Cheng-Wei Qiu

Three-dimensional optical holography using a plasmonic metasurface

Benefitting from the flexibility in engineering their optical response, metamaterials have been used to achieve control over the propagation of light to an unprecedented level, leading to highly unconventional and versatile optical functionalities compared with their natural counterparts. Recently, the emerging field of metasurfaces, which consist of a monolayer of photonic artificial atoms, has offered attractive functionalities for shaping wave fronts of light by introducing an abrupt interfacial phase discontinuity. Here we realize three-dimensional holography by using metasurfaces made of subwavelength metallic nanorods with spatially varying orientations. The phase discontinuity takes place when the helicity of incident circularly polarized light is reversed. As the phase can be continuously controlled in each subwavelength unit cell by the rod orientation, metasurfaces represent a new route towards high-resolution on-axis three-dimensional holograms with a wide field of view. In addition, the undesired effect of multiple diffraction orders usually accompanying holography is eliminated.

Dual-polarity plasmonic metalens for visible light

Surface topography and refractive index profile dictate the deterministic functionality of a lens. The polarity of most lenses reported so far, that is, either positive (convex) or negative (concave), depends on the curvatures of the interfaces. Here we experimentally demonstrate a counter-intuitive dual-polarity flat lens based on helicity-dependent phase discontinuities for circularly polarized light. Specifically, by controlling the helicity of the input light, the positive and negative polarity are interchangeable in one identical flat lens. Helicity-controllable real and virtual focal planes, as well as magnified and demagnified imaging, are observed on the same plasmonic lens at visible and near-infrared wavelengths. The plasmonic metalens with dual polarity may empower advanced research and applications in helicity-dependent focusing and imaging devices, angular-momentum-based quantum information processing and integrated nano-optoelectronics.

Plasmonic Color Palettes for Photorealistic Printing with Aluminum Nanostructures

We introduce the first plasmonic palette utilizing color generation strategies for photorealistic printing with aluminum nanostructures. Our work expands the visible color space through spatially mixing and adjusting the nanoscale spacing of discrete nanostructures. With aluminum as the plasmonic material, we achieved enhanced durability and dramatically reduced materials costs with our nanostructures compared to commonly used plasmonic materials such as gold and silver, as well as size regimes scalable to higher-throughput approaches such as photolithography and nanoimprint lithography. These advances could pave the way toward a new generation of low-cost, high-resolution, plasmonic color printing with direct applications in security tagging, cryptography, and information storage.

Resolution Enhancement for Inversed Synthetic Aperture Radar Imaging Under Low SNR via Improved Compressive Sensing

The theory of compressed sampling (CS) indicates that exact recovery of an unknown sparse signal can be achieved from very limited samples. For inversed synthetic aperture radar (ISAR), the image of a target is usually constructed by strong scattering centers whose number is much smaller than that of pixels of an image plane. This sparsity of the ISAR signal intrinsically paves a way to apply CS to the reconstruction of high-resolution ISAR imagery. CS-based high-resolution ISAR imaging with limited pulses is developed, and it performs well in the case of high signal-to-noise ratios. However, strong noise and clutter are usually inevitable in radar imaging, which challenges current high-resolution imaging approaches based on parametric modeling, including the CS-based approach. In this paper, we present an improved version of CS-based high-resolution imaging to overcome strong noise and clutter by combining coherent projectors and weighting with the CS optimization for ISAR image generation. Real data are used to test the robustness of the improved CS imaging compared with other current techniques. Experimental results show that the approach is capable of precise estimation of scattering centers and effective suppression of noise.

Achieving Higher Resolution ISAR Imaging With Limited Pulses via Compressed Sampling

Recent theory of compressed sampling (CS) suggests that exact recovery of an unknown sparse signal with overwhelming probability can be achieved from very limited number of samples. In this letter, we adapt this idea and present a framework of high-resolution inverse synthetic aperture radar imaging with limited measured data. During the framework, we mathematically convert the imaging into a problem of signal reconstruction with orthogonal basis; hence, a conceptive upper bound of the cross-range resolution is presented based on the CS theory. Real data results show that the CS imaging approach outperforms the conventional range-Doppler one in resolution.

