Inverse-Designed Photonic Crystal Cavities with Controllable Far-Field Numerical Aperture

Abstract: Photonic crystal cavities confine light to subwavelength volumes, enabling strong light-matter interactions for applications in low-power photonics, opto-electronics, nonlinear optics, and quantum information. These applications demand cavities that combine high quality factors, low mode volumes, and high coupling efficiencies. However, optimizing across these metrics requires exploring a large design space, motivating the use of inverse design strategies. Previous inverse design efforts targeted high quality factors and low mode volumes, sacrificing the coupling efficiency or lacking the ability to precisely control the far-field radiation pattern. In this work, we present an inverse design framework that simultaneously optimizes cavity quality factor and far-field numerical aperture, both specified as design targets. Using this method, we design L3 photonic crystal cavities, with different far-field numerical apertures, in the visible wavelength and fabricate them in silicon nitride. Photoluminescence measurements confirm experimental control of the far-field numerical aperture and reveal a 28-fold and 3.9-fold improvement in the coupling efficiency and quality factor respectively when compared to the standard L3 cavity. Disorder analysis further shows that the designs retain significant performance despite nanofabrication imperfections. Our work demonstrates a versatile inverse design framework for multi-objective optimization of photonic crystal cavities to attain high quality factors and coupling efficiency.