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The term Poincaré beam, which describes the space-variant polarization of a light beam carrying spin angular momentum (SAM) and orbital angular momentum (OAM), plays an important role in various optical applications. Since the radius of a Poincaré beam conventionally depends on the topological charge number, it is difficult to generate a stable and high-quality Poincaré beam by two optical vortices with different topological charge numbers, as the Poincaré beam formed in this way collapses upon propagation. Here, based on an all-dielectric metasurface platform, we experimentally demonstrate broadband generation of a generalized perfect Poincaré beam (PPB), whose radius is independent of the topological charge number. By utilizing a phase-only modulation approach, a single-layer spin-multiplexed metasurface is shown to achieve all the states of PPBs on the hybrid-order Poincaré Sphere for visible light. Furthermore, as a proof-of-concept demonstration, a metasurface encoding multidimensional SAM and OAM states in the parallel channels of elliptical and circular PPBs is implemented for optical information encryption. We envision that this work will provide a compact and efficient platform for generation of PPBs for visible light, and may promote their applications in optical communications, information encryption, optical data storage and quantum information sciences., Well-controlled Poincaré beams are potentially useful in optical communications applications. Here, the authors present phase-only metasurfaces that generate broadband, perfect Poincaré beams in the visible, with radius independent of the topological number.

Monochromatic light can be characterized by its three fundamental properties: amplitude, phase, and polarization. In this work, we propose a versatile, transmission-mode all-dielectric metasurface platform that can independently manipulate the phase and amplitude for two orthogonal states of polarization in the visible frequency range. For proof-of-concept experimental demonstration, various single-layer metasurfaces composed of subwavelength-spaced titanium-dioxide nanopillars are designed, fabricated, and characterized to exhibit the ability of polarization-switchable multidimensional light-field manipulation, including polarization-switchable grayscale nanoprinting, nonuniform cylindrical lensing, and complex-amplitude holography. We envision the metasurface platform demonstrated here to open new possibilities toward creating compact multifunctional optical devices for applications in polarization optics, information encoding, optical data storage, and security.

Structural colors emerge when a particular wavelength range is filtered out from a broadband light source. It is regarded as a valuable platform for color display and digital imaging due to the benefits of environmental friendliness, higher visibility, and durability. However, current devices capable of generating colors are all based on direct transmission or reflection. Material loss, thick configuration, and the lack of tunability hinder their transition to practical applications. In this paper, a novel mechanism that generates high-purity colors by photon spin restoration on ultrashallow plasmonic grating is proposed. We fabricated the sample by interference lithography and experimentally observed full color display, tunable color logo imaging, and chromatic sensing. The unique combination of high efficiency, high-purity colors, tunable chromatic display, ultrathin structure, and friendliness for fabrication makes this design an easy way to bridge the gap between theoretical investigations and daily-life applications.

Metasurfaces used in the manipulation of light beams have attracted growing interests owing to their unique electromagnetic properties in the subwavelength regime. However, most previously demonstrated single-layer metasurfaces are normally designed to realize one-fold function of either polarization or phase manipulation and suffer from low cross-polarization conversion efficiency and high-background, especially for transmissive metasurfaces. Here, a metasurface based on metal-insulator-metal (MIM) subwavelength grating is proposed to simultaneously achieve polarization filtering and phase controlling. The transmission coefficient reaches up to 78.9% and the polarization extinction ratio (ER = 20*log(T TM /T TE) is larger than 16.1 dB. A local abrupt phase difference covering 0–2π is introduced into transmitted light with the polarization direction vertical to the grating by artificially tailoring the geometrical parameters of MIM grating. Furthermore, background-free wavefront control and high-purity radial/azimuthal polarization are realized by the metasurfaces based on the MIM grating. This flexible and high-efficient scheme of full control wavefront and polarization promises an unprecedented progress of spatial vectorial beams modulation and enable the realization of novel optical components.

Simultaneous manipulation of light polarization and phase possesses great significance for modern optical technologies. Here we propose metal-dielectric gratings capable of tailoring the polarization and phase of light based on polarization filtering with the polarization extinction ratio exceeding 10 dB and the transmittance higher than 65%. The standard radially polarized (RP) light with axially symmetric polarization distribution can be generated by using arrayed grating with gradient phase under the illumination of circularly polarized (CP) light. Besides, a RP metasurface lens with high numerical aperture is presented, which can convert the CP light into the focused radially polarized light at wavelength of 632.8 nm. This compact and efficient approach would have potential applications in beam manipulation, super-resolution imaging and integrated optics system.

