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Scientific Reports volume 14, Article number: 28000 (2024 ) Cite this article long lasting performance
An infrared (IR) absorber is a crucial component for thermal detectors, requiring high absorptance over a broad wavelength range while maintaining low heat capacity for optimal performance. Most thermal detectors use a thin film IR absorber that is suspended in air, supported by a layer beneath it for mechanical stability. However, this support layer increases heat capacity without contributing to IR absorptance, thereby reducing the performance of thermal detectors. In this paper, we introduce a polarization-independent nanowire array absorber using flanged nanowires with a C-shaped cross-section. This C-shaped design provides mechanical stability, eliminating the need for a support layer. Although nanowire array is generally known to exhibit polarization characteristics, the unique structure of the proposed flanged nanowires enables them to achieve polarization-independent properties, resulting in high absorptance similar to that of film absorbers. We theoretically analyzed the polarization-independent characteristics of the flanged nanowires using an optical circuit model and optimized the flanged nanowire structure using finite-difference time-domain (FDTD) simulations. Finally, we experimentally demonstrated the polarization-independent characteristics of the flanged nanowires and confirmed their high absorptance comparable to that of film absorbers.
Infrared (IR) absorbers are essential components in devices such as energy harvesters1,2, thermophotovoltaics3,4, and thermal detectors5, as they absorb infrared radiation emitted from heat sources. To be applicable across a wide range of applications, IR absorbers require high absorptance over a broad wavelength range. To achieve this, research is continuously being conducted on various material and structural approaches, such as multilayers6,7,8 and metamaterials9,10. Multilayers, composed of alternating metal and insulator layers, cancel out the reflected wavelengths from each layer, leading to high absorptance due to the absence of reflected infrared. However, designing the thickness and materials of each layer to cover a broad wavelength range is challenging11. Metamaterials are artificial materials that enable phenomena not possible in nature. They are formed by arranging sub-wavelength-sized metal nanowires in various structures such as rings12, squares13, and textiles14 on an insulator–metal layer. The upper metal nanowires utilize plasmonic effects to achieve high absorptance, and by adjusting the width, length, and spacing of the nanowires, high absorptance over a broad wavelength range can be attained. However, the thick insulator layer in metamaterials increases heat capacity15,16, which raises noise equivalent temperature difference (NETD) and degrades the performance of thermal detectors like bolometers, making their application difficult.
IR absorbers used in bolometers also face the issue of increased heat capacity17,18. Most bolometers use thin-film IR absorbers suspended in air, with a support layer present to mechanically stabilize the thin IR absorber. This support layer increases heat capacity, leading to an increase in NETD and thus reducing the performance of the thermal detectors.
We considered flanged nanowires with excellent mechanical stability, eliminating the need for a support layer as IR absorbers. Nanowires with flanges at both ends, forming a ‘C’ cross-section, possess high mechanical stability, allowing them to float stably without a support layer19,20. Additionally, flanged nanowire array exhibit polarization-independent characteristics, achieving high absorptance similar to that of film IR absorbers. Generally, horizontally aligned nanowire array shows high absorptance for transverse electronic (TE) mode waves oscillating along the nanowire length. In contrast, transverse magnetic (TM) mode waves oscillating perpendicular to the nanowire length mostly transmit through, as the nanowire width is much smaller than the incident light wavelength, preventing the nanowire electrons from interacting with the incident light. Therefore, conventional nanowire array absorbers had the limitation of being applicable only to IR polarized in the TE mode21,22,23. However, flanged nanowire array, with widened side areas and dense spacing due to the flanges, has low impedance between nanowires. Thus, unlike conventional nanowire array, flanged nanowire array interacts with TM mode waves across the entire array layer, resulting in high absorptance.
In this work, we introduce flanged nanowire array with polarization-independent characteristics in the IR wavelength range for the first time. First, we used an optical circuit model, capable of calculating the absorptance of nanostructures24,25,26, to analyze why conventional nanowire array without flanges do not absorb TM mode waves in the IR range and exhibit polarization characteristics. To enhance TM mode wave absorptance, we designed the cross-section of flanged nanowires and optimized the structure using Ansys Lumerical finite-difference time-domain (FDTD) simulations by adjusting the nanowire thickness, flange width and length, and nanowire spacing. Finally, to experimentally verify the polarization-independent characteristics of the flanged nanowire array, we measured and compared the IR absorptance of flanged and flat nanowire array and films using Fourier transform infrared (FT-IR) spectroscopy. The results showed high absorptance for both TE and TM mode waves in films and flanged nanowire array, similar to the simulation results, while flat nanowire array exhibited low absorptance for TM mode waves, confirming polarization characteristics. These results challenge the conventional belief that nanowire array exhibit polarization characteristics in the IR wavelength range and suggest that they could be utilized as a new platform for IR absorbers similar to film-type IR absorbers.
