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Abstract
Absorptive metamaterials made of epsilon-near-zero indium tin oxide and silicon dioxide films are designed and fabricated for wideband perfect light absorption near the epsilon-near-zero wavelength. By increasing the number of bilayers, we achieve over 90% absorption in a spectral bandwidth of 0.95 microns.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Perfect light absorption in metamaterials is significant for a variety of applications in optics and photonic systems. Metamaterial perfect absorbers made of patterned metal-dielectric-metal subwavelength structures have been extensively investigated and demonstrated in different spectral regimes from microwave and terahertz [1–4] to optical frequencies [5–17]. Patterned metal-dielectric-metal perfect light absorbers intrinsically have narrow operation bands due to the nature of electromagnetic resonance. By using multiple mode resonance complex structures, wideband perfect light absorbers have also been demonstrated [5–14]. However, patterned metal-dielectric-metal light absorbers require complex fabrication processes. Since multilayer metal-dielectric-metal thin-film light absorber structures have advantages as they are easy to fabricate without lithography and can be integrated with almost any optical device, the multilayer thin film absorbers were investigated to achieve wideband absorption by the multiple resonances from the metal and dielectric layer cavities [18–23]. In this work, we replace the metal layer in the metal-dielectric-metal cavities by using an epsilon-near-zero (ENZ) material, indium tin oxide (ITO). ENZ materials are materials which the real part of the electric permittivity approaches zero at a certain wavelength (ENZ wavelength) [24]. Previously, numerical simulation results show that ENZ material and dielectric bilayer structures gave strong absorption resonance around the ENZ wavelengths [25,26]. Thus, in this work, we design, optimize, and fabricate the epsilon-near-zero thin-film metamaterials for wideband near-perfect light absorption. A 10-bilayer structure device is experimentally demonstrated to achieve over 90% absorption in the wavelength range from 1.5-2.4 µm.
2. Thin-film metamaterial structure and simulation results
Figure 1 shows the structure of the ITO and SiO2 thin film absorptive metamaterials. An optically thick layer of TiN forms the bottom layer of the structure and is deposited on any substrate. This layer is used to prevent light transmission into the substrate. Each bilayer made of a pair of ITO and SiO2 thin films. An N-bilayer structure consists of N pairs of ITO and SiO2 bilayers on an optical thick TiN substrate.
To enable valid simulations, complex electric permittivity data of TiN, SiO2, and ITO were measured by using a J.A. Woollam V-Vase spectroscopic ellipsometer over a spectral range of 0.27-3.0 µm. The thicknesses of the films were determined by simultaneously fitting reflectance and ellipsometric data. For ITO films, the ENZ wavelength varies dramatically with thickness for ultra-thin films (≤ 20 nm) [27]. Therefore, the films investigated initially targeted a thickness between 50-100 nm, where the ENZ wavelength of the films has small variations. The permittivity of ITO from 100 nm thickness ITO thin film is presented in Fig. 2(a). For TiN and SiO2 films, the permittivities of these two materials have small variations due to the layer thicknesses. Electric permittivities of TiN and SiO2 films of 100 nm thickness are measured and shown in Fig. 2(b) and (c). The complex permittivities of films have been observed to vary slightly between deposited films, but the effect has no observable effect on the measured reflectance spectra or in the simulated model.
First, we calculated the absorption spectra of SiO2/ITO thin film structures for different number of bilayers by using a commercial FDTD simulation software (Lumerical, Inc.). In the simulations, the measured electric permittivities of the materials were used. We fix the layer thicknesses of each layer at 75 nm, the optical absorptance were calculated for different number of bilayers. Figure 3 shows calculated optical absorptance versus wavelength for different number of bilayers. The strongest absorption occurs around the ENZ wavelength of 1.6 µm. The absorption from 1 bilayer and 3 bilayers is less than 90%. As the number of bilayers is increased, peak absorption increases as well as the absorption bandwidth, due to the multiple absorption resonance modes. The absorption bandwidth is heavily influenced by the number of bilayers present in the structure. As shown in Fig. 3, more absorption resonance modes appear as increasing the number of bilayers. These absorption resonance modes raise the total absorption and the absorption bandwidth. The simulation result shows ultra-wide absorption bandwidth for 20 bilayers, although such a thick multilayer structure may be challenging to fabricate. A structure with 10 bilayers was chosen as the optimal number of bilayers in maximizing the absorption bandwidth while minimizing the complexity of fabrication.
Fig. 3. Calculated optical absorptance versus wavelength for different number of bilayers. Each bilayer consists of a 75 nm SiO2 layer and a 75 nm ITO layer with an overall thickness of 150 nm.
