Flat Lens Research Articles

Generalized metric for broadband flat lens performance comparison

Abstract A plethora of metalenses and diffractive lenses (“flat lenses”) have been demonstrated over the years. Recently, attempts have been made to stretch their performance envelope, particularly in the direction of wide-band achromatic performance. While achromatic behavior has been demonstrated, showing an actual improvement in imaging performance relative to conventional (non-chromatically corrected) flat lenses has remained a major challenge. The reasons for this are use of inappropriate performance metrics, lack of comparison to a baseline conventional design, and lack of a performance metric that combines signal-to-noise ratio and resolution. An additional problem is that different published flat lens designs use different first order parameters, so they cannot be compared. In this work we present an overall performance metric that will allow comparison of different types of flat lenses, even if their first order optical parameters are not the same. We apply this metric to several published achromatic flat lens designs and compare them to the equivalent conventional flat lens, which we consider as the lower bound for achromatic flat lens performance. We found that the performance of the achromatic flat lenses studied does not surpass that of a conventional diffractive lens. Use of this metric paves the way for future developments in the field of achromatic flat lenses, which will display proven progress.

Broadband metasurface superstrate for polarization-independent wave focusing and gain enhancement at Ka-band

A broadband metasurface flat lens is proposed as a polarization-independent wideband superstrate for wave focusing and gain enhancement at Ka-band. The proposed metasurface structure consists of four metal layers and is designed with diagonally symmetric unit cells to accommodate both the vertical and horizontal polarizations. The focusing ability of the proposed metasurface flat lens is validated via simulation and measurement, where normally incident plane waves are shown to be enhanced by up to 11 dB as a result of wave focusing. Also, the radiation gain enhancement due to the proposed metasurface flat lens is demonstrated via simulation and measurement, where a gain enhancement of up to 10.5 dB is achieved. The results show that the proposed structure maintains the wave focusing and gain enhancement characteristics over a bandwidth of 28–32 GHz. Furthermore, to demonstrate the utility of the proposed metasurface for circular polarization (CP), the gain enhancement of a CP patch antenna as a result of implementing the proposed metasurface as a superstrate is demonstrated via simulation and measurement. It is shown that the proposed metasurface superstrate provides a CP gain enhancement of nearly 10 dB.

An investigation into potential improvements in the design of lead glasses for protecting the eyes of interventional cardiologists

The lens of the eye can be damaged by ionising radiation, so individuals whose eyes are exposed to radiation during their work may need to protect their eyes from exposure. Lead glasses are widely available, but there are questions about their efficiency in providing eye protection. In this study, Monte Carlo simulations are used to assess the efficiency of lead glasses in protecting the sensitive volume of the eye lens. Two designs currently available for interventional cardiologists are a wraparound (WA) style and ones with flat frontal lenses with side shielding. These designs were considered together with four modifications that would impact upon their efficiency: changing the lead equivalent thickness, adding lead to the frames, elongating the frontal lenses, and adding a closing shield to the bottom rim. For the eye closest to the source, standard models of lead glasses only decrease the radiation reaching the most sensitive region of the eye lens by 22% or less. Varying the lead thickness between 0.4 mm and 0.75 mm had little influence on the protection provided in the simulation of clinical use, neither did adding lead to the frames. Improved shielding was obtained by elongating the frontal lens, which could reduce radiation reaching the eye lens by up to 76%. Glasses with lenses that had a rim at the base, extending towards the face of the user, also provided better shielding than current models, decreasing the dose by up to 80%. In conclusion, elongating the frontal lens of lead glasses, especially of the WA design, could provide a three-fold increase in shielding efficiency and this is still valid for lenses with 0.4 mm lead equivalence.

To what extent can space be compressed? Bandwidth limits of spaceplates

Spaceplates are novel flat-optic devices that implement the optical response of a free-space volume over a smaller length, effectively “compressing space” for light propagation. Together with flat lenses such as metalenses or diffractive lenses, spaceplates have the potential to enable the miniaturization of any free-space optical system. While the fundamental and practical bounds on the performance metrics of flat lenses have been well studied in recent years, a similar understanding of the ultimate limits of spaceplates is lacking, especially regarding the issue of bandwidth, which remains as a crucial roadblock for the adoption of this platform. In this work, we derive fundamental bounds on the bandwidth of spaceplates as a function of their numerical aperture and compression ratio (ratio by which the free-space pathway is compressed). The general form of these bounds is universal and can be applied and specialized for different broad classes of space-compression devices, regardless of their particular implementation. Our findings also offer relevant insights into the physical mechanism at the origin of generic space-compression effects and may guide the design of higher performance spaceplates, opening new opportunities for ultra-compact, monolithic, planar optical systems for a variety of applications.

