Hyperbolic metamaterials: fusing artificial structures to natural 2D materials

Optical metamaterials have presented an innovative method of manipulating light. Hyperbolic metamaterials have an extremely high anisotropy with a hyperbolic dispersion relation. They are able to support high-k modes and exhibit a high density of states which produce distinctive properties that have been exploited in various applications, such as super-resolution imaging, negative refraction, and enhanced emission control. Here, state-of-the-art hyperbolic metamaterials are reviewed, starting from the fundamental principles to applications of artificially structured hyperbolic media to suggest ways to fuse natural two-dimensional hyperbolic materials. The review concludes by indicating the current challenges and our vision for future applications of hyperbolic metamaterials.

Surface Dyakonov–Cherenkov radiation

Recent advances in engineered material technologies (e.g., photonic crystals, metamaterials, plasmonics, etc.) provide valuable tools to control Cherenkov radiation. In all these approaches, however, the particle velocity is a key parameter to affect Cherenkov radiation in the designed material, while the influence of the particle trajectory is generally negligible. Here, we report on surface Dyakonov–Cherenkov radiation, i.e. the emission of directional Dyakonov surface waves from a swift charged particle moving atop a birefringent crystal. This new type of Cherenkov radiation is highly susceptible to both the particle velocity and trajectory, e.g. we observe a sharp radiation enhancement when the particle trajectory falls in the vicinity of a particular direction. Moreover, close to the Cherenkov threshold, such a radiation enhancement can be orders of magnitude higher than that obtained in traditional Cherenkov detectors. These distinct properties allow us to determine simultaneously the magnitude and direction of particle velocities on a compact platform. The surface Dyakonov–Cherenkov radiation studied in this work not only adds a new degree of freedom for particle identification, but also provides an all-dielectric route to construct compact Cherenkov detectors with enhanced sensitivity.

Highly sensitive force measurements in an optically generated, harmonic hydrodynamic trap

The use of optical tweezers to measure forces acting upon microscopic particles has revolutionised fields from material science to cell biology. However, despite optical control capabilities, this technology is highly constrained by the material properties of the probe, and its use may be limited due to concerns about the effect on biological processes. Here we present a novel, optically controlled trapping method based on light-induced hydrodynamic flows. Specifically, we leverage optical control capabilities to convert a translationally invariant topological defect of a flow field into an attractor for colloids in an effectively one-dimensional harmonic, yet freely rotatable system. Circumventing the need to stabilise particle dynamics along an unstable axis, this novel trap closely resembles the isotropic dynamics of optical tweezers. Using magnetic beads, we explicitly show the existence of a linear force-extension relationship that can be used to detect femtoNewton-range forces with sensitivity close to the thermal limit. Our force measurements remove the need for laser-particle contact, while also lifting material constraints, which renders them a particularly interesting tool for the life sciences and engineering.

Phonon scattering and exciton localization: molding exciton flux in two dimensional disorder energy landscape

Two dimensional excitonic devices are of great potential to overcome the dilemma of response time and integration in current generation of electron or/and photon based systems. The ultrashort diffusion length of exciton arising from ultrafast relaxation and low carrier mobility greatly discounts the performance of excitonic devices. Phonon scattering and exciton localization are crucial to understand the modulation of exciton flux in two dimensional disorder energy landscape, which still remain elusive. Here, we report an optimized scheme for exciton diffusion and relaxation dominated by phonon scattering and disorder potentials in WSe2 monolayers. The effective diffusion coefficient is enhanced by > 200% at 280 K. The excitons tend to be localized by disorder potentials accompanied by the steadily weakening of phonon scattering when temperature drops to 260 K, and the onset of exciton localization brings forward as decreasing temperature. These findings identify that phonon scattering and disorder potentials are of great importance for long-range exciton diffusion and thermal management in exciton based systems, and lay a firm foundation for the development of functional excitonic devices.

Phyllotaxis-inspired nanosieves with multiplexed orbital angular momentum

Nanophotonic platforms such as metasurfaces, achieving arbitrary phase profiles within ultrathin thickness, emerge as miniaturized, ultracompact and kaleidoscopic optical vortex generators. However, it is often required to segment or interleave independent sub-array metasurfaces to multiplex optical vortices in a single nano-device, which in turn affects the device’s compactness and channel capacity. Here, inspired by phyllotaxis patterns in pine cones and sunflowers, we theoretically prove and experimentally report that multiple optical vortices can be produced in a single compact phyllotaxis nanosieve, both in free space and on a chip, where one meta-atom may contribute to many vortices simultaneously. The time-resolved dynamics of on-chip interference wavefronts between multiple plasmonic vortices was revealed by ultrafast time-resolved photoemission electron microscopy. Our nature-inspired optical vortex generator would facilitate various vortex-related optical applications, including structured wavefront shaping, free-space and plasmonic vortices, and high-capacity information metaphotonics.

Large-scale phase retrieval

High-throughput computational imaging requires efficient processing algorithms to retrieve multi-dimensional and multi-scale information. In computational phase imaging, phase retrieval (PR) is required to reconstruct both amplitude and phase in complex space from intensity-only measurements. The existing PR algorithms suffer from the tradeoff among low computational complexity, robustness to measurement noise and strong generalization on different modalities. In this work, we report an efficient large-scale phase retrieval technique termed as LPR. It extends the plug-and-play generalized-alternating-projection framework from real space to nonlinear complex space. The alternating projection solver and enhancing neural network are respectively derived to tackle the measurement formation and statistical prior regularization. This framework compensates the shortcomings of each operator, so as to realize high-fidelity phase retrieval with low computational complexity and strong generalization. We applied the technique for a series of computational phase imaging modalities including coherent diffraction imaging, coded diffraction pattern imaging, and Fourier ptychographic microscopy. Extensive simulations and experiments validate that the technique outperforms the existing PR algorithms with as much as 17dB enhancement on signal-to-noise ratio, and more than one order-of-magnitude increased running efficiency. Besides, we for the first time demonstrate ultra-large-scale phase retrieval at the 8K level ($$7680\times 4320$$ 7680 × 4320 pixels) in minute-level time.

Highlighting photonics: looking into the next decade

Let there be light–to change the world we want to be! Over the past several decades, and ever since the birth of the first laser, mankind has witnessed the development of the science of light, as light-based technologies have revolutionarily changed our lives. Needless to say, photonics has now penetrated into many aspects of science and technology, turning into an important and dynamically changing field of increasing interdisciplinary interest. In this inaugural issue of eLight, we highlight a few emerging trends in photonics that we think are likely to have major impact at least in the upcoming decade, spanning from integrated quantum photonics and quantum computing, through topological/non-Hermitian photonics and topological insulator lasers, to AI-empowered nanophotonics and photonic machine learning. This Perspective is by no means an attempt to summarize all the latest advances in photonics, yet we wish our subjective vision could fuel inspiration and foster excitement in scientific research especially for young researchers who love the science of light.

Planar liquid crystal polarization optics for augmented reality and virtual reality: from fundamentals to applications

Planar and ultrathin liquid crystal (LC) polarization optical elements have found promising applications in augmented reality (AR), virtual reality (VR), and photonic devices. In this paper, we give a comprehensive review on the operation principles, device fabrication, and performance of these optical elements. Optical simulations methods for optimizing the device performance are discussed in detail. Finally, some potential applications of these devices in AR and VR systems are illustrated and analyzed.