Session P6: Graphene Optical Phenomena

Graphene-based photonic crystals

A novel type of photonic crystals formed by embedding a periodic array graphene and dielectric material into a background dielectric medium is discussed. One-dimensional (1D) photonic crystal formed by an array of periodically located stacks of alternating graphene and dielectric stripes, while the two-dimensional (2D) one is formed by constituent stacks of alternating graphene and dielectric discs. The electromagnetic wave propagation in 1D crystal analyzed in the framework of the Kronig-Penney model. The frequency band structure of 1D graphene-based photonic crystal is obtained analytically as a function of the filling factor and the thickness of the dielectric between graphene stripes. The photonic frequency corresponding to the electromagnetic wave localized by a defect that breaks the symmetry of the system is obtained. For 2D crystal the photonic band structure and transmittance are calculated. The graphene-based photonic crystals can be used effectively as the frequency filters and waveguides for the far infrared region of electromagnetic spectrum. Due to substantial suppression of absorption of low-frequency radiation in doped graphene the damping and skin effect in the photonic crystal are also suppressed. The advantages of the graphene-based photonic crystal are discussed.   

Graphene Plasmonic Terahertz Filters and Polarizers

Graphene has remarkably strong interaction with light, especially in the terahertz frequency range. Free carriers in graphene exhibit Drude behavior and the Drude weight can be tuned by electrostatic or chemical doping. Graphene can support surface plasmons. In this paper, we'll show that with multiple stacked CVD graphene layers, terahertz filters and polarizers can be realized by patterning them into micro-disks and ribbons. The influence of dipole-dipole interaction on the extinction spectra will be discussed.   

Surface-plasmon polaritons on graphene-metal surface

In this presentation we present a new application of graphene in the field of plasmonics. We studied excitation of surface-plasmon polaritons on graphene-metal surface. The metal surface is functionalized by transfer printing of a graphene layer grown by chemical vapor deposition on copper foils. Surface plasmon resonance (SPR) characteristics of monolayer and multilayer graphene on the metal surface are presented. The results reveal the essential features predicted by the calculations based on transfer matrix method. As an application, we fabricated a surface plasmon resonance sensor integrated with a microfluidic device to study nonspecific physical interaction between graphene layer and proteins. We obtained association and dissociation coefficient of BSA adsorbed on graphene layer. We believe that graphene functionalized SPR sensors could provide a new platform to study interactions between graphene and molecules.   

New Aspects of Photocurrent Generation at Graphene pn Junctions Revealed by Ultrafast Optical Measurements

The remarkable electrical and optical properties of graphene make it a promising material for new optoelectronic applications. However, one important, but so far unexplored, property is the role of hot carriers in charge and energy transport at graphene interfaces. Here we investigate the photocurrent (PC) dynamics at a tunable graphene pn junction using ultrafast scanning PC microscopy. Pump-probe measurements show a temperature dependent relaxation time of photogenerated carriers that increases from 1.5ps at 290K to 4ps at 20K; while the amplitude of the PC is independent of the lattice temperature. These observations imply that it is hot carriers, not phonons, which dominate ultrafast energy transport. Gate dependent measurements show many interesting features such as pump induced saturation, enhancement, and sign reversal of probe generated PC. These observations reveal that the underlying PC mechanism is a combination of the thermoelectric and built-in electric field effects. Our results enhance the understanding of non-equilibrium electron dynamics, electron-electron interactions, and electron-phonon interactions in graphene. They also determine fundamental limits on ultrafast device operation speeds ($\sim $500 GHz) for graphene-based photodetectors.   

Relaxation dynamics of photoexcited carriers in graphene probed by optical pump-THz probe spectroscopy

The relaxation of hot carriers in graphene is a subject of much current interest. The role of electronic and phonon relaxation channels is a topic of particular focus, with phenomena such as carrier multiplication, in which multiple charge carriers are generated from absorption of a single photon, having been predicted.\footnote{T. Winzer, A. Knorr, and E. Malic, Nano Letts. 10, 4839 (2010).} In this work, we apply the optical pump-THz probe spectroscopy to investigate the relaxation dynamics of photoexcited carriers in large-area single-layer graphene samples grown by chemical vapor deposition (CVD). The complex optical conductivity induced by optical excitation is determined over a broad range of THz frequencies as a function of the pump-probe delay time. To probe carrier relaxation dynamics, we compare the transient optical conductivity to a single-particle model for the intraband response. The effect of pump fluence and the static doping density on the carrier dynamics will be discussed.   

