Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R5: Graphene: Transport and Optical Properties: THz and Plasmons |
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Sponsoring Units: DCMP Chair: Elbert Chia, Nanyang Technological University Room: 301 |
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R5.00001: Terahertz conductivity of twisted bilayer graphene Elbert E.M. Chia, Xingquan Zou, Jingzhi Shang, Jianing Leaw, Zhiqiang Luo, Liyan Luo, Siew Ann Cheong, Haibin Su, Jian-Xin Zhu, A.H. Castro Neto, Ting Yu Using terahertz time-domain spectroscopy, the real part of optical conductivity [$\sigma_{1}(\omega)$] of twisted bilayer graphene was obtained at different temperatures (10 -- 300~K) in the frequency range 0.3 -- 3~THz. On top of a Drude-like response, we see a strong and narrow peak in $\sigma_{1} (\omega)$ at $\sim$2.7~THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7~THz to an enhanced density of states at that energy, that is caused by the presence of van Hove singularities arising from a commensurate twisting of the two graphene layers. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R5.00002: Theory of optical responses in the bilayer and trilayer graphene in the quantum Hall regime Takahiro Morimoto, Mikito Koshino, Hideo Aoki In the graphene physics, there are growing interests toward bilayer and trilayer graphene, whose electronic structures are distinct from that of monolayer graphene. It is then interesting to ask how the variety of low-lying electronic structures will affect optical responses, i.e., optical longitudinal and optical Hall conductivities, where the former describes the absorption while the latter the Faraday and Kerr rotations. Thus we study the optical conductivities in bilayer and trilayer graphene systems. We shall show for bilayer graphene that the Lifshitz transition associated with the trigonal warping greatly affects the resonance structures in Faraday rotation not only on low-energy scale where Dirac cones emerges but also in the higher-energy range with parabolic bands as a sequence of satellite resonances. For trilayer graphene, on the other hand, we shall show that the optical conductivities are dominated by the difference in the stacking order. In ABA trilayer, the resonance spectrum is a superposition of effective monolayer and bilayer contributions with band gaps, while ABC trilayer exhibits a distinct spectrum peculiar to the cubic-dispersed bands. In the latter, the trigonal warping effect becomes strong with a larger Lifshitz transition energy ($\sim$10 meV). [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R5.00003: Dynamical Conductivity of AA-Stacked Bilayer Graphene Calvin Tabert, Elisabeth Nicol Motivated by the potential availability of AA-stacked bilayer graphene samples[1,2], we investigate the optical conductivity of this stacking variation[3]. We find the band structure to be made of bonding and antibonding orbitals which are linear at low energy and decoupled for the longitudinal response; this causes the conductivity to behave as the sum of an electron-doped and hole-doped monolayer graphene system. We find a low energy Drude response at charge neutrality and two step features which can be tuned by varying the chemical potential. We find that the interlayer hopping energy plays an important role in determining the onset of these steps. We compute the partial optical sum and find that the Drude weight also depends on the value of chemical potential relative to the interlayer hopping parameter. \\[4pt] [1] J. K. Lee et al. J. Chem. Phys. 129 234709 (2008) [2] W. Norimatsu et al. Phys. Rev. B 81 161410 (2010) [3] C. J. Tabert et al. Phys. Rev. B 86 075439 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R5.00004: Graphene's Dynamic Conductivity in THz Regime Sufei Shi, Tsung-Ta Tang, Bo Zeng, Long Ju, Feng Wang Graphene, a single layer of carbon atoms arranged in honeycomb structure, has linear dispersion relation. The conductivity of graphene in the THz regime is highly tunable due to its gapless dispersion relation, which makes graphene a promising candidate for THz application. Using optical excitation as the pump, we probe graphene with a THz beam and study the THz conductivity in the time domain. This study sheds light on the carrier relaxation in graphene after intense optical excitation and provides information for designing future graphene-based opto-electronic device. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R5.00005: Mid-Infrared Graphene Photoresponse Allen Hsu, Patrick Herring, Yong Cheol Shin, Ki Kang Kim, Jing Kong, Charlie Marcus, Nathaniel Gabor, Tomas Palacios, Pablo Jarillo-Herrero Graphene is a two-dimensional (2D) material that has attracted great interest for electronic devices since its discovery in 2004. Due to its zero band gap band structure, it has a broad-band optical absorption ranging from the far-infrared all the way to the visible making it potentially useful for infrared photodetectors. Electrostatically gated p-n junctions have demonstrated photocurrents in the near-IR ($\lambda =$ 850nm), primarily due to hot carrier mechanisms. In order to study these mechanisms at longer wavelengths ($\lambda =$ 10 $\mu $m), high quality chemically vapor grown (CVD) graphene is necessary to fabricate electrostatically controlled p-n junctions due to the longer optical length scales. Moreover, at these low energies ($\sim$ 125 meV), optical phonon scattering is suppressed and is predicted to lead to increased carrier lifetimes and enhanced photo-response. Using electrostatic gating, we are able to study the absorption mechanisms in graphene by selecting between conventional photovoltaic effects and photo-thermoelectric effects. Experiments suggest that the photocurrent signal is enhanced by electrostatic gating near the Dirac peak and reduced disorder in the graphene sample. