Orbital Motion From Optical Spin: The Extraordinary Momentum Of Circularly Polarized Light Beams

Abstract

We provide a vivid demonstration of the mechanical effect of transverse spin momentum in an optical beam in free space. This component of the Poynting momentum was previously thought to be virtual, and unmeasurable. Here, its effect is revealed in the inertial motion of a probe particle in a circularly polarized Gaussian trap, in vacuum. Transverse spin forces combine with thermal fluctuations to induce a striking range of non-equilibrium phenomena. With increasing beam power we observe (i) growing departures from energy equipartition, (ii) the formation of coherent, thermally excited orbits and, ultimately, (iii) the ejection of the particle from the trap. Our results complement and corroborate recent measurements of spin momentum in evanescent waves, and extend them to a new geometry, in free space. In doing so, we exhibit fundamental, generic features of the mechanical interaction of circularly polarized light with matter. The work also shows how observations of the under-damped motion of probe particles can provide detailed information about the nature and morphology of momentum flows in arbitrarily structured light fields as well as providing a test bed for elementary non-equilibrium statistical mechanics.

References in corpus (17)

• Laser cooling of a nanomechanical oscillator into its quantum ground state
• Extraordinary momentum and spin in evanescent waves
• Millikelvin cooling of an optically trapped microsphere in vacuum
• Momentum of an electromagnetic wave in dielectric media
• Lateral Optical Force On Chiral Particles Near a Surface
• Dual electromagnetism: Helicity, spin, momentum, and angular momentum
• Internal flows and energy circulation in light beams
• Cavity cooling a single charged nanoparticle
• Nanoscale temperature measurements using non-equilibrium Brownian dynamics of a levitated nanosphere
• Measuring the Transverse Spin Density of Light
• Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever
• Dynamic Relaxation of a Levitated Nanoparticle from a Non-Equilibrium Steady State
• Direct Measurement of Kramers Turnover with a Levitated Nanoparticle
• Dynamic Consequences of Optical Spin-Orbit Interaction
• Optical forces and torques in non-uniform beams of light
• Orbital rotation without orbital angular momentum: mechanical action of the spin part of the internal energy flow
• Nonlinear mode-coupling and synchronization of a vacuum-trapped nanoparticle