Plasmonic color generation utilizing ultra-thin metasurfaces as well as metallic nanoparticles hold a great promise for a wide range of applications, including color displays, data storage, and information encryption due to its high spatial resolution and mechanical/chemical stability. Most of the recently demonstrated systems generate static colors; however, more advanced applications such as data storage require fast and flexible means to tune the plasmonic colors, while keeping them vibrant and stable. Here, a surface-relief aluminum metasurface that reflects polarization-tunable plasmonic colors is designed and experimentally demonstrated. Excitation of localized surface plasmons encodes discrete combinations of the incident and reflected polarized light into diverse colors. A single storage unit - a nanopixel - stores a multiple-bit piece of information in the orientation of its constituent nanoantennae. This information is then reliably retrieved by inspecting the reflected color sequence with two linear polarizers. It is the broad color variability and high spatial resolution of the proposed encoding approach that supports a strong promise for rapid parallel read-out and encryption of high-density optical data. Our method also enables the robust generation of dynamic kaleidoscopic images with no detrimental "cross-talk" effect. The approach opens up a new route for advanced dynamic steganography, high-density parallel-access optical data storage, and optical information encryption.

Student contribution: Plasmonic systems are efficient in converting optical energy into heat hence show technological significance in solar thermophotovoltaics, nanoparticle manipulation, and photocatalysis, etc. Conventional techniques to characterize plasmonic heaters are mostly thermal camera- and thermographic phosphor (TGP)- based. In this work, we present our results of characterizing a plasmonic heater using thermoreflectance imaging (TRI). The TRI technique presented here outperforms thermal camera-based technique in spatial resolution due to the visible light utilized for illumination, and does not require special sample preparation as in TGP-based technique. We chose to use a gap plasmon structure to maximize the optical absorption, and fabricated structures with various dimensions that exhibit varying optical absorptions at a fixed wavelength of 825 nm, which is the wavelength of pump light used in the TRI measurement. The TRI setup uses a millisecond-modulated continuous-wave pump laser to induce local temperature fluctuation on the sample surface, a 530 nm LED probe light then senses the change in the temperature-dependent material reflectance between high and low temperatures, which combined with a pre-calibrated thermoreflectance coefficient can be used to calculate the temperature rise on each image pixel. This technique grants us a resolution of ~200 nm. The experimentally obtained temperature rise on various gap plasmon structures correlates well with their optical absorption, and we compare the results against a finite element heat transfer model. Using a separate pump-probe thermoreflectance technique, we experimentally obtain the heat transfer dynamics of such gap plasmon structure under laser irradiation with picosecond resolution.

In order to develop metasurfaces and fully analyze their functionalities, it is important to have an accurate and efficient model to describe a unit cell, as well as possibility to realize large scale device performance analysis. In this work we propose bi-anisotropic homogenization method suitable for large-scale simulations in frequency and time domains. First, the proposed method is probed with various types of metasurface designs at normal incidence in frequency domain. Then, we demonstrate its generalization for oblique incidence and extend this technique to time-domain simulations.

We demonstrate an optimization process to obtain subtractive reflective color filters based on plasmonic metasurface arranged of silver cylindrical nanoantennas. In our study, colors are produced by absorbing light of specific wavelengths and reflecting the remaining spectrum. Accordingly, we can get different subtractive color filters from the reflected spectrum such as yellow, magenta, and cyan. Our results show that we obtain higher absorption resonance peaks (reflection dips) which give good performance for the proposed devices.

Terahertz (THz) absorber with dynamically tunable bandwidth possesses huge application value in the fields of switches, sensors, and THz detection. However, the perfect absorbers based on photonic crystals and metamaterials are not intelligent enough to capture the electromagnetic wave in a tunable way. In this paper, we utilized only patterned graphene to tune the absorption positions and the bandwidth in the terahertz regime. More distinguished than some dynamic absorbers proposed before, the performances with peak frequency relative tuning range of 68 % and nearly unity absorbance are obtained by a single cross-shaped graphene layer. Additionally, the working bandwidth can be broadened with stacked structured graphene. The almost perfect absorption shifted from 2.36∼3.2 to 3.26∼3.99 THz continuously via changing the chemical potential of graphene.

The past decade has witnessed a great deal of optical systems designed for exceeding the Abbe's diffraction limit. Unfortunately, a deep subwavelength spot is obtained at the price of extremely short focal length, which is indeed a near-field diffraction limit that could rarely go beyond in the nanofocusing device. One method to mitigate such a problem is to set up a rapid oscillatory electromagnetic field that converges at the prescribed focus. However, abrupt modulation of phase and amplitude within a small fraction of a wavelength seems to be the main obstacle in the visible regime, aggravated by loss and plasmonic features that come into function. In this paper, we propose a periodically repeated ring-disk complementary structure to break the near-field diffraction limit via plasmonic Fano resonance, originating from the interference between the complex hybrid plasmon resonance and the continuum of propagating waves through the silver film. This plasmonic Fano resonance introduces a pi phase jump in the adjacent channels and amplitude modulation to achieve radiationless electromagnetic interference. As a result, deep subwavelength spots as small as 0.0045 lambda(2) at 36 nm above the silver film have been numerically demonstrated. This plate holds promise for nanolithography, subdiffraction imaging and microscopy.

