Chromatic Dispersion Manipulation Based on Metalenses

Metasurfaces for generating higher-order Poincaré beams by polarization-selective focusing and overall elimination of co-polarization components

Focused higher-order Poincaré (HOP) beams are of particular interest because they facilitate understanding the exotic properties of structured light and their applications in classical physics and quantum information. However, generating focused HOP beams using metasurfaces is challenging. In this study, we proposed a metasurface design comprising two sets of metal nanoslits for generating coaxially focused HOP beams. The nanoslits were interleaved on equispaced alternating rings. The initial rings started at the two adjacent Fresnel zones to provide opposite propagation phases for overall elimination of the co-polarization components. With the designed hyperbolic and helical profiles of the geometric phases, the two vortices of the opposite cross-circular-polarizations were formed and selectively focused, realizing HOP beams of improved quality. Simulations and experimental results demonstrated the feasibility of the proposed metasurface design. This study is of significance in the integration of miniaturized optical devices and enriches the application areas of metasurfaces.

Broadband achromatic and wide field-of-view single-layer metalenses in the mid-infrared

Metalenses are considered a promising solution for miniaturizing numerous optical systems due to their light weight, ultrathin thickness and compact size. However, it remains a challenge for metalenses to achieve both wide field-of-view and broadband achromatic imaging. In this work, a single-layer achromatic metalens with a wide field-of-view of 160° in the 3800 nm-4200 nm band is designed and analyzed. The quadratic phase profile of the metalens and the propagation phase of each meta-atom are used to increase the field-of-view and compensate for chromatic aberration, respectively. In addition, the metalens is capable of transverse achromatic imaging. The design can be extended to other optical frequencies, which is promising for applications in unmanned vehicles, infrared detection, etc.

Tunable Polarization-Multiplexed Achromatic Dielectric Metalens

Tunable metasurfaces provide a compact and efficient strategy for optical active wavefront shaping. Varifocal metalens is one of the most important applications. However, the existing tunable metalens rarely serves broadband wavelengths restricting their applications in broadband imaging and color display due to chromatic aberration. Herein, an electrically tunable polarization-multiplexed achromatic metalens integrated with twisted nematic liquid crystals (TNLCs) in the visible region is demonstrated. The phase profiles at different wavelengths under two orthogonal polarization channels are customized by the particle swarm optimization algorithm and matched with the dielectric metaunits database to achieve polarization-multiplexed achromatic performance. By combining the broadband linear polarization conversion ability of TNLC, the tunability of varifocal achromatic metalens is realized by applying different voltages. Further, the electrically tunable customized dispersion-manipulated metalens and switchable color metaholograms are demonstrated. The proposed devices will accelerate the application of metasurfaces in broadband zoom imaging, AR/VR displays and spectral detection.

The Impact of Manufacturing Imperfections on the Performance of Metalenses and a Manufacturing-Tolerant Design Method

Metalenses play an important role in optoelectronic integrated devices, given their advantages in miniaturization and integration. Due to its high aspect ratio subwavelength structure, fabricating metalenses requires a high-level dry etching technology. Consequently, structure deformation of the metalens will exist if the etching process of the material is not mature enough, which will impair the metalens' performance. In this paper, a polarization-independent InP dielectric metalens is designed to focus the incident light from air into the substrate, which is used for monolithically integrating with the InGaAs/InP photodetector in the future. Subsequently, with the simulation method, we investigated the impact of the structure deformation on the metalens' performance, which was found in our InP dry etching process development. We have found that the sidewall slope and aspect ratio-dependent etching effect greatly impaired the focusing efficiency because of the phase modulation deviation. To solve this problem, we proposed a manufacturing-tolerant design method, which effectively improved the performance of the device with structural deformation. Our work is instructive for developing metalenses and can accelerate their integration application.

Spectrum dispersion element based on the metasurface with parabolic phase

New kinds of dispersion elements are required for the minimization of the spectrometers. Metasurfaces offer new methods for a novel type of spectrometers due to their ultra-thin property and great ability to manipulate the electromagnetic field. Here, we propose and demonstrate a spectral modulated metasurface as a miniaturized dispersion element that possesses parabolic phase profile. Different wavelengths of the incident light can be dispersed to different spatial positions due to the accumulation of the dynamic phase varies with the wavelengths from metasurface. Detailed theoretical spectrum dispersion ability is analyzed and experimental demonstration is achieved. The polarization conversion efficiency is high, which is promising to be used in practical applications. Such metasurface provides a new and simple way to design dispersion devices and has the potential to be used in spectrometers, variable filters, spectrum tomography, etc.

