Omnidirectional Gradient Force Optical Trapping in Dielectric Nanocavities by Inverse Design
Beñat Martinez de Aguirre Jokisch, Benjamin Falkenberg Gøtzsche, Philip Trøst Kristensen, Martijn Wubs, Ole Sigmund, Rasmus E. Christiansen
Abstract
Optical trapping enables precise control of individual particles of different sizes such as atoms, molecules, or nanospheres. Optical tweezers provide free-space omnidirectional optical trapping of objects in laboratories around the world. As an alternative to standard macroscopic setups based on lenses, which are inherently bound by the diffraction limit, plasmonic and photonic nanostructures promise trapping by near-field optical effects on the extreme nanoscale. However, the practical design of lossless waveguide-coupled nanostructures capable of trapping subwavelength-sized particles in all spatial directions has until now proven insurmountable. In this work, we demonstrate an omnidirectional optical trap realized by inverse-designing fabrication-ready integrated dielectric nanocavities. The subwavelength optical trap is designed to rely solely on the gradient force and is thus particle-size-agnostic. In particular, we show how a trapped particle with a radius of 15 nm experiences a force strong enough to overcome room-temperature thermal fluctuations. Furthermore, through the robust inverse-design framework, we tailor manufacturable devices operating at short-wave-infrared and near-infrared wavelengths. Our results open a new regime of levitated optical trapping by achieving a deep trapping potential capable of trapping single subwavelength particles in all directions using optical gradient forces. We anticipate potentially groundbreaking applications of the optimized optical trapping system for biomolecular analysis in aqueous environments, levitated cavity optomechanics, and cold atom physics, constituting an important step toward realizing integrated bionanophotonics and mesoscopic quantum mechanical experiments.