To compare the polarization characteristics of a conventional nanowire array, represented by a schematic flat nanowire array, and the proposed flanged nanowire array, we conducted FDTD simulations. Figure 1a,c show the schematic diagrams of the flat nanowire array and flanged nanowire array used for the simulations. The flat nanowires and flanged nanowires were designed with identical widths (W) and thicknesses (T) of 370 nm and 10 nm, respectively, and a distance (D) of 30 nm between the nanowires, except for the presence of flanges in the latter. Additionally, both the flat and flanged nanowire arrays are designed to be 2 µm away from the substrate to form a Fabry–Perot cavity to enhance IR absorptance, and the substrate surface is deposited with an aluminum (Al) layer to act as an IR reflecting layer. The inset of Fig. 1a shows the cross-section of a flat nanowire. The conventional flat nanowire’s rectangular shape, without structural design, results in a smaller facing area between adjacent nanowires. This leads to a lower capacitance value between the nanowires, acting as a high impedance (Zc = 1/jωC) to TM mode waves. High impedance hinders the interaction of TM mode waves with the absorber, causing it to pass through rather than being absorbed. In contrast, the inset of Fig. 1c shows the cross-section of a flanged nanowire, where the flanged structure increases the facing area between adjacent nanowires. The increased area raises the capacitance value between nanowires, lowering the impedance to TM mode waves. Consequently, the flanged nanowire array can be expected to have increased absorptance of IR light for TM mode waves. Figure 1b presents the results of FDTD simulations performed for both flat and flanged nanowire array across the 4–14 µm wavelength range for TE and TM mode waves. As expected, both arrays show similarly high absorptance rates for TE mode waves, with maximum absorptance of 94.73% and 92.58%, respectively. However, a stark difference is observed for TM mode waves, with maximum absorptance of 44.24% for the flat nanowire array and 90.9% for the flanged nanowire array. This confirms that designing the nanowire structure to control an impedance between nanowires can enhance the absorptance for TM mode waves.
Comparison of polarization characteristics between the flat nanowire array and the flanged nanowire array. (a) Schematic of the flat nanowire array, (b) Simulation results of TE and TM mode wave absorptance for the flat nanowire array and flanged nanowire array in the MWIR and LWIR ranges, (c) Schematic of the flanged nanowire array.
To theoretically analyze the differences in absorptance due to the structure of the IR absorber, we applied the optical circuit model to calculate the IR absorptance of a film, a flat nanowire array, and a flanged nanowire array24,25,26. Converting the absorber into an optical circuit model necessitates knowledge of the material’s permittivity values, as these determine how the materials can be modeled as lumped components like resistors, inductors, and capacitors. The NiCr material used for the absorber, exhibiting negative real permittivity values and non-zero imaginary permittivity values in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) wavelength range, is modeled as a parallel connection of a resistor and an inductor, as shown in Fig. 2a,b. Meanwhile, the air between adjacent nanowires with positive real permittivity and zero imaginary permittivity, is modeled as a capacitor, as shown in Fig. 2a,b. When connecting each component in the optical circuit, the film is treated the same for both TE and TM modes. However, due to the structural characteristics of the nanowire array, the optical circuit model differs for TE and TM modes. For TE mode waves, which travel parallel to the nanowire length, the lumped component of air between adjacent nanowires is connected in parallel to that of NiCr, as shown in Fig. 2a. Since the impedance of air is generally higher than that of NiCr, most of the TE mode wave is absorbed by the nanowire rather than the air, as indicated by the dashed arrows in Fig. 2a. For TM mode waves, traveling perpendicular to the nanowire length, the lumped component of nanowire and air is connected in series (Fig. 2b), and the fringing field-induced capacitor between adjacent nanowires is also considered for accuracy in TM mode wave calculations. With the lumped component of air and nanowire in series, maximum absorptance is achieved when their impedances match, as shown by the dashed arrows in Fig. 2b. Therefore, it is crucial to adjust the high impedance of air between adjacent nanowires to be similar to that of the nanowire for effective absorptance of TM mode waves. The IR absorptance is then calculated based on the power applied to the absorber material’s resistor after converting the structure into an optical circuit.
