MEMS-Based Wavelength-Selective Bolometers

We propose and experimentally demonstrate a compact design for membrane-supported wavelength-selective infrared (IR) bolometers. The proposed bolometer device is composed of wavelength-selective absorbers functioning as the efficient spectroscopic IR light-to-heat transducers that make the amorphous silicon (a-Si) bolometers respond at the desired resonance wavelengths. The proposed devices with specific resonances are first numerically simulated to obtain the optimal geometrical parameters and then experimentally realized. The fabricated devices exhibit a wide resonance tunability in the mid-wavelength IR atmospheric window by changing the size of the resonator of the devices. The measured spectral response of the fabricated device wholly follows the pre-designed resonance, which obviously evidences that the concept of the proposed wavelength-selective IR bolometers is realizable. The results obtained in this work provide a new solution for on-chip MEMS-based wavelength-selective a-Si bolometers for practical applications in IR spectroscopic devices.


Introduction
Microelectromechanical systems (MEMS, also known as micromachines) technology has been rapidly growing since the 1970s and early 1980s owing to its versatile application in a broad range of devices such as sensors, actuators, micropower generators and microfluidic systems [1][2][3][4][5][6]. Recently, MEMS have been identified as one of the most promising technologies for the industrial internet of things (IoT) market in the 21st century. Recent advances in photonics and nanofabrication techniques have proved the possibility in controlling optical properties of the photonic and metamaterial structures A short discussion on the potential applications of the proposed wavelength-selective bolometer is given in Section 3.6. Finally, Section 4 concludes the work.

