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. Custom-designed metasurfaces can act as compact and efficient generators of special light beams known as Bessel beams. These beams are of great interest because unlike other kinds of light beams they do not diffract as they propagate. Wei Ting Chen and co-workers from Harvard University in the USA have made a metasurface that functions as an axicon — a special type of lens with a conical front surface that converts plane waves into Bessel beams. The metasurfaces consists of tiny TiO2 fins arranged in concentric ring-like patterns; each fin has a precisely defined orientation. On striking this metasurface axicon, a plane wave of light is transformed into a Bessel beam. The team fabricated and tested two ‘meta-axicons’ designed for operation with blue and green light and found that the beams' intensity profiles were independent of wavelength.