Article pubs.acs.org/journal/apchd5
Hole Array Perfect Absorbers for Spectrally Selective Midwavelength Infrared Pyroelectric Detectors Thang Duy Dao,*,†,‡ Satoshi Ishii,†,‡ Takahiro Yokoyama,†,‡ Tomomi Sawada,†,‡ Ramu Pasupathi Sugavaneshwar,†,‡ Kai Chen,†,‡ Yoshiki Wada,§ Toshihide Nabatame,†,‡ and Tadaaki Nagao*,†,‡,∥ †
International Center for Materials Nanoarchitectonics and §Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ∥ Department of Condensed Matter Physics, Graduate School of Science, Hokkaido University, Kita 8, Nishi 5, Kita-ku, Sapporo 060-0810, Japan S Supporting Information *
ABSTRACT: We demonstrate a hybrid plasmonic−pyroelectric device operating as an uncooled midwavelength 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 predesigned plasmonic resonances, which are tunable by changing the Au hole diameters. The results obtained here open up a route for realizing a new type of uncooled spectroscopic infrared detectors with a compact design and simple fabrication process. KEYWORDS: plasmonic metamaterial, perfect absorber, colloidal lithography, pyroelectric detector, midwavelength infrared detector
U
multicolor IR imaging. In this context, a number of dispersive photonic structures have been proposed such as micromachined Fabry−Perot interferometers,4,5 photonic crystals,6,7 or plasmonic structures.8−18 Plasmonic perfect absorbers (PAs) are another type of structure that exhibits nearly 100% absorptivity at desired wavelengths. Plasmonic absorbers have been successfully used in a wide range of applications such as in thermophotovoltaics,19,20 photodetectors,15,21 molecular sensing,22−24 and thermal emitters.25−27 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 heat,28−32 integration of a MWIR PA and a pyroelectric
ncooled midwavelength infrared (MWIR) detectors, especially pyroelectric detectors, enjoy widespread use ranging from human motion sensors, to thermal imaging, to Fourier transform infrared spectroscopy (FTIR). Among different types of infrared detectors, pyroelectric detectors are robust and simple and can be operated at room temperature without cooling. They can exhibit wideband and flat spectral responses with high sensitivity as well as a simple design without adopting elaborate thermal insulation, which makes them advantageous for industrial applications together with their low fabrication costs.1−3 To realize high performances as well as making them more industrial compatible, the search for efficient pyroelectric materials with nontoxic earth-abundant elements has been done. Also, integration with photonic structures is expected to result in spectral selectivity at desired wavelengths for potential applications such as nondispersive infrared spectroscopy (NDIR), infrared chemical imaging, and © 2016 American Chemical Society
Received: April 10, 2016 Published: June 6, 2016 1271
DOI: 10.1021/acsphotonics.6b00249 ACS Photonics 2016, 3, 1271−1278
ACS Photonics
Article
Figure 1. (a) Schematic diagram of the PA−PIR with definition of the simulation domain (dashed 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 resonating at 3.88 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. The electric field polarized in the x direction propagates along the −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 PA−PIRs by tuning the Au hole size (1.4, 1.8, 2.4, and 2.7 μm), whereas the periodicity and the ZnO thickness are 3.0 and 0.68 μm, respectively, for all four samples.
wavelengths. The mechanism and detailed optical properties of the trilayered metal hole−insulator−metal PA have been discussed in previous reports.25,36,37 Here we chose a hexagonal geometry of the plasmonic holes at the top metallic layer since the structure with this azimuthal symmetry exhibits polarization-independent 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 the 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 states 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 states.36 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 a ZnO dielectric spacer. These induced dipoles at the Pt surface oscillate against and quench dipole oscillations at the surface of the Au hole layer. In addition, oscillating dipoles at Au and Pt surfaces are sensitive to damping due to the 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 Information, Figure S1c), periodicity (p) (Supporting Information Figure S1d), or the diameter of the Au hole (d) (Figure 1b). Here we focus on the tunability of the PA−PIR by changing the diameter of the Au
detector is a promising strategy for realizing wavelengthselective devices such as spectroscopic IR sensing and multicolor 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 material,33−35 sandwiched between a Pt film and a 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 colloidalmask lithography combined with a 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 narrowband IR detection in the MWIR region. By tuning the diameter of the Au holes, the responsivity curve of the detector can be tuned flexibly, evidencing its suitability for spectroscopic infrared sensing and multicolor MWIR imaging.
