Theoretical investigation of all-metal-based mushroom plasmonic metamaterial absorbers at infrared wavelengths

Abstract. High-performance wavelength-selective infrared (IR) sensors require small pixel structures, a low-thermal mass, and operation in the middle-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) regions for multicolor IR imaging. All-metal-based mushroom plasmonic metamaterial absorbers (MPMAs) were investigated theoretically and were designed to enhance the performance of wavelength-selective uncooled IR sensors. All components of the MPMAs are based on thin layers of metals such as Au without oxide insulators for increased absorption. The absorption properties of the MPMAs were investigated by rigorous coupled-wave analysis. Strong wavelength-selective absorption is realized over a wide range of MWIR and LWIR wavelengths by the plasmonic resonance of the micropatch and the narrow-gap resonance, without disturbance from the intrinsic absorption of oxide insulators. The absorption wavelength is defined mainly by the micropatch size and is longer than its period. The metal post width has less impact on the absorption properties and can maintain single-mode operation. Through-holes can be formed on the plate area to reduce the thermal mass. A small pixel size with reduced thermal mass and wideband single-mode operation can be realized using all-metal-based MPMAs.


Introduction
There has been increasing interest in uncooled infrared (IR) sensors with a microelectromechanical system-based pixel structure due to their low-cost fabrication and wide range of applications. 1,2 We have recently developed uncooled IR sensors with advanced functions using plasmonics 3,4 and metamaterials 5,6 to expand their field of applications, which includes fire detection, gas analysis, hazardous material recognition, multicolor imaging, and polarimetric imaging. First, a wavelength-selective uncooled IR sensor was developed for the middle-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) regions using a twodimensional plasmonic absorber (2-D PLA). 7-10 A 2-D PLA is an Au-based periodic dimple structure, in which surface plasmon resonance is produced and strong wavelengthselective absorption occurs according to the surface periodicity. A polarization-selective uncooled IR sensor was then developed using a 2-D PLA with ellipsoidal dimples 11 and one-dimensional grating absorbers, 12 whereby polarization could be selectively detected according to the asymmetry on the surface of the absorbers.
Some advantages of 2-D PLAs are a simple fabrication procedure and structural robustness. Wavelength-and polarization-selective functions can be realized for PLAs without the attachment of filters, 13 multilayer structures, 14 mirrors, 15 or polarizers, 16,17 which effectively reduces the cost and additional space requirements of optical systems. Multicolor/ polarimetric imaging can, therefore, be realized by the integration of various 2-D PLAs with different wavelength/ polarization-detection specifications. 2-D PLAs inevitably require a large number of periods and a large thickness, which results in a large pixel size and a high-thermal mass. 7,8 However, high-performance wavelength-selective uncooled IR sensors for multicolor IR imaging require a small pixel size and low-thermal mass to achieve high resolution and fast response. Metal-insulator-metal structures [18][19][20][21] based on plasmonic metamaterials are important candidates for high-performance absorbers, which control the absorption wavelength according to the surface resonator size beyond the surface array period. However, insulator layers such as SiO 2 and SiN cause an additional absorption peak, restrict the operation wavelength to near the MWIR region due to their intrinsic absorption, 22 and also increase the thermal mass. Therefore, we have proposed three-dimensional (3-D) or mushroom plasmonic metamaterial absorbers (MPMAs), 23 as well as a suitable fabrication method. 24 MPMAs with Si posts were also developed for dual-and single-band detection. 25 However, MPMAs with Si posts require precise control of the post width to achieve single-mode operation. Therefore, we have investigated the detailed absorption properties of MPMAs based on all-metal structures to realize high-performance wavelength-selective uncooled IR sensors with wideband and single-mode operation.
In this study, we report a theoretical investigation of MPMAs for high-performance uncooled IR sensors with advanced functions.  Figure 1 shows schematic illustrations of the structure investigated in this study. The micropatches are arrayed and connected to the bottom plate with posts. This structure is referred to as a mushroom metamaterial. 26,27 In this work, all of the components were assumed to be made of Au. The dielectric constant of Au used in our calculations was taken from Ref. 28. The minimum thickness of all components is approximately twice the skin depth to prevent light penetration and achieve ultrathin absorbers with low-thermal mass. The structural parameters of the period, the width (diameter) and thickness of the micropatch, the thickness of the bottom plate, and the width (diameter) and height of the posts are defined as p, w m ; t m , t b ; w p ; and h p , respectively. The gap between the micropatches and the bottom plate is equal to h p . The thickness t b has no impact on the absorption, as long as it is sufficiently thick to reflect incident IR radiation. Thus, t b is considered to be at least >50 nm in the IR wavelength region for practical applications 29 and was set as a semi-infinite thickness for the simulation. The parameters t m , w p , and h p were fixed at 50, 200, and 150 nm, respectively. The incident angle is set to normal incidence for the fundamental evaluation of MPMAs conducted in this study. The polarization direction used is defined as that parallel to the basic lattice vector of the square lattice structure, which corresponds to the xand y-directions, as shown in Fig. 1(b). The results obtained represent the averaged total absorption for all polarizations due to the symmetry of the MPMAs.
We first investigated the absorption properties of a circular-shaped micropatch using rigorous coupled-wave analysis (RCWA). The period p was fixed at 4.0 μm and w m was varied.  Figure 2 indicates that sufficient absorption of over 90% was obtained for the MPMAs with most of the w m values, and the absorption wavelength could be controlled primarily by w m , regardless of p. The absorption wavelength is almost proportional to w m ; however, a forbidden band where the absorption is suppressed appears for some w m values, such as 1.8, 2.5, 2.8, and 3.2 μm. The full width at half maximum (FWHM) also increases with w m . As the absorption wavelength approaches the period, the FWHM is improved due to the reduction of the gap resonance between the micropatches.
The absorption properties of square-shaped micropatches were also investigated by RCWA; a schematic illustration of an MPMA with square-shaped micropatches is provided in Fig. 3. As with the circular-shaped micropatch, the period p was again fixed at 4.0 μm and w m was varied. Figure 4 shows the wavelength absorption for this MPMA for various values of w m from 1.0 to 4.0 μm. Sufficient absorption of over 90% was obtained for the MPMA with all of the w m values, and the absorption wavelength could be controlled according to w m , regardless of p. The absorption wavelength is almost proportional to w m , and a uniform absorption peak is maintained over a wide range of MWIR and LWIR wavelengths compared with that for the MPMA with circular-shaped micropatches. Each circular-shaped micropatch forms an unequal gap with the adjacent micropatches, which produces broader resonance. In contrast, the squareshaped micropatches maintain a parallel gap with the adjacent micropatches, which produces narrower resonance. When the absorption wavelength is smaller than p, diffraction starts to occur for both the circular-and square-shaped  micropatches. Therefore, the absorption abruptly decreases for wavelengths <4 μm.
The time-averaged electromagnetic field amplitude distribution was calculated at the resonance using the 2-D finitedifference time-domain (FDTD) method. Here, w m was fixed at 2.0 μm, whereas the values of the other parameters were the same as in the RCWA calculation. Figures 5(a)-5(c) show the normalized amplitude of the electric field and the magnetic field distribution, and the power, respectively. X, Y, and Z are defined as parallel to w m , vertical to the x-direction, and parallel to h p , respectively. Figure 5 clearly demonstrates that plasmonic resonance occurs only at the perimeter of the micropatch. These results confirm that localized plasmonic resonance rather than propagating plasmonic resonance occurs in the MPMAs. The absorption wavelength is defined only by the size of the micropatch beyond the pattern period due to this strong confinement of such localized plasmonic resonance, and thus contributes to a smaller pixel size.

