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