Chirality sorting using two-wave-interference–induced lateral optical force

We demonstrate that a lateral optical force (LOF) can be induced on chiral nanoparticles immersed in the two interfering plane waves. The LOF can push the chiral nanoparticle sideways, in the direction relying on the helicity of the particle as well as the phase difference between two waves. Analytical theory reveals that the LOF comes mostly from the direct coupling of the optical vorticity with the particle chirality. In particular, the LOF has a magnitude comparable to the gradient force and radiation pressure when the particle chirality is sufficiently large. The LOF may serve for chirality sorting due to its unusual properties and also provide an opportunity for passive chiral spectroscopy.

Figure 1

(a) Schematic illustration of a LOF on a chiral particle immersed in the interference field of two plane waves. (b) The LOF $F_y$ on a nanoparticle versus the particle displacement along the $x$ axis obtained from the FWS. The blue and red lines denote the particle with $k=0$ and $k=0.3$, respectively. The two plane waves have the same incident wavelength of $\lambda =1.064\mu \mathrm{m}$ and incident angle of $\alpha ={45}^{\circ }$. The particle has the radius $r_s=0.1\mu \mathrm{m}$, $\varepsilon =2.53$, and $\mu =1$ for the above cases.

Figure 2

LOF $F_y$ on a chiral particle with $\kappa =0.3$ versus the particle displacement along the $x$ axis. The red, green, and blue lines denote the two plane waves with incident angles $\alpha ={20}^{\circ },{50}^{\circ },\text{and}{80}^{\circ }$, respectively. Other parameters are the same as those in Fig. 1.

Figure 3

The terms in Eq. (6) with their $y$ components contributing significantly to the LOF on the chiral particle with $\kappa =0.3$. The accurate total LOF ${\mathbf{F}}_{\text{FWS}}$ calculated with the FWS in Fig. 1 is replotted for comparison. All other parameters are the same as those in Fig. 1.

Figure 4

Time-averaged Poynting vectors for a nanoparticle with (a) $\kappa =0$, and (b) $\kappa =0.3$ immersed in the two interfering plane waves. Other parameters are the same as those in Fig. 1 except that the particle is located at $x=0.2\mu \mathrm{m}$.

Figure 5

Optical forces versus the chirality parameter $\kappa$. All other parameters are the same as those in Fig. 4.