Relaxation dynamics of the Holstein polaron.

Keeping the full quantum nature of the problem, we compute the relaxation time of the Holstein polaron in one dimension after it was driven far from the equilibrium by a strong oscillatory pulse. Just after the pulse, the polaron's kinetic energy increases and subsequently exhibits a relaxation-type decrease with simultaneous emission of phonons. In the weak coupling regime, partial tunneling of the electron from the polaron self-potential is observed. The inverse relaxation time is for small values of electron-phonon coupling λ linear with λ, while it deviates downwards from the linear regime at λ>/~0.1/ω(0). The imaginary part of the equilibrium self-energy shows good agreement with the inverse relaxation time obtained from nonequilibrium simulations.

Ultrafast hopping dynamics of 5f electrons in the Mott insulator UO2 studied by femtosecond pump-probe spectroscopy.

We describe a femtosecond pump-probe study of ultrafast hopping dynamics of 5f electrons in the Mott insulator UO2 following Mott-gap excitation at temperatures of 5-300 K. Hopping-induced response of the lattice and electrons is probed by transient reflectivity at mid- and above-gap photon energies, respectively. These measurements show an instantaneous hop, subsequent picosecond lattice deformation, followed by acoustic phonon emission and microsecond relaxation. Temperature-dependent studies indicate that the slow relaxation results from Hubbard excitons formed by U³âº-U5⁺ pairs.

Tuning the resonance in high-temperature superconducting terahertz metamaterials.

In this Letter, we present resonance properties in terahertz metamaterials consisting of a split-ring resonator array made from high-temperature superconducting films. By varying the temperature, we observe efficient metamaterial resonance switching and frequency tuning. The results are well reproduced by numerical simulations of metamaterial resonance using the experimentally measured complex conductivity of the superconducting film. We develop a theoretical model that explains the tuning features, which takes into account the resistive resonance damping and additional split-ring inductance contributed from both the real and imaginary parts of the temperature-dependent complex conductivity. The theoretical model further predicts more efficient resonance tuning in metamaterials consisting of a thinner superconducting split-ring resonator array, which are also verified in subsequent experiments.

Ultrafast spectroscopy of the uranium(IV) and thorium(IV) bis(ketimide) complexes (C5Me5)2An[-N=C(Ph)(CH2Ph)]2 (An = Th, U).

Ultrafast pump-probe spectroscopic studies have been performed on (C 5Me 5) 2U[- N=C(Ph)(CH 2Ph)] 2 and (C 5Me 5) 2Th[- N=C(Ph)(CH 2Ph)] 2 including, for the uranium complex, the first direct measurement of dynamics of electronic deactivation within a 5f-electron manifold. Evidence has been found for strong coupling between the electronic ground state and the f-electron manifold which dominates the dynamics of the excited states of the bis(ketimide) uranium complex. These also demonstrate strong singlet-f manifold coupling, which assists in the deactivation of the photoexcited state of the uranium complex, and provide information on intersystem crossing and internal conversion processes in both complexes.

Electronic control of extraordinary terahertz transmission through subwavelength metal hole arrays.

We describe the electronic control of extraordinary terahertz transmission through subwavelength metal hole arrays fabricated on doped semiconductor substrates. The hybrid metal-semiconductor forms a Schottky diode structure, where the active depletion region modifies the substrate conductivity in real-time by applying an external voltage bias. This enables effective control of the resonance enhanced terahertz transmission. Our proof of principle device achieves an intensity modulation depth of 52% by changing the voltage bias between 0 and 16 volts. Further optimization may result in improvement of device performance and practical applications. This approach can be also translated to the other optical frequency ranges.