Supporting Information Infrared Perfect Absorbers Fabricated by Colloidal Mask Etching of Al-Al 2 O 3 -Al Trilayers Thang Duy Dao 1,2,3,*, Kai Chen 1,2, Satoshi Ishii 1,2, Akihiko Ohi 1,2, Toshihide Nabatame 1,2, Masahiro Kitajima 1,2, and Tadaaki Nagao 1,2,* 1 International Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. 2 CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan. 3 Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan. *Corresponding Authors: Dao.Duythang@nims.go.jp, NAGAO.Tadaaki@nims.go.jp
1. Simulations of the MIM perfect absorbers using different plasmonic materials Figure S1. (a) Dielectric functions of Al, Au, Ag and W 1. (b) Simulated absorptivities of geometrically identical PAs but with four different metallic materials. The resonances of the PAs show little dependence on the materials. A comparison of dielectric functions between several plasmonic materials including Au, Ag, W and Al is shown in Figure S1a 1. The simulated absorptivites of geometrically identical PAs but with four different metallic materials (Au, Ag, W, Al) are shown in Figure S1b. Even though Al shows high loss in IR region compared to Au and Ag, the simulated absorptivity of Al-PA shows almost identical performance (absorptivity and bandwidth) compared to those PAs made of noble metals of Au and Ag (Figure S1b). The result indicates the possibility of using the earthabundant Al for high-performance IR plasmonic devices such as Al-PA. 2. Electromagnetic field distributions of the Al-PA excited at SP-photonic coupling mode As discussed in Figure 2, the origin of the short-wavelength resonance (M 1 ) is the coupling between SP and photonic mode of the periodic structures. The electromagnetic field distributions of an Al-PA with the parameters p = 4.4 µm, d = 3.3 µm, and t = 0.2 µm excited at M 1 (4.16 µm) are shown in Figure S2. The electric field E x enhanced by 8 folds (Figure S2a) indicates a weak
SP resonance compared to the LSP resonance at M 2. In addition, the electric loop-likes and the magnetic hot-pots are observed between top and bottom Al layers (Figure S2b) indicating that the coupling SP is excited and coupled to photonic property of the Al-PA. Figure S2. Electromagnetic field distributions of an Al-PA with parameters of p = 4.4 µm, d = 3.3 µm, and t = 0.2 µm excited at SP-photonic coupling mode M 1 (4.16 µm). (a) Electric field distribution of E x at the incident wave plane shows electric field located at the edges of Al disk resonators is enhanced by a factor of 8, indicating a weak SP resonance. (b) Three magnetic hot-pots in the insulating layer indicate the magnetic fields are oscillating in different phases, which are induced by the coupling between SP and photonic mode when the diameter of the Al disk (3.3 µm) is comparable with the resonant wavelength at M 1 (4.16 µm). (c) Induced electric field E z shows the hot-pots related to the charge motion in third order mode. White arrows indicate the charge motions in the top and bottom Al layers.
3. Simulation of the dependence of the resonant wavelengths on the resonator thickness and on the incident light polarization Figure S3. (a) Simulated resonator thickness dependence of the Al-PAs (b) Simulated polarization dependence of the Al-PAs. The periodicity, diameter, and thicknesses of insulator layer and bottom Al film were 4.4 µm, 3.3 µm, 0.2 µm and 0.1 µm, respectively for both figures. The thickness of Al disk in (b) is 0.1 µm. Figure S3a shows the simulated result of the thickness-dependent resonant wavelengths of the Al-PAs. Even though the resonator thickness changes in a wide-range from 0.05-0.4 µm, both the SP-photonic coupling mode and the magnetic mode are red-shifted only by 0.4 µm and the absorptivities are also slightly reduced. As shown in Figure S3b, the Al-PA exhibits polarizationindependent resonance because of azimuthal symmetry. 4. Controllable etching of PS mask In the fabrication of Al-PAs, the etching of PS mask determines the size of Al disk resonator and thus determines the resonance wavelength of M 2. We chose an etching recipe (oxygen 20sccm, APC pressure 1 Pa, antenna RF power 200 W, bias RF power 5 W) which showed a
linear etching rate of ~3.1 nm/ second (Figure S4d). It should be noted that the etching process of Al film (etching rate 0.82 nm/ second) using dual-gases BCl 3 /Cl 2 (3/3 sccm, APC pressure 0.15 Pa, antenna RF power 50 W, bias RF power 10 W) hardly affects the PS masks. Figure S4. SEM images of PS marks (a) Before and after etching of (b) 360 s and (c) 480 s. The scale bar in each image is 5 µm (d) Reactive ion etching of PS mask controlled the diameter of Al disk resonator. The red line is the linear fit to the data. Diameter of PS mask is linearly dependent on the etching time and the estimated etching rate is 3.1 nm/ second.