Materials and Methods
Simulations: To determine the optimal geometrical parameters (i.e., size of square resonators, the periodicity, and the thickness of insulator-and metal films) for the proposed bolometers to have resonances at desirable wavelengths, numerical simulations were performed. Once the optimal geometries were obtained, we proceeded with fabrication to realize the device. Optical spectra including reflectance, transmittance and absorptivity of the proposed wavelength-selective bolometers were numerically simulated using the rigorous coupled-wave analysis (RCWA) method (DiffractMOD, Synopsys' RSoft). The electromagnetic field distributions and absorption maps of the proposed device were simulated using full-wave simulations based on the finite-difference time-domain (FDTD) method (FullWAVE, Synopsys' RSoft). The geometry of the device model was constructed to be identical to the design of the wavelength-selective bolometer using a computer-aided design (CAD) layout (RSoft CAD Environment™, Version 2017.09, Ossining, NY, USA). In the simulation, a 5-nm-thick Ti film serving as the adhesive layers was also added between each interface of Au and dielectric films. The dielectric functions of Au, Ti, Si, SiO 2 and Al 2 O 3 were taken from literatures [36,37]. The dielectric function of Si 3 N 4 was from the reference [38]. For the FDTD simulations, periodic boundary conditions were applied to both the xand y-directions and perfectly matched layers were applied to the z-axis. In all the simulations, the incident electric field propagated along the z-axis and oscillated along the x-axis, the incident field amplitudes were normalized to unity.
Fabrications: The a-Si films were fabricated using a DC sputtering (sputter i-Miller CFS-4EP-LL, Shibaura, Yokohama, Japan) involving a boron-doped Si target (0.02 Ω·cm). For the a-SiGe alloys, a co-sputtering deposition was processed by adding a Ge target. The sputtering conditions of a-Si and a-SiGe alloys films were the same as follows; DC power of 100 W, Ar gas flow of 20 sccm, pressure of 0.304 Pa, sample holder rotation speed of 20 rpm and at room temperature. The membrane-supported wavelength-selective bolometers were processed on a 3-inch double side polished Si wafer. Prior to the fabrication of the devices, a 100-nm-thick SiO 2 layer was formed on both sides of the 3-inch Si wafer by the dry thermal oxidation at 1150 • C. Subsequently, a-350 nm thick Si 3 N 4 film was sputtered on both sides of the SiO 2 /Si wafer following a DC (200 W) reactive sputtering recipe from a boron-doped Si target in a mixture of Ar/N 2 (18/10 sccm) gases. The quality (hardness) of the Si 3 N 4 films was then amended by a rapid thermal annealing (RTA) process in N 2 atmosphere (heating rate of 5 • C·s −1 , keeping constant at 1000 • C for 1 min, then naturally cooling down). The fabrication of the MEMS devices used several steps of lithography processes including the direct laser writing (µPG 101 Heidelberg Instruments, Heidelberg, Germany) and electron beam writing (Elionix, ESL-7500DEX, Tokyo, Japan) combined with electron beam depositions of metals (UEP-300-2C, ULVAC, Yokohama, Japan) and reactive-ion etching (RIE) of Si 3 N 4 (CHF 3 gas, Ulvac CE-300I). The elaborate fabrication of the membrane-supported wavelength-selective bolometers is detailed in the next section.
Characterizations: The amorphous structural property of fabricated a-Si films was verified using an X-ray diffractometer (XRD) with the Cu(Kα) line (SmartLab, Rigaku, Tokyo, Japan). The carrier characteristic of the fabricated bolometers was conducted using a Hall measurement system (Toyo Corporation, ResiTest 8400, Tokyo, Japan). The temperature-dependent resistance measurement was carried out using a source meter (Agilent Technologies B1500A, Santa Clara, CA, USA) combined with a temperature-controlled heating stage. The percent errors of all resistance measurements were less than 2%. The morphological characteristic of the fabricated membrane-supported bolometers was investigated using a scanning electron microscope (SEM) (Hitachi SU8230, Tokyo, Japan) operating at an accelerating voltage of 5 kV. The reflectance spectra of the fabricated devices were measured using an FTIR spectrometer (Nicolet iS50R FT-IR Thermo Scientific, Madison, WI, USA) equipped with a liquid N 2 -cooled mercury cadmium telluride (MCT) detector and a KBr beam splitter. Then the absorptivity spectra were calculated by 1-reflectance, since the transmittance through the absorbers is zero with a 100-nm Au film serving as the bottom mirror. For the spectral response measurement, a tunable IR laser system (104 fs, 1 kHz repetition rate, Spectra-Physics) was used as a tunable excitation source [30]. Figure 1a shows the proposed MEMS-supported wavelength-selective bolometer. The absorber layer placed on top of the bolometers composes of three layers; a top Au square antennas array serving as plasmonic resonators is isolated from a bottom Au planar mirror via an Al 2 O 3 dielectric film. The absorbed IR energy in the absorber is converted to heat and then transfers to the a-Si bolometer film. The heat induces the change of the electrical resistance of the bolometer under an external bias applied on the two lateral Pt electrodes. The wavelength-selective absorber is thermally isolated to the Si substrate by a Si 3 N 4 membrane.  Figure 1a shows the proposed MEMS-supported wavelength-selective bolometer. The absorber layer placed on top of the bolometers composes of three layers; a top Au square antennas array serving as plasmonic resonators is isolated from a bottom Au planar mirror via an Al2O3 dielectric film. The absorbed IR energy in the absorber is converted to heat and then transfers to the a-Si bolometer film. The heat induces the change of the electrical resistance of the bolometer under an external bias applied on the two lateral Pt electrodes. The wavelength-selective absorber is thermally isolated to the Si substrate by a Si3N4 membrane. The geometrical parameters of the device including the width of Au square antenna-w, the periodicity of the square lattice-p, the thickness of the Al2O3 dielectric-t was optimized for certain desirable resonance. The thicknesses of the top Au antenna, the bottom Au film were fixed at 0.07 µm, 0.1 µm, respectively, which are much larger than the skin depth of Au (i.e., 0.03 µm) in the IR region. It is worth noting that the bottom Au film with a 0.1 µm thick is opaque in the MIR region, therefore the optical property of the absorber does not depend on layers underneath. Therefore, the resonance of the absorber relies mainly on p, t and w. Figure 1b plots the simulated reflectance (black curve), transmittance (blue curve) and absorptivity (red curve) of a device having the geometrical parameters of p = 1.2 µm, t = 0.045 µm and w = 0.72 µm. The device exhibits an evident resonance at The geometrical parameters of the device including the width of Au square antenna-w, the periodicity of the square lattice-p, the thickness of the Al 2 O 3 dielectric-t was optimized for certain desirable resonance. The thicknesses of the top Au antenna, the bottom Au film were fixed at 0.07 µm, 0.1 µm, respectively, which are much larger than the skin depth of Au (i.e., 0.03 µm) in the IR region. It is worth noting that the bottom Au film with a 0.1 µm thick is opaque in the MIR region, therefore the optical property of the absorber does not depend on layers underneath. Therefore, the resonance of the absorber relies mainly on p, t and w. Figure 1b plots the simulated reflectance (black curve), transmittance (blue curve) and absorptivity (red curve) of a device having the geometrical parameters of p = 1.2 µm, t = 0.045 µm and w = 0.72 µm. The device exhibits an evident resonance at 3.73 µm with almost unity absorptivity. If the periodicity and insulator layer are unchanged, the resonance wavelength of the device is directly proportional to the width (w) of the resonator while it retains perfect absorption in a wide spectral range of the MIR atmospheric window (Figure 1c). This provides a simple way to tune the active wavelength of the bolometer for spectroscopic applications such as for multi-wavelength detection. Further simulations were also performed to verify the effects of other geometrical parameters on optical spectra of the proposed device ( Figure S1, Supplementary Information), including the resonator's thickness ( Figure S1a), the insulator's thickness ( Figure S1b), the periodicity ( Figure S1c). In particular, with the symmetric resonator (square), the device's resonance does not depend on the polarization ( Figure S1d). The proposed device can be also applied for the near IR and the long-wavelength IR regions ( Figure S2, Supplementary Information). The angle-resolved absorptivity was also simulated, and the result is summarized in Figure 1d. Although there exists another resonance in the shorter wavelength region which is attributed to the surface plasmon polariton (SPP) at Au/air interface of the periodic Au square lattice array [38,39], the main resonance (magnetic mode) remains unchanged up to 70 • incidence. This large working angle of the proposed device is essential for the practical applications.

