Hole Array Perfect Absorbers for Spectrally Selective Midwavelength

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Hole Array Perfect Absorbers for Spectrally Selective Mid-Wavelength Infrared Pyroelectric Detectors Thang Duy Dao, Satoshi Ishii, Takahiro Yokoyama, Tomomi Sawada, Ramu Pasupathi Sugavaneshwar, Kai Chen, Yoshiki Wada, Toshihide Nabatame, and Tadaaki Nagao ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00249 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 6, 2016

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Hole Array Perfect Absorbers for Spectrally Selective Mid-Wavelength Infrared Pyroelectric Detectors Thang Duy Dao1,2,*, Satoshi Ishii1,2, Takahiro Yokoyama1,2, Tomomi Sawada1,2, Ramu Pasupathi Sugavaneshwar1,2, Kai Chen1,2, Yoshiki Wada3, Toshihide Nabatame1,2, and Tadaaki Nagao1,2,4,* 1

International Center for Materials Nanoarchitectonics, National Institute for Materials

Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. 2

CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-

0012, Japan. 3

Research Center for Functional Materials, National Institute for Materials Science (NIMS),

1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. 4

Department of Condensed Matter Physics, Graduate School of Science, Hokkaido University,

Kita 8, Nishi 5, Kita-ku, Sapporo 060-0810, Japan *

Corresponding Authors: [email protected], [email protected]

SYNOPSIS TOC

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ABSTRACT: We demonstrate a hybrid plasmonic-pyroelectric device operating as an uncooled mid-wavelength infrared detector with narrowband spectral selectivity. The device consists of a plasmonic perfect absorber with a built-in pyroelectric ZnO layer: It consists of a ZnO layer sandwiched by a Au microhole array as a top electrode and a Pt bottom electrode as a template for the uniaxially grown ZnO film. The geometrical design of the plasmonic Au (hole array)/ZnO/Pt system is determined by the numerical electromagnetic simulation and then fabricated by colloidal-mask lithography combined with reactive-ion etching. The fabricated detectors exhibit excellent spectral selectivity at the pre-designed plasmonic resonances which are tunable by changing the Au hole diameters. The results obtained here open up a route for realizing new type of uncooled spectroscopic infrared detectors with compact design and simple fabrication process.

KEYWORDS:

plasmonic

metamaterial,

perfect

absorber,

colloidal

lithography,

pyroelectric detector, mid-wavelength infrared detector. Uncooled mid-wavelength infrared (MWIR) detectors, especially pyroelectric detectors, enjoy widespread use ranging from human motion sensor, thermal imaging to Fourier transform infrared spectroscopy (FTIR). Among different types of infrared detectors, pyroelectric detectors are robust, simple, and can be operated at room temperature without cooling. They can exhibit wideband and flat spectral response with high sensitivity as well as simple design without adopting elaborate thermal insulation, which makes them advantageous for industrial applications together with their low fabrication costs1–3. To realize high performances as well as making them more industrial compatible, search for efficient pyroelectric materials with nontoxic earth abundant elements have been done. Also, integration with photonic structures are expected to embody spectral selectivity at desired wavelengths for potential applications such as non-dispersive infrared spectroscopy (NDIR),


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infrared chemical imaging as well as multi-color IR imaging. In this context, a number of dispersive photonic structures have been proposed such as micromachined Fabry-Perot interferometers4,5, photonic crystals6,7, or plasmonic structures8–18.

Plasmonic perfect absorbers (PA) is another type of structure which exhibit nearly 100% absorptivity at desired wavelengths. Plasmonic absorbers have been successfully used in a wide range of applications such as in thermophotovoltaics19,20, photodetectors15,21, molecular sensing22–24 and in thermal emitters25–27. By tuning the geometrical parameters, the absorption band of the PAs can be readily tuned. Since the infrared light absorbed by a PA is effectively converted into heat28–32, integration of a MWIR PA and a pyroelectric detector is a promising strategy for realizing wavelength-selective devices such as spectroscopic IR sensing and multi–color IR imaging.

