Maximizing the quality factor to mode volume ratio for ultra-small photonic crystal cavities

TL;DR

This paper presents a topology optimization approach for designing ultra-small photonic crystal cavities that significantly improve the Q/V ratio, achieving deep-subwavelength light confinement with manufacturing tolerance considerations.

Contribution

It introduces a systematic topology optimization method to enhance the Q/V ratio in small photonic crystal cavities, including designs that surpass standard and shape-optimized cavities.

Findings

Topology-optimized cavities increase Q/V ratio by up to two orders of magnitude.

Bow-tie shaped cavities confine light beyond the diffraction limit.

Designs maintain efficiency under manufacturing tolerances.

Abstract

Small manufacturing-tolerant photonic crystal cavities are systematically designed using topology optimization to enhance the ratio between quality factor and mode volume, Q/V. For relaxed manufacturing tolerance, a cavity with bow-tie shape is obtained which confines light beyond the diffraction limit into a deep-subwavelength volume. Imposition of a small manufacturing tolerance still results in efficient designs, however, with diffraction-limited confinement. Inspired by numerical results, an elliptic ring grating cavity concept is extracted via geometric fitting. Numerical evaluations demonstrate that for small sizes, topology-optimized cavities enhance the Q/V-ratio by up to two orders of magnitude relative to standard L1 cavities and more than one order of magnitude relative to shape-optimized L1 cavities. An increase in cavity size can enhance the Q/V-ratio by an increase of the Q-factor without significant increase of V. Comparison between optimized and reference cavities illustrates that significant reduction of V requires big topological changes in the cavity.