Holographic optical tweezers (HOTs) have been utilized for trapping microscopic particles in three dimensions with multiple foci generated by wavefront shaping of light, which can manipulate three-dimensional (3-D) positions of colloidal particles as well as exerting an optical force on particles. So far, most experiments using HOTs have been conducted for trapping spherical particles because optical principles can easily predict optical forces and the responding motion of microspheres. For non-spherical particles, however, calculation of optical forces and torques exerting on samples is very complicated, and the orientation control of non-spherical particles is limited since the non-spherical particles tend to align along the optic axis of the trapping beam.
Here, we propose and experimentally demonstrate 3-D trapping of non-spherical particles by wavefront shaping of light based on the measurement of 3-D refractive index (RI) distribution of samples. The 3-D RI distribution of non-spherical particles was measured by optical diffraction tomography and the phase hologram which can generate stable optical traps for the samples was calculated by iterative 3-D Gerchberg-Saxton algorithm from the measured 3-D RI distribution. We first validate the proposed method for stable trapping and orientation control of 2-m colloidal PMMA ellipsoids. The proposed method is also exploited for rotating, folding and assembly of red blood cells.
Kyoohyun Kim and YongKeun Park, "Holographic trapping of non-spherical particles with 3D refractive index measurements (Conference Presentation)," Proc. SPIE 10074, Quantitative Phase Imaging III, 1007405 (Presented at SPIE BiOS: January 29, 2017; Published: 24 April 2017); https://doi.org/10.1117/12.2251571.5380600148001.
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