Single gradientless light beam drags particles as tractor beams.

Usually a light beam pushes a particle when the photons act upon it. This is due to that the electric-dipole particle in the paraxial beam is considered. We investigate the scattering forces in non-paraxial gradientless beams and find that the forces can drag certain particles towards the beam source. The major criterion to be carried out to get the attractive force is the strong nonparaxiality of the light beam. The cone angle denoting the non-paraxiality has been investigated to unveil its importance on achieving dragging force. We hope the attractive forces will be very useful in nanoparticle manipulation.

Ultrathin pancharatnam-berry metasurface with maximal cross-polarization efficiency.

Novel ultrathin dual-functional metalenses are proposed, fabricated, tested, and verified in the microwave regime for the first time. The significance is that their anomalous transmission efficiency almost reaches the theoretical limit of 25%, showing a remarkable improvement compared with earlier ultrathin metasurface designs with less than 5% coupling efficiency. The planar metalens proposed empowers significant reduction in thickness, versatile focusing behavior, and high transmission efficiency simultaneously.

Homogeneous Thermal Cloak with Constant Conductivity and Tunable Heat Localization

Invisible cloak has long captivated the popular conjecture and attracted intensive research in various communities of wave dynamics, e.g., optics, electromagnetics, acoustics, etc. However, their inhomogeneous and extreme parameters imposed by transformation-optic method will usually require challenging realization with metamaterials, resulting in narrow bandwidth, loss, polarization-dependence, etc. In this paper, we demonstrate that thermodynamic cloak can be achieved with homogeneous and finite conductivity only employing naturally available materials. It is demonstrated that the thermal localization inside the coating layer can be tuned and controlled robustly by anisotropy, which enables an incomplete cloak to function perfectly. Practical realization of such homogeneous thermal cloak has been suggested by using two naturally occurring conductive materials, which provides an unprecedentedly plausible way to flexibly realize thermal cloak and manipulate heat flow with phonons.

Three-dimensional plasmonic stereoscopic prints in full colour

Metal nanostructures can be designed to scatter different colours depending on the polarization of the incident light. Such spectral control is attractive for applications such as high-density optical storage, but challenges remain in creating microprints with a single-layer architecture that simultaneously enables full-spectral and polarization control of the scattered light. Here we demonstrate independently tunable biaxial colour pixels composed of isolated nanoellipses or nanosquare dimers that can exhibit a full range of colours in reflection mode with linear polarization dependence. Effective polarization-sensitive full-colour prints are realized. With this, we encoded two colour images within the same area and further use this to achieve depth perception by realizing three-dimensional stereoscopic colour microprint. Coupled with the low cost and durability of aluminium as the functional material in our pixel design, such polarization-sensitive encoding can realize a wide spectrum of applications in colour displays, data storage and anti-counterfeiting technologies.

Hybrid bilayer plasmonic metasurface efficiently manipulates visible light

Two highly coupled plasmonic metasurfaces exhibit much higher conversion efficiency and extinction ratio than individual ones. Metasurfaces operating in the cross-polarization scheme have shown an interesting degree of control over the wavefront of transmitted light. Nevertheless, their inherently low efficiency in visible light raises certain concerns for practical applications. Without sacrificing the ultrathin flat design, we propose a bilayer plasmonic metasurface operating at visible frequencies, obtained by coupling a nanoantenna-based metasurface with its complementary Babinet-inverted copy. By breaking the radiation symmetry because of the finite, yet small, thickness of the proposed structure and benefitting from properly tailored intra- and interlayer couplings, such coupled bilayer metasurface experimentally yields a conversion efficiency of 17%, significantly larger than that of earlier single-layer designs, as well as an extinction ratio larger than 0 dB, meaning that anomalous refraction dominates the transmission response. Our finding shows that metallic metasurface can counterintuitively manipulate the visible light as efficiently as dielectric metasurface (~20% in conversion efficiency in Lin et al.’s study), although the metal’s ohmic loss is much higher than dielectrics. Our hybrid bilayer design, still being ultrathin (~λ/6), is found to obey generalized Snell’s law even in the presence of strong couplings. It is capable of efficiently manipulating visible light over a broad bandwidth and can be realized with a facile one-step nanofabrication process.