Metasurface engineering for enhanced chiral absorption in the visible region

Abstract

Chirality, defined as the property of an object which cannot be superimposed onto its mirror image, is a fundamental characteristic observed in natural materials such as DNA, proteins, and sugars. It plays a significant role in various optical phenomena, including circular dichroism (CD) and optical activity. However, the chiroptical responses in natural materials are very weak. To overcome this limitation, metasurfaces designed with precise geometric asymmetries have emerged as an effective platform for enhancing light–matter interactions. In this thesis, we present the development of ultra-narrowband chiral absorbers in the visible spectrum, utilizing Brillouin zone folding (BZF) metasurfaces to achieve strong chiroptical effects. By focusing on two-dimensional (2D) planar chiral structures, which simplify fabrication compared to three-dimensional (3D) designs, we demonstrate significantly enhanced CD responses. The optical resonances within these metasurfaces are precisely engineered to selectively absorb specific circularly polarized light, achieving superior circular polarization selectivity. Unlike conventional 3D chiral structures that face fabrication challenges, the proposed 2D planar designs exhibit strong intrinsic chiroptical responses under normal incidence. We experimentally achieve a substantial differential absorptance (ΔA = ARCP – ALCP) of 0.75 with a linewidth of 1.4 nm in the visible region. Furthermore, by leveraging handedness-dependent strong field enhancement, high levels of chiral emission with a luminescence linewidth of 1.75 nm are demonstrated from quantum dots coated on the absorber surface. This work introduces a novel approach to tailoring chiral optical properties via metasurface structuring, resulting in ultra-narrow linewidths and high circular polarization purity. Our findings establish a foundation for future advancements in chiral photonics by providing an efficient solution for high-performance chiral light manipulation across various wavelength ranges.