Tracking the Time Evolution of the Electron Distribution Function in Copper by Femtosecond Broadband Optical Spectroscopy.

Multitemperature models are nowadays often used to quantify the ultrafast electron-phonon (boson) relaxations and coupling strengths in advanced quantum solids. To test their applicability and limitations, we perform systematic studies of carrier relaxation dynamics in copper, a prototype system for which the two-temperature model (TTM) was initially considered. Using broadband time-resolved optical spectroscopy, we study the time evolution of the electron distribution function, f(E), over a large range of excitation densities. Following intraband optical excitation, f(E) is found to be athermal over several 100 fs, with a substantial part of the absorbed energy already being transferred to the lattice. We show, however, that the electron-phonon coupling constant can still be obtained using the TTM analysis, provided that the data are analyzed over the time window where the electrons are already quasithermal, and the electronic temperature is determined experimentally.

Energy scales and dynamics of electronic excitations in functionalized gold nanoparticles measured at the single particle level.

The knowledge of the electronic structure and dynamics of nanoparticles is a prerequisite to develop miniaturized single-electron devices based on nanoparticles. Low-temperature transport measurements of individual stable metallic nanoparticles enable unravelling the system specific electronic structure while ultrafast optical spectroscopy gives access to the electron dynamics. In this work, we investigate bare and thiol-functionalized gold nanoparticles. For the latter, we employ a fast and low-cost fabrication technique which yields nanoparticles with narrow size distribution. Using relatively long thiol-ended alkane chains for the functionalization modifies the electronic density of states of the nanoparticles. The study of decay dynamics of surface-plasmon-related hot electrons reveals the presence of electronic states at the interface which serve as a fast decay channel for electronic relaxation. By low-temperature scanning tunnelling microscopy we precisely investigate the energy scales and electronic interactions relevant for the tunnel charge transport through this system. We observe that the interaction between the functional ligand and the substrate on which the nanoparticles reside also influences the electronic transport. The procedure that we employ can be easily adapted to other metallic nanoparticles. Our findings are therefore important for incorporating them into single-electron devices.

Manipulation of charge transfer and transport in plasmonic-ferroelectric hybrids for photoelectrochemical applications.

Utilizing plasmonic nanostructures for efficient and flexible conversion of solar energy into electricity or fuel presents a new paradigm in photovoltaics and photoelectrochemistry research. In a conventional photoelectrochemical cell, consisting of a plasmonic structure in contact with a semiconductor, the type of photoelectrochemical reaction is determined by the band bending at the semiconductor/electrolyte interface. The nature of the reaction is thus hard to tune. Here instead of using a semiconductor, we employed a ferroelectric material, Pb(Zr,Ti)O3 (PZT). By depositing gold nanoparticle arrays and PZT films on ITO substrates, and studying the photocurrent as well as the femtosecond transient absorbance in different configurations, we demonstrate an effective charge transfer between the nanoparticle array and PZT. Most importantly, we show that the photocurrent can be tuned by nearly an order of magnitude when changing the ferroelectric polarization in PZT, demonstrating a versatile and tunable system for energy harvesting.