Achromatic Metalens Research Articles

Mid-Infrared Broadband Achromatic Metalens with Wide Field of View.

Metasurfaces have the ability to flexibly control the light wavefront, and they are expected to fill the gaps of traditional optics. However, various aberrations pose challenges for the application of metasurfaces in the wide angle and wide spectral ranges. The previous multi-aberration simultaneous optimization works had shortcomings such as large computational load, complex structure, and low generality. Here, we propose a metalens design method that corrects both monochromatic and chromatic aberrations simultaneously. The monochromatic aberration-corrected phase distribution is obtained by the optical design, and the chromatic aberration is reduced by using the original search algorithm combined with dispersion engineering. The designed single-layered wide-angle achromatic metalens has a balanced and efficient focusing effect in the mid-infrared band from 3.7 μm to 5 μm and a wide angle of ±30°. The design method proposed has the advantages of low computational load, wide application range, and easy experimental fabrication, which provides new inspiration for the development of generalized software for the design and optimization of metasurfaces.

A polarization-insensitive infrared broadband achromatic metalens consisting of all-silicon anisotropic microstructures

Infrared lenses have a wide range of applications in thermal imaging and detection. Here, by designing a single layer metasurface consisting of anisotropic microstructures, we experimentally demonstrate a polarization-insensitive long-wavelength infrared achromatic metalens. The focal lengths of the fabricated metalens with a numerical aperture of 0.33 remain unchanged and maintain diffraction-limited performance for the incident wavelength varying from 8.5 to 11.5 μm, demonstrating the elimination of chromatic aberration with 30% operation bandwidth. We envision this metalens may pave the way to the development of ultracompact achromatic thermal imaging systems.

Compact vertical grating coupler with an achromatic in-plane metalens on a 220-nm silicon-on-insulator platform.

We proposed a new type of vertical grating couplers (VGCs) with a compact footprint on the 220-nm silicon-on-insulator platform. The overall size of the device containing the L-shaped coupling grating and the taper with achromatic in-plane metalens is only 45 × 15 µm2, and the measured coupling efficiency at 1550 nm is -5.2 dB with a 1 dB bandwidth of 38 nm, around 1.6 dB higher than the VGC without metalens. The incidence angle mismatch has a 1 dB bandwidth of roughly 4°, whereas the displacement mismatch along the x-/y- axis has a bandwidth of around 3/4 µm. Furthermore, we experimentally show that such a design is compatible with VGCs operating in the S, C, and L bands.

A Broadband Achromatic Dielectric Planar Metalens in Mid-IR Range

Metalens are planar lenses composed of the subwavelength arrays, which have unconventional and versatile functionalities to manipulate the light fields compared with the traditional lens. It is noted that the most metalens are designed in a monochromatic mode in the visible or mid-infrared range (mid-IR), however, the broadband range is needed in many practical applications, such as spectroscopy, sensing, and imaging. Here, we design and demonstrate a broadband achromatic dielectric metalens in the mid-IR range of 4 µm–5 µm for near diffraction-limited (1.0λ) focusing. The broadband achromatic propagation and focusing of the metalens are designed and simulated by constructing and optimizing the phase profile. The Pancharatnam-Berry (P-B) phases of all the elements contribute to the main phase increment of the whole phase profile of the metalens. The additional phase is constructed and optimized by using the random search algorithm to obtain the optimized size of all the elements. The focusing efficiency of the achromatic metalens is also optimized and averaged as the result of phase optimization within a wide band for the building elements, while it is lowered comparing with the regular metalens without broadband achromatic designing. Using this combined designing approach, various flat achromatic devices with the broadband metalens can find a new way for full-color detection and imaging.

All-Dielectric Metasurface Lenses for Achromatic Imaging Applications

Metasurface can use artificial microstructures to manipulate electromagnetic waves more accurately and flexibly. All-dielectric metalens have a wide range of materials and low cost so it has a wide application prospect. Herein, we propose a all-dielectric achromatic metalens built with Si as the structural unit that can operate over a broadband of wavelengths in the visible region. It controls the wavefront of light through the Pancharatnam–Berry phase and propagation phase to eliminate the chromatic aberration. Meanwhile, we also use Gerchberg–Saxton algorithm and its improved algorithm to iterate over multiple design wavelengths and obtain holographic phases suitable for broadband. Thus, both the metalenses and holographic metasurfaces can achieve achromatic broadband in the visible light range, which provides a new method for the development of meta-optical imaging devices.

Robust Achromatic All-Dielectric Metalens for Infrared Detection in Intelligent Inspection.