Optical circuit model of the nanowire array and calculation and simulation results of MWIR and LWIR absorptance. Optical circuit model of the nanowire array for (a) TE and (b) TM mode waves. Calculation and simulation results of MWIR and LWIR absorptance for the (c) film, (d) flat nanowire array, and (e) flanged nanowire array.
To verify the IR absorptance according to the absorber structure, we designed film, flat nanowire array, and flanged nanowire array structures. Each structure is suspended in air without a Fabry–Perot cavity or reflecting layer. The maximum absorptance calculated using the optical circuit model is approximately 50% due to the structure lacking a reflecting layer. The thickness of each structure is designed to be 10 nm to enhance absorptance in the MWIR and LWIR wavelength range, considering the skin effect of NiCr. The nanowire width is set at 370 nm, significantly smaller than the IR wavelength, and the distance between adjacent nanowires is varied according to the structural characteristics. The flat nanowire array, which generally exhibits polarization characteristics, has a designed gap of 70 nm between nanowires, while the flanged nanowire array, aimed at creating a polarization-independent nanowire absorber, has a 30 nm gap to reduce air’s impedance between nanowires for TM mode waves. Additionally, to align the impedance of air between nanowires with that of the nanowire for TM mode waves, the flange width and height of the flanged nanowire are designed as 2 nm and 100 nm, respectively (Fig. S1).
Figure 2c–e present the results of calculating the IR absorptance for film, flat nanowire array, and flanged nanowire array across the 4–14 µm wavelength range for TE and TM mode waves using the optical circuit model, alongside FDTD simulation result. The formula for calculating the IR absorptance of each structure through the optical circuit model are shown in Fig. S2. Initially, the film shows matching results for TE and TM mode waves due to identical optical circuits, aligning closely with simulation outcomes (Fig. 2c). Figure 2d displays the IR absorptance results for flat nanowire array for TE and TM mode waves, revealing that while TE mode wave absorptance (ATE) is similar to that of the film, the TM mode wave absorptance (ATM) is notably lower compared to the TE mode wave absorptance, as confirmed through optical circuit modeling and simulation. Conversely, for the flanged nanowire array, results for TE mode waves are similar to those of the film and flat nanowire array, but TM mode wave absorptance increases significantly (Fig. 2e). The degree of polarization (DOP), calculated as [(ATE − ATM)/(ATE + ATM)]27,28, shows a polarization degree of 0.65 for the flat nanowire array and 0.08 for the flanged nanowire array at a 6 µm wavelength, the peak absorptance wavelength for the flanged nanowire array from Fig. 1b, indicating an 8.15 times lower polarization for the flanged nanowire array. This demonstrates that by designing the flanged structure and spacing between nanowires, impedance between nanowires for TM mode waves can be adjusted, and designing the impedance between nanowires similar to that of the nanowire itself enables the implementation of a polarization-independent absorber. Furthermore, the similarity between the optical circuit model calculations and simulation results for Fig. 2d,e indicates that the IR polarization characteristics observed in nanowire array are due to the high impedance between nanowires in TM mode.
Next, we aimed to analyze the changes in absorptance resulting from alterations in the flanged nanowire array structure using FDTD simulation in MWIR and LWIR wavelength range (4–14 µm). Here, absorptance was considered as the average value between the absorptance for TE mode and TM mode waves. The material for the flanged nanowire array is NiCr which has high absorptance in MWIR and LWIR wavelength range29,30, and to achieve maximum absorptance, the flanged nanowire array was positioned 2 µm away from the aluminum (Al) reflective layer to form a Fabry–Perot cavity. The width (W) of the flanged nanowires was fixed at 370 nm, significantly smaller than the IR wavelength, while the thickness (T), the distance (D) between nanowires, and the flange width (FW) and height (FH) were varied to determine absorptance through FDTD simulation. Varying only the thickness from 2 to 18 nm while keeping other variables constant, we simulated absorptance (Fig. 3a). The highest absorptance was observed at T = 4 nm, with increased reflectance and decreased absorptance as T increased. At T = 2 nm, the thinness seemed to increase transmittance, leading to reduced absorptance. When simulating absorptance with distances between nanowires increasing from 10 to 120 nm, the highest absorptance was observed at D = 10 nm, with a tendency for absorptance to decrease as D increased (Fig. 3b). As D increases, the air capacitance for TM mode waves between the nanowires decreases, leading to higher impedance and reduced absorptance.
