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.
A microelectromechanical torsional oscillator was used to obtain new constraints in the search for new Yukawa-like
interactions at the ~ 100 nm range. A new heterodyne technique was used to enhance the possible contributions of
hypothetical forces, while electromagnetic interactions (including the ones associated with vacuum fluctuations),
remained the same. In particular, the force between a Au-coated sphere and a Au film deposited on the oscillator was
subtracted in situ from the force between the same sphere and a composite film made out of Ge and Au. The
combination of the high quality factor Q of the oscillator and this new approach that greatly reduced the Casimir
background yielded improvements in the constraints close to one order of magnitude over the 50-400 nm interaction
We describe a new system for controlling the tip-to-sample separation in a Near Field Scanning Optical Microscope. A tapered Al coated fiber was glued to a high-Q Si paddle mechanical oscillator. The paddle is capacitively driven at one of its resonances, and the amplitude of the movement is detected through another electrode. As the tip approaches the surface, the viscous drag acting on its increases, causing the amplitude of oscillation of the paddle-tip system to reduce. A signal proportional to the amplitude of oscillation is used as feedback to control the tip-to-sample distance. This is accomplished in the 0 - 200 nm range, with a stability better than 1 nm. We present a complete characterization of the system. In order to determine the capabilities of our setup we provide shear force images of different samples as well as simultaneous topographic and near field images of GaAs/AlGaAs 1.55 micrometers waveguides.