Probing the Casimir force with optical tweezers

Abstract

We propose to use optical tweezers to probe the Casimir interaction between microspheres 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. This line of investigation has the potential for bringing together different fields including classical and quantum optics, statistical physics and colloid science, while paving the way for novel quantitative applications of optical tweezers in cell and molecular biology.

Subfemtonewton force fields measured with ergodic Brownian ensembles

“…For instance, colloids can be involved in reconstructing complex topological force fields, in particular in the near field where momentum exchanges are enhanced by near-field inhomogeneities [5,8]. As recently proposed, colloidal systems can also become pertinent tools for weak force measurements in the context of Casimir physics [28]. The advent of nano-and micro-structured and functionalized colloids [29][30][31] can lead to new types of dynamical responses to external fields, as exemplified with chiral optical forces [32][33][34][35][36][37].…”

Role of diffraction in the Casimir effect beyond the proximity force approximation

“…where the scattering angle Θ is given by (14), and p = 1(2) stands for ⊥ ( ) polarization. The leading corrections in (22) in order 1/R are found as [34] s ⊥ = c 2ξ cos(Θ) sin 3…”

“…The interaction for such geometry has been probed for a number of different materials over the last decade [6][7][8][9][10][11][12]. The Casimir interaction between two spherical surfaces is also of great interest [13][14][15] given its applications in colloids and surface sciences [16].…”

Theory of optical-tweezers forces near a plane interface

“…An example in interface science is the measurement of surface interactions between a microsphere and a glass slide [2] or between two microspheres [3,4]. In biological applications, transparent microspheres are usually employed as force transducers, with laser light of wavelength λ ∼ 1 μm to avoid damage to living cells.…”