From Free Space to Guided Modes: Tunable and Directional Coupling with Microsphere Resonators

Abstract

Efficiently interfacing free-space light with subwavelength waveguides remains a major challenge in integrated photonics, particularly at visible wavelengths relevant for spectroscopy and photocatalysis. We demonstrate a lithography-free coupling strategy using individual silica (SiO2) microspheres integrated onto suspended silicon nitride (Si3N4) membranes. When illuminated with a focused Gaussian beam, the microspheres excite resonances that evanescently couple into the underlying waveguide with efficiencies up to 50%, as measured by integrating-sphere microscopy. The coupled spectrum spans 500–780 nm, and axial beam translation reproducibly shifts the dominant wavelength from 550 to 710 nm, providing straightforward spectral tunability. Crucially, both experiments and full-field simulations reveal that the in-coupled light is launched directionally, while the wavelength-matched out-coupling and in-coupling efficiencies show excellent agreement, consistent with reciprocity. This combination of broadband operation, postfabrication tunability, and controlled directionality establishes microsphere coupling as a reconfigurable route for visible-light integrated photonics, enabling selective light delivery for chip-based photocatalysis and multiplexed lab-on-chip spectroscopy.