Metasurfaces are artificially structured thin films with unusual properties on demand. Different from metamaterials, the metasurfaces change the electromagnetic waves mainly by exploiting the boundary conditions, rather than the constitutive pa-rameters in three dimensional (3D) spaces. Despite the intrinsic similarities in the operational principles, there is not a universal theory available for the understanding and design of metasurface-based devices. In this article, we propose the concept of metasurface waves (M-waves) and provide a general theory to describe the principles of them. Most importantly, it is shown that the M-waves share some fundamental properties such as extremely short wavelength, abrupt phase change and strong chromatic dispersion, which make them different from traditional bulk waves. It is shown that these properties can enable many important applications such as subwavelength imaging and lithography, planar optical devices, broadband anti-reflection, absorption and polarization conversion. Our results demonstrated unambiguously that traditional laws of diffraction, refraction, reflection and absorption should be revised by using the novel properties of M-waves. The theory provided here may pave the way for the design of new electromagnetic devices and further improvement of metasurfaces. The exotic properties of metasurfaces may also form the foundations for two new sub-disciplines called “subwavelength surface electromagnetics” and “subwavelength electromagnetics”.

Principles of electromagnetic waves in metasurfaces

Control over the dispersion of the refractive index is essential to the performance of most modern optical systems. These range from laboratory microscopes to optical fibres and even consumer products, such as photography cameras. Conventional methods of engineering optical dispersion are based on altering material composition, but this process is time-consuming and difficult, and the resulting optical performance is often limited to a certain bandwidth. Recent advances in nanofabrication have led to high-quality metasurfaces with the potential to perform at a level comparable to their state-of-the-art refractive counterparts. In this Review, we introduce the underlying physical principles of metasurface optical elements (with a focus on metalenses) and, drawing on various works in the literature, discuss how their constituent nanostructures can be designed with a highly customizable effective index of refraction that incorporates both phase and dispersion engineering. These metasurfaces can serve as an essential component for achromatic optics with unprecedented levels of performance across a broad bandwidth or provide highly customized, engineered chromatic behaviour in instruments such as miniature aberration-corrected spectrometers. We identify some key areas in which these achromatic or dispersion-engineered metasurface optical elements could be useful and highlight some future challenges, as well as promising ways to overcome them. Flat metasurface optics provides an emerging platform for combining semiconductor foundry methods of manufacturing and assembling with nanophotonics to produce high-end and multifunctional optical elements. This Review highlights the design of metasurfaces, recent advances in the field and initial promising applications.

Flat optics with dispersion-engineered metasurfaces

Metasurfaces, two-dimensional versions of metamaterials, retain the great capabilities of three-dimensional counterparts in manipulating electromagnetic wave behaviors, while reducing the challenges in fabrication. By judiciously engineering parameters of individual building blocks (such as geometry, size, and material) and selecting specific design algorithms, metasurfaces are promising to replace conventional electromagnetic elements in nanoplasmonic/photonic devices. Significantly, such concept can be readily promoted to other disciplines, such as acoustics, thermal physics, and seismology. In this article, the latest advances in full control of electromagnetic waves with metasurfaces are briefly reviewed from a functionality perspective. A broad avenue towards real-life applications of metamaterials has been opened up, although they are still at their infant stage. At the end, several promising approaches are suggested to extend the applications of metasurfaces.

Advances in Full Control of Electromagnetic Waves with Metasurfaces

Intelligence at either the material or metamaterial level is a goal that researchers have been pursuing. From passive to active, metasurfaces have been developed to be programmable to dynamically and arbitrarily manipulate electromagnetic (EM) wavefields. However, the programmable metasurfaces require manual control to switch among different functionalities. Here, we put forth a smart metasurface that has self-adaptively reprogrammable functionalities without human participation. The smart metasurface is capable of sensing ambient environments by integrating an additional sensor(s) and can adaptively adjust its EM operational functionality through an unmanned sensing feedback system. As an illustrative example, we experimentally develop a motion-sensitive smart metasurface integrated with a three-axis gyroscope, which can adjust self-adaptively the EM radiation beams via different rotations of the metasurface. We develop an online feedback algorithm as the control software to make the smart metasurface achieve single-beam and multibeam steering and other dynamic reactions adaptively. The proposed metasurface is extendable to other physical sensors to detect the humidity, temperature, illuminating light, and so on. Our strategy will open up a new avenue for future unmanned devices that are consistent with the ambient environment.

