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Jan 27, 2018 - Through incorporation of a 12 nm thick ITO layer between the patterned gold nanodisks and the SiO2 dielectric layer, a 240 nm wide, fla...
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Letter Cite This: ACS Photonics 2018, 5, 776−781

Coupling of Epsilon-Near-Zero Mode to Gap Plasmon Mode for FlatTop Wideband Perfect Light Absorption Joshua R. Hendrickson,*,† Shivashankar Vangala,†,‡ Chandriker Dass,†,§ Ricky Gibson,†,⊥ John Goldsmith,†,§ Kevin Leedy,† Dennis E. Walker, Jr.,† Justin W. Cleary,† Wonkyu Kim,∥ and Junpeng Guo*,∥ †

Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, Dayton, Ohio 45433, United States Azimuth Corporation, Dayton, Ohio 45431, United States § KBRwyle Laboratories, Inc., Dayton, Ohio 45431, United States ⊥ University of Dayton Research Institute, Dayton, Ohio 45469, United States ∥ Department of Electrical and Computer Engineering, University of Alabama in Huntsville, Huntsville, Alabama 35899, United States ‡

S Supporting Information *

ABSTRACT: Epsilon-near-zero (ENZ) materials, when probed at or near wavelengths corresponding to their zero permittivity crossing points, have unique and interesting properties that can be exploited for enhancing nanoscale light−matter interactions. Here, we experimentally show that over an order of magnitude increase in the absorption band of a periodically patterned metal− dielectric−metal structure can be obtained by integrating an indium tin oxide (ITO) subwavelength nanolayer into the insulating dielectric gap region. Through incorporation of a 12 nm thick ITO layer between the patterned gold nanodisks and the SiO2 dielectric layer, a 240 nm wide, flat-top perfect (>98%) absorption centered at 1550 nm wavelength is enabled. The demonstrated wideband, perfect absorption resonance is shown to be due to coupling between the gap plasmon mode of the metasurface and the ENZ mode in the nanoscale ITO film. KEYWORDS: epsilon-near-zero, perfect light absorption, plasmonics, metasurface, light−matter coupling

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absorber device was reported by using electrically induced carrier depletion in an ENZ material layer.15 Recently, a stack of indium tin oxide (ITO) layers with different doping concentrations was shown to provide broadband perfect absorption, albeit only for oblique angle of incidence.16 In this work, we show that a nanofilm of ITO can significantly broaden the absorption band near the ENZ wavelength when an ITO nanolayer is integrated into the gap region of a metal− insulator−metal plasmonic structure. Epsilon-near-zero materials are a class of materials that have their real part of the electric permittivities crossing zero at one or multiple wavelengths. Certain materials naturally possess this property, such as metals at their plasma frequencies or at strong phonon interaction wavelengths in dielectrics such as SiO2 and

ight absorption is one of the most fundamental properties of materials and plays a very important role in modern optoelectronics. Photodetectors, solar cells, and imaging sensors all rely on absorption of light through conversion of photon energy to other forms such as electrical and thermal energies. Being able to tailor the absorption spectrum in terms of absorption strength, bandwidth, and spectral selectivity provides obvious benefits to these devices and expands the range of applications. With this in mind, a number of schemes have been proposed over the past decade to engineer the absorption of materials; examples include metamaterials, plasmonic-based optical antennas, ultrathin semiconductors, nanostructured films, time-reversed lasing and coherent absorption, and epsilon-near-zero (ENZ) materials.1−12 By multiplexing different sizes of optical antennas within the unit cell of a metasurface, perfect light absorption has been observed over a wide spectral band.13,14 A dynamically tunable light © 2018 American Chemical Society

Received: December 6, 2017 Published: January 27, 2018 776

DOI: 10.1021/acsphotonics.7b01491 ACS Photonics 2018, 5, 776−781

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ACS Photonics

Figure 1. (a) Schematic and (b) cross section of the device consisting of a TiN metallic thick film, a silicon dioxide dielectric layer, an ITO nanofilm, and a patterned periodic array of metal squares, all on top of a silicon substrate. (c) Schematic of a reference “traditional” perfect light absorber without an ENZ layer.

