Broadband optical chirality with nano-kirigami metasurfaces

Broadband chiral metasurfaces facilitate ultracompact devices for chiral light generation, manipulation, and detection across a wide spectral range, rendering them important for various photonics applications. Here, we present an unconventional approach to realizing broadband optical chirality using stereoscopically twisted nano-kirigami metasurfaces. By increasing the geometric asymmetry between the two pairs of arms of the metamolecules, mode degeneracy on the arms is lifted, leading to a gradual broadening of the spectral profile of the circular polarization transmission differences (ΔT). In the spectral range from 1.46 to 2.24 μm, ΔT consistently exceeds 0.5, and the circular polarization extinction ratio surpasses 10 within the 1.62 to 2.25 μm wavelength range. Owing to the unique closed-loop configuration incorporating bi-chiral substructures, this metasurface can generate stably localized broadband near-field optical chirality. This explains that the broad-spectrum spin-selective transmission is formed by the combination of reflection and absorption circular dichroism. Moreover, the metasurface exhibits a significant asymmetric chirality characteristic, with a noticeable difference in ΔT values between forward and backward incidences. This strategy of achieving broadband chiroptical responses through geometric asymmetry modulation in nano-kirigami metasurfaces provides a potential pathway for developing ultrathin, compact, highly integrated, and easily fabricable broadband chiral photonic devices.