# An L3-type silicon photonic crystal cavity with a quality factor exceeding 20 million

Authorea preprint 02/22/2017

Abstract: We present an L3-type photonic crystal cavity in silicon with a theoretical quality factor of 20.9 million. This highly-optimized design is made by shifting the positions of the holes surrounding the cavity, and was obtained through an automated global optimization procedure.

In Ref. (Minkov 2014) we presented a global optimization of the quality factors ($$Q$$) of several of the most widely-used photonic crystal (PhC) cavities. This resulted in a tremendous improvement – typically of one or more orders of magnitude – in this important figure of merit. In particular, the L3-type cavity, consisting of three missing holes in the PhC, was optimized to a theoretical $$Q$$ of 4.1 million, and a $$Q$$ of 2 million for this design was measured in a subsequent experiment (Lai 2014). Here, we present an additional improvement of the theoretical $$Q$$ of this cavity, accomplished by including three extra parameters in the optimization. We use the same overall PhC parameters as in Ref. (Minkov 2014): slab refractive index $$n=3.46$$ (as for silicon); hole radius $$R=0.25a$$; slab thickness $$d=0.55a$$, with $$a$$ the PhC lattice constant.