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[Thien Duc Ngo](https://orcid.org/0000-0001-5123-9001), [Hai Dang Ngo](https://orcid.org/0000-0002-6205-0850), [Toan Phuoc Tran](https://orcid.org/0000-0002-4540-7360), Hiroshi Harada, [Tadaaki Nagao](https://orcid.org/0000-0002-6746-2686)

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[Infrared Thermophotonics: Theoretical Benchmarking of Ni–Al Superalloys](https://mdr.nims.go.jp/datasets/03056736-a3bb-4898-aae1-29ae4b864822)

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1 Infrared Thermophotonics: Theoretical benchmarking of Ni–Al superalloys Thien Duc Ngo*, Hai Dang Ngo, Toan Phuoc Tran, Hiroshi Harada, and Tadaaki Nagao*  T. D. Ngo, H. D. Ngo, T. P. Tran, T. Nagao Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Email: DucThien.NGO@nims.go.jp; NAGAO.Tadaaki@nims.go.jp  T. D. Ngo, H. D. Ngo, T. P. Tran, T. Nagao Department of Condensed Matter Physics, Graduate School of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo 060-0810, Japan  H. Harada Distinguished Emeritus Researcher, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan   Keywords: high-temperature superalloys, intermetallics, nickel–aluminium, thermophotonics, thermal emitters Abstract Ni–Al intermetallic compounds are highly stable, heat-resistant superalloys that have been studied as possible materials for automotive and aerospace applications. However, no reports of their thermophotonic applications, such as high-temperature IR emitters, have been reported. Herein, we report an integrated approach that combines a theoretical investigation of the optical properties of Ni–Al compounds with an optimized geometrical microdevice design. Benchmarking of the optical properties and device performance revealed that NiAl exhibited the best optical properties among the surveyed families of Ni–Al intermetallic compounds, comparable to those of conventional plasmonic materials in the IR region. Additionally, the NiAl-based microdevices exhibited an excellent quality of 692, making them promising high-temperature plasmonic superalloys for IR thermophotonic applications. In addition, the experimental dielectric function in the IR region was consistent with the simulated value. Simultaneously, we successfully modeled various designs of plasmonic metamaterial   2 structures based on NiAl, demonstrating the good performance of this material as a perfect spectroscopic absorber and emitter operating in the IR region.  1. Introduction In recent years, interest in the development of IR nano/microdevices to control thermal radiation and meet sustainable development goals (SDGs) has increased, particularly in terms of enhancing energy efficiency and cost-effectiveness. Key performance factors include the choice of the optimum plasmonic material and the surface nano/microstructure design. Although noble metals and base metals such as Al are suitable plasmonic elements for applications in the UV–vis or IR regions,[1–3] their role in photothermal applications is limited because of their low thermal stability and inherent ohmic and collisional losses in the IR region.[4–6] Expanding studies on plasmonic thin-film materials beyond the abovementioned elemental metals to novel high-temperature superalloys has the potential to achieve thermally stable materials with excellent spectral tunability. This is significant for the practical applications of these materials and devices, particularly in harsh chemical environments and at high temperatures.  In this study, we investigated Ni–Al intermetallic compounds as thermally stable plasmonic building blocks for IR photothermal applications. Ni- or Al-based superalloys are thermally stable, heat-resistant materials that have been widely employed in automobile and aviation technologies.[7][8–11] Among them are nickel–aluminum intermetallic compounds, promising superalloys with extremely high melting points (> 1400 °C),[12] which are materials of choice for aircraft engine components, such as high-pressure turbine blades. In the form of thin films, Ni–Al intermetallic compounds have also been studied as barrier coatings. Previous atomic-level research on Ni–Al intermetallic compounds comprehensively examined their crystal and electronic structures, mechanical properties, and thermodynamics;[13–18] some early studies also investigated the optical properties of these compounds.[19–26] Moreover, several studies have focused on crystal growth and thin-film deposition.