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A theoretical frame for two-photon photoemission is derived from the general theory of pump-probe photoemission, assuming that not only the probe but also the pump pulse is sufficiently weak. This allows us to use a perturbative approach to compute the lesser Green function within the Keldysh formalism. Two-photon photoemission spectroscopy is a widely used analytical tool to study non-equilibrium phenomena in solid materials. Our theoretical approach aims at a material-specific, realistic and quantitative description of the time-dependent spectrum based on a picture of effectively independent electrons as described by the local-density approximation in band-structure theory. To this end we follow Pendrys one-step theory of the photoemission process as close as possible and heavily make use of concepts of multiple-scattering theory, such as the representation of the final state by a time-reversed low-energy electron diffraction state. The formalism is fully relativistic and allows for a quantitative calculation of the time-dependent photocurrent for moderately correlated systems like simple metals or more complex compounds like topological insulators. An application to the Ag(100) surface is discussed in detail.
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A theoretical frame for pump-probe photoemission is presented. The approach is based on a general formulation using the Keldysh formalism for the lesser Greens function to describe the real-time evolution of the electronic degrees of freedom in the initial state after a strong pump pulse that drives the system out of equilibrium. The final state is represented by a time-reversed low-energy electron diffraction state. Our one-step description is related to Pendrys original formulation of the photoemission process as close as possible. The formalism allows for a quantitative calculation of time-dependent photocurrent for simple metals where a picture of effectively independent electrons is assumed as reliable. The theory is worked out for valence- and core-electron excitations. It comprises the study of different relativistic effects as a function of the pump-probe delay.
We provide here a roadmap for modeling silicon nano-devices with one or two group V donors (D). We discuss systems containing one or two electrons, that is, D^0, D^-, D_2^+ and D_2^0 centers. The impact of different levels of approximation is discussed. The most accurate instances -- for which we provide quantitative results -- are within multivalley effective mass including the central cell correction and a configuration interaction account of the electron-electron correlations. We also derive insightful, yet less accurate, analytical approximations and discuss their validity and limitations -- in particular, for a donor pair, we discuss the single orbital LCAO method, the Huckel approximation and the Hubbard model. Finally we discuss the connection between these results and recent experiments on few dopant devices.
Single atoms absorb and emit light from a resonant laser beam photon by photon. We show that a single atom strongly coupled to an optical cavity can absorb and emit resonant photons in pairs. The effect is observed in a photon correlation experiment on the light transmitted through the cavity. We find that the atom-cavity system transforms a random stream of input photons into a correlated stream of output photons, thereby acting as a two-photon gateway. The phenomenon has its origin in the quantum anharmonicity of the energy structure of the atom-cavity system. Future applications could include the controlled interaction of two photons by means of one atom.
A novel Bloch-waves based one-step theory of photoemission is developed within the augmented plane wave formalism. Implications of multi-Bloch-wave structure of photoelectron final states for band mapping are established. Interference between Bloch components of initial and final states leads to prominent spectral features with characteristic frequency dispersion experimentally observed in VSe_2 and TiTe_2. Interference effects together with a non-free-electron nature of final states strongly limit the applicability of the common direct transitions band mapping approach, making the tool of one-step analysis indispensable.
Employing femtosecond laser pulses in front and back side pumping of Au/Fe/MgO(001) combined with detection in two-photon photoelectron emission spectroscopy we analyze local relaxation dynamics of excited electrons in buried Fe, injection into Au across the Fe-Au interface, and electron transport across the Au layer at 0.6 to 2.0 eV above the Fermi energy. By analysis as a function of Au film thickness we obtain the electron lifetimes of bulk Au and Fe and distinguish the relaxation in the heterostructures constituents. We also show that the excited electrons propagate through Au in a superdiffusive regime and conclude further that electron injection across the epitaxial interface proceeds ballistically by electron wavepacket propagation.
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