We point out that plasmons in doped graphene simultaneously enable low-losses and significant wave localization for frequencies below that of the optical phonon branch ω Oph ≈ 0.2 eV. Large plasmon losses occur in the interband regime (via excitation of electron-hole pairs), which can be pushed towards higher frequencies for higher doping values. For sufficiently large dopings, there is a bandwidth of frequencies from ω Oph up to the interband threshold, where a plasmon decay channel via emission of an optical phonon together with an electron-hole pair is nonegligible. The calculation of losses is performed within the framework of a random-phase approximation and number conserving relaxation-time approximation. The measured DC relaxation-time serves as an input parameter characterizing collisions with impurities, whereas the contribution from optical phonons is estimated from the influence of the electron-phonon coupling on the optical conductivity. Optical properties of plasmons in graphene are in many relevant aspects similar to optical properties of surface plasmons propagating on dielectric-metal interface, which have been drawing a lot of interest lately because of their importance for nanophotonics. Therefore, the fact that plasmons in graphene could have low losses for certain frequencies makes them potentially interesting for nanophotonic applications.

Near-field thermophotovoltaic (TPV) systems with carefully tailored emitter-PV properties show large promise for a new temperature range (600 – 1200K) solid state energy conversion, where conventional thermoelectric (TE) devices cannot operate due to high temperatures and far-field TPV schemes suffer from low efficiency and power density. We present a detailed theoretical study of several different implementations of thermal emitters using plasmonic materials and graphene. We find that optimal improvements over the black body limit are achieved for low bandgap semiconductors and properly matched plasmonic frequencies. For a pure plasmonic emitter, theoretically predicted generated power density of 14 W/cm2 and efficiency of 36% can be achieved at 600K (hot-side), for 0.17eV bandgap (InSb). Developing insightful approximations, we argue that large plasmonic losses can, contrary to intuition, be helpful in enhancing the overall near-field transfer. We discuss and quantify the properties of an optimal near-field photovoltaic (PV) diode. In addition, we study plasmons in graphene and show that doping can be used to tune the plasmonic dispersion relation to match the PV cell bangap. In case of graphene, theoretically predicted generated power density of 6(120) W/cm2 and efficiency of 35(40)% can be achieved at 600(1200)K, for 0.17eV bandgap. With the ability to operate in intermediate temperature range, as well as high efficiency and power density, near-field TPV systems have the potential to complement conventional TE and TPV solid state heat-to-electricity conversion devices.

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It is shown that thermally excited plasmon-polariton modes can strongly mediate, enhance and \emph{tune} the near-field radiation transfer between two closely separated graphene sheets. The dependence of near-field heat exchange on doping and electron relaxation time is analyzed in the near infra-red within the framework of fluctuational electrodynamics. The dominant contribution to heat transfer can be controlled to arise from either interband or intraband processes. We predict maximum transfer at low doping and for plasmons in two graphene sheets in resonance, with orders-of-magnitude enhancement (e.g. $10^2$ to $10^3$ for separations between $0.1\mu m$ to $10nm$) over the Stefan-Boltzmann law, known as the far field limit. Strong, tunable, near-field transfer offers the promise of an externally controllable thermal switch as well as a novel hybrid graphene-graphene thermoelectric/thermophotovoltaic energy conversion platform.Comment: 4 pages, 3 figure

We predict the existence of transverse electric (TE) plasmons in bilayer graphene. We find that their plasmonic properties are much more pronounced in bilayer than in monolayer graphene, in a sense that they can get more localized at frequencies just below ħω = 0.4 eV for adequate doping values. This is a consequence of the perfectly nested bands in bilayer graphene which are separated by ∼ 0.4 eV.

