Carbon nanotube sponges as tunable materials for electromagnetic applications.

The microwave conductivity and permittivity of both single-walled and multi-walled carbon nanotube (SWCNT and MWCNT) sponges were measured while compressing the samples. Compression leads to a huge variation of the absorptance, reflectance, and transmittance of the samples. The dependence of the microwave conductivity on the sponge density follows a power-law relation with exponents 1.7 ± 0.1 and 2.0 ± 0.2 for MWCNT and SWCNT sponges, respectively. These exponents can be decreased slightly by the addition of a non-conducting component which partly electrically separates adjacent tubes within the samples. The conductivity of MWCNT sponge was measured in the terahertz range while heating in air from 300 to 513 K and it increased due to an increase of a number of conducting channels in MWCNTs.

Soft cutting of single-wall carbon nanotubes by low temperature ultrasonication in a mixture of sulfuric and nitric acids.

To decrease single-wall carbon nanotube (SWCNT) lengths to a value of 100-200 nm, aggressive cutting methods, accompanied by a high loss of starting material, are frequently used. We propose a cutting approach based on low temperature intensive ultrasonication in a mixture of sulfuric and nitric acids. The method is nondestructive with a yield close to 100%. It was applied to cut nanotubes produced in three different ways: gas-phase catalysis, chemical vapor deposition, and electric-arc-discharge methods. Raman and Fourier transform infrared spectroscopy were used to demonstrate that the cut carbon nanotubes have a low extent of sidewall degradation and their electronic properties are close to those of the untreated tubes. It was proposed to use the spectral position of the far-infrared absorption peak as a simple criterion for the estimation of SWCNT length distribution in the samples.

Matter coupling to strong electromagnetic fields in two-level quantum systems with broken inversion symmetry.

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.

Dipole polarizability of onion-like carbons and electromagnetic properties of their composites.

Onion-like carbons (OLC) obtained by thermal transformation of nanodiamonds are agglomerates of multi-shell fullerenes, often covered by an external graphitic mantle. For the present work, elemental OLC units were constructed on the computer by coalescence of several two-layer fullerenes, in a structure similar to carbon peapods with a corrugated external wall. The electrical polarizability of such pod-of-peas fullerenes has been computed by a classical monopole-dipole atomistic theory. The description of pod-of-peas fullerenes was further simplified by representing them as linear arrays of point-like objects, whose polarizability matches that of the starting molecules. Calculations demonstrated that the static polarizability of spherically shaped assemblies of these arrays, modeling real OLC materials, is weakly dependent on the geometry of its constituent molecules and is chiefly proportional to the volume of the whole cluster. It increases with increasing filling fraction of the pod-of-peas fullerenes in the OLC aggregate. The polarizability so obtained can be used in Maxwell-Garnett theory to predict the permittivity of OLC-based composites, at least for static excitations. Experimental results obtained at GHz frequencies reveal a weak attenuation for OLC- and nanodiamond-based polydimethylsiloxane composites. In these silicone composites, we did not find long chains of coupled OLCs. Quite separated clusters were found instead, which contribute little to the polarizability and to the dielectric properties, in good agreement with our theoretical predictions.

Thermal radiation from carbon nanotubes in the terahertz range.

The thermal radiation from an isolated finite-length carbon nanotube (CNT) is theoretically investigated both in near- and far-field zones. The formation of the discrete spectrum in metallic CNTs in the terahertz range is demonstrated due to the reflection of strongly slowed-down surface-plasmon modes from CNT ends. The effect does not appear in semiconductor CNTs. The concept of a CNT as a thermal nanoantenna is proposed.

Spontaneous decay of excited atomic states near a carbon nanotube.

The spontaneous decay process of an excited atom placed inside or outside a carbon nanotube is analyzed. Calculations have been performed for various achiral nanotubes. The effect of the nanotube surface is shown to increase the atomic spontaneous decay rate by up to 6 orders of magnitude compared with that of the same atom in vacuum. This increase is associated with nonradiative decay via surface excitations in the nanotube.

Group-velocity dispersion in an all-pass Bragg filter.

On the basis of previously performed theoretical analysis it is shown that a slanted volume grating acts as an all-pass filter with nonlinear dispersion when the diffracted wave is evanescent. An analytical expression for the grating transfer function applicable over a wide angular range of mutual orientation of the initial wave and grating vectors is presented. It is demonstrated that group-velocity dispersion (GVD) in an all-pass Bragg filter is a frequency-dependent quasi-periodic function provided that GVD levels are sufficiently high.