# Distributed feedback in and distributed Bragg reflection from periodic chiral structures

#### Abstract

In this study, we examine the combined effects of electromagnetic chirality and periodicity on wave interactions in active and passive media. Isotropic chiral media have the unique property of possessing two intrinsic indices of refraction without the additional complication of anisotropy. The two refractive indices arise from different propagation velocities for right-circularly-polarized (RCP) and left-circularly-polarized (LCP) waves (i.e., circular birefringence), and their existence suggests that chiral media may be used in conjunction with the coherent feedback characteristics of periodic structures to create structures that favor either RCP or LCP solutions. In this study, we develop the theory that describes the wave interactions in such periodic chiral structures, and we examine several representative cases, including active devices, to identify specific characteristics.^ Bandgap characteristics are developed here for finite-length slabs of periodic chiral media using coupled-mode theory. The theory shows that stratified, periodic, isotropic media fall into one of three categories: subchiral, chiral, or superchiral. The superchiral periodic medium possesses three stop bands for the fundamental Bragg condition where the three stop bands are associated with LCP $\leftrightarrow$ RCP coupling, LCP $\to$ LCP coupling, and RCP $\to$ RCP coupling. We examine the reflectivity for superchiral periodic media to show the degree of polarization mode isolation for both passive and active structures. We also study the relationships among the chiral constitutive parameters (i.e., permittivity, permeability, and chirality admittance) based on a canonical chiral media model, and we address how the presence of chirality may enhance the reflectivity from periodic slabs.^ In addition to periodic media, we investigate the role of chirality for mode coupling in symmetric planar waveguides with periodic boundaries. Such structures are of current interest as mode converters, filters, and resonant devices for integrated optics applications. We show that the periodic chiral waveguides support coupling between guided modes predominantly composed of counter-propagating, circularly polarized plane waves. Also, planar waveguides using a chiral cladding material exhibit single-mode operation. This is the only symmetric waveguide geometry using isotropic media that can do so. ^

#### Subject Area

Electrical engineering|Condensed matter physics|Optics

#### Recommended Citation

Flood, Kevin Michael, "Distributed feedback in and distributed Bragg reflection from periodic chiral structures" (1995). Dissertations available from ProQuest. AAI9615041.
https://repository.upenn.edu/dissertations/AAI9615041

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