Large Chromatic Aberration Research Articles

A Holographic Broadband Achromatic Metalens

AbstractMetalens with nanostructured optical elements are designed to control wavefront shaping at the wavelength‐scale thickness. However, most metalens still suffer from large chromatic aberrations due to phase dispersion, limiting their wide applications for multiple wavelengths. Here, avoiding complex resonant designs or multiple‐layer nanostructures, a simple broadband achromatic metalens by holographic diffraction in the visible spectrum is demonstrated. The hyperbolic phase distributions of multiple discrete wavelengths within a shared aperture and fabricate a broadband achromatic diffractive metalens by flexible direct‐laser‐writing technique is randomly interleaved. The designed polarization‐insensitive metalens modulate the light field by changing the height of the artificial neuron phase units to achieve average focusing efficiencies of 55% in the visible spectrum. This holographic design concept can pave the way to expand achromatic metalenses to multiple spectra.

Sb2S3-Based Dynamically Tuned Color Filter Array via Genetic Algorithm.

Color displays have become increasingly attractive, with dielectric optical nanoantennas demonstrating especially promising applications due to the high refractive index of the material, enabling devices to support geometry-dependent Mie resonance in the visible band. Although many structural color designs based on dielectric nanoantennas employ the method of artificial positive adjustment, the design cycle is too lengthy and the approach is non-intelligent. The commonly used phase change material Ge2Sb2Te5 (GST) is characterized by high absorption and a small contrast to the real part of the refractive index in the visible light band, thereby restricting its application in this range. The Sb2S3 phase change material is endowed with a wide band gap of 1.7 to 2 eV, demonstrating two orders of magnitude lower propagation loss compared to GST, when integrated onto a silicon waveguide, and exhibiting a maximum refractive index contrast close to 1 at 614 nm. Thus, Sb2S3 is a more suitable phase change material than GST for tuning visible light. In this paper, genetic algorithms and finite-difference time-domain (FDTD) solutions are combined and introduced as Sb2S3 phase change material to design nanoantennas. Structural color is generated in the reflection mode through the Mie resonance inside the structure, and the properties of Sb2S3 in different phase states are utilized to achieve tunability. Compared to traditional methods, genetic algorithms are superior-optimization algorithms that require low computational effort and a high population performance. Furthermore, Sb2S3 material can be laser-induced to switch the transitions of the crystallized and amorphous states, achieving reversible color. The large chromatic aberration ∆E modulation of 64.8, 28.1, and 44.1 was, respectively, achieved by the Sb2S3 phase transition in this paper. Moreover, based on the sensitivity of the structure to the incident angle, it can also be used in fields such as angle-sensitive detectors.

End-to-End Optimization of an Achromatic Diffractive Optical Element Array for Integral Imaging Three-Dimensional Display

Diffractive optical element (DOE) array promises compact shape for full-parallax integral imaging three-dimensional (3D) display. However, DOEs suffer from large chromatic aberration due to the strong wavelength-dependent nature of diffraction phenomena that degrade the quality of reconstructed 3D images. An end-to-end DOE optimization approach is proposed to reduce chromatic aberration for integral imaging. The end-to-end optimization framework includes RGB pre-processing convolutional neural networks and achromatic optics optimization design of rotationally symmetric DOE. An optical display model based on diffractive optics is proposed to analyze the integral imaging 3D display process for achromatic optical optimization design. The pre-processed elemental image arrays are modulated by an optimized DOE array to reconstruct the achromatic 3D images. A 3D artifacts scene without chromatic aberration is reconstructed in different views with the proposed method, and both peak signal to noise ratio (PSNR) and structural similarity (SSIM) are improved compared to the conventional Fresnel lens DOE.

Large aperture phase-coded diffractive lens for achromatic and 16° field-of-view imaging with high efficiency