Dual-band flat lens with negative index for silicon photonics

Dual-band flat lens with negative index for silicon photonics

Multi-optical effects in two-dimensional photonic crystals of metallic pairs

Beam steering in photonic crystals is significant in manipulating light beams. In this paper, we design self-collimation, beam splitting, and negative refraction effects in photonic crystals based on metallic pairs immersed in dielectric background. By using a single array of metallic pairs, a broadband and wide-angle self-collimation effect is achieved. Meanwhile, due to the extreme anisotropy, a beam splitting effect is achieved by utilizing the kissing of equi-frequency contours. We then restructure the unit cell with glide symmetry to achieve negative refraction effect in a wide frequency range and realize the effect of flat lens. To illustrate this phenomenon, a flat lens based on the restructured unit cell is designed and numerically demonstrated.

11‐3: Invited Paper: New Materials for Film Optics and Game‐Changing Head‐Mount Devices

The recent drive towards high performance AR and VR headsets has generated a new demand for materials designed for the targeted requirements of this sector. Many of the most pressing challenges associated with this technology relate to improving the user experience through improved optical components in the combiner. This paper will present the challenges and status of new materials specifically designed for this application. Materials for both passive components, such as flat lenses and gratings as well as for active components will be discussed.

Seipin transmembrane segments critically function in triglyceride nucleation and lipid droplet budding from the membrane.

Lipid droplets (LDs) are organelles formed in the endoplasmic reticulum (ER) to store triacylglycerol (TG) and sterol esters. The ER protein seipin is key for LD biogenesis. Seipin forms a cage-like structure, with each seipin monomer containing a conserved hydrophobic helix and two transmembrane (TM) segments. How the different parts of seipin function in TG nucleation and LD budding is poorly understood. Here, we utilized molecular dynamics simulations of human seipin, along with cell-based experiments, to study seipin's functions in protein-lipid interactions, lipid diffusion, and LD maturation. An all-atom simulation indicates that seipin TM segment residues and hydrophobic helices residues located in the phospholipid tail region of the bilayer attract TG. Simulating larger, growing LDs with coarse-grained models, we find that the seipin TM segments form a constricted neck structure to facilitate conversion of a flat oil lens into a budding LD. Using cell experiments and simulations, we also show that conserved, positively charged residues at the end of seipin's TM segments affect LD maturation. We propose a model in which seipin TM segments critically function in TG nucleation and LD growth.

Microacoustic Metagratings at Ultra-High Frequencies Fabricated by Two-Photon Lithography.

The recently proposed bianisotropic acoustic metagratings offer promising opportunities for passive acoustic wavefront manipulation, which is of particular interest in flat acoustic lenses and ultrasound imaging at ultra‐high frequency ultrasound. Despite this fact, acoustic metagratings have never been scaled to MHz frequencies that are common in ultrasound imaging. One of the greatest challenges is the production of complex microscopic structures. Owing to two‐photon polymerization, a novel fabrication technique from the view of acoustic metamaterials, it is now possible to precisely manufacture sub‐wavelength structures in this frequency range. However, shrinking in size poses another challenge; the increasing thermoviscous effects lead to a drop in efficiency and a frequency downshift of the transmission peak and must therefore be taken into account in the design. In this work three microacoustic metagrating designs refracting a normally incident wave toward −35° at 2 MHz are proposed. In order to develop meta‐atoms insensitive to thermoviscous effects shape optimization techniques incorporating the linearized Navier–Stokes equations discretized with finite element method are used. The authors report for the first time microscopic acoustic metamaterials manufactured using two‐photon polymerization and, subsequently, experimentally verify their effectiveness using an optical microphone as a detector in a range from 1.8 to 2.2 MHz.

Feed corrective lenslets for enhanced beamscan in flat lens antenna systems.

A method for improving beamscan performance of flat lens antenna systems is proposed, wherein small gradient index (GRIN) lenses are included in the feed apertures to correct spillover losses and improve scan collimation. Given a lens system with a flat feed surface, these feed-corrective-lenslets (FCLs) sit in the apertures of offset feed elements and squint the feed pattern toward the center of the lens, reducing spillover radiation and increasing gain at scan. Furthermore, the FCLs shift the effective phase center of the feed to be closer to the Petzval surface, improving scanned collimation and enhancing beam angle. A GRIN lens and FCLs for three offset positions are designed, fabricated and demonstrated in the Ku-band. The FCLs improve the gain of the scanned beam by up to 2 dB out to 50°, reducing the scan loss exponent from 5.0 to 2.5 at 18 GHz.

