Probing the Casimir force with optical tweezers

Abstract

We propose to use optical tweezers to probe the Casimir interaction between\nmicrospheres inside a liquid medium for geometric aspect ratios far beyond the\nvalidity of the widely employed proximity force approximation. This setup has\nthe potential for revealing unprecedented features associated to the\nnon-trivial role of the spherical curvatures. For a proof of concept, we\nmeasure femtonewton double layer forces between polystyrene microspheres at\ndistances above $400$ nm by employing very soft optical tweezers, with\nstiffness of the order of fractions of a fN/nm. As a future application, we\npropose to tune the Casimir interaction between a metallic and a polystyrene\nmicrosphere in saline solution from attraction to repulsion by varying the salt\nconcentration. With those materials, the screened Casimir interaction may have\na larger magnitude than the unscreened one. This line of investigation has the\npotential for bringing together different fields including classical and\nquantum optics, statistical physics and colloid science, while paving the way\nfor novel quantitative applications of optical tweezers in cell and molecular\nbiology.\n