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Effective dynamic modulation of visible light properties has been significantly desired for advanced imaging and sensing technologies. In particular, phase-change materials have attracted much attention as active material platforms owing to their broadband tunability of optical dielectric functions induced by the temperature-dependent phase-changes. However, their uses for visible light modulators are still limited to meet multi-objective high performance owing to the low material quality factor and active tunability in the visible regime. Here, a design strategy of phase-change metafilm absorber is demonstrated by making the use of the material drawbacks and extending design degree of freedom. By engineering tunability of effective anisotropic permittivity tensor of VO2-Ag metafilm around near-unity absorption conditions, strong dynamic modulation of reflection wave is achieved with near-unity modulation depth at desired wavelength regions without sacrificing bandwidth and efficiency. By leveraging effective medium theory of metamaterial and coupled mode theory, the intuitive design rules and theoretical backgrounds are suggested. It is also noteworthy that the dynamic optical applications of intensity modulation, coloring, and polarization rotation are enabled in a single device. By virtue of ultrathin flat configuration of a metafilm absorber, design extensibility of reflection spectrum is also verified. It is envisioned that our simple and powerful strategy would play a robust role in development of miniaturized light modulating pixels and a variety of photonic and optoelectronic applications.

Topography measurement is essential for surface characterization, semiconductor metrology, and inspection applications. To date, performing high-throughput and accurate topography remains challenging due to the trade-off between field-of-view (FOV) and spatial resolution. Here we demonstrate a novel topography technique based on the reflection-mode Fourier ptychographic microscopy, termed Fourier ptychograhpic topography (FPT). We show that FPT provides both a wide FOV and high resolution, and achieves nanoscale height reconstruction accuracy. Our FPT prototype is based on a custom-built computational microscope consisting of programmable brightfield and darkfield LED arrays. The topography reconstruction is performed by a sequential Gauss-Newton-based Fourier ptychographic phase retrieval algorithm augmented with total variation regularization. We achieve a synthetic numerical aperture (NA) of 0.84 and a diffraction-limited resolution of 750 nm, increasing the native objective NA (0.28) by 3×, across a 1.2 × 1.2 mm2 FOV. We experimentally demonstrate the FPT on a variety of reflective samples with different patterned structures. The reconstructed resolution is validated on both amplitude and phase resolution test features. The accuracy of the reconstructed surface profile is benchmarked against high-resolution optical profilometry measurements. In addition, we show that the FPT provides robust surface profile reconstructions even on complex patterns with fine features that cannot be reliably measured by the standard optical profilometer. The spatial and temporal noise of our FPT system is characterized to be 0.529 nm and 0.027 nm, respectively.

One of the most important research topic in optics and photonics is the design of metasurfaces to substitute conventional optical elements that demonstrate unprecedented merits in terms of performance and form factor. In this context, full-space control of metasurfaces that makes it possible to manipulate scattered light in transmission and reflection spaces simultaneously, is proposed as the next-generation scheme in optics, with a potential for applications such as 360° holographic images and novel optical systems. However, previously designed metasurfaces lacked functionality because the desired operation occurs under preconditioned light; therefore, they are difficult to use in real applications. Here, we present a design method that enables polarization-dependent full-space control, in which two independent and arbitrary phase profiles can be addressed to each space. Upon introducing a phase gradient value to realize the critical angle condition, conversion of transmissive into reflective operation is realized. Then, rectangular nanopillars are utilized to facilitate polarization beam splitting with the desired phase. Three samples were fabricated and measured based on the proposed scheme.

Metasurfaces have received enormous attention thanks to their unique ability to modulate electromagnetic properties of light in various frequency regimes. Recently, exploiting its fabrication ease and modulation strength, unprecedented and unique controlling of light that surpasses conventional optical devices has been suggested and studied a lot. Here, in this paper, we discuss some parts of this trend including holography, imaging application, dispersion control, and multiplexing, mostly operating for optical frequency regime. Finally, we will outlook the future of the devices with recent applications of these metasurfaces.

Since Leith and Upatnieks demonstrated the first optical hologram in 1964, hologram technology has attracted a great deal of interest in a wide range of optical fields owing to its potential use in future optical applications such as holographic imaging and optical data storage. Although there have been considerable efforts to develop holographic technologies using conventional optics, critical issues still hinder future development. Recently, metasurfaces composed of artificially fabricated subwavelength structures have been considered as novel holographic devices that show an unprecedented ability to control electromagnetic waves. In this review, we outline the recent progress in metasurface holography. A general introduction to several types of metasurface holography categorized based on their physics and application is provided. Then, our personal perspective on the future of this field is discussed.

