**1. Introduction**

Electromagnetic (EM) wave absorbers are drawing significant interest from aspects of both fundamental science and industry applications. Typical EM wave absorbers are essentially based on the intrinsic loss of the material and thus require a long optical path, which results in large volume and poor design flexibility. EM wave absorbers with absorption properties that can be efficiently controlled by their structures have thus been studied for many years. Such EM wave absorbers were first studied in the microwave range and are roughly classified into two groups, according to Reference [1], as broadband absorbers and resonant absorbers. The broadband absorbers are further categorized into two groups: geometric transition absorbers and low-density absorbers [1]. Geometric transition absorbers consist of two-dimensional (2D) periodic pyramids that cause a gradual change in the dielectric constant from the free space to the absorbers [2,3]. Low-density absorbers utilize porous materials [4,5] and the multi-reflections that occur in these pores has led to significant absorption, which was realized using thin absorbers.

The resonance absorbers are classified into three types, according to Reference [6]. Figure 1a–f shows schematic illustrations and the reflectance of the Salisbury screen, Jaumann absorber, and circuit analog (CA) absorber. All of these resonance absorbers use a quarter-wavelength gap from the top material to the bottom substrate. The Salisbury screen uses a non-periodic resistive sheet in front of a ground plate [7]. The Jaumann absorber uses two or more resistive sheets in front of each other and is a basic resonance absorber [8]. These two absorbers use purely resistive sheets. The CA absorber uses a periodic surface made of a lossy material with three layers: the top periodic metal patterns, a middle dielectric layer, and a continuous metallic bottom layer [6]. The concept of CA absorbers is the basis of recent metamaterial absorbers for a wide range of wavelength regions, from visible to microwave wavelengths.

**Figure 1.** Schematic illustrations and reflectance of resonant absorbers: (**<sup>a</sup>**,**b**) the Salisbury screen; (**<sup>c</sup>**,**d**) the Jaumann absorber; and (**<sup>e</sup>**,**f**) the circuit analog CA absorber. "d" represents the quarter-wavelength gap [6].

Recent advances in plasmonics [9] and metamaterials [10] research together with the progress in nanotechnological fabrication techniques has led to novel EM absorbers at visible and infrared (IR) wavelengths. These absorbers uses localized surface plasmon polaritons (LSPPs) [11] with a metamaterial concept to achieve much smaller absorber volumes, sufficient performance, and design flexibility based on geometry rather than the materials used. SPPs are the collective oscillation of electrons between metals and dielectrics that can go beyond the diffraction limit [12]. LSPPs are key to realizing small and thin absorbers for the visible and IR wavelength regions. Therefore, much significant research has been performed on EM wave absorbers using SPPs or LSPPs at visible and IR wavelengths.

There are roughly two categories of absorbers that employ plasmonics and metamaterials: conventional periodic structures such as plasmonic crystals [13–15] and gratings [16–19], and metamaterial-based structures, where periodicity has less impact on the optical properties [20]. In particular, metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) are the most promising and widely studied for a wide range of wavelengths due to their high performance, such as high absorbance, incident angle, and polarization insensitivity, as well as their design flexibility and simple fabrication. Although their fundamental principles are basically the same, a wide range of applications is expected, such as solar cells [21], refractive index sensors [22], optical camouflage [23], cloaking [24], optical switches [25], color pixels [26,27], thermal IR sensors [28–31], mechanical thermal sensors [32], surface-enhanced spectroscopy [33,34], and gas sensing [35]. Therefore, this review paper aims to clarify the fundamental principle, characteristics, possibilities, and challenges of MIM-PMAs at visible and IR wavelengths to contribute to future research and the expansion of their applications.

Please note that MIM-based thermal emitters are considered as MIM-PMAs at IR wavelengths [36] because absorbance is equal to emissivity, as given by Kirchhoff's law. To the best of our knowledge, MIM-PMAs were first demonstrated as thermal IR emitters by Puscasu and Schaich [37].

#### **2. Structures and Materials**

In this section, the fundamental structures and materials of MIM-PMAs are explained with a simple introduction of the basic optical properties. The detailed optical characteristics are discussed in a later section.

MIM-PMAs consist of three layers: a bottom metal layer, a middle dielectric layer, and a top periodic metal micropatches. Figure 2a,b shows schematic illustrations of conventional MIM-PMAs with two-dimensional (2D) and one-dimensional (1D) periodic micropatches, respectively. The absorption wavelength is fundamentally defined by the micropatch size. The 2D configuration is symmetric for two orthogonal directions, the *x* and *y* directions; therefore, the optical properties are polarization insensitive. On the other hand, the 1D configuration is asymmetric in the *x* and *y* directions, so the optical properties are polarization sensitive. Figure 2c,d shows cross-sectional views of the conventional MIM-PMAs with flat and isolated dielectric layers, respectively. The middle dielectric layers underneath the micropatches are required, so that both structures function as MIM-PMAs.

