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 ...

Plasmon resonance is expected to occur in metallic and doped semiconducting carbon nanotubes in the terahertz frequency range, but its convincing identification has so far been elusive. The origin of the terahertz conductivity peak commonly observed for carbon nanotube ensembles remains controversial. Here we present results of optical, terahertz, and DC transport measurements on highly enriched metallic and semiconducting nanotube films. A broad and strong terahertz conductivity peak appears in both types of films, whose behaviors are consistent with the plasmon resonance explanation, firmly ruling out other alternative explanations such as absorption due to curvature-induced gaps.

We describe a film of highly-aligned single-walled carbon nanotubes that acts as an excellent terahertz linear polarizer. There is virtually no attenuation (strong absorption) when the terahertz polarization is perpendicular (parallel) to the nanotube axis. From the data we calculated the reduced linear dichrosim to be 3, corresponding to a nematic order parameter of 1, which demonstrates nearly perfect alignment as well as intrinsically anisotropic terahertz response of single-walled carbon nanotubes in the film.

We demonstrate a terahertz polarizer built with stacks of aligned single-walled carbon nanotubes (SWCNTs) exhibiting ideal broadband terahertz properties: 99.9% degree of polarization and extinction ratios of 10(-3) (or 30 dB) from ~0.4 to 2.2 THz. Compared to structurally tuned and fragile wire-grid systems, the performance in these polarizers is driven by the inherent anistropic absorption of SWCNTs that enables a physically robust structure. Supported by a scalable dry contact-transfer approach, these SWCNT-based polarizers are ideal for emerging terahertz applications.

Hexagonal Se nanowires were synthesized using a simple vapor-phase growth with the assistance of the silicon powder as a source material, which turned out to be very important in the growth of the Se nanowires. The morphology, microstructure, and chemical compositions of the nanowires were characterized using various means (XRD, SEM, TEM, XPS, and Raman spectroscopy). The possible growth mechanism of the Se nanowires was explained. The as-grown Se nanowires may find wide applications in biology and optoelectronics.

No abstract

We study macroscopically-aligned single-wall carbon nanotube arrays with uniform lengths via polarization-dependent terahertz and infrared transmission spectroscopy. Polarization anisotropy is extreme at frequencies less than ∼3 THz with no sign of attenuation when the polarization is perpendicular to the alignment direction. The attenuation for both parallel and perpendicular polarizations increases with increasing frequency, exhibiting a pronounced and broad peak around 10 THz in the parallel case. We model the electromagnetic response of the sample by taking into account both radiative scattering and absorption losses. We show that our sample acts as an effective antenna due to the high degree of alignment, exhibiting much larger radiative scattering than absorption in the mid/far-infrared range. Our calculated attenuation spectrum clearly shows a non-Drude peak at ∼10 THz in agreement with the experiment.

We have observed cyclotron resonance in a high-mobility GaAs/AlGaAs two-dimensional electron gas by using the techniques of terahertz time-domain spectroscopy combined with magnetic fields. From this, we calculate the real and imaginary parts of the diagonal elements of the magnetoconductivity tensor, which in turn allows us to extract the concentration, effective mass, and scattering time of the electrons in the sample. We demonstrate the utility of ultrafast terahertz spectroscopy, which can recover the true linewidth of cyclotron resonance in a high-mobility (>10(6) cm(2)V(-1)s(-1)) sample without being affected by the saturation effect.