Interferometric Time-resolved Two-photon Photoemission Research Articles

Plasmonic Spin-Hall Effect in Surface Plasmon Polariton Focusing

Surface plasmon polaritons (SPPs) are fundamental collective charge density excitations at surfaces of solid-state plasmas. They can energize physical and chemical processes at the nanoscale, but control of the spatiotemporal evolution of their energy, momentum, and spin is necessary to effectively transfer their quantum properties to molecules and other nanoscopic objects. Thus, to design the coupling of SPPs into other modes of an electronic system, it is important to describe and control their nanofemto spatiotemporal distributions. We investigate the spatial SPP field and spin distributions when launched by illuminating a converging lens coupling structure in an Ag film with obliquely incident linearly or circularly polarized femtosecond light pulses. The propagation and focusing of SPPs are recorded as movies by scanning the pump–probe pulse delay and recording the evolving interference patterns between the SPP and optical fields by ultrafast interferometric time-resolved two-photon photoemission ele...

Excited electron dynamics of bismuth film grown on Si(1 1 1) surface by interferometric time-resolved two-photon photoemission spectroscopy

We have carried out studies on the excited-electron dynamics in the single and poly crystalline Bi films by interferometric time-resolved two-photon photoemission spectroscopy (ITR-2PPE). The fast energy relaxation times of the excited electrons due to the electron–electron scattering were observed. The energy relaxation times were less than 15 fs and those of the single crystalline film are faster than those of the poly crystalline film. The faster energy relaxation times of the single crystalline Bi film are attributed to the higher scattering rate induced by the higher densities of states by appearance of the surface states in the vicinity of the Fermi level.

Interface electronic structure and dynamics of tris-(8-hydroxyquinoline) aluminum on Cu(100)

We have investigated the evolution of occupied and unoccupied electronic states of tris-(8-hydroxyquinoline) aluminum (Alq3) molecules adsorbed on Cu(100) using ultraviolet photoelectron spectroscopy and interferometric time-resolved two-photon photoemission (ITR-2PPE). The adsorption of Alq3 lowers the work function of the surface, which allows an unambiguous observation of Cu(100) surface resonance coupled with d-band state and molecular interband transition in our ITR-2PPE setup. No evidence of an anion state, which used to be formed by dynamic charge transfer from metal to a molecular unoccupied state, is identified. Analysis of relaxation dynamics of the transiently occupied electronic states shows that the lifetimes for those excited states at this surface are independent of the Alq3 thickness. These results indicate that the Alq3–Cu(100) interface is described as a weak van der Waals interaction.

Collective and single-particle dynamics in time-resolved two-photon photoemission

A general model for time-resolved two-photon photoemission from solids is presented comprising both collective and single-electron dynamics. In combination with interferometric time-resolved two-photon photoemission, this allows one to determine the shape of the collective response function across the excitation spectrum and the single-particle lifetime of excited electrons. Ag nanoparticles supported on graphite exhibit a strong collective resonance and serve as model system to demonstrate experimentally this separation of collective and single-particle dynamics in two-photon photoemission.

Femtosecond electron dynamics studied by interferometric time-resolved two-photon photoemission

Electron–hole excitation and relaxation in the bulk, at interfaces, and surfaces of solid state materials play a key role in a variety of physical and chemical phenomena that are also important for optoelectronic devices and surface photochemistry. Femtosecond time-resolved two-photon photoemission has attracted strong interest because of its capability to measure energy and momentum resolved charge carrier dynamics. Moreover, interferometric measurements have opened the possibility of probing a variety of coherent phenomena. Some of the coherent phenomena observed by interferometric time-resolved two-photon photoemission are reported.

Optical intersubband transitions and femtosecond dynamics in Ag/Fe(100) quantum wells.

The optical intersubband transitions and femtosecond dynamics of electrons in quantum well states in Ag/Fe(100) are investigated by interferometric time-resolved two-photon photoemission. The quantum well wave functions and transition probabilities are evaluated from the two-photon photoemission resonance energies and intensities using an extended phase accumulation model. Direct femtosecond pump-probe correlation measurements elucidate the importance of interfaces in confined structures.

