90 research outputs found
Fresnel's equations in statics and quasistatics
Fresnel's equations describe reflection and transmission of electromagnetic
waves at an interface between two media. It turns out that these equations can
be used in quasistatics and even statics, for example to straightforwardly
calculate magnetic forces between a permanent magnet and a bulk medium. This
leads to a generalization of the classical image method
The magnetic permeability in Fresnel's equation
Magnetic permeabilities derived for infinite, periodic media are used in the
Fresnel equation to calculate the reflection from corresponding semi-infinite
media. By comparison to finite-difference-time-domain (FDTD) simulations, we
find that the Fresnel equation gives accurate results for 2D metamaterials
which mimic natural magnetism, in a frequency range where the magnetic moment
density dominates the part of the total Landau--Lifshitz
permittivity. For a 1D layered structure, or for large frequencies, the
correspondence is poor. We also demonstrate that even if a medium is described
accurately by a local permittivity and permeability, the Fresnel equation is
not necessarily valid
Secure detection in quantum key distribution by real-time calibration of receiver
The single photon detection efficiency of the detector unit is crucial for
the security of common quantum key distribution protocols like Bennett-Brassard
1984 (BB84). A low value for the efficiency indicates a possible eavesdropping
attack that exploits the photon receiver's imperfections. We present a method
for estimating the detection efficiency, and calculate the corresponding secure
key generation rate. The estimation is done by testing gated detectors using a
randomly activated photon source inside the receiver unit. This estimate gives
a secure rate for any detector with non-unity single photon detection
efficiency, both inherit or due to blinding. By adding extra optical components
to the receiver, we make sure that the key is extracted from photon states for
which our estimate is valid. The result is a quantum key distribution scheme
that is secure against any attack that exploits detector imperfections.Comment: 7 pages, 4 figure
Inverse scattering of 2d photonic structures by layer-stripping
Design and reconstruction of 2d and 3d photonic structures are usually
carried out by forward simulations combined with optimization or intuition.
Reconstruction by means of layer-stripping has been applied in seismic
processing as well as in design and characterization of 1d photonic structures
such as fiber Bragg gratings. Layer-stripping is based on causality, where the
earliest scattered light is used to recover the structure layer-by-layer.
Our set-up is a 2d layered nonmagnetic structure probed by plane polarized
harmonic waves entering normal to the layers. It is assumed that the dielectric
permittivity in each layer only varies orthogonal to the polarization. Based on
obtained reflectance data covering a suitable frequency interval,
time-localized pulse data are synthesized and applied to reconstruct the
refractive index profile in the leftmost layer by identifying the local,
time-domain Fresnel reflection at each point. Once the first layer is known,
its impact on the reflectance data is stripped off, and the procedure repeated
for the next layer.
Through numerical simulations it will be demonstrated that it is possible to
reconstruct structures consisting of several layers. The impact of evanescent
modes and limited bandwidth is discussed
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