[27–30] However, no studies have focused on their thermophotonic properties and potential IR device applications, such as perfect plasmonic absorbers, thermal emitters, molecular sensors, and thermophotovoltaics. To enable research in this unexplored field, we used Kohn–Sham density functional theory (DFT) calculations and Maxwellian electromagnetic (EM) design optimization to develop   3 microdevices based on Ni–Al intermetallic compounds for high-temperature IR applications.[31,32] First, DFT calculations were employed to investigate the chemical compositions, crystal structures, and optical properties of the Ni–Al compounds. The candidates that exhibited the best optical properties were chosen for experimental characterization, including X-ray diffraction (XRD), electrical properties, and dielectric function measurements. The experimental results were compared with those obtained theoretically. Second, electromagnetic microdevice optimization was performed to determine the design of IR microdevices. We also examined the “material–device relationship” of the Ni–Al intermetallic compounds by benchmarking the microdevice performance and selected NiAl as the best material among the candidate Ni–Al compounds. Additionally, we benchmarked both the optical properties of NiAl and the performance of its microdevices against other well-known metals to highlight the potential of NiAl as a replacement for conventional plasmonic materials. Finally, NiAl-based thermophotonic microdevices were simulated and demonstrated. We examined their suitability as perfect IR absorbers, molecular sensors, and high-temperature thermal emitters. 2. Results and Discussion 2.1. Crystal structures and optical properties of Ni–Al intermetallic compounds   Figure 1. Optimized conventional unit cells of pure bulk (a) Ni and (b) Al, along with their Ni–Al intermetallic compounds (c–j), which have been experimentally observed in different space groups. The intermetallic compounds include (c) NiAl (#221), (d) Ni2Al3 (#164), (e)   4 NiAl3 (#62), (f) Ni3Al (#225), (g) Ni3Al (#221), (h) Ni3Al (#123), (i) Ni5Al3 (#65), and (j) Ni3Al4 (#230). Blue and red spheres represent Ni and Al atoms, respectively.  First, the crystal structures of the Ni–Al intermetallic compounds were collected from the Inorganic Crystal Structure Database (ICSD) and the Materials Project as initial geometrical inputs for DFT calculations. [37][38][44] Conventional cells of Ni–Al intermetallic compounds that crystallize in various space groups are shown in Figure 1. These structures exhibit distinct atomic arrangements based on their stoichiometry and phase, which influence their electronic and optical properties. The atomic structures of pure bulk Ni (Figure 1(a)) and Al (Figure 1(b)) are also presented, highlighting the differences in crystal symmetry and atomic packing between the elemental metals and their intermetallic compositions. The optimized lattice constants and cell volumes with respect to the PBE of NiAl are listed in Table 1. The values in parentheses are the experimental measurements of the lattice constant and volume columns. With the exception of Ni3Al (P4/mmm, #123), which had a volume deviation of 4% from the reference, the remaining compounds had volume deviations of less than 1%. Therefore, the structural parameters are consistent with the reported experimental and calculated results. Among these Ni–Al intermetallic compounds, Ni3Al (Pm-3m, #221) and Ni3Al (P4/mmm, #123) exhibited magnetic moments, which are consistent with the observations of Min et al.[45] No magnetic moment was observed for any of the remaining compounds. However, further studies on the magnetic properties of these compounds are beyond the scope of this study.    5 Table 1. Materials, space groups, lattice constants, and volumes of two pure Ni and Al bulks, as well as their compositions in Ni–Al intermetallic compounds, both simulated and experimental. Material (Identified code) Space group Lattice constants [Å] Volume [Å3] Ref. Ni  (ICSD 37502) Fm-3m (#225) 3.578, 3.578, 3.578 (3.504, 3.504, 3.504) 45.800 (43.040) [46] Al  (ICSD 18839) Fm-3m (#225) 4.049, 4.049, 4.049 (4.039, 4.039, 4.039) 66.400 (65.910) [47] NiAl (ICSD 191110) Pm-3m (#221) 2.886, 2.886, 2.886  (2.885, 2.885, 2.885) 24.037 (24.010) [48] Ni2Al3 (ICSD 191103) P-3m1 (#164) 4.032, 4.032, 4.901 (4.036, 4.036, 4.896) 69.001 (69.060) [48] NiAl3 (ICSD 608787) Pnma (#62) 6.610, 7.378, 4.812 (6.598, 7.352, 4.802) 234.679 (232.94) [49] Ni3Al (MP-672232) Fm-3m (#225) 5.654, 5.654, 5.654 (5.658, 5.658, 5.658) 180.722  (181.088) [44] Ni3Al (ICSD 608811) Pm-3m (#221)  3.556, 3.556, 3.556 (3.566, 3.566, 3.566) 44.951 (45.350) [50] Ni3Al (ICSD 107862) P4/mmm (#123) 3.556, 3.556, 3.554 (3.780, 3.780, 3.280) 44.951 (46.870) [49] Ni5Al3 (ICSD 191113) Cmmm (#65) 7.454, 6.653, 3.753 (7.554, 6.522, 3.789) 186.078 (186.670) [48] Ni3Al4 (ICSD 58042) Ia-3d (#230) 11.397, 11.397, 11.397 (11.408, 11.408, 11.408) 1480.376 (1484.67) [51]  The primary objective of this study was to investigate and evaluate the optical properties of Ni–Al intermetallic compounds, particularly in the IR region. The Drude model was used to predict the optical properties of the compounds, for which the plasma frequency and scattering rate were the key parameters, as given by Equation (2). The plasma frequencies of the Ni–Al compounds were calculated from the band structures using Equation (3), and the values obtained are presented in Table 2 along three coordinates. Owing to the isotropic natures of NiAl (Pm-3m, #221), Ni3Al (Fm-3m, #225), Ni3Al (Pm-3m, #221), and Ni3Al4 (Ia-3d, #230), which crystallize in cubic space groups, the same plasma frequencies along the three primary coordinates are exhibited. In contrast, Ni2Al3 (P-3m1, #164), NiAl3 (Pnma, #62), Ni3Al (P4/mmm, #123), and Ni4Al3 (Cmmm, #65) have anisotropic structures where the three lattice constants are not equal and the plasma frequencies along the three primary coordinates differ for these materials. We selected the highest plasma frequency for each material to compare the optical properties of the compounds fairly. The plasma frequencies and dielectric functions of pure Ni and Al, as reported by Rakić et al.,[52] are included for reference and comparison with their respective intermetallic compounds, utilizing the same Drude-Lorentz model.   6  Table 2. Calculated plasma frequencies (ωp) along the x-, y-, z-directions of Ni–Al intermetallic compounds compared with values for pure Ni and Al reported in literature. Material Space group Crystal system ωp (xx) [eV] ωp (yy) [eV] ωp (zz) [eV] Nia) Fm-3m (#225) Cubic 15.92 15.92 15.92 Ala) Fm-3m (#225) Cubic 14.98 14.98 14.98 NiAl Pm-3m (#221) Cubic 7.143 7.143 7.143 Ni2Al3 P-3m1 (#164) Trigonal 5.679 5.679 4.455 NiAl3 Pnma (#62) Orthorhombic 4.579 5.290 4.624 Ni3Al Fm-3m (#225) Cubic 4.385 4.385 4.385 Ni3Al Pm-3m (#221) Cubic 4.262 4.262 4.262 Ni3Al P4/mmm (#123) Tetragonal 4.261 4.261 4.268 Ni5Al3 Cmmm (#65) Orthorhombic 4.227 3.371 4.049 Ni3Al4 Ia-3d (#230) Cubic 3.330 3.330 3.330 a)Taken from literature.[52]  Figure 2 illustrates the calculated total dielectric functions of the Ni–Al intermetallic compounds compared with those of pure Ni and Al. The real part of the dielectric function (ε₁), a quantity that describes the optical response, is illustrated in Figure 2(a). Among the Ni–Al compounds, NiAl (Pm-3m, #221) (red curve) yielded the largest negative value, indicating the best optical response. In contrast, Ni3Al4 (Ia-3d, #230) exhibits the worst optical properties. The quantity describing the imaginary parts of the optical loss is illustrated in Figure 2(b).    7  Figure 2. (a) Calculated real and (b) imaginary parts of the total dielectric functions (including interband and intraband transitions) for eight Ni–Al intermetallic compounds, compared with values for pure Ni and Al reported in literature. (c) QLSP and (d) QSPP figures-of-merits, providing a comparative analysis of their optical performance.  Ideally, a plasmonic material should have a high optical response (large negative real part) and low optical loss (small imaginary part). Therefore, two figures-of-merit (FOMs), QLSP (quality factor for localized surface plasmon; 𝑄𝐿𝑆𝑃 = −𝜀1/𝜀2) and QSPP (quality factor for surface plasmon polariton; 𝑄𝑆𝑃𝑃 = 𝜀12/𝜀2 ), which are frequently used in the plasmonic research community, were used for the comparison of the plasmonic materials. Figures 2(c) and 2(d) illustrate the QLSP and QSPP values of the Ni–Al intermetallic compounds, respectively, compared with those of pure Ni and Al. With respect to the FOMs, NiAl (Pm-3m, #221) was consistently the best material for the compounds in this study. Therefore, based on the predicted optical properties, NiAl (Pm-3m, #221) was expected to exhibit the best performance among the Ni–Al compounds. Moreover, in comparison with the elemental metals Ni and Al, the QLSP of Al is exceptionally high in the UV–vis region. However, the near- and mid-IR   8 regions are comparable to those of NiAl and exhibit the same orders of magnitude. In addition, the QLSP of NiAl surpassed that of Ni in the IR region by approximately 5  μm, and the QSPP of NiAl exceeded that of Ni by approximately 8  μm. These results indicate that the optical properties of NiAl are comparable to those of pure Ni and Al. Notably, owing to its high melting point, NiAl can be utilized under high-temperature conditions where Ni and Al may not be suitable. In this context, NiAl (Pm-3m, #221) was selected as the most promising candidate, and its superior thermophotonic performance was verified by designing an IR microdevice. 