Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y40: Plasmons in Graphene and Optical Properties of 2D Materials |
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Sponsoring Units: DCMP Chair: Rahul Rao, UES, Inc. Room: LACC 501C |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y40.00001: Ballistic plasmon polaritons in graphene GuangXin Ni, Alex McLeod, Zhiyuan Sun, Lei Wang, Lin Xiong, Kirk Post, Sai Sunku, Bor-Yuan Jiang, James Hone, Cory Dean, Michael Fogler, Dimitri Basov Polaritons are hybrid excitations of light and matter that can confine the energy of long-wavelength radiation at the nano-scale. The polaritons can enable many enigmatic quantum effects including condensation and superfluidity, lasing, topological protection, and dipole-forbidden absorption. A necessary condition for realizing these novel phenomena is a long polariton lifetime, which is notoriously difficult to meet. Surface plasmon polaritons in graphene – hybrid modes of Dirac quasiparticles and infrared photons – have emerged as an outstanding platform for exploring light-matter interaction at the nano-scale. However, plasmonic dissipation in graphene has remained significant and its fundamental limits remained undetermined. Here we exploit nano-infrared imaging to investigate propagating plasmons in high mobility graphene at cryogenic temperatures where electrons exhibit ballistic transport. In this regime, plasmons likewise travel over distances limited largely by sample dimensions. Our analysis reveals an intrinsic plasmonic propagation length beyond ten micrometers at liquid nitrogen temperatures that are commonplace in infrared technology. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y40.00002: Origin of plasmon damping in pristine graphene Zhiyuan Sun, Dimitri Basov, Michael Fogler The physical origin and the fundamental limits of plasmonic losses in graphene is a problem that attracted much attention. Recent near-field experiments have provided a key input for solving it, namely, the temperature dependence of the losses. We identify electron-phonon coupling through pseudomagnetic field as the dominant mechanism that limits the frequency-dependent transport scattering rate of Dirac electrons and therefore plasmon damping. We calculate scattering rate due to two types of phonons, the acoustic and the A′1 zone-boundary ones. The former induces intravalley scattering. It dominates at low temperatures and has a weak frequency dependence. The latter causes intervalley scattering of Dirac electrons. It has an activated dependence on temperature, an exponential dependence on plasmon frequency, and it becomes dominant at room temperature. We also consider additional small contributions from electron-electron scattering. Previously studied electron-phonon coupling due to deformation potential is shown to be negligible. Our theoretical results are in quantitative agreement with the experiments. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y40.00003: Plasmons and demons in graphene Michael Fogler, Zhiyuan Sun We study theoretically the optical response of graphene in a range of temperature and doping where both electrons and holes are present. We show that two types of collective modes should exist in this systems. Besides the higher-frequency mode, which is the familiar plasmon, graphene should support a lower-frequency mode, which is variously referred to as the sound, acoustic plasmon, energy wave, or, as we prefer, the demon. The demon can be studied using the relativistic hydrodynamics approach. We discuss linear and nonlinear phenomena enabled by the demons and how they can be observed in nano-optics experiments. We also address nonlinear electric and thermal transport in graphene in the hydrodynamic regime. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y40.00004: Effect of particle-hole continuum on two-stream instability modes in graphene Ben Hu, Mitchell Duffer Recent theoretical studies [1,2] have shown that under conditions in graphene that are analogous to the two-stream instability in plasma physics, some plasmons may be unstable in the sense that their amplitudes increase with time. These studies used approximate forms of the dielectric functions that excluded the effects of the particle-hole excitation continuum (PHEC), which typically leads to decay of plasmons. For parabolic band systems the electron density can be tuned so that the unstable mode lies outside the PHEC. However, in graphene, it is not possible to do this (because the fermi velocity does not change with electron density) and therefore it is important that theoretical studies take into account the effects of the PHEC. We report theoretical studies of these unstable plasmon modes in single-layer graphene in which we utilize an approximation of the dielectric function which includes the effect of the PHEC. Our preliminary results suggest that while the PHEC does indeed suppress the formation of the unstable plasmons, there is a range of experimental parameters where unstable plasmons still exist. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y40.00005: Plasmon reflections by topological electronic boundaries in bilayer graphene Bor-Yuan Jiang, GuangXin Ni, Zachariah Addison, Jing Shi, Xiaomeng Liu, Shu Yang Frank Zhao, Philip Kim, Eugene Mele, Dimitri Basov, Michael Fogler Domain walls separating regions of AB and BA interlayer stacking in bilayer graphene have attracted attention as novel examples of structural solitons, topological electronic boundaries, and nanoscale plasmonic scatterers. We show that strong coupling of domain walls to surface plasmons observed in infrared nanoimaging experiments is due to topological chiral modes confined to the walls. