Prof Arzhang Ardavan

Abstract:

The focusing of the radiation generated by a polarization current with a superluminally rotating distribution pattern is of a higher order in the plane of rotation than in other directions. Consequently, our previously published [J. Opt. Soc. Am. A24, 2443 (2007)] asymptotic approximation to the value of this field outside the equatorial plane breaks down as the line of sight approaches a direction normal to the rotation axis, i.e., is nonuniform with respect to the polar angle. Here we employ an alternative asymptotic expansion to show that, though having a rate of decay with frequency (mu) that is by a factor of order mu(2/3) slower, the equatorial radiation field has the same dependence on distance as the nonspherically decaying component of the generated field in other directions: It, too, diminishes as the inverse square root of the distance from its source. We also briefly discuss the relevance of these results to the giant pulses received from pulsars: The focused, nonspherically decaying pulses that arise from a superluminal polarization current in a highly magnetized plasma have a power-law spectrum (i.e., a flux density S infinity mu(alpha)) whose index (alpha) is given by one of the values -2/3, -2, -8/3, or -4.

Abstract:

A photo-switchable fullerene dimer and its analogous nitrogen endohedral species have been synthesized and characterized. Irradiation by ultraviolet and visible light has been used to switch between the trans and cis isomers of both the C60- and N@C60- based dimers. Environmental perturbations experienced by the encapsulated nitrogen atom upon switching between the two isomers in degassed carbon disulfide has been determined by pulse electron paramagnetic resonance. Both T1 and T2 electron spin relaxation times of the two isomers of the endohedral fullerene containing dimer revealed a biexponential decay. Although the zero field splitting parameter Deff for both isomers in solution was similar, around 13.0 MHz, the molecular rotation correlation time τC of the trans and cis isomers was calculated to be 37.2 ± 1.6 and 34.8 ± 2.7 ps, respectively. © 2008 American Chemical Society.

Abstract:

Observational data imply the presence of superluminal electric currents in pulsar magnetospheres. Such sources are not inconsistent with special relativity; they have already been created in the laboratory. Here we describe the distinctive features of the radiation beam that is generated by a rotating superluminal source and show that (i) it consists of subbeams that are narrower the farther the observer is from the source: subbeams whose intensities decay as 1/R instead of 1/R2 with distance (R), (ii) the fields of its subbeams are characterized by three concurrent polarization modes: Two modes that are 'orthogonal' and a third mode whose position angle swings across the subbeam bridging those of the other two, (iii) its overall beam consists of an incoherent superposition of such coherent subbeams and has an intensity profile that reflects the azimuthal distribution of the contributing part of the source (the part of the source that approaches the observer with the speed of light and zero acceleration), (iv) its spectrum (the superluminal counterpart of synchrotron spectrum) is broader than that of any other known emission and entails oscillations whose spacings and amplitudes respectively increase and decrease algebraically with increasing frequency, and (v) the degree of its mean polarization and the fraction of its linear polarization both increase with frequency beyond the frequency for which the observer falls within the Fresnel zone. We also compare these features with those of the radiation received from the Crab pulsar.