Effect of the Post Height
The effect of h p was investigated for an MPMA with squareshaped micropatches. Figure 6 shows the wavelength absorption for the MPMA as a function of h p from 0 to 500 nm with a fixed w m and p of 2.0 and 4.0 μm, respectively. Sufficient absorption of over 90% was obtained for h p ranging from ∼80 to 200 nm, where the shift of the absorption wavelength was small. This result indicates that the narrow-gap resonance between the micropatches and the bottom plate is especially important to obtain sufficient absorption.

Effect of the Post Width
The effect of w p was also investigated for an MPMA with square-shaped micropatches. Figure 7 shows the wavelength absorption for the MPMA as a function of w p from 100 nm to 1.0 μm with a fixed w m and p of 2.0 and 4.0 μm, respectively. Sufficient absorption of over 90% was obtained, and a single absorption mode was achieved over a wide range of w p values from 100 nm to 1.0 μm, which is half that of w m at 2.0 μm. This result is attributed to the strongly localized plasmonic resonance at the edge of the micropatches. The post width has less impact on the absorption properties, and single-mode operation can be maintained, in contrast to the 2-D PLA with Si-post structures, which produces a gap resonant mode inside the Si post.

Effect of Through-Holes
High-performance uncooled IR sensors require a small absorber volume to achieve fast responsivity. Through-holes are thus typically formed on absorbers to reduce the thermal mass. They are also beneficial for etchant or etching gas to fabricate thermal isolation structures by bulk micromachining. Therefore, the use of through-holes with the MPMA was investigated; Figure 8 shows a schematic illustration of the MPMA with square-shaped through-holes. The absorption properties of an MPMA with square-shaped micropatches and square-shaped through-holes were calculated by RCWA, in which p and w m were fixed at 4.0 and 2.0 μm, respectively. Figure 9 shows the wavelength absorption of the MPMA as a function of the through-hole width (w t ) from 100 nm to 1.0 μm. The presence of throughholes had no influence on the absorption properties of the MPMA in the studied w t range. This indicates that the micropatches induced an antenna effect, because the plasmonic resonance mainly occurs at the edge of the micropatch, and narrow-gap resonance is formed, as shown in Fig. 5. The same effect was also confirmed for an MPMA with circular micropatches and circular through-holes. The through-holes did significantly reduce the absorber volume of the pixels, which leads to fast response and provides an advantage for surface micromachining techniques, such as under etching of the absorber area. 30

Conclusions
Au-based MPMAs were theoretically investigated to aid the development of high-performance wavelength-selective uncooled IR sensors. The RCWA calculations demonstrated that wavelength-selective absorption could be realized depending on the shape and size of the micropatches. Broadband absorption was achieved with circular micropatches, whereas unity absorption was realized with square-shaped micropatches over a wide range of MWIR and LWIR wavelengths. The FDTD calculation demonstrated that the strong plasmonic resonance was confined at the edges of the micropatches, which leads to a small pixel size. A post height of ∼80 to 200 nm was appropriate to obtain a narrow-gap resonance. However, the post width has less impact on the absorption properties and can maintain single-mode absorption, in contrast to that of Si-post structures. Through-holes can be formed on the bottom plate due to this 3-D plasmonic resonance and can reduce the thermal mass of the pixel as well as providing advantages for the fabrication of cavities.    All-metal-based MPMAs can be applied for any type of uncooled IR sensors, such as thermopiles, bolometers, and silicon-on-insulator diodes. 31,32 The results obtained here will contribute to the development of high-performance single-mode wavelength-selective uncooled IR sensors for multicolor imaging in the wideband MWIR and LWIR regions.