Surface Femtochemistry: Frustrated Desorption of Alkali Atoms from Noble Metals

The electronic excitation of chemisorbed Cs atoms through the coherent photoinduced charge transfer from the Cu(111) surface turns on repulsive forces that initiate the dissociative wave packet motion. Due to unusually slow electronic relaxation, with the interferometric time-resolved two-photon photoemission technique it is possible to follow the characteristic change in the surface electronic structure due to the ensuing nuclear dynamics for ∼200 fs after excitation. This direct observation of the incipient bond breaking process on a metal surface provides information on a series of complex electronic and nuclear events that previously could be gleaned only through analysis of the reaction products. The metal band structure, the polarizability of the Cs atom, and the mechanical properties of the surface strongly influence the electronic and nuclear relaxation, and therefore, the reaction efficiency. The importance of time-resolved methods in studying of surface reactions is underscored by significant deviations in the electronic and nuclear dynamics of Cs/Cu(111) from the standard models for surface photochemistry.

Parallel excitation pathways in ultrafast interferometric pump-probe correlation measurements of hot-electron lifetimes in metals

Hot electron (E-EFermi=0.75 to 1.55 eV) lifetimes for cesiated Cu(100) and Cu(111) surfaces are measured via interferometric time-resolved two-photon photoemission with a 19-fs intensity FWHM mode locked Ti:sapphire laser at 1.55 eV. The data are analyzed using the optical Bloch equations and a laser pulse characterized in situ via surface second-harmonic generation interferometric autocorrelation. It is found that the retrieved hot-electron lifetimes are unphysically fast, and have a strong dependence on the temperature of the sample and the polarization of the laser. A simple explanation for the data is that the measured signal consists of contributions from transitions through both virtual and real intermediate states.

Quantum Control of Nuclear Motion at a Metal Surface

The possibility of quantum control of surface photochemical reactions is demonstrated for the system of Cs/Cu(111). Coherent nuclear wave packet motion is induced by resonant dipole excitation with 3.08 eV light from an occupied surface state intrinsic to the Cu(111) surface to an unoccupied Cs antibonding state for a Cs atom coverage of 0.1 monolayer. Interferometric time-resolved two-photon photoemission measurements show that the polarization induced between the two states decays in ∼25 fs. This makes it possible to control the position and momentum of the excited state wave packet through the phase of the excitation field. Quantum control is demonstrated for excitation with phase-related <20 fs pulse pairs or single chirped laser pulses. The phase dependence of two-photon photoemission spectra demonstrates the control of desorptive motion of Cs atoms.

Electronic relaxation of alkali metal atoms on the Cu(111) surface

Interferometric time-resolved two-photon photoemission measurements are performed on Cs and Rb chemisorbed on Cu(111). The population decay of the alkali-atom-induced anti-bonding state at 35 K is observed on 50 fs and 20 fs time scales for Cs and Rb respectively. Factors that contribute to such unusually long lifetimes, and in general determine the lifetimes of adsorbates on metal surfaces, are discussed in light of related measurements by Bauer et al. [Phys. Rev. B, 60 (1999) 5016] and theoretical simulations by Borisov et al. [Surf. Sci. 430 (1999) 165].

Optical decoherence and quantum beats in Cs/Cu(111)

Electron dynamics induced by optical excitation from the occupied Shockley surface state to an unoccupied state localized on Cs atoms on Cu(111) is studied by interferometric time-resolved two-photon photoemission. With near-resonant excitation, beat structures are observed in interferometric two-pulse correlation measurements, where the two-photon photoemission intensity is measured as a function of pump-probe delay time. The quantum beats that originate from the frequency differences among initial-intermediate and intermediate-final states, and the carrier frequency are reproduced qualitatively by a three-level optical Bloch equation model.

Phase and Energy Relaxation in an Antibonding Surface State: Cs/Cu(111)

Electron dynamics induced by direct photoexcitation of the antibonding state on the Cs/Cu(111) surface are studied by interferometric time-resolved two-photon photoemission. Femtosecond resolution pump-probe correlation measurements indicate phase and energy decay, respectively, on 15 and 50 fs time scales for the Cs antibonding state at 33 K. Both the polarization and phase dynamics are nonexponential and strongly temperature dependent. The energy dependence of the antibonding state population dynamics is consistent with Cs photodesorption.