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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 Reviewed, Postprint (author's final draft)

We show that slow light in hyperbolic waveguides is linked to topological transitions in the dispersion diagram as the film thickness changes. The effect appears in symmetric planar structures with type II films, whose optical axis (OA) lies parallel to the waveguide interfaces. The transitions are mediated by elliptical mode branches that coalesce along the OA with anomalously ordered hyperbolic mode branches, resulting in a saddle point. When the thickness of the film increases further, the merged branch starts a transition to hyperbolic normally ordered modes propagating orthogonally to the OA. In this process, the saddle point transforms into a branch point featuring slow light for a broad range of thicknesses, and a new branch of ghost waves appears., Agència de Gestió d’Ajuts Universitaris i de Recerca (2017-SGR-1400); Ministerio de Economía y Competitividad (PGC2018-097035-B-I00, SEV-2015-0522); H2020 Marie Sklodowska-Curie Actions (GA665884); Fundación Cellex; Fundació Mir-Puig. National Science Foundation with a CAREER Grant No. ECCS-1749177., Award-winning, Postprint (author's final draft)

Bound states in the continuum (BICs) exist in a variety of physical systems where they appear as lossless propagating states surrounded by radiating modes. However, in the case of open systems, they coexist with continuous families of guided states, which may be modes or other BICs, located in different regions of the frequency-momentum parameter space. Here we report that waveguiding structures comprising anisotropic materials with two radiation channels where continuous families of modes are not possible whatsoever can support discrete BICs that exist for a single frequency and a single propagation direction. The results are isolated spectrally-discrete BIC states existing as lossless needles emerging from a sea of radiating leaky modes., H2020 Marie Sk lodowska-Curie Action GA665884; Government of Spain (grants PGC2018- 097035-B-I00; Severo Ochoa CEX2019-000910-S); Fundaci ´o Cellex; Fundaci´o Mir-Puig; Generalitat de Catalunya (CERCA and AGAUR 2017-SGR-1400)., Postprint (published version)

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 Reviewed, Postprint (author's final draft)

Bound states in the continuum (BICs), first predicted in the field of quantum physics [1], are localized radiationless states existing in the part of the spectrum that corresponds to radiative modes. BICs are a general wave phenomenon and have recently been found in photonic systems [2]. They are known to exist in photonic structures in almost-pure transverse-electric (TE) or transverse-magnetic (TM) states, at fixed frequencies and propagation directions [3]. More recently, we have shown the existence of fully hybrid BICs alongside pure TE/TM BICs in planar structures containing uniaxial materials in both symmetric and asymmetric geometries [4]. BICs occur when we decouple the radiation mode from the continuum by suppressing the radiation channel. This can happen in anisotropic planar structures due to polarization separation or destructive interference. The fact that interferometric BICs change propagation direction with frequency suggests that the structure gives itself to potential applications as spatial and spectral filters., Peer Reviewed, Postprint (published version)

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 Reviewed, Postprint (published version)

Hyperbolic metamaterials (HMMs) are uniaxial materials that have hyperbolic dispersion curves. There are two types of hyperbolic metamaterials, type I that is described by a permittivity tensor with e|| < 0 and e¿ > 0, and type II that has e|| > 0 and e¿ < 0. Planar waveguides with hyperbolic materials have been mostly studied with propagation direction along the principal axis of the dielectric tensor only. Such studies showed that modes feature anomalous ordering in such a way that cut-off appears for lower order modes as light frequency increases. Interestingly, stopped light featuring null group velocity at specific wavelengths has been predicted for modes near cut-off in type II HMMs planar waveguides with optical axis (OA) oriented along the propagation direction [1]. Here, we theoretically study light propagation in type II HMMs planar waveguides for any orientation ¿ of the OA contained in a plane parallel to the waveguides interfaces. The results shows that a change in the wavelength only changes the OA direction ¿ at which stopped light exist. The process is a consequence of the lack of mode cut-off and it is caused by a mode spectroscopy transition from hyperbolic modes with anomalous order propagating along the OA (¿ = 0º), to modes with normal order, propagating orthogonally to the OA (¿ = 90º)., Peer Reviewed, Postprint (published version)

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 Reviewed, Postprint (published version)

Waveguiding structures made of anisotropic media support bound states in the continuum (BICs) that arise when the radiation channel of otherwise semileaky modes is suppressed. Hitherto, only structures with optical axes aligned in symmetric orientations inside the waveguide plane, where BICs appear as lines in the momentum-frequency dispersion diagram, have been considered. Here we address settings where such symmetry is broken and unveil a number of fundamental different features. Weak and strong symmetry-breaking regimes are identified, corresponding to azimuthal and polar optical axes orientation asymmetries, respectively. The azimuthal symmetry breaking is found to still preserve the existence loci of BICs in the momentum-frequency dispersion diagram as lines. However, all possible BICs become interferometric, while the polarization separable states that occur in symmetric settings cease to exist. The polar symmetry breaking has stronger effects and transforms the BICs’ existence loci from lines to points, which correspond to full-vector states that exist at discrete values of the optical axis orientation for a given wavelength. Such transformation results in fundamental changes in the topological properties of the radiated field around the BICs., Peer Reviewed, Postprint (published version)

En col·laboració amb la Universitat Autònoma de Barcelona (UAB), la Universitat de Barcelona (UB) i l’Institut de Ciències Fotòniques (ICFO)

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 Reviewed, Postprint (author's final draft)

We report theoretical and experimental observation of radiationless bound states in the continuum (BIC) in waveguiding anisotropic structures. BICs may have all possible polarizations and be the only possible boundstates in the structure., Peer Reviewed, Postprint (published version)

Optical forces can set tiny objects in states of mechanical self-sustained oscillation, spontaneously generating periodic signals by extracting power from steady sources. Miniaturized self-sustained coherent phonon sources are interesting for applications such as mass-force sensing, intra-chip metrology and intra-chip time-keeping among others. In this paper, we review several mechanisms and techniques that can drive a mechanical mode into the lasing regime by exploiting the radiation pressure force in optomechanical cavities, namely stimulated emission, dynamical back-action, forward stimulated Brillouin scattering and self-pulsing., Peer Reviewed, Postprint (author's final draft)

Optical forces can set tiny objects in states of mechanical self-sustained oscillation, spontaneously generating periodic signals by extracting power from steady sources. Miniaturized self-sustained coherent phonon sources are interesting for applications such as mass-force sensing, intra-chip metrology and intra-chip time-keeping among others. In this paper, we review several mechanisms and techniques that can drive a mechanical mode into the lasing regime by exploiting the radiation pressure force in optomechanical cavities, namely stimulated emission, dynamical back-action, forward stimulated Brillouin scattering and self-pulsing., Peer Reviewed, Postprint (author's final draft)