Diffractive lenses (DLs) can realize high-resolution imaging with light weight and compact size. Conventional DLs suffer large chromatic and off-axis aberrations, which significantly limits their practical applications. Although many achromatic methods have been proposed, most of them are used for designing small aperture DLs, which have low diffraction efficiencies. In the designing of diffractive achromatic lenses, increasing the aperture and improving the diffraction efficiency have become two of the most important design issues. Here, a novel phase-coded diffractive lens (PCDL) for achromatic imaging with a large aperture and high efficiency is proposed and demonstrated experimentally, and it also possesses wide field-of-view (FOV) imaging at the same time. The phase distribution of the conventional phase-type diffractive lens (DL) is coded with a cubic function to expand both the working bandwidth and the FOV of conventional DL. The proposed phase-type DL is fabricated by using the laser direct writing of grey-scale patterns for a PCDL of a diameter of 10 mm, a focal length of 100 mm, and a cubic phase coding parameter of 30π. Experimental results show that the working bandwidth and the FOV of the PCDL respectively reach 50 nm and 16° with over 8% focusing efficiency, which are in significant contrast to the counterparts of conventional DL and in good agreement with the theoretical predictions. This work provides a novel way for implementing the achromatic, wide FOV, and high-efficiency imaging with large aperture DL.

Full-visible achromatic imaging with a single dual-pinhole-coded diffractive photon sieve.

Conventional diffractive optical elements suffer from large chromatic aberration due to its nature of severe dispersion so that they can only work at a single wavelength with near zero bandwidth. Here, we propose and experimentally demonstrate an achromatic imaging in the full-visible wavelength range with a single dual-pinhole-coded diffractive photon sieve (PS). The pinhole pattern (i.e., distribution of the position and size of each pinhole) is generated with dual wavelength-multiplexing coding (WMC) and wavefront coding (WFC), in which WMC makes multiple wavelengths that are optimally selected within the full visible range focus coherently on a common designed focal length while WFC expands the bandwidth of the diffracted imaging at each of the selected wavelengths. Numerical simulations show that when seven wavelengths (i.e., 484.8, 515.3, 547.8, 582.4, 619.1, 658.1 and 699.5 nm) within the visible range between 470 nm to 720 nm and a cubic wavefront coding parameter α = 30π are selected, a broadband achromatic imaging can be obtained within the full range of visible wavelength. Experimental fabrication of the proposed dual-pinhole-coded PS with a focal length of 500 mm and a diameter of 50 mm are performed using the mask-free UV-lithography. The experimental imaging results agree with the numerical results. The demonstrated work provides a novel and practical way for achieving achromatic imaging in the full visible range with features of thin, light and planar.

Achromatic flat lens performance limits

In recent years there has been a lot of interest in flat lenses, a category that includes diffractive lenses and metalenses. These lenses have the potential of reducing the size and cost of optical systems by replacing conventional refractive optical elements. A major obstacle to the widespread use of flat lenses is their inherent large chromatic aberration, associated with diffraction effects. To meet this challenge, achromatic diffractive lenses and metalenses have been developed. In this paper we review previously reported metalens performance limits, derive limits on the performance of achromatic diffractive lenses, and compare the two. We show that such lenses can support a wide spectral range, limited only by loss of efficiency caused by manufacturing limitations related to feature depth and size. On the other hand, we show that these lenses can provide near-diffraction-limited performance only at very low Fresnel numbers, i.e., they cannot provide large focusing power and broadband response simultaneously. We then go on to compare the limits of achromatic metalenses and diffractive lenses, in attempt to understand the potential of different types of flat lenses. Our findings facilitate better understanding of flat lens capabilities and limitations, and the exploration of novel design concepts and applications.

Broadband Achromatic Transmission–Reflection‐Integrated Metasurface Based on Frequency Multiplexing and Dispersion Engineering

AbstractTransmission–reflection‐integrated metasurfaces (TRIMs) provide an effective avenue to realize functionality integration and miniaturization of imaging and communication systems. However, most of the previous works suffer from large chromatic aberrations due to the intrinsic dispersive properties of the resonators constituting metasurfaces, which results in the meta‐devices working in a small operation bandwidth. Here, a broadband achromatic TRIM is proposed based on the frequency‐multiplexing scheme and dispersion engineering in transmission and reflection mode, respectively. As a proof of the scheme, 1D achromatic transmission–reflection‐integrated focusing metasurface and common‐caliber transmitarray and reflectarray flat plate antenna are demonstrated over a broad frequency range. The achromatic focusing in transmission mode and reflection mode with simulated focusing efficiencies of 13.12–17.21% and 14.57–20.86% are simultaneously realized at 12–18 and 19–24 GHz, respectively. The radiation gains increase by an average of 17.53 and 23.57 dB for the transmission mode and reflection mode relative to the bare standard rectangle waveguides. The scheme for achromatic and independent electromagnetic manipulation in transmission and reflection mode can also be applied to other frequency ranges and promise unprecedented progress for optical imaging and wireless telecommunication.

