We have fabricated a centimeter-size single-layer graphene device, with a gate electrode, which can modulate the transmission of terahertz and infrared waves. Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range (10-10000 cm -1 ), we measured the dynamic conductivity change induced by electrical gating and thermal annealing.Both methods were able to effectively tune the Fermi energy, E F , which in turn modified the Drude-like intraband absorption in the terahertz as well as the '2E F onset' for interband absorption in the midinfrared. These results not only provide fundamental insight into the electromagnetic response of Dirac fermions in graphene but also demonstrate the key functionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics. KEYWORDS: graphene, Fermi level, terahertz dynamics, infrared spectroscopyThe AC dynamics of Dirac fermions in graphene have attracted much recent attention. The influence of linear dispersions, two-dimensionality, electron-electron interactions, and disorder on the dynamic conductivity, σ(ω), has been theoretically investigated, 1-11 whereas unique terahertz (THz) and mid-infrared (MIR) properties have been identified for novel optoelectronic applications. 12-17 For example, it has been predicted that the response of Dirac fermions to an applied AC electric field of frequency ω would automatically contain all odd harmonics of (2n+1)ω, where n is an integer, implying extremely high nonlinearity. 13,14 Furthermore, creation of electrons and holes through interband optical pumping is expected to lead to population inversion near the Dirac point, resulting in negative σ(ω), or gain, in the THz to MIR range. 12,17 While initial experimental investigations on graphene have concentrated on DC characteristics, these recent theoretical studies have instigated a flurry of new experimental activities to uncover unusual AC properties. A number of experiments have already confirmed the so-called universal optical conductivity σ 0 = e 2 /4 ! (e: electronic charge and ! : reduced Planck constant) for interband transitions in a wide spectral range. [18][19][20][21] On the other hand, experimental studies of the intraband conductivity have been very limited, [21][22][23][24] Here, we describe our THz and MIR spectroscopy study of large-area (centimeter scale), single-layer graphene with an electrically tunable Fermi level. In a field-effect transistor configuration consisting of graphene on a SiO 2 /p-Si substrate, the transmitted intensity of THz and MIR electromagnetic waves was observed to change with the gate voltage. The Drude-like intraband conductivities and the '2E F onset' of the interband transitions, monitored through time-domain THz spectroscopy (TDTS) and Fourier-transform IR (FTIR) spectroscopy, respectively, were both modulated by the gate voltage. By analyzing the spectral shape of the induced changes with appropriate models, we were able to determine ...

Single-wall carbon nanotubes (SWCNTs), with their uniquely simple crystal structures and chirality-dependent electronic and vibrational states, provide an ideal laboratory for the exploration of novel 1D physics, as well as quantum engineered architectures for applications in optoelectronics. This article provides an overview of recent progress in optical studies of SWCNTs. In particular, recent progress in post-growth separation methods allows different species of SWCNTs to be sorted out in bulk quantities according to their diameters, chiralities, and electronic types, enabling studies of (n,m)-dependent properties using standard macroscopic characterization measurements. Here, a review is presented of recent optical studies of samples enriched in 'armchair' (n = m) species, which are truly metallic nanotubes but show excitonic interband absorption. Furthermore, it is shown that intense ultrashort optical pulses can induce ultrafast bandgap oscillations in SWCNTs, via the generation of coherent phonons, which in turn modulate the transmission of a delayed probe pulse. Combined with pulse-shaping techniques, coherent phonon spectroscopy provides a powerful method for studying exciton-phonon coupling in SWCNTs in a chirality-selective manner. Finally, some of the basic properties of highly aligned SWCNT films are highlighted, which are particularly well-suited for optoelectronic applications including terahertz polarizers with nearly perfect extinction ratios and broadband photodetectors.

The ability to confine light into tiny spatial dimensions is important for applications such as microscopy, sensing, and nanoscale lasers. Although plasmons offer an appealing avenue to confine light, Landau damping in metals imposes a trade-off between optical field confinement and losses. We show that a graphene-insulator-metal heterostructure can overcome that trade-off, and demonstrate plasmon confinement down to the ultimate limit of the length scale of one atom. This is achieved through far-field excitation of plasmon modes squeezed into an atomically thin hexagonal boron nitride dielectric spacer between graphene and metal rods. A theoretical model that takes into account the nonlocal optical response of both graphene and metal is used to describe the results. These ultraconfined plasmonic modes, addressed with far-field light excitation, enable a route to new regimes of ultrastrong light-matter interactions.