Generation of wavelength-independent subwavelength Bessel beams using metasurfaces

Bessel beams are of great interest due to their unique non-diffractive properties.Using a conical prism or an objective paired with an annular aperture are two typical approaches for generating zeroth-order Bessel beams.However,the former approach has a limited numerical aperture(NA),and the latter suffers from low efficiency,as most of the incident light is blocked by the aperture.Furthermore,an additional phase-modulating element is needed to generate higher-order Bessel beams,which in turn adds complexity and bulkiness to the system.We overcome these problems using dielectric metasurfaces to realize meta-axicons with additional functionalities not achievable with conventional means.We demonstrate meta-axicons with high NA up to 0.9 capable of generating Bessel beams with full width at half maximum about as small as~λ/3(λ=405 nm).Importantly,these Bessel beams have transverse intensity profiles independent of wavelength across the visible spectrum.These meta-axicons can enable advanced research and applications related to Bessel beams,such as laser fabrication,imaging and optical manipulation.

Giant intrinsic chiro-optical activity in planar dielectric nanostructures

The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics,stereochemistry and spintronics.However,these intrinsically chiral metamaterials are restricted to a complicated three-dimensional(3D)geometry,which leads to significant fabrication challenges,particularly at visible wavelengths.Their planar two-dimensional(2D)counterparts are limited by symmetry considerations to operation at oblique angles(extrinsic chirality)and possess significantly weaker chiro-optical responses close to normal incidence.Here,we address the challenge of realizing strong intrinsic chirality from thin,planar dielectric nanostructures.Most notably,we experimentally achieve near-unity circular dichroism with~90%of the light with the chosen helicity being transmitted at a wavelength of 540 nm.This is the highest value demonstrated to date for any geometry in the visible spectrum.We interpret this result within the charge-current multipole expansion framework and show that the excitation of higher-order multipoles is responsible for the giant circular dichroism.These experimental results enable the realization of high-performance miniaturized chiro-optical components in a scalable manner at optical frequencies.