Hawking radiation from "phase horizons" in laser filaments?

Belgiorno et al have reported on experiments aiming at the detection of (the analogue of) Hawking radiation using laser filaments [F. Belgiorno et al, Phys. Rev. Lett. 105, 203901 (2010)]. They sent intense focused Bessel pulses into a non-linear dielectric medium in order to change its refractive index via the Kerr effect and saw creation of photons orthogonal to the direction of travel of the pluses. Since the refractive index change in the pulse generated a "phase horizon" (where the phase velocity of these photons equals the pulse speed), they concluded that they observed the analogue of Hawking radiation. We study this scenario in a model with a phase horizon and a phase velocity very similar to that of their experiment and find that the effective metric does not quite correspond to a black hole. The photons created in this model are not due to the analogue of black hole evaporation but have more similarities to cosmological particle creation. Nevertheless, even this effect cannot explain the observations -- unless the pulse has significant small scale structure in both the longitudinal and transverse dimensions.Comment: 13 pages RevTeX, 2 figure

Breaking of the overall permutation symmetry in nonlinear optical susceptibilities of one-dimensional periodic dimerized Huckel model

Based on infinite one-dimensional single-electron periodic models of trans-polyacetylene, we show analytically that the overall permutation symmetry of nonlinear optical susceptibilities is, albeit preserved in the molecular systems with only bound states, no longer generally held for the periodic systems. The overall permutation symmetry breakdown provides a fairly natural explanation to the widely observed large deviations of Kleinman symmetry for periodic systems in off-resonant regions. Physical conditions to experimentally test the overall permutation symmetry break are discussed.Comment: 7 pages, 1 figur

Searching in HI for Massive Low Surface Brightness Galaxies: Samples from HyperLeda and the UGC

A search has been made for 21 cm HI line emission in a total of 350 unique galaxies from two samples whose optical properties indicate they may be massive The first consists of 241 low surface brightness (LSB) galaxies of morphological type Sb and later selected from the HyperLeda database and the the second consists of 119 LSB galaxies from the UGC with morphological types Sd-m and later. Of the 350 unique galaxies, 239 were observed at the Nancay Radio Telescope, 161 at the Green Bank Telescope, and 66 at the Arecibo telescope. A total of 295 (84.3%) were detected, of which 253 (72.3%) appear to be uncontaminated by any other galaxies within the telescope beam. Finally, of the total detected, uncontaminated galaxies, at least 31 appear to be massive LSB galaxies, with a total HI mass $\ge$ 10$^{10}$ M$_{sol}$, for H$_0$ = 70 km/s/Mpc. If we expand the definition to also include galaxies with significant total (rather than just gas) mass, i.e., those with inclination-corrected HI line width W$_{50}$,cor > 500 km/s, this bring the total number of massive LSB galaxies to 41. There are no obvious trends between the various measured global galaxy properties, particularly between mean surface brightness and galaxy mass.Comment: 71 pages, including all tables and figures; Accepted by A

A photonic basis for deriving nonlinear optical response

Nonlinear optics is generally first presented as an extension of conventional optics. Typically the subject is introduced with reference to a classical oscillatory electric polarization, accommodating correction terms that become significant at high intensities. The material parameters that quantify the extent of the nonlinear response are cast as coefficients in a power series - nonlinear optical susceptibilities signifying a propensity to generate optical harmonics, for example. Taking the subject to a deeper level requires a more detailed knowledge of the structure and properties of each nonlinear susceptibility tensor, the latter differing in form according to the process under investigation. Typically, the derivations involve intricate development based on time-dependent perturbation theory, assisted by recourse to a set of Feynman diagrams. This paper presents a more direct route to the required results, based on photonic rather than semiclassical principles, and offers a significantly clearer perspective on the photophysics underlying nonlinear optical response. The method, here illustrated by specific application to harmonic generation and down-conversion processes, is simple, intuitive and readily amenable for processes of arbitrary photonic order. © 2009 IOP Publishing Ltd

Optically induced forces and torques:Interactions between nanoparticles in a laser beam

Distinctive optical forces and torques arise between nanoparticles irradiated by intense laser radiation. These forces, associated with a pairwise process of stimulated scattering, prove to enable the possibility of producing significant modifications to both the form and magnitude of interparticle forces, with additional contributions arising in the case of dipolar materials. Moreover, such forces have the capacity to generate unusual patterns of nanoscale response, entirely controlled by the input beam characteristics- principally the optical frequency, intensity, and polarization. Based on quantum electrodynamical theory, a general result is secured for the laser-induced force under arbitrary conditions, incorporating both static and dynamic coupling mechanisms. Specific features of the results are identified for pairs of particles with prolate cylindrical symmetry, e.g., carbon nanotubes, where it is shown that the laser-induced forces and torques are sensitive functions of the pair spacing and orientation, and the laser beam geometry; significantly, they can be either repulsive or attractive according to conditions. For nanoparticles trapped in a Laguerre-Gaussian laser beam the results also reveal additional and highly distinctive torques that suggest further possibilities for nanomanipulation with light. The paper concludes with a discussion on several potential applications of such forces. © 2005 The American Physical Society

Is the Butcher-Oemler effect a function of the cluster redshift ?

Using PSPC {\it Rosat} data, we measure x-ray surface brightness profiles, size and luminosity of the Butcher-Oemler (BO) sample of clusters of galaxies. The cluster x-ray size, as measured by the Petrosian r_{\eta=2} radius, does not change with redshift and is independent from x-ray luminosity. On the other hand, the x-ray luminosity increases with redshift. Considering that fair samples show no-evolution, or negative luminosity evolution, we conclude that the BO sample is not formed from the same class of objects observed at different look-back times. This is in conflict with the usual interpretation of the Butcher-Oemler as an evolutionary (or redshift-dependent) effect, based on the assumption that we are comparing the same class of objects at different redshifts. Other trends present in the BO sample reflect selection criteria rather than differences in look-back time, as independently confirmed by the fact that trends loose strength when we enlarge the sample with x-ray selected sample of clusters. The variety of optical sizes and shapes of the clusters in the Butcher-Oemler sample, and the Malmquist-like bias, are the reasons for these selection effects that mimic the trends usually interpreted as changes due to evolution.Comment: ApJ, in press, scheduled on May, 10 issue. 17 pages & 11 figure