Metasurface-Assisted Quantum Nonlocal Weak-Measurement Microscopy.

In standard quantum weak measurements, preselection and postselection of quantum states are implemented in the same photon. Here we go beyond this restrictive setting and demonstrate that the preselection and postselection can be performed in two different photons, if the two photons are polarization entangled. The Pancharatnam-Berry phase metasurface is incorporated in the weak measurement system to perform weak coupling between probe wave function and spin observable. By introducing nonlocal weak measurement into the microscopy imaging system, it allows us to remotely switch different microscopy imaging modes of pure-phase objects, including bright-field, differential, and phase reconstruction. Furthermore, we demonstrate that the nonlocal weak-measurement scheme can prevent almost all environmental noise photons from detection and thus achieves a higher image contrast than the standard scheme at a low photon level. Our results provide the possibility to develop a quantum nonlocal weak-measurement microscope for label-free imaging of transparent biological samples.

Surface topography detection based on an optical differential metasurface.

Surface topography detection can extract critical characteristics from objects, playing an important role in target identification and precision measurement. Here, an optical method with the advantages of low power consumption, high speed, and simple devices is proposed to realize the surface topography detection of low-contrast phase objects. By constructing reflected light paths, a metasurface can perform spatial differential operation via receiving the light directly reflected from a target. Therefore, our scheme is experimentally demonstrated as having remarkable universality, which can be used not only for opaque objects, but also for transparent pure phase objects. It provides a new, to the best of our knowledge, application for optical differential metasurfaces in precise detection of microscale surface topography.

Optical analog computing enabled broadband structured light.

Mathematically, any function can be expressed as the operation form of another function. Here, the idea is introduced into an optical system to generate structured light. In the optical system, a mathematical function is represented by an optical field distribution, and any structured light field can be generated by performing different optical analog computations for any input optical field. In particular, optical analog computing has a good broadband performance, as it can be achieved based on the Pancharatnam-Berry phase. Therefore, our scheme can provide a flexible way to generate broadband structured light, and this is theoretically and experimentally demonstrated. It is envisioned that our work may inspire potential applications in high-resolution microscopy and quantum computation.

Computing metasurfaces enabled chiral edge image sensing.

Computing metasurfaces have shown the extraordinary ability to precisely perform optical analog operations to the input light wave, and therefore exhibit greater potentials toward sensing applications. Here, we propose a unique application of computing metasurface for chiral edge sensing by incorporating a weak-value amplification technique. The computing metasurface performs the spatial differentiation operations of phase objects and extracts the edge-enhanced images, because the phase gradient generally occurs at the edge. The chirality-induced polarization rotation acts as the preselection state and the spatial differentiation operations in the metasurface provide weak coupling. The amplified pointer shift related to the tiny polarization rotation will eventually lead to an asymmetric edge-enhanced image. Owing to the high sensitivity of the weak-value amplification, we experimentally demonstrate a high-contrast recognition of chirality by edge detection, which may have potential applications in real-time measurement and separation of chiral enantiomers.

Intrinsic Optical Spatial Differentiation Enabled Quantum Dark-Field Microscopy.

By solving the Maxwell's equations in Fourier space, we find that the cross-polarized component of the dipole scattering field can be written as the second-order spatial differentiation of the copolarized component. This differential operation can be regarded as intrinsic which naturally arises as consequence of the transversality of electromagnetic fields. By introducing the intrinsic spatial differentiation into heralded single-photon microscopy imaging technique, it makes the structure of pure-phase object clearly visible at low photon level, avoiding any biophysical damages to living cells. Based on the polarization entanglement, the switch between dark-field imaging and bright-field imaging is remotely controlled in the heralding arm. This research enriches both fields of optical analog computing and quantum microscopy, opening a promising route toward a nondestructive imaging of living biological systems.

Examining the optical model of graphene via the photonic spin Hall effect.

In modern optics, there are two general models to describe the behavior of light in graphene: the zero-thickness model and the slab model. The difference in physical phenomena predicted by the two models is very small, which is hardly distinguished by traditional measurement methods. Therefore, which model can describe the light-matter interaction in graphene more exactly is still a challenging issue. In this work, based on the sensitive optical phenomenon called the photonic spin Hall effect, the small difference can be magnified to a detectable level by the weak-value amplification. The experimental results show that the zero-thickness model can more accurately describe the interaction between light and monolayer or bilayer graphene, while the case of more than two layers, which can no longer be regarded as two-dimensional thickness, should be described by the slab model. Our results may provide information on light interacting with graphene for future investigation in photonics and optoelectronics.

Realization of tunable edge-enhanced images based on computing metasurfaces.