Since ancient times, plasmonic structural coloring has inspired humanity; glassmakers achieved vibrant colors by doping glass with metal nanoparticles to craft beautiful objects such as the Roman Lycurgus cup and stained glass. These lovely color filtering effects are a consequence of the resonant coupling of light and free electrons in metal nanoparticles, known as surface plasmons. Thanks to the continuing improvement of nanofabrication technology, the dimensions of nanoparticles and structures can now be precisely engineered to form “optical nanoantennas,” allowing for control of optical response at an unprecedented level. Recently, the field of plasmonic structural coloring has seen extensive growth. In this review, we provide an up-to-date overview of various plasmonic color filtering approaches and highlight their uses in a broad palette of applications. Various surface plasmon resonance modes employed in the plasmonic color filtering effect are discussed. We first review the development of the pioneering static plasmonic colors achieved with invariant optical nanoantennas and ambient environment, then we address a variety of emerging approaches that enable dynamic color tuning, erasing, and restoring. These dynamic color filters are capable of actively changing the filtered colors and carrying more color information states than the static systems. Thus, they open an avenue to high-density data storage, information encryption, and plasmonic information processing. Finally, we discuss the challenges and future perspectives in this exciting research area.

Plasmonic color generation based on gap-plasmon resonators finds an increasing role in subwavelength fade-free color printing, data storage and information encoding due to its high spatial resolution and mechanical/chemical stability. However, most of the current designs are limited to trivial spectral responses, leading to subtractive colors with restricted ranges of their color palettes. Here, we design a plasmonic color metasurface made of an aluminum resonator array, producing strong gap plasmon resonance and nearly perfect light absorption that results in enhanced color saturation. The range of color palette is broadly increased by introducing polarization-dependent supercell of plasmonic resonators, forming “i-patterned” dimers. Such a plasmonic color metasurface holds a great promise for color displays, polarization-multiplexing system, image encryption, and steganography.

We develop and explore a nonlinear regression modeling approach to designing subtractive color filters (SCFs) based on plasmonic metasurfaces. The approach opens up the possibility of rapidly choosing a desired optimized SCF design with high color saturation and brightness using an analytical expression. In this Letter, colors are produced by absorbing the light of specific wavelengths and reflecting the remaining spectrum with silver gap-plasmon nanoantennas deposited on a silver film. First, we design three different SCFs-yellow, magenta, and cyan. Then, by adjusting the design parameters of the nanoantennas, we optimize their high absorption resonance peaks (reflections dips), which are tunable over the visible spectrum. Finally, by using nonlinear regression analysis, we fit our results to a cubic regression model. Accordingly, a SCF for a color of choice can be designed in a straightforward way. This is a promising technique that provides a methodology to design preoptimized filters for practical applications such as color printing, high-resolution chromatic displays, and multi-spectral imaging.

Manipulation of the polarization state using electromagnetic metasurface has attracted considerable attention in recent years. However, most previously demonstrated single-layer polarization conversion metasurfaces suffer from low polarization conversion efficiency, narrow operation bandwidth, or huge fabrication challenges, especially for the visible and near-infrared frequencies. In this letter, a broadband and high-efficiency reflective linear polarization converter composed of ellipse-shaped plasmonic planar resonator was demonstrated in the near-infrared region. A polarization conversion ratio in power larger than 91.1% is achieved from 730 nm to 1870 nm. Furthermore, orthogonal interference lithography is adopted to prepare the large-area optical polarization conversion metasurface. The fabrication strategy unplugs the bottleneck of the fabrication of the large-area metasurface in the optical regime, promising an unprecedented progress for optical communication and integrated optics.

Polarization conversion efficiency is vitally important to highly efficient wavefront manipulation based on phase discontinuities. However, previous single-layer phase gradient metasurfaces have suffered from low polarization conversion efficiency (at most 25%). Here we present a three-layer structure to enhance polarization conversion efficiency. The average efficiency is 76% for circularly polarized incident light converted to its opposite handedness. By arraying metallic antennas with varied optical axes for circularly polarized incident light, the efficiency of anomalous refraction is apparently increased, and the focused intensity of flat lenses can be significantly enhanced in the terahertz regime. It is expected that this scheme has potential applications in detection, focusing, and imaging.

We propose an efficient way to realize Fano-type resonances at optical frequencies based on a low-loss dielectric nanorod array. An ultrahigh Q factor (larger than 10000) is numerically demonstrated, which is attributed to the mode interference between the broadband Fabry-Perot (FP) resonance and the narrowband guided mode stemming from coupled quadrupoles. A wide gap region for field enhancement is formed between adjacent rods, making such a structure an ideal platform for related applications such as biological sensing and nonlinear devices. (C) 2014 The Japan Society of Applied Physics