Plasmons describe collective oscillations of electrons. They have a fundamental role in the dynamic responses of electron systems and form the basis of research into optical metamaterials. Plasmons of two-dimensional massless electrons, as present in graphene, show unusual behaviour that enables new tunable plasmonic metamaterials and, potentially, optoelectronic applications in the terahertz frequency range. Here we explore plasmon excitations in engineered graphene micro-ribbon arrays. We demonstrate that graphene plasmon resonances can be tuned over a broad terahertz frequency range by changing micro-ribbon width and in situ electrostatic doping. The ribbon width and carrier doping dependences of graphene plasmon frequency demonstrate power-law behaviour characteristic of two-dimensional massless Dirac electrons. The plasmon resonances have remarkably large oscillator strengths, resulting in prominent room-temperature optical absorption peaks. In comparison, plasmon absorption in a conventional two-dimensional electron gas was observed only at 4.2 K (refs 13, 14). The results represent a first look at light-plasmon coupling in graphene and point to potential graphene-based terahertz metamaterials.

Integrated optical modulators with high modulation speed, small footprint and large optical bandwidth are poised to be the enabling devices for on-chip optical interconnects. Semiconductor modulators have therefore been heavily researched over the past few years. However, the device footprint of silicon-based modulators is of the order of millimetres, owing to its weak electro-optical properties. Germanium and compound semiconductors, on the other hand, face the major challenge of integration with existing silicon electronics and photonics platforms. Integrating silicon modulators with high-quality-factor optical resonators increases the modulation strength, but these devices suffer from intrinsic narrow bandwidth and require sophisticated optical design; they also have stringent fabrication requirements and limited temperature tolerances. Finding a complementary metal-oxide-semiconductor (CMOS)-compatible material with adequate modulation speed and strength has therefore become a task of not only scientific interest, but also industrial importance. Here we experimentally demonstrate a broadband, high-speed, waveguide-integrated electroabsorption modulator based on monolayer graphene. By electrically tuning the Fermi level of the graphene sheet, we demonstrate modulation of the guided light at frequencies over 1 GHz, together with a broad operation spectrum that ranges from 1.35 to 1.6 µm under ambient conditions. The high modulation efficiency of graphene results in an active device area of merely 25 µm(2), which is among the smallest to date. This graphene-based optical modulation mechanism, with combined advantages of compact footprint, low operation voltage and ultrafast modulation speed across a broad range of wavelengths, can enable novel architectures for on-chip optical communications.

Inelastic light scattering spectroscopy has, since its first discovery, been an indispensable tool in physical science for probing elementary excitations, such as phonons, magnons and plasmons in both bulk and nanoscale materials. In the quantum mechanical picture of inelastic light scattering, incident photons first excite a set of intermediate electronic states, which then generate crystal elementary excitations and radiate energy-shifted photons. The intermediate electronic excitations therefore have a crucial role as quantum pathways in inelastic light scattering, and this is exemplified by resonant Raman scattering and Raman interference. The ability to control these excitation pathways can open up new opportunities to probe, manipulate and utilize inelastic light scattering. Here we achieve excitation pathway control in graphene with electrostatic doping. Our study reveals quantum interference between different Raman pathways in graphene: when some of the pathways are blocked, the one-phonon Raman intensity does not diminish, as commonly expected, but increases dramatically. This discovery sheds new light on the understanding of resonance Raman scattering in graphene. In addition, we demonstrate hot-electron luminescence in graphene as the Fermi energy approaches half the laser excitation energy. This hot luminescence, which is another form of inelastic light scattering, results from excited-state relaxation channels that become available only in heavily doped graphene.

Electrons moving in graphene behave as massless Dirac fermions, and they exhibit fascinating low-frequency electrical transport phenomena. Their dynamic response, however, is little known at frequencies above one terahertz (THz). Such knowledge is important not only for a deeper understanding of the Dirac electron quantum transport, but also for graphene applications in ultrahigh speed THz electronics and IR optoelectronics. In this paper, we report the first measurement of high-frequency conductivity of graphene from THz to mid-IR at different carrier concentrations. The conductivity exhibits Drude-like frequency dependence and increases dramatically at THz frequencies, but its absolute strength is substantially lower than theoretical predictions. This anomalous reduction of free electron oscillator strength is corroborated by corresponding changes in graphene interband transitions, as required by the sum rule.Our surprising observation indicates that many-body effects and Dirac fermion-impurity interactions beyond current transport theories are important for Dirac fermion electrical response in graphene.

A facile method is reported for the direct (polymer-free) transfer of layer-area graphene from metal growth substrates to selected target substrates. The direct route, by avoiding several wet chemical steps and accompanying mechanical stresses and contamination common to all presently reported layer-area graphene transfer methods, enables fabrication of layer-area graphene devices with unprecedented quality. To demonstrate, we directly transfer layer-area graphene from Cu growth substrates to holey amorphous carbon transmission electron microscopy (TEM) grids, resulting in robust, clean, full-coverage graphene grids ideal for high resolution TEM.

