materials Review Metal-Insulator-Metal-Based Plasmonic Metamaterial Absorbers at Visible and Infrared Wavelengths: A Review Shinpei Ogawa 1,* and Masafumi Kimata 2 1 Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan 2 College of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-6-6497-7533 Received: 23 February 2018; Accepted: 17 March 2018; Published: 20 March 2018 Abstract: Electromagnetic wave absorbers have been investigated for many years with the aim of achieving high absorbance and tunability of both the absorption wavelength and the operation mode by geometrical control, small and thin absorber volume, and simple fabrication. There is particular interest in metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) due to their complete fulfillment of these demands. MIM-PMAs consist of top periodic micropatches, a middle dielectric layer, and a bottom reflector layer to generate strong localized surface plasmon resonance at absorption wavelengths. In particular, in the visible and infrared (IR) wavelength regions, a wide range of applications is expected, such as solar cells, refractive index sensors, optical camouflage, cloaking, optical switches, color pixels, thermal IR sensors, IR microscopy and gas sensing. The promising properties of MIM-PMAs are attributed to the simple plasmonic resonance localized at the top micropatch resonators formed by the MIMs. Here, various types of MIM-PMAs are reviewed in terms of their historical background, basic physics, operation mode design, and future challenges to clarify their underlying basic design principles and introduce various applications. The principles presented in this review paper can be applied to other wavelength regions such as the ultraviolet, terahertz, and microwave regions. Keywords: plasmonics; metamaterials; metal-insulator-metal; absorbers 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]. Figure1a–f shows schematic illustrations and the reflectance of the Salisbury screen, Jaumann absorber, and circuit Materials 2018, 11, 458; doi:10.3390/ma11030458 www.mdpi.com/journal/materials Materials 2018, 11, x FOR PEER REVIEW 2 of 18 Materials 2018, 11, 458 2 of 18 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 circuitanalog analog (CA) absorber.(CA) absorber. All of All these of these resonance resonance absorbers absorbers use ause quarter-wavelength a quarter‐wavelength gap gap from from the the top topmaterial material to theto the bottom bottom substrate. substrate. The The Salisbury Salisbury screen screen uses uses a non-periodic a non‐periodic resistive resistive sheet sheet in front in front of a ofground a ground plate plate [7]. [7]. The The Jaumann Jaumann absorber absorber uses uses two two or more or more resistive resistive sheets sheets in front in front of each of each other other and andis a is basic a basic resonance resonance absorber absorber [8]. These [8]. These two absorbers two absorbers use purely use resistivepurely resistive sheets. Thesheets. CA The absorber CA absorberuses a periodic uses a periodic surface madesurface of made a lossy of material a lossy material with three with layers: three the layers: top periodicthe top periodic metal patterns, metal patterns,a middle a dielectric middle dielectric layer, and layer, a continuous and a continuous metallic bottommetallic layer bottom [6]. layer The concept [6]. The of concept CA absorbers of CA absorbersis the basis is ofthe recent basis of metamaterial recent metamaterial absorbers absorbers for a wide for range a wide of wavelengthrange of wavelength regions, fromregions, visible from to visiblemicrowave to microwave wavelengths. wavelengths. (a) (c) (e) d d d (b) (d)d (f) 1.0 1.0 1.0 Reflectance Reflectance Reflectance Frequency Frequency Frequency FigureFigure 1. 1. SchematicSchematic illustrations andand reflectancereflectance of of resonant resonant absorbers: absorbers: (a ,b(a),b the) the Salisbury Salisbury screen; screen; (c,d ) (cthe,d) Jaumannthe Jaumann absorber; absorber; and ( eand,f) the (e, circuitf) the analogcircuit CAanalog absorber. CA absorber. “d” represents “d” represents the quarter-wavelength the quarter‐ wavelengthgap [6]. gap [6]. RecentRecent advances advances in in plasmonics plasmonics [9] [9 ]and and metamaterials metamaterials [10] [10 ]research research together together with with the the progress progress inin nanotechnological nanotechnological fabrication fabrication techniques techniques has has led led to novel to novel EM EMabsorbers absorbers at visible at visible and infrared and infrared (IR) wavelengths.(IR) wavelengths. These These absorbers absorbers uses useslocalized localized surface surface plasmon plasmon polaritons polaritons (LSPPs) (LSPPs) [11] [11 ]with with a a metamaterialmetamaterial concept concept to to achieve achieve much much smaller smaller absorber absorber volumes, volumes, sufficient sufficient performance, performance, and and design design flexibilityflexibility based based on on geometry geometry rather rather than than the the materials materials used. used. SPPs SPPs are are the the collective collective oscillation oscillation of of electronselectrons between between metals metals and and dielectrics dielectrics that that can can go go beyond beyond the the diffraction diffraction limit limit [12]. [12]. LSPPs LSPPs are are key key toto realizing realizing small small and and thin thin absorbers absorbers for for the the visible visible and and IR IR wavelength wavelength regions. regions. Therefore, Therefore, much much significantsignificant research research has has been been performed performed on on EM EM wave wave absorbers absorbers using using SPPs SPPs or or LSPPs LSPPs at at visible visible and and IRIR wavelengths. wavelengths. ThereThere are are roughly roughly two two categories categories of of absorbers absorbers that that employ employ plasmonics plasmonics and and metamaterials: metamaterials: conventionalconventional periodic periodic structures structures such such as asplasmonic plasmonic crystals crystals [13–15] [13 –and15] gratings and gratings [16–19], [16 and–19], metamaterialand metamaterial-based‐based structures, structures, where where periodicity periodicity has hasless lessimpact impact on onthe the optical optical properties properties [20]. [20]. InIn particular, particular, metal metal-insulator-metal-based‐insulator‐metal‐based plasmonic plasmonic metamaterial metamaterial absorbers absorbers (MIM (MIM-PMAs)‐PMAs) are are the the mostmost promising promising and and widely widely studied studied for for a a wide wide range range of of wavelengths wavelengths due due to to their their high high performance, performance, suchsuch as as highhigh absorbance,absorbance, incident incident angle, angle, and and polarization polarization insensitivity, insensitivity, as well as as well their as design their flexibility design flexibilityand simple and fabrication. simple fabrication. Although theirAlthough fundamental their fundamental principles are principles basically are the basically same, a wide the same, range a of wideapplications range of is applications expected, such is expected, as solar cells such [21 as], solar refractive cells index[21], refractive sensors [ 22index], optical sensors camouflage [22], optical [23], camouflagecloaking [24 [23],], optical cloaking switches [24], [ optical25], color switches pixels [ 26[25],,27 color], thermal pixels IR [26,27], sensors thermal [28–31], IR mechanical sensors [28–31], thermal mechanicalsensors [32 ],thermal surface-enhanced sensors [32], spectroscopy surface‐enhanced [33,34], and spectroscopy gas sensing [[33,34],35]. Therefore, and gas this sensing review paper[35]. Therefore,aims to clarify this review the fundamental paper aims principle, to clarify characteristics, the fundamental possibilities, principle, and characteristics, challenges of possibilities, MIM-PMAs at visible and IR wavelengths to contribute to future research and the expansion of their applications. Materials 2018, 11, x FOR PEER