**Figure 2.** Schematic illustrations of metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs); oblique view of (**a**) two-dimensional (2D) and (**b**) one-dimensional (1D) periodic micropatches. Cross-sectional views of (**c**) continuous and (**d**) isolated middle dielectric layers.

The thickness of the metal in the bottom layer and the top micropatches is required to be more than twice the depth of the operating wavelength region, e.g., 100 nm thickness is sufficient for the IR wavelength region [38]. The thickness of the middle dielectric layer can be less than the operating wavelength/50 due to the strong confinement of the waveguide mode of SPPs [39].

The possible lattice structures for 2D periodic micropatches are square, triangular, and honeycomb. However, the lattice structures have less impact on absorption properties because each micropatch acts as a single isolated resonator [40].

The shape of micropatches are roughly classified into symmetric in the two orthogonal directions, such as squares [28,41], circles [42], and crosses [38,43], and nano-particles [44–46], or asymmetric, such as ellipses [47], rectangles [48], wedges [49], bow-ties [50], split-ring resonators [51], and asymmetric crosses [52,53]. Nanocubes have also been used as micropatches [54,55]. The shape of the corners and the sidewall angles have an important role in defining single and multiband

resonances [56]. The end shape of the micropatch produces the difference of optical modes formed in the middle dielectric layer because MIM structures can be considered as waveguides and the shape of the waveguide end defines the waveguide mode [57]. The first three micropatch shapes are symmetric in the *x* and *y* directions, and absorption occurs at a single wavelength. On the other hand, the latter four micropatch shapes are asymmetric in the *x* and *y* directions and produce two absorption wavelengths. As discussed in Section 4, the symmetry is an important parameter for polarization dependence.

It is also important to consider the temperature tolerance and compatibility of the complementary metal oxide semiconductor (CMOS) process for the choice of materials used for MIM-PMAs [43,58]. Tables 1 and 2 show the properties of the metals and dielectrics used in MIM-PMAs [43].

The top and the bottom layers are typically based on metals such as gold (Au) [38,41], silver (Ag) [49], and aluminum (Al) [28], which are common materials for SPPs. Titanium nitride (TiN) [59,60], molybdenum (Mo) [43], and tungsten (W) [58], or highly-doped silicon (Si) [61], can be used for the bottom and top micropatches. Graphene can also be used for top micropatches in the IR wavelength region [62]. TiN or Mo have recently been used for thermal IR emitters due to requirements of high-temperature tolerance. However, absorbers require less temperature tolerance. Therefore, Al is widely employed due to its compatibility with the CMOS process and its low cost.



**Table 2.** Properties of insulators used in MIM-PMAs. Adapted with permission from Reference [43]. © 2017 American Chemical Society.


The middle layer is roughly classified into two groups: oxides or nitrides such as Al2O3 [41], SiO2 [40], CeO2 [63], SiN [28], and AlN [43], and semiconductors such as Si [64], germanium (Ge) [65], and ZnS [66]. Another material often used is MgF2 [33]. Phase transition materials of germanium antimony telluride [67] and VO2 [68] have been used for thermal switching. The loss is an important factor of the middle dielectric layer, as discussed in a later section. Lossless materials such as Si, Ge, ZnS, and CeO2 should be used for the IR wavelength region in the vicinity of 10 μm to maintain the linear tunability of the absorption wavelength. However, these materials are not compatible with the CMOS process. The alternative choice to avoid the influence of intrinsic loss materials is mushroom-PMAs, where small post structures connect the micropatches and the bottom layer without a continuous middle dielectric layer [69–71].

A perforated metal plate can be used as a top layer, which is a complementary structure of periodic micropatches [72] that provides optical properties similar to MIM-PMAs. However, there

is less design flexibility due to the need for periodicity. 1D grating structures with ultra-narrow groove widths (ca. 100 nm) and high aspect ratios (>10) [16,18,73] can be classified as MIM-PMAs because such a slit is considered to be the waveguide, which is equivalent to the insulator layer in MIM structures [74,75]. However, high aspect ratios and narrow groove widths require complicated fabrication procedures. In this paper, these structures are excluded and instead focus is made on the conventional MIM-PMAs, shown as Figure 2. Other structures such as core-shell nanoparticles [76] and multi-flat-layer structures [77–79] can be considered to have a principle similar to that of MIM-PMAs.

#### **3. Basic Optical Properties**