The interaction of Li(+) with single and few layer graphene is reported. In situ Raman spectra were collected during the electrochemical lithiation of the single- and few-layer graphene. While the interaction of lithium with few layer graphene seems to resemble that of graphite, single layer graphene behaves very differently. The amount of lithium absorbed on single layer graphene seems to be greatly reduced due to repulsion forces between Li(+) at both sides of the graphene layer.

Surface plasmon, with its unique capability to concentrate light into sub-wavelength volume, has enabled great advances in photon science, ranging from nano-antenna 1 and single-molecule Raman scattering 2 to plasmonic waveguide 3 and metamaterials 4,5 . In many applications it is desirable to control the surface plasmon resonance in situ with electric field. Graphene, with its unique tunable optical properties 6,7 , provides an ideal material to integrate with nanometallic structures for realizing such control. Here we demonstrate effective modulation of the plasmon resonance in a model system composed of hybrid graphene-gold nanorod structure. Upon electrical gating the strong optical transitions in graphene can be switched on and off, which leads to significant modulation of both the resonance frequency and quality factor of plasmon resonance in gold nanorods. Hybrid graphene-nanometallic structures, as exemplified by this combination of graphene and gold nanorod, provide a general and powerful way for electrical control of plasmon resonances. It holds promise for novel active optical devices and plasmonic circuits at the deep subwavelength scale.Surface plasmon resonance in nanoscale metal structures has attracted tremendous interest due to its unique capability to concentrate light into deep subwavelength scale. Recent advances in plasmonics have enabled a wide spectrum of applications, such as single-molecule Raman scattering 2 , sub-diffraction-limit imaging 8 , and optical cloaking 4,5 . All these applications rely on our capability to control the surface plasmon resonance through structural design, as embodied in shape-controlled metal nanoparticles 9 , nano-antennas 1 , and plasmonic metamaterials 4,5 . New opportunities can emerge if we can develop new capability in surface plasmon control, such as high-speed, in-situ modulation of plasmon resonances in an existing nanometallic structure.

Fano resonances are features in absorption, scattering or transport spectra resulting from the interaction of discrete and continuum states. They have been observed in a variety of systems [1][2][3][4][5][6] . Here, we report a many-body Fano resonance in bilayer graphene that is continuously tunable by means of electrical gating. Discrete phonons and continuous exciton (electronhole pair) transitions are coupled by electron-phonon interactions, yielding a new hybrid phonon-exciton excited state. It may also be possible to control the phonon-exciton coupling with an optical field. This tunable phonon-exciton system could allow novel applications such as phonon lasers.Two-dimensional graphene shows remarkable electrical 7,8 , vibrational 9-12 and optical properties [13][14][15][16][17] . Many of these properties can be modified in graphene by means of electrical gating, which allows control of the intricate interplay between electrons, phonons and photons. Such control is exemplified in gated bilayer graphene structures in which a gating electrical field can modify phonon-light interactions by inducing infrared activity (that is, the ability to emit and absorb infrared radiation) in an otherwise infrared-inactive phonon vibration (Fig. 1a,b), as well as modifying electron-light interactions by generating a tunable electronic bandgap [17][18][19][20][21][22][23] (Fig. 1c,d). At the same time, electron-phonon interactions are also accompanying the electronic bandgap opening and phonon dipole changes [10][11][12] . This tunable coupling among electrons, vibrations and light offers exciting opportunities in the exploration of new physical phenomena. One example is the control of quantum interference between vibration-and electron-mediated optical absorption ( Fig. 1e), which provides an unusual tunable Fano system involving three different elementary excitations: photon, exciton and phonon.A Fano resonance describes the quantum interference effects on the optical absorption through coupled discrete and continuum transitions, in which the excited eigenstates are mixtures of the discrete and continuum states 1,2 . It gives rise to characteristic asymmetric lineshapes in absorption spectra 1 . First introduced to describe atomic photo-ionization 2 , the Fano lineshape has been observed ubiquitously in neutron scattering 3 , Raman scattering 4 , photo-absorption in quantum wells 5 and electrical transport through nanostructures 6 . In particular, Fano lineshapes have been observed in the Raman spectra of a number of graphitic materials, such as graphite intercalation compounds 24 , fullerene systems 25 , doped carbon nanotube bundles 26 and metallic carbon nanotubes 27 , and are believed to arise from interference between the G-mode phonons and either plasmons or a multi-phonon continuum.However, a tunable Fano system characterized by controlled interference between distinct many-body excitations has been lacking up to now. Here, bilayer graphene provides a unique many-body system in which Fano interference between coupl...