超表面透镜的宽带消色差成像（特邀）

Metalens, the specific type of lens designed with the surfaces mading of two dimensional array at the subwavelength scale, has shown great flexibilities to control the light field, including the arbitrary modulation abilities of amplitude, phase and polarization at the subwavelength scale. Moreover, the metalens possesses the unique advantages of low loss, integratable and conformable design and ultrathin, therefore attracts immense attentions in recent years. However, in most cases, the metalens designed for a specific wavelength may penetrate through the large chromatic aberration, which limits their usefulness in multi-wavelength or broadband applications. On the other hand, the metalens has renewed new degrees of freedom due to its two-dimensional planar structure, which has the potential in the elimination of chromatic aberration. Some different typical achromatic metalens designs and their achromatic modulation mechanism were reviewed, the existing achromatic metalens were classified from the types of modulated light bands, such as the achromatic matelens for discrete and continuous wavelength respectively, and the latter can be classified as transmissive and reflective from the working mode. Finally, the application of metalenses array in imaging and their prospect of broadband achromatic devices of large depth of field were introduced.

On-chip multiwavelength achromatic thin flat lens

Here we demonstrate a periodic array of thin flat lens defined on a silicon substrate on a standard silicon-on-insulator (SOI), which can focus light of broadband wavelength on the same focal plane. The on-chip thin flat lens has been implemented a periodic structure in the y-direction , but due to the high degree of dispersion of its structural units, the lens significantly suffers from large chromatic aberrations; And we also designed a fixed structure to keep the focal length of the thin flat lens unchanged at 1450 nm, 1550 nm, and 1650 nm, i.e. realizing the on-chip multiwavelength achromatic lens. In this way, functions previously unavailable for various on-chip imaging devices will be realized on this multi-band, which will help innovation in integration and imaging.

Squeezing a Prism into a Surface: Emulating Bulk Optics with Achromatic Metasurfaces

AbstractMetasurfaces promise to replace bulky prisms and lenses with 2D surfaces, revolutionizing wavefront control with technologically significant advantages in size, weight, and planar fabrication. However, conventional implementations suffer from large chromatic aberrations and cannot sustain performance over practical bandwidths of real‐world signals because of the limited phase modulation margin available in a surface. How can an infinitely thin surface generate the arbitrarily large, broadband phase delay that bulk phase accumulation can provide? Here, equivalence between bulk optics and certain multiresonant metasurfaces is demonstrated, where phase delay arises from trains of multiple resonances in the effective sheet conductivities of the surface itself instead of accumulation of propagation phase. The fundamentally required electromagnetic surface conductivities of a purely achromatic metasurface are derived and general design rules for arbitrarily broadband beam steering and lensing are obtained. Both operation in transmission and reflection can be achieved in a unified way by proper alignment of the resonances, enabling 360° directionality. The arbitrary spectral bandwidth, dual reflection/transmission operation, and complete dispensability of propagation phase constitute a major advance in the state of the art of achromatic metasurfaces for wavefront manipulation.

DMD-based three-dimensional chromatic confocal microscopy.

In this paper, a digital mirror device (DMD)-based chromatic confocal microscopy is proposed and demonstrated for three-dimensional (3D) surface profiling without any mechanical scanning. In this method, the DMD works as the multipoint source and multi-pinhole at the same time to achieve the lateral scanning. Moreover, axial scanning is realized through the chromatic aberration of the confocal optics. Since the micromirror array of the DMD is not perpendicular to the confocal imaging axis, a corresponding calibration is needed to eliminate the tilt effects and perform accurate 3D imaging. The measurement range with the current optical system is 45 µm over 505-650 nm working spectrum and can be increased by using a custom objective with large chromatic aberration. The system performance has been demonstrated with a multistep sample.

Spatially truncated Gaussian pulsed beam and its application in modeling diffraction of ultrashort pulses from hard apertures.