2.2. Comparison of DFT results with experimental data from single-crystal NiAl For verification, we compared the theoretical lattice constant and electrical and optical properties of NiAl with the experimental results. Before characterizing the electrical and optical properties, we determined the crystal structure of the single-crystal NiAl using XRD.  Figure 3. (a) Calculated and experimental real and (b) imaginary parts and (c) FOMs of the total dielectric functions. Vertical grey lines separate three distinct regions: IR (ε < 1.7 eV), VIS (1.7 eV < ε < 3.3 eV), and UV (ε > 3.3 eV).  Figure 3 illustrates the simulated and measured dielectric functions of the NiAl single crystal in the photon energy range (0–6 eV). The total dielectric function of NiAl is a combination of the interband and intraband parts, as mentioned in Section 4. The blue line represents the total dielectric function. The total dielectric functions calculated using the DFT + U approximation are plotted for comparison (orange dashed line). The experimental dielectric function fitted with the Drude–Lorentz model is indicated by the solid red line. Figure 3(a) illustrates the real part (𝜀1) of the dielectric function. The crossover energy (screened plasma energy), which is   9 defined as the photon energy at which 𝜀1 = 0, where the material starts to exhibit metallic behavior, is shown in Figure 3(a) at 1.74 eV (λ = 711 nm) in the near-IR region. However, in the simple IPA framework and standard PBE approach, the spin-orbit, local-field, and quasiparticle effects were not included. This causes a deviation of approximately 0.5 eV in the peak positions in the visible-to-UV range.[53] This problem can be solved using advanced approaches such as DFT + U, HSE06,[46][47] or Green’s function screening (GW) approximation.[56] In the DFT+U framework, the Hubbard U parameter can be tuned until the imaginary peak matches the experimental peak. For example, in Figure 3(b), there is an experimental peak at 3.14 eV for the imaginary part. Using the standard PBE functional, the simulated value was 2.46 eV, when U = 6.4 eV for the Ni d-orbital, a peak at 2.88 eV, which is close to the observed peak at 3.14 eV, was observed. However, in the IR region of interest, where intraband excitations dominate the dielectric function, the standard DFT approach and IPA model yield good agreement with the experimental values and are computationally economical.[31,32,53] In Figure 3(c), the strong consistency between the simulated and experimental results for the real and imaginary parts of the IR region indicates that QLSP exhibits a strong correlation in the corresponding wavelength range.  Table 3. Lattice constants, plasma frequencies, scattering rates, screened plasma wavelengths, and maximum FOM of crystalline NiAl; both simulated and experimental results are shown.  PBE DFT + U Expts. Lattice constant [Å] 2.886 2.860 2.885a), 2.890b) Volume [Å3] 24.037 23.400 24.013a), 24.138b) Plasma frequency [eV] 7.14 8.02 7.49b) Scattering rate [eV] 0.1 0.1 0.12b) Electrical conductivity [107 Ω-1 m-1] 0.69 0.87 0.66b), 1.0c), 1.02d) Screened plasma wavelength [nm] 711 599 700b) Max QLSP 5.3 7.1 4.3b) a)Taken from literature.[48] b)This study. c)Taken from literature.[13] d)Taken from literature.[57]  Table 3 lists a comparison of the physical parameters of NiAl calculated using first-principles DFT with the experimental values. The volume calculated using DFT + U is slightly lower than the experimental value (2.6%) (Table 3). We also estimated the experimental plasma frequency, scattering rate, and conductivity of single-crystal NiAl using ellipsometry, finding   10 7.49 eV, 0.12 eV, and 0.66 × 107 Ω-1 m-1, respectively. Our experimental scattering rate matches the empirical estimated value (γ = 0.1 eV) well. Notably, the PBE functional provided results closer to the experimental values, particularly for the lattice constant and plasma frequency. The DFT + U formalism can improve the exchange-correlation and interband dielectric functions; however, it does not significantly affect the dielectric function in the IR region, where the intraband contribution dominates the total dielectric function. Therefore, in this study, we used the dielectric function obtained at the PBE level for IR microdevice optimization for thermophotonic applications in the IR region. 