82 research outputs found
Nonhermitian transport effects in coupled-resonator optical waveguides
Coupled-resonator optical waveguides (CROWs) are known to have interesting
and useful dispersion properties. Here, we study the transport in these
waveguides in the general case where each resonator is open and asymmetric,
i.e., is leaky and possesses no mirror-reflection symmetry. Each individual
resonator then exhibits asymmetric backscattering between clockwise and
counterclockwise propagating waves, which in combination with the losses
induces non-orthogonal eigenmodes. In a chain of such resonators, the coupling
between the resonators induces an additional source of non-hermiticity, and a
complex band structure arises. We show that in this situation the group
velocity of wave packets differs from the velocity associated with the
probability density flux, with the difference arising from a non-hermitian
correction to the Hellmann-Feynman theorem. Exploring these features
numerically in a realistic scenario, we find that the complex band structure
comprises almost-real branches and complex branches, which are joined by
exceptional points, i.e., nonhermitian degeneracies at which not only the
frequencies and decay rates coalesce but also the eigenmodes themselves. The
non-hermitian corrections to the group velocity are largest in the regions
around the exceptional points.Comment: 11 pages, 9 figure
Revisiting the hierarchical construction of higher-order exceptional points
Higher-order exceptional points in the spectrum of non-Hermitian Hamiltonians
describing open quantum or wave systems have a variety of potential
applications in particular in optics and photonics. However, the experimental
realization is notoriously difficult. Recently, Q. Zhong et al. [Phys. Rev.
Lett. 125, 203602 (2020)] have introduced a robust construction where a
unidirectional coupling of two subsystems having exceptional points of the same
order leads generically to a single exceptional point of twice the order. Here,
we investigate this scheme in a different manner by exploiting the nilpotency
of the traceless part of the involved Hamiltonians. We generalize the scheme
and derive a simple formula for the spectral response strength of the composite
system hosting a higher-order exceptional point. Its relation to the spectral
response strengths of the subsystems is discussed. Moreover, we investigate
nongeneric perturbations. The results are illustrated with an example.Comment: 6 pages, 2 figure
Rotating optical microcavities with broken chiral symmetry
We demonstrate in open microcavities with broken chiral symmetry,
quasi-degenerate pairs of co-propagating modes in a non-rotating cavity evolve
to counter-propagating modes with rotation. The emission patterns change
dramatically by rotation, due to distinct output directions of CW and CCW
waves. By tuning the degree of spatial chirality, we maximize the sensitivity
of microcavity emission to rotation. The rotation-induced change of emission is
orders of magnitude larger than the Sagnac effect, pointing to a promising
direction for ultrasmall optical gyroscopes.Comment: 5 pages, 5 figure
Nonlinear dynamical tunneling of optical whispering gallery modes in the presence of a Kerr nonlinearity
The effect of a Kerr nonlinearity on dynamical tunneling is studied, using
coupled whispering gallery modes in an optical microcavity. The model system
that we have chosen is the 'add-drop filter', which comprises an optical
microcavity and two waveguide coupled to the cavity. Due to the evanescent
field's scattering on the waveguide, the whispering gallery modes in the
microcavity form doublets, which manifest themselves as splittings in the
spectrum. As these doublets can be regarded as a spectral feature of dynamical
tunneling between two different dynamical states with a spatial overlap, the
effect of a Kerr nonlinearity on the doublets is numerically investigated in
the more general context of the relationship between cubic nonlinearity and
dynamical tunneling. Within the numerical realization of the model system, it
is observed that the doublets shows a bistable transition in its transmission
curve as the Kerr-nonlinearity in the cavity is increased. At the same time,
one rotational mode gets dominant over the other one in the transmission, since
the two states in the doublet have uneven linewidths. By using coupled mode
theory, the underlying mode dynamics of the phenomena is theoretically modelled
and clarified.Comment: 7 pages, 5 figure
Unidirectional light emission from high-Q modes in optical microcavities
We introduce a new scheme to design optical microcavities supporting high-Q
modes with unidirectional light emission. This is achieved by coupling a low-Q
mode with unidirectional emission to a high-Q mode. The coupling is due to
enhanced dynamical tunneling near an avoided resonance crossing. Numerical
results for a microdisk with a suitably positioned air hole demonstrate the
feasibility and the potential of this concept.Comment: 4 pages, 6 figures (in reduced resolution
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