Abstract
Scientists in the USA have developed powerful ultrathin planar lenses made from 30-nm-thick perforated gold films. These plasmonic metalenses, created by Xingjie Ni and co-workers at Purdue University, focus a beam of visible light into a spot measuring only slight larger than the wavelength of operation. The devices rely on a concentric pattern of Babinet-inverted nano-antennas (nano-avoids) in a metal film that manipulate the phase of the incident light. For example, a 4-μm-diameter lens provides a focal length of 2.5 μm at a wavelength of 676 nm. The lenses are designed to work at two orthogonal linear polarizations and in future could be suitable for use as miniature couplers or light concentrators in on-chip optical devices.
Highlights
The convergence or divergence of an optical beam in a traditional, refraction-based lens depends on the phase change of the light propagating inside the lens
Fresnel zone plates, which consist of concentric rings (Fresnel zones) and use diffraction instead of refraction or reflection, can be used to focus light, but it is impossible to shrink the size down to only a few wavelengths since the radius differences between the neighboring opaque and transparent rings must be at least half of the wavelength of the incident light, and typically a large number of rings is required for good performance
Those designs suffer from limited phase control, which restricts their minimum sizes and thicknesses: either the size of the lens cannot be further reduced because the design is based on the diffraction of the light through transparent/opaque regions, or the thickness of the lens must be comparable to the operational wavelength because the phase change is obtained by light propagating inside the lens material
Summary
The convergence or divergence of an optical beam in a traditional, refraction-based lens depends on the phase change of the light propagating inside the lens. A number of plasmonic lenses have been developed recently based on superoscillation and mode-index manipulation of guided waves inside nano-apertures (slits or holes).. A number of plasmonic lenses have been developed recently based on superoscillation and mode-index manipulation of guided waves inside nano-apertures (slits or holes).3–6 Those designs suffer from limited phase control, which restricts their minimum sizes and thicknesses: either the size of the lens cannot be further reduced because the design is based on the diffraction of the light through transparent/opaque regions, or the thickness of the lens must be comparable to the operational wavelength because the phase change is obtained by light propagating inside the lens material A number of plasmonic lenses have been developed recently based on superoscillation and mode-index manipulation of guided waves inside nano-apertures (slits or holes). those designs suffer from limited phase control, which restricts their minimum sizes and thicknesses: either the size of the lens cannot be further reduced because the design is based on the diffraction of the light through transparent/opaque regions, or the thickness of the lens must be comparable to the operational wavelength because the phase change is obtained by light propagating inside the lens material
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