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Wu Zhixiang, Jin Qijian, Zhang Kun, et al. Binary-amplitude modulation based super-oscillatory focusing planar lens for azimuthally polarized wave[J]. Opto-Electronic Engineering, 2018, 45(4): 170660. doi: 10.12086/oee.2018.170660
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Binary-amplitude modulation based super-oscillatory focusing planar lens for azimuthally polarized wave
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Abstract
Tight focus of azimuthally polarized wave finds its applications in optical super-resolution, particle trapping and so on. To overcome the disadvantages of conventional optics, including bulky size and difficult for integration, a binary-amplitude (0, 1) super-oscillatory planar lens is designed for sub-diffraction focusing of azimuthally polarized wave at wavelength of 632.8 nm. The lens radius is 650λ, and its focal length is 200λ. The corresponding numerical aperture is 0.96. The experimental results demonstrate the generation of a hollow spot with circular ring shape on the focal plane. The inner full-width-at-half-maximum of the hollow spot is 0.368λ, smaller than the super-oscillatory criterion (0.398λ), and the maximum sidelobe ratio is about 36.7%. Such planar lenses are easy to fabricate. Their small size and ultra-thin thickness make them promising in system minimization and integration for different applications, such as optical microscopy, optical trapping and ultra-high density data storage.-
Keywords:
- super-oscillatory focusing /
- planar lens /
- azimuthally polarized wave /
- dark spot
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References
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Overview
Overview: The generation of optical dark spots is attractive for various applications, such as optical microscopy, optical tweezers and nanolithography. Due to its unique focusing properties, azimuthally polarized wave is used to generate tight focused dark spot with conventional optics. However, high numerical optical lenses are bulky and expensive, and more importantly, the conventional optics are diffraction-limited. In recent years, there has been growing interest in developing planar lenses with small size, thin thickness and light weight. To further reduce the focusing spot size, the idea of super-oscillation was proposed to overcome this restriction. In addition, super-oscillation optical fields consist of only propagating waves and can generate sub-diffraction optical features in far field. Although, super-oscillatory dark spot has been demonstrated by focusing azimuthally polarized wave with a binary-phase (0, π) lens, it requires comparatively high precision in the growth of the dielectric layer with proper thickness to ensure the correct phase delay. In this paper, a binary-amplitude (0, 1) super-oscillatory planar lens is proposed for the focusing of azimuthally polarized wave and generation of optical dark spots with super-oscillatory size. Utilizing vectoral-angular-spectrum method and particle-swarm algorithms, a planar lens was designed with a radius of 650λ and focal length of 200λ for azimuthally polarized wave at wavelength of 632.8 nm. The corresponding numerical aperture is 0.96. In the experiment, a test system based on high-numerical-aperture microscope was used to obtain the 2-dimentinal optical intensity distribution. With a nano-positioner, the objective lens can scan the 2-dimentinal optical intensity distribution at different position along the optical axis. The experimental results demonstrate the generation of a hollow spot with circular ring shape on the focal plane. The inner full-width-at-half-maximum of the hollow spot is 0.368λ, smaller than the super-oscillatory criterion (0.398λ), and the maximum sidelobe ratio is about 36.7%. Such planar lenses are easy to fabricate. Their small size and ultra-thin thickness make them promising in system minimization and integration for different applications, such as optical microscopy, optical trapping and ultra-high density data storage.
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Wu Zhixiang, Jin Qijian, Zhang Kun, et al. Binary-amplitude modulation based super-oscillatory focusing planar lens for azimuthally polarized wave[J]. Opto-Electronic Engineering, 2018, 45(4): 170660. doi: 10.12086/oee.2018.170660
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Figure 1.
Binary-amplitude modulation based super-oscillatory focusing planar lens for azimuthally polarized wave. (a) Working principle; (b) Basic structures, (top) top view and (bottom) cross-section view
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Figure 2.
The distribution of the focusing optical field obtained in numerical design: The optical intensity (blue) and phase distribution (red) on the focal plane at z=200λ
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Figure 3.
The simulation results obtained with Comsol Multiphysics. (a) The optical intensity distribution on the focal plane; (b) The optical intensity distribution along the radial coordinate on the focal plane; (c) The peak intensity (red), full-width-at-half-maximum (blue) and sidelobe ratio (green) distribution along the optical axis, where the red-dotted line and the black-dotted line represent the diffraction-limit (0.5λ/NA) and the super-oscillation criterion (0.38λ/NA), respectively
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Figure 4.
SEM pictures of the super-oscillatory focusing planar lens
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Figure 5.
The schematic diagram of the testing system for super-oscillatory focusing planar lens
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Figure 6.
The experimental results of the super-oscillatory planar lens. (a) The 2-dimensional intensity distribution on the focal plane at z=200.89λ; (b) the intensity distribution along the x-axis (blue curve) and y-axis (red curve), respectively, on the focal plane; (c) The values of the hollow spot FWHM in different direction, where the black-dashed line, red-dashed line and blue dash-dot line are the diffraction-limit, super-oscillation criterion and average FWHM
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Figure 7.
The comparison between experimental and theoretical results. In the propagation plane, the optical intensity distribution between z=194λ and z=206λ obtained by (a) experiment and (b) Comsol Multiphysics simulation, respectively. The corresponding distributions of (c) peak intensity, (d) inner full-width-at-half-maximum and (e) sidelobe ratio obtained by (a) Comsol Multiphysics simulation (red-solid) and experiment (blue-solid), where the black-dotted line and the red-dotted line indicate the diffraction-limit (0.5λ/NA) and super-oscillation criterion (0.38λ/NA), respectively
