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Abstract
A mid-infrared broadband absorber with high visible light transmittance is proposed in this paper. The absorber is composed of layered ITO nanorod arrays with increasing angles fabricated by oblique angle deposition technique. The experimental results show that the average transmittance of the absorber reaches 80% in the 400-800 nm band and the integrated absorption reaches 82.9% in the 3-5 µm band, when the QCM thickness of the first layer of film is 100 nm and the deposition angle θ is 10°, the QCM heights of the second to fifth layers of nanorods are all 330 nm, and their deposition angles are 55°, 68°, 80°, and 87°, respectively. The high transmittance in the visible band is attributed to the gradient of the refractive index. The broadband absorption in the mid-infrared band results from different resonances in the empty cavities with different sizes. Such a simple and large-area absorber has potential applications in window materials and infrared cloaking.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mid-infrared broadband absorbers with high transmittance in the visible band have play an important role in windshields of stealth fighters, window materials such as air force helmets [1], and infrared cloaking [2]. In terms of enhancing visible light transmittance, many different methods have been proposed by researchers. Gao et al. [3] fabricated an Ag nanopore array film coated by glass with the help of polystyrene sphere (PS) array template. Liang et al. [4] separated a VO2 film into VO2 nanoparticles by acidification, and the separated VO2 nanoparticles usually have localized surface plasmon resonance (LSPR) properties to enhance the transmittance of visible light. Zheng et al. [5] proposed a TiO2/VO2/TiO2 multilayer film with its transmittance increased by 30.1% in the visible band compared with a TiO2/VO2 film. Aluminium-doped ZnO films were fabricated by Barhoumi et al. [6] with DC sputtering deposition to improve visible light transmittance, with its average values varying between 78% and 90%. Various Ag-Ti composite nanorod structures were prepared by Dai et al. [7] using oblique angle co-deposition technique, and the final visible transmittance was greater than 90% at different polarization directions by adjusting the mixing ratio of the materials. N. Tajik et al. [8] proposed four-layer ZnS films with gradual refractive index by glancing angle deposition (GLAD) technique, with increased transmittance in the visible band and an average transmittance of up to 93%. The guided-mode resonance (GMR) effect can also achieve high transmittance of visible light by the modulation of incident light. Lin et al. [9] demonstrated a pd-coated GMR-based device to achieve a broadband transmittance spectrum with an average transmittance of 80% under certain gas injection conditions. In the enhancement of mid-infrared absorption, different approaches have also been proposed. The first approach is a resonant absorption structure consisting of subunits of different sizes [10–14]. The different subunits have different resonances. The absorption spectra generated by these resonances are superimposed, resulting in the broadening of absorption spectra. The second one is to use metal/dielectric stacks of different sizes [15–17]. When longer wavelength light is incident, resonant absorption occurs in the wider part of the stacked structure, and when shorter wavelength one is incident, resonant absorption happens in the narrower part of the stacked structure. Their combination brings to broadening absorption spectra. The third one is to design complex broadband absorption structures by means of genetic algorithms [18,19]. For each optimization, a cost function is to be defined and the shape with the lowest cost is used for each optimization. After several iterations, a structural shape with broadband absorption can be produced. The fourth one utilizes impedance matching to achieve broadband absorption [20–22]. The fifth one employs multi-mode field resonance structures [23–26]. The resonances of the structures at different wavelengths form different resonance modes, each of which leads to resonance absorption. Their combination results in the broadening of absorption spectra.
However, it was reported that it was difficult to achieve both high transmittance in the visible band and good absorption in the mid-infrared band at the same time. To address this problem, this paper adopts oblique angle deposition (OAD) technique [27–29], which is suitable for mass production and easy to fabricate large-area structural materials, and introduces ITO which is good mid-infrared plasmonic material [30] and transparent in the visible band to design and fabricate our desired structure. The structure is composed of large-area layered nanorod arrays, which have good absorption covering the 3-5 µm atmospheric window and high transmittance in the visible band. By optimizing the structure parameters, an average transmittance of 80% in the 400-800 nm range and an integrated absorption of 82.9% in the 3-5 µm atmospheric window were obtained. Such a structure has potential applications in window-like materials and infrared cloaking.
