Ultrafast Study of Layered 3D Topological Insulators, Graphene-Gold Hybrid Plasmonic Nanostructure and Ultrathin Gold Nanowires

Abstract

In a typical time-resolved pump-probe spectroscopy, ultrafast laser pulses (pulse width ~ 100 fs) are utilized to induce a perturbation in the material of study within a time much shorter than the characterisitic lifetime of its constituents such as charge carriers, phonons and other excitations. Using a probe pulse much weaker in intensity, the temporal evolution of the nonequilibrium state of the material and the different relaxation pathways through which it restores are studied. In this thesis, time-resolved studies of layered three-dimensional (3D) topological insulators, graphene-gold hybrid nanostructure and ultrathin gold nanowires is reported. we have investigated the carrier dynamics in high aspect ratio ultrathin gold nanowires (Au-UNWs) of average diameter 2 nm using pump (3.1 eV) and coherent white light continuum as probe in the spectral range of 1.15 eV to 2.75 eV. The dynamics in the Au-UNWs is systematically probed in weak to strong excitation regime. The results show that under extreme excitation regime, the cooling dynamics of thermalized hot conduction electrons at elevated electronic temperature is slower than that predicted by the well established two-temperature model (TTM). Surface e ects in the carrier dynamics are found to play important role. We show that the screened electron-electron interactions due to spilling of conduction band and localization of core (d band) electrons at the surface promotes many-body interaction leading to nonequilibrium carrier density dependent Auger heating which slows the carrier cooling dynamics. In a quest to understand the e ect of size on carrier dynamics in metals, previous reports on femtosecond pump-probe study of metals have only focused on thin metal lms of varying thickness [14] and metal nanoparticles (MNs) of varying size [15{21]. Our study of electron dynamics in ultrathin metal nanowires which con ne the conduction electrons only in two dimensions instead of three dimensions like in MNs lls the missing and yet unexplored gap in this quest. Our results are of potential importance for devices and sensors based on Au-UNWs whose performance depends on the electron dynamics in gold at reduced dimensions.