1,014 research outputs found
Bound states in the continuum in symmetric and asymmetric photonic crystal slabs
We develop a semi-analytical model to describe bound states in the continuum
(BICs) in photonic crystal slabs. We model leaky modes supported by photonic
crystal slabs as a transverse Fabry-Perot resonance composed of a few
propagative Bloch waves bouncing back and forth vertically inside the slab.
This multimode Fabry-Perot model accurately predicts the existence of BICs and
their positions in the parameter space. We show that, regardless of the slab
thickness, BICs cannot exist below a cut-off frequency, which is related to the
existence of the second-order Bloch wave in the photonic crystal. Thanks to the
semi-analyticity of the model, we investigate the dynamics of BICs with the
slab thickness in symmetric and asymmetric photonic crystal slabs. We evidence
that the symmetry-protected BICs that exist in symmetric structures at the
{\Gamma}-point of the dispersion diagram can still exist when the horizontal
mirror symmetry is broken, but only for particular values of the slab
thickness
Topological nature of bound states in the radiation continuum
Bound states in the continuum (BICs) are unusual solutions of wave equations
describing light or matter: they are discrete and spatially bounded, but exist
at the same energy as a continuum of states which propagate to infinity. Until
recently, BICs were constructed through fine-tuning parameters in the wave
equation or exploiting the separability of the wave equation due to symmetry.
More recently, BICs that that are both robust and not symmetry-protected
(accidental) have been predicted and experimentally realized in periodic
structures; the simplest such system is a periodic dielectric slab, which also
has symmetry-protected BICs. Here we show that both types of BICs in such
systems are vortex centers in the polarization direction of far-field
radiation. The robustness of these BICs is due to the existence of conserved
and quantized topological charges, defined by the number of times the
polarization vectors wind around the vortex centers. Such charges can only be
generated or annihilated by making large changes in the system parameters, and
then only according to strict rules, which we derive and test numerically. Our
results imply that laser emission based on such states will generate vector
beams
Reflectionless evanescent-wave amplification by two dielectric planar waveguides
Utilizing the underlying physics of evanescent wave amplification by a
negative-refractive-index slab, it is shown that evanescent waves with specific
spatial frequencies can also be amplified without any reflection simply by two
dielectric planar waveguides. The simple configuration allows one to take
advantage of the high resolution limit of a high-refractive-index material
without contact with the object.Comment: 4 pages, 3 figures, v2: accepted by Optics Letters, v3: included the
Erratum submitted to Optics Letter
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