Varifocal diffractive lenses for multi-depth microscope imaging.

Flat optical elements enable the realization of ultra-thin devices able to either reproduce or overcome the functionalities of standard bulky components. The fabrication of these elements involves the structuration of material surfaces on the light wavelength scale, whose geometry has to be carefully designed to achieve the desired optical functionality. In addition to the limits imposed by lithographic design-performance compromises, their optical behavior cannot be accurately tuned afterward, making them difficult to integrate in dynamic optical systems. Here we show the realization of fully reconfigurable flat varifocal diffractive lens, which can be in-place realized, erased and reshaped directly on the surface of an azopolymer film by an all-optical holographic process. Integrating the lens in the same optical system used as standard refractive microscope, results in a hybrid microscope capable of multi-depth object imaging. Our approach demonstrates that reshapable flat optics can be a valid choice to integrate, or even substitute, modern optical systems for advanced functionalities.

Vector light field display based on an intertwined flat lens with large depth of focus

The depth of focus (DOF) of a lens is a crucial parameter in glasses-free 3D displays that affects the viewing distance range. Here we proposed a vector light field display based on intertwined flat lens for extended viewing distance. The gray-scale diffractive lens (GDL) is designed and fabricated with extended DOF for red (658 nm), green (532 nm), and blue (450 nm) colors. By integrating the intertwined GDLs with a liquid crystal display, four views form a smooth horizontal parallax with a cross talk below 26% over a viewing distance from 24 to 90 cm. The enhancement of the DOF is 1.8 × 1 0 4 -fold. The light efficiency of the pixelated GDL reaches 82%. The proposed vector light field 3D display has the advantages of a thin form factor, high efficiency, high color fidelity, and large viewing distance. The potential applications include portable electronics, 3D TVs, and tabletop displays.

Influence of thickness on the focusing of terahertz metasurface

Terahertz (THz) focusing metasurface plays a key role in micro-focusing and micro-imaging. Most of the existing focus sets only show the number and deflection of focuses. However, few researches are concerned about the adjustment of focus intensity. Herein we designed a complementary metasurface based on a C-ring and cross-shaped composite slot structures, and the effect of structural thickness on focusing intensity and focal length were studied. We find that the angle of anomalously transmitted beam can be scanned from [Formula: see text] to [Formula: see text] when the frequency changes from 1.2 THz to 1.8 THz, while the focal length of the focusing metasurface can be linearly changed from 1000 [Formula: see text] to 1650 [Formula: see text]. With increase in the thickness of the C-ring area, more interestingly, the focus intensity decreased linearly from 8.0 dB (V/m) to 4.5 dB (V/m). However, the focus intensity showed no obvious change when the thickness of the cross-shaped area was increased from 0.2 [Formula: see text] to 5 [Formula: see text]. Therefore, our work would enhance the application of THz in high-resolution imaging, new THz device, and flat lens with adjustable focus intensity.

Standardizing flat lens characterization

Standardizing flat lens characterization

Visible and near-infrared programmable multi-level diffractive lenses with phase change material Sb2S3.

In this paper, we discuss flat programmable multi-level diffractive lenses (PMDL) enabled by phase change materials working in the near-infrared and visible ranges. The high real part refractive index contrast (Δn ∼ 0.6) of Sb2S3 between amorphous and crystalline states, and extremely low losses in the near-infrared, enable the PMDL to effectively shift the lens focus when the phase of the material is altered between its crystalline and amorphous states. In the visible band, although losses can become significant as the wavelength is reduced, the lenses can still provide good performance as a result of their relatively small thickness (∼ 1.5λ to 3λ). The PMDL consists of Sb2S3 concentric rings with equal width and varying heights embedded in a glass substrate. The height of each concentric ring was optimized by a modified direct binary search algorithm. The proposed designs show the possibility of realizing programmable lenses at design wavelengths from the near-infrared (850 nm) up to the blue (450 nm) through engineering PMDLs with Sb2S3. Operation at these short wavelengths, to the best of our knowledge, has not been studied so far in reconfigurable lenses with phase-change materials. Therefore, our results open a wider range of applications for phase-change materials, and show the prospect of Sb2S3 for such applications. The proposed lenses are polarization insensitive and can have the potential to be applied in dual-functionality devices, optical imaging, and biomedical science.

