Optical screwdriving induced by the quantum spin Hall effect of surface plasmons near an interface between strongly chiral material and air

We reveal a behavior of screwdriving of a surface plasmonic field formed near a surface of strongly chiral material, where lateral force and torque can be provided by the quantum spin Hall effect of light simultaneously. We study the role of chirality strength of the material in optical manipulating a particle with electric and magnetic dipole and demonstrate the effect of the size and chirality strength of the particle on manipulation. We hope these results may point out a unique routine to sort and separate chiral particles and biomolecules optically.

Figure 1

(a) Snapshot of the SPs propagating along an air-negatively refractive chiral interface. Parameters are chosen as ${\varepsilon }_c=2, \kappa =3$. (b) Snapshot spin density distribution for the transverse SAM across the interface. The plot illustrates that the transverse spins have opposite signs in the two media because the direction of propagation, the direction of decay, and the direction of transverse spin form a right-handed system. (c) Snapshot distribution of the time averaged Poynting vector in the $x-z$ plane.

Figure 2

(a) Geometry of a dipolar chiral particle located above the interface between a negatively refractive chiral material and air. The spherical particle undergoes linear and spinning motions which exerted by the SPs propagating along the interface. The optical manipulation is similar to a screw turned by a screwdriver. (b) Side view of the distribution of the scattered $E_z$ and $H_y$ field components, where ${\varepsilon }_s=2.1, {\kappa }_s=0.2$.

Figure 3

Total contribution of the gradient force and radiation pressure ${\mathbf{F}}_{\mathbf{gr}}$, acting on a dipolar chiral particle (${\varepsilon }_s=2.1$ and $r=20$ nm) as a function of the particle's chirality ${\kappa }_s$ when located in the air at $d=30\mathrm{nm}$ above the interface.

Figure 4

(a) Scattering recoil force ${\mathbf{F}}_{\mathbf{s}}$, and (c) optical torques acting on a dipolar chiral particle (${\varepsilon }_s=2.1$ and $r=20$ nm) as a function of the chiral particle's chirality ${\kappa }_s$. Opposite enantiomers experience opposite forces in the lateral direction, namely the $y$ direction. Moreover, opposite enantiomers undergo identical torques in the lateral direction but opposite torques in the propagating direction of the SPs. (b) Poynting vectors around the particles with different chirality in the plane as indicated in each corresponding inset. The backgrounds are $E_z$ field components. Particles are located at $d=30$ nm in the air above the interface.

Figure 5

(a) Scattering recoil forces ${\mathbf{F}}_{\mathbf{s}}$ and optical torques acting on dipolar chiral particles (${\varepsilon }_s=2.1$ and ${\kappa }_s=0.2$) as a function of the radius. Particles are located at $d=30$ nm in the air above the interface. (b) Scattering recoil forces and torques acting on a dipolar chiral particle ($r=20$ nm) as a function of the chirality ($\kappa$) of the strongly chiral material.