Microbubble trapping in inverted optical tweezers

We have developed an inverted microscope optical tweezers for trapping and manipulation of microscopic gas bubbles. Trapping is achieved by a time-averaged optical trap using a rapidly-scanning Gaussian laser beam. Unlike holographic optical tweezers for microbubbles that employ a Laguerre-Gaussian beam, in this configuration the backwards-directed optical gradient force is sufficient to confine a microbubble against both the optical scattering force and the microbubble buoyancy. We have calibrated the optical trapping forces for microbubbles with a range of sizes, and determined the scanning trap configuration that produces the strongest confinement. Our system also includes a real-time “point-and-click” user interface for interactive selection, capture and isolation of individual microbubbles with optimal trap stiffness.