Nanophotonics 2018; 7(6): 959–987 Review article Sajid M. Choudhury, Di Wang, Krishnakali Chaudhuri, Clayton DeVault, Alexander V. Kildishev, Alexandra Boltasseva and Vladimir M. Shalaev* Material platforms for optical metasurfaces https://doi.org/10.1515/nanoph-2017-0130 metal oxides. We identify the key advantages of each Received December 20, 2017; revised March 6, 2018; accepted March material platform and review the breakthrough devices 22, 2018 that were made possible with each material. Finally, we provide an outlook for emerging metasurface devices and Abstract: Optical metasurfaces are judicously engineered the new material platforms that are enabling such devices. electromagnetic interfaces that can control and manipu- late many of light’s quintessential properties, such as Keywords: materials platforms; metasurface; plasmonics; amplitude, phase, and polarization. These artificial sur- dielectric metasurface. faces are composed of subwavelength arrays of optical antennas that experience resonant light-matter inter- action with incoming electromagnetic radiation. Their ability to arbitrarily engineer optical interactions has 1 Introduction generated considerable excitement and interest in recent Harnessing, controlling, and understanding light have years and is a promising methodology for miniaturizing been long-standing pursuits of human civilization, dating optical components for applications in optical commu- back to ancient times. After years of exploration and dis- nication systems, imaging, sensing, and optical manipu- covery, we find ourselves now surrounded with optical lation. However, development of optical metasurfaces technologies that have advanced our ability to detect requires progress and solutions to inherent challenges, early signs of disease, transmit data across the world at namely large losses often associated with the resonant the speed of light, and stream high-definition movies on structures; large-scale, complementary metal-oxide-sem- our phones during class lectures. Optics has revolution- iconductor-compatible nanofabrication techniques; and ized the world – yet, after so much progress and discov- incorporation of active control elements. Furthermore, ery, most of our modern devices still resemble and rely practical metasurface devices require robust operation in on basic and bulky optical components such as lenses, high-temperature environments, caustic chemicals, and mirrors, and prisms for steering light. With current intense electromagnetic fields. Although these challenges trends to progressively miniaturize technology, it is now are substantial, optical metasurfaces remain in their essential to look for alternative methods to control light infancy, and novel material platforms that offer resilient, at extremely small dimensions. This miniaturization low-loss, and tunable metasurface designs are driving requires compact and planar devices with novel function- new and promising routes for overcoming these hurdles. alities that can now be realized via novel approaches that In this review, we discuss the different material platforms utilize artificial composite optical materials. Metamateri- in the literature for various applications of metasurfaces, als (MMs) are artificial materials composed of periodic or including refractory plasmonic materials, epitaxial noble specially arranged metal/dielectric structural elements metal, silicon, graphene, phase change materials, and with deeply subwavelength dimensions. Engineered MMs exhibit artificial optical properties that are very different from the properties of their constituent materials. The *Corresponding author: Vladimir M. Shalaev, School of Electrical and Computer Engineering and Birck Nanotechnology Center, electromagnetic properties of MMs can be tailored and Purdue University, 1205 W State St. West Lafayette, IN 47907, USA, manipulated almost at will via smart design techniques, e-mail: [email protected] making MMs a promising platform to overcome many of Sajid M. Choudhury, Di Wang, Krishnakali Chaudhuri, the limitations of conventional optical elements. MMs can Alexander V. Kildishev and Alexandra Boltasseva: School of couple to the electric and magnetic fields of incident light Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA and demonstrate effective properties for electric (per- Clayton DeVault: Department of Physics and Birck Nanotechnology mittivity) and magnetic (permeability) field interactions Center, Purdue University, West Lafayette, IN 47907, USA that are not usually found in nature. Such materials can Open Access. © 2018 Vladimir M. Shalaev et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution- NonCommercial-NoDerivatives 4.0 License. 