Active wavefronts control with graphene-functionalized terahertz Metasurfaces

Metasurfaces, a new kind of artificial subwavelength structures, have exhibited unprecedented capabilities in wavefront control of electromagnetic waves. Currently, a series of active metasurfaces have been explored for engineering applications in anomalous refraction, ultrathin flat lens, holograms, and vortex beam generation, However, to date, most reported metasurfaces are mainly concentrated on active control on the polarization conversion and wavefront of circular polarized waves. Here, graphene-functionalized metasurfaces consisting of metallic double split ring resonators integrated with graphene (G-DSRRs) are proposed to actively control polarization conversion and wavefronts of linearly polarized waves at the terahertz region. By carefully designing spatially phase profile, anomalous reflection of linearly cross-polarized terahertz waves is demonstrated in a broadband range of frequencies. Moreover, the amplitude of the anomalous reflection terahertz wave can be dynamically tuned by varying Fermi energy via electrically doping graphene. Based on the tunable graphene-functionalized metasurface, several polarization conversion metasurface devices including vortex beam generators, invisibility cloaks, and metalens, are successfully designed and numerical results show that their amplitudes can be actively tuned to a significant extent by doping graphene. Therefore, such graphene-functionalized metasurfaces may have potential applications in THz telecommunications, high-resolution terahertz displays, and advanced THz imaging devices.

Corrections to “Millimeter-Wave Holographic Flat Lens Antenna for Orbital Angular Momentum Multiplexing” [Aug 21 4289-4303

Corrections to “Millimeter-Wave Holographic Flat Lens Antenna for Orbital Angular Momentum Multiplexing” [Aug 21 4289-4303

Elliptical-Shaped Fresnel Lens Design through Gaussian Source Distribution

A novel three-dimensional elliptical-shaped Fresnel lens (ESFL) analytical model is presented to evaluate and maximize the solar energy concentration of Fresnel-lens-based solar concentrators. AutoCAD, Zemax and Ansys software were used for the ESFL design, performance evaluation and temperature calculation, respectively. Contrary to the previous modeling processes, based on the edge-ray principle with an acceptance half-angle of ±0.27° as the key defining parameter, the present model uses, instead, a Gaussian distribution to define the solar source in Zemax. The results were validated through the numerical analysis of published experimental data from a flat Fresnel lens. An in-depth study of the influence of several ESFL factors, such as focal length, arch height and aspect ratio, on its output performance is carried out. Moreover, the evaluation of the ESFL output performance as a function of the number/size of the grooves is also analyzed. Compared to the typical 1–16 grooves per millimeter reported in the previous literature, this mathematical parametric modeling allowed a substantial reduction in grooves/mm to 0.3–0.4, which may enable an easy mass production of ESFL. The concentrated solar distribution of the optimal ESFL configuration was then compared to that of the best flat Fresnel lens configuration, under the same focusing conditions. Due to the elliptical shape of the lens, the chromatic aberration effect was largely reduced, resulting in higher concentrated solar flux and temperature. Over 2360 K and 1360 K maximum temperatures were found for ESFL and flat Fresnel lenses, respectively, demonstrating the great potential of the three-dimensional curved-shaped Fresnel lens on renewable solar energy applications that require high concentrations of solar fluxes and temperatures.

Multifocal flat lens using thin anisotropic surface in visible light spectrum

In this study, a novel design methodology of multifocal flat lens using a thin anisotropic surface is proposed and investigated in visible light spectrum. The unit cell of the anisotropic surface consists of two identical electric field coupled resonators (ELCRs) separated by a dielectric substrate. When the wave-front of transmitted wave through the anisotropic surface has a shape of parabola, the focusing spot can be generated at focal point. The transverse and longitudinal multi-focusing can be achieved by a separated mode and a chessboard alternating mode, respectively. The feasibility of the methodology is confirmed by both analytic and simulated results for circular polarization incidence with wavelength of 600 nm.

Design framework for polarization-insensitive multifunctional achromatic metalenses

Abstract 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 metalenses still remains a challenge. Here, we develop a systematic design method enabling a simultaneous, polarization-insensitive control of the phase and the group delay of a light beam based on libraries of transmission-mode dielectric meta-elements. Mid-infrared achromatic metalenses are designed and theoretically analyzed to have diffraction-limited focal spots with vanishing chromatic aberrations in the operating wavelength range of 6–8.5 μm, while maintaining high focusing efficiencies of 41% on average. The proposed methodology, which can be used as a general design rule for all spectra, also provides a versatile design scheme for ultrashort pulse focusing and achromatic vortex-beam generation (orbital angular momentum), representing a major advance toward practical implementations of functional metalenses.