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R5.00006: Ga Nanoparticle/Graphene Platforms: Plasmonic and Charge Transfer Interactions Congwen Yi, Tong-Ho Kim, Yang Yang, Maria Losurdo, April S. Brown Metal nanoparticle (NP) -- graphene multifunctional platforms are of great interest for numerous applications, such as sensing and catalysis, and for fundamental studies on charge transfer and light-matter interactions. To understand platform-photon interactions, it is important to articulate the coupling of photon-based excitations, such as the interaction between plasmons in each of the material components, as well as their charge-based interactions dependent upon the energy alignment at the metal/graphene interface. Herein, we use liquid metal Ga nanoparticles, which can be deposited at 300K on graphene, to explore the surface-enhanced Raman spectroscopy modulation induced by the NPs,. The localized charge transfer between Ga NPs and graphene are investigated, and enhancement of the graphene Raman modes is correlated with metal coverage the transfer of electrons from Ga to graphene creating local regions of enhanced electron concentration which modify the electron-phonon interaction in graphene. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R5.00007: Terahertz and mid-infrared reflectance of epitaxial graphene Cristiane N. Santos, Benoit Hackens, Fr\'ed\'eric Joucken, Robert Sporken, Jessica Campos Delgado, Jean-Pierre Raskin, Domingos De Sousa Meneses, Patrick Echegut Epitaxial graphene grown by thermal decomposition on SiC substrate has been widely investigated as a promising material for electronics and optics. Here, we investigate the infrared (IR) optical properties of few-layer (FL) and multilayer (ML) graphene on the C-terminated face of 6H-SiC substrates [1]. Contrary to IR transmission spectroscopy, which is hampered over a large part of the IR range by the SiC reststrahlen band and multiphonon absorption, IR reflectance gives access to invaluable information from terahertz (THz) to mid-infrared (MIR). Experimental data are well fitted with an explicit model over the entire spectral range using the SiC dielectric function and the graphene optical conductivity, taking into account both intraband and interband transitions. The number of layers extracted from our data in the FL and ML graphene corroborates with the X-ray photoelectron spectroscopy (XPS) measurements. We demonstrate that this consistent and simultaneous analysis leads to precise information on the carrier properties, doping level and the number of layers, even in the case of thick ML (30 layers or more). MIR microscopy was also used to check the sample homogeneity. [1] F. Joucken et al., Phys. Rev. B 85, 161408(R) (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R5.00008: Self-Energy and Excitonic Contributions to the Drude Conductivity of Doped Graphene Felipe Jornada, Steven Louie There has been a growing interest in the far infrared AC conductivity of doped graphene because of possible applications in optoelectronics, but there is still disagreement between recent experiments [1,2] and theories [3] with respect to the Drude weight. In this work we study from an ab-inito GW-BSE perspective the effects of the electron-electron interactions and excitons in the renormalization of the Drude weight. We discuss the role of quasiparticle lifetimes due to electron-electron and electron-phonon interactions, and we determine the AC conductivity in the forbidden region (i.e., for $\omega < 2 E_F$). This work was supported by NSF grant No. DMR10-1006184, U.S. DOE under Contract No. DE-AC02-05CH11231 and the U.S. DOD - Office of Naval Research under RTC Grant No. N00014-09-1-1066. Computational resources have been provided by NERSC. [1] J. Horng et al., PRB 83, 165113 (2011). [2] H. Yan et al., ACS Nano 5, 9854 (2011). [3] S. H. Abedinpour et al., PRB 84, 045429 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R5.00009: Tuning optical conductivity of large-scale CVD graphene by strain engineering Guangxin Ni, Jing Wu, Orhan Kahya, Chee Tat Toh, Jong Hyun Ahn, Vitor M. Pereira, Barbaros \"Ozyilmaz Strain engineering has been widely recognized as an effective way to tailor the electrical properties of graphene. In the optical domain, the strain effect is also predicted to alter the optical conductivity of graphene, making graphene possible for the atomically thin optical elements. However, a direct experimental observation is still missing. Using the nanopillar structure, here we show that optical conductivity of CVD graphene under nonuniform strain exhibits periodic modulation as a function of polarization. The optical absorption can be further modulated via the application of an external uniaxial strain, which is confirmed by Raman spectroscopy as well as AFM images. Our experimental observations are quantitatively interpreted within the Kubo-Greenwood formalism. The manipulation of the optical properties of graphene demonstrated in this study can be effectively utilized in the novel type of optical devices and strain sensor applications. