Session H26: Graphene: Plasmons and Fluorescence

Plasmon damping in graphene out of equilibrium

Motivated by recent experiments with graphene under high photoexcitation, we study theoretically plasmons of graphene in the two-temperature regime, i.e., the regime where electrons are much hotter than the lattice. We calculate the plasmon damping due to scattering of electrons by acoustic phonons, which is the dominant intrinsic contribution in clean graphene. As the system relaxes to equilibrium, the plasmon frequency adiabatically changes with time. We show that this causes a partial compensation of the plasmon damping. A similar mechanism may apply to another collective mode (the energy wave) predicted to exist in graphene in the low-frequency hydrodynamic regime. Implications for infrared and THz pump-probe experiments are discussed.   

Plasmonics of graphene laced stratified media.

Strong overlap of fields of graphene physics and photonics drawn a lot of attention recently. Not only graphene~possesses intrinsic highly tunable plasmons but a combination of grapheme with noble metal~nano structures promises a variety of existing applications for conventional plasmonics , such as novel optical devices working in a broad range from THz to visible spectra. We report simulations of those devices using combination of discrete dipole approximation (DDA) and~ boundary element methods (BEM). While DDA is an essential tool for modeling large molecule polarizabilities and scattering the BEM provides necessary Green's function tensors when those molecules are in close proximity to the nano-structures. As an example of that technique we study electron energy loss and Raman spectra for~complex molecules in presence of metal plasmon active~nano particles~embedded into a stratified graphene laced medium.   

Nano-plasmonic phenomena in graphene nanoribbons

We report on infrared nano-imaging studies of confined plasmon modes inside patterned graphene nanoribbons. The confined geometry of these ribbons leads to distinct mode patterns and strong field enhancement, both of which evolve systematically with the ribbon width and excitation laser frequency. In addition, broadband nano-imaging in a wide mid-infrared region allowed us to evaluate in real space the effect of the plasmon-phonon coupling. Our data and modeling show that the plasmon damping rate increases significantly when approaching the substrate phonon. Furthermore, we observed one-dimensional edge plasmons that propagate strictly along the edges of our patterned graphene nanostructures. These edge modes appear to have a relatively shorter wavelength compared to two-dimensional plasmons.   

Resonantly Enhanced Nonlinear Response of Graphene Plasmons

Sub-wavelength graphene structures support plasmonic resonances in terahertz and mid-infrared part of the spectrum. The strong field confinement at plasmon resonance significantly enhances the light-graphene interaction and can lead to a very strong nonlinear optical response. This feature of graphene plasmons can enable nonlinear optics with low field intensity in miniaturized sub-wavelength devices. However, to date, the nonlinear response of graphene plasmons and their energy loss dynamics have not been studied experimentally. Here we present an experimental and theoretical study of the nonlinear terahertz response of plasmon resonances and their energy relaxation dynamics in graphene nanoribbons. Using THz pump-THz probe measurements at the plasmon frequency (9.4 THz), we observe a strong saturation of plasmon absorption followed by a 10 ps relaxation time. The observed nonlinearity is found to be significantly higher than that of graphene with no plasmon resonance. We further present a thermal model for nonlinear plasmonic absorption in graphene nanoribbons that supports the experimental results.   

Fluorescence Intermittency and Nanodot Evolution in Graphene Oxide

In recent experiments, micron-sized reduced graphene oxide (rGO) flakes were observed to exhibit strong photoluminescence intensity fluctuations, or blinking. Although blinking has been observed in a wide variety of nanoscale emitters, and striking universalities exist across these very different systems, rGO is the first quasi-two dimensional emitter that shows blinking. Despite the widespread presence of blinking at nanoscale, a microscopic mechanism behind this phenomenon remains elusive. Here we provide density functional theory results, analytical calculations, and Monte Carlo simulations to connect the fluorescence trajectories observed in the experiment to microscopic processes. Through Monte Carlo simulations of chemical processes occurring on the graphene oxide surface, we observe the formation and destruction of carbon nanodots. Finally, we use emission characteristics of carbon nanodots from Ab Initio methods to reconstruct the photoluminescence of the macroscopic flake. In particular, we are investigating whether fluorescence intermittency in reduced graphene oxide is an intrinsic optoelectronic property of the nanodot constituents or the result of reversible chemical processes capable of changing the size and number of graphene nanodots.   

