Generation of a Nondiffracting Superchiral Optical Needle for Circular Dichroism Imaging of Sparse Subdiffraction Objects

Chirality describes not only the structural property of three-dimensional objects, but also an intrinsic feature of electromagnetic fields. Here we report a strategy to realize a Bessel beam superchiral “needle” by focusing a twisted radially polarized beam on a planar dielectric interface. By tailoring the light spatial distribution in the pupil plane of a high numerical aperture lens, the chirality of the local field at the focus can be enhanced by 11.9-fold than that of a circular polarized beam. Through a combined interaction of chiral and achiral transitions, the dimension of the region with enhanced chiral sensitivity can be shrunk down to $\lambda /25$. This theoretical work paves the way towards a completely new label-free imaging technique using the enhanced circular dichroism for sparse subdiffraction chiral objects (e.g., individual molecules).

Figure 1

(a) Schematic of the system to generate a superchiral field by focusing a twisted RP beam. (b) Theoretical model to calculate the EM field distribution near the focus. $f$ is the focus length of the lens, and $h$ is the lateral position of the incident ray.

Figure 2

(a) The $g/g_{\mathrm{CPL}}$ at the focus point ($\rho =0$, $z=0$) when the incident angle is varied from 0 to ${\theta }_c$. The black curve represents the prediction function of $g(0,\theta )/g_{\mathrm{CPL}}=1/\mathrm{\Theta }(\theta )$, and the red dots represent the simulation results. The green dashed curve represents the sine condition of $h=f\sin \theta$. The highlighted region represents the range of incident angle [$\alpha {\theta }_c$, ${\theta }_c$]. (b) The lateral distribution of $g/g_{\mathrm{CPL}}$ for different ranges of incident angle when the topological charge of the SPP is $m=1$.

Figure 3

(a)–(d) represent the distributions of $|{\mathit{E}}_{\parallel }|$, $|{\mathit{E}}_z|$, $|{\mathit{H}}_{\parallel }|$, and $g/g_{\mathrm{CPL}}$ on the air-glass interface $z=0$), respectively. (e)–(h) represent the distributions of $|{\mathit{E}}_{\parallel }|$, $|{\mathit{E}}_z|$, $|{\mathit{H}}_{\parallel }|$, and $g/g_{\mathrm{CPL}}$ on the $x\text{−}z$ plane ($y=0$), respectively.

Figure 4

The chiral imaging of nano-objects with diameters of 600, 200, and 50 nm in (a), (b), and (c), respectively. The chirality of the molecule in (b) is opposite from those in (a) and (c). The $g$ factor of the chiral disk is calculated by $g(\mathbf{R})=2(A^{+}-A^{-})/(A^{+}+A^{-})$ (see Sec. III of the Supplemental Material [10]). (d) The relationship between the nano-object diameter and FWHM of the $g$-factor image.