In this work, we propose a design for wavelength-selective pyroelectric MWIR detectors by taking advantage of the spectrally-selective light-heat conversion of PAs. The device is integrated with a c-axis oriented textured ZnO film as a pyroelectric material33–35, sandwitched between a Pt film and an Au film patterned with a hexagonal array of microholes. The electromagnetic simulations are performed based on the rigorous coupled-wave analysis (RCWA) to optimize the geometrical parameters of the device. The hybrid plasmonic-pyroelectric detectors (PA-PIRs) are fabricated using colloidal-mask lithography combined with reactive-ion etching (RIE) process to pattern periodic arrays of Au microholes on ZnO films. Spectral responsivity curves of the integrated metamaterial PA-PIRs exhibit perfect agreement with the absorptivity curves of the designed devices and clearly demonstrate their high performance for the narrowband IR detection in the MWIR region. By tuning the diameter of the Au holes, the responsivity curve of the detector can be tuned


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flexibly, evidencing its suitability for spectroscopic infrared sensing and multi-color MWIR imaging.

RESULTS AND DISCUSSION

Figure 1. (a) A schematic diagram of the PA-PIR with definition of simulation domain (dash rectangle) and geometrical parameters: periodicity - p, Au hole diameter - d and pyroelectric ZnO thickness - t. (b) Simulated spectra of a PA-PIR (p = 3.0 µm, d = 1.8 µm, and t = 0.68 µm) reveal a dual-band PA resonated at 3.88 µm and 5.50 µm, which match to two vibrational bands of N2O and NO gases, respectively. (c) Electric field distributions excited at these two resonant peaks. Electric field polarized in X direction propagates along -Z axis and the amplitude of the incident electric field was normalized to unity. (d) Simulated absorptivity spectra of PA-PIRs show the tunability of the PAPIRs by tuning Au hole sizes (1.4, 1.8, 2.4 and 2,7 µm) whereas the periodicity and the ZnO thickness are 3.0 µm and 0.68 µm, respectively for all the four samples.


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Our proposed PA-PIR is illustrated in Figure 1a where a hexagonal array of plasmonic Au hole and bottom Pt film is insulated by a ZnO layer. The thicknesses of Au hole array and Pt films are fixed at 0.1 µm and 0.2 µm, respectively. The diameter - d, the periodicity - p of Au hole array, and the thickness - t of the ZnO film were optimized using the RCWA simulation to have a perfect absorptivity in the range of 3 - 7 µm (MWIR region) where most of the molecules have their vibrational wavelengths in this region. The mechanism and detailed optical properties of the trilayered metal hole-insulator-metal PA have been discussed in previous reports25,36,37. Here we chose hexagonal geometry of the plasmonic holes at the top metallic layer since the structure with this azimuthal symmetry exhibits polarizationindependent perfect absorptivity (Supporting Information Figure S1a). The incident angle dependence of the absorptivity is also simulated and shown in the Supporting Information Figure S1b. As mentioned later, hexagonal periodicity can be patterned by colloidal-mask lithography combined with RIE process. In a plasmonic hole PA, the coupling between the highly confined surface-plasmon polariton (SPP) and the incident light can be enhanced by employing leaky Bloch modes in the periodic structure, which enables an increased density of state of the coupled SPP-photonic modes.36 Here, the photonic effect in the periodic Au hole array works as a perturbation, resulting in hybrid coupling modes as the linear combinations of the SPPs and photonic states36. In addition, the electric dipoles of these hybrid coupling modes at the top Au hole layer also induce electric dipoles at the Pt surface bridged by ZnO dielectric spacer. These induced dipoles at the Pt surface oscillate against and quench dipole oscillations at the surface of Au hole layer. In addition, oscillating dipoles at Au and Pt surfaces are sensitive to damping due to lossy nature of metals in the IR region, resulting in a high absorptivity at the resonant wavelengths of PA-PIRs. By controlling the thickness of the ZnO layer, nearly perfect absorptivity at these resonant modes can be achieved. The resonance of the PA-PIR can be tuned by changing the ZnO thickness – t (Supporting