With a fixed SiO2 layer thickness, the optical absorptance of thin-film structures of different number of bilayers (1, 3, 5, and 10) was calculated for varying ITO layer thickness. The simulation results are plotted and shown in Fig. 4. Figure 4 shows the spectral absorption as a function of ITO layer thicknesses for different number of bilayers: (a) 1 bilayer, (b) 3 bilayers, (c) 5 bilayers, and (d) 10 bilayers. It is seen that by increasing the number of bilayers, the absorption band increases. The absorptive thin-film metamaterial structure of 10 bilayers on TiN substrate gives nearly unit absorption over a wavelength range from 1.4-1.8 µm.
Fig. 4. Calculated optical absorptance versus wavelength and ITO layer thickness for devices with different number of bilayers: (a) 1 bilayer, (b) 3 bilayers, (c) 5 bilayers, and (d) 10 bilayers.
3. Experimental results
ITO films of various thicknesses were deposited by a reactive sputter at an elevated temperature in oxygen and argon plasma. The ITO target used has a composition of 80:20 In2O3:SnO2. The ENZ wavelength of the ITO layer can be tuned by adjusting the deposition temperature, sputter power, and the ratio of oxygen to argon in the chamber [28,29]. We utilize high precision mass flow controllers to adjust the oxygen to argon flow rate ratio between 0-10% with a precision of about 0.5% and be able to shift the ENZ wavelength from 1.2 µm to 2.2 µm. We have observed that for our system this is the most reliable and reproducible mechanism for effectively tuning the ENZ wavelength. ITO films were deposited at 500 °C using 3% O2 in Ar. Variations were minimized by maintaining a uniform low base pressure, deposition pressure, sputter power, and deposition time for all depositions.
Titanium nitride was deposited onto silicon substrates and characterized. After this, bilayers of ITO and SiO2 thin films were sputtered in succession without breaking vacuum. ITO was deposited at elevated temperatures as noted above, but the system was allowed to cool prior to the deposition of the SiO2 layer. The sample was removed from the system after 1, 3, 5, and 10 bilayers were deposited so that the absorption can be measured and compared to the simulation results. Reflectance data were measured using a Bruker FTIR spectrometer with a reflecting microscope objective. Although the sample was measured at normal incidence, the reflecting objective has a light cone from about 12°-24° which is radially symmetric. An optically thick gold film was the reference. Since the transmittance is zero due to the optically thick TiN layer at the bottom of the structure, the absorptance (A) can be obtained from measured reflectance (R) as A=1-R.
Figure 5(a) shows the measured optical absorptance of fabricated devices with different number of bilayers. The 1-bilayer device has high reflectivity, although a small resonance peak is observed at 1.6 µm, which is roughly at the ENZ point of the ITO film. The 3-bilayer device gives a maximum absorption that increases to above 70% and the resonance shifts to 2 µm. In this case, a lower harmonic is observed at 1.1 µm, likely due to Fabry-Perot interference from the multiple interfaces. The absorption of the 5-bilayer device is over 90% in a 500 nm bandwidth, and two distinct resonances are observed, one just below 1.5 µm and one about 1.7 µm, close to the resonance of the first layer. The 10-bilayer device has an absorption bandwidth of 0.95 µm over 90% absorption, with maximum absorption of 99.8% at 1.66 µm.
Fig. 5. (a) Measured optical absorptance of structures with 1, 3, 5, and 10 bilayers. (b) The cross-sectional SEM image of the 10-bilayer device. The entire structure sits on a silicon wafer substrate (bottom). The optically thick TiN is the first dark layer on the Si substrate. The conductive ITO layers appear bright, while the insulating SiO2 thin film layers appear dark in the SEM image of the device cross section.
The measured absorption band of the 10-bilayer device has wider absorption spectral band compared to the simulation results. The absorption bandwidth is wider, as it extends towards the mid-wave IR (the absorption is just under 50% at 3 µm). The simulation and experiment results have shown multiple interference fringes in the shorter wavelength region, but the simulation predicts these fringes will not oscillate as strongly as they are observed from the fabricated sample. The discrepancy between these results is attributed to the roughness of the thin films and a difference in the optical properties of the deposited ITO films. The permittivity of ITO films in the simulations was assumed to be uniform across all bilayers and have an ENZ wavelength of 1.49 µm. However, ITO films have been shown that it is tunable in post-deposition annealing [28,29]. The deposition of each subsequent bilayer in the structure acts as a post-deposition anneal of each previously deposited ITO film, which results in a difference in the optical properties of the deposited ITO films. The roughness and the differences in optical properties of ITO films result in an enhancement of optical absorption in the mid-wave IR region.