Bright-field imaging and edge imaging can extract different characteristics from objects, and therefore play important roles in image processing and pattern recognition. Here, we propose a fast, convenient, and electrically driven adjustable scheme to achieve tunable edge-enhanced images based on computing metasurfaces. The computing metasurface can perform spatial differential operation as optical waves propagate through it. This optical differential operation is polarization-dependent, thus any desirable contrast can be realized by the interplay between two orthogonal polarization components. By regulating the external voltages applied on the liquid-crystal phase plate, different phase retardances between two orthogonal polarization components are introduced; this allows us to quickly switch between the bright-field image and the edge image.

Visualization of transparent particles based on optical spatial differentiation.

Optical analog computing operates on the amplitude, phase, polarization, and frequency distributions of the electromagnetic field through the interaction of light and matter. The differentiation operation is widely used in all-optical image processing technology, such as edge detection. Here, we propose a concise way to observe transparent particles, incorporating the optical differential operation that occurs on a single particle. The particle's scattering and cross-polarization components combine into our differentiator. We achieve high-contrast optical images of transparent liquid crystal molecules. The visualization of aleurone grains (the structures that store protein particles in plant cells) in maize seed was experimentally demonstrated with a broadband incoherent light source. Avoiding the interference of stains, our designed method provides the possibility to observe protein particles directly in complex biological tissues.

Realization of all-optical higher-order spatial differentiators based on cascaded operations.

Cascaded operations play an important role in traditional electronic computing systems for the realization of advanced strategies. Here, we introduce the idea of cascaded operations into all-optical spatial analog computing. The single function of the first-order operation has difficulty meeting the requirements of practical applications in image recognition. The all-optical second-order spatial differentiators are implemented by cascading two first-order differential operation units, and the image edge detection of amplitude and phase objects are demonstrated. Our scheme provides a possible pathway toward the development of compact multifunctional differentiators and advanced optical analog computing networks.

Two-dimensional optical spatial differentiation and high-contrast imaging.

Optical analog signal processing technology has been widely studied and applied in a variety of science and engineering fields, with the advantages of overcoming the low-speed and high-power consumption associated with its digital counterparts. Much attention has been given to emerging metasurface technology in the field of optical imaging and processing systems. Here, we demonstrate, for the first time, broadband two-dimensional spatial differentiation and high-contrast edge imaging based on a dielectric metasurface across the whole visible spectrum. This edge detection method works for both intensity and phase objects simply by inserting the metasurface into a commercial optical microscope. This highly efficient metasurface performing a basic optical differentiation operation opens up new opportunities in applications of fast, compactible and power-efficient ultrathin devices for data processing and biological imaging.

Metasurface enabled quantum edge detection.

Metasurfaces consisting of engineered dielectric or metallic structures provide unique solutions to realize exotic phenomena including negative refraction, achromatic focusing, electromagnetic cloaking, and so on. The intersection of metasurface and quantum optics may lead to new opportunities but is much less explored. Here, we propose and experimentally demonstrate that a polarization-entangled photon source can be used to switch ON or OFF the optical edge detection mode in an imaging system based on a high-efficiency dielectric metasurface. This experiment enriches both fields of metasurface and quantum optics, representing a promising direction toward quantum edge detection and image processing with remarkable signal-to-noise ratio.

Optical analog computing of two-dimensional spatial differentiation based on the Brewster effect.

Optical analog computing has attracted widespread attention in recent decades due to its advantages of lower consumption, higher efficiency, and real-time imaging in image processing. Here, we propose a two-dimensional optical analog computing scheme based on the Brewster effect. We experimentally demonstrate two-dimensional edge detection with high efficiency. By combining microscopy, our approach may develop some significant applications in cellular and molecular imaging.

Vortex generation in the spin-orbit interaction of a light beam propagating inside a uniaxial medium: origin and efficiency.

It has been known that an optical vortex with a topological charge ±2 can be generated as a circularly polarized (CP) light beam propagates in a bulk uniaxial crystal, but its physical origin remains obscure which also hinders its practical applications. Here, through a rigorous full-wave analyses on the problem, we show that, as a CP beam possessing a particular spin (handedness) propagates inside a uniaxial crystal, two beams with opposite spins can be generated caused by the unique spin-sensitive light-matter interactions in the anisotropic medium. Flipping the spin can offer the light beam an vortex phase with a topological charge of ±2 owing to the Pancharatnam-Berry mechanism, with efficiency dictated by the material properties of the uniaxial medium and the topological structure of the beam itself. With its physical origin fully uncovered, we finally discuss how to improve the efficiency of such effect, and compare the mechanisms of vortex generations in different systems. Our findings not only provide deeper understandings on such an intriguing effect, but also shed light on other spin-orbit-interaction-induced effects.

Spatial differential operation and edge detection based on the geometric spin Hall effect of light.

Unlike the conventional spin Hall effect of light (SHEL) originating from the light-matter interaction, the spin-dependent splitting in the geometric SHEL is purely a geometric effect and independent from the properties of matter. Here it is shown that the geometric SHEL is not only of fundamental theoretical interest in understanding the spin-orbit interaction of light, but also sheds light on important technological applications. This Letter describes the theoretical foundation and experimental realization of optical differential operation and one-dimensional edge detection based on the geometric SHEL.

