Tracking ultrafast hot-electron diffusion in space and time by ultrafast thermomodulation microscopy.

The ultrafast response of metals to light is governed by intriguing nonequilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them leads to nonlinear diffusion behavior on ultrashort time scales. Here, we use scanning ultrafast thermomodulation microscopy to image the spatiotemporal hot-electron diffusion in thin gold films. By tracking local transient reflectivity with 20-nm spatial precision and 0.25-ps temporal resolution, we reveal two distinct diffusion regimes: an initial rapid diffusion during the first few picoseconds, followed by about 100-fold slower diffusion at longer times. We find a slower initial diffusion than previously predicted for purely electronic diffusion. We develop a comprehensive three-dimensional model based on a two-temperature model and evaluation of the thermo-optical response, taking into account the delaying effect of electron-phonon coupling. Our simulations describe well the observed diffusion dynamics and let us identify the two diffusion regimes as hot-electron and phonon-limited thermal diffusion, respectively.

Ultrafast generation and control of an electron vortex beam via chiral plasmonic near fields.

Vortex-carrying matter waves, such as chiral electron beams, are of significant interest in both applied and fundamental science. Continuous-wave electron vortex beams are commonly prepared via passive phase masks imprinting a transverse phase modulation on the electron's wavefunction. Here, we show that femtosecond chiral plasmonic near fields enable the generation and dynamic control on the ultrafast timescale of an electron vortex beam. The vortex structure of the resulting electron wavepacket is probed in both real and reciprocal space using ultrafast transmission electron microscopy. This method offers a high degree of scalability to small length scales and a highly efficient manipulation of the electron vorticity with attosecond precision. Besides the direct implications in the investigation of nanoscale ultrafast processes in which chirality plays a major role, we further discuss the perspectives of using this technique to shape the wavefunction of charged composite particles, such as protons, and how it can be used to probe their internal structure.

Holographic imaging of electromagnetic fields via electron-light quantum interference.

Holography relies on the interference between a known reference and a signal of interest to reconstruct both the amplitude and the phase of that signal. With electrons, the extension of holography to the ultrafast time domain remains a challenge, although it would yield the highest possible combined spatiotemporal resolution. Here, we show that holograms of local electromagnetic fields can be obtained with combined attosecond/nanometer resolution in an ultrafast transmission electron microscope (UEM). Unlike conventional holography, where signal and reference are spatially separated and then recombined to interfere, our method relies on electromagnetic fields to split an electron wave function in a quantum coherent superposition of different energy states. In the image plane, spatial modulation of the electron energy distribution reflects the phase relation between reference and signal fields. Beyond imaging applications, this approach allows implementing quantum measurements in parallel, providing an efficient and versatile tool for electron quantum optics.

Stimulated electron energy loss and gain in an electron microscope without a pulsed electron gun.

We report on a novel way of performing stimulated electron energy-loss and energy-gain spectroscopy (sEELS/sEEGS) experiments that does not require a pulsed gun. In this scheme, a regular scanning transmission electron microscope (STEM) equipped with a conventional continuous electron gun is fitted with a modified EELS detector and a light injector in the object chamber. The modification of the EELS detector allows one to expose the EELS camera during tunable time intervals that can be synchronized with nanosecond laser pulses hitting the sample, therefore allowing us to collect only those electrons that have interacted with the sample under light irradiation. Using  ∼ 5 ns laser pulses of  ∼ 2 eV photon energy on various plasmonic silver samples, we obtain evidence of sEELS/sEEGS through the emergence of up to two loss and gain peaks in the spectra at  ±â€¯2 and  ±â€¯4 eV. Because this approach does not involve any modification of the gun, our method retains the original performances of the microscope in terms of energy resolution and spectral imaging with and without light injection. Compared to pulsed-gun techniques, our method is mainly limited to a perturbative regime (typically no more that one gain event per incident electron), which allows us to observe resonant effects, in particular when the plasmon energy of a silver nanostructure matches the laser photon energy. In this situation, EELS and EEGS signals are enhanced in proportion to n+1 and n, respectively, where n is the average plasmon population due to the external illumination. The n term is associated with stimulated loss and gain processes, and the term of 1 corresponds to conventional (spontaneous) loss. The EELS part of the spectrum is therefore an incoherent superposition of spontaneous and stimulated EEL events. This is confirmed by a proper quantum-mechanical description of the electron/light/plasmon system incorporating light-plasmon and plasmon-electron interactions, as well as inelastic plasmon decay.

Graphene-Based Active Random Metamaterials for Cavity-Free Lasing.

Manipulating and controlling the optical energy flow inside random media is a research frontier of photonics and the basis of novel laser designs. Here, we show that a metamaterial consisting of randomly dispersed graphene nanoflakes embedded within an optically pumped gain medium (rhodamine 6G) can operate as a cavity-free laser thanks to its extraordinarily low threshold for saturable absorption. The emitted light is self-organized into a well-determined spatial pattern, which depends on the graphene flake density and can be externally controlled through the optical pump. We provide different examples of tunable laser operation ranging from stable single-mode to chaoticlike behavior. Our metamaterial design holds great potential for the optical control of light amplification, as well as for the development of single-mode beam-engineered cavity-free lasers.

