Nano Guiding of Light Using Dyakonov Waves

Postprint (published version

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

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

STED imaging performance estimation by means of Fourier transform analysis

Due to relatively high powers used in STED, biological samples may be affected by the illumination in the process of image acquisition. Similarly, the performance of the system may be limited by the sample itself. Optimization of the STED parameters taking into account the sample itself is therefore a complex task as there is no clear methodology that can determine the image improvement in an objective and quantitative manner. In this work, a method based on Fourier transform formalism is presented to analyze the performance of a STED system. The spatial frequency distribution of pairs of confocal and STED images are compared to obtain an objective parameter, the Azimuth Averaged Spectral Content Spread (AASCS), that is related to the performance of the system in which the sample is also considered. The method has been first tested on samples of beads, and then applied to cell samples labeled with multiple fluorescent dyes. The results show that a single parameter, the AASCS, can be used to determine the optimal settings for STED image acquisition in an objective way, only by using the information provided by the images from the sample themselves. The AASCS also helps minimize the depletion power, for better preservation of the samples.Peer ReviewedPostprint (published version

Soliton content with quadratic nonlinearities

Summary form only given. Quadratic solitons, that form through cascading in materials with second-order nonlinearities, are a topic of current intense investigation. Solitons have been observed in second harmonic generation (SHG) and in parametric amplification schemes, and many of their basic properties are well established. In particular, families of spatial and temporal solitons existing in waveguides and in bulk geometries are known, including those existing in settings with a small Poynting vector walk-off and/or temporal group-velocity mismatch. Under conditions where modulational instabilities can not grow, such families of solitons are stable under propagation and robust against several perturbations.Peer ReviewedPostprint (published version

Large Diameter of Palytoxin-induced Na/K Pump Channels and Modulation of Palytoxin Interaction by Na/K Pump Ligands

Palytoxin binds to Na/K pumps to generate nonselective cation channels whose pore likely comprises at least part of the pump's ion translocation pathway. We systematically analyzed palytoxin's interactions with native human Na/K pumps in outside-out patches from HEK293 cells over a broad range of ionic and nucleotide conditions, and with or without cardiotonic steroids. With 5 mM internal (pipette) [MgATP], palytoxin activated the conductance with an apparent affinity that was highest for Na+-containing (K+-free) external and internal solutions, lowest for K+-containing (Na+-free) external and internal solutions, and intermediate for the mixed external Na+/internal K+, and external K+/internal Na+ conditions; with Na+ solutions and MgATP, the mean dwell time of palytoxin on the Na/K pump was about one day. With Na+ solutions, the apparent affinity for palytoxin action was low after equilibration of patches with nucleotide-free pipette solution. That apparent affinity was increased in two phases as the equilibrating [MgATP] was raised over the submicromolar, and submillimolar, ranges, but was increased by pipette MgAMPPNP in a single phase, over the submillimolar range; the apparent affinity at saturating [MgAMPPNP] remained ∼30-fold lower than at saturating [MgATP]. After palytoxin washout, the conductance decay that reflects palytoxin unbinding was accelerated by cardiotonic steroid. When Na/K pumps were preincubated with cardiotonic steroid, subsequent activation of palytoxin-induced conductance was greatly slowed, even after washout of the cardiotonic steroid, but activation could still be accelerated by increasing palytoxin concentration. These results indicate that palytoxin and a cardiotonic steroid can simultaneously occupy the same Na/K pump, each destabilizing the other. The palytoxin-induced channels were permeable to several large organic cations, including N-methyl-d-glucamine+, suggesting that the narrowest section of the pore must be ∼7.5 Å wide. Enhanced understanding of palytoxin action now allows its use for examining the structures and mechanisms of the gates that occlude/deocclude transported ions during the normal Na/K pump cycle

Propuesta de ordenación y manejo de los recursos naturales para la conservación y restauración del tramo bajo del río Guadairilla (Sevilla)

Se realiza una propuesta de ordenación para la conservación y restauración de la ribera del curso bajo del río Guadarilla, afluente del Guadaíra (cuenca del Guadalquivir). Esta propuesta se basa en una metodología de ordenación de los recursos naturales que identifica unidades ambientales objeto de ordenación partiendo del análisis de los elementos geomorfológicos, hidrológicos y biogeográficos y sus cambios temporales. Estos cambios establecen cuales son las áreas de progresión, estabilidad o regresión de la superficie de distribución de las comunidades vegetales de ribera que quedan de este río. En función de este análisis de cambio y del conocimiento de campo de la composición florística y estructural de las comunidades vegetales riparias del río Guadairilla, se propone una zonificación en tres niveles para: manejo, conservación y propuesta de restauración de una ribera mediterránea. El análisis muestra la capacidad de regeneración ambiental que tienen las riberas fluviales termomediterráneas cuando disminuye la presión del hombre y cuál sería la respuesta que el medio natural poseería al realizarse una ordenación del territorio con criterios ambientales sostenibles.This paper shows a proposal of management for the conservation and restoration of the lower basin of Guadarilla River, a tributary of Guadaíra (Guadalquivir basin). This is based on a methodology for natural resources management focused on the identification of environmental units under management from the analysis of geomorphological, hydrological and biogeographic elements and their changes along the time line. These changes produce areas of progression, stability or regression of the surface distribution of riparian plant communities that still remain in this river. A proposal of zoning based on the analysis of changes and the field knowledge about floristic and structural composition of riparian plant communities of Guadairilla River. This zoning is focused on three levels: management, conservation and restoration. The analysis shows the ability of environmental regeneration of thermo-mediterranean riverbanks when human pressure decreases and what would be the response of the environment to a land use planning designed with a sustainable criterion

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