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Timestamp: 2019-04-20 15:20:26+00:00

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Here we present a tunable chiral metamaterial platform that incorporates both metal and dielectric components, where the sign and magnitude of the circular dichroism (CD) response depend on the refractive index of the dielectric component. Using finite-difference time-domain simulations, we show that non-resonant scattering interactions between the components of the system reverse the sign of the CD signal by changing the dissymmetry in absorption of circularly polarized light by the individual plasmonic components of the system. The platform exhibits tunable CD signal regardless of the shape and dimension of the dielectric scatterer, and the magnitude of the CD signal is enhanced by improving the scattering cross section of the dielectric structure. Finally, we show that the structure can be modified to incorporate other materials without diminishing the reversal in dissymmetry in transmission. These results indicate that controlled, off-resonant interactions between different materials in chiral metamaterials may be used to create tailored and tunable chiral platforms.
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Fig. 1 (a) Schematic of the assembly. The assembly is composed of two gold nanorods with a dielectric disk on top. The individual rods have length = 380 nm, width = 70 nm, and height = 40 nm and are positioned 70 nm above the glass substrate. The dielectric disk has a diameter of 180 nm and thickness of 60 nm, and is placed over the center of the rods at a 30 nm vertical distance. The disk is 70 nm below the surface of the cladding medium that has a total thickness of 270 nm. The assemblies are arranged with a C4 rotation symmetry, and the lattice constant is 2000 nm. (b) Simulated transmission spectra of the device under LCP and RCP illumination with ndisk = 3.5 and nmedium = 1.55.
Fig. 2 (a) CD response ( T RCP − T LCP ) of the system as a function of the refractive index of the dielectric disk. The medium has a refractive index of 1.55, and the color scale denotes the difference between the refractive index of the disk and the medium. The magnitude and sign of n disk − n medium determines the magnitude and sign of the CD response. (b) Calculated scattering cross section of the disk at the high energy CD peak ( λ=1815 nm), where the refractive index of the disk is increased from n disk =1 to n disk =2.1. These values represent the lower and upper limit of the refractive index of the disk in our CD calculations.
Fig. 3 (a) The dissymmetry in absorption (continuous line) and transmittance (dotted line) of the unit cell of the platform have similar magnitude and sign for systems with n disk =2.1 (red) and n disk =1 (blue). (b) Absorption spectra of the individual nanorods for the case without a disk on top. The dotted line is the absorption of the nanorod that contains the disk in the chiral geometry, while the dashed line is the absorption of the nanorod that does not contain the disk in the chiral geometry, as shown in the schematic inset. For the achiral system, the difference in absorption by the individual nanorods have the same magnitude but opposite sign and thus the composite system is not optically active, signified by the solid black line. (c) Absorption spectrum of individual gold nanorods in systems where n disk =2.1 (red) and n disk =1 (blue). The dotted lines denote the differential absorption by the nanorod with the disk on top, while the segmented lines indicate the dissymmetry in absorption by the nanorod without the disk.
Fig. 4 Calculations of the optical chirality enhancement for a system with n disk =1 at λ=1815 nm (high energy CD resonance peak). A cross section is shown at 10 nm above the top side of the disks under (a) RCP and (b) LCP illumination.
Fig. 5 CD response ( T RCP − T LCP ) of systems with disks and squares of varying diameters and diagonals (denoted by d) with n = 1 (blue) and n = 2.1 (red). The reversal of the CD response when n disk − n medium switches sign is not affected when the shape of the scatterer changes from a disk to a square.
Fig. 6 (a) CD response for systems where the dielectric components are rectangles with the same dimensions (200 nm x 380 nm x 70 nm), but one of them is along (continuous line) and the other one is perpendicular (dashed line) relative to the long axis of the nanorod underneath. (b) CD response for systems where the dielectric components are ellipsoids with the same dimensions (long axis = 400 nm, short axes = 100 nm), but different orientation relative to the long axis of the nanorod underneath.
Fig. 7 The shape of the dielectric component is varied between a disk, ellipsoid, square, or rectangle with n = 1. The magnitude of the CD peak ( | T RCP − T LCP | max ) at 1815 nm (a) and at 2217 nm (b) for each of these assemblies are plotted against the scattering cross section of the isolated dielectric scatterer. The square scatterers were either elongated along all their sides simultaneously (red), or one side was kept constant while the opposite side was elongated (purple and orange). The simulated edge lengths ranged from 200 nm to 380 nm with a step size of 30 nm. We also simulated systems with disks with varying diameters (green), where the diameters ranged from 100 nm to 380 nm with a step size of 40 nm. Lastly, we calculated the response in systems where the scatterer is a sphere with 100 nm diameter that is elongated along the direction parallel to the rod (dark blue), and perpendicular to the rod (light blue). The elongated diameter of the resulting ellipsoid varied from 100 nm (sphere) to 400 nm with a step size of 60 nm.
Fig. 8 CD response ( T RCP − T LCP ) for modified versions of the original chiral system. In (a), the height of the cladding medium is changed to 200nm and the top side of the disk is covered in a film with n = 1.65. The CD response is reversed upon changing the refractive index of the disk In (b), the system is perforated with holes. Changing the refractive index of a 70 nm film added on top of the modified system from 1.01 to 2.1 reverses the CD signal.

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