Flat Optics Research Articles

Broadband polarization-insensitive metalens with excellent achromaticity and high efficiency for the entire visible spectrum

Metasurface-based flat optics is continuously attracting interest owing to its ability for miniaturization of optical devices and systems. Over the last few years, broadband achromatic metalenses have been extensively studied for focusing and imaging in the visible band. However, it is still a substantial challenge to design an achromatic metalens that can produce high-focusing efficiency and high-consistency focal length over the entire visible spectrum. In this Letter, we propose a polarization-independent achromatic metalens operating at wavelengths from 400 to 760 nm, which comprises only a single layer of cross-shaped and square-ring silicon nitride-based nanopillars on a glass substrate in the transmission mode. Numerical results demonstrate that the achromatic metalens has an average efficiency of 75.9% and a maximum focal shift of less than 0.3λ (λ being the operating wavelength) over the entire visible band, which indicate significant improvement over the previously reported achromatic metalenses for focusing visible light in terms of focal-shift suppression and wideband operation with high efficiency. This study recommends a means to develop a high-quality achromatic metalens for the whole visible spectrum. The proposed approach is also suitable for achromatic metalens designs of other wavebands.

Ultrasensitive in-plane excitons-dominated pseudo-Brewster angle of transition metal dichalcogenides monolayers

Strong light-matter interactions rooted in excitons enable the transitional metal dichalcogenides (TMDCs) monolayers to exhibit unique optical properties that are promising for breaking the limitation of dimensions to create new nanophotonic techniques and applications. Herein, we theoretically and experimentally investigated the fundamental pseudo-Brewster angle (pBA) of TMDCs monolayers (MoS2, MoSe2, WS2, and WSe2) coated on dielectric substrates. We found the pBA undergoes significant angular shifts up to 3.1–4.7° in the visible region by inserting the TMDCs monolayers on a dielectric sapphire substrate, which exhibit close association with their excitonic transitions. Moreover, we demonstrated the pBA has surprisingly deep sub-monolayer resolution of by investigating the MoS2 sub-monolayers with varied surface coverage ratios (CRs). Experiment and calculation results show the sub-monolayer sensitivity of ∼ 0.047°/1% CRs, which is super advanced in resolving subtle differences of disconnected monolayers. Our results provide a basic reference for the studies on nano-optics, flat optics, and a wide range of low-dimensional materials towards creating functional and outstanding nano-photonic devices and sensors.

Airy-type X-ray states generated using 3/2 flat diffractive optics.

X-rays have developed into an essential tool in variety of fields, such as biology, materials, chemistry, and physics etc. Numerous X-ray types, including the orbital angular momentum (OAM), the Laguerre-Gauss, and the Hermite-Gauss states, have been proposed. This greatly enhances the depth of application of X-ray. The X-ray states described above are mostly produced by binary amplitude diffraction elements. In light of this, this paper proposes a flat X-ray diffraction grating based on caustic theory to generate Airy-type X-ray. It is proved by the simulation of multislice method that the proposed grating can generate the Airy beam in the X-ray field. The results show that the generated beams have a secondary parabolic trajectory deflection with the propagation distance, which is consistent with the theory. Inspired by the success of Airy beam in light-sheet microscope, the Airy-type X-ray can be anticipated to enable novel image capability for bio or nanoscience.

Toward Arbitrary Spin‐Orbit Flat Optics Via Structured Geometric Phase Gratings (Laser Photonics Rev. 17(5)/2023)

Toward Arbitrary Spin-Orbit Flat Optics via Structured Geometric Phase Gratings In article number 2200800, Peng Chen, Zhi-Han Zhu, Yan-Qing Lu, and colleagues demonstrate a new type of flat-optics elements, termed structured geometric phase gratings, capable of complex-amplitude control with polarization switchable behavior. Such an advanced spin-orbit device, compared with the present elements, unlocks the control of paraxial structured light in all spatial dimensions and paves the way towards arbitrary SOC conversion via flat optics.

High-Quality Infrared Metalenses Based on Germanium Dimers

Modern methods of nanophotonics allow creating miniature devices that change the direction of light propagation, modulate the phase front, and control the outcoming state of the polarization. One of the promising areas of research is the development of flat optics elements based on planar analogues of metamaterials—dielectric metasurfaces, which are two-dimensional arrays of subwavelength nanoparticles with a high refractive index and low absorption coefficient. However, the resonances of such nanoscatterers have usually a low quality factor. Symmetry breaking of particle can lead to the excitation of a high-Q quasi-bound states in the continuum. In this work, we numerically study infrared metasurfaces that support such resonances and are formed by dimers of germanium nanocuboids. The possibility of focusing radiation to a point and line by 300‑nm-thick spherical and cylindrical metalenses is shown.

