Tunable Chiral Optics in All-Solid-Phase Reconfigurable Dielectric Nanostructures

Abstract

Subwavelength nanostructures with tunable compositions and geometries show favorable optical functionalities for the implementation of nanophotonic systems. Precise and versatile control of structural configurations on solid substrates is essential for their applications in on-chip devices. Here, we report all-solid-phase reconfigurable chiral nanostructures with silicon nanoparticles and nanowires as the building blocks in which the configuration and chiroptical response can be tailored on-demand by dynamic manipulation of the silicon nanoparticle. We reveal that the optical chirality originates from the handedness-dependent coupling between optical resonances of the silicon nanoparticle and the silicon nanowire via numerical simulations and coupled-mode theory analysis. Furthermore, the coexisting electric and magnetic resonances support strong enhancement of optical near-field chirality, which enables label-free enantiodiscrimination of biomolecules in single nanostructures. Our results not only provide insight into the design of functional high-index materials but also bring new strategies to develop adaptive devices for photonic and electronic applications.

Keywords: biosensing; dielectric materials; optical coupling; optical nanofabrication; reconfigurable chiral metamaterials.

Figure 1.

Assembly of solid-phase dielectric chiral nanostructures. (a) Schematic showing the assembly of LH chiral structure. (b) The measured CDS spectra of the assembled chiral nanostructures in panels c and d. (c,d) Schematic, optical, and SEM images of the assembled LH (c) and RH (d) chiral nanostructures. Scale bars: 1 μm.

Figure 2.

Building blocks of chiral structures. (a,b) Experimental scattering spectra of a SiNP (a) and a SiNW (b). The insets show the corresponding optical and SEM images. Scale bars: 1 μm. (c) Scattering spectra of a 500 nm SiNP along with multipole decomposition calculated with Mie theory. MD, magnetic dipole; ED, electric diploe; MQ, magnetic quadrupole; EQ, electric quadrupole; MO, magnetic octupole; EO, electric octupole; and MH, magnetic hexadecapole. (d) Calculated scattering spectra and multipole decomposition of a SiNW with a diameter of 165 nm. Inset shows the electric and magnetic field distribution in the cross section of the SiNW at 700 nm.

Figure 3.

Reconfigurable construction on a solid substrate. (a) Schematic illustration of on-demand assembly of the SiNP-SiNW nanostructure with LH, achiral, or RH configuration. (b–e) Optical images of dispersed building blocks (b), LH (c), achiral (d), and RH (e) structures. All scale bars are 5 μm. (f) CDS spectra of LH, achiral, and RH structures. (g–i), Optical scattering spectra of LH (g), achiral (h), and RH (i) structures under LCP and RCP illumination.

Figure 4.

FDTD simulation and coupled-mode theory analysis. (a) Simulated scattering spectra of the LH structure under LCP and RCP illumination. (b) Simulated CDS spectra of LH, achiral, and RH structures. The black dashed lines are the fitting curves via the coupled-mode theory. (c,d) The electric field distributions in the LH structure at 740 nm induced by LCP (c) and RCP (d) incidence. (e,f) The electric field distributions in achiral structure at 740 nm induced by LCP (e) and RCP (f) incidence. All electric field distributions are cut at the cross-sectional plane of the SiNW passing through the center of the SiNP.

Figure 5.

Enhanced chiral sensing. (a) Differential optical chirality mapping in the LH structure at 740 nm under LCP and RCP illumination. C₀ is the chirality for circularly polarized light without the nanostructure. (b) CDS spectra of the LH and RH structures before and after the adsorption of l-phenylalanine. (c) ΔΔλ values for l-Phenylalanine and d-Phenylalanine. The opposite signs of ΔΔλ values reveal the opposite handedness of l-phenylalanine and d-phenylalanine. Insets show the chemical structures of the chiral molecules.