Improved design and experimental demonstration of ultrahigh-Q C${}_\text{6}$-symmetric H1 hexapole photonic crystal nanocavities

Abstract

An H1 photonic crystal nanocavity is based on a single point defect and has eigenmodes with a variety of symmetric features. Thus, it is a promising building block for photonic tight-binding lattice systems that can be used in studies on condensed matter, non-Hermitian and topological physics. However, improving its radiative quality (𝑄) factor has been considered challenging. Here, we report the design of a hexapole mode of an H1 nanocavity with a 𝑄 factor exceeding 10 8 . We achieved such extremely high-𝑄 conditions by designing only four structural modulation parameters thanks to the C 6 symmetry of the mode, despite the need of more complicated optimizations for many other nanocavities. The fabricated silicon photonic crystal nanocavities exhibited a systematic change in their resonant wavelengths depending on the spatial shift of the air holes in units of 1 nm. Out of 26 such samples, we found eight cavities with loaded 𝑄 factors over one million (1.2 × 10 6 maximum). We examined the difference between the theoretical and experimental performances by conducting a simulation of systems with input and output waveguides and with randomly distributed radii of air holes. Automated optimization using the same design parameters further increased the theoretical 𝑄 factor by up to 4.5 × 10 8 , which is two orders of magnitude higher than in the previous studies. Our work elevates the performance of the H1 nanocavity to the ultrahigh-𝑄 level and paves the way for its large-scale arrays with unconventional functionalities.

H1 hexapole photonic crystal nanocavities with theoretical and measured quality factors exceeding 10^8 and 10^6

“…This value is more than a hundred times those in the previous studies 18,19 . Details will be discussed in the presentation and elsewhere 21 .…”