Nonlocal electro-optic metasurfaces for free-space light modulation

Abstract Dynamic optical metasurfaces with ultrafast temporal response, i.e., spatiotemporal optical metasurfaces, provide attractive solutions and open fascinating perspectives for modern highly integrated optics and photonics. In this work, electro-optically controlled optical metasurfaces operating in reflection and utilizing resonant waveguide mode excitation are demonstrated from the viewpoint of free-space propagating light modulation. The modulation of reflected light power with superior characteristics in comparison with prior research is achieved by identifying a suitable low-loss waveguide mode and exploiting its resonant excitation. The electro-optic Pockels effect in a 300 nm-thick lithium niobate (LN) film sandwiched between a continuous thick gold film and an array of gold nanostripes, serving also as control electrodes, is exploited to realize fast and efficient light modulation. The fabricated compact (active area <1000 µm2) modulators operate in the wavelength range of 850–950 nm, featuring a maximum intensity modulation depth of 42 % at the driving voltage of ±10 V within the bandwidth of 13.5 MHz (with the potential bandwidth of 6.5 GHz). The introduced nonlocal electro-optic metasurface configuration opens new avenues towards the realization of ultrafast, efficient, and robust free-space light modulators based on an LN flat optics approach.

Electrical Phase Modulation Based on Mid‐Infrared Intersubband Polaritonic Metasurfaces

Electrically reconfigurable metasurfaces that overcome the static limitations in controlling the fundamental properties of scattered light are opening new avenues for functional flat optics. This work proposes and experimentally demonstrates electrically phase-tunable mid-infrared metasurfaces based on the polaritonic coupling of Stark-tunable intersubband transitions in semiconductor heterostructures and electromagnetic modes in plasmonic nanoresonators. In the applied voltage range of -3 to +3V, the local phase tuning of the light reflects from the metasurface, which enables the electrical control of the polarization state and wavefront of the reflected wave. Electrical beam polarization control, electrical beam diffraction control, and electrical beam steering are experimentally demonstrated as applications for local phase tunability. The proposed electrically tunable metasurfaces can easily tune the operating wavelength and function at relatively low voltages, which will enable various applications in the mid-infrared region.

High-Quality Infrared Metalenses Based on Germanium Dimers

Modern methods of nanophotonics allow creating miniature devices that change the direction of light propagation, modulate the phase front, and control the outcoming state of the polarization. One of the promising areas of research is the development of flat optics elements based on planar analogues of metamaterials—dielectric metasurfaces, which are two-dimensional arrays of subwavelength nanoparticles with a high refractive index and low absorption coefficient. However, the resonances of such nanoscatterers have usually a low quality factor. Symmetry breaking of particle can lead to the excitation of a high-Q quasi-bound states in the continuum. In this work, we numerically study infrared metasurfaces that support such resonances and are formed by dimers of germanium nanocuboids. The possibility of focusing radiation to a point and line by 300‑nm-thick spherical and cylindrical metalenses is shown.

28.2: Chiral Liquid Crystal Flat Optics Devices for Optical Components and Augmented Reality Displays

Chiral liquid crystal with a short pitch can form gratings with very small period and lead to efficient diffraction over large angels. With this approach flat optical devices may be created for incoupling and outcoupling of light in waveguides. The efficiency in a narrow wavelength range is high, which makes this approach very promising for augmented reality displays.

Photothermal Effect and Phase Transition in VO2 Enhanced by Plasmonic Particles

Phase change metasurfaces based on VO2, which are pre-heated with electric current and optically addressed by projected structured light hologram, are considered to become a new paradigm in programmed THz/middle IR flat optics. Macroscopic quasi-homogeneous arrays of Au nanoparticles show large near IR absorption and a significant photothermal effect capable of boosting a light-triggered switching of VO2 and are to be carefully examined. We propose a new approach to simultaneously probe the altered temperature and electric conductivity of a hybrid Au particle-VO2 film composite by monitoring a phase shift and attenuating a surface acoustic wave in a YX128° cut LiNbO3 substrate. The method shows a temperature resolution of 0.1 °C comparable with the best existing techniques for studying nanoobjects and surfaces. The laser-induced photothermal effects were characterized in a macroscopic array of Au nanostars (AuNSts) with different surface coverage. In a monolayer of 10 nm Au, coupled plasmonic nanoparticles were deposited on the LiNbO3 substrate. An optically triggered insulator-metal transition assisted by photothermal effect in AuNSts/VO2/TiO2/LiNbO3 composites was studied at varied light power. We believe that the proposed SAW-based method is of significant importance for the characterization and optimization of radiation absorbing or/and electrically heated elements of metasurfaces and other devices for lab-on-chip and optical communication/processor technology.

