Planar super-oscillatory lens for sub-diffraction optical needles at violet wavelengths

Abstract

Planar optical lenses are fundamental elements of miniaturized photonic devices. However, conventional planar optical lenses are constrained by the diffraction limit in the optical far-field due to the band-limited wavevectors supported by free-space and loss of high-spatial-frequency evanescent components. As inspired by Einstein's radiation ‘needle stick', electromagnetic energy can be delivered into an arbitrarily small solid angle. Such sub-diffraction optical needles have been numerically investigated using diffractive optical elements (DOEs) together with specially polarized optical beams, but experimental demonstration is extremely difficult due to the bulky size of DOEs and the required alignment precision. Planar super-oscillatory lenses (SOLs) were proposed to overcome these constraints and demonstrated that sub-diffraction focal spots can actually be formed without any evanescent waves, making far-field, label-free super-resolution imaging possible. Here we extend the super-oscillation concept into the vectorial-field regime to work with circularly polarized light, and experimentally demonstrate, for the first time, a circularly polarized optical needle with sub-diffraction transverse spot size (0.45λ) and axial long depth of focus (DOF) of 15λ using a planar SOL at a violet wavelength of 405 nm. This sub-diffraction circularly polarized optical needle has potential applications in circular dichroism spectroscopy, super-resolution imaging, high-density optical storage, heat-assisted magnetic recording, nano-manufacturing and nano-metrology.

Creation of Sub-diffraction Longitudinally Polarized Spot by Focusing Radially Polarized Light with Binary Phase Lens

“…In range from z = 197.4λ to 203.2λ, the average FWHM is less than the diffraction limit 0.54λ, resulting in a 5λ-long sub-diffraction optical needle with longitudinal polarization. Although sub-diffraction optical needles with linear polarization35 and circular polarizaiton2024 have been experimentally demonstrated previously, the optical needle reported here has a unique longitudinally polarization along the optical axis.…”

“…Although super-resolution can be achieved by near-field optics1718, super-oscillation provides a promising approach to realize far-field sub-diffraction focusing and imaging. Many planar diffractive lenses have been proposed for focusing and imaging beyond the diffraction limit192021222324252627. Although metasurfaces2628 have been reported for focusing purposes, their amplitude transmission rate is very low and they are difficult to be fabricated in visible wavelength range.…”

Reconfigurable phase-change photomask for grayscale photolithography

“…The SOL is a device capable of sub-diffraction-limited focusing of light 25 , and is here designed for operation at wavelength of 1550 nm. The SOL is composed of a sequence of concentric rings whose width and diameter are intricately designed to produce a focal spot with the required properties 26 . Interference of the waves transmitted through the SOL leads to a complex diffraction pattern with subwavelength hotspot at the center.…”

Recent Progress in Far-Field Optical Metalenses

“…The super-oscillation concept can be extended into the vectorial-field regime to work with circularly polarized light [25].…”