Pinças ópticas: compensação de aberrações ópticas via modulação espacial de luz

Abstract

In this dissertation, we evaluate a method for compensating optical aberrations in na optical trap built with a Gaussian beam, using the transverse elastic constant as a probe. The methodology is based on determining the optical trap’s elastic constant via the Stokes– Faxén relation from the displacement of the initial equilibrium position of the trapped sphere’s center of mass caused by a constant viscous force. The study aimed to evaluate the effect of aberrations on the transverse elastic constant, as well as the effectiveness of the proposed method for correcting optical aberrations in the system used. For this purpose, measurements were carried out on microspheres with radii within the range where aberrations tend to reduce the transverse trapping constant. Wavefront modifications were made using a spatial light modulator, applying first-order Zernike coefficients for the following aberrations: spherical aberration, astigmatism, coma, and trefoil. A theoretical model for optical traps in the Mie scattering regime, including spherical aberration introduced by a glass–water interface, was used to evaluate the elastic constant. The effect of aberrations is assessed by comparing situations with and without the compensations made by the phase modulator. It was found that the main aberrations reducing the elastic constant were spherical aberration and astigmatism. An improvement in the performance of the optical trap was observed, especially for the smallest microsphere. Furthermore, we demonstrate that the criterion of maximizing the elastic constant can only be applied within specific radii ranges, based on theoretical results. Comparing the theoretical model with experimental results, it is found that, despite the performance improvement for spheres with radii smaller than half the wavelength of the trapping laser, and the agreement for spheres with radii close to the wavelength, the optimization condition was not achieved. Imperfections in polarization or modulated phase control may be responsible for this result.