Perspectives on Imaging with Diffractive Flat Optics

Flat optics have been applied successfully for focusing and imaging for many decades. Their recent resurgence is fueled by advances in computational electromagnetics and nanofabrication. In this brief perspective, we summarize the key connections of flat lenses to holograms, describe the current state of diffractive flat lenses, propose an effective metric to compare their performance (the generalized strehl ratio), and muse on potential areas for future research.

Toward Arbitrary Spin‐Orbit Flat Optics Via Structured Geometric Phase Gratings

AbstractReciprocal spin‐orbit coupling (SOC) via geometric phase with flat optics provides a promising platform for shaping and controlling paraxial structured light. Current devices, from the pioneering q‐plates to the recent J‐plates, provide only spin‐dependent wavefront modulation without amplitude control. However, achieving control over all the spatial dimensions of paraxial SOC states requires spin‐dependent control of corresponding complex amplitude, which remains challenging for flat optics. Here, to address this issue, a new type of flat‐optics elements termed structured geometric phase gratings is presented, that is capable of conjugated complex‐amplitude control for orthogonal input circular polarizations. By using a microstructured liquid crystal photoalignment technique, a series of flat‐optics elements is engineered and their excellent precision in arbitrary SOC control is shown. This principle unlocks the full‐field control of paraxial structured light via flat optics, providing a promising way to develop an information exchange and processing units for general photonic SOC states, as well as extra‐/intracavity mode convertors for high‐precision laser beam shaping.

Varifocal Metalens Using Tunable and Ultralow-loss Dielectrics.

The field of flat optics that uses nanostructured, so-called metasurfaces, has seen remarkable progress over the last decade. Chalcogenide phase-change materials (PCMs) offer a promising platform for realizing reconfigurable metasurfaces, as their optical properties can be reversibly tuned. Yet, demonstrations of phase-change metalenses to date have employed material compositions such as Ge2 Sb2 Te5 , which show high absorption in the visible to near-IR wavelengths particularly in their crystalline state, limiting the applicability. Here, by using a low-loss PCM Sb2 Se3 , for the first time, active polarization-insensitive phase-change metalenses at near-IR wavelengths with comparable efficiencies in both material states are shown. An active metalens with a tunable focusing intensity of 95% and a focusing efficiency of 23% is demonstrated. A varifocal metalens is then demonstrated with a tunable focal length from 41 to 123µm with comparable focusing efficiency (5.7% and 3%). The ultralow-loss nature of the material introduces exciting new possibilities for optical communications, multi-depth imaging, beam steering, optical routing, and holography.

Enabling new applications with flat optics

Over the past decade, flat optics, also known as flat metasurfaces, have been a new hot topic in the photonics community. The application opportunities seem nearly limitless. In this article, we will discuss the exciting potential of flat optics for the photonics industry and market. We will also discuss some of the requirements and challenges

Understanding wide field-of-view flat lenses: an analytical solution [Invited

Wide field-of-view (FOV) optics are widely used in various imaging, display, and sensing applications. Conventional wide FOV optics rely on complicated lens assembly comprising multiple elements to suppress coma and other Seidel aberrations. The emergence of flat optics exemplified by metasurfaces and diffractive optical elements (DOEs) offers a promising route to expand the FOV without escalating complexity of optical systems. To date, design of large FOV flat lenses has been relying upon iterative numerical optimization. Here, we derive, for the first time, to the best of our knowledge, an analytical solution to enable computationally efficient design of flat lenses with an ultra-wide FOV approaching 180°. This analytical theory further provides critical insights into working principles and otherwise non-intuitive design trade-offs of wide FOV optics.

Nonlocal flat optics

In electromagnetics and photonics, ‘nonlocality’ refers to the phenomenon by which the response/output of a material or system at a certain point in space depends on the input field across an extended region of space. Although nonlocal effects and the associated wavevector/momentum dependence have often been neglected or seen as a nuisance in the context of metasurfaces, the emerging field of nonlocal flat optics seeks to exploit strong effective nonlocality to enrich and enhance their response. Here we summarize the latest advances in this field, focusing on its fundamental principles and various applications, from optical computing to space compression. The convergence of local and nonlocal flat optics may open exciting opportunities in the quest to control light, in real and momentum space, using ultra-thin platforms. Nonlocal effects—in which the optical response of a system at a given spatial point depends on the field in the surrounding space—are reviewed in the context of metasurfaces and flat optics. Nonlocal flat optics may be useful for controlling light in ultra-thin platforms.

