Remote excitation and detection of surface-enhanced Raman scattering from graphene.

We demonstrate the remote excitation and detection of surface-enhanced Raman scattering (SERS) from graphene using a silver nanowire as a plasmonic waveguide. By investigating a nanowire touching a graphene sheet at only one terminal, we first show the remote excitation of SERS from graphene by propagating surface plasmon polaritons (SPPs) launched by a focused laser over distances on the order of 10 µm. Remote detection of SERS is then demonstrated for the same nanowire by detecting light emission at the distal end of the nanowire that was launched by graphene Raman scattering and carried to the end of the nanowire by SPPs. We then show that the transfer of the excitation and Raman scattered light along the nanowire can also be visualized through spectrally selective back focal plane imaging. Back focal plane images detected upon focused laser excitation at one of the nanowire's tips reveal propagating surface plasmon polaritons at the laser energy and at the energies of the most prominent Raman bands of graphene. With this approach the identification of remote excitation and detection of SERS for nanowires completely covering the Raman scatterer is achieved, which is typically not possible by direct imaging.

Phase retrieval of ultrashort laser pulses using a MIIPS algorithm.

We developed a new method for retrieving the group delay dispersion of a laser from Multiphoton Intra-pulse Interference Phase Scan (MIIPS) data. The method takes into account the spectral amplitude of the laser pulse and provides a direct feedback on the accuracy of the retrieval. The main advantage of the method derives from providing sufficiently high accuracy to avoid the need for multiple experimental iterations. Another advantage is that the new method can discriminate among pulses with different spectral phase and amplitude profiles, in which MIIPS traces occupy the same position in the phase-frequency MIIPS map.

Graphene Near-Degenerate Four-Wave Mixing for Phase Characterization of Broadband Pulses in Ultrafast Microscopy.

We investigate near-degenerate four-wave mixing in graphene using femtosecond laser pulse shaping microscopy. Intense near-degenerate four-wave mixing signals on either side of the exciting laser spectrum are controlled by amplitude and phase shaping. Quantitative signal modeling for the input pulse parameters shows a spectrally flat phase response of the near-degenerate four-wave mixing due to the linear dispersion of the massless Dirac Fermions in graphene. Exploiting these properties we demonstrate that graphene is uniquely suited for the intrafocus phase characterization and compression of broadband laser pulses, circumventing disadvantages of common methods utilizing second or third harmonic light.

Microscopic view on the ultrafast photoluminescence from photoexcited graphene.

We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. On the basis of a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: besides the well-known incoherent contribution driven by nonequilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence.

New materials for tunable plasmonic colloidal nanocrystals.

We present a review on the emerging materials for novel plasmonic colloidal nanocrystals. We start by explaining the basic processes involved in surface plasmon resonances in nanoparticles and then discuss the classes of nanocrystals that to date are particularly promising for tunable plasmonics: non-stoichiometric copper chalcogenides, extrinsically doped metal oxides, oxygen-deficient metal oxides and conductive metal oxides. We additionally introduce other emerging types of plasmonic nanocrystals and finally we give an outlook on nanocrystals of materials that could potentially display interesting plasmonic properties.

Two-photon-induced blue shift of core and shell optical transitions in colloidal CdSe/CdS quasi-type II quantum rods.

The spectral dependence of the two-photon absorption in CdSe/CdS core/shell nanocrystal heterorods has been studied via two-photon-induced luminescence excitation spectroscopy. We verified that the two-photon absorption in these samples is a purely nonlinear phenomenon, excluding the contribution from multistep linear absorption mediated by defect states. A large absorption cross section was observed for CdSe/CdS core/shell quantum rods, in the range of 10(5) GM (1 GM = 10(-50) cm(4) s phot(-1)), scaling with the total nanocrystal volume and thus independent of the core emission wavelength. In the two-photon luminescence excitation spectra, peaks are strongly blue-shifted with respect to the one-photon absorption peaks, for both core and shell transitions. The experimental results are confirmed by k·p calculations, which attribute the shift to both different parity selection rules that apply to one-photon and two-photon transitions and a low oscillator strength for two-photon transitions close to the ground-state one-photon absorption. In contrast with lead chalcogenide quantum dots, we found no evidence of a breakdown of the optical selection rules, despite the presence of band anisotropy, via the anisotropic hole masses, and the explicitly induced reduction of the electron wave function symmetry via the rod shape of the shell. The anisotropy does lead to an unexpected splitting of the electron P-states in the case of a large CdSe core encapsulated in a thin CdS shell. Hence, tuning of the core and shell dimensions and the concurrent transition from type I to quasi-type II carrier localization enables unprecedented control over the band-edge two-photon absorption.

