Engineering the chiroptical responses of artificial nanostructures is vital for realizing applications in the fields of optical devices, enantioselective separation and bio-sensing. Here, by utilizing the nano-kirigami based meta-molecule arrays, the flexible engineering of circular dichroism (CD) is achieved in the near-infrared wavelength region by the excitation of chiral surface lattice resonances (SLRs). It is found that the chiral SLRs can be flexibly tailored by a tiny structural perturbation. As a result, the wavelength, intensity, and sign of CD peak/dip can be abruptly engineered. Specifically, a CD peak with the value of +0.44 is evolved into a CD dip with the intensity of -0.66, when the etching silt length of the meta-molecules is simply decreased by 190 nm. Importantly, such CD reversal is experimentally demonstrated with the nano-kirigami method without requiring the inversion of geometric chirality. Moreover, it is found that the asymmetric dielectric environment around the meta-molecules can significantly suppress the chiral SLRs, providing an insightful understanding of the chiral SLR. Such flexible tailoring of the CD with chiral SLRs paves a versatile way towards the manipulation of chiral light-matter interactions and chiroptical functional devices.

Circular dichroism (CD), as one of the most representative chiroptical effects, provides a simple strategy for the detection and characterization of the molecular chirality. The enhancement and sign reversal of CD are of great importance for its practical applications in chiral bio-sensing, chirality switching and optical filtering, etc. Here, we realize considerable adjustments and the sign reversal of CD in quasi-three-dimensional (quasi-3D) combined Archimedean spiral nanostructures. With special local and lattice configurations, the nanostructures have both right-handed and left-handed geometric chirality, which are designed based on the proximity effect of stencil lithography. We find that the CD response of the nanostructures becomes obvious once its height exceeds 200 nm and can be adjusted by the further increase of the height or the change of the blade spacing of the nanostructures. The CD reversal is achieved by utilizing the competition of two chiral centers when the height or blade spacing exceeds a critical value. Further analysis of the scattering power of multipole moments reveals that the CD modulation is determined by both magnetic dipole moment and electric quadrupole moment. Benefiting from the highly sensitive CD response to the height, the extreme sign reversal of CD is achieved when a sub-10-nm ultrathin medium layer is anchored on the surface of the nanostructures, which provides a promising strategy for ultra-sensitive chiral bio-sensing.

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Graphene oxide (GO) is a new two-dimensional carbon nano-material with large specific surface area and outstanding hydrophilicity. Its refractive index varies significantly with humidity, which can be used to improve the sensitivity of optical fiber humidity sensor. A kind of fiber Bragg grating (FBG) humidity sensor based on GO is proposed and experimentally demonstrated. GO is uniformly coated on the FBG surface by deposition. When the ambient humidity changes, the GO film adsorbs or releases water molecules and its refractive index changes. Thus the effective refractive index of FBG changes and the transmission wavelength also changes. The experimental results show that the sensor has a linear response of 2.53 pm/% RH in the range of 20%-70% RH, so it has the advantages of high sensitivity and simple structure.