Abstract
Flat optics, which could planarize and miniaturize the traditional optical elements, possesses the features of extremely low profile and high integration for advanced manipulation of light. Here we proposed and experimentally demonstrated a planar metalens to realize an ultra-long focal length of ~240λ with a large depth of focus (DOF) of ~12λ, under the illumination of azimuthally polarized beam with vortical phase at 633 nm. Equally important is that such a flat lens could stably keep a lateral subwavelength width of 0.42λ to 0.49λ along the needle-like focal region. It exhibits one-order improvement in the focal length compared to the traditional focal lengths of 20~30λ of flat lens, under the criterion of having subwavelength focusing spot. The ultra-long focal length ensures sufficient space for subsequent characterization behind the lens in practical industry setups, while subwavelength cross section and large DOF enable high resolution in transverse imaging and nanolithography and high tolerance in axial positioning in the meantime. Such planar metalens with those simultaneous advantages is prepared by laser pattern generator rather than focused ion beam, which makes the mass production possible.
Highlights
Flat optics, which could planarize and miniaturize the traditional optical elements, possesses the features of extremely low profile and high integration for advanced manipulation of light
Correspondence and requests for materials should be addressed to M.H.H. www.nature.com/scientificreports/. This doughnut spot can be changed into a significantly sharper focal spot when a vortical phase encoded on the azimuthally polarized beam[11,12,13,14,15], which shows the intriguing prospect in practical applications due to its sub-wavelength lateral spot size and purely transverse electric field[16,17,18,19]. This azimuthally polarized beam with vortical phase (APV) was applied in solid immersion lens system for achieving a sub-wavelength focusing spot because of its immunity to the presence of the medium interface, and was able to maintain this spot for several wavelengths after passing through interface[20], which is superior to other polarizations
The planarization of traditional optical elements has been another emerging field applied in nano-photonics, and planar focusing devices have been demonstrated during the past few years by metasurface flat lenes[21,22,23,24], binary optics[25,26], and SOL27–30 etc
Summary
Flat optics, which could planarize and miniaturize the traditional optical elements, possesses the features of extremely low profile and high integration for advanced manipulation of light. Large-scaled lens with fine details is a challenging issue due to their rather complicated fabrication processes, so that the dimensions of all reported SOL are around 40 μm in diameter while suffering from a small focal length around 10 μm[27,32], which is a big obstacle for the scanning process in nano-imaging if the surface roughness of target sample is large To address these challenges, we present a high NA planar metalens with a focal length of ~240λ under the illumination of azimuthally polarized beams with vortical phase. This work paves a viable path for the industrial application of planar metalens
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