Phonon-mediated mid-infrared photoresponse of graphene.

The photoresponse of graphene at mid-infrared frequencies is of high technological interest and is governed by fundamentally different underlying physics than the photoresponse at visible frequencies, as the energy of the photons and substrate phonons involved have comparable energies. Here, we perform a spectrally resolved study of the graphene photoresponse for mid-infrared light by measuring spatially resolved photocurrent over a broad frequency range (1000-1600 cm(-1)). We unveil the different mechanisms that give rise to photocurrent generation in graphene on a polar substrate. In particular, we find an enhancement of the photoresponse when the light excites bulk or surface phonons of the SiO2 substrate. This work paves the way for the development of graphene-based mid-infrared thermal sensing technology.

Dichroism in the interaction between vortex electron beams, plasmons, and molecules.

We study the transfer of orbital angular momentum between vortex electron beams and chiral samples, such as staircase plasmonic nanostructures and biomolecules. Inelastic electron scattering from these samples produces large dichroism in the momentum-resolved electron energy-loss spectra. We illustrate this phenomenon with calculations for chiral and nonchiral clusters of silver spheres using both focused and extended electron beams, which exhibit ∼10% difference between channels of opposite angular momentum. In addition to its fundamental interest, this remarkably high dichroism suggests a way of spatially resolving chiral optical excitations, including dark plasmons. We also predict a dichroic response when probing a chiral biomolecule, which suggests the use of these electron beams for resolving different enantiomers.

Tunable plasmons in atomically thin gold nanodisks.

The ability to modulate light at high speeds is of paramount importance for telecommunications, information processing and medical imaging technologies. This has stimulated intense efforts to master optoelectronic switching at visible and near-infrared frequencies, although coping with current computer speeds in integrated architectures still remains a major challenge. As a partial success, mid-infrared light modulation has been recently achieved through gating patterned graphene. Here we show that atomically thin noble metal nanoislands can extend optical modulation to the visible and near-infrared spectral range. We find plasmons in thin metal nanodisks to produce similar absorption cross-sections as spherical particles of the same diameter. Using realistic levels of electrical doping, plasmons are shifted by about half their width, thus leading to a factor-of-two change in light absorption. These results, which we substantiate on microscopic quantum theory of the optical response, hold great potential for the development of electrical visible and near-infrared light modulation in integrable, nanoscale devices.

Controlled interaction of surface quantum-well electronic states.

We report on the construction of well-defined surface quantum well arrangements by combining self-assembly protocols and molecular manipulation procedures. After the controlled removal of individual porphyrin molecules from dense-packed arrays on Ag(111), the surface state electrons are confined at the bare silver patches. These act as quantum wells that show well-defined unoccupied bound surface states. Scanning tunneling spectroscopy and complementary boundary element method calculations are performed to characterize the interaction between the bound states of adjacent quantum wells and reveal a hybridization of wave functions resulting in bonding and antibonding states. The interwell coupling can be tuned by the deliberate choice of the molecules acting as potential barriers. The fabrication method is shown to be ideally suited to engineer specific configurations as one-dimensional chains or two-dimensional artificial molecules.

Excitation of confined modes on particle arrays.

We describe both theoretically and experimentally the existence and excitation of confined modes in planar arrays of gold nanodisks. Ordered 2D lattices of monodispersive nanoparticles are manufactured, embedded in a silica matrix, and exposed to evanescent prism-coupling illumination, leading to dark features in the reflectivity, which signal the presence of confined modes guided along the arrays. We find remarkable agreement between theory and experiment in the frequency-momentum dispersion of the resonances. Direct excitation of these modes reveals long propagation distances and deep extinction features. This combined experimental and theoretical characterization of guided modes shows a good understanding of the optical response of metallic particles arrays, which can be beneficial in future designs of optical-signal and distant-sensing applications.

Universal distance-scaling of nonradiative energy transfer to graphene.

The near-field interaction between fluorescent emitters and graphene exhibits rich physics associated with local dipole-induced electromagnetic fields that are strongly enhanced due to the unique properties of graphene. Here, we measure emitter lifetimes as a function of emitter-graphene distance d, and find agreement with a universal scaling law, governed by the fine-structure constant. The observed energy transfer rate is in agreement with a 1/d(4) dependence that is characteristic of two-dimensional lossy media. The emitter decay rate is enhanced 90 times (energy transfer efficiency of ~99%) with respect to the decay in vacuum at distances d ≈ 5 nm. This high energy transfer rate is mainly due to the two-dimensionality and gapless character of the monatomic carbon layer. Graphene is thus shown to be an extraordinary energy sink, holding great potential for photodetection, energy harvesting, and nanophotonics.

Single-photon nonlinear optics with graphene plasmons.