■
RESULTS AND DISCUSSION Our proposed PA−PIR is illustrated in Figure 1a, where a hexagonal array of plasmonic Au holes and bottom Pt film is insulated by a ZnO layer. The thicknesses of Au hole array and Pt films are fixed at 0.1 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 3−7 μm (MWIR region), where most of the molecules have their vibrational 1272
DOI: 10.1021/acsphotonics.6b00249 ACS Photonics 2016, 3, 1271−1278
ACS Photonics
Article
Figure 2. (a) XRD pattern of the ZnO film fabricated on a Pt-coated Si substrate indicating a highly c-axis oriented hexagonal film of ZnO. The inset in (a) illustrates the spontaneous polarization of the ZnO film along the c-axis when irradiated by IR light. (b) Complex permittivity of the fabricated ZnO film ranging from the UV to FIR region. (c) Raman spectra of the ZnO film averaged from a 20 × 20 μm area on sample S2 by using 2D scanning Raman specrtroscopy. (d) 2D Raman spectroscopic mapping of sample S2 integrated at the 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.
also designed with a hole diameter of 1.4 μm (black curve in Figure 1d). The PA−PIRs were fabricated by colloidal-mask lithography combined with the RIE process (described in detail in the Methods section). 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] direction.38 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 a 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. The complex dielectric function of the 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 to 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 about 4, which is in good agreement with the previous report in the same spectral range.39 In the MWIR region of our interest, the complex dielectric function of the fabricated film is nonabsorbing (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. 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 and 0.68
hole (d), whereas the ZnO thickness (t) and periodicity (p) were fixed at 0.68 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 gases, it is desirable to have a dual-band resonant detector that 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 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, and p = 3.0 μm exhibiting a dual-band perfect absorption with resonances at 3.88 and 5.50 μm, which match 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 dualband 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 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 1273
DOI: 10.1021/acsphotonics.6b00249 ACS Photonics 2016, 3, 1271−1278
ACS Photonics
Article
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, the quality of the pyroelectric ZnO layer was further examined by Raman spectroscopic mapping. Figure 2c presents Raman spectra of the ZnO film averaged from a twodimensional (2D) scanning Raman spectroscopy taken on a 20 × 20 μm area of S2. The result reveals two strong peaks at 99 cm−1 (Elow mode) and 437 cm−1 (Ehigh mode) and three 2 2 lower peaks at 332 cm−1 (Elow − Ehigh mode), 380 cm−1 2 2 (A1(TO) mode), and 582 cm−1 (E1(LO) mode), by which the whole ZnO film was confirmed to be of high crystallinity.40 The strength and the sharpness of the Elow and Ehigh modes 2 2 evidence that the ZnO film grew highly uniaxially with its c-axis aligned to the film’s perpendicular direction. These results confirm the observation from the XRD. A 2D Raman image was constructed by integrating the 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 the 2D Raman image clearly reflects the micropattern of the fabricated PR−PIR, indicating the hexagonal periodic Au hole array patterned on the ZnO film. The measurement setup is shown in Figure 3a, and the details are described in the Methods section. Before the
decrease; thus the surface charge also decreases. Similarly, when the chopper frequency increases and with 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 is as high as 3.25 V/W at the chopper frequency of 1 Hz, and it decreases by increasing the chopper frequency. The working frequency range of the fabricated device covers a broad range from very low frequency (