2. Experiment
2.1 Materials
The substrate is polished sapphire, which was bought from Yuanjing Electronic Technology Co. Ltd. ITO (m(In2O3) : m(SnO2) 90 : 10) and Al2O3 target materials (99.9%) were purchased from Dingwei New Materials Co., Ltd.
2.2 Fabrication of samples
The flow chart of the structure preparation is shown in Fig. 1. A coordinate system was established with the vertical sample upwards as the Z-axis, the horizontal sample inwards as the Y-axis, and the origin of the coordinates at the centre of the lower surface of the first layer of nanorods. θ represents as the deposition angle. An electron beam evaporation system (DE500, Technology Inc.) was used during the deposition. Experimental material preparation: Firstly, an 1.5 × 1.5 cm sapphire substrate was dipped in acetone, anhydrous ethanol, and pure water in turn for five minutes for ultrasonic cleaning, and then dried in flowing nitrogen. Finally, the clean substrate was placed on the sample table in the evaporation system, and the Al2O3 and ITO targets were placed in the two crucibles 50 cm below the sample table. Experimental procedure: The deposition angle θ was set to 10° and an Al2O3 film with 100 nm thickness was deposited using the OAD technique. Then the deposition angles were set to 55°, 68°, 80°, and 87°, respectively. All the thicknesses were 330 nm and the deposition material was ITO. The deposition rate was 1 Å/s. The thickness and rate of deposition were monitored by two separate 6 MHz quartz crystal microbalances (QCMs). Evaporation deposition started when the vacuum level in the chamber reached below 5 × 10−7 Torr and the chamber vacuum level was not higher than 5 × 10−5 Torr throughout the experiment. After evaporation, the sample was removed and annealed at 550°C for 30 min in a constant temperature oven in air atmosphere, and warmed up at 1°/min.
2.3 Characterization
The transmittance T of the sample in the visible band was measured using a UV/VIS/NIR spectrophotometer [31] (Lambda 950) with air as the scanning background. For the measurement of its absorption in the 3-5 µm band, a Fourier infrared spectrometer [32] (Bruker Tensor 27) was used. Its transmittance T in the 3-5 µm band was firstly measured. Its reflectance R was obtained by using a standard gold mirror as the scanning background. The resulting absorption was described as follows.
Figure 2(a) shows the macroscopic view of the sample under sunlight. It can be seen that it has a high transmittance and the text below can be clearly seen. The morphology of the sample was observed using a field emission scanning electron microscope (SEM) (SU8010, Hitachi). The SEM image of the sample surface is shown in Fig. 2(b), which shows that the surface is columnar and has many pores. Figure 2(c) shows the SEM image of the sample side, which clearly shows that the sample is a layered nanorod array structure with different tilted angles. The first layer is an Al2O3 film with an actual deposition thickness of 86 nm. The actual tilt angles of the nanorods formed from the second to fifth layers are 27°, 35°, 43°, and 49°, respectively. Their thicknesses are 337 nm, 329 nm, 292 nm, and 253 nm, respectively. The actual thicknesses of the films deposited at the same QCM thickness decrease as the deposition angles increase. The principle behind the formation of this structure is that in the initial stage of deposition, atoms of gaseous matter condense randomly on the surface of the substrate to form nuclei. As the matter continues to accumulate, the nuclei on the substrate gradually grow and the areas without nuclei are obscured. The areas form gaps, i.e. the shadow effect. Thus, ordered nanorod arrays can be formed. After the first layer of nanorods is formed, the second to fifth layers are also similarly formed with increasing angles of deposition.