Metalens has the advantages of high design freedom, light weight and easy integration, thus provides a powerful platform for infrared detection. Here, we numerically demonstrated a broadband achromatic infrared all-dielectric metalens over a continuous 800 nm bandwidth, with strong environmental adaptability in air, water and oil. By building a database with multiple 2π phase coverage and anomalous dispersions, optimizing the corrected required phase profiles and designing the sizes and spatial distributions of silicon nanopillars, we numerically realized the design of broadband achromatic metalens. The simulation results of the designed metalens show nearly constant focal lengths and diffraction-limited focal spots over the continuous range of wavelengths from 4.0 to 4.8 μm, indicating the ability of the designed metalens to detect thermal signals over a temperature range from various fault points. Further simulation results show that the metalens maintains good focusing performance under the environment of water or oil. This work may facilitate the application of metalens in ultra-compact infrared detectors for power grid faults detection.

Polarization-multiplexing achromatic metasurfaces for manipulation of terahertz waves

Metasurfaces that can effectively manipulate electromagnetic waves provide a novel solution to realize efficient terahertz functional devices. However, the chromatic aberration of metasurfaces is a remarkable challenge for their extensive applications. Multi-functional metasurfaces are highly desirable in practical applications. Here, we demonstrate polarization-multiplexing achromatic metasurfaces working in the terahertz regime based on anisotropic meta-atoms which exhibit the form birefringence. Specifically, a polarization-multiplexing achromatic metalens working in the frequency range from 0.8 THz to 1.2 THz is proposed and verified numerically, which can focus terahertz waves within the working bandwidth to different focal planes by varying the polarization state of an incident terahertz wave; the deviation of focal length is less than 8.88% across the whole working bandwidth. In addition, a polarization-multiplexing achromatic deflector working in the frequency range from 0.8 THz to 1.2 THz is also designed and simulated, which can deflect a terahertz wave within the working bandwidth to different directions by manipulating the polarization state of an incident terahertz wave. The metasurfaces demonstrated here are of great significance for the development of ultra-compact, flexible, and multi-functional terahertz devices based on metasurfaces.

Achromatic acoustic generalized phase-reversal zone plates

We report an achromatic acoustic generalized phase-reversal zone plate by harnessing the response of dipole and monopole, which eliminate the chromatic aberration of conventional zone plates. The focusing properties of the proposed metalens are compared with that of the conventional Soret-type Fresnel zone plate (FZP) in both experiments and simulations. Due to the combination of the phase-reversal characteristic and the tunable transmission phase induced by dipole and monopole, an achromatic high efficient focusing is confirmed by experiment in the frequency range from 3350 to 3950 Hz, with the focal intensity of achromatic metalens being approximately twice that of Soret-type FZP. The proposed achromatic metalens has potential applications in the broad field of acoustics, such as imaging and energy harvesting.

An achromatic metafiber for focusing and imaging across the entire telecommunication range

Dispersion engineering is essential to the performance of most modern optical systems including fiber-optic devices. Even though the chromatic dispersion of a meter-scale single-mode fiber used for endoscopic applications is negligible, optical lenses located on the fiber end face for optical focusing and imaging suffer from strong chromatic aberration. Here we present the design and nanoprinting of a 3D achromatic diffractive metalens on the end face of a single-mode fiber, capable of performing achromatic and polarization-insensitive focusing across the entire near-infrared telecommunication wavelength band ranging from 1.25 to 1.65 µm. This represents the whole single-mode domain of commercially used fibers. The unlocked height degree of freedom in a 3D nanopillar meta-atom largely increases the upper bound of the time-bandwidth product of an achromatic metalens up to 21.34, leading to a wide group delay modulation range spanning from −8 to 14 fs. Furthermore, we demonstrate the use of our compact and flexible achromatic metafiber for fiber-optic confocal imaging, capable of creating in-focus sharp images under broadband light illumination. These results may unleash the full potential of fiber meta-optics for widespread applications including hyperspectral endoscopic imaging, femtosecond laser-assisted treatment, deep tissue imaging, wavelength-multiplexing fiber-optic communications, fiber sensing, and fiber lasers.

Manipulation force analysis of nanoparticles with ultra-high numerical aperture metalens.

Metalens optical tweezers technology has several advantages for manipulating micro-nano particles and high integration. Here, we used particle swarm optimization (PSO) to design a novel metalens tweezer, which can get 3-dimensional trapping of particles. The numerical aperture (NA) of the metalens can reach 0.97 and the average focusing efficiency is 44%. Subsequently, we analyzed the optical force characteristics of SiO2 particles with a radius of 350 nm at the focal point of the achromatic metalens. We found the average maximum force of SiO2 particles in the x-direction and z-direction to be 0.88 pN and 0.72 pN, respectively. Compared with the dispersive metalens, it is beneficial in maintaining the constant of optical force, the motion state of trapped particles, and the stability of the trapping position.

