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Zilong Wu1 Yuebing Zheng1 Mingsong Wang1

1, University of Texas at Austin, Austin, Texas, United States

Plasmonic chiral metamaterials with strong optical chirality and high tunability in visible and near-infrared light regimes have emerged as promising candidates for photonic sensors and devices. Here, we demonstrate a new type of chiral metamaterials, known as moiré chiral metamaterials (MCMs), to overcome limits in current chiral metamaterials that rely on local structural chirality or site-specific symmetry breaking. Consisting of two layers of identical achiral Au nanohole arrays stacked into moiré patterns, the ultrathin (~70 nm, which is only ≈1/10 of the operation wavelength) MCMs exhibit strong chiroptical effects. The optical chirality can be precisely tuned by the relative rotation between the lattice directions of the two Au nanohole arrays. We have further demonstrated that the MCMs can distinguish a therapeutic chiral drug, R-thalidomide, from its medically toxic enantiomer (S-thalidomide) at picogram level in a label-free manner.

Moreover, we have exploit Fano coupling as a new mechanism to achieve ultrathin active chiral metamaterials of highly tunable chiroptical responses by adding a dielectric spacer layer in MCMs. Our simulations and experiments reveal that spacer-dependent Fano coupling exists in the MCMs, which significantly enhances the spectral shift and line shape change of the circular dichroism (CD) spectra of the MCMs. We further use a silk fibroin thin film as an active spacer layer in the MCMs. With the solvent-controllable swelling of the silk fibroin thin films, we demonstrate tunable Fano coupling and chiroptical responses of the silk-MCMs using different solvents and their mixtures. Impressively, we have achieved the spectral shift over a wavelength range that is more than one full width at half maximum and the sign inversion of the CD spectra in a single ultrathin (1/5 of wavelength in thickness) MCM. Finally, we apply the silk-MCMs as ultrasensitive sensors to detect trace amount of solvent impurities down to 200 ppm, corresponding to an ultrahigh sensitivity of >105 nm/refractive index unit (RIU) and a figure of merit of 105 /RIU. With their strong and tunable optical chirality, in combination with robust cost-effective fabrication, the MCMs will become critical components for chiroptical devices. Our results also pave a way towards active chiral metamaterials of high tunability, ultrathin thickness and large-scale fabrication for a wide range of applications.

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