We propose to use optical tweezers to probe the Casimir interaction between micro-spheres inside a liquid medium for geometric aspect ratios far beyond the validity of the widely employed proximity force approximation. This setup has the potential for revealing unprecedented features associated to the non-trivial role of the spherical curvatures. For a proof of concept, we measure femtonewton double-layer forces between polystyrene microspheres at distances above 400 nm by employing very soft optical tweezers, with stiffness of the order of fractions of a fN/nm. As a future application, we propose to tune the Casimir interaction between a metallic and a polystyrene microsphere in saline solution from attraction to repulsion by varying the salt concentration. With those materials, the screened Casimir interaction may have a larger magnitude than the unscreened one.
We report on the nonlinear effects of light propagation through a fluorescent nanocolloid, where self-collimated beams are formed. The medium is constituted by a bidisperse suspension of fluorescent and nonfluorescent nanospheres of similar diameters (60nm and 62nm, respectively) in distilled water. A CW laser beam (532 nm wavelength) was focused into the nano-suspension. The threshold power and focusing conditions to create a self-collimated beam are analyzed as a function of the incident power, and a hysteresis effect is observed for the size of the output beam when the power is increasing and decreasing. We also discuss other effects associated to the presence of the fluorescent nanospheres.
A Bessel beam generated with an axicon lens is focused by a low numerical aperture objective lens to create an optical levitation trap. This configuration allows a full three dimensional trapping of solid glass spheres of few microns in diameter immerse in water. We establish a comparison with an optical levitation trap generated with a Gaussian beam under the same focusing conditions. In both cases the particles are lifted up in an axial position above the focal region that depends on the incident power. The spatial stability is investigated in both cases as a function of axial position, which in turn depends on the input optical power.