Broadband achromatic metalens with polarization insensitivity in the mid-infrared range

Broadband achromatic metalens (BAML) has been successfully demonstrated in the mid-infrared (mid-IR) owing to the importance in numerous applications ranging from night vision to biomolecular sensing. However, these BAMLs only works for specific polarized light, hindering their practical applications. In this paper, we propose an achromatic metalens that works in the transmission mode with polarization insensitivity and continuously provide a near-constant focal length over a broad wavelength region of λ = 3.7–4.7 μm. The proposed metalens is comprised of just a single layer of silicon meta-units and has a numerical aperture of 0.84 with a diameter of 77.4 μm. Moreover, we design an achromatic axilens in the same wavelength region to demonstrate the proposed concept. The performances of the achromatic metalens and axilens are numerically analyzed in detail. Our study represents a major step towards practical implementations of mid-IR devices.

Band-tunable achromatic metalens based on phase change material.

Achromatic metalens have the potential to significantly reduce the size and complexity of broadband imaging systems. A large variety of achromatic metalens has been proposed and most of them have the fixed achromatic band that cannot be actively modified. However, band-tunable is an important function in practical applications such as fluorescence microscopic imaging and optical detection. Here, we propose a bilayer metalens that can switch achromatic bands by taking the advantage of the high refractive index contrast of Sb2S3 between amorphous and crystalline state. By switching the state of Sb2S3, the achromatic band can be reversibly switched between the red region of visible spectrum (650-830 nm) and the near-infrared spectrum (830-1100 nm). This band-tunable design indicates a novel (to our knowledge) method to solve the problem of achromatic focusing in an ultrabroad band. The metalens have an average focusing efficiency of over 35% and 55% in two bands while maintaining diffraction-limited performance. Moreover, through proper design, we can combine different functionalities in two bands such as combining achromatic focusing and diffractive focusing. The proposed metalens have numerous potential applications in tunable displaying, detecting devices and multifunctional devices.

RGB Achromatic Metalens Doublet for Digital Imaging.

Chromatic aberration is a major challenge faced by metalenses. Current methods to achieve broadband achromatic operation in metalenses usually suffer from limited size, numerical aperture, and working bandwidth due to the finite group delay of meta-atoms, thus restricting the range of practical applications. Multiwavelength achromatic metalenses can overcome those limitations, making it possible to realize larger numerical aperture (NA) and sizes simultaneously. However, they usually require three layers, which increases their fabrication complexity, and have only been demonstrated in small sizes, with low numerical aperture and modest efficiencies. Here, we demonstrate a 1 mm diameter red-green-blue achromatic metalens doublet with a designed NA of 0.8 and successfully apply the metalens in a digital imaging system. This work shows the potential of the doublet metasurfaces, extending their applications to digital imaging systems such as digital projectors, virtual reality glasses, high resolution microscopies, etc.

Design of the all-silicon long-wavelength infrared achromatic metalens based on deep silicon etching.

An all-silicon long-wavelength infrared (LWIR) achromatic metalens based on deep silicon etching is designed in this paper. With a fixed aperture size, the value range of the equivalent optical thickness of the non-dispersive meta-atoms constructing the achromatic metalens determines the minimum f-number. The fabrication characteristic with high aspect ratio of deep silicon etching amplifies the difference value of optical thickness between different meta-atoms by increasing the propagation distance of the propagation mode, which ensures a small f-number to obtain a better imaging resolution. A 280-µm-diameter silicon achromatic metalens with a f-number of 1 and the average focusing efficiency of 27.66% has been designed and simulated to validate the feasibility of this strategy. The simulation results show that the maximum focal length deviation percentage from the target value between the wavelength of 8.6 and 11.4 µm is 1.61%. This achromatic metalens design is expected to play a role in the field of LWIR integrated optical system.

Design of a polarization insensitive achromatic metalens with a high NA and uniform focusing efficiency based on a double layer structure of silicon and germanium

A polarization insensitive achromatic metalens (PIAML) was designed to realize a high NA of 0.5 and uniform focusing efficiency in the visible range based on a double layer structure of silicon (Si) and germanium (Ge). Due to their high refractive indices as well as the opposite characteristics of group delay in the visible wavelength, the combination of Si and Ge can contribute to the high NA and achromatic performances. In addition, an isotropic cylindrical unit cell structure was applied to confirm polarization sensitivity. From the finite difference time domain (FDTD) simulation results, it was confirmed that the designed PIAML shows good optical performance of both polarization insensitivity and achromatic performance with uniform focusing efficiency of 27% and high NA of 0.5 in the visible wavelength.

Arbitrary active control of the Pancharatnam-Berry phase in a terahertz metasurface.