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y40.00006: Binding energies of trions and biexcitons in layered semiconductors from diffusion quantum Monte Carlo calculations Elaheh Mostaani, Marcin Szyniszewski, Neil Drummond, Andrea Ferrari, Vladimir Falko Excitonic effects play a key role in the optoelectronic behavior of layered materials semiconductors (LM-S). To facilitate the interpretation of experimental photoabsorption and photoluminescence spectra we provide statistically exact diffusion quantum Monte Carlo binding energy (BE) data for Mott-Wannier models of excitons, trions, and biexcitons in LM-S. We also provide interpolation formulas giving the BE and contact density as functions of the electron and hole effective masses and in-plane polarizability. Our results suggest that experimental spectra have been misclassified, because the trion BE should exceed the biexciton BE, but they also indicate that the Keldysh interaction fails to give a quantitative description of the observed excitonic properties of LM-S. The contact pair density data that we provide will enable the theoretical and experimental exploration of the role played by contact interactions between charge carriers and intervalley scattering in LM-S [1]. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y40.00007: Strong Optical Nonlinearity of Graphene Nanoribbons Farhad Karimi, Irena Knezevic Many nonlinear nanophotonics applications require strong nonlinear light-matter interaction; hence, there is ongoing research on finding materials with a strong nonlinear optical response. Here, we calculate the third-order Kerr susceptibility and the third-harmonic-generation susceptibility of graphene nanoribbons (GNRs) and show that, in the long-wavelength regime, GNRs have a remarkably strong nonlinear optical response, particularly the Kerr optical response, in a broad frequency range: from terahertz to near-infrared. The strong optical nonlinearity of GNRs in the long-wavelength region enables stacking them on top of one another and embedding them in the on-chip semiconductor waveguides, and therefore makes GNRs a promising material for integrated nonlinear nanophotonics applications. Our analysis is based on perturbatively solving a Lindblad-type quantum-master equation. We show that electron scattering plays a nontrivial role in the nonlinear optical response of GNRs and semiclassical approaches, such as the relaxation-time approximation, are not accurate for calculating the GNRs optical nonlinearity. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y40.00008: Nonlinear Optical Nano-imaging of Graphene Tao Jiang, Vasily Kravtsov, Molly May, Sultan Almutairi, Alexey Belyanin, Markus Raschke Nonlinear optical response offers a powerful tool for investigating a wide range of phenomena in physics, chemistry, and biology and provides a basis for future photonic and optoelectronic applications. Nonlinear optics of graphene is particularly interesting due to the unique Dirac-cone band structure, with the associated strong nonlinear response over a broad spectral range. Here, we apply the adiabatic plasmonic nanofocusing technique to study the mechanism and nanoscale spatial heterogeneity of the nonlinear optical response of graphene. We generate strong four-wave mixing (FWM) signal in monolayer and few-layer graphene with compressed surface plasmon polaritons (SPPs) at a Au nano-tip apex and investigate its heterogeneity by nonlinear imaging with nanoscale spatial resolution. We find a pronounced enhancement of the graphene FWM response at crystal edges associated with the formation of a tip-edge resonant cavity and study its mechanism and properties by spectral tuning and electrostatic gating. Our results reveal highly localized and plasmon-enhanced nonlinear optical interactions in graphene and suggest exciting possibilities of graphene-plasmon-mediated nonlinearities, with applications in photonic circuits, all-optical information processing, and quantum optics. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y40.00009: First-Principles Study of Phonon Anharmonicity in Atomically-Thin Black Phosphorus Andrew Cupo, Damien Tristant, Vincent Meunier Black phosphorus (BP) is of interest due to its semiconducting band gap which remains direct independent of the number of layers, as well as its high carrier mobility. It was shown experimentally that the Raman active modes Ag1, B2g, and Ag2 have frequencies which downshift with increasing temperature. In this work we study this phenomenon for the single-layer using first-principles density functional theory (DFT) calculations. To model the temperature-induced shift of the phonon frequencies, we carry out ab initio molecular dynamics simulations with varied temperature. The normal mode frequencies are located at the peak positions in the power spectrum, which is calculated as the magnitude of the Fourier transform of the total velocity autocorrelation. Anharmonicity induces a frequency shift for each mode individually as well as from phonon-phonon coupling, with the latter interpreted classically as an effective damping of a given mode due to its interaction with all other modes. The effect of thermal expansion is also included by imposing the temperature dependent lattice constant in each simulation as calculated within the quasiharmonic approximation. In general we obtain frequency downshifts with increasing temperature in agreement with experiment. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y40.00010: Stacking-induced Second Harmonic Generation in Graphene Trilayers Yuwei Shan, Yingguo Li, Di Huang, Qingjun Tong, Wang Yao, Weitao Liu, Shiwei Wu Van der Waals stacking of layered two-dimensional crystals, such as few-layer graphene, can lead to profound effects on electronic, optical and mechanic properties. It has been reported that the Bernal (ABA) and rhombohedral (ABC) stacking graphene trilayers have dramatically different electronic, transport and optical properties. So it is crucial to develop an effective method to identify the stacking order and domain of graphene trilayers. Previously, Raman scattering and infrared absorption spectroscopies have been employed to investigate the stacking order of graphene few-layers. However, all these techniques have either low imaging throughput or poor spatial resolution. In this talk, we report the use of second harmonic generation (SHG) microscopy to image the ABA and ABC stacking order of graphene trilayers due to their distinct structural symmetry. We found that the stacking-induced SHG response from ABA stacked trilayer is surprisingly strong, comparable to other non-centrosymmetric layered materials. Furthermore, the crystalline orientation of ABA trilayer could be unambiguously determined by polarization-resolved SHG measurement. Therefore, our work is expected to expedite the exploration of stacking dependent physics in graphene. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y40.00011: Gate Tunability of Third Harmonic Generation in CVD Graphene Steven Drapcho, Hsin-Zon Tsai, Iqbal Utama, Chan-Shan Yang, Michael Crommie, Feng Wang One of the key features that has driven interest in graphene since its discovery is the ability to control its properties through electrostatic doping. It has been widely established that many optical properties of graphene, such as absorption and Raman scattering, can be tuned by applying a gate voltage to modulate the carrier concentration. We show that the nonlinear third order response of graphene can also be modulated by measuring third harmonic generation spectra in CVD graphene while controlling the doping level using ion gel gating. We measure the magnitude and relative phase change of the third harmonic light and compare our results to theories of the third order optical response in graphene. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y40.00012: Novel Raman Spectroscopy Instrumentation for Investigating Quantum Materials Amber McCreary, Jeffrey Simpson, Heather Hill, Angela Hight Walker Raman spectroscopy, imaging, and mapping are powerful non-contact, non-destructive optical methods to probe the fundamental physics of two-dimensional (2D) layered materials. An amazing amount of information can be quantified from the spectra such as layer thickness, disorder, grain boundaries, doping, strain, and thermal conductivity. Most interestingly for the 2D materials is that Raman efficiently probes the evolution of the electronic structure and the electron-phonon interactions as a function of temperature, laser energy, and polarization. Our unique magneto-Raman spectroscopic capabilities will be detailed, which allows for diffraction-limited, spatially-resolved Raman measurements while simultaneously varying the temperature (4K to 400 K), laser wavelength (tunability from visible to near infrared), and magnetic field (up to 9 T) to study the photo-physics of nanomaterials. Additionally, coupling to a triple grating spectrometer provides access to low-frequency phonon modes. Current results on novel 2D materials will be presented to highlight our instrumentation capabilities, including resonant Raman studies of anisotropic ReS2, temperature- and magnetic field-dependent studies of ferromagnetic CrBr3, and an investigation on the Raman spectra of 2D GaN. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y40.00013: Quantum dots and quantum devices in 2d-semiconductors Carmen Palacios-Berraquero, Dhiren Kara, Mete Atature, Alejandro Montblanch, Matteo Barbone, Marko Loncar, Pawel Latawiec, Andrea Ferrari The atomically-thin semiconductor family of transition metal dichalcogenides (2d-TMDs) offer technological advantages as platforms for quantum devices. Lack of dangling bonds, atomically-precise interfaces, absence of nuclear spins and ease of integration to form 2d heterostructures are some. I will present how we deterministically create quantum dots (QDs) in 2d-TMDs, in large-scale arrays. The QDs are single-photon sources, and exhibit better stability than their random counterparts. They are formed by stamping the monolayer flakes onto silica substrates patterned with nanopillars, over which the flake tents. I will report on the method and on the magneto-optical characterization of the emitters. The QDs can be embedded within 2d-heterostructures to form functional quantum devices: we use 1L-WSe2 and WS2, along with graphene and hexagonal boron nitride, to create quantum light-emitting diodes that produce electrically driven single-photons. Finally, I will introduce our current work: achieving charge-tunability of the WSe2-QDs using field-effect type 2d-devices. We are able to controllably charge the QDs with single-electrons and single-holes - first step towards the use of spin and valley pseudospin of single carriers as optically addressable matter qubits in 2d materials. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y40.00014: ARPES studies and band structure modulation of thermoelectric material SnSe Kenan Zhang, Ke Deng, Jiaheng Li, Haoxiong Zhang, Yang Wu, Duan Wenhui, Shuyun Zhou SnSe, a group IV-VI monochalcogenide with similar lattice structure to black phosphorus, has recently attracted particular interest due to its excellent thermoelectric properties and potential applications. Here, we present the anisotropic valence bands and multi-valley dispersions which are closely related to its thermoelectric properties on intrinsically semiconducting SnSe crystals by systematic ARPES experiments. Moreover, we carried out the band structure modulation of SnSe and novel phenomena were observed. |
Friday, March 9, 2018 2:03PM - 2:15PM |
Y40.00015: WS2 in ultrafast laser pulse S. Azar Oliaei Motlagh, Jhih-Sheng Wu, Vadym Apalkov, Mark Stockman We theoretically investigate the interaction of a circularly polarized ultrafast laser pulse with a monolayer of WS2 which belongs to a class of materials known as transition metal dichalcogenides. The applied pulse has a duration of about 4fs. The amplitude of the electric field is about 0.1-0.3 V/Å, and it is comparable to the internal fields acting on electrons. This causes strong interband coupling which results in strong redistribution of electrons between the valence and conduction bands. The monolayer of WS2 has two inequivalent valleys in reciprocal space. We predict that one-cycle circular ultrafast laser pulse populates these valleys unequally leading to the high valley polarization in this monolayer. |
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