This method is particularly useful to probe the chiral optical response of single nanoparticles, each and every with its unique handedness and optical properties.

The absolute calibration of optical tweezers has been demonstrated by including the aberration effects of the optical system into the theoretical framework, thereby opening the door to an improved instrument design, reliable force measurements, and a better control over experiments.

We show that the optical force field in optical tweezers with elliptically polarized beams has the opposite handedness for a wide range of particle sizes and for the most common configurations. Our method is based on the direct observation of the particle equilibrium position under the effect of a transverse Stokes drag force, and its rotation around the optical axis by the mechanical effect of the optical torque. We find overall agreement with theory, with no fitting, provided that astigmatism, which is characterized separately, is included in the theoretical description. Our work opens the way for characterization of the trapping parameters, such as the microsphere complex refractive index and the astigmatism of the optical system, from measurements of the microsphere rotation angle.

We measure the colloidal interaction between two silica microspheres in an aqueous solution in the distance range from 0.2 to $0.5\mu \mathrm{m}$ with the help of optical tweezers. When employing a sample with a low salt concentration, the resulting interaction is dominated by the repulsive double-layer interaction which is fully characterized. The double-layer interaction is suppressed when adding $0.22\mathrm{M}$ of salt to our sample, thus leading to a purely attractive Casimir signal. When analyzing the experimental data for the potential energy and force, we find good agreement with theoretical results based on the scattering approach. At the distance range probed experimentally, the interaction arises mainly from the unscreened transverse magnetic contribution in the zero-frequency limit, with nonzero Matsubara frequencies providing a negligible contribution. In contrast, such unscreened contribution is not included by the standard theoretical model of the Casimir interaction in electrolyte solutions, in which the zero-frequency term is treated separately as an electrostatic fluctuational effect. As a consequence, the resulting attraction is too weak in this standard model, by approximately one order of magnitude, to explain the experimental data. Overall, our experimental results shed light on the nature of the thermal zero-frequency contribution and indicate that the Casimir attraction across polar liquids has a longer range than previously predicted.

We extend a previous proposal for absolute calibration of optical tweezers by including optical setup aberrations into the first-principles theory, with no fitting parameters. Astigmatism, the dominant term, is determined from images of the focused laser spot. Correcting it can substantially increase stiffness. Comparison with experimental results yields agreement within error bars for a broad range of bead sizes and trap heights, as well as different polarizations. Absolute calibration is established as a reliable and practical method for applications and design of optical tweezers systems.

Optical pulling forces or tractor beams can pull particles against light propagation by redirecting the incident photons forward. This is typically achieved using Bessel beams with very small half-cone angles, which considerably limits their applicability. One can circumvent such an issue by using a superposition of plane waves. In order to investigate optical pulling forces exerted by a pair of noncollinear plane waves, we develop a theoretical framework based on Mie theory, Debye potentials, and Wigner rotation matrices. We apply this framework to calculate the optical pulling force on metallodielectric composite particles, which we put forward as an alternative material platform to optimize and tailor tractor beams. Indeed, we demonstrate that by adding a few plasmonic inclusions to low-refractive-index dielectric particles of arbitrary sizes, we are able to produce polarization-dependent optical pulling forces that cannot occur in the corresponding homogeneous particles. Altogether, our findings not only provide innovative theoretical methods to compute optical pulling forces but also provide strategies to tailor and optimize them, paving the way to increase their applicability.