Optical trapping and axial manipulation with annular laser beams

In a quest to find an optimal laser beam shape for the most stable optical trap, here we investigate relative trapping efficiencies for a set of concentric annular beams, comparing with the conventionally used Gaussian beam. Combining an intuitive theoretical model considering a focused Gaussian beam decomposed into concentric light cones with an experiment implementing laser beam-shaping, we show that contrary to the preconception that tighter focusing would lead to better trapping, the efficiency depends on a balancing act not only among gradient force and scattering force but aberration effects as well. Furthermore, we present a new technique for controlled axial manipulation of a trapped particle by utilizing the axial focus shifts that are intrinsic to each annular beam in conjunction with real-time switching between ring beams through the use of a spatial light modulator. This approach paves the way for practical applications requiring controlled three-dimensional particle manipulation by enabling stable axial positioning across long distances.