Optimization of the flanged nanowire array structure for MWIR and LWIR absorptance. (a) Simulation results of MWIR and LWIR absorptance for the flanged nanowire array with a fixed width (W) of 370 nm, distance (D) of 30 nm, flange width (FW) of 5 nm, and flange height (FH) of 100 nm, varying the thickness (T) from 2 to 18 nm, (b) Simulation results of MWIR and LWIR absorptance for the flanged nanowire array with a fixed width (W) of 370 nm, thickness (T) of 10 nm, flange width (FW) of 5 nm, and flange height (FH) of 100 nm, varying the distance (D) from 10 to 120 nm, (c) Simulation results of MWIR and LWIR absorptance for the flanged nanowire array with a fixed width (W) of 370 nm, thickness (T) of 10 nm, distance (D) of 30 nm, and flange height (FH) of 100 nm, varying the flange width (FW) from 0 to 10 nm, (d) Simulation results of MWIR and LWIR absorptance for the flanged nanowire array with a fixed width (W) of 370 nm, thickness (T) of 10 nm, distance (D) of 30 nm, and flange width (FW) of 5 nm, varying the flange height (FH) from 0 to 140 nm.
Subsequently, we simulated the absorptance for flat nanowires without flanges and flanged nanowires with flange widths increasing from 2 to 10 nm (Fig. 3c). Flat nanowires showed a significant decrease in absorptance compared to flanged nanowires, due to polarization characteristics that reduce TM mode wave absorptance. Conversely, flanged nanowires exhibited high absorptance without polarization characteristics, but showed a decreasing trend in absorptance as FW increased. The decrease in absorptance with increasing FW is attributed to the increased impedance of the flange for both TE and TM mode waves. Finally, we simulated the absorptance for flat nanowires and flanged nanowires with flange heights increasing from 20 to 140 nm (Fig. 3d). Flat nanowires without flanges showed the lowest absorptance, and increasing FH resulted in increased absorptance. This is because the increase in FH reduces the impedance of air for TM mode waves.
Fabrication results of the film, flat nanowire array, and flanged nanowire array. (a) Schematics of the fabrication process for the NiCr flanged nanowire array on an amorphous carbon (a-C) nanograting substrate, (b) Cross-sectional scanning electron microscope (SEM) image of a-C film substrate (scale bar: 1 µm), (c) Cross-sectional SEM image of a-C nanograting substrate (scale bar: 1 µm), (d) Cross-sectional SEM image of the enlarged a-C nanograting substrate (scale bar : 200 nm), (e) Cross-sectional SEM image of HfO2 deposited on the a-C nanograting substrate (scale bar: 200 nm), (f) Cross-sectional SEM image of the NiCr film on the HfO2-deposited a-C film substrate (scale bar: 1 µm), (g) Cross-sectional SEM image of the NiCr flat nanowire array on the HfO2-deposited a-C nanograting substrate (scale bar: 500 nm), (h) Cross-sectional SEM image of the NiCr flanged nanowire array on the HfO2-deposited a-C nanograting substrate (scale bar: 500 nm), (i) Cross-sectional SEM image of the enlarged NiCr film (shaded area: NiCr) (scale bar: 200 nm), (j) Cross-sectional SEM image of the enlarged NiCr flat nanowire array (shaded area: NiCr) (scale bar: 100 nm), (k) Cross-sectional SEM image of the enlarged NiCr flanged nanowire array (shaded area: NiCr) (scale bar: 100 nm).
To compare the IR absorptance of NiCr film, flat nanowire array, and flanged nanowire array, we first checked whether the a-C substrate and HfO2 affect the absorptance of NiCr (Fig. S3). As a result, since the a-C substrate and HfO2 do not influence the IR absorptance of NiCr, we measured the IR absorptance without etching away the a-C and HfO2 layers. The IR absorptance measurements were performed in the MWIR wavelength range of 3.5 µm to 6 µm using FT-IR spectroscopy. We separately measured the absorptance for TE mode waves and TM mode waves, then calculated the total absorptance as the average of the TE and TM mode absorptance. Additionally, we compared the measurements with FDTD simulation results of models designed to replicate the fabricated absorber structures (Fig. S4). In this study, all simulated, calculated, and measured absorptance values were determined by subtracting reflectance and transmittance from 1. Reflectance and transmittance were measured and calculated in the normal direction. Figure 5a displays the total absorptance measurement results for the film, flat nanowire array, and flanged nanowire array. The film showed a maximum total absorptance of 89.17%, while the flat nanowire array exhibited a 19.7% lower total absorptance of 71.55% compared to the film’s maximum. In contrast, the flanged nanowire array reached a maximum total absorptance of 87.37%, similar to the film’s maximum and 122% of the maximum total absorptance of the flat nanowire array. This indicates that the flanged nanowire array, despite having a nanowire array structure, achieves a similar absorptance to the film, as verified through the TE and TM mode absorptance measurements. Figure 5b, which shows the TE mode absorptance measurement results for the film, flat nanowire array, and flanged nanowire array, reveals