Fig. 2. (a) Macroscopic view of a layered nanorod array structure under sunlight. (b) SEM images of the surface and side (c) of the structure.
3. Results and discussion
The ITO material chosen for this paper has a small extinction coefficient in its complex refractive index in the visible band, which makes the ITO material transparent in the visible band. As shown in Fig. 3(a), the transmittance curve of the sample in the visible band shows that the sample has an average transmittance of 80% and a maximum value of 89.7% in the 400-800 nm band. The structure used is layered tilted ITO nanorod arrays, with each layer of ITO nanorods tilted at a certain angle, resulting in different equivalent refractive indices for different layers of tilted nanorods. The larger the tilt angle, the smaller the equivalent refractive index becomes, satisfying the refractive index gradient and facilitating the enhancement of transmittance [33]. Figure 3(b) shows the transmittance, reflection, and absorption curves of the sample in the 3-5 µm atmospheric window. It can be seen that both the transmittance and the reflection are smaller, resulting in a higher absorption according to Eq. (1). The integrated absorption reaches in 82.9%. In the mid-infrared band, ITO is a direct leap broadband n-type semiconductor. When the mid-infrared light transmits in laminar tilted ITO nanorod arrays, carrier oscillations in the ITO are excited and resonant absorption phenomena occur, which will be demonstrated in the simulations of the electric field intensity distributions.
Fig. 3. (a) Transmittance spectrum in the visible band; (b) Absorption, transmittance and reflection spectra in the 3-5 µm band.
In order to prove that the layered nanorod array structure has advantages in transmittance in the visible band and in absorption in the mid-infrared band, we fabricated a ITO film with the same thickness as that of layered nanorod arrays in the structure. The transmittance in the visible band and the absorption in the mid-infrared band are also plotted in Fig. 3. It can be found that althougth the absorption of the ITO film in mid-infrared band is strong, the transmittance in the visible band is low.
3.1 Effects of different angles on visible light transmittance
The optimum deposition angle for each layer is selected using a layer-by-layer optimization approach to meet the gradual increase of refractive index from air to substrate, thus achieving high transmittance in the visible band. A sapphire substrate was used in this experiment, and the epitaxial growth material was ITO. In order to better condense the gaseous material atoms on the substrate surface to form nuclei, it was necessary to match the evaporation material with the substrate. Hence, the Al2O3 with a thickness of 100 nm was deposited as a spacer, and a deposition angle of 10° was set. The conditions of the first layer was fixed unchanged. The third, fourth, and fifth layers were deposited with deposition angles of 72°, 80°, and 87°, respectively, all with a thickness of 270 nm. Four samples with the second layer tilted at 53°, 55°, 58°, and 65° with a thickness of 270 nm were prepared. As shown in Fig. 4(a), the highest visible transmittance was achieved when the second layer was deposited at the angle of 55°. Then, the first layer was fixed and the second layer was kept at a deposition angle of 55°. The fourth and fifth layers were still deposited at 80° and 87° with a thickness of 270 nm to explore the best angle for the third layer. The third layer was set at 65°, 68°, 70°, and 72°, with a thickness of 270 nm. As shown in Fig. 4(b), the highest visible transmittance was achieved when the third layer was deposited at the angle of 68°. The optimum angle for the fourth layer was then explored, with the first layer remaining the same and the second, third, and fifth layers set at 55°, 68°, and 87°, respectively, with a thickness of 270 nm. Figure 4(c) shows that the deposition angle of 80° is slightly better. As the deposition angle had to be large in order to make the outermost refractive index small enough to be close to the refractive index of air, the fifth layer was finally set at an angle of 87°. The best deposition angles of 55°, 68°, 80°, and 87° were obtained for the second to fifth ITO layers, respectively.
