26,221 research outputs found
Dipolar Interactions in Superconductor-Ferromagnet Heterostructures
We consider a simple model for a superlattice composed of a thin magnetic
film placed between two bulk superconductors. The magnetic film is modelled by
a planar but otherwise arbitrary distribution of magnetic dipoles and the
superconductors are treated in the London approximation. Due to the linearity
of the problem, we are able to compute the magnetic energy of the film in the
presence of the superconductors. We show that in the case of small wavenumbers
compared to the inverse London penetration depth, the magnetic energy resembles
the energy of a distribution of magnetisation in a two dimensional space.
Possible experimental applications of these results are discussed.Comment: RevTeX, 29 pages, 3 figures. To be published in Phys. Rev. B. Changes
were made (including adding fig. 3), to conform with the referee's report.
Reference 54 was also adde
Fundamental limits to optical response in absorptive systems
At visible and infrared frequencies, metals show tantalizing promise for
strong subwavelength resonances, but material loss typically dampens the
response. We derive fundamental limits to the optical response of absorptive
systems, bounding the largest enhancements possible given intrinsic material
losses. Through basic conservation-of-energy principles, we derive
geometry-independent limits to per-volume absorption and scattering rates, and
to local-density-of-states enhancements that represent the power radiated or
expended by a dipole near a material body. We provide examples of structures
that approach our absorption and scattering limits at any frequency, by
contrast, we find that common "antenna" structures fall far short of our
radiative LDOS bounds, suggesting the possibility for significant further
improvement. Underlying the limits is a simple metric, for a material with susceptibility , that enables
broad technological evaluation of lossy materials across optical frequencies.Comment: 21 pages and 6 figures (excluding appendices, references
Model for quantitative tip-enhanced spectroscopy and the extraction of nanoscale-resolved optical constants
Near-field infrared spectroscopy by elastic scattering of light from a probe
tip resolves optical contrasts in materials at dramatically sub-wavelength
scales across a broad energy range, with the demonstrated capacity for chemical
identification at the nanoscale. However, current models of probe-sample
near-field interactions still cannot provide a sufficiently quantitatively
interpretation of measured near-field contrasts, especially in the case of
materials supporting strong surface phonons. We present a model of near-field
spectroscopy derived from basic principles and verified by finite-element
simulations, demonstrating superb predictive agreement both with tunable
quantum cascade laser near-field spectroscopy of SiO thin films and with
newly presented nanoscale Fourier transform infrared (nanoFTIR) spectroscopy of
crystalline SiC. We discuss the role of probe geometry, field retardation, and
surface mode dispersion in shaping the measured near-field response. This
treatment enables a route to quantitatively determine nano-resolved optical
constants, as we demonstrate by inverting newly presented nanoFTIR spectra of
an SiO thin film into the frequency dependent dielectric function of its
mid-infrared optical phonon. Our formalism further enables tip-enhanced
spectroscopy as a potent diagnostic tool for quantitative nano-scale
spectroscopy.Comment: 19 pages, 9 figure
Nonlinear resonance reflection from and transmission through a dense glassy system built up of oriented linear Frenkel chains: two-level models
A theoretical study of the resonance optical response of assemblies of
oriented short (as compared to an optical wavelength) linear Frenkel chains is
carried out using a two-level model. We show that both transmittivity and
reflectivity of the film may behave in a bistable fashion and analyze how the
effects found depend on the film thickness and on the inhomogeneous width of
the exciton optical transition.Comment: 26 pages, 9 figure
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