A new kind of pulsed beam, which we call a spatially truncated Gaussian pulsed beam, is defined to represent a Gaussian pulsed beam that is diffracted from a semi-infinite hard aperture. The analytical equations for the propagation of the spatially truncated Gaussian pulsed beam through a nonrotationally symmetric paraxial system with second-order dispersion is derived starting from the generalized spatiotemporal Huygens integral. The spatially truncated Gaussian pulsed beam is then combined with the conventional Gaussian pulsed beam decomposition method to enable the modeling of diffraction of a general ultrashort pulse from an arbitrarily shaped hard aperture. The accuracy of the analytical propagation equation derived for the propagation of the truncated Gaussian pulsed beam is evaluated by a numerical comparison with diffraction results obtained using the conventional pulse propagation method based on the Fourier transform algorithm. The application of the modified Gaussian pulsed beam decomposition method is demonstrated by propagating an ultrashort pulse after a circular aperture through a dispersive medium and a focusing aspherical lens with large chromatic aberration.

Gaussian pulsed beam decomposition for propagation of ultrashort pulses through optical systems.

Many applications of ultrashort laser pulses require manipulation and control of the pulse parameters by propagating them through different optical components before the target. This requires methods of simulating the pulse propagation taking into account all effects of dispersion, diffraction, and system aberrations. In this paper, we propose a method of propagating ultrashort pulses through a real optical system by using the Gaussian pulsed beam decomposition. An input pulse with arbitrary spatial and temporal (spectral) profiles is decomposed into a set of elementary Gaussian pulsed beams in the spatiospectral domain. The final scalar electric field of the ultrashort pulse after propagation is then obtained by performing the phase correct superposition of the electric fields all-Gaussian pulsed beams, which are propagated independently through the optical system. We demonstrate the application of the method by propagating an ultrashort pulse through a focusing aspherical lens with large chromatic aberration and a Bessel-X pulse generating axicon lens.

Chromatic Dispersion Manipulation Based on Metalenses.

Metasurfaces are 2D metamaterials composed of subwavelength nanoantennas according to specific design. They have been utilized to precisely manipulate various parameters of light fields, such as phase, polarization, amplitude, etc., showing promising functionalities. Among all meta-devices, the metalens can be considered as the most basic and important application, given its significant advantage in integration and miniaturization compared with traditional lenses. However, the resonant dispersion of each nanoantenna in a metalens and the intrinsic chromatic dispersion of planar devices and optical materials result in a large chromatic aberration in metalenses that severely reduces the quality of their focusing and imaging. Consequently, how to effectively suppress or manipulate the chromatic aberration of metalenses has attracted worldwide attention in the last few years, leading to variety of excellent achievements promoting the development of this field. Herein, recent progress in chromatic dispersion control based on metalenses is reviewed.

Fiber optic array as an alternative to the optical lens in microscopy: A proof-of-concept study

The majority of optical microscopes rely heavily on the lens group because of the low cost and ease of use. However, this type of microscope is limited by the relative long working distance, large chromatic aberration and complex structure. As an alternative to the lens, fiber optic array (FOA) can overcome the above limitations and achieve better results in the same magnification scale. This paper demonstrated that the microscope based on the FOA had a comparable resolution (3 μm) with the traditional lens group microscope, while the field-of-view (FOV) of FOA was almost four times larger than the latter. Without any manual focus, the volume and distortion of our FOA microscope were obviously smaller than that of the lens group microscope. To obtain a better microscopy imaging performance based on the FOA, the coupling efficiency and signal-to-noise ratio (SNR) of the FOA microscope were analyzed and some related measures for improvement were proposed.

Large-aperture UV (250~400 nm) imaging spectrometer based on a solid Sagnac interferometer.

Developing an ultraviolet (UV) imaging spectrometer is challenging due to a low level of incident power of photon flux, large chromatic aberration, and relatively low quantum efficiency of imaging sensor in UV waveband. In this paper, a large-aperture UV (250~400 nm) Fourier transform imaging spectrometer is presented for close-range hyperspectral sensing with high spatial resolution and decent spectral resolution. An advanced design based on a modified solid Sagnac interferometer working in UV waveband of 250~400 nm is introduced to improve the interferometric stability. A large-aperture and a reflective-transmissive filtering system are used to increase the spectral purity of the incident UV radiation, and air-spaced achromatic doublets are designed to address the chromatic aberration. The finished spectrometer has a spatial resolution of 23.44 μm on the target plane, a wavelengths resolution of 1.59 nm at 250 nm, and can provide approximately 59 wavelength samples over the waveband of 250~400 nm. The proposed imaging spectrometer acquires a hyperspectral data cube through push-broom scanning in a few minutes. Examples of UV hyperspectral imaging are demonstrated with a sample of resolution test chart, and a cotton sample with vitamin C (VC) and vitamin B6 (VB6) traces. Based on the analysis of spectra, monochromatic images, and k-Means clustering results, it can be concluded that the spectrometer is capable of UV hyperspectral imaging with excellent spectral accuracy, spatial performance, compactness, and robustness. The potential applications of the proposed instrument include materials analysis and traces detection with UV spectral characteristics.