that the maximum TE mode absorptance for the film is 89.17%, with the flat and flanged nanowire array showing not significantly different maximum TE mode absorptance of 80.06% and 80.58%, respectively. However, as shown in Fig. 5c, which presents the TM mode absorptance measurements, while the film’s absorptance remains the same as in TE mode, the maximum TM mode absorptance for the flat nanowire array is 66.45%, and for the flanged nanowire array, it is remarkably higher at 95.7%. Despite general expectations for nanowire array to exhibit lower TM mode absorptance in the IR spectrum, the TM mode absorptance of the flanged nanowire array is higher than that of the film. It is attributed to its unique flanged structure and the narrow spacing reduce the impedance between nanowires for TM mode waves. Additionally, interference effects from the periodicity of the nanograting shifted the peak absorptance wavelength of the nanowire array, contributing to its higher absorptance compared to the film. Lastly, the similarity between the measured results and the simulation absorptance results for the film, flat nanowire array, and flanged nanowire array, enhances the reliability of the measurements. Therefore, it can be concluded that the flanged nanowire array improves TM mode absorptance, exhibiting polarization-independent characteristics that differ from the typical IR absorptance properties of nanowire array.
MWIR absorptance measurements and simulation results. (a) Measurement and simulation results of absorptance for the film, flat nanowire array, and flanged nanowire array under unpolarized MWIR, (b) Measurement and simulation results of absorptance for the film, flat nanowire array, and flanged nanowire array under TE mode, (c) Measurement and simulation results of absorptance for the film, flat nanowire array, and flanged nanowire array under TM mode.
In conclusion, we have proposed and demonstrated for the first time a flanged nanowire array that does not exhibit polarization in the MWIR and LWIR wavelength range and possesses absorptance similar to that of films. Through the optical circuit model, we confirmed that polarization characteristics in the nanowire array arise due to high air impedance between adjacent nanowires. To reduce the air impedance, we optimized the flanged nanowire structure using FDTD simulations. To verify the polarization-independent characteristics and absorptance of the flanged nanowire array, we fabricated flanged nanowire arrays, flat nanowire arrays, and films on an HfO2-deposited a-C substrate and measured the TE and TM mode wave absorptance in the MWIR range. The flat nanowire array, like typical nanowire arrays, shows lower TM mode absorptance compared to TE mode. However, the flanged nanowire array reduces the high air impedance between the nanowires and, due to interference effects from its periodic structure, demonstrates higher TM mode absorptance than TE mode. Furthermore, comparing the total absorptance, the film demonstrated an absorptance of 89.17%, while the flanged nanowire array exhibited 87.37%. This suggests that the flanged nanowire array, despite its nanowire structure, attains absorptance comparable to that of films. This not only breaks the conventional limitation of using nanowire array solely as polarizers or IR absorbers for polarized IR but also suggests the potential for utilizing the unique properties of flanged nanowires as absorbers in MWIR and LWIR wavelength range.
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I would like to express my deep gratitude to Dr. Duk-Hyung Lee for his valuable assistance in the calculations using the optical circuit model. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2022M3H4A1A04098829). This work was supported by the National Research and Development Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (RS-2023-00222166).
These authors are contributed equally: Beom-Jun Kim and Je-Min Kim.
School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea
Beom-Jun Kim, Je-Min Kim, Min-Seung Jo, Sung-Ho Kim & Jun-Bo Yoon
Center for Bio-Integrated Electronics, Northwestern University, 633 Clark St, Evanston, IL, 60208, USA
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B.-J.K. and J.-M.K. contributed equally. B.-J.K. conceived the idea. J.-M.K. and S.-H.K. contributed to the experiments and analyzed data. B.-J.K. and M.-S.J. contributed to the FDTD simulation. J.-B.Y. inspired the research, with guidance, and participated in data analysis. The manuscript and all figures were written and drawn, respectively, by B.-J.K., J.-M.K., M.-S.J., S.-H.K. and J.-B.Y. All authors discussed the results and commented on the manuscript.
The authors declare no competing interests.
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Kim, BJ., Kim, JM., Jo, MS. et al. Theoretical and experimental study on the polarization-independent flanged nanowire array infrared absorber. Sci Rep 14, 28000 (2024). https://doi.org/10.1038/s41598-024-79631-5
DOI: https://doi.org/10.1038/s41598-024-79631-5
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