Excitonic Effects and Optical Absorption Spectrum of Doped Graphene

First-principles calculations based on the GW-Bethe-Salpeter Equation (GW-BSE) approach and subsequent experiments have shown large excitonic effects in the optical absorbance of graphene. Here we employ the GW-BSE formalism to probe the effects of charge carrier doping and of having an external electric field on the absorption spectrum of graphene. We show that the absorbance peak due to the resonant exciton exhibits systematic changes in both its position and profile when graphene is gate doped by carriers, in excellent agreement to very recent measurements\footnote{Tony F. Heinz, private communications.}. We analyze the various contributions to these changes in the absorption spectrum, such as the effects of screening by carriers to the quasiparticle energies and electron-hole interactions. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and the U.S. DOD - Office of Naval Research under RTC Grant No. N00014-09-1-1066. Computer time was provided by NERSC.   

Magneto-phonon resonance in graphene

Recently, much attention has been paid to electron-phonon coupling in graphene. In particular, significant re-normalization and broadening of long-wavelength optical phonons are predicted to occur through resonant interaction with Landau-quantized Dirac fermions. We report a high-field magneto-Raman spectroscopy study of single-layer graphene in magnetic fields up to 45 T. The Raman G peak exhibits clear splitting at approximately 30 T, which we attribute to the fundamental magneto-phonon resonance associated with (0,1) inter Landau level transitions. The coupled electron-phonon modes exhibit characteristic anti-crossing behavior allowing for an accurate determination of the electron-phonon coupling strength in graphene.   

Coherent photocurrent control in a graphene bilayer in a magnetic field

We consider theoretically the coherent control of a Bernal-stacked graphene bilayer in a perpendicular magnetic field. When the system is exposed to a two-color optical pulse, photocurrents of electrons and holes are induced through interference between one- and two-photon excitation processes. The generated photocurrents are time-dependent as a result of the two processes placing electrons or holes in different Landau levels. The direction and phase of the terahertz current oscillation can be tuned through the polarization and relative phase parameter of the optical pulses. We compare the results to those obtained in the absence of a magnetic field [1] and also to the results for monolayer graphene. \\[4pt] [1] J. Rioux, G. Burkard and J. E. Sipe, Phys. Rev. B 83, 195406 (2011).   

Giant optical nonlinearity of graphene in a strong magnetic field

We demonstrate theoretically that graphene placed in a strong magnetic field possesses by far the highest third-order optical nonlinearity among all known materials. The giant nonlinearity originates from unique electronic properties and selection rules near the Dirac points, which gives rise to resonantly enhanced nonlinear response. We present rigorous and intuitive quantum-mechanical density-matrix formalism for calculating linear and nonlinear optical properties of graphene, valid for arbitrarily strong magnetic and optical field [1]. The calculated magnitude of the third-order nonlinearity is of the order of 0.01 esu for the field of several Tesla in the mid/far-infrared spectral range. Due to this giant nonlinearity, even one monolayer of graphene gives rise to appreciable nonlinear frequency conversion efficiency for incident mid/far-infrared radiation. \\[4pt] [1] X. Yao and A. Belyanin, Giant optical nonlinearity of graphene in a strong magnetic field, Phys. Rev. Lett. submitted; arXiv: 1110.4869.   

Mid-Infrared Magneto-Optical Kerr and Faraday Studies in Gated Monolayer and Rotated Bilayer CVD Graphene

Previous studies of multi-layer graphene grown on C-face SiC have proven that mid-IR (111-135 meV) Polar Magneto-Optical Kerr Effect (PMOKE) measurements provide a unique and sensitive way to probe the Landau Level (LL) structure of graphene. Graphene PMOKE measurements, which are proportional to the Hall conductivity ($\sigma _{xy})$, have revealed large changes in the Kerr angle due to the chiral nature of LL transitions in mono- and multi-layer graphene, as well as the dependence of these Kerr features on the position of the Fermi level. In this work we present new results that extend these measurements to more ideal samples consisting of large area (5x5mm), epitaxial single layer CVD grown graphene that has been deposited onto a Si/SiO$_{2}$ substrate. Unlike epitaxial SiC graphene these new samples can be back-gated and also allow magneto-optical measurements to be made in transmission (Faraday geometry). The ability to tune parameters such as the Fermi energy, probing photon energy, magnetic field strength, and temperature allows us to better understand graphene through its mid-IR Hall conductivity and to test for theoretical predictions of an infrared quantum Hall effect in graphene (Morimoto, PRL, 2009). This work is supported by NSF-DMR1006078 and by the Office of Naval Research.   

