Enhanced beam shifts mediated by Bound States in Continuum

TL;DR

This paper explores how Bound States in Continuum (BIC) can significantly enhance beam shifts like Goos-Hanchen and Imbert-Fedorov in stratified optical structures, revealing new possibilities for optical sensing and manipulation.

Contribution

It develops a generic formalism for analyzing beam interactions with stratified media exhibiting BICs, demonstrating giant beam shifts near high-Q resonances in dielectric layered structures.

Findings

Giant Goos-Hanchen shifts observed near BIC resonances

Enhanced Imbert-Fedorov shifts for circular polarization

Reflected beam distortion with satellite spot formation

Abstract

The interaction of light beams with resonant structures has led to the development of various optical platforms for sensing, particle manipulation, and strong light-matter interaction. In the current study, we investigate the manifestations of the bound states in continuum (BIC) on the in plane and out of plane shifts (referred to as Goos-Hanchen (GH) and Imbert-Fedorov (IF) shifts, respectively) of a finite beam with specific polarization incident at an arbitrary angle. Based on the angular spectrum decomposition, we develop a generic formalism for understanding the interaction of the finite beam with an arbitrary stratified medium with isotropic and homogeneous components. it is applied to the case of a Gaussian beam with p and circularly polarized light incident on a symmetric structure containing two polar dielectric layers separated by a spacer layer. For p-polarized plane wave incidence one of the coupled Berreman modes of the structure was recently shown to evolve to the bound state with infinite localization and diverging quality factor coexisting with the other mode with large radiation leakage (Remesh et al. Optics Communications, 498:127223, 2021). A small deviation from the ideal BIC resonance still offers resonances with very high quality factors and these are exploited in this study to report giant GH shifts. A notable enhancement in the IF shift for circularly polarized light is also shown. Moreover, the reflected beam is shown to undergo distortion leading to a satellite spot. The origin of such a splitting of the reflected beam is traced to a destructive interference due to the left and right halves of the corresponding spectra.