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Smart metasurface with self-adaptively reprogrammable functions

Hyperbolic materials enable numerous surprising applications that include far-field subwavelength imaging, nanolithography, and emission engineering. The wavevector of a plane wave in these media follows the surface of a hyperboloid in contrast to an ellipsoid for conventional anisotropic dielectric. The consequences of hyperbolic dispersion were first studied in the 50’s pertaining to the problems of electromagnetic wave propagation in the Earth’s ionosphere and in the stratified artificial materials of transmission lines. Recent years have brought explosive growth in optics and photonics of hyperbolic media based on metamaterials across the optical spectrum. Here we summarize earlier theories in the Clemmow’s prescription for transformation of the electromagnetic field in hyperbolic media and provide a review of recent developments in this active research area.

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Hyperbolic metamaterials: new physics behind a classical problem

Plasmonic Metasurfaces for Simultaneous Thermal Infrared Invisibility and Holographic Illusion

Poor aspect profiles of plasmonic lithography patterns are suffering from evanescent waves' scattering loss in metal films and decaying exposure in photoresist. To address this issue, we experimentally report plasmonic cavity lens to enhance aspect profile and resolution of plasmonic lithography. The profile depth of half-pitch (hp) 32 nm resist patterns is experimentally improved up to 23 nm, exceeding in the reported sub-10 nm photoresist depth. The resist patterns are then transferred to bottom resist patterns with 80 nm depth using hard-mask technology and etching steps. The resolution of plasmonic cavity lens up to hp 22 nm is experimentally demonstrated. The enhancement of the aspect profile and resolution is mainly attributed to evanescent waves amplifying from the bottom silver layer and scattering loss reduction with smooth silver films in plasmonic cavity lens. Further, theoretical near-field exposure model is utilized to evaluate aspect profile with plasmonic cavity lens and well illustrates th...

Enhancing aspect profile of half-pitch 32 nm and 22 nm lithography with plasmonic cavity lens

Dynamic color tuning is a very important and fascinating direction in the field of structural colorations due to its possible applications in stealth, anticounterfeiting, displaying techniques, etc. However, the most recent work about structural colors either have limitations on dynamic frequency tunability or require special conditions. In this work, a tensile substrate (e.g., polydimethylsiloxane) is introduced into conventional plasmonic structures to demonstrate dynamic tunable structural colors. The proposed idea is experimentally realized by fabricating samples via interference photolithography. The results show that resonance-induced colors can be dynamically tuned from green to fuchsia by stretching the tensile substrate. Moreover, a further thematic analysis of the physical mechanism in the proposed structure is conducted. The authors envisage that their method has potential applications in anticounterfeiting, biometric sensors, and mechanics measurements. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Actively Tunable Structural Color Rendering with Tensile Substrate

Nanofabrication technology with high-resolution, high-throughput and low-cost is essential for the development of nanoplasmonic and nanophotonic devices. At present, most metasurfaces are fabricated in a point by point writing manner with electron beam lithography or a focused ion beam, which imposes a serious cost barrier with respect to practical applications. Near field optical lithography, seemingly providing a high-resolution and low-cost way, however, suffers from the ultra shallow depth and poor fidelity of obtained photoresist patterns due to the exponential decay feature of evanescent waves. Here, we propose a method of surface plasmonic imaging lithography by introducing a reflective plasmonic lens to amplify and compensate evanescent waves, resulting in the production of nano resist patterns with high fidelity, contrast and enhanced depth beyond that usually obtained by near field optical lithography. As examples, a discrete and anisotropically arrayed nano-slots mask pattern with different orientations and a size of 40 nm × 120 nm could be imaged in photoresist and transferred successfully onto a metal layer through an etching process. Evidence for the pattern quality is given by virtue of the fabricated metasurface lens devices showing good focusing performance in experiments. It is believed that this method provides a parallel, low-cost, high-throughput and large-area nanofabrication route for fabricating nanostructures of holograms, vortex phase plates, bio-sensors and solar cells etc.

Fabrication of anisotropically arrayed nano-slots metasurfaces using reflective plasmonic lithography.

Abstract 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.

https://www.degruyter.com/downloadpdf/j/nanoph.2018.7.issue-1/nanoph-2017-0062/nanoph-2017-0062.xml

Kaipeng Liu

Papers

Plasmonic Metasurfaces for Simultaneous Thermal Infrared Invisibility and Holographic Illusion

Enhancing aspect profile of half-pitch 32 nm and 22 nm lithography with plasmonic cavity lens

Actively Tunable Structural Color Rendering with Tensile Substrate

Fabrication of anisotropically arrayed nano-slots metasurfaces using reflective plasmonic lithography.

Color display and encryption with a plasmonic polarizing metamirror