Direct‐Printing Hydrogel‐Based Platform for Humidity‐Driven Dynamic Full‐Color Printing and Holography

AbstractHydrogel materials endow the flat optics platform with active tuning capability, owing to their remarkable humidity‐responsive swelling behavior. Despite recent advances in hydrogel‐based devices for spectral tuning, their complex patterning processing and limited functionality obstruct them from practical applications. Herein, a single‐step direct‐printing technique is originally demonstrated with an active hydrogel material platform for realizing unprecedented multi‐field full‐color display dynamics. Through exploring the dose‐induced shrinkage on polyvinyl alcohol, the stepwise hydrogel nanocavities sandwiched by ultrathin metallic films can be directly printed by grayscale e‐beam lithography. Due to the tunable structural coloration from hydrogel nanocavity scaling, multi‐functionalities are successfully created for optical concealment, dynamic coloring, and dynamic full‐color holography. By encoding the cavity‐dependent transmission phase into the direct‐printed hydrogel platform, it originally enables the projected full‐color holographic dynamics in real‐time by simply exhalation, beyond the static holographic display or monochromatic holographic switching. The proposed active displays can rapidly respond to the surrounding humidity change at a millisecond‐level (< 150 ms). Such a direct‐printing strategy for hydrogel nanocavity represents a critical advance toward the unprecedented dynamic full‐color display, and suggests promising applications in optical security, gas sensing, multispectral imaging, full‐color holography, and next‐generation display techniques.

Electrically controllable diffractive optical elements fabricated by direct laser writing on a carbon nanotube network film

Abstract A randomly connected single-walled carbon nanotube (CNT) network film is suggested as an optically homogenous thin film to implement a tunable diffractive optical element with a subwavelength thickness. A Fresnel zone plate (FZP) as a thin-film lens is successfully realized by mask-free direct laser writing onto the CNT network film with a thickness of 450 nm. The fabricated FZP exhibits an intense three-dimensional focus having lateral and axial focal sizes of 0.95λ and 7.10λ, respectively, at the wavelength of 1550 nm. Furthermore, we show that the intensities at focal points of the first and second diffraction orders can be significantly modulated by 72% and 40% through ion-gel gating between +1.8 V and −1.8 V. These results may offer the potential for electro-optic tunability in multifocal diffraction flat optics and the like.

Diffractive lensless imaging with optimized Voronoi-Fresnel phase.

Lensless cameras are a class of imaging devices that shrink the physical dimensions to the very close vicinity of the image sensor by replacing conventional compound lenses with integrated flat optics and computational algorithms. Here we report a diffractive lensless camera with spatially-coded Voronoi-Fresnel phase to achieve superior image quality. We propose a design principle of maximizing the acquired information in optics to facilitate the computational reconstruction. By introducing an easy-to-optimize Fourier domain metric, Modulation Transfer Function volume (MTFv), which is related to the Strehl ratio, we devise an optimization framework to guide the optimization of the diffractive optical element. The resulting Voronoi-Fresnel phase features an irregular array of quasi-Centroidal Voronoi cells containing a base first-order Fresnel phase function. We demonstrate and verify the imaging performance for photography applications with a prototype Voronoi-Fresnel lensless camera on a 1.6-megapixel image sensor in various illumination conditions. Results show that the proposed design outperforms existing lensless cameras, and could benefit the development of compact imaging systems that work in extreme physical conditions.

Polarization-Tunable Perovskite Light-Emitting Metatransistor.

Emerging immersive visual communication technologies require light sources with complex functionality for dynamic control of polarization, directivity, wavefront, spectrum, and intensity of light. Currently, this is mostly achieved by free space bulk optic elements, limiting the adoption of these technologies. Flat optics based on artificially structured metasurfaces that operate at the sub-wavelength scale are a viable solution, however, their integration into electrically driven devices remains challenging. Here, a radically new approach to monolithic integration of a dielectric metasurface into a perovskite light-emitting transistor is demonstrated. It is shown that nanogratings directly structured on top of the transistor channel yield an 8-fold increase of electroluminescence intensity and dynamic tunability of polarization. This new light-emitting metatransistor device concept opens unlimited opportunities for light management strategies based on metasurface design and integration.