5. Wide angle range perfect absober Figure S5. (a) Simulated and (b) Measured incident angle dependence of the resonant wavelengths of an Al-PA with resonance at 8.65 µm (sample S3). The resonance at 8.65 µm shows perfect absorption in a wide angle range up to 60. The parameters of the Al-PA are p = 4.4 µm, d = 3.3 µm and t = 0.2 µm. The Al-PAs reveal high absorptivity in a wide incident angle range as shown in both simulation and measurement results of sample S3 with geometrical parameters of p = 4.4 µm, d = 3.3 µm and t = 0.2 µm (Figure S5). The main magnetic mode shows perfect absorption and unchanged peak position in a wide angle range up to 60. In contrast, the SP-photonic coupling mode, which can be excited due to the matching between SP momentum with the momentum of the excited photon and periodic structure, are strongly dependent on the incident angle.
6. Al-PA based selective thermal emitters Figure S6. (a-d) SEM images of the Al-PAs used for thermal emission experiments. The scale bar in each image is 5 µm. (e) A photo of a typical Al-PA thermal emitter device. (f) Schematic of an Al-PA device having two Au electrodes. In the thermal emitter application, we designed and fabricated four Al-PAs on 0.5 3 cm 2 Si (100) substrates named S3a, S3b, S3c and S3d. Their resonant wavelengths were 6.73 µm (S3a), 7.46 µm (S3b), 8.15 µm (S3c) and 8.65 µm (S3d). Their SEM images are shown in Figure S6a-d, respectively. Gold contacts of 0.1 µm in thickness were deposited at the two ends of the substrates as shown in Figure S6e. The Al-PA thermal emitters were then heated by applying sufficient currents in the bottom Al film through Au contacts as illustrated in Figure S6f.
Figure S7. (a) Simulated (blue) and measured (red) reflectance spectra of an Al-PA with magnetic resonance M 2 of 7.26 µm (p = 4.4 µm, d = 2.3 µm, t = 0.2 µm). (d) Emission from the Al-PA thermal emitter at different temperatures ranging from 75 300 C. (c) Black-body emissions at different temperatures. We measured emission from an Al-PA (p = 4.4 µm, d = 2.3 µm, t = 0.2 µm) with resonant wavelength of 7.26 µm at different temperatures (Figure S7a-b). The two resonances at 3.85 µm
and 2.94 µm are attributed to the first- and second-order of the SP-photonic coupling modes, respectively. A shoulder at 5.95 µm might be attributed to the electric mode where the electric dipoles in the top Al disk and bottom Al film oscillate in same phases. Although the SP-photonic modes are remarkable in the simulation, they showed lower absorptivities in the measured results due to the lattice imperfection of the fabricated samples, thus Al-PA exhibits single emission at the main magnetic mode M 2. By changing the applied current through the Al film, the Al-PAs were heated to 75 C, 150 C, 200 C and 300 C as shown in Figure S7b. The relative emission (at M 2 ) from Al-PA increased as the temperature increased. Figure S7c shows the plots of blackbody radiations at different temperatures ranging from 75 300 C, which corresponds to the maximum emission ranging from 8.5 5 µm. The Al-PAs based thermal emitters in our experiment were designed in this wavelength region. Reference: (1) Rakic, A. D.; Djurišic, A. B.; Elazar, J. M.; Majewski, M. L. Optical Properties of Metallic Films for Vertical-Cavity Optoelectronic Devices. Appl. Opt. 1998, 37 (22), 5271 5283.