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Information, Figure S1c), periodicity – p (Supporting Information Figure S1d) or the diameter of Au hole – d (Figure 1b). Here we focus on the tunability of the PA-PIR by changing the diameter of Au hole – d whereas the ZnO thickness – t and periodicity – p were fixed at 0.68 µm and 3.0 µm, respectively. For example, in the NDIR spectroscopy for detecting nitrous oxide (N2O) and nitric oxide (NO) gases, both of which are air pollutant greenhouse gasses, it is desirable to have a dual-band resonant detector which can detect a band at 2500 - 2600 cm-1 of N2O gas and another band at 1750 - 1950 cm-1 of NO gas. By using the present architecture of the PA-PIR detector, a dual-band resonant PA-PIR matching to these two vibrational bands can be designed. As a result, Figure 1b shows the simulated spectra of a PA-PIR with optimized parameters of d = 1.8 µm, t = 0.68 µm, p = 3.0 µm exhibiting a dual-band perfect absorption with resonances at 3.88 µm and 5.50 µm, which match to the vibrational bands of N2O (at 2577 cm-1 or 3.88 µm) and NO (at 1818 cm-1 or 5.5 µm) gases, respectively. The electric field distributions at these two resonant peaks shown in Figure 1c reveal that the electric fields are confined around Au fishnets and Au holes, respectively, which efficiently enhance the light-heat conversion and transfer to the adjacent ZnO pyroelectric film at these wavelengths. Together with the use of the dual-band resonant IR detector, a single-band resonant PA-PIR is achievable also by simply tuning the Au hole diameter while fixing the periodicity and the ZnO thickness at 3 µm and 0.68 µm, respectively. For example, the simulated absorptivity of PA-PIR with a Au hole diameter of 2.7 µm reveals a single-band perfect absorption at 4.42 µm (blue curve in Figure 1d). On the other hand, a broadband multiple absorption (green curve in Figure 1d) can be achieved with a hole diameter of 2.4 µm. Finally, a narrowband perfect absorption at 3.66 µm, accompanied by a secondary absorption band at 5.48 µm, is also designed with a hole diameter of 1.4 µm (black curve in Figure 1d).


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The PA-PIRs were fabricated by colloidal-mask lithography combined with RIE process (described in detail in the Methods session). It has been found that the hexagonal array of the atoms in the (111) plane of Pt promotes the nucleation and growth of the hexagonal ZnO oriented along the [0002] direction38. Prior to the fabrication of PA-PIRs, the crystallographic and optical properties of the fabricated ZnO film were investigated by X-ray diffraction (XRD) analysis. As shown in Figure 2a, a θ - 2θ diffraction pattern of a 100 nm ZnO film deposited on Pt/Si substrate reveals a strong (002) diffraction peak at 34.8° (JCPDS card No 36-1451), indicating that the fabricated film is highly oriented along its c-axis, which enables good pyroelectricity of the ZnO film. Complex dielectric function of ZnO film was experimentally retrieved from the ellipsometric data and adopted for the RCWA simulation to optimize the geometrical parameters of the PA-PIRs. Figure 2b illustrates the complex dielectric function of the ZnO film from 0.2 – 25 µm. The sputtered ZnO film shows an interband transition peak at 0.36 µm and exhibits high transparency in the visible region where the real part of the permittivity is of about 4 which is in good agreement with the previous report in the same spectral range39. In the MWIR region of our interest, the complex dielectric function of the fabricated film is non-absorbing (imaginary part is almost zero) and the real part is ~3.8. These properties make ZnO suitable for using it as an insulating layer in a PA-PIR.


