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Abstract
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Optical trapping is a powerful technique in physics and nanotechnology for the precise control and manipulation of microscopic and nanoscale particles. Based on intensity-gradient forces and radiation pressure, it has found widespread applications in biotechnology, nanoscience, and fundamental physics research. Laguerre--Gaussian (LG) beams, with their annular intensity profile and helical phase structure, provide unique conditions for optical trapping, particularly for inducing and studying rotational motion. In this work, we investigate the rotational dynamics of dielectric particles trapped in high--topological-charge LG beams under strong focusing conditions and in the presence of spherical aberration. The transverse trap stiffness is quantified using two complementary approaches---Boltzmann statistics and the equipartition theorem---through a comprehensive analysis of the particle’s radial and angular position time series. In contrast to the conventional quadrant photodiode (QPD) method, which suffers from limited angular collection of scattered light, our imaging-based approach allows accurate stiffness measurements even when the trapped particle experiences significant slowdown or asymmetric intensity distributions in the beam ring. The method offers several advantages: (i) it enables reliable characterization of stiffness anisotropy for LG traps with topological charges up to l = 7; (ii) it remains robust in the presence of aberrations that distort the beam’s circular symmetry; and (iii) it provides simultaneous insight into both radial stability and angular-velocity variations. The proposed approach can enhance the accuracy of optical trapping measurements, improve the design of high-order LG optical traps, and deepen the understanding of particle behavior in complex optical potential landscapes.
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