310 research outputs found
Dynamically encircling exceptional points: in situ control of encircling loops and the role of the starting point
The most intriguing properties of non-Hermitian systems are found near the
exceptional points (EPs) at which the Hamiltonian matrix becomes defective. Due
to the complex topological structure of the energy Riemann surfaces close to an
EP and the breakdown of the adiabatic theorem due to non-Hermiticity, the state
evolution in non-Hermitian systems is much more complex than that in Hermitian
systems. For example, recent experimental work [Doppler et al. Nature 537, 76
(2016)] demonstrated that dynamically encircling an EP can lead to chiral
behaviors, i.e., encircling an EP in different directions results in different
output states. Here, we propose a coupled ferromagnetic waveguide system that
carries two EPs and design an experimental setup in which the trajectory of
state evolution can be controlled in situ using a tunable external field,
allowing us to dynamically encircle zero, one or even two EPs experimentally.
The tunability allows us to control the trajectory of encircling in the
parameter space, including the size of the encircling loop and the starting/end
point. We discovered that whether or not the dynamics is chiral actually
depends on the starting point of the loop. In particular, dynamically
encircling an EP with a starting point in the parity-time-broken phase results
in non-chiral behaviors such that the output state is the same no matter which
direction the encircling takes. The proposed system is a useful platform to
explore the topology of energy surfaces and the dynamics of state evolution in
non-Hermitian systems and will likely find applications in mode switching
controlled with external parameters.Comment: 15 pages, 11 figure
Acoustic circular dichroism in a three-dimensional chiral metamaterial
Circular dichroism (CD) is an intriguing chiroptical phenomenon associated
with the interaction of chiral structures with circularly polarized lights.
Although the CD effect has been extensively studied in optics, it has not yet
been demonstrated in acoustic systems. Here, we demonstrate the acoustic CD
effect in a three-dimensional chiral metamaterial supporting circularly
polarized transverse sound. We find that the effect is negligible in the lossy
metamaterial possessing rotational symmetry but can be strongly enhanced
in the -symmetric system with inhomogeneous loss. The phenomena can be
understood based on the properties of the metamaterial's complex band structure
and the quality factors of its eigenmodes. We show that the enhanced CD in the
-symmetric system is attributed to the polarization bandgaps and the
non-Hermitian exceptional points appearing near the Brillouin-zone center and
boundaries. The results contribute to the understanding of chiral sound-matter
interactions and can find applications in acoustic sensing of chiral structures
and sound manipulations based on its vector properties.Comment: 9 pages, 9 figure
Multiplexing spectral line shape of waveguide transmission by photonic spin-orbit interaction
Manipulating the spectral line shape exhibits great potential in realizing
active optical circuits with switching, sensing, and modulation capabilities.
Exploring unusual line shapes, such as Fano resonance and electromagnetically
induced transparency (EIT), has attracted substantial interest. Conventional
methods of engineering the spectral line shape have limited tunability and face
challenges in multiplexing different spectral line shapes. Here, we propose and
numerically demonstrate a new mechanism to tailor the transmission line shape
almost at will by exploiting the interference of frequency-dependent chiral
dipolar states in two helix particles sitting above a dielectric waveguide. We
show that, by tuning the polarization of the chiral dipoles and exploiting
transverse spin-orbit interaction, one can control the asymmetric
Pancharatnam-Berry geometric phase for the excited guided waves propagating in
opposite directions. The interference of the guided waves respectively excited
by the two particles can give rise to transmissions with various line shapes,
including Lorentzian-like, antiresonance-like, Fano-like, and EIT-like line
shapes, which carry an intriguing property of line shape-momentum locking,
i.e., the transmissions in opposite directions have different line shapes. Our
findings open new possibilities for multiplexed and multifunctional
nanophotonic designs with unprecedented capability of spectral-line shaping.
The proposed structures can be conveniently integrated with optical circuits
for on-chip applications.Comment: 9 pages, 5 figure
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