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: nonstoichiometric 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.

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.

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.

The aim of this study is to determine the minimum amount of dopant that prevents the occurrence, near room temperature, of a JahnÈTeller (JÈT) transition in the M-doped lithium manganese spinel of composition with 0.00 \ x O 0.06 and M \ Ni2`, Co3`, Cr3`or Ti4`. EPR spectra and magnetic Li 1.02 M x Mn 1.98~x O 4 susceptibility data are related to the valence state of M and Mn, and the homogeneous distribution of the dopant. We Ðnd that the spinel framework is remarkably sensitive to displaying low electronic and magnetic changes in its cationic sublattice due to cation substitution. The JÈT distortion, which is associated with a sudden drop in conductivity with decreasing temperature, is suppressed by substituting 3% of Mn with Co3ò r Cr3`, or by adding an even smaller amount of Ni2`(x \ 0.02, or 1% substitution). However, this inhibition occurs only in samples with a ratio r \ [Mn4`]/[Mn3`] P 1.18, i.e., a value larger than the ratio r \ 1.106 we have with no doping (x \ 0). As a consequence, doping with the tetravalent cation Ti4`, which always decreases the r value, does not suppress the JÈT transition. We suggest that both the dopant ion and the Li`in excess over the stoichiometric composition are located in 16d sites. The removal of the JÈT transition in the Co3`(x \ 0.06) sample is also due to local disorder.

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.

The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an xray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta. Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers.1 Author to whom correspondence should be addressed; electronic mail: afbartelt@lbl.gov. 2 Author to whom correspondence should be addressed; electronic mail: a_scholl@lbl.gov. 2Ultrafast magnetic storage and processing is founded on our ability to control magnetism on picosecond and femtosecond time scales. Magnetic phase transitions conserve the total angular momentum and usually involve the crystal lattice as a quasi-infinite reservoir of angular momentum. A prototypical ultrafast magnetic phenomenon is the demagnetization after excitation by an intense laser pulse [1][2][3][4][5]. Here, the orbital momentum is crucial as it links the electron spin, which carries most of the magnetic moment, to the lattice via the spinorbit interaction. In this letter, we investigate the orbital momentum dynamics during an ultrafast demagnetization in the model system Fe/Gd using X-ray magnetic circular dichroism (XMCD) [6].The Fe/Gd multilayer consists of two metals of very different electronic structure. Fe has exchange-split 3d spin bands which intersect the Fermi surface, allowing both low-energy spin-flip (Stoner) and spin wave excitations (magnons). The spin momentum dominates the total angular momentum while the orbital momentum is quenched by the strong ligand field and only partially restored by the spin-orbit interaction. The coupling of the orbital momentum to the anisotropic ligand field enables the flow of angular momentum from the spin system to the lattice during the demagnetization. A direct photon-driven exchange of spin and orbital momentum as proposed by Hübner [7] would, for example, appear as a temporary accumulation of orbital and concomitant reduction of spin momentum. In contrast, a bottleneck caused by the spin-orbit interaction would be visible as a reduced orbital to spin momentum ratio. The second component of the multilayer, Gd, is best described as a Heisenberg ferromagnet with localized 4f electrons. Gd does not exhibit an orbital momentum in the 4f shell, which is half full. A large exchange energy of about 11 eV separates the majority and minority 4f states, inhibiting low-energy spin-flip excitations. Magnetic long range order in Gd is established via 4f-5d exchange with the Gd 5d valence states and their exchange interaction with Gd 5d orbitals of nearest neighbors [8]. Therefore, the Gd 4f demagnetization occurs indirectly via 4f-5d exchange and subsequent 5d electronphonon scattering while the Fe demagnetization occurs directly via 3d electron-phonon scattering. The Gd orbital momentum should t...

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.

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.