in a spatially modulated GaAs crystal enabled us to explain the experimentally observed features and extract the modulation coefficient of quadratic electrooptic nonlinearity.

Femtosecond transient absorption dynamics of close-packed gold nanocrystal monolayer arrays

Femtosecond transient absorption spectroscopy is used to investigate hot electron dynamics of close-packed 6 nm gold nanocrystal monolayers. Morphology changes of the monolayer caused by the laser pump pulse are monitored by transmission electron microscopy. At low pump power, the monolayer maintains its structural integrity. Hot electrons induced by the pump pulse decay through electron–phonon (e–ph) coupling inside the nanocrystals with a decay constant that is similar to the value for bulk films. At high pump power, irreversible particle aggregation and sintering occur in the nanocrystal monolayer, which cause damping and peak shifting of the transient bleach signal.

The relaxation rate in hot-electron dynamics: beyond the first-order Born approximation inkinetic theory

The validity of a perturbative approach to studying the lifetime of excited electronsin an electron gas is analysed in a range of metallic densities. The relaxationrate is calculated using a kinetic theory framework with second-order Born andpartial-wave approximations for the scattering amplitude. The comparison of theterms obtained by physically motivated Thomas–Fermi-type potentials showsthat the next-to-leading correction to the first-order result is small only for highdensities of the electron gas. At low densities, a nonperturbative estimation ofthe relaxation rate is needed. An analytic expression of practical use whichincorporates both the role of screening and scattering is derived. A comparison withexperimental data obtained for an Ag(111) surface is made. The agreement foundsupports an inherent consistency, under the experimental conditions and down to(E−EF) = 0.4 eV,of the theoretical description based on a three-dimensional Fermi-liquid model.

Dynamics of electron scattering between bulk states and the C 1 surface state of InP(100)

Hot electron dynamics was investigated, with a focus on scattering between bulk states and the C1 surface state that is formed on the (2×4)-reconstructed In-rich surface of InP(100). The latter surface was prepared via metal organic chemical vapor deposition (MOCVD) and monitored by reflectance anisotropy spectroscopy (RAS/RDS). Two-color twophoton photoemission (2PPE) was employed with laser pulses of about 50 fs duration. Hot electrons were generated in bulk states about 0.5 eV above the C1 surface state, thereby avoiding any significant direct optical population of the surface state. A time constant of 35 fs was determined from the experimental data for electron scattering from isoenergetic bulk states to the C1 surface state by analyzing the rise of a C1-specific peak in the 2PPE spectrum. The decay of this C1 peak was ascribed to energy relaxation of the photo-generated electrons to bulk states below the surface state. Analogous measurements were carried out with a (2×1/2×2)-reconstructed P-rich surface of InP(100) that was also grown via MOCVD. No sign of a surface statewas detected in the 2PPE spectra for the latter surface in the corresponding energy range of the conduction band.