Metasurface color filters (MCFs) have attracted considerable attention thanks to their compactness and functionality as a candidate of an optical element in a miniaturized image sensor. However, conventional dielectric and plasmonic MCFs that have focused on color purity and efficiency cannot avoid reflection in principle, which degrades image quality by optical flare. Here, we introduce absorptive-type MCFs through truncated-cone hyperbolic metamaterial absorbers. By applying a particle swarm optimization method to design multiple parameters simultaneously, the proposed MCF is theoretically and numerically demonstrated in perceptive color on CIELAB and CIEDE2000 with suppressed-reflection. Then, a color filter array is numerically proven in 255 nm of sub-pixel pitch.

Nano-structural colour pixels have attracted much attention as promising solutions for compact display devices. However, it is difficult to miniaturize and integrate conventional transmissive colour filtering components for high resolution pixels within subwavelength scale without sacrificing colour depth. Here, we propose a novel colour pixel structure using bi-layered hybrid metasurfaces that are composed of aluminum nanograting and amorphous silicon nanorod layers. The independent high-contrast control of resonance intensity and spectral position is achieved by anisotropic Mie resonances and cavity effect between stacked two metasurfaces. Moreover, the proposed structures permit the wide colour gamut through the novel physical principles. In addition, a meta-pixel which can provide gradual tuning of colour is demonstrated to obtain high colour depth. The proposed structures are expected to be fruitful for the development of next generation display and imaging systems.

Modern imaging and spectroscopy systems require to implement diverse functionalities with thin thickness and wide wavelength ranges. In order to meet this demand, polarization-resolved imaging has been widely investigated with integrated circular polarizers. However, the circular polarizers which operate at the entire visible wavelengths and have a thickness of several tens of nanometers have not been developed yet. Here, a circular polarizer, operating at the entire visible wavelength range, is demonstrated using helically stacked aluminum nano-grating layers. High extinction ratio and broad operation bandwidth are simultaneously achieved by using non-resonant anisotropic characteristics of the nano-grating. It is theoretically verified that the averaged extinction ratio becomes up to 8 over the entire visible wavelength range while having a thickness of 390 nm. Also, the feasibility of the proposed structure and circular polarization selectivity at the visible wavelength range are experimentally verified. It is expected that the proposed structure will lead to extreme miniaturization of a circular polarizer and contribute greatly to the development of mobile/wearable imaging systems such as virtual reality and augmented reality displays. (C) 2017 Optical Society of America

Active switching of near-field directivity, which is an essential functionality for compact integrated photonics and small optoelectronic elements, has been challenging due to small modulation depth and complicated fabrication methods for devices including active optical materials. Here, we theoretically and experimentally realize a nanoscale active directional switching of surface plasmon polaritons (SPPs) using a phase transition material for the first time. The SPP switching device with noticeable distinction is demonstrated based on the phase transition of vanadium dioxide (VO2) at the telecom wavelength. As the insulator-to-metal phase transition (IMT) of VO2 induces the large change of VO2 permittivity at telecom wavelengths, the plasmonic response of a nanoantenna made of VO2 can be largely tuned by external thermal stimuli. The VO2-insulator-metal (VIM) nanoantenna and its periodic array, the VIM metagrating, are suggested as optical switches. The directional power distinction ratio is designed to change from 8.13:1 to 1:10.56 by the IMT and it is experimentally verified that the ratio changes from 3.725:1 to 1:3.132 as the VIM metagratings are heated up to 90 °C. With an electro-thermally controllable configuration and an optimized resonant design, we expect potential applications of the active switching mechanism for integrable active plasmonic elements and reconfigurable imaging.

Ultra-compact dynamically reconfigurable modulation of optical transmission has been widely studied by using subwavelength-spaced resonant metasurface structures containing reconfigurable optical materials. However, it has been difficult to achieve high transmissivity, large modulation depth, and broad bandwidth simultaneously with the conventional resonance-based metasurface schemes. Here, we propose a reconfigurable phase-transition diffractive grating, made of thick VO2 ridge waveguides, for achieving the above-mentioned three goals simultaneously in the near-infrared range. Based on the large dielectric-to-plasmonic transition characteristic of VO2 in the near-infrared range, diffraction directivity of dual-VO2 ridge waveguide is designed to be tuned by thermally driven phase transition of VO2 for transverse electrically polarized illumination. Then, the diffractive VO2 ridge waveguide grating composed of the periodically arranged dual VO2 ridge waveguides is designed with on-state efficiency around 0.3 and minimum modulation depth about 0.35 over a broad bandwidth of 550 nm (1100-1650 nm). The working principle and excellent modulation performance are thoroughly verified through numerical and experimental studies.

In this paper, the broadband transmissive modulation of polarization and amplitude is demonstrated with high efficiency and tunability using multi-layered aluminum metasurfaces. Broadband and nondispersive optical rotation in the optical frequency region is realized by using Fabry-Pérot-like cavity and phase compensation. Simultaneously, the transmission amplitude can be independently controlled by adjusting the twist angle of the anisotropic metasurfaces. The proposed polarization-amplitude modulators are numerically demonstrated to achieve large tunability with an amplitude modulation depth of 0.95 and maximum rotation angle of 180°.