2.3. Proposed high-temperature Ni–Al intermetallic thermophotonic microdevices As heat-resistant and thermally stable candidate materials, Ni–Al intermetallic materials have high-temperature applications such as high-pressure turbine blades and aircraft engine components, as described earlier. According to Kirchhoff’s law of thermal equilibrium,[58] the emissivity is equivalent to the absorptivity. Therefore, the absorptivity of the devices can be simulated to assess the performance of IR emitters and absorbers. We expect that Ni–Al compounds can be used in IR applications, especially as high-temperature thermal emitters. First, we examined the relationship between the optical properties of the candidate materials and the absorptivity of their IR microdevices, focusing on the grating-type absorber structure. Subsequently, the optimal candidate was used to design typical microdevices for the absorbers and thermal emitters.   11 2.3.1. Benchmarking the photothermal performance of microdevices prepared with different Ni–Al intermetallic compounds and conventional plasmonic materials  Figure 4. (a) 3D model of the grating configuration and desired absorptivity spectrum peaking at 4.25 μm to match the absorption band of CO2 gas. Comparison of two FOMs: (b) QLSP and (c) QSPP for NiAl and conventional plasmonic materials (Ag, Au, W, and Mo). The red area represents the near-IR region (0.78–2.5 μm) The vertical dashed line indicates the resonant wavelength at 4.25 μm. (d) Quality factors (QPA) (blue bar plot) and FWHMs (red line plot) of all Ni–Al compounds-based grating devices constructed using the optimal structural parameters.  Notably, the performance of a plasmonic absorber, including its absorptivity and wavelength resolution, is strongly influenced by the inherent optical properties of the materials used in microdevices. To emphasize this effect, we benchmarked not only the candidate Ni–Al intermetallic compounds but also four well-established plasmonic materials (Ag, Au, W, and Mo) as perfect absorbers with a resonant wavelength of 4.25 μm.[59] This wavelength matches the absorption band of CO2 gas and can be used as an IR light source in nondispersive infrared (NDIR) sensors for CO2. A simple grating configuration exhibiting Wood’s anomaly and the structural parameters P (periodicity), W (grating width), and H (grating height) were set to define the perfect absorbers (Figure 4(a)).[60] The thickness of the bottom metallic layer was fixed at 0.2 μm to prevent incident light from passing through the device, thereby maximizing   12 the absorption efficiency. For each grating, a grid search was performed by varying the W and H values until absorptivity peaks were found at 4.25 μm. The simulation domain was set from 3.25 to 5.25 μm around the desired absorptivity peak. We assessed the performance or quality factor of a perfect absorber using Equation (4). 𝑄𝑃𝐴 = 𝐴 ×𝜆𝑟𝑒𝑠𝐹𝑊𝐻𝑀. (4) Here, A is absorptivity, 𝜆𝑟𝑒𝑠 is the resonant wavelength (4.25 μm), and FWHM is the full-width at half maximum of the absorptivity peak.  Table 4 displays the FWHMs and quality factors (QPA) of the absorptivity spectra, as well as the optimal geometric parameters, obtained from the grid search of the Ni–Al intermetallic compounds and four well-established plasmonic materials, aimed at 4.25 μm in the grating configuration, for comparison. The materials of the Ni–Al compounds in Table 4 are listed in descending order of their FOMs, as illustrated in Figure 2. The results indicate that the improvement in the optical properties corresponds to a narrower FWHM and higher Q-factors.  Table 4. FWHMs and Q-factors of the absorptivity spectra, and optimal parameters of Ni–Al intermetallic compounds and conventional plasmonic materials (Ag, Au, W, and Mo) aimed at 4.25 μm in the grating configuration. Material Space group FWHM [nm] QPA P [m] W [m] H [m] Ag Fm-3m (#225) 5.70 731.05 4.25 1.46 0.16 Au Fm-3m (#225) 5.75 714.34 4.25 1.46 0.16 W Im-3m (#229) 5.35 718.33 4.25 1.13 0.13 Mo Im-3m (#229) 5.60 747.70 4.25 1.46 0.14 NiAl Pm-3m (#221) 6.05 692.43 4.25 1.44 0.16 Ni2Al3 P-3m1 (#164) 6.25 618.87 4.25 1.22 0.18 NiAl3 Pnma (#62) 7.95 478.51 4.25 1.23 0.23 Ni3Al P4/mmm (#123) 9.65 394.27 4.25 1.14 0.25 Ni3Al Pm-3m (#221) 9.70 392.42 4.25 1.14 0.25 Ni3Al Fm-3m (#225) 12.80 287.70 4.25 1.18 0.25 Ni5Al3 Cmmm (#65) 13.25 273.90 4.25 1.18 0.25 Ni3Al4 Ia-3d (#230) 14.10 251.07 4.25 1.22 0.25  In Figure 4(b) and 4(c), we compare the optical