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Original Article Open Access
A waveguide metasurface based quasi-far-field transverse-electric superlens
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Abstract
The imaging capability of conventional lenses is mainly limited by the diffraction of light, and the so-called superlens has been developed allowing the recovery of evanescent waves in the focal plane. However, the remarkable focusing behavior of the superlens is greatly confined in the near-field regime due to the exponential decay of evanescent waves. To tackle this issue, we design a waveguide metasurface-based superlens with an extraordinary quasi-far-field focusing capability beyond the diffraction limit in the present work. Specifically, we analyze the underlying physical mechanism and provide experimental verification of the proposed superlens. The metasurface superlens is formed by an array of gradient nanoslits perforated in a gold slab, and supports transverse-electric (TE) waveguide modes under linearly polarized illumination along the long axis of the slits. Numerical results illustrate that exciting such TE waveguide modes can modulate not only optical phase but also evanescent waves. Consequently, some high-spatial-frequency waves can contribute to the focusing of the superlens, leading to the quasi-far-field super-resolution focusing of light. Under 405 nm illumination and oil immersion, the fabricated superlens shows a focus spot of 98 nm (i.e. λ/4.13) at a focal distance of 1.49 μm (i.e. 3.68λ) using an oil immersion objective, breaking the diffraction limit of λ/2.38 in the quasi-far field regime. The developed metasurface optical superlens with such extraordinary capabilities promises exciting avenues to nanolithography and ultra-small optoelectronic devices. -
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References
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Figure 1.
(a) The illustration of the superlens formed by an array of width-varied nanoslits perforated in a gold film on a glass substrate. All the nanoslits have the same length l. The width of each nanoslit w is variable. (b) Schematic focusing of the superlens based on the principle of optical interference under the normal illumination of a TE-polarized plane wave. d is the thickness of gold film and f is the focal distance.
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Figure 2.
Electric field distributions and Fourier spectra of a metasurface superlense. The FDTD simulated intensity distributions (a) in the y-z plane at x=0, (b) at the focal plane, and (c) in k-space near the superlens focus. The plane z=0 is the exit surface of the superlens. Corresponding theoretically calculated electric field intensity distributions (d) in the y-z plane at x=0, (e) at the focal plane, and (f) in k-space near the superlens focus.
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Figure 3.
Experimental demonstration of the quasi-far-field super-resolution focusing superlens. (a) A scanning electron micrograph of the fabricated superlens using FIB. (b) Measured intensity distributions in the y-z plane at x=0. (c) The measured and FDTD simulated intensity distribution at the focal plane.
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