Micro-Optical Line Generator Metalens for a Visible Wavelength Based on Octagonal Nanopillars Made of Single-Crystalline Silicon

Conventionally, silicon is less often selected as the material of dielectric metasurfaces in the visible band than other lossless materials, including titanium dioxide, silicon nitride, and gallium nitride. The reason is its relatively high extinction coefficient and resulting low transmittance. This study demonstrated that accurately designed nanopillars made of single-crystal silicon could be used satisfactorily on a metasurface, even in the visible band. Four line-focusing metasurface lenses were designed to verify the lens performance effectiveness of silicon nanopillars in the visible band. In addition, a combination of the character projection and the variable-shaped beam modes in the electron beam lithography was operated to evaluate the compatibility between mass productivity and accuracy. We successfully obtained a highly efficient line-focusing metasurface lens composed of single crystalline silicon nanopillars. The parameters of the metasurface lens at a wavelength of 532 nm were as follows: lens thickness, 300 nm; focal length, 3.91 mm; square aperture, 2 mm; numerical aperture, 0.25; measured transmittance, 38.4% to 46.8%; and measured beam spot width,
$3.68\,\, \mu \text{m}$
(full width at half maximum, FWHM) at the focal point. The results obtained in this study show a promising use of silicon metasurface for optical sensor applications in the visible band.