Optimization design of a stereo-photographic system based on achromatic double-prism arrays.

We present an optimization design for a stereo-photographic system where achromatic double-prism arrays are used in front of the camera lens to reduce chromatic aberration and distortion. Due to the optical properties of the prism, the image formed by a single-prism array exhibits large chromatic aberration and distortion. The chromatic aberration and distortion for both single-prism array and double-prism array stereo-photographic systems were analyzed by the CODE V software. The double-prism arrays have larger distortion, but the chromatic aberration is small. The distortion arising from the double-prism arrays can be compensated by the camera lens design. The design results show that the maximum spot size is reduced from 17.7μm to 8.8μm after optimization. The proposed stereo-photographic system has high-quality stereo images.

Multispectral and large bandwidth achromatic imaging with a single diffractive photon sieve.

Conventional photon sieves suffer from large chromatic aberration due to diffractive nature and can image only at a single designed wavelength with near zero bandwidth. Here, a novel photon sieve that can image achromatically and simultaneously at multiple wavelengths with wide spectral bandwidth is proposed and demonstrated experimentally. The multispectral achromatic imaging with a single diffractive photon sieve is implemented with harmonic diffraction and wavefront coding, in which harmonic diffraction makes different diffracted orders of multiple harmonic wavelengths on a common focus while wavefront coding through the coded distribution of the pinholes expands the bandwidth of diffracted imaging. Numerical simulations show that when four spectral bands centered at 437.5, 500, 583.3, and 700 nm in the visible range is designed with a cubic wavefront coding parameter α = 30π and a harmonic diffraction order of 5, the bandwidth at the corresponding wavelength band can reach ± 8, ± 9, ± 11 and ± 14 nm respectively, and the total working bandwidth of the harmonic diffraction wavefront coded photon sieve reaches ~84 nm compared with 0.39 nm of the conventional one. Experimental validation was performed using an UV-lithography fabricated wavefront coded photon sieve of a focal length of 500 mm and a diameter of 50 mm at a designed wavelength of 700 nm. The results show excellent agreement with the theoretical predictions.

Broadband achromatic optical metasurface devices

Among various flat optical devices, metasurfaces have presented their great ability in efficient manipulation of light fields and have been proposed for variety of devices with specific functionalities. However, due to the high phase dispersion of their building blocks, metasurfaces significantly suffer from large chromatic aberration. Here we propose a design principle to realize achromatic metasurface devices which successfully eliminate the chromatic aberration over a continuous wavelength region from 1200 to 1680 nm for circularly-polarized incidences in a reflection scheme. For this proof-of-concept, we demonstrate broadband achromatic metalenses (with the efficiency on the order of ∼12%) which are capable of focusing light with arbitrary wavelength at the same focal plane. A broadband achromatic gradient metasurface is also implemented, which is able to deflect wide-band light by the same angle. Through this approach, various flat achromatic devices that were previously impossible can be realized, which will allow innovation in full-color detection and imaging.

Composite functional metasurfaces for multispectral achromatic optics

Nanostructured metasurfaces offer unique capabilities for subwavelength control of optical waves. Based on this potential, a large number of metasurfaces have been proposed recently as alternatives to standard optical elements. In most cases, however, these elements suffer from large chromatic aberrations, thus limiting their usefulness for multiwavelength or broadband applications. Here, in order to alleviate the chromatic aberrations of individual diffractive elements, we introduce dense vertical stacking of independent metasurfaces, where each layer is made from a different material, and is optimally designed for a different spectral band. Using this approach, we demonstrate a triply red, green and blue achromatic metalens in the visible range. We further demonstrate functional beam shaping by a self-aligned integrated element for stimulated emission depletion microscopy and a lens that provides anomalous dispersive focusing. These demonstrations lead the way to the realization of ultra-thin superachromatic optical elements showing multiple functionalities—all in a single nanostructured ultra-thin element.