M. E. Portnoi

Terahertz science and technology of carbon nanomaterials

The diverse applications of terahertz (THz) radiation and its importance to fundamental science makes finding ways to generate, manipulate and detect THz radiation one of the key areas of modern applied physics. One approach is to utilize carbon nanomaterials, in particular, single-wall carbon nanotubes and graphene. Their novel optical and electronic properties offer much promise to the field of THz science and technology. This article describes the past, current, and future of THz science and technology of carbon nanotubes and graphene. We will review fundamental studies such as THz dynamic conductivity, THz nonlinearities and ultrafast carrier dynamics as well as THz applications such as THz sources, detectors, modulators, antennas and polarizers.

Smooth electron waveguides in graphene

We present exact analytical solutions for the zero-energy modes of two-dimensional massless Dirac fermions fully confined within a smooth one-dimensional potential V(x)=-{alpha}/cosh({beta}x), which provides a good fit for potential profiles of existing top-gated graphene structures. We show that there is a threshold value of the characteristic potential strength {alpha}/{beta} for which the first mode appears, in striking contrast to the nonrelativistic case. A simple relationship between the characteristic strength and the number of modes within the potential is found. An experimental setup is proposed for the observation of these modes. The proposed geometry could be utilized in future graphene-based devices with high on/off current ratios.

Carbon nanotubes as a basis for terahertz emitters and detectors

We propose and justify two schemes utilizing the unique electronic properties of carbon nanotubes for novel THz applications including tunable THz generation by hot electrons in quasi-metallic nanotubes and THz radiation detection by armchair nanotubes in strong magnetic fields.

Superlattice properties of carbon nanotubes in a transverse electric field

Electron motion in a

Carbon nanotubes: A new type of emitter in the terahertz range

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Zero-energy states in graphene quantum dots and rings

carbon nanotube is shown to correspond to a de Broglie wave propagating along a helical line on the nanotube wall. This helical motion leads to periodicity of the electron potential energy in the presence of an electric field normal to the nanotube axis. The period of this potential is proportional to the nanotube radius and is greater than the interatomic distance in the nanotube. As a result, the behavior of an electron in a

Terahertz processes in carbon nanotubes

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Two-phonon scattering of magnetorotons in fractional quantum Hall liquids

nanotube subject to a transverse electric field is similar to that in a semiconductor superlattice. In particular, Bragg scattering of electrons from the long-range periodic potential results in the opening of gaps in the energy spectrum of the nanotube. Modification of the band structure is shown to be significant for experimentally attainable electric fields, which raises the possibility of applying this effect to nanoelectronic devices.

Tuning gaps and phases of a two-subband system in a quantizing magnetic field

It is theoretically demonstrated that the electric-field-induced heating of the electron gas in carbon nanotubes can lead to population inversion in the electron subbands, which results in the generation of electromagnetic waves in the terahertz range by hot electrons. This phenomenon can be used for the creation of radiators of a new type, based on carbon nanotubes, for the terahertz frequency range.

Searching for confined modes in graphene channels: The variable phase method

School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom(Dated: May 5, 2011)We present exact analytical zero-energy solutions for a class of smooth decaying potentials, show-ing that the full confinement of charge carriers in electrostatic potentials in graphene quantum dotsand rings is indeed possible without recourse to magnetic fields. These exact solutions allow usto draw conclusions on the general requirements for the potential to support fully confined states,including a critical value of the potential strength and spatial extent.