Fig. 4. (a) Transmittance spectra corresponding to different deposition angles θ2, where θ3 = 72°, θ4 = 80°, and θ5 = 87°. The thicknesses of deposition layers were all set to 270 nm.; (b) Transmittance spectra corresponding to different deposition angles θ3, where θ2 = 55°, θ4 = 80°, and θ5 = 87°. The thicknesses of deposition layers were all set to 270 nm. ; (c) Transmittance spectra corresponding to different deposition angles θ4, where θ2 = 55°, θ3 = 68°, and θ5 = 87°. The thicknesses of deposition layers were all set to 270 nm.
Figure 5 shows the surface and side SEM images of four annealed ITO films with deposition angles of 55°, 68°, 80°, and 87°, respectively. It can be seen that as the deposition angle increases, the tilts of the nanorods become larger and the porosities within the films increases. An ellipsometric polarimeter (RC2 XI+) was used to measure the refractive index of the four ITO films in the 400-800 nm band, as shown in Fig. 6. The refractive indexes of the ITO films decrease from 1.8 to 1.21 at 400 nm and from 1.83 to 1.25 at 800 nm as the deposition angles change from 55° to 87°. It is clear that as the deposition angles increase, the refractive indexes decrease due to the increases of the porosities within films.
Fig. 5. Images of four annealed ITO films with deposition angles θ of 55°, 68°, 80°, and 87°, respectively. (a) The side SEM with θ = 55°; (b) the side SEM with θ = 68°; (c) the side SEM with θ = 80°; (d) the side SEM with θ = 87°; (e) the surface SEM with θ = 55°; (f) the surface SEM with θ = 68°; (g) the surface SEM with θ = 80°; (h) the surface SEM with θ = 87°.
3.2 Influence of each layer thickness on mid-infrared absorption
Transmittance of the sample in the visible band is required to be high. Meanwhile, it is also necessary to improve the absorbance of the sample in the 3-5 µm band. According to Eq. (1), the increase in absorbance is simultaneously determined by the decreases of reflectance and transmittance. Since the optimal tilt angle of each layer has been determined, it means that the refractive index of each layer of the sample is determined. Hence, the reflectivity remains the same. To reduce the transmittance, the film thickness d needs to be increased to enhance the absorption. The increase of the thickness of each layer means increasing the transmission distance of the electromagnetic wave in the sample, which indicates the increase of the absorption. Figure 7 shows absorption spectra of samples with their 2-5 layer thicknesses synchronously changed. According to the comparison it is found that the greater the film thickness, the greater the absorption. This is also confirmed by the integral absorption of these samples shown in Fig. 8. Figure 9 shows the transmittance curves in the visible band for these samples with different thickness layers. It can be seen that the transmittance decreases as the thickness increases. Therefore, in order to maintain good transmittance in the visible band and good absorption in the 3-5 µm band, it is appropriate to choose a thickness of 330 nm for each layer. The average transmittance of the sample is 80% in the 400-800 nm band, and the integrated absorption is 82.9% in the 3-5 µm band.
Fig. 7. Absorption spectra of samples with their 2-5 layer thicknesses synchronously changed in the 3-5 µm band.
Fig. 9. Transmittance spectra of samples with their 2-5 layer thicknesses synchronously changed in the 400-800 nm band.
3.3 Structural optical property simulations
The structure shown in Fig. 10(a) was simulated by the finite difference time domain (FDTD) method, The first layer was an Al2O3 film the same as substrate material. The second layer was an ITO nanorods with a length of 338 nm and a radius of 25 nm at a tilt angle of 27°. The third layer was an ITO nanorods with a length of 322.5 nm and a radius of 22.5 nm at a tilt angle of 35°. The fourth layer was an ITO nanorods with a length of 290.7 nm and a radius of 17 nm at a tilt angle of 43°. The fifth layer was an ITO nano rods with a length of 237 nm and a radius of 14.5 nm at a tilt angle of 49°. As shown in Fig. 10(b), since the structure was a hexagonal lattice arrangement, only one unit cell was simulated. Figure 10(c) was an X-Y cross-sectional view when z = 0. The boundary surfaces in the perpendicular electromagnetic wave propagation direction were set as periodic boundary conditions, and the boundary surfaces in the direction parallel to the electromagnetic wave propagation direction were set as perfect absorption boundary conditions (PMLs). The permittivity of ITO was expressed by the Drude Lorentz model [34],
Fig. 10. Simulation construction diagram. (a) 3D view; (b) Vertical view; (c) X-Y cross-sectional view at z = 0.