Figure 5(b) shows a SEM cross sectional image of a fabricated 10-bilayer structure device. The substrate of the structure is a silicon wafer, which is the dark region on the bottom. An optically thick TiN layer is deposited onto the silicon substrate; the TiN is the dark layer at the bottom of the stack. Small roughness is seen in this layer and the roughness propagates throughout the rest of the thin film layers during the deposition process. The alternating bright/dark layers are the ITO and SiO2, respectively. The 10-bilayer device has a total thickness of about 1.5 µm indicating that each layer is about 75 nm thick. Each layer has an RMS roughness in the range of a few nanometers, although some peaks deviate by as much as 20 nm, which is typical for reactively sputtered ITO thin films [30,31]. Some roughness features propagate into subsequent layers, creating the appearance of increased layer roughness overall, however there are no apparent systematic roughness or thickness abnormalities; each layer has a relatively uniform thickness and uniformly distributed roughness. This is corroborated with AFM measurements of similarly deposited TiN and ITO witness samples which had RMS roughness values between 2 and 5 nm.
4. Discussions
To investigate the difference between the measured and simulated absorption spectra, we simulated a structure that included ITO films with differing optical permittivities. All structures had 10 bilayers of ITO/SiO2. First, all 10 bilayers have the same ITO permittivity with an ENZ wavelength at 1.5 µm. In the second simulation, the bottom 5 bilayers remained the same- the ITO had an ENZ of 1.5 µm, while the top 5 ITO layers have 1.74 µm ENZ wavelength. In a third simulation, 4 different ITO permittivities were assumed in the ENZ thin film metamaterial structure. From bottom to top, the ENZ thin film metamaterial has 3 ITO layers with 1.29 µm ENZ wavelength, 2 ITO layers with 1.5 µm ENZ wavelength, 2 ITO layers with 1.74 µm ENZ wavelength, and 3 ITO layer with 2.11 µm ENZ wavelength. All of the permittivity data was obtained from spectroscopic ellipsometry of alternate ITO films deposited and annealed under different conditions; therefore the wavelengths chosen in these simulations do not necessarily match what is in the 10-bilayer fabricated sample, but all are real and plausible permittivities of ITO in such a structure.
Figure 6 shows the absorptance determined from these simulations plotted against the experimental data shown in Fig. 5. With a single electric permittivity of ITO films in the thin film metamaterial, the absorption bandwidth is only 0.62 µm. With two different electric permittivities of ITO films, the absorption bandwidth extends into longer wavelengths, while keeping the shorter end of the spectrum unchanged. The ENZ thin metamaterial with two different electric permittivities of ITO films has an absorption bandwidth of 0.95 µm. The metamaterial with four different electric permittivities of ITO films proves an even broader absorption bandwidth of 1.4 µm, although this is blue-shifted from the other spectra. This data most closely resembles the experimental data, although the bandwidth is even wider. By engineering a structure in which the ENZ increases with each deposited layer of ITO, the absorption bandwidth could extend from below the telecommunication wavelength up to the mid-IR to achieve a very broadband near perfect absorption.
Fig. 6. Plots showing the difference in absorptance that results from different ITO permittivities in the 10-bilayer thin film structure. The black curve is the experimentally measured absorptance of 10 bilayers structure device shown in Fig. 5. The curve labeled as “1 ENZ” has a uniform permittivity for all 10 bilayers. The other two curves have 2 and 4 different permittivities of the ITO films in the 10-bilayer stack.
5. Conclusion
In this work, we have designed and fabricated a new kind of absorptive metamaterials using multiple SiO2 and ITO bilayer thin-film structures, for wideband near-perfect light absorption in the short wavelength IR region. A 10-bilayer device was experimentally demonstrated to give an over 90% optical absorption in a wide bandwidth of 0.95 µm.
Funding
Air Force Office of Scienticif Research (FA9550-19RYCOR048); Alabama Graduate Research Scholarship Program; Individual Investigator Distinguished Research Award of the University of Alabama in Huntsville.
Acknowledgments
The work was partially supported by the Air Force Office of Scientific Research (Program Manager Dr. Gernot Pomrenke) under award number FA9550-19RYCOR048. J. Chen acknowledges the support from Alabama Graduate Research Scholarship Program. J. Guo acknowledges the support of the Individual Investigator Distinguished Research Award of the University of Alabama in Huntsville.
Disclosures
The authors declare no conflicts of interest.
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