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

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

Optimal second best taxation of addictive goods in dynamic general equilibrium: a revenue raising perspective

In this paper we derive conditions under which optimal tax rates for addictive goods exceed tax rates for non-addictive consumption goods within a rational addiction framework where exogenous government spending cannot be financed with lump sum taxes. We reexamine classic results on optimal commodity taxation and find a rich set of new findings. Two dynamic effects exist. First, households anticipating higher future addictive tax rates reduce current addictive consumption, so they will be less addicted when the tax rate increases. Therefore, addictive tax revenue falls prior to the tax increase. Surprisingly, the optimal tax rate on addictive goods is generally decreasing in the strength of tolerance, since strong tolerance strengthens this tax anticipation effect. Second, high current tax rates on addictive goods make households less addicted in the future, affecting all future tax revenues in a way which depends on how elasticities are changing over time. Classic results on uniform commodity taxation emerge as special cases when elasticities are constant and the addiction function is homogeneous of degree one. Finally, we also study features of addictive goods such as complementarity to leisure that, while not directly related to the definition of addiction, are nonetheless properties many addictive goods display

Anomalies and Wess-Zumino Terms in an Extended, Regularized Field-Antifield Formalism

Quantization of anomalous gauge theories with closed, irreducible gauge algebra within the extended Field-Antifield formalism is further pursued. Using a Pauli-Villars (PV) regularization of the generating functional at one loop level, an alternative form for the anomaly is found which involves only the regulator. The analysis of this expression allows to conclude that recently found ghost number one cocycles with nontrivial antifield dependence can not appear in PV regularization. Afterwards, the extended Field-Antifield formalism is further completed by incorporating quantum effects of the extra variables, i.e., by explicitly taking into account the regularization of the extra sector. In this context, invariant PV regulators are constructed from non-invariant ones, leading to an alternative interpretation of the Wess-Zumino action as the local counterterm relating invariant and non-invariant regularizations. Finally, application of the above ideas to the bosonic string reproduces the well-known Liouville action and the shift $(26-D)\rightarrow(25-D)$ at one loop.Comment: 30 pages, Latex file, KUL-TF-93/50, UB-ECM-PF 93/14, UTTG-16-93. (shortened version, to appear in Nucl. Phys. B

Brane-Intersection Dynamics from Branes in Brane Backgrounds

We derive the dynamics of M-brane intersections from the worldvolume action of one brane in the background supergravity solution of another one. In this way we obtain an effective action for the self-dual string boundary of an M2-brane in an M5-brane, and show that the dynamics of the 3-brane intersection of two M5-branes is described by a Dirac-Born-Infeld action.Comment: 12 pages,latex. Typos corrected, reference adde

Infinitely many rigid symmetries of kappa-invariant D-string actions

We show that each rigid symmetry of a D-string action is contained in a family of infinitely many symmetries. In particular, kappa-invariant D-string actions have infinitely many supersymmetries. The result is not restricted to standard D-string actions, but holds for any two-dimensional action depending on an abelian world-sheet gauge field only via the field strength. It applies thus also to manifestly $SL(2,Z)$ covariant D-string actions. Furthermore, it extends analogously to $d$-dimensional actions with $(d-1)$-form gauge potentials, such as brane actions with dynamical tension.Comment: 5 pages, RevTex; one reference added, complete names of author