properties of NiAl, the best-performing material among the Ni–Al intermetallic compounds, with those of conventional plasmonic materials (Ag, Au, W, and Mo) using common FOMs, QLSP and QSPP. Notably, Ag and Au exhibit exceptionally high FOMs, particularly in the visible region. However, in the mid-IR region, the optical properties of NiAl are comparable to those of Ag, Au, W, and Mo.   13 Specifically, in the near-IR region (0.78–2.5 μm), as indicated by the red area in Figures 4(b) and 4(c), NiAl exhibits higher FOMs compared to W and Mo. According to Wien's law, this wavelength range corresponds to peak blackbody radiation temperatures between approximately 886 °C and 3442 °C. This suggests that heat-resistant NiAl superalloy-based microdevices are suitable for high-performance applications under high-temperature operating conditions. In Table 4, a comparison among the best NiAl-based microdevice and devices constructed using Ag, Au, W, and Mo reveals that the NiAl-based microdevices can achieve a performance equivalent to that of conventional plasmonic materials.  In summary, based on its optical properties and microdevice performance, NiAl, which crystallizes in a CsCl-type structure with the Pm-3m (#221) space group, was identified as the best candidate among Ni–Al intermetallic compounds. Furthermore, by benchmarking NiAl-based microdevices against their Ag, Au, W, and Mo counterparts, we assessed the balance between optical performance and practical considerations, such as high-temperature stability, demonstrating their potential as a robust alternative to conventional plasmonic materials. 2.3.2. Proposed NiAl-based microdevices for IR applications Upon assessing the effects of different materials on the performance of the IR microdevices, NiAl (Pm-3m, #221) was found to have excellent optical properties and enhanced microdevice performance. Next, we focused on the geometrical optimization of three different microdevice designs using NiAl as the most promising thin-film material for IR photothermal applications with different spectral emission features. The configurations selected for this study included gratings, microtrenches, and metal–insulator–metal (MIM) stripes.[32,61,62] As before, in all microdevices, a NiAl layer with a thickness of 0.2 μm was used as a perfect reflector to prevent the passage of light.    14  Figure 5. (a) Grating 3D model, (b) absorptivity aimed at 4.25 μm, (c) P-dependent, and (d) angle-dependent absorptivity of a NiAl grating. (e) Microtrench 3D model, (f) dual-band absorptivity/emissivity aimed at 4.60 and 13.56 μm, (g) H-dependent, (h) angle-dependent absorptivity of a NiAl microtrench configuration. (i) 3D model of MIM stripes (j) dual-band absorptivity aimed at 5.83 and 8.22 μm, (k) W-dependent absorptivity, and (l) spectral radiance at 224 C of this NiAl MIM trench.   Figure 5(a) illustrates the NiAl-based grating and its absorptivity (Figure 5(b)) with the optimized structural parameters (Table 3). A narrow FWHM of 6.05 nm results in a high QPA of 692.43, indicating excellent absorption performance. The periodicity-dependent absorptivity in Figure 5(c) reveals a linear dependence of the resonant wavelength on the   15 periodicity. At normal incidence, the resonant mode achieves peak absorptivity at 4.25 μm (Figure 5(d)). However, the absorption efficiency drops sharply for incident angles above 1°, and the resonant peaks split and shift, indicating strong angle sensitivity, which is ideal for ultrahigh-directivity IR applications. The periodicity-dependent SPP(-1) and SPP(-2) curves in Figure 5(c) and the angular-dependent SPP(-1) and SPP(1) curves in Figure 5(d) originate from surface plasmon polariton (SPP) excitation, explained by the phase-matching condition at the metal-dielectric interface.[32,63,64]  Figure 5(e) illustrates the 3D scheme of a NiAl microtrench with deep trenches. Using P = 4 μm, W = 3.7 μm, and H = 2.86 μm, it achieves two absorption peaks at M1 = 4.60 μm and M2 = 13.56 μm with efficiencies of 0.93 and 1.00, respectively (Figure 5(f)). The resonant wavelengths depend on H (Figure 5(g)), allowing tunability across the mid-IR region for key molecular vibrations. Figure 5(h) illustrates the angle-dependent absorptivity. The M1 mode remains efficient up to 10° and is influenced by SPP(-1), whereas M2 is angularly insensitive, making it suitable for omnidirectional IR applications. Despite requiring intricate fabrication, its monolithic geometry makes it ideal for ultrahigh-temperature applications such as IR heaters and thermal emitters.  