960 S.M. Choudhury et al.: Material platforms for optical metasurfaces Transparent 2D conducting Si materials oxides based Refractory Meta surface Dielectric Diamond plasmonic Plasmonic materials Semimetals Noble Transition metals Phase metal Other metals changing oxides materials Figure 1: Conventional and emerging material platforms for optical metasurfaces including noble and other commonly used in plasmon- ics metals; semimetals and intermetallic compounds exhibiting metallic behavior (such as metal nitrides, hydrides, oxides, borides, etc.); transparent conducting oxides; and dielectrics. realize effective negative refractive index [1] and thereby metasurfaces, namely new materials for specialized appli- can be utilized to make new devices such as lenses that cations such as transition metal nitrides, transparent con- can image beyond the diffraction limit [2] or invisibility ducting oxides, high-K dielectrics, and tunable materials cloaks [3]. Practical realization of such MMs is limited due such as Ga:ZnO and ultra-low footprint materials such as to numerous challenges such as harsh microfabrication graphene. Finally, Section 5 discusses emerging applica- and nanofabrication requirement for layered structures tions and potential future developments for metasurfaces and significant optical losses in the materials. and how the materials play a role in it. Optical metasurfaces that represent a special class of In this review, we primarily emphasize on the emerg- two-dimensional (2D) MMs can overcome the size limi- ing material platforms of metasurface beyond noble metals tations of both conventional optical elements as well as and high-index dielectric studied before and compare fabrication challenges of bulk 3D MMs. A metasurface them with the traditional material platforms, identifying manipulates light using a 2D array of optical scatterers the key advantages and limitations of each. Table 1 gives a with a subwavelength distance between the scatterers. general overview of the material classes that are described These scatterers themselves are smaller than the wave- in detail throughout the article. length scale, and the metasurfaces can modulate incom- ing light in both amplitude and phase having their total thickness much smaller than the wavelength. The scat- terers can be either metallic or dielectric structured 2 Plasmonic metasurfaces nanoparticles or subwavelength apertures in a metallic or dielectric thin film. While for phase control, a conven- Tiny nanoparticles of noble metals have been used to color tional optical element relies on continuous phase accu- glass since ancient times [16]. A fourth century Roman mulation through light propagation, the optical scatterers Lycurgus cup shows different colors when the light is in a metasurface can introduce an abrupt phase change to shone through it and when the light is reflected off from the electromagnetic waves that they interact with. it. Polychrome luster decorations from the Abbasid era [17] There are numerous review articles related to the and Cassius purple and ruby glass from the renaissance development and seminal works of metasurfaces as well period [18] demonstrate colorful pottery by using the char- as applications and metasurfaces for specific material acteristic red color of spherical gold nanoparticles. Such platforms [4–15]. For our review, we focus on the different coloration can be explained by the interaction between material platforms of metasurfaces, shown in Figure 1. In nanoparticles and light, which forms the fundamental a broad context, all metasurfaces can be divided into two principle of plasmonics and plasmonic metasurfaces. general classes – plasmonic and dielectric metasurfaces, Plasmonics deals with the coupling between the electro- which are discussed in Sections 2 and 3, respectively. magnetic field and the electronic oscillations of mate- Section 4 describes the emerging material platforms for rial. Plasmon is a quantum of free electron oscillation. S.M. Choudhury et al.: Material platforms for optical metasurfaces 961 Table 1: Different material platforms with their wavelength range, advantages, limitations and applications. Material Examples Wavelength Advantages Limitations/challenges Application examples type range Plasmonic Au, Ag, Al, Visible–mid IR Established fabrication Relatively low melting point; Proof-of-concept metal Cu… Plasmonic in the UV part of Lack of tunability demonstrations of compact the spectrum (Ag, Al) High solubility/diffusion at and flat optical devices Biocompatible (Au) elevated temperature Biosensing (Au) Small device footprint Low chemical stability (Ag, Al) Structured coloration and High field concentration holography (Al) Refractory TiN, ZrN,