Heterogeneous fluorescence intermittency in single layer reduced graphene oxide

Fluorescence intermittency, or blinking, has been observed in a wide range of systems, including quantum dots, nanorods, and nanowires. Striking similarities have been documented in the optical response of these nanoscale emitters. However, the mechanism behind blinking still remains elusive. For the first time, blinking has been observed in a two-dimensional system in recent experiments on reduced graphene oxide (rGO). Here we reveal the power spectral density (PSD) of the blinking in rGO shares the same 1/f-like behavior of previously known blinking systems; meanwhile, the heterogeneous dynamic evolution and spatial correlation make rGO a unique blinking system. To investigate the origin of blinking, we self-consistently explain the evolution of rGO blinking using the phenomenological multiple recombination center (MRC) model that captures common features of nanoscale blinking. Furthermore, tight binding method and ab-initio method calculations of carbon nanodots are utilized to look for the microscopic structure corresponding to the RCs in the MRC model.   

Photoemission from Graphene on Copper and Cesium Antimonide: Theory and Experiment

The work function is calculated using DFT for a substrate of flat copper on which a single layer of graphene is deposited. These calculations show a reduced work function, compared to bare copper, when graphene is deposited on a cathode. Based on our DFT-calculated results, a simple model using the transfer matrix approach gives the transmission probability near and above the barrier maximum. An important element of our model is the DFT-calculated, macroscopically-averaged electrostatic potential. Using this potential, graphene behaves as a resonant well for electrons transmitted between the substrate and vacuum regions. Another system to be discussed is graphene atop cesium antimonide, which has very low work function making it technologically useful, in particular for the development of an x-ray free electron laser. On cesium antimonide, we examine whether graphene may allow for the retention of an underlying cesium layer that is often damaged in high-field applications. A discussion of these results in light of recent experimental characterization at LANL will be given.   

Optical properties of graphene nanoribbons

We calculate the dielectric function and optical conductivity of ultra-narrow armchair graphene nanoribbons (AGNRs) and zigzag graphene nanoribbons (ZGNRs) by a self- consistent-field approach within a Markovian master-equation formalism (SCF-MMEF) coupled with full-wave electromagnetic equations. Based on third-nearest-neighbor tight-binding, with appropriate modifications for AGNRs and ZGNRs, we calculate electron dispersions and Bloch wave functions in excellent agreement with the local spin-density approximation (LSDA) results. A generalized Markovian master equation of the Lindblad form, which maintains the positivity of the density matrix, is derived to describe the interaction of the electronic system with an external electromagnetic field (to first order) and with a dissipative environment (to second order). Not only does the SCF-MMEF capture the interband electron-hole-pair generation, but it also accurately accounts for concurrent interband and intraband electron scattering with phonons and impurities. We employ the SCF-MMEF to calculate the dielectric function, complex conductivity, and loss function for both suspended and supported AGNRs and ZGNRs with different widths. Then, we obtain the plasmon dispersion and propagation length from the loss-function maximum.   

Thermally managed $f$s Z-scan methods investigation of the size-dependent nonlinearity of Graphene Oxide in different solvents