We predict the existence of coupled plasmon-phonon excitations in graphene by using the selfconsistent linear response formalism. The unique electron-phonon interaction in graphene leads to unconventional mixing of plasmon and optical phonon polarizations. We find that longitudinal plasmons couple exclusively to transverse optical phonons, whereas graphene's transverse plasmons couple only to longitudinal optical phonons. This coupling can serve as a magnifier for exploring the electron-phonon interaction in graphene, and it offers novel electronical control over phonon frequencies.PACS numbers: 73.20. Mf,73.22.Lp,63.22.Rc,78.67.Wj The interaction of electrons and crystal lattice vibrations (phonons) has fundamental implications on properties of materials and leads to diverse many-body phenomena such as superconductivity and charge-density waves. The electron-phonon interaction takes an unusual form in graphene, a recently discovered two-dimensional (2D) material [1] (see, e.g., [2] for a review), and its implications are far from being explored. These include the breakdown of the Born-Oppenheimer approximation [3], anomaly of the optical phonon [4], and nonadiabatic Khon anomaly [5]. However, the interaction of collective electron excitations (plasmons) and optical phonons has not yet been presented for graphene. Plasmons in graphene are of fundamental scientific interest [6][7][8][9][10][11][12], but they also hold potential for technological applications (e.g., in the context of plasmon lasers [9] and metamaterials [12]). Besides the ordinary longitudinal plasmons (transverse magnetic modes) [6][7][8][9]12], graphene also supports unusual transverse plasmons (transverse electric modes) [8]. The hybridization of plasmon and phonon modes is a striking manifestation of the breakdown of the Born-Oppenheimer approximation, because it occurs when phonons and electrons are on comparable energy scales. Plasmon-phonon coupling has been studied in bulk semiconductors [13,14], systems with reduced dimensionality (see e.g. [15][16][17][18]), and in the context of graphene, plasmons were shown to couple to surface optical phonons of the substrate (e.g., SiC, which is a polar material) [11,19,20]. However, to the best of our knowledge, the phenomenon of plasmon-phonon coupling was not yet presented for an isolated 2D material. Here we predict the coupling of plasmons with intrinsic optical phonons in graphene by using the selfconsistent linear response formalism. We find that, in contrast to all other known systems in nature, longitudi- * Electronic address: mjablan@phy.hr † Electronic address: soljacic@mit.edu ‡ Electronic address: hbuljan@phy.hr nal plasmons (LP) couple only to transverse optical (TO) phonons [21], while transverse plasmons (TP) couple only to longitudinal optical (LO) phonons. The LP-TO coupling is stronger for larger concentration of carriers, in contrast to the TP-LO coupling (which is fairly weak). The former could be measured via current experimental techniques. Thus, plasmon-phonon resonance cou...

We show that graphene possesses a strong nonlinear optical response in the form of multiplasmon absorption, with exciting implications in classical and quantum nonlinear optics. Specifically, we predict that graphene nanoribbons can be used as saturable absorbers with low saturation intensity in the farinfrared and terahertz spectrum. Moreover, we predict that two-plasmon absorption and extreme localization of plasmon fields in graphene nanodisks can lead to a plasmon blockade effect, in which a single quantized plasmon strongly suppresses the possibility of exciting a second plasmon. DOI: 10.1103/PhysRevLett.114.236801 PACS numbers: 73.20.Mf, 42.50.Hz, 78.67.Wj, 79.20.Ws The field of nonlinear optics ranges from fundamental questions concerning light-matter interactions to exciting technological applications [1]. However, usually very large field intensities are required to observe nonlinear effects. One is thus always looking for systems that will exhibit nonlinear phenomena at lower powers, with the ultimate limit being strong interactions between just two quanta of light [2]. One possibility to increase nonlinear effects is to use the strong localization and enhancement of electromagnetic fields in the form of surface plasmon excitations [3]. In that regard, we note that graphene [4] has been demonstrated to support extremely localized plasmons [5][6][7][8][9][10][11][12][13][14]. While optical nonlinearities in graphene have been studied by several authors [15][16][17][18][19][20][21][22][23][24][25][26], here we predict a novel nonlinear effect in the form of multiplasmon absorption. We also show how this effect leads to saturable absorption in graphene nanoribbons at low input powers in the farinfrared and terahertz spectrum. Moreover, we predict that the extreme localization of plasmon fields in graphene nanodisks leads to such a strong two-plasmon absorption that it becomes nearly impossible to excite a second quantized plasmon in the system. This plasmon blockade effect would cause the nanodisk to behave essentially like a quantum two-level system, which is observable in its resonance fluorescence spectrum.Graphene is a two-dimensional hexagonal lattice of carbon atoms [4]. The low-energy band structure of graphene is described by Dirac cones with the electron dispersion E nk ¼ nℏv F jkj, where v F ¼ 10 6 m=s and n ¼ AE1 stands for the conduction (valence) band [27]. In its intrinsic form graphene is a zero-gap semiconductor; however, it can also be easily doped with free carriers and as such it supports plasmon modes [5][6][7]. At low frequencies, one can get a rather accurate description of these modes by using a simple Drude conductivitywhere E F is the Fermi energy of graphene and γ is a phenomenological damping rate that takes into account various decay channels like impurity or phonon scattering [7]. The resulting plasmon dispersion is given byand we assume the average dielectric permittivityε r ≈ 2.5, which roughly corresponds to the case of graphene on a SiO 2 substrate and air on top. This...

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