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R5.00010: Tunable magneto-plasmons in graphene: an infrared study Zhiguo Chen, Hugen Yan, Xuesong Li, Wenjuan Zhu, Phaedon Avouris, Fengnian Xia, Zhiqiang Li Plasmons, collective oscillations of electrons, in graphene have attracted much attention due to their important roles in understanding the intriguing physics of graphene and potential applications in optoelectronic devices. Using infrared spectroscopy, we investigated the optical response of the plasmons in micrometer-sized graphene disks in high magnetic fields up to 18 T. Our study shows that the plasmon resonance splits into edge and bulk modes in magnetic fields. Due to the linear band structure of graphene, the splitting exhibits a sensitive doping dependence, which is not observed in conventional two-dimensional electron gas systems. Moreover, the lifetime of the two modes can be dramatically modified by magnetic fields, with the edge plasmons developing increasingly longer lifetimes in high fields. The latter behavior can be understood from the suppression of backscattering at the edges. Our work not only opens an avenue to explore the magneto-plasmons and edge physics in graphene but also supports the great potential of graphene for tunable magneto-optical devices.\\[4pt] [1] Hugen Yan, Zhiqiang Li, Xuesong Li, Wenjuan Zhu, Phaedon Avouris, and Fengnian Xia, Nano letters, 12, 3766 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R5.00011: Long-range plasmon-assisted energy transfer over doped graphene Kirill Velizhanin, Tigran Shahbazyan F\"orster resonance energy transfer (FRET) between spatially separated donor and acceptor fluorophores, such as dye molecules or semiconductors quantum dots, underpins diverse phenomena in physics, chemistry and biology. However, the range of present and potential applications of FRET is limited by its intrinsically short-range nature ($\sim 1/R^6$). We demonstrate that longitudinal plasmons in doped monolayer graphene can mediate highly efficient long-range ($\sim 1/R$) energy transfer between nearby fluorophores, e.g., semiconductor quantum dots. We derive a simple analytical expression for the energy transfer efficiency that incorporates all the essential processes involved. We perform numerical calculations of the transfer efficiency for a pair of PbSe quantum dots near graphene for inter-fluorophore distances of up to 1 $\mu$m and find that the plasmon-assisted long-range energy transfer can be enhanced by up to a factor of $\sim 10^4$ relative to FRET in vacuum. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R5.00012: Graphene multilayers as hyperbolic metamaterials Ashley DaSilva, Allan MacDonald Graphene and multilayer graphene systems show promise for numerous electronic and optical applications in part due to the extraordinary tunability of graphene via gate voltage. We discuss the optical properties of electrically decoupled multilayer graphene systems. These can be described by the reflection and transmission coefficients, which we calculate using a transfer matrix approach. This point of view allows an explicit comparison between graphene multilayers and metal/dielectric multilayer metamaterials. In particular, we will compare multilayer graphene systems to hyperbolic metamaterials which have extreme anisotropy in the effective dielectric constant: $\epsilon_{x}=\epsilon_{y}<0$ and $\epsilon_{z}>0$. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R5.00013: Magnetoplasmons in quasi-neutral epitaxial graphene nanoribbons Jean-Marie Poumirol, Wenlong Yu, Claire Berger, Walter de Heer, Michael Smith, Taisuke Ohta, Wei Pan, Dmitry Smirnov, Zhigang Jiang We report on infrared transmission spectroscopy study of magnetoplasmons in quasi-neutral epitaxial graphene nanoribbon arrays. The energy of the $ L_{0(-1)}\to L_{1(0)}$ inter-Landau level transitions deviates from the characteristic $\sqrt{B}$ dependence observed in two-dimensional graphene. This behavior is explained as a signature of the upper hybrid mode formed between the Landau level transition and the plasmon resonance. Studying the hybrid mode allows us to probe the zero magnetic field plasmon resonance in the interacting regime, when coupling to electron-holes excitations results in strong decay of plasmons. We observe a deviation of the plasmon frequency from the standard $\omega_{pl}\propto q^{1/2}$ dispersion relation, and attribute it to the finite length of the graphene ribbons. [Preview Abstract] |
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