The NiAl-based MIM microstripe array was designed by adding an Al₂O₃ layer between the NiAl metallic mirror and stripe array (Figure 5(i)). Optimized structural parameters were P = 5.8 μm, W = 2.34 μm, H = 0.2 μm, and T = 0.2 μm. Similar to the microtrench, it exhibits dual-band resonance at 5.83 μm (M1) and 8.22 μm (M2) (Figure 5(j)). The influence of W on the absorptivity spectra was examined while keeping P, H, and T constant (Figure 5(k)). M1 remained nearly unchanged, indicating a grating-coupled SPP-like mode, whereas M2 varied linearly with W and behaved as a localized surface plasmon resonance-like (LSPR) mode. Increasing W widened the spectral separation, allowing flexible tuning of the dual peaks for selective molecular targeting. As a proof of concept, the NiAl MIM microstripe was designed to match acetone’s vibrational frequencies at 5.83 μm and 8.22 μm. According to Kirchhoff’s law, these dual-band absorbers function as selective thermal emitters upon heating. Unlike blackbody heaters that excite a broad range of molecules, these devices efficiently heat specific molecules by matching their vibrational absorption bands. Notably, the NiAl microstripe at 224 °C exhibits sharp thermal emission at 5.83 μm (Figure 5(l)), according to Wien’s law   16 formula (𝜆𝑝𝑒𝑎𝑘 = 𝑏/𝑇, where b = 2.898 mm, K is the Wien’s displacement constant, and T is the absolute temperature in K).  We examined three types of NiAl-based microdevices: gratings, microtrenches, and MIM stripes. The grating structure (Figure 5(a)) exhibited a narrow absorptivity band; however, it was highly angle-dependent, making it suitable for high-directivity applications such as NDIR gas sensors, but unsuitable for omnidirectional applications. To address this limitation, the microtrench structure (Figure 5(f)) offers dual-band absorption, with the M2 mode being angle-insensitive, and its peak remaining stable regardless of the incident angle (Figure 5(h)). The resonant wavelength was also tunable by varying H (Figure 5(g)), although longer wavelengths increased the fabrication time and cost. The MIM microstripe device (Figure 5(i)) allowed precise tuning of structural parameters, such as a NiAl stripe as thin as 0.2 µm, reducing deposition and etching times. In summary, different NiAl metasurface designs and resonant modes can be optimized for various IR applications based on specific requirements. 3. Conclusions In this study, we integrated DFT and electromagnetic simulations to optimize the chemical composition of Ni–Al intermetallic compounds and the device geometry for IR photothermal absorbers/emitters. First, we explored the correlation between the crystal structure and optical properties of the Ni–Al intermetallic compounds. Our findings revealed that NiAl, a high-temperature superalloy, exhibits excellent optical properties on par with conventional plasmonic materials in the IR region. The measured structural parameters, electrical characteristics, and optical properties of the single-crystal NiAl sample were consistent with the predicted results, confirming the accuracy and effectiveness of the proposed approach for predicting the optical properties in the IR region. Second, an examination of the relationship between the performance of the absorber/emitter devices and the constituent Ni–Al intermetallic compounds revealed that the best candidate material, NiAl, exhibited excellent microdevice performance, potentially achieving an excellent quality factor of 692. NiAl has emerged as a novel high-temperature material in the IR region owing to its thermal stability and excellent plasmonic properties. As a proof of concept, we designed NiAl-based metasurfaces with gratings, microtrenches, and MIM microstripe structures. The proposed microdevices exhibited resonances with narrowband near-perfect absorptivity, covering a broad spectrum in the mid-IR region. Therefore, these thermophotonic microdevices have   17 various potential applications, such as in highly sensitive IR sensors/detectors, surface-enhanced infrared absorption (SEIRA), and thermal emitters for heaters and thermophotovoltaic applications. In the long term, our research aims to bridge theoretical and simulation studies with experimental advancements in superalloy-based thermophotonic devices. We adopted a stepwise approach, beginning with theoretical studies, followed by the successful uniaxial and smooth surface preparation of NiAl thin films and the fabrication of NiAl-based microdevices. Additionally, key fabrication challenges such as precise etching techniques for structuring NiAl metasurfaces are being actively addressed to ensure the feasibility of high-performance thermophotonic applications. 