Plasmon dynamics in colloidal Au2Cd alloy-CdSe core/shell nanocrystals.

Metal-semiconductor nanocrystal heterostructures are model systems for understanding the interplay between the localized surface plasmon resonances in the metal domain and the relaxation of the excited carriers in the semiconductor domain. Here we report the synthesis of colloidal Au2Cd (core)/CdSe (shell) nanocrystal heterostructures, which were characterized extensively with several structural and optical techniques, including time-resolved fluorescence and broad-band transient absorption spectroscopy (both below and above the CdSe band gap). The dynamics of the transient plasmon peak was dominated by the relaxation of hot carriers in the metal core, its spectral shape was independent of the pump wavelength, and the bleaching lifetime was about half a picosecond, comparable with the value found in the AuCd seeds used for the synthesis.

Plasmon bleaching dynamics in colloidal gold-iron oxide nanocrystal heterodimers.

Colloidal nanocrystal heterodimers composed of a plasmonic and a magnetic domain have been widely studied as potential materials for various applications in nanomedicine, biology, and photocatalysis. One of the most popular nanocrystal heterodimers is represented by a structure made of a Au domain and a iron oxide domain joined together. Understanding the nature of the interface between the two domains in such type of dimer and how this influences the energy relaxation processes is a key issue. Here, we present the first broad-band transient absorption study on gold/iron oxide nanocrystal heterodimers that explains how the energy relaxation is affected by the presence of such interface. We found faster electron-electron and electron-phonon relaxation times for the gold "nested" in the iron oxide domain in the heterodimers with respect to gold "only" nanocrystals, that is, free-standing gold nanocrystals in solution. We relate this effect to the decreased electron screening caused by spill-out of the gold electron distribution at gold/iron oxide interface.

Plasmon dynamics in colloidal Cu2-xSe nanocrystals.

The optical response of metallic nanostructures after intense excitation with femtosecond-laser pulses has recently attracted increasing attention: such response is dominated by ultrafast electron-phonon coupling and offers the possibility to achieve optical modulation with unprecedented terahertz bandwidth. In addition to noble metal nanoparticles, efforts have been made in recent years to synthesize heavily doped semiconductor nanocrystals so as to achieve a plasmonic behavior with spectrally tunable features. In this work, we studied the dynamics of the localized plasmon resonance exhibited by colloidal Cu(2-x)Se nanocrystals of 13 nm in diameter and with x around 0.15, upon excitation by ultrafast laser pulses via pump-probe experiments in the near-infrared, with ∼200 fs resolution time. The experimental results were interpreted according to the two-temperature model and revealed the existence of strong nonlinearities in the plasmonic absorption due to the much lower carrier density of Cu(2-x)Se compared to noble metals, which led to ultrafast control of the probe signal with modulation depth exceeding 40% in transmission.

Optical and electrical properties of colloidal (spherical Au)-(spinel ferrite nanorod) heterostructures.

We report here a simple synthetic route to Au-Fe(x)O(y) heterostructures in which spinel ferrite (Fe(x)O(y)) grows as a nanorod on a spherical gold (Au) seed. The large red shift in the plasmon resonance in the heterostructures could be explained by a dielectric effect (although we could not entirely exclude a contribution due to electron transfer from Au to defect states at the Au-Fe(x)O(y) interface), while the magnetic properties of the Au-Fe(x)O(y) heterostructures were basically the same as those of the corresponding nanocrystals after Au leaching. In films of Au-Fe(x)O(y) heterostructures the electrical conductivity appeared to be mediated by the Au domains.