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Principle of designing an achromatic metalens. (a) An achromatic metalens with dispersionless focusing. (b) Schematic of three different metalens elements consisting of Si nanopillars with the height of h = 8 μm and unit cell periodicity p = 2 μm. The displayed shape of elements has varying in-plane geometrical parameters (l, l h , l t ). (c) Phase and group delay for all elements in our library at the center frequency ω0 = 42.62 THz (λ ≈ $\approx $ 7 μm). (d) A comparison of phase response for different meta-elements from eigenmode solutions versus finite-element method (FEM) calculations. The red curve represents a “solid” element; the green a “hole” element; and the blue a “star” element whose library positions are specified in (c).
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Controlling the wavefront of light, especially on a subwavelength scale, is pivotal in modern optics. Metasurfaces present a unique platform for realizing flat lenses, called metalenses, with thicknesses on the order of the wavelength. Despite substantial effort, however, suppressing the chromatic aberrations over large operational bandwidths of me...
Citations
... As HRI materials are highly dispersive and the subwavelength nanoantennae also induce strong scattering in such metadevices, the concomitant realization of metalenses with (i) high numerical aperture (NA), (ii) broadband (achromatic), and (iii) high efficiency in the visible range is hindered. Some of the recent works on achromatic metalenses tackle these three points in the near-or mid-infrared (IR) range, where the material dispersion is substantially lower and material absorption is negligible (20)(21)(22). ...
Flat optics consisting of nanostructures of high–refractive index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a trade-off between the numerical aperture (NA) and bandwidth, which limits performance. Here, we propose a new approach to design high-NA, broadband, and polarization-insensitive multilayer achromatic metalenses (MAMs). We combine topology optimization and full-wave simulations to inversely design MAMs and fabricate the structures in low–refractive index materials by two-photon polymerization lithography. MAMs measuring 20 μm in diameter operating in the visible range of 400 to 800 nm with 0.5 and 0.7 NA were achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light and RGB narrowband illuminations. These results highlight the potential of the 3D-printed multilayer structures for realizing broadband and multifunctional meta-devices with inverse design.
... Achromatic metalenses are first designed for optical imaging based on dielectric materials, working from infrared to the visible band [14][15][16][17][18][19][20]. The reported achromatic metalenses require close-packed meta-atoms [14][15][16] or meta-atoms with complex shapes [17,19,20] to provide the desired phase compensation and group delay. ...
... Achromatic metalenses are first designed for optical imaging based on dielectric materials, working from infrared to the visible band [14][15][16][17][18][19][20]. The reported achromatic metalenses require close-packed meta-atoms [14][15][16] or meta-atoms with complex shapes [17,19,20] to provide the desired phase compensation and group delay. Moreover, some achromatic metalenses only work for circularpolarization waves as the design employs geometrical phase [14][15][16]. ...
Achromatic lenses, which have the same focal length regardless of the illumination frequency, find strong applications in imaging, sensing, and communication systems. Making achromatic lenses with metasurfaces is highly desirable because they are flat, ultrathin, relatively light, and easily fabricable. However, existing metalenses experience combinations of limitations which include single polarization operation, narrow bandwidth, and small numerical aperture (NA). In this work, we propose a dual polarized, broadband and high NA achromatic metalens based on the Huygens’ metasurface. We use Huygens’ metasurface unit cells with three tunable resonances to realize a stable group delay over a large bandwidth, while also achieving high transparency and large phase tunability. With these cells, we construct a dual-polarized achromatic Huygens’ metalens with an NA of 0.64 that works from 22 to 26 GHz. Our achromatic metalens achieves diffraction-limited focusing with 2 % maximum focal length deviation and 70 % average focusing efficiency over a bandwidth of 16.7 %. Most key performance metrics for this lens surpass or are comparable with the best of previous metalenses. An achromatic metalens simultaneously achieving broad bandwidth, large NA, and polarization-independent operation will open wide-ranging opportunities for microwave and mm-wave imaging and communication applications.
... Recent advances suggest such a challenge can be addressed by adopting symmetric nanostructures or their inverse nanostructures to constitute a metasurface, [24][25][26][27][28][29] which, however, usually comes with the price of a vast of optimization effort and time consumption, and losing a degree of freedom in the design space due to the symmetry of these constituent elements. Most recently, Capasso's group counterintuitively opened up an unprecedented solution to realize an achromatic metalens capable of focusing any incident polarization in the visible range by limiting the rotation angle of each anisotropic element to either 0 deg or 90 deg, which, however, still involves multitudinous optimization efforts. ...
... The variation of the focusing efficiency can probably stem from the transmission amplitude change and the nearest-neighbor effects. 29 We note that the focusing efficiencies of the BAPIML are not fully on par with monochromatic designs; 4,40,41 however, they are comparable and even outperform other recent dielectric achromatic metalens demonstrations. 1,20,30,42 Conferring broadband achromatic metalens with polarization-insensitive focusing characteristics is highly desirable for many applications, especially in imaging fields. ...
... Among the dielectric materials, silicon (Si) is selected as the most constituent material of metasurfaces in the THz range for its merits of low loss, high refractive index, and ease of fabrication [22,23]. It has been reported that Si meta-atoms can be implemented for versatile applications, such as polarization-independent metalenses [24,25], polarization-dependent THz wavefront manipulation [26], polarization-sensitive modulated optical vortices generation [27,28], achromatic metalenses [29,30], and reconfigurable THz metasurface pure phase holograms [31]. In addition, traditional metasurface holograms were achieved based on iterative optimization algorithms (the most commonly used being the Gerchberg-Saxton (GS) algorithms) [31][32][33][34]. ...
There is a growing trend towards the development of high resolution and multiplexing metasurface holograms. In this paper, we propose the reconstruction of polarization multiplexing terahertz (THz) holographic images based on transmissive metasurface. The metasurface composed of all-dielectric meta-atoms is designed as a multi-foci metalens and the focal points of the metalens are utilized as the pixels of a reconstructed image. We analyze the effects of focal length and phase
pixel number of the metalens on focal point to achieve high-resolution holographic images. In addition, by switching the polarization of incident lights, holographic images with different patterns are reconstructed on its focal plane. Such high-resolution and polarization multiplexing metasurface holograms is promising for applications in THz communications, information engineering, and encryption.
Flat optics consisting of nanostructures of high-refractive-index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a trade-off between the numerical aperture (NA) and bandwidth, which limits performance. Here we propose a new approach to design high NA, broadband and polarization-insensitive multilayer achromatic metalenses (MAM). We combine topology optimization and full wave simulations to inversely design MAMs and fabricate the structures in low-refractive-index materials by two-photon polymerization lithography. MAMs measuring 20 micrometer in diameter operating in the visible range of 400-800 nm with 0.5 NA and 0.7 NA were achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light, and RGB narrowband illuminations. These results highlight the potential of the 3D printed multilayer structures for realizing broadband and multi-functional meta-devices with inverse design.
A metasurface is a planar structure that can flexibly control the polarization, phase, and amplitude of incident electromagnetic waves. An all‐dielectric hexagonal cell structure to reduce the interaction between cells and achieve efficient parametric regulation of electromagnetic waves is proposed. The single resonant transmission phase and the geometric phase are superimposed to realize the composite phase unit structure and construct a multifunctional flat metasurface device. The designed metasurface can flexibly tune the phase and polarization of incident electromagnetic waves and enable multifunctional integration phenomena, such as polarization separation, light intensity convergence, and vector light generation, on a single metasurface.