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Figure 2. (a) XRD pattern of the ZnO film fabricated on a Pt-coated Si substrate indicates highly caxis oriented hexagonal film of ZnO. The inset in (a) illustrates the spontaneous polarization of the ZnO film along c-axis when irradiated by IR light. (b) Complex permittivity of the fabricated ZnO film ranging from UV to FIR region. (c) Raman spectra of ZnO film averaged from a 20 × 20 µm area on the sample S2 by using 2D scanning Raman specrtroscopy. (d) 2D Raman spectroscopic mapping of sample S2 integrated at 437 cm-1 peak of ZnO Raman spectra. The inset in (d) represents an optical image of sample S2. The scale bars in (d) and its inset are 3 µm.

Next, to demonstrate the performance of the fabricated device, we designed and fabricated three different PA-PIRs using the same periodicity and ZnO thickness of 3.0 µm and 0.68 µm, respectively, and with different Au hole diameters: S1 (d = 1.9 µm), S2 (d = 1.6 µm), and S3 (d = 1.4 µm). The SEM images of these theree fabricated samples are shown in Figure S2a-c of the Supporting Information. Before investigating the IR-resonant property and the spectral responsivity of the device, quality of the pyroelectric ZnO layer was further examined by


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Raman spectroscopic mapping. Figure 2c presents Raman spectra of ZnO film averaged from a two-dimensional (2D) scanning Raman spectroscopy taken on a 20 × 20 µm area of S2. The result reveals two strong peaks at 99 cm-1 ( E2low mode) and 437 cm-1 ( E2high mode), and three lower peaks at 332 cm-1( E2low − E2high mode), 380 cm-1 (A1(TO) mode), 582 cm-1 (E1(LO) mode), by which the whole ZnO film was confirmed to be in high crystallinity40. The strength and the sharpness of the E2low and E2high modes evidence that the ZnO film grew highly uniaxially with its c-axis aligned to the film perpendicular direction. These results confirm the observation from the XRD. A 2D Raman image was constructed by integrating Raman signal at 437 cm-1 of ZnO (Figure 2d) and mapped for a 20 × 20 µm scanned area of S2. The intensity distribution of 2D Raman image clearly reflects the micro-pattern of the fabricated PR-PIR indicating the hexagonal periodic Au hole array patterned on ZnO film. The measurement setup is shown in Figure 3a, and the details are described in the Methods section. Before the measurements, each fabricated PA-PIRs were mounted onto an N-channel junction gate field-effect transistor (JFET; 2SK2751, Panasonic) with a load resistor of 47 kΩ to work at the voltage mode. A picture and schematic circuit of an integrated PR-PIR are shown in Figures 3b and 3c, respectively. The frequency-dependent responsivity of the fabricated PA-PIRs were measured using a blackbody light source heated up at 300° C combined with a mechanical chopper. For a pyroelectric detector, when the chopper frequency increases, the irradiation time (light-on) and the induced heat in the pyroelectric film decrease, thus the surface charge also decreases. Similarly, when the chopper frequency increases and the light-off, the opposite surface charge also decreases. Therefore, the responsivity of the pyroelectric detector is inversely proportional to the chopper frequency. As shown in Figure 3d, the responsivity of sample S2 exhibits as high as 3.25 V/W at the chopper frequency of 1 Hz and it decreases by increasing the chopper frequency. The


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working frequency range of the fabricated device covers a broad range from very low frequency (< 1Hz) up to kilohertz.

Figure 3. (a) Setups of the responsivity measurements of PA-PIRs include two measurements using two excitation sources, the frequency-dependent responsivity measurement uses a blackbody emitter combined with a mechanical chopper, and spectral response measurement uses a tunable IR laser . (b) A photo of a fabricated PA-PIR integrated with a charge-coupled signal extraction circuit as a buffer using a JFET and (c) Schema of the buffer circuit. (d) The frequency-dependent responsivity of PAPIR S2.