Acoustic and thermal diffusion effects are often ignored in Z-scan measurements resulting in misinterpretation of the nonlinear index of refraction and nonlinear absorption. Thermally managed Z-scan using a modified chopper was compared to utilizing a pulsepicker with the common calibration material CS$_{2}$ and then extended to Graphene Oxide (GO) in different solvents. The chopper reveals properties of the material in time and is an inexpensive alternative to changing the repetition rate with a pulsepicker. The pulsepicker allows for much faster rise-times and therefore measurements can be taken before thermal effects have overwhelmed the nonlinear electronic response. GO in DI water using pulsepicked $f$s laser excitation yielded a value of (-1.79$+$/-.6)x10$^{-15}$ cm$^{2}$/W for nanometer particles and (-1.09$+$/-.6)x10$^{-15}$ cm$^{2}$/W for micrometer sized particles. Open aperture Z-scan of GO in THF using the modified chopper shows a flip from reverse saturable absorption to saturable absorption in time, previously shown to be intensity dependent, potentially resulting from thermal effects. Both measurements indicate smaller particles have larger negative nonlinearities originating from thermal effects or from defects in lattice structure at the edges.   

Infrared spectroscopy of vertical heterostructures of graphene and hexagonal boron nitride

We suggest that optical absorption of monolayer and bilayer graphene on hexagonal boron nitride will provide meaningful information about the moiré characteristics. In particular, study of the absorption spectrum as a function of the doping for an almost completely full first miniband will distinguish between various theoretical proposals for the physically realistic interaction. Also, for bilayer graphene, the ability to compare spectra for the opposite signs of the interlayer asymmetry induced by an external electric field might provide additional information about the moiré parameters.   

Quantized beam shifts in graphene

We show that the magneto-optical response of a graphene-on-substrate system in the presence of an external magnetic field strongly affects light beam shifts. In the quantum Hall regime, we predict quantized Imbert-Fedorov, Goos-H\"anchen, and photonic spin Hall shifts. The Imbert-Fedorov and photonic spin Hall shifts are given in integer multiples of the fine structure constant $\alpha$, while the Goos-H\"anchen ones in discrete multiples of $\alpha^2$. Due to time-reversal symmetry breaking the IF shifts change sign when the direction of the applied magnetic field is reversed, while the other shifts remain unchanged. We investigate the influence on these shifts of magnetic field, temperature, and material dispersion and dissipation. An experimental demonstration of quantized beam shifts could be achieved at terahertz frequencies for moderate values of the magnetic field.   

Auger mediated positron sticking on graphene and highly oriented pyrolytic graphite.

Positron annihilation induced Auger electron spectroscopy (PAES) measurements on 6-8 layers graphene grown on polycrystalline copper and the measurements on a highly oriented pyrolytic graphite (HOPG) sample have indicated the presence of a bound surface state for positrons. Measurements carried out with positrons of kinetic energies lower than the electron work function for graphene or HOPG have shown emission of low energy electrons possible only through the Auger mediated positron sticking (AMPS) process. In this process the positron makes a transition from a positive energy scattering state to a bound surface state. The transition energy is coupled to a valence electron which may then have enough energy to get ejected from the sample surface. The positrons which are bound to surface state are highly localized in a direction perpendicular to surface and delocalized parallel to it which makes this process highly surface sensitive and can thus be used for characterizing graphene or graphite surfaces for open volume defects and surface impurities. The measurements have also shown an extremely large low energy tail for the C KVV Auger transition at 263eV indicative of another physical process for low energy emission.   

Impurity signatures in two-dimensional materials in atomic-resolution valence-electron-energy-loss spectroscopic maps.

The local atomic configurations and electronic states of impurities in 2D materials can be probed directly by several microscopy techniques. Probes of electronic excitations, however, lack spatial resolution. Here we demonstrate that valence-electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope yields atomic-resolution maps of electronic excitations that provide unique signatures of distinct bonding configuration impurities in 2D materials. We report simulations of the maps based on density functional theory and dynamical scattering theory, which agree with and provide direct interpretation of the observed features. The maps differentiate between different bonding configurations of impurities in graphene and hexagonal boron nitride. The theoretical analysis yields information on local electronic excitations, corresponding to impurity-induced bound, resonant and antiresonant states. The method stands to benefit from new monochromators and detectors that enhance spatial and energy resolution and constitutes a powerful alternative to optical spectroscopies for probing electronic and magnetic signatures related with impurities and defects.