Abstract
Optical forces acting on particles - controlled by the intensity, polarization and direction of optical beams - have become an important tool in manipulation, sorting and analysis of nano/micro-particles. The nature of these forces has been well understood in reciprocal structures exhibiting time-reversal symmetries. Here, we investigate the nature of optical forces in non-reciprocal structures with non-degenerate counter-propagating modes. We consider the specific case of non-reciprocity induced via translational motion and show that the two counter-propagating modes in a moving slab-waveguide are not degenerate which results in a non-zero lateral and longitudinal force on a nanoparticle. We prove that these anomalous forces are fundamentally connected to near-field photonic spin in optical waveguides and explain their directionality using universal spin-momentum locking of evanescent waves. The presented results show that the interplay of photon spin and non-reciprocity can lead to unique avenues of controlling nanoscale optical forces on-chip.
Highlights
The use of controlled optical forces has evolved into an important tool for analysis, manipulation and sorting of nanoparticles [1,2,3,4,5]
Among the optical forces, an anomalous lateral force acting on chiral particles is of particular interest, as it strongly depends on the chirality of the particle and acts in a direction transverse to the radiation pressure and the gradient force [6,7,8,9,10,11,12,13,14]
Locking of the optical spin with the direction of propagation is similar to the phenomena known for electronic surface states [18, 19], and explains the polarization-dependent directional propagation of optical modes and surface plasmon polaritons reported in recent experiments [20,21,22,23]
Summary
The use of controlled optical forces has evolved into an important tool for analysis, manipulation and sorting of nanoparticles [1,2,3,4,5]. We show that the degeneracy of positive- and negative-momentum modes in a non-reciprocal (moving) waveguide is broken, and results in an unbalanced transverse and longitudinal optical force. It is physically possible for a fast moving relativistic particle to observe the waveguide as moving with velocity greater than the Cherenkov velocity (vmotion > ω/kx) since the phase velocity in a stationary waveguide is less the velocity of light. A similar transformation for plasmonic mode in moving metal-insulator-metal waveguide structure is explained in detail in [41] This essentially means that the slab-waveguide is moving fast enough for the backward propagating mode to appear as forward propagating in the reference frame of the particle; switching the sign of momentum from negative to positive at the Cherenkov velocity. This corresponds to the approximate Poynting vector of 17 μW/m2 at the center of the stationary waveguide
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