The wavelength-dependent responsivity of the PA-PIRs was investigated using a tunable IR laser source with the repetition rate of 1 kHz. A set of the spectra taken from the three PAPIR samples, including absorptivity spectra and spectral responses, are summarized in Figure 4. Herein, we demonstrated the tunable spectral selectivity of the PA-PIRs aiming at a dualband PA-PIR to a nearly single-band PA-PIR by tuning the diameter of Au holes. Figure 4a-c present the simulated absorptivity spectra of the designed PA-PIRs S1, S2 and S3, respectively, where all of the devices show a dual-band resonance with different peak


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intensities and peak positions. As seen from the results, when the Au hole size decreases, the short-wavelength resonance is gradually blue-shifted with a relatively increased absorptivity, whereas the long-wavelength resonance is red-shifted with a relatively decreased absorptivity. S1 with largest Au hole size (d = 1.9 µm) has a short-wavelength resonance at 3.86 µm (absorptivity of 86 %) and a long-wavelength resonance at 5.24 µm (absorptivity of 77 %). Interestingly, compared to S1, with smaller Au hole sizes, the short-wavelength resonances of S2 (d = 1.6 µm) and S3 (d = 1.4 µm) are blue-shifted to 3.74 µm and 3.66 µm, respectively, wherein their absorptivities are increased to near-perfect absorption: 97.8 (S2) and 98.7 % (S3). By contrast, the long-wavelength resonances of S2 and S3 are red-shifted to 5.40 µm (absorptivity of 74 %) and 5.48 µm (absorptivity of 55 %), respectively. Figure 4d-f show measured absorptivity spectra of S1, S2 and S3, the peak positions of which are highly consistent with the designed PA-PIRs. It should be noted that due to the small working angle of long-wavelength resonance (Supporting Information Figure S1b), lower absorptivities at long-wavelength resonances observed in the measurement compared to the simulation are attributed to the non-collimated and non-normal nature of the incident beam in the reflectance measurement configuration. In Figure 4g-i, the spectral responsivity curves of the three fabricated devices S1, S2 and S3 are shown respectively. All three devices exhibit excellent spectral response which are effectively embodied by the designed wavelength-dependent absorptivities of the PAs. As seen in Figure 4g, the spectral response of S1 (d = 1.9 µm) exhibits two peaks at 3.86 µm and 5.24 µm, which are highly consistent with the short-wavelength and long-wavelength resonances, respectively, of the designed PA-PIR S1 as shown in the simulated (Figure 4a) and measured (Figure 4d) spectra. As seen for the figures, the spectral responses of the PAPIRs are readily controlled by tuning the diameter of Au hole from 1.9 µm (S1) to 1.6 µm (S2) and 1.4 µm (S3). As seen that the short-wavelength peak in the spectral responsivity


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curve is blue-shifted from 3.86 µm (S1, Figure 4g) to 3.74 µm (S2, Figure 4h) and 3.66 µm (S3, Figure 4i). By contrast, the long-wavelength peak of the spectral response is blue-shifted from 5.24 µm (S1) to 5.40 µm (S2) and 5.48 µm (S3). Interestingly, the responsivity ratio between the short-wavelength resonance and long-wavelength resonance increases from 1.04 of S1 to 1.5 of S2 and to 2.74 of S3, indicating the potential for flexibly switching from a dual-band IR detector (S1) to a nearly single-band IR detector (S3). The perfect agreement between the optical absorptivity and the responsivity curve has evidenced that the ZnO film is effectively heated when the infrared light was resonantly absorbed by the PAs.

Figure 4. Performance of three fabricated PA-PIRs; S1, S2, S3. (a) - (c) Simulated absorptivity spectra. (d) - (f) Measured absorptivity spectra. (g) - (i) Spectral responsivity curves.

