Dielectric chiral metasurfaces for enhanced circular dichroism spectroscopy at near infrared regime

Abstract

Numerous applications of chiro-optical effects can be found in nanophotonics, including imaging and spin-selective absorption, particularly in sensing for separating and detecting chiral enantiomers. Flat single-layer metasurfaces composed of chiral or achiral sub-wavelength structures offer unique properties to manipulate the light due to their extraordinary light-matter interaction. However, at optical wavelengths, the generation of strong chirality is found to be challenging via conventional chiral metasurface approaches. This work intends to design and optimize a dielectric chiral meta-nano-surface based on a diatomic design strategy to comprehend giant chiro-optical effects in the near-infrared (NIR) regime for potential application in circular dichroism (CD) spectroscopy. Instead of using a single chiral structure that limits the CD value at optical wavelengths, the proposed metasurface used a diatomic (two meta-atoms with distinct geometric parameters) chiral structure as a building block to significantly enhance the chiro-optical effect. Combining both meta-atoms in a single periodicity of the building block introduces constructive and destructive interferences to attain the maximum circular dichroism value exceeding 75%. Moreover, using multipolar resonance theory, the physics behind the generation of giant chiro-optical effects have also been investigated. The proposed dielectric chiral meta-platform based on the extra degree of freedom can find application in compact integrated optical setups for CD spectroscopy, enantiomer separation and detection, spin-dependent color filters, and beam splitters.

Fig. 1. The artistic view of the working principle and the design strategy of the proposed diatomic flat nano-surface. (a) The reflective nano-surface composed of hydrogenated amorphous silicon-based meta-atoms produces CD signals for enhanced spectroscopy. Inset shows the diatomic chiral structure comprises a pair of symmetry-breaking meta-atoms. (b) The design strategy of the proposed diatomic system.

Fig. 2. Parametric optimization of proposed diatomic chiral structure to realize the optimum value of length and width for CP illumination. (a) The perspective and (b) the top view of the chiral structure with design parameters. For the optimum selection of length and width parameters, the co-polarized reflectance is demonstrated at the working wavelengths of (c and g) 896 nm (d and h) 970 nm (e and i) 996 nm (f and j) 1030 nm for RCP and LCP excitation, respectively. The black dots denote the optimum value for maximum circular dichroism at the working wavelengths. R_RR: Co-polarized reflectance for RCP incidence, R_LL: Co-polarized reflectance for LCP incidence.

Fig. 3. Parametric optimization of proposed diatomic chiral structure to realize the optimum periodicity, the center distance between both meta-atoms, length (L₂), and height. For the optimum selection of periodicity (P) and the central distance (d) parameter, the co-polarized reflectance is demonstrated at the working wavelengths of (a and e) 896 nm (b and f) 970 nm (c and g) 996 nm (d and h) 1030 nm for RCP and LCP excitation, respectively. Similarly, for the optimum selection of length (L₂) and height (H) parameters, the co-polarized reflectance is demonstrated at the working wavelengths of (i and m) 896 nm (j and n) 970 nm (k and o) 996 nm (l and p) 1030 nm for RCP and LCP excitation, respectively. The black dots denote the optimum value for maximum circular dichroism at the working wavelengths.

Fig. 4. Demonstration of reflection parameters and circular dichroism of single atoms compared to diatomic unit-atom. The reflection parameters for (a) L₁-meta-atom and (b) L2-meta-atom at the optimal design parameters. The circular dichroism for (e) L₁-meta-atom and (f) L₂-meta-atom. (c) The reflection parameters and (g) the CD for the diatomic chiral structure (enantiomer A) shows the enhancement in chirality compared to a single atom structure. (d) The reflection parameters and (h) the circular dichroism for the enantiomer B of the proposed diatomic chiral system.

Fig. 5. The chirality dependence on the illumination angle of light. The reflectance for (a) RCP and (b) LCP illumination concerning illumination angle in azimuth-plane in a range of 0–80°. Meanwhile, the reflectance for illumination angle in the elevation plane is demonstrated for (c) RCP and (d) LCP light. The inset dotted lines show the working wavelengths.

Fig. 6. Spin-dependent multipole decomposition for the diatomic structure. Multipolar decomposition of scattering power for designed chiral diatomic chiral system in terms of (a) electric dipole, (b) magnetic dipole, (c) toroidal dipole, (d) electric quadrupole, (e) magnetic quadrupole for CP illumination. The spin-dependent multipolar resonances demonstrate the significant contribution towards giant circular dichroism in the NIR regime.

Fig. 7. Reflectance in visible, NIR, and MIR regimes. The reflectance parameters for the proposed structure are in (a) visible, (b) near-infrared (NIR), and (c) mid-infrared (MIR) regimes. It shows the chirality in visible and NIR regimes at multiple wavelengths but no chirality in MIR regimes.