Synchronization of colloidal rotors through angular optical binding

Abstract

A mechanism for the synchronization of driven colloidal rotors via optical coupling torques is presented and analyzed. Following our recent experiments [Brzobohatý et al., Opt. Express 23, 7273 (2015)], we consider a counterpropagating optical beam trap that carries spin angular momentum, but no net linear momentum, operating in an aqueous solvent. The angular momentum carried by the beams causes the continuous low-Reynolds-number rotation of spheroidal colloids. Due to multiple scattering, the optical torques experienced by these particles depend on their relative orientations, while the effect of hydrodynamic interaction is negligible. This results in frequency pulling, which causes weakly dissimilar spheroids to synchronize their rotation rates and lock their relative phases. The effect is qualitatively captured by a coupled dipole model and quantitatively reproduced by T-matrix calculations. For pairs of rotors, the relative torque τ is shown to vary with relative phase φ according to τ ≈ A sin(2 φ + δ) + B for constants A,B,δ, so the resulting motion is governed by the well-known Adler equation. We show that this behavior can be preserved for larger numbers of particles. The application of these phenomena to the inertial motion of particles in vacuum could provide a route to the sympathetic cooling of mesoscopic particles.

Rotranslational cavity cooling of dielectric rods and disks

“…If the laser intensity is reduced adiabatically [60], on the other hand, a free quantum state of low kinetic and rotational energy can be generated. Finally, aligning many anisotropic particles in a single cavity mode might give rise to novel phenomena, such as a non-polar version of a gas of interacting dipoles [61], where synchronization of the dielectric's motion may be observable [22].…”

“…In the case of small molecules with a sharp internal transition, the ro-translational motion can be laser cooled by exploiting the methods developed for atoms [12][13][14]. Micron-sized particles in solution and low vacuum can be trapped and manipulated rotationally with optical tweezers and vortex beams [15][16][17][18][19][20][21][22][23]. A first step towards controlling the ro-translational state of a nanometer-sized rod in high vacuum was demonstrated only recently [24].…”

Spatio-orientational decoherence of nanoparticles

“…Recently, there has been some effort to exploit that anisotropically shaped nanoparticles enhance the interaction with the cavity field [19][20][21][22]. While such nanoparticles can be well controlled in solution and low vacuum [23][24][25][26][27][28][29][30][31][32], coherence experiments involving the orientational degrees of freedom [21,22,[33][34][35][36] are still pending. Understanding the spatio-orientational decoherence processes in such experiments with ultra-cold anisotropic nanoparticles is a prerequisite for exploiting the quantum motion of the center of mass and the orientation.…”

Hydrodynamic synchronization and collective dynamics of colloidal particles driven along a circular path

“…In this paper, we theoretically address the dynamics of colloidal particles driven along a circular path. Nontrivial collective motion have been found in experiments using an optical vortex [14][15][16][17][18][19] or optical tweezers [20][21][22][23][24][25]. An optical vortex can drive multiple particles simultaneously along a single circular trajectory [26], while optical tweezers manipulate the particles individually.…”