Exceptional Bound States in the Continuum

Bound states in the continuum (BICs) and exceptional points (EPs) are unique singularities of non-Hermitian systems. BICs demonstrate enhancement of the electromagnetic field at the nanoscale, while EPs exhibit high sensitivity to small perturbations. Here, we demonstrate that several BICs can be merged into one EP, forming an EP-BIC. The resulting state inherits properties from both BICs and EP, namely, it does not radiate and shows extremely high sensitivity to perturbations. We validate the developed theory with numerical simulations and demonstrate the formation of second and third-order EP-BICs in stacked dielectric metasurfaces. We also show that the losses of the resulting leaky resonances exhibit an anomalous behavior when the unit cell is broken, which differs from the asymptotics commonly attributed to BICs

Optical resonances in graded index spheres: a resonant-state-expansion study and analytic approximations

Recent improvements in the resonant-state expansion (RSE), focusing on the static mode contribution, have made it possible to treat transverse-magnetic (TM) modes of a spherically symmetric system with the same efficiency as their transverse-electric (TE) counterparts. We demonstrate here that the efficient inclusion of static modes in the RSE results in its quick convergence to the exact solution regardless of the static mode set used. We then apply the RSE to spherically symmetric systems with continuous radial variations of the permittivity. We show that in TM polarization, the spectral transition from whispering gallery to Fabry-Pérot modes is characterized by a peak in the mode losses and an additional mode as compared to TE polarization. Both features are explained quantitatively by the Brewster angle of the surface reflection which occurs in this frequency range. Eliminating the discontinuity at the sphere surface by using linear or quadratic profiles of the permittivity modifies this peak and increases the Fabry-Pérot mode losses, in qualitative agreement with a reduced surface reflectivity. These profiles also provide a nearly parabolic confinement for the whispering gallery modes, for which an analytical approximation using the Morse potential is presented. Both profiles result in a reduced TE-TM splitting, which is shown to be further suppressed by choosing a profile radially extending the mode fields. Based on the concepts of ray optics, phase analysis of the secular equation, and effective quantum-mechanical potential for a wave equation, we discuss a number of useful approximations which shed light on the physical phenomena observed in the spectra of graded-index systems