It is worth noting that we chose the current hole array design because the top Au and the bottom Pt layers can be used as the electrodes that significantly reduces the number of


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fabrication steps and shows excellent heat conduction from the metallic layers of the PA to the ZnO pyroelectric layer. Since the pyroelectric ZnO layer is embedded inside the perfect absorber itself, plasmonically enhanced electromagnetic near-field located at the surfaces of the Au hole array and coupled to the bottom Pt layer plays a key role in directly inducing heat in the ZnO layer. For example, the calculated electric field distributions in sample S3 excited at the resonant wavelength of 3.66 µm (Figure 5a and Figure S3a-c in the Supporting Information) show that the electric field is not only confined and enhanced at the metal surfaces but also confined in the ZnO layer, which can efficiently induce the heat in the ZnO layer of the PA-PIR detector. Compared to the blackbody painted Au electrode (Supporting Information Figure S4), the responsivity of our device PA-PIR (S3) is 8 times higher than that of the blackbody painted PIR, and shows a linear response to the input power (Figure 5c), exhibiting a high performance of a selective MWIR detector. In addition, here the complicated microstructure designs for the thermal isolation often used in commercialized micorborometers or thermopiles are not really necessary, that greatly reduces the fabrication process. While ZnO used in the current study has the advantages of being nontoxic and compatible to the current semiconductor processes, many other pyroelectric materials can be applicable to the current design as well. For example, replacing ZnO by lithium tantalate (LiTaO3) or deuterated triglycine sulfate (DTGS) is expected to improve the responsivity of the current PA-PIR by a few orders of magnitude.


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Figure 5. (a) Simulated electric field distribution in PA-PIR S3 excited at 3.66 µm resonant wavelength reveals high field enhancement at the Au surface and in the interior of the ZnO layer. (b) Comparison of the responsivity (on-off excitation) between a blackbody painted electrode PIR and PA-PIR S3 at 3.66 µm excitation. (c) Input power dependence of PA-PIR S3, at the excitation wavelength of 3.66 µm matching with the resonant wavelength of S3.

CONCLUSION We demonstrate a simple design and facile fabrication process to realize low-cost, wavelength-selective pyroelectric MWIR detectors. The PA-PIRs were systematically designed to have narrowband and spectrally selective perfect absorptivity in the MWIR region to demonstrate effective integration with pyroelectric ZnO films. To realize high pyroelectric performance with the ZnO film, we have adopted Pt(111) bottom electrode as the template substrate for the uniaxial growth of ZnO film. The resonances of PA-PIRs show excellent controllability by tuning the diameter of the holes in the top Au electrodes and also yield high sensitivity without having any thermal isolation. The IR spectral detection realized in our PA-PIRs can be used for various applications such as temperature sensing, IR-color imaging, NDIR spectroscopy, and IR material sensors. In addition, the proposed design of the


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IR spectral sensors can also be applied for other type of IR detectors such as photoconductive or thermoelectric detectors. METHODS Fabrication of PA-PIRs. The PA-PIRs were fabricated using colloidal-mask lithography combined with RIE process. A 0.2-µm thick Pt film was deposited on a 1×1 cm2 Si (100) substrate using a sputtering instrument (i-Miller CFS-4EP-LL, Shibaura) where a 10-nm thick Ti layer was sputtered prior to the Pt film as an adhesion layer. Subsequently, a 0.68-µm thick ZnO film was then sputtered on the Pt film. To prepare the Au hole array on the ZnO film, a monolayer of polystyrene spheres (PS) (Polybead Polystyrene Microspheres Polysciences) was deposited on the ZnO film as a mask layer using the following process.27 First, the ZnO/Pt/Si substrates were aligned in a 10° tilted plastic container filled up with deionized water. A monolayer of PS spheres was prepared on the surface of water by using a slow rate flow of PS solution from a glass slide tilled 30° to the water surface. Finally, a monolayer of PS spheres was formed on ZnO/Pt films by using a micro-pump to drain the water in the plastic container. The size of the PSs was shrunk by RIE using oxygen plasma at low power (O2/20 sccm, APC pressure 1 Pa, antenna RF power 200 W, bias RF power 5 W, Ulvac CE-300I) where the etching times were varied to have different PS mask sizes. It should be noted that we repeated multiple short RIE times with each etching time of 90 second to prevent overheating by ion bombardment during the etching process, which can melt the PS mask and damage the ZnO film. Using this optimized etching condition, the etching rate was found to be 2 nm/second and prevented any damages on the ZnO. To prepare the Au film layer, a 0.1-µm thick Au film with a 5-nm thick Cr adhesive layer was deposited on PS-masked the ZnO film using electron beam deposition technique (UEP-300-


