Exact stationary solutions of the electron-photon Dirac equation are obtained to describe the strong interaction between massless Dirac fermions in graphene and circularly polarized photons. It follows from them that this interaction forms bound electron-photon states which should be considered as a kind of charged quasiparticles. The energy spectrum of the quasiparticles is of dielectric type and characterized by an energy gap between the valence and conductivity bands. Therefore the electron-photon interaction results in metalinsulator transition in graphene. The stationary energy gap, induced by photons, and concomitant effects can be observed for graphene exposed to a laser-generated circularly polarized electromagnetic wave.

It is shown that the electron coupling to photons in field-dressed nanostructures can result in the ground electron-photon state with a nonzero electric current. Since the current is associated with the ground state, it flows without the Joule heating of the nanostructure and is nondissipative. Such a dissipationless electron transport can be realized in strongly coupled electron-photon systems with the broken time-reversal symmetry--particularly, in quantum rings and chiral nanostructures dressed by circularly polarized photons.

We demonstrate theoretically the parametric oscillator behavior of a two-level quantum system with broken inversion symmetry exposed to a strong electromagnetic field. A multitude of resonance frequencies and additional harmonics in the scattered light spectrum as well as an altered Rabi frequency are predicted to be inherent to such systems. In particular, dipole radiation at the Rabi frequency appears to be possible. Since the Rabi frequency is controlled by the strength of the coupling electromagnetic field, the effect can serve for the frequency-tuned parametric amplification and generation of electromagnetic waves. Manifestation of the effect is discussed for III-nitride quantum dots with strong built-in electric field breaking the inversion symmetry. Terahertz emission from arrays of such quantum dots is shown to be experimentally observable.

We formulate and justify several proposals utilizing unique electronic properties of carbon nanotubes for a broad range of applications to THz optoelectronics, including THz generation by hot electrons in quasi-metallic nanotubes, frequency multiplication in chiral-nanotube-based superlattices controlled by a transverse electric field, and THz radiation detection and emission by armchair nanotubes in a strong magnetic field. c 2007 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Terahertz radiationCreating a compact reliable source of terahertz (THz) radiation is one of the most formidable tasks of contemporary applied physics [1]. One of the latest trends in THz technology [2] is to use carbon nanotubes -cylindrical molecules with nanometer diameter and micrometer length [3-5] -as building blocks of novel high-frequency devices. There are several promising proposals of using carbon nanotubes for THz applications including a nanoklystron utilizing extremely efficient high-field electron emission from nanotubes [2,6,7], devices based on negative differential conductivity in large-diameter semiconducting nanotubes [8,9], high-frequency resonant-tunneling diodes [10] and Schottky diodes [11][12][13][14] [20].In this paper we formulate and discuss several novel schemes to utilize the physical properties of single-wall carbon nanotubes (SWNTs) for generation and detection of THz radiation.

We demonstrate theoretically that quasi-metallic carbon nanotubes emit terahertz radiation induced by an applied voltage. It is shown that in the ballistic transport regime their spontaneous emission spectra have a universal frequency and bias voltage dependence, which raises the possibility of utilizing this effect for high-frequency nanoelectronic devices.