4. Experimental Section 4.1. First-principles DFT calculations DFT calculations were performed using the Vienna ab initio Simulation Package (VASP) code with projector-augmented wave (PAW) pseudopotentials and the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) functional.[33][34][35] The energy cutoff was set to 520 eV for all the calculations. The total energy convergence was set to 10-6 eV atom-1. The lattice constants were fully relaxed until the residual force was less than 0.005 eV Å-1. The Monkhorst–Pack scheme,[36] having reciprocal space resolutions of 2π × 0.02 and 2π × 0.006 Å-1, were set for structural optimization and optical calculations, respectively. The smearing width was set to 0.1 eV. We used the Drude-Lorentz model for optical calculations and ellipsometry fitting of the NiAl single crystal. The dielectric function of the metal is a combination of the interband and intraband components:  𝜀(𝜔) = 𝜀𝑖𝑛𝑡𝑒𝑟(𝜔) + 𝜀𝑖𝑛𝑡𝑟𝑎(𝜔). (1) The interband dielectric function (Lorentz part) was calculated using independent particle approximation (IPA),[37] as implemented in the VASP code. The intraband dielectric function is obtained from the Drude expression 𝜀𝑖𝑛𝑡𝑟𝑎(𝜔) = 1 −𝜔𝑝2𝜔2 + 𝑖Γ𝐷𝜔, (2) where 𝜔𝑝 and Γ𝐷 are the plasma frequency and Drude scattering rate, respectively.  The plasma frequency (𝜔𝑝) was calculated from the band structure as follows:[38] 𝜔𝑃2(𝛼, 𝛽) = −4𝜋𝑒2𝑉∑ 2𝜕𝑓(𝐸𝑛)𝜕𝐸𝑛𝑛𝑘〈𝑒𝛼𝜕𝐸𝑛𝑘𝜕𝑘〉 〈𝑒𝛽𝜕𝐸𝑛𝑘𝜕𝑘〉, (3)   18 where 𝑒 is the electron charge, V is the cell volume, 𝐸𝑛𝑘 is the band energy for the 𝑛-th band at k-points 𝑘, 𝑓 represents the occupation number for band n, and 𝑒𝛼 and 𝑒𝛽 denote the unit vectors in the Cartesian space. In the IR region, the scattering rate was roughly chosen as an empirical parameter at Γ𝐷 = 0.1 eV.[32,39] To select a suitable functional for comparison with the experimental optical properties in the IR region, the DFT + U formalism (U = 6.4 eV was applied to the d-electrons of nickel to improve the exchange correlation and compared with the PBE results.[40,41] 4.2. Electromagnetic simulations The optical spectral absorptivity of the microdevices was simulated using a Rigorous Coupled-Wave Analysis (RCWA) method implemented in the DiffractMOD package (Synopsys RSoft Photonic Device Tools). 4.3. Material characterization The theoretical results for NiAl (Pm-3m, #221) were compared with the experimental data to examine the accuracy of the simulation method. Single-crystal NiAl was characterized to determine its lattice constant, dielectric function, and electrical conductivity. The cubic lattice constant of NiAl (denoted as a) was calculated using X-ray diffraction (XRD; Rigaku Smartlab X-ray Diffractometer, Japan). The frequency-dependent dielectric function 𝜀(𝜔)  of NiAl single crystal was obtained by spectroscopic ellipsometry. The FTIR spectroscopic ellipsometers SENTECH (UV–Near-IR) and SENDIRA (Mid-IR) were used for measurements in the spectral region of interest (0.2–25 μm). The Drude-Lorentz model, which combines the Drude expression of free charge carriers (Equation (2)) and Lorentz oscillators, was used for the ellipsometry fitting. Once the plasma frequency ( 𝜔𝑝 ) and scattering rate ( Γ𝐷 ) were estimated from Ellipsometry fitting via Drude-Lorentz model, the electrical conductivity (𝜎) was calculated as 𝜎(𝑐𝑚−1) = 𝜔𝑝2/(4𝜋Γ𝐷) . The observed structural parameters and the electrical and optical properties of the single-crystal NiAl are listed in Table 3.  Acknowledgements This study was supported by KAKENHI (Grant No. 16H06364). DFT calculations were performed using a Numerical Material Simulator at the National Institute for Materials Science (NIMS). We are grateful to M. Kitajima and S. Otani for the fruitful discussions.    19 Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))  References [1] N. Yu, F. Capasso, Nat. Mater. 2014, 13, 139, https://doi.org/10.1038/nmat3839. [2] M.W. Knight, N.S. King, L. Liu, H.O. Everitt, P. Nordlander, N.J. 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Phys. 32 (1991) 173–183. https://doi.org/10.1080/00107519108211048.     26 The table of contents entry should be 50–60 words long and should be written in the present tense. The text should be different from the abstract text.  T. D. Ngo*, H. D. Ngo, T. P. Tran, H. Harada, T. Nagao*   Infrared Thermophotonics: Theoretical benchmarking of Ni–Al superalloys  ToC figure ((Please choose one size: 55 mm broad × 50 mm high or 110 mm broad × 20 mm high. Please do not use any other dimensions))