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2C, ULVAC). A Au hole array was finally achieved by removing the PS mask in ethanol under ultrasonication and rinsed by toluene and ethanol solutions. Characterizations. The crystalline structural property of the ZnO film was characterized by an X-ray diffractometer with Cu Kα line (SmartLab, Rigaku). The optical characterization of the sputtered ZnO film was carried out using two ellipsometers (Sentech SE 850 DUV for UV-SWIR region and Sentech Sendira for MWIR-FIR region). SEM images of the fabricated PA-PIRs were taken using an SEM (Hitachi SU8200) at the accelerating voltage of 5 kV. Two dimensional Raman spectroscopy imaging was performed using a Raman microscope (WITec Alpha 300S microscope combined with a RayShield coupler excited at 532 nm laser). The reflectance spectra of the fabricated PA-PIRs were taken using a variable angle reflectance accessory of a conventional FTIR spectrometer (Nicolet iS50R FT-IR Thermo Scientific) with a liquid-nitrogen-cooled mercury cadmium telluride detector and a KBr beam splitter. Then the absorptivity spectra of the fabricated devices were simply carried out using the formula: absorptivity = 1 – reflectance, since the transmittance is almost zero for a thick Pt film of 0.2 µm. The chopper frequency-dependent responsivity of the fabricated PA-PIR was taken using a blackbody light source heated up at 300° C where the output light was modulated by a mechanical chopper (5584A, NF Corporation) and collimated by a Si convex lens. The output signal from PA-PIR was first amplified using a pre-amplifier (RS560, Stanford Research Systems) and then finally gained using a lock-in amplifier (LI5640, NF Corporation). For the spectral response measurement, a tunable IR laser system was used as a tunable excitation source. The output IR laser was obtained by the non-collinear difference frequency generation (NDFG, Light Conversion) of the signal and idler outputs of an optical parametric amplifier (TOPAS Prime, Spectra-Physics), which was excited by a Ti:Sapphire regenerative


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amplifier (Solstice, Spectra-Physics). The output laser from the Ti:Sapphire regenerative amplifier had a pulse width of 104 fs and a repetition rate of 1kHz. Simulations. The optical spectra and the electric field distributions of the PA-PIRs were calculated using RCWA (DiffractMOD, Synopsys' RSoft) and finite-difference time-domain (FDTD) (FullWAVE, Synopsys' RSoft) methods. The dielectric functions of Au, Pt and Si were taken from the literature41,42 (Ref. 41 for Au and Pt, Ref. 42 for Si). Only the dielectric function of ZnO was retrieved by the ellipsometry measurements as shown in Figure 2b. The model of PA-PIR device was performed in a CAD layout (RSoft CAD), with the unit cell as shown in Figure 1a and the mesh size of 20 nm. For both RCWA and FDTD simulations, the excitation electromagnetic field propagated along the –Z axis and the electric field oscillated along the X axis. In the RCWA simulation, transmittance (T) and reflectance (R) were calculated over the simulation domain of the whole system included Au-hole/ZnO/Pt absorber and Si substrate. The absorptivity (A) of the Au-hole/ZnO/Pt absorber on Si substrate was calculated by A = 1 – (T+R). ASSOCIATED CONTENT Supporting Information. Further simulated optical properties, SEM images of fabricated PA-PIRs. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org REFERENCES (1)

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182x72mm (150 x 150 DPI)


Hole Array Perfect Absorbers for Spectrally Selective Midwavelength

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