Ultra-compact snapshot spectral light-field imaging

Ideal imaging, which is constantly pursued, requires the collection of all kinds of optical information of the objects in view, such as three-dimensional spatial information (3D) including the planar distribution and depth, and the colors, i.e., spectral information (1D). Although three-dimensional spatial imaging and spectral imaging have individually evolved rapidly, their straightforward combination is a cumbersome system, severely hindering the practical applications of four-dimensional (4D) imaging. Here, we demonstrate the ultra-compact spectral light-field imaging (SLIM) by using a transversely dispersive metalens array and a monochrome imaging sensor. With only one snapshot, the SLIM presents advanced imaging with a 4 nm spectral resolution and near-diffraction-limit spatial resolution. Consequently, visually indistinguishable objects and materials can be discriminated through SLIM, which promotes significant progress towards ideal plenoptic imaging.

The authors demonstrate compact spectral light-field imaging by using a transversely dispersive metalens array and a monochrome imaging sensor. They show that 4D images can be obtained in a single shot, and demonstrate discrimination of visually indistinguishable objects.

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.

Simulation for multiwavelength large-aperture all-silicon metalenses in long-wave infrared

Long-wave infrared imaging systems are widely used in the field of environmental monitoring and imaging guidance. As the core components, the long-wave infrared lenses suffer the conditions of less available materials, difficult processing, large volume and mass. Metalens composed of sub-wavelength structures is one of the most potential candidates to achieve a lightweight and planar optical imaging systems. Meanwhile, it is essential to obtain large-aperture infrared lenses with high power and high resolution. However, it is difficult to use the finite-difference time-domain method to simulate a large-aperture metalens with the diameter of 201 mm due to the large amount of computational memory and computational time required. Here, to solve the mentioned problem, we firstly propose a simulation method for designing a large-aperture metalens, which combines the finite-difference time-domain algorithm and diffraction integration. The finite-difference time-domain algorithm is used to simulate the meta-atom's transmitted complex amplitude and the one-dimensional simplification of the diffraction integral is to calculate the focused field distributions of the designed metalens. Furthermore, the meta-atom spatial multiplexing is applied to design the all-silicon metalenses with the aperture of 201 mm to realize dual-wavelength (10 and 11μm) achromatic focusing, super anomalous dispersion focusing and super normal dispersion focusing. The designed metalenses are numerically confirmed, which reveal the feasibility of all-silicon sub-wavelength structures to accomplish the multiwavelength dispersion control. The designed all-silicon metalenses have the advantage of lightweight and compact. The proposed method is effective for the development of large-aperture imaging systems in the long-wave infrared.

Compact metalens-based integrated imaging devices for near-infrared microscopy

With current trends to progressively miniaturize optical systems, it is now essential to look for alternative methods to control light at extremely small dimensions. Metalenses are composed of subwavelength nanostructures and have an excellent ability to manipulate the polarization, phase, and amplitude of incident light. Although great progress of metalenses has been made, the compact metalens-integrated devices have not been researched adequately. In the study, we present compact imaging devices for near-infrared microscopy, in which a metalens is exploited. The indicators including resolution, magnification, and image quality are investigated via imaging several specimens of intestinal cells to verify the overall performance of the imaging system. The further compact devices, where the metalens is integrated directly on the CMOS imaging sensor, are also researched to detect biomedical issues. This study provides an approach to constructing compact imaging devices based on metalenses for near-infrared microscopy, micro-telecopy, etc., which can promote the miniaturization tending of futural optical systems.

Design of Polarization-Independent Reflective Metalens in the Ultraviolet-Visible Wavelength Region

Flat lens or metalens, as one of the most important application branches of metasurfaces, has recently been attracting significant research interest. Various reflective and transmissive metalenses have been demonstrated in the terathertz, infrared and visible wavelength range. However, metalens operating in the ultraviolet (UV) wavelength range is rare. Moreover, the development of reflective UV metalens, the important counterpart of transmissive ones, falls far behind. In this work, with thorough investigation of material properties, we propose a reflective metalens based on silicon dioxide (SiO₂) and aluminum (Al) that operates in the vacuum ultraviolet (VUV) to visible wavelength region. Four reflective metalenses were designed and optimized for wavelengths of 193, 441, 532 and 633 nm, and prominent focusing capability was observed, especially for the VUV wavelength of 193 nm. Dispersion characteristics of the metalenses were also studied within ±50 nm of the design wavelength, and negative dispersion was found for all cases. In addition, the SiO₂ + Al platform can be, in principle, extended to the mid-infrared (IR) wavelength range. The reflective VUV metalens proposed in this work is expected to propel miniaturization and integration of UV optics.