Optical BICs in a NW GSL. (a) Geometry of a NW GSL under TE-polarized plane wave illumination, where the length of each segment is p/2. (b) Q factors of two GSL eigenmodes with varying p in a NW GSL with d = 200nm, e = 170nm. Modes are labeled with angular numbers m = 0 or m = 1. (c) Hy pattern of m = 0 GSL eigenmode. (d) Hy, upper, and Ey, lower, patterns of m = 1 GSL eigenmode. (e) Qsca spectrum of a NW GSL with d = 200nm, e = 170nm, and p = 400nm, solid black curve, and of a NW with d = 185nm, gray dashed curve. (f), (g) Heat maps of (f) Qsca and (g) log(U/U0) for a NW GSL with varying p for fixed d = 200nm and e = 170nm. Single spectra for a uniform NW with d = 185nm are presented on top of each heat map.

Geometric Superlattices

Perfect trapping of light in a subwavelength cavity is a key goal in nanophotonics. Perfect trapping has been realized with optical bound states in the continuum, BIC, in waveguide arrays and photonic crystals; yet the formal requirement of infinite periodicity has limited the experimental realization to structures with macroscopic planar dimensions.

In work published in Physical Review Letters, Seokhyoung Kim and Kyoung-Ho Kim in the Cahoon Group, characterize BICs in a silicon nanowire, NW, geometric superlattice, GSL, that exhibits one-dimensional periodicity in a compact cylindrical geometry with a subwavelength diameter.

Seokhyoung Kim and James Cahoon

Seokhyoung Kim and James Cahoon

The team members analyzed the scattering behavior of NW GSLs by formulating temporal coupled mode theory to include Lorenz-Mie scattering, and they show that GSL-based BICs can trap electromagnetic energy for an infinite lifetime and exist over a broad range of geometric parameters.

Using synthesized NW GSLs tens of microns in length and with variable pitch, the Cahoon Group members demonstrate the progressive spectral shift and disappearance of Fano resonances in experimental single-NW extinction spectra as a manifestation of BIC GSL modes.