We show that it is possible to realize significant nonlinear optical interactions at the few photon level in graphene nanostructures. Our approach takes advantage of the electric field enhancement associated with the strong confinement of graphene plasmons and the large intrinsic nonlinearity of graphene. Such a system could provide a powerful platform for quantum nonlinear optical control of light. As an example, we consider an integrated optical device that exploits this large nonlinearity to realize a single photon switch.

Magnetic polarization in the optical absorption of metallic nanoparticles.

We find remarkably strong absorption due to magnetic polarization in common colloidal and lithographic metallic nanoparticles. Our analysis is based upon a thorough examination of the dipolar electric and magnetic polarizabilities for representative combinations of nanoparticle composition, size, and morphology. We illustrate this concept by first discussing absorption in metallic spheres and then exploring ellipsoids, disks, and rings. Magnetic polarization reaches ~ 90% of the total absorption in 100 nm disks and rings for wavelengths above 1 µm under co-linear electric and magnetic irradiation. Our results demonstrate that the magnetic contribution to absorption cannot be naively overlooked, as it can largely exceed the contribution of electric polarization.

Looking through the mirror: optical microcavity-mirror image photonic interaction.

Although science fiction literature and art portray extraordinary stories of people interacting with their images behind a mirror, we know that they are not real and belong to the realm of fantasy. However, it is well known that charges or magnets near a good electrical conductor experience real attractive or repulsive forces, respectively, originating in the interaction with their images. Here, we show strong interaction between an optical microcavity and its image under external illumination. Specifically, we use silicon nanospheres whose high refractive index makes well-defined optical resonances feasible. The strong interaction produces attractive and repulsive forces depending on incident wavelength, cavity-metal separation and resonance mode symmetry. These intense repulsive photonic forces warrant a new kind of optical levitation that allows us to accurately manipulate small particles, with important consequences for microscopy, optical sensing and control of light by light at the nanoscale.

Plasmon scattering from single subwavelength holes.

We map the complex electric fields associated with the scattering of surface plasmon polaritons by single subwavelength holes of different sizes in thick gold films. We identify and quantify the different modes associated with this event, including a radial surface wave with an angularly isotropic amplitude. This wave is shown to arise from the out-of-plane electric dipole induced in the hole, and we quantify the corresponding polarizability, which is in excellent agreement with electromagnetic theory. Time-resolved measurements reveal a time delay of 38±18 fs between the surface plasmon polariton and the radial wave, which we attribute to the interaction with a broad hole resonance.

Power transfer between neighboring planar waveguides.

The ability to control light over very small distances is a problem of fundamental importance for a vast range of applications in communications, nanophotonics, and quantum information technologies. For this purpose, several methods have been proposed and demonstrated to confine and guide light, for example in dielectric and surface plasmon polariton (SPP) waveguides. Here, we study the interaction between different kinds of planar waveguides, which produces dramatic changes in the dispersion relation of the waveguide pair and even leads to mode suppression at small separations. This interaction also produces a transfer of power between the waveguides, which depends on the gap and propagation distances, thus providing a mechanism for optical signal transfer. We analytically study the properties of this interaction and the power transfer in different structures of interest including plasmonic and particle-array waveguides, for which we propose an experimental realization of these ideas.

Lifshitz transition across the Ag/Cu(111) superlattice band gap tuned by interface doping.

The two-dimensional, free-electron-like band structure of noble metal surfaces can be radically transformed by appropriate nanostructuration. A case example is the triangular dislocation network that characterizes the epitaxial Ag/Cu(111) system, which exhibits a highly featured band topology with a full band gap above E(F) and a hole-pocket-like Fermi surface. Here we show that controlled doping of the Ag/Cu(111) interface with Au allows one to observe a complete Lifshitz transition at 300 K; i.e., the hole pockets fill up, the band gap entirely shifts across E(F), and the Fermi surface becomes electron-pocket-like.

Quantum plexcitonics: strongly interacting plasmons and excitons.

We present a fully quantum mechanical approach to describe the coupling between plasmons and excitonic systems such as molecules or quantum dots. The formalism relies on Zubarev's Green functions, which allow us to go beyond the perturbative regime within the internal evolution of a plasmonic nanostructure and to fully account for quantum aspects of the optical response and Fano resonances in plasmon-excition (plexcitonic) systems. We illustrate this method with two examples consisting of an exciton-supporting quantum emitter placed either in the vicinity of a single metal nanoparticle or in the gap of a nanoparticle dimer. The optical absorption of the combined emitter-dimer structure is shown to undergo dramatic changes when the emitter excitation level is tuned across the gap-plasmon resonance. Our work opens a new avenue to deal with strongly interacting plasmon-excition hybrid systems.

Tunable quantum dot arrays formed from self-assembled metal-organic networks.

The confinement of Ag(111) surface-state electrons by self-assembled, nanoporous metal-organic networks is studied using low-temperature scanning tunneling microscopy and spectroscopy as well as electronic structure calculations. The honeycomb networks of Co metal centers and dicarbonitrile-oligophenyl linkers induce surface resonance states confined in the cavities with a tunable energy level alignment. We find that electron scattering is repulsive on the molecules and weakly attractive on Co. The tailored networks represent periodic arrays of uniform and coupled quantum dots.