19 research outputs found
Unidirectional Lasing Emerging from Frozen Light in Non-Reciprocal Cavities
We introduce a class of unidirectional lasing modes associated with the
frozen mode regime of non-reciprocal slow-wave structures. Such asymmetric
modes can only exist in cavities with broken time-reversal and space inversion
symmetries. Their lasing frequency coincides with a spectral stationary
inflection point of the underlying passive structure and is virtually
independent of its size. These unidirectional lasers can be indispensable
components of photonic integrated circuitry.Comment: 5 pages, 3 figure
Microwave detection of buried mines using non-contact, synthetic near-field focusing
Existing ground penetrating radars (GPR) are limited in their 3-D resolution. For the detection of buried land-mines, their performance is also seriously restricted by `clutter'. Previous work by the authors has concentrated on removing these limitations by employing multi-static synthetic focusing from a 2-D real aperture. This contribution presents this novel concept, describes the proposed implementation, examines the influence of clutter and of various ground features on the system's performance, and discusses such practicalities as digitisation and time-sharing of a single transmitter and receiver. Experimental results from a variety of scenarios are presented
Anomalous optical phonons in FeTe pnictides: spin state, magnetic order, and lattice anharmonicity
Polarized Raman-scattering spectra of non-superconducting, single-crystalline
FeTe are investigated as function of temperature. We have found a relation
between the magnitude of ordered magnetic moments and the linewidth of A1g
phonons at low temperatures. This relation is attributed to the intermediate
spin state (S=1) and the orbital degeneracy of the Fe ions. Spin-phonon
coupling constants have been estimated based on microscopic modeling using
density-functional theory and analysis of the local spin density. Our
observations show the importance of orbital degrees of freedom for the Fe-based
superconductors with large ordered magnetic moments, while small magnetic
moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions
in those systems.Comment: 17 pages, 3 figure
Optical Limiter Based on PT-Symmetry Breaking of Reflectionless Modes
The application of parity-time (PT) symmetry in optics, especially
PT-symmetry breaking, has attracted considerable attention as a novel approach
to controlling light propagation. Here, we report optical limiting by two
coupled optical cavities with a PT-symmetric spectrum of reflectionless modes.
The optical limiting is related to broken PT symmetry due to light-induced
changes in one of the cavities. Our experimental implementation is a
three-mirror resonator of alternating layers of ZnS and cryolite with a
PT-symmetric spectral degeneracy of two reflectionless modes. The optical
limiting is demonstrated by measurements of single 532-nm 6-ns laser pulses. At
fluences below 10 mJ/cm2, the multilayer exhibits a flat-top passband at 532
nm. At higher fluences, laser heating combined with the thermo-optic effect in
ZnS leads to cavity detuning and PT-symmetry breaking of the reflectionless
modes. As a result, the entire multilayer structure quickly becomes highly
reflective, protecting itself from laser-induced damage. The cavity detuning
mechanism can differ at much higher limiting thresholds and include
nonlinearity.Comment: 17 pages, 5 figure
Inverse Borrmann effect in photonic crystals
The Borrmann effect, which is related to the microscopic distribution of the
electromagnetic field inside the primitive cell, is studied in photonic and
magnetophotonic crystals. This effect, well-known in x-ray spectroscopy, is
responsible for the enhancement or suppression of various linear and nonlinear
optical effects when the incidence angle and/or the frequency change. It is
shown that by design of the primitive cell this effect can be suppressed and
even inverted
A reflective mm-wave photonic limiter
Millimeter wave (mm-wave) communications and radar receivers capable of
processing small signals must be protected from high-power signals, which can
damage sensitive receiver components. Many of these systems arguably can be
protected by using photonic limiting techniques, in addition to electronic
limiting circuits in receiver front-ends. Here we demonstrate, experimentally
and numerically, a free-space, reflective mm-wave limiter based on a multilayer
structure involving a nanolayer of vanadium dioxide (VO2), experiencing a
thermal insulator-to-metal transition. The multilayer acts as a variable
reflector, controlled by the input power. At low input power levels, VO2
remains dielectric, and the multilayer exhibits resonant transmittance. When
the input power exceeds a threshold level, the emerging metallic phase renders
the multilayer highly reflective while dissipating a small portion of the input
power without damage to the limiter. In the case of a Gaussian beam, the
limiter has a nearly constant output above the limiting threshold input.Comment: 18 pages, 6 figures, 3 supplementary figures and 1 supplementary
tabl