A beam of light may possess both spin and orbital angular momentum. In non-paraxial conditions part of the spin converts into orbital angular momentum through the spin-orbit angular momentum conversion phenomenon. This effect has important consequences at the nanoscale, particularly in nano-manipulation and nano-photonics. In this work, we thoroughly analyze the rotation of microscopic beads subjected to a tightly focused Laguerre-Gaussian beam. Particularly, we observe the rotation of particles along circular trajectories that will depend strongly on the combination of topological charges and the state of polarization. Based on Richard and Wolf theory for non-paraxial beam focusing, we found a very good agreement between the experimental results and the theoretical model based on calculation of the optical forces using the generalized Lorenz-Mie theory.
The accurate measurement of microscopic force fields is crucial in many branches of science and technology, from biophotonics and mechanobiology to microscopy and optomechanics. These forces are often probed by analysing their influence on the motion of Brownian particles. Here we introduce a powerful algorithm for microscopic force reconstruction via maximum-likelihood-estimator analysis (FORMA) to retrieve the force field acting on a Brownian particle from the analysis of its displacements [1]. FORMA estimates accurately the conservative and non-conservative components of the force field with important advantages over established techniques, being parameter-free, requiring ten-fold less data and executing orders-of-magnitude faster. We demonstrate FORMA performance using optical tweezers, showing how, outperforming other available techniques, it can identify and characterise stable and unstable equilibrium points in generic force fields. Thanks to its high performance, FORMA can accelerate the development of microscopic and nanoscopic force transducers for physics, biology and engineering.
[1] García, Laura Pérez, Jaime Donlucas Pérez, Giorgio Volpe, Alejandro V. Arzola, and Giovanni Volpe. "High-performance reconstruction of microscopic force fields from Brownian trajectories." Nature Communications 9, no. 1 (2018): 5166. https://doi.org/10.1038/s41467-018-07437-x
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