Fig. 11. (a) Simulated absorption spectrum in the 3-5 µm band. (b) Simulated transmittance spectrum in the 400-800 nm band.
To investigate the mid-infrared broadband absorption mechanism, the electric field intensity distributions were simulated at the 3 µm, 4 µm, and 5 µm wavelengths, respectively. The cross sections were taken in the XY plane in the Z direction with 150 nm as an interval. The electric field intensity distributions in eight cross sections are given, as shown in Fig. 12. Black dot curves in the figure denote the cross sections of ITO nanorods. Evidently, there are different distributions at different heights. The electric field is mainly distributed in the cavities between the nanorods. For the 3 µm incidence, the electric field is mainly distributed between z = 300-600 nm and 900-950 nm. For the 4 µm incidence, the electric field is mainly distributed between z = 0-300 nm and 600-950 nm. For the 5 µm incidence, the electric field is mainly distributed between z = 0-150 nm and 600-950 nm. Between z = 0-450 nm, the electric field becomes stronger with smaller z as the wavelength increases. Between z = 450-750 nm, the electric field enhances with larger z with increasing wavelength. Between z = 750-950 nm, the electric field enhances with larger z at 3-5µm incidence. It can also be found that there are strong electric field distributions at the connecting regions of the nanorods at different tilt angles. The larger z value indicates the smaller radius and greater tilt of the nanorods. The nanorods thus become sparser and larger cavities between the nanorods are produced, resulting in strong electric field due to strong resonance in the cavities. According to the above analyses, the multi-layer nanorod array structure will stimulate the cavity resonances at different sizes. These resonances result in carrier oscillations in the ITO at the different cavity regions, thus producing a more broadband mid-infrared light absorption.
Fig. 12. Electric field intensity distributions at eight different interfaces spaced at 150 nm at three different wavelengths of 3 µm, 4 µm, and 5 µm. (a) z = 0 nm; (b) z = 150 nm; (c) z = 300 nm; (d) z = 450 nm; (e) z = 600 nm; (f) z =750 nm; (g) z = 900 nm; (h) z = 950 nm.
4. Conclusion
In this paper, a layered nanorod array structure with different tilt angles, which has high transmittance in the visible band and strong absorption in the mid-infrared band, was prepared. The first layer of Al2O3 with a deposition angle of 10° and an 100 nm QCM thickness was deposited on a sapphire substrate using OAD technology. The QCM heights of the second to fifth layers of nanorods are all 330 nm, and their deposition angles are 55°, 68°, 80°, and 87°, respectively. The tilt angles of the four-layer nanorods actually formed were 27°, 35°, 43°, and 49°. The actual heights of five layers were 86 nm, 337 nm, 329 nm, 292 nm, and 253 nm, respectively. The experimental results show that the structure has an average transmittance of 80% in the wavelength range of 400-800 nm and an integrated absorption of 82.9% in the wavelength range of 3-5 µm. The high transmittance comes from the refractive index gradient. The absorption broadening originates from the resonances in the empty cavities with different sizes. Such a mid-infrared absorber with high transmittance in the visible band provides a reference for the design and fabrication of a new generation of stealth window materials.
Funding
National Natural Science Foundation of China (61771227, 62071208); Graduate Research and Innovation Projects of Jiangsu Province (2021XKT1223); Priority Academic Program Development of Jiangsu Higher Education Institutions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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