Optical activity in diffraction from a planar array of achiral nanoparticles

We prepare diffractive planar arrays of metal nanoparticles that are chiral because of either the shape of individual particles (“molecular” chirality) or the orientation of achiral particles in the array (“structural” chirality). Both sorts of samples are shown to lead to comparable polarization changes in the diffracted light. For the case of structural chirality, one might assume that these effects can occur only through interparticle interactions, as would be the case for transmission measurements (zero-order diffraction). However, we show that the results can be explained by a simple model in which the polarization effects are based on independent scattering by individual particles, with no interparticle coupling, and with the array structure simply determining the direction of the diffraction maximum. We thus conclude that structural and molecular chiralities are indistinguishable in diffraction experiments.

Figure 1

(Color online) Experimental setup and sample layout.

Figure 2

(Color online) (a) Polarization azimuth rotation and (b) ellipticity of light in diffraction from the planar structures. Data for $p$- and $s$-polarized incident light are shown as squares and circles, respectively. The results for the two enantiomerically opposite forms are connected by the arrows. The offset of the zero level set by the achiral patterns 1 and 2 is within $1°$. It has been shifted to coincide with the zero level of the graphs for both polarization rotation and ellipticity.

Figure 3

(Color online) Scattering from a tilted cross. (a) Top view of the cross. (b) Side view of the scattering geometry.

Figure 4

(Color online) Polarization azimuth rotation of the diffracted light as a function of the tilt angle $\alpha$ of the crosses with respect to the lattice axes, calculated using the wire model. The solid and dashed curves correspond to the $p$ and $s$ polarizations of the incident light, respectively.