arXiv subjects

Petar Andrejić

Publications and source records attributed to Petar Andrejić.

4 recordsLinked to original sources

arXiv2025

Collective nuclear excitation and pulse propagation in single-mode x-ray waveguides

Waveguides offer a means to controllably couple atomic ensembles to the electromagnetic field therein. Here, we demonstrate x-ray propagation in planar thin-film waveguides coupled to Mössbauer nuclei under collective resonant excitation by short pulses of synchrotron radiation. We record x-ray photons that have been emitted into resonant modes of the waveguide. Depending on the geometry and mode of excitation, two fundamentally different signatures of the collective emission are observed, for which we present a unifying theoretical model. Our results form a new platform for waveguide quantum electrodynamics in the hard x-ray regime with the potential to provide a coherent narrowband source of x-rays on the nanometer scale.

quant-ph↗

arXiv2021

Superradiance and anomalous hyperfine splitting in inhomogeneous ensembles

Collective effects in the interaction of light with ensembles of identical scatterers play an important role in many fields of physics. However, often the term ``identical'' is not accurate due to the presence of hyperfine fields which induce inhomogeneous transition shifts and splittings. Here we develop a formalism based on the Green function method to model the linear response of such inhomogeneous ensembles in one-dimensional waveguides. We obtain a compact formula for the collective spectrum, which exhibits deviations from the uniform frequency shift and broadening expected of two level systems. In particular, if the coherent contribution to the collective coupling is large, the effect of inhomogeneous broadening can be suppressed, with the linewidth approaching that of the superradiant value. We apply this formalism to describe collective effects in x-ray scattering off thin-film waveguides for inhomogeneous hyperfine parameters.

quant-ph↗

arXiv2018

Convergent measure of focal extent, and largest peak intensity for non-paraxial beams

Second moment beam widths are commonly used in paraxial optics to define the focal extent of beams. However, second moments of arbitrary beams are not guaranteed to be finite. I propose the focal concentration area as a measure of beam focal area, defined to be the ratio of total radial intensity to radial intensity regulated by a unit area Gaussian distribution. I use the Dirac delta limit of this distribution to establish a rigorous upper bound on the peak intensity of non-paraxial beams of a given total intensity, and show that this is achieved by the recently proposed `proto- beam' solution. I discuss the generalisation to electromagnetic beams, and find the same lower bound as the scalar case. This bound cannot be achieved for any physical beam, and as such the physical lower bound must be higher.

physics.optics↗

arXiv2017

Lorentz transformation of electromagnetic pulses derived from Hertz potentials

Electric and magnetic Hertz potentials are a formalism for obtaining solutions of Maxwell's equations from solutions of the inhomogeneous wave equation, with polarisation and magnetisation as the sources. We provide an overview of their covariant transformation properties, and examine an application of Hertz potentials to the Lorentz transformation of localised pulses by Lekner, who obtained a result seeming to contradict Von Laue's theorem. We show that Lekner's result of total energy-momentum not transforming as a four-vector was due to an erroneous transformation of Hertz potentials, that did not take into account their bivector nature.

physics.class-ph↗