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DTSTART:20260415T080000Z
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TRANSP:OPAQUE
LOCATION:Auditorium
SUMMARY:ICFO | MARIA DEL PILAR PUJOL CLOSA
CLASS:PUBLIC
DESCRIPTION:Low-loss waveguides are essential for energy-efficient photonic
  circuits\, optical communications\, and sensing applications. Over the pa
 st century\, two lossless phenomena&mdash\;Dyakonov modes and Bound States
  in the Continuum (BICs)&mdash\;have been discovered in anisotropic wavegu
 ides\, where permittivities differ but share the same sign. Hyperbolic met
 amaterials (HMMs) exhibit extreme anisotropy\, with ordinary and extraordi
 nary permittivities of opposite signs\, enabling unconventional light mani
 pulation. Their unique properties have attracted broad interest for applic
 ations including subdiffraction imaging\, spontaneous emission control\, a
 nd enhanced light-matter interactions. This raises a fundamental question:
  can extreme hyperbolic anisotropy support novel confinement mechanisms or
  new regimes of lossless propagation? Prior research on HMM waveguides has
  been constrained to simplified models or propagation along principal axes
 \, leaving systematic exploration of arbitrary propagation directions\, an
 d the phenomena they may reveal\, as a critical gap.\nTo address this gap\
 , this thesis develops a semi-analytical computational framework that comb
 ines a transfer-matrix formulation with a complex-plane Newton-Raphson roo
 t finder\, enabling stable tracking of guided and leaky modes for arbitrar
 y propagation directions. This tool allows systematic exploration of a wid
 e range of parameters and configurations previously difficult to study.\nT
 his thesis provides the most comprehensive investigation to date of light 
 propagation in planar HMM waveguides. For the first time\, the work analyz
 es both type I and type II HMM waveguides across all in-plane propagation 
 directions and with arbitrary optic axis orientations. The analysis reveal
 s how hyperbolic anisotropy fundamentally influences polarization\, confin
 ement\, polarization exchange between modes\, mode ordering\, radiation me
 chanisms\, and slow light arising from topological transitions. This estab
 lishes general trends\, identifies new guiding regimes\, and maps the land
 scape of wave phenomena in these extreme anisotropic systems.\nThe explora
 tion of leaky modes enabled a key discovery: Dirac points embedded in the 
 Continuum (DECs)\, a novel class of topological degeneracy in non-Hermitia
 n systems. DECs emerge when a symmetry-protected BIC and an interferometri
 c BIC intersect linearly. At this intersection\, the system exhibits a rea
 l eigenvalue\, two orthogonal modes\, and zero radiation loss&mdash\;local
 ly Hermitian behavior despite being embedded in a non-Hermitian system. Th
 e presence of both BICs suppresses Exceptional Points (EPs) and collapses 
 the Fermi arc to a single point. Because DECs arise from universal BIC int
 eractions rather than material-specific properties\, this phenomenon exten
 ds beyond hyperbolic media\, with implications in the fields of topologica
 l photonics and non-Hermitian physics.\nThis thesis demonstrates the frame
 work&rsquo\;s generality and reliability through application to anisotropi
 c liquid-crystal waveguides\, where predicted BIC trajectories match exper
 imental observations\, and to $\\sigma$-near-zero metasurfaces\, where the
  framework accurately reproduces published dispersion diagrams. These vali
 dations confirm its applicability beyond hyperbolic systems.\nThis thesis 
 establishes a comprehensive theoretical and computational understanding of
  wave propagation in planar HMM waveguides for both type I and type II con
 figurations and discovers DECs as a novel physical phenomenon with implica
 tions beyond hyperbolic media. By revealing how extreme anisotropy enables
  new guiding regimes and loss suppression\, this work advances the underst
 anding of light confinement in open\, strongly anisotropic systems and pro
 vides new routes for designing low-loss photonic devices..\nWednesday Apri
 l 15\, 10:00 h. ICFO Auditorium \nThesis Director: Prof. Dr. David Artigas
DTSTAMP:20260417T113227Z
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