insulator−metal structures without integrated ENZ nanofilms.27 Second, if the spacer layer is too thick, the coupling of the gap plasmon mode to the ENZ mode will become too weak due to reduced spatial overlap of the modes, and, conversely, if it is too thin, the device can enter the strong coupling regime and the two resonances will split into two fully resolved hybrid resonance modes.28 The exact spacer layer thickness for this crossover depends on the specific thickness of the ITO layer itself. In order to optimize broadband absorption, the coupling between the gap plasmon mode and the ENZ mode should be on the verge of strong coupling, but still in the weak coupling regime. In this scenario, the two resonance modes will couple and repel each other; however, the splitting will not be large enough to resolve the two coupled resonance modes. Essentially, the upper limit for the coupling strength in the situation considered here is where the splitting to line width ratio is equal to one-half, so that there is maximum splitting/ broadening of the coupled resonances while still not being able to resolve either hybrid resonance. It is with these considerations that the specific device sizes were designed. Note that for other applications strong coupling would be advantageous; for example, in multispectral absorption two resolved resonance modes may be preferred. To calculate the absorption in the device structure, a commercial FDTD software29 (Lumerical FDTD) was used to calculate the reflectivity from the device at normal incidence. In the devices considered here where an optically thick metallic ground plane is used, the reflectivity R and absorption A are simply related by A = 1 − R. Electric permittivity values of the TiN, SiO2, and ITO materials were obtained from ellipsometry measurements, while the gold electric permittivities were taken from Palik’s Handbook of Optical Constants.30 Figure 2 shows the measured real and imaginary parts of the electric permittivity of a 12 nm thick ITO film, where the ENZ wavelength was found to be at 1450 nm. As a reference, the reflectivity in the absence of an ITO nanofilm is shown in Figure 3(a) using the following device parameters: optically thick TiN ground plane, variable SiO2 spacer layer thickness spanning 0−300 nm, and a gold square array of width 345 nm, height 80 nm, and period 900 nm. As the spacer layer thickness is increased up to 120 nm, the gap plasmon absorption resonance blue-shifts from 2.4 μm to 1.6 μm. For thicknesses greater than 120 nm the absorption becomes extremely weak. With integration of a 12 nm ITO nanofilm between the patterned gold square array and the dielectric spacer layer, the reflectivity spectrum drastically changes, as shown in Figure 3(b). A new resonance near 1.35 μm is now observed, due to excitation of the ENZ mode in the subwavelength ITO

Al2O3. Using nanofabrication techniques, electric permittivity and magnetic permeability can be tuned, allowing for artificial engineering of epsilon-near-zero materials.17 The most common ENZ materials are transparent conducting oxides and doped semiconductors. By varying the doping concentration and/or growth conditions, the ENZ wavelength can be deterministically controlled within a certain spectral range throughout the near- and mid-infrared regions.18,19 One such ENZ material is ITO, with an ENZ wavelength that can be made around 1550 nm. The ENZ mode, also known as a Berreman mode, can only be excited when films of ENZ materials are made sufficiently thin.20 The film thickness requirement for ENZ mode excitation is that it must be on the order of λENZ/50, where λENZ is the wavelength where the real part of the electric permittivity equals zero.21 ENZ modes were shown to have very large densities of states, which makes them attractive for enhancing light−matter interactions. However, ENZ modes have electric field profiles oriented normal to the film surface and, therefore, cannot be accessed with normal incidence light. Previously, ENZ modes in nanofilms have been shown to strongly couple to metamaterial resonators, dipole antennas, inter-sub-band quantum well transitions, and phonon modes.22−25 In this work, we show that the ENZ mode can be excited when being incorporated into the gap region of a metal−insulator−metal structure, and the coupling of the gap plasmon mode to the ENZ mode significantly broadens the perfect light absorption of the gap plasmon structure.

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DEVICE STRUCTURE AND SIMULATIONS

The device structure is a patterned metal−insulator−metal with an ENZ nanofilm placed in the gap region, as shown in Figure 1. The device consists of an optically thick TiN metal film as a ground plane, a SiO2 dielectric spacer layer, an ITO ENZ nanofilm, and a periodic array of gold squares. Since the structure supports a gap plasmon resonance mode localized in the gap spacer layer region which has a strong electric field oriented in the out-of-plane direction,26 the gap plasmon resonance mode can couple efficiently to the ENZ mode of the ITO nanofilm. The thickness of the dielectric spacer layer and the thickness of the ITO nanofilm are critical in the device design. As stated earlier, for the ENZ mode to be excited, the ITO nanofilm thickness must be on the order of λENZ/50 or less. For the device considered here with operation at a telecommunication wavelength of 1550 nm, the maximum thickness of the ITO layer is 30 nm. As for the dielectric spacer layer, there are two considerations to take into account. First, the spacer layer thickness affects the absorption strength of the gap plasmon resonance, as seen in numerous previous studies of metal− 777