Active phase-control metasurfaces show outstanding capability in the active manipulation of light propagation, while the previous active phase control methods have many constraints in the cost of simulation or the phase modulation range. In this paper, we design and demonstrate a phase controlled metastructure based on two circular split ring resonators (CSRRs) composed of silicon and Au with different widths, which can continuously achieve an arbitrary Pancharatnam-Berry (PB) phase between -π and π before or after active control. The PB phase of such a metasurface before active control is determined by the rotation angle of the Au-composed CSRR, while the PB phase after active control is determined by the rotation angle of the silicon-composed CSRR. And active control of the PB phase is realized by varying conductivity of silicon under an external optical pump. Based on this metastructure, active control of light deflection, metalens with arbitrary reconfigurable focal points and achromatic metalens under selective frequencies are designed and simulated. Moreover, the experimental results demonstrate that focal spots of metalens can be actively controlled by the optical pump, in accord with the simulated ones. Our metastructure implements a plethora of metasurfaces' active phase modulation and provides applications in active light manipulation.

Broadband achromatic longitudinal bifocal metalens in the visible range based on a single nanofin unit cell

Metasurfaces provide a remarkable platform to manipulate over phase, amplitude, and polarization flexibly and precisely. Bifocal metalens draws great research interest due to its ability of converging wavefronts to different focal positions horizontally and longitudinally. However, as wavelength of incident light changes, chromatic aberration will cause the focal lengths reliance on the incident wavelength, which will affect the performance of metasurface, especially for longitudinal bifocal metalens. In this work, a broadband achromatic longitudinal bifocal metalens (BALBM) based on single nanofin unit cell is demonstrated. Pancharatnam-Berry (PB) phase is used to converge the incident light. Cross commixed sequence distribution (CCSD) is introduced to control the positions of focal points FLand FRwhen left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) incident. Propagation phase is used to compensate the phase difference caused by chromatic aberration. Simulation results show that in the continuous wavelength range from 500 nm to 700 nm, the focal point shifts caused by chromatic dispersion are reduced 81% for FL and 83% for FR, respectively. The focal length variations are stabilized to 6.21% for FLand 4.8% for FRcomparing with the focal lengths at the initial wavelength 500 nm. The proposed BALBM brings advances to bifocal metasurfaces in versatile application areas including machine vision, optical computed tomography and microimaging.

Single-layer metalens for achromatic focusing with wide field of view in the visible range

Metalenses have attracted extensive attention due to their capability of flexible wavefront manipulation by subwavelength nanostructures. However, the development of metalenses has been deeply limited by the field of view (FOV) and spectral bandwidth. Existing wide FOV and achromatic metalenses usually adopt cascaded metasurfaces to correct aberrations, resulting in inevitable challenges for nano-fabrication technologies. In this paper, we design a wide FOV metalens with a numerical aperture of 0.24 which can provide a diffraction-limited FOV for over 40° at three discrete wavelengths of 473 nm (blue), 532 nm (green), and 632.8 nm (red). The metalens involves only a single layer of metasurface integrated on a flat substrate to correct aberration. This method considerably simplifies the architectures compared with the cascaded metalenses and conventional bulky optical systems. In addition, an achromatic wide FOV metalens with an operating wavelength range from 492 to 592 nm is further designed to demonstrate the generality of the proposed method. The simulated results indicate that metalenses have a great achromatic and diffraction-limited focusing performance within a large incident angle region, which is expected to have promising applications in imaging, holography, machine vision, and other fields.

Genetic algorithms designed ultra-broadband achromatic metalens in the visible

Chromatic aberration is a significant phenomenon in optical systems, and it significantly affects quality imaging. Intelligent algorithms provide a powerful new approach for complex optimization. Here, we propose a broadband achromatic polarization-insensitive metalens in the wide wavelength region from 400 nm to 740 nm using a genetic algorithm. The genetic algorithm can optimize the parameter space through selection, crossover, and mutation operations. The incident light is modulated by different diameters and side lengths of circular and square titanium dioxide nanopillars. Using a powerful genetic algorithm, various geometric parameters of the desired nanopillar can be optimized simultaneously and effectively to match the target phase compensation in a broad wavelength region. The focal lengths all converged to a record narrow location range in the whole wavelength range from 400 nm to 740 nm.

Design of an achromatic optical polarization-insensitive zoom metalens.

A diffractive lens based on metasurfaces has many advantages such as flatness, small aberrations, and compactness. The focal length can be adjusted by changing the lateral displacement between a pair of conjugate metasurfaces while fixing their axial distance, thereby forming a very compact zoom lens. However, chromatic aberration of diffractive optical elements restricts this system to working at one wavelength. This Letter proposes a metalens design method based on novel three-layer polarization-insensitive nanoposts, which can improve transmission amplitude and satisfy the achromatic zoom function with wavelengths ranging from 1310 nm to 1550 nm. The focal length can be adjusted from 31.2 µm to 19.8 µm, corresponding to extremal wavelength-dependent focal length relative deviation of 8.98%. This achromatic zoom metalens design method could have applications in various fields including augmented reality and integrated optical systems.