52 research outputs found
Anisotropy-induced photonic bound states in the continuum
Bound states in the continuum (BICs) are radiationless localized states embedded in the part of the parameter space that otherwise corresponds to radiative modes. Many decades after their original prediction1, 2, 3 and early observations in acoustic systems4, such states have been demonstrated recently in photonic structures with engineered geometries5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18. Here, we put forward a mechanism, based on waveguiding structures that contain anisotropic birefringent materials, that affords the existence of BICs with fundamentally new properties. In particular, anisotropy-induced BICs may exist in symmetric as well as in asymmetric geometries; they may form in tunable angular propagation directions; their polarization may be pure transverse electric, pure transverse magnetic or full vector with tunable polarization hybridity; and they may be the only possible bound states of properly designed structures, and thus appear as a discrete, isolated bound state embedded in a whole sea of radiative states.Peer ReviewedPostprint (author's final draft
Transition from Dirac points to exceptional points in anisotropic waveguides
We uncover the existence of Dirac and exceptional points in waveguides made of anisotropic materials, and study the transition between them. Dirac points in the dispersion diagram appear at propagation directions where the matrix describing the eigenvalue problem for bound states splits into two blocks, sorting the eigenmodes either by polarization or by inner mode symmetry. Introducing a non-Hermitian channel via a suitable leakage mechanism causes the Dirac points to transform into exceptional points connected by a Fermi arc. The exceptional points arise as improper hybrid leaky states and, importantly, are found to occur always out of the anisotropy symmetry planes.Peer ReviewedPostprint (published version
Unidirectional guided resonances in anisotropic waveguides
We show that anisotropic planar anti-guiding waveguide structures with two radiation channels toward the surrounding cladding materials can support unidirectional guided resonances (UGRs), where radiation is canceled in one of the radiation channels and redirected into the other. Their formation is subtle as it requires breaking the so-called polar anisotropy-symmetry of the structures. Then, UGRs appear at specific wavelengths and light propagation directions, are robust, and are characterized by phase singularities in the channel in which radiation is canceled. The mechanism we describe allows for ready selection of the radiation direction, as well as tuning of the wavelength and the propagation angle at which UGRs occur.H2020 Marie Sklodowska-Curie Action GA665884; Government of Spain (grants PGC2018-097035-B-I00; Severo Ochoa CEX2019-000910-S); Fundació Cellex; Fundació Mir-Puig; Generalitat de Catalunya (CERCA and AGAUR 2017-SGR-1400).Peer ReviewedPostprint (author's final draft
Quantifying the robustness of topological slow light
Low-dimensional nanostructured materials can guide light propagating with
very low group velocity vg. However, this slow light is significantly sensitive
to unwanted imperfections in the critical dimensions of the nanostructure. The
backscattering mean free path, xi, the average ballistic propagation length
along the waveguide, quantifies the robustness of slow light against this type
of structural disorder. This figure of merit determines the crossover between
acceptable slow-light transmission affected by minimal scattering losses and a
strong backscattering-induced destructive interference when xi exceeds the
waveguide length L. Here, we calculate the backscattering mean free path for a
topological photonic waveguide for a specific and determined amount of disorder
and, equally relevant, for a fixed value of the group index ng which is the
slowdown factor of the group velocity with respect to the speed of light in
vacuum. These two figures of merit, xi and ng, should be taken into account
when quantifying the robustness of topological and conventional
(non-topological) slow-light transport at the nanoscale. Otherwise, any claim
on a better performance of topological guided light over conventional one is
not justified
Angular control of anisotropy-induced bound states in the continuum
Radiation of leaky modes existing in anisotropic waveguides can be cancelled by destructive
interference at special propagation directions relative to the optical axis orientation, resulting in
fully bound states surrounded by radiative modes. Here we study the variation of the loci of such
special directions in terms of the waveguide constitutive parameters. We show that the angular
loci of the bound states is sensitive to several design parameters, allowing bound states to exist for
a broad range of angular directions and wavelengths and suggesting applications in filtering and
sensing.Peer ReviewedPostprint (author's final draft
Topology Transitions of Anisotropy Induced Bound States in the Continuum
We demonstrate that Bound states In the Continuum (BICs) are supported in planar
anisotropic structures where the optic axes are arbitrarily oriented. Moreover, we reveal
fundamental new topological properties of these BICs that depend on the relative orientation of the
optic axes in the core and substrate.Peer ReviewedPostprint (published version
Modification of Akhieser mechanism in Si nanomembranes and thermal conductivity dependence of the Q-factor of high frequency nanoresonators
We present and validate a reformulated Akhieser model that takes into account the reduction of thermal conductivity due to the impact of boundary scattering on the thermal phonons' lifetime. We consider silicon nanomembranes with mechanical mode frequencies in the GHz range as textbook examples of nanoresonators. The model successfully accounts for the measured shortening of the mechanical mode lifetime. Moreover, the thermal conductivity is extracted from the measured lifetime of the mechanical modes in the high-frequency regime, thereby demonstrating that the Q-factor can be used as an indication of the thermal conductivity and/or diffusivity of a mechanical resonator
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