DOI: 10.1021/acsphotonics.7b01491 ACS Photonics 2018, 5, 776−781

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nanofilm. This resonance is shifted from the ENZ wavelength of ∼1.45 μm due to strong coupling between the gap plasmon mode and the ENZ mode,28 albeit with relatively large detuning between the two resonances. As the gap plasmon resonance again blue-shifts with increasing spacer layer thickness, the detuning is reduced; however, the strength of the coupling is also reduced due to reduced spatial mode overlap. At a spacer layer thickness of 140 nm, where the detuning is minimal and the coupling has now transitioned from the strong to the weak regime, an absorption of >99% is obtained over a spectral band of 246 nm. Figure 3(c) shows the 2D plot of optical reflectivity versus ITO layer thickness and wavelength for a fixed 140 nm SiO2 spacer layer. As the ITO nanofilm thickness is increased, the absorption bandwidth also increases. The absorption strength, however, is maximized at 12 nm of ITO and then gradually reduces as the thickness is further increased. Therefore, there is a trade-off between absorption strength and absorption bandwidth. As our goal is to demonstrate flat-top wideband perfect absorption, we focused on the parameter set that gives the largest bandwidth with greater than 99% absorption. The gap plasmon mode−ENZ mode coupled wideband perfect absorption spectrum is shown in Figure 4 for the

Figure 2. Real (blue) and imaginary (green) parts of the measured electric permittivity of a 12 nm ITO film.

Figure 4. Simulated reflectivity curves from devices of integrated ENZ perfect absorber (solid black line), absorber without ENZ layer (dashed black line), and an optimized “traditional” gap-plasmon perfect light absorber without ENZ layer (solid red line). The integrated ENZ perfect absorber has flat, wideband perfect absorption over a 246 nm spectral band.

optimized structure of a SiO2 layer thickness of 140 nm and ITO layer thickness of 12 nm. The patterned gold square array has the same parameters as listed above, i.e., width of 345 nm, thickness of 80 nm, and period of 900 nm. Optimization of the integrated ENZ perfect absorber results in a flat-top wideband absorption of greater than 99% over a wavelength range of 246 nm. As a comparison, the reflectivity spectrum for a device without the ITO layer is also shown (dashed black curve). Without the 12 nm ENZ film, the reflectivity of the device increases substantially. In order to reobtain perfect absorption in a device without the ENZ layer, herein called a “traditional” perfect absorber, it is necessary to change the device parameters to 30 nm SiO2 spacer layer and 270 nm gold square width with all other parameters remaining the same. The calculated bandwidth for >99% absorption is 16 nm for the “traditional” perfect absorber (red curve). Comparing this traditional device

Figure 3. Reflectivity versus SiO2 spacer layer thickness and wavelength for (a) devices without an ENZ ITO nanofilm and (b) devices with a 12 nm ITO nanofilm. (c) Reflectivity versus ITO nanofilm thickness and wavelength for a fixed 140 nm SiO2 spacer layer.

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DOI: 10.1021/acsphotonics.7b01491 ACS Photonics 2018, 5, 776−781

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Figure 5. Simulated reflectivity of the optimized ENZ perfect light absorber as a function of angle for (a) TE- and (b) TM-polarized light.

Figure 6. (a) Experimentally measured absorption spectrum (black curve) compared with simulated absorption spectrum (red curve). Our FTIR spectrometer was configured for operation at wavelengths of 1250 nm and longer. In the shorter wavelength range, the FTIR signals are extremely weak, resulting in noisy data. (b) SEM image of the fabricated device.

beam lithography and lift-off process. The gold squares have a thickness of 80 nm, are arranged in a period of 900 nm, and are 100 × 100 μm2 in total device area. Since the gap plasmon mode’s resonant wavelength is directly related to the width of the gold squares, the detuning between the gap plasmon mode and the ENZ mode can be controlled by adjusting the square sizes. Initial simulations showed that zero detuning could be obtained with square sizes of 350 nm. In order to compensate for any potential fabrication-related errors and discrepancy between simulation results and experimental results, several devices were fabricated with sizes ranging from 320 to 360 nm. Reflectivity measurements were performed with a microscope-coupled Fourier transform infrared (FTIR) spectrometer. A 0.4 NA reflecting objective with a collection angle of 12−24 degrees was used. All measured data were normalized to a gold mirror. Figure 6 shows the measured reflectivity from a device with gold squares of about 345 nm in width as measured with a scanning electron microscope (SEM). The measured absorption was found to be >98% over a bandwidth of 240 nm. Overlaid is the simulated reflectivity curve, which matches very well with the experimental result. As a comparison, a “traditional” multiresonator-based structure at a resonant wavelength of 3500 nm displayed an increased absorption bandwidth of 100 nm,13 a value that would be further decreased upon scaling the device down to telecom wavelengths. To illustrate the coupling of the gap-plasmon resonance mode to the ENZ mode, we calculated the electric and magnetic field distributions at a wavelength of 1500 nm within the absorption band. Figure 7 shows the results of these calculations. It can be seen that at the perfect absorption wavelength the electric field is strongly enhanced and confined into the 12 nm ITO nanolayer, indicating the excitation of the