A spin controlled wavefront shaping metasurface with low dispersion in visible frequencies.

Similar to amplitude and phase, optical spin plays an important and non-trivial role in optics, which has been widely demonstrated in wavefront engineering, creation of new optical components, and sensitive optical metrology. In this work, we propose and experimentally demonstrate a new type of spin controlled wavefront shaping metasurface. The proposed geometric phase metasurface is designed by employing the integrated and interleaved structures to independently control the left-handed and right-handed spin components. As an exemplary demonstration, our experimental results show that such a composite metasurface can convert a plane wave into a vortex beam and a Hermite beam for left-handed and right-handed polarized light, respectively. Because such a metasurface is made from non-resonant dielectric structures, it can work for broadband frequencies with very low dispersion. The proposed metasurface is fabricated by the laser writing method inside transparent glass with a low cost, which avoids the typical high-resolution lithography process. This spin dependent broadband wavefront shaping metasurface may find potential applications in optical communications, information processing, and optical metrology.

Optical edge detection based on high-efficiency dielectric metasurface.

Optical edge detection is a useful method for characterizing boundaries, which is also in the forefront of image processing for object detection. As the field of metamaterials and metasurface is growing fast in an effort to miniaturize optical devices at unprecedented scales, experimental realization of optical edge detection with metamaterials remains a challenge and lags behind theoretical proposals. Here, we propose a mechanism of edge detection based on a Pancharatnam-Berry-phase metasurface. We experimentally demonstrated broadband edge detection using designed dielectric metasurfaces with high optical efficiency. The metasurfaces were fabricated by scanning a focused laser beam inside glass substrate and can be easily integrated with traditional optical components. The proposed edge-detection mechanism may find important applications in image processing, high-contrast microscopy, and real-time object detection on compact optical platforms such as mobile phones and smart cameras.

Large in-plane asymmetric spin angular shifts of a light beam near the critical angle.

The photonic spin Hall effect (SHE) manifests itself as the transverse and in-plane spin-dependent shifts of a light beam. Normally, the spin shifts are tiny due to the weak spin-orbit coupling. Therefore, it is very important and interesting to explore some effective methods for enhancing this phenomenon. In this Letter, we theoretically propose and experimentally verify a simple method for obtaining large and asymmetric in-plane spin angular shifts when an arbitrary linearly polarized beam reflects near the critical angle (for total internal reflection). The universal expressions of spatial and angular shifts are deduced. Remarkably, by modulating the incident and polarization angles, the left- and right-handed circularly polarized components can be distinguished directly.

Transitional Goos-Hänchen effect due to the topological phase transitions.

We examine the Goos-Hänchen (GH) effect for a Gaussian beam impinging on the surface of silicene whose topological phase transitions can be modulated by external electric field and/or irradiating circular polarized light. It is shown that both the spatial and angular shifts in GH effect present a sharp jump due to the topological phase transitions. The transitional GH effect can be attributed to transitional optical conductivity, which relates to Berry curvature and Chern numbers. These results can be extensively extended to other two-dimensional atomic crystals in graphene family. We believe that the transitional GH effect may offer a possible way to determine the Berry curvature, Chern numbers, and topological phase transition by a direct optical measurement.

Electrically driven generation of arbitrary vector vortex beams on the hybrid-order Poincaré sphere.

We propose a simple, efficient, and fast tunable method to generate arbitrary vector vortex beams on the hybrid-order Poincaré sphere in an electrically driven way. The scheme incorporates the tunability and switching capabilities of liquid crystals into dielectric metasurfaces to form an efficient vector vortex beam generator. By applying certain voltages on the liquid crystal phase retarder, the generator converts a linearly polarized Gaussian beam into any desirable vector vortex beams. We demonstrate that the evolution route of the corresponding vector vortex states is just a closed circuit on the hybrid-order Poincaré sphere when the phase retardation varies from 0 to 2π. Several special cases are selected to demonstrate our scheme, and the experimental results coincide well with the theoretical predictions.

Structured thermal surface for radiative camouflage.

Thermal camouflage has been successful in the conductive regime, where thermal metamaterials embedded in a conductive system can manipulate heat conduction inside the bulk. Most reported approaches are background-dependent and not applicable to radiative heat emitted from the surface of the system. A coating with engineered emissivity is one option for radiative camouflage, but only when the background has uniform temperature. Here, we propose a strategy for radiative camouflage of external objects on a given background using a structured thermal surface. The device is non-invasive and restores arbitrary background temperature distributions on its top. For many practical candidates of the background material with similar emissivity as the device, the object can thereby be radiatively concealed without a priori knowledge of the host conductivity and temperature. We expect this strategy to meet the demands of anti-detection and thermal radiation manipulation in complex unknown environments and to inspire developments in phononic and photonic thermotronics.