Substrate effects and photon energy dependence of ultrafast carrier relaxation in graphene

Ultrafast photo-excitation in graphene creates non-equilibrium carrier distributions and provides an avenue for the measurement of couplings between electronic and lattice degrees of freedom. Previous studies have explored the carrier relaxation dynamics in graphene on various substrates albeit primarily in the linear dispersion regime. We investigate the ultrafast carrier dynamics of graphene both in the linear band regime and near the saddle point. We perform femtosecond-resolved degenerate pump-probe differential transmission experiments to extract the timescales for electronic relaxation from different starting points on the band structure. The use of degenerate pump-probe allows us to obtain exact relaxation timescales corresponding to the local band structure without contributions from other carriers. We use multiple transparent substrates, such as fused silica, quartz, sapphire etc. to probe the substrate-graphene interactions at various points along the local electronic band structure. We find that in the UV regime, the relaxation dynamics in graphene shows remarkable dependence on the substrate, with relaxation timescales ranging from a few to hundreds of picoseconds We will also compare our measurements with those obtained in freestanding graphene samples.   

Excitonic Effects on Optical Absorption Spectra of Doped Graphene

We have performed first-principles calculations to study optical absorption spectra of doped graphene with many-electron effects included. Both self-energy corrections and electron-hole interactions are reduced due to the enhanced screening in doped graphene. However, self-energy corrections and excitonic effects nearly cancel each other, making the prominent optical absorption peak fixed around 4.5 eV under different doping conditions. On the other hand, an unexpected increase of the optical absorbance is observed within the infrared and visible-light frequency regime (1 $\sim $ 3 eV). Our analysis shows that a combining effect from the band filling and electron-hole interactions results in such an enhanced excitonic effect on the optical absorption. These unique variations of the optical absorption of doped graphene are of importance to understand relevant experiments and design optoelectronic applications.   

Dirac Plasmas in Graphene

Collective excitation of Dirac Fermions in graphene has many similarities to that of conventional semiconductor two dimensional electron gas (2DEG). For instance, the resonance frequency is proportional to q1/2, where q is the wave-vector. However, there are some fundamental differences. In this paper, we'll present our far-infrared spectroscopy studies of Dirac plasma in graphene. Localized plasmons in graphene disks and micro-ribbons are excited by far-infrared photons. The resonance frequency scales as n1/4 , where n is the sheet carrier density. This is distinctively different from conventional semiconductor. In addition, plasmons in graphene superlattice are compared to their counterparts in semiconductor superlattice.   

Experimental and theoretical investigations on SERS enhancement mechanism of graphene

Graphene has recently been shown to improve the Surface-enhanced Raman Scattering (SERS) performance of traditional nanostructured metallic substrates. Here we present an experimental and theoretical study on its SERS enhancement mechanism. We observed that SERS enhancement of graphene can be tuned by changing its Fermi level via doping. Both molecular doping and gate doping experiments show that hole-doped graphene yields a larger SERS enhancement in methylene blue (MB) than electron-doped one. The MB-graphene system is then modeled using both a fully quantum mechanical (QM) description as well as a QM-polarizable force field model wherein the graphene is modeled using a Drude-Lorentz function. In the first model, charge transfer (CT) excitations between MB and a graphene cluster can be accounted for, while in the second model we can account for the ``infinite'' size of the graphene sheet. Both of the models confirm the role of graphene Fermi level on its SERS enhancement. Our preliminary results suggest that graphene SERS enhancement would likely be coming from a ground-state chemical enhancement.   

Exciting and probing plasmons in graphene by local defects

The short wavelength of collective excitations, i.e., plasmons, in doped graphene ($\sim$10-20 nm) is very attractive for multiple applications. However, the same short wavelength makes photoexcitation of plasmons in graphene a very challenging task. In this work, we discuss various types of local defects including semiconductor quantum dots, metallic nanoclusters, edges and holes in graphene as means to ``squeeze'' the large wavelength of optical excitation down to the nanometer scale, thus, providing an effective coupling between free photons and plasmons in graphene. In the case of semiconductor quantum dots, we show how plasmons in graphene can be excited and probed by Forster resonance energy transfer from the optically excited quantum dot to the graphene sheet. Specifically, we demonstrate how the calculated dispersion relation of plasmons in graphene as well as of other electronic excitations can be accurately extracted by controlling the backgate voltage and the distance between the quantum dot and graphene [1]. \\[4pt] [1] K. A. Velizhanin, A. Efimov``Probing plasmons in graphene by resonance energy transfer,'' Phys. Rev. B, 085401, 84 (2011).