to the ENZ coupled device, which has an absorption band of 246 nm (solid black curve), integration of a 12 nm ENZ ITO layer into the gap results in a factor of 15× improvement of the absorption band. Additionally, the absorption band of the ITOintegrated perfect absorber is flat over the perfect absorption range. The optical response of nanostructured surfaces generally depends upon the angle of incidence, which leads to degraded performance for large oblique angles. To better understand the performance of the optimized ENZ coupled wideband absorber, we calculated the reflectivity as a function of incident angle over a range of 0−45 degrees for both TE and TM polarizations. The simulation results are shown in Figure 5. For TE polarization, there is essentially no change in the absorption bandwidth for angles less than 30 degrees, while for TM polarization this range is reduced to angles of less than 15 degrees. For both TE and TM polarization, as the incident angle increases above these ranges, the reflectivity on the shorter wavelength side of the absorption bandwidth begins to increase, while the long-wavelength edge of the absorption band remains constant.

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EXPERIMENTAL RESULTS AND DISCUSSIONS For experimental studies, an optically thick TiN film of 400 nm was sputtered onto a silicon substrate followed by a 140 nm thermally evaporated SiO2 spacer layer and then a 12 nm ITO layer applied using pulsed laser deposition. While any metal could potentially serve as the ground plane, we choose TiN since its melting temperature is well above the ITO deposition temperature of 300 °C; initial trials using gold resulted in pitting of the gold layer. On top of the layer structure a periodic array of gold squares was fabricated using a standard electron 779

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near-zero ITO nanofilm. A flat-top spectral band with greater than 98% absorption over a 240 nm spectral range has been demonstrated experimentally, while simulations show that >99% absorption over a 246 nm band is possible. While the study was performed in the near-infrared region around 1550 nm wavelength, the concept is applicable across a wide spectral range by device scaling and selection of epsilon-near-zero materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01491. Details of ellipsometry measurements and additional electromagnetic field profiles (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Joshua R. Hendrickson: 0000-0002-5342-0346 Notes

Figure 7. (a) Simulated electric field magnitude profile (in units of V/ m) and (b) magnetic field magnitude profile (in units of A/m) at 1500 nm wavelength within the perfect absorption band.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H. and J.C. acknowledge support from the Air Force Office of Scientific Research (Program Manager Dr. Gernot Pomrenke) under contract number 15RYCOR159 and 15RYCOR162, respectively. J.G. acknowledges support by the National Science Foundation under the award no. 1158862.

ENZ mode. At the same wavelength, the magnetic field enhancement is strongly located in the dielectric gap region between the gold squares and the ground TiN film, indicating that the gap plasmon resonance mode is simultaneously excited. The physical explanation for this effect is that when an electromagnetic wave propagates in the structure, electric fields and currents are induced in the patterned gold layer and the ITO layer. The fields can become very strong near the edges of the gold structures, due to high charge concentration at the edges of the metal. In the conductive TiN ground plane, electric fields and currents are induced in the opposite direction to that of the fields in the patterned gold squares due to the near-field coupling. The antiparallel currents then create a magnetic field in the gap SiO2 layer. Field profiles taken at either edge of the absorption band show very similar results to those taken at the center of the band; see Supporting Information. It can be seen that there is slightly more (less) electric field penetration into the SiO2 layer at the long (short)-wavelength edge. This indicates that in the short-wavelength side of the absorption band the ENZ mode slightly dominates and in the long wavelength side the gap plasmon mode slightly dominates. We also simulated the electric and magnetic field profiles at a wavelength of 2000 nm outside the absorption band. Substantially less electric field enhancement is seen in the ITO layer, with the majority of the field being located either in air above the ITO layer or in the underlying SiO2 layer, indicating that at this wavelength the ENZ mode is not excited. Therefore, it can be concluded that the wideband perfect absorption is due to the coupling of the gap plasmon mode to the ENZ mode. In this work, we have shown that the bandwidth of a gap plasmon resonance perfect light absorber can be made flat and increased significantly through the integration of an epsilon-

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REFERENCES

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DOI: 10.1021/acsphotonics.7b01491 ACS Photonics 2018, 5, 776−781