Structural Design and Simulated Optical Properties
To further understand the origin of the perfect resonant absorption in the device, we performed FDTD simulations to calculate electromagnetic field distributions of the absorber used in the proposed bolometer. Figure 1e-g shows the distributions of the electric fields (E x and E z ) and magnetic field (H y ) of the device excited at the resonance (3.73 µm). In this metal-insulator-metal absorber, the top resonator functions as the sub-wavelength electric dipole antenna which defines the resonance of the absorber. The excited electric field of the electric dipole at the top resonator induces an inverse dipole at the bottom metal film. The oscillation of this antiparallel electric dipoles pair along with the excited electric field creates an electric current loop through the top resonator and bottom metal film, resulting in a large magnetic field enhancement between them (Figure 1g). The resonance of the absorber is so-called magnetic resonance. Interestingly, due to the optical loss of the metal, the absorbed energy at the resonance arises at the top Au resonator and bottom Au film subsequently converts to heat through Joule heating that follows Poynting's theorem [38,40]. The resonantly generated heat is then conductively transferred to the bolometer film via a thin Si 3 N 4 layer to trigger the change of the electrical resistance for IR sensing. Here it is worthy that we adopted a 100-nm-thick Si 3 N 4 layer to electrically isolate the metal film with the bolometer.

Amorphous Silicon and Silicon-Germanium Alloys for Bolometer
Prior to the fabrication of the membrane-supported bolometers, we fabricated bolometer films and examined their temperature coefficient of resistance (TCR) characteristics. In the past two decades, the a-Si and a-SiGe alloys bolometers (thermistors) have been intensively investigated in both scientific study and practical application due to its great success in achieving a high TCR with simple fabrication processes [20,[41][42][43]. In this work, we also intended to choose a-Si film fabricated by sputtering for our MEMS-based wavelength-selective bolometers. Figure 2a displays XRD patterns of two a-Si (p-type) films deposited on the 100-nm-thick SiO 2 /Si substrates by DC sputtering using a boron-doped Si target; an as-deposited film shown in blue color and an annealed film for a comparison (500 • C in H 2 atmosphere) shown in red color. Both two films reveal the amorphous phase of the silicon with two broad 2θ peaks at 27.5 • and 54 • [44]. It is worth noting that the sharp peaks observed in the XRD patters are attributed to the Laue diffraction features of the Si substrate. The carrier characteristics of the as-deposited and annealed a-Si films were carried out via Hall measurements, which indicated that both films showed p-type carriers with concentrations of 7.23 × 10 15 (cm −3 ) and 7.08 × 10 16 (cm −3 ), respectively. The temperature-dependent resistance property of the a-Si films was performed to evaluate the TCR performance. In the semiconductor, the resistivity is the exponential function of thermal activation conductance expressed by [18]: where ρ, ρ 0 are the resistivities at a certain temperature and at the initial measured temperature, respectively. E a , k B and T are the activation energy, Boltzmann constant and temperature (K), respectively. Figure 2b presents the temperature-dependent resistivity of the as-deposited and annealed a-Si films. As seen, the resistivities of both the as-deposited and the annealed a-Si films decreases rapidly when the temperature increases. The resistance changes are plotted in ln(R/R 0 ), where R and R 0 are the resistances at temperature T and at the initial temperature, respectively, are inversely proportional to the increase of temperature (Figure 2c). The activation energies (E a ) of the two a-Si films were then obtained from the slope of ln(R/R 0 ) against the temperature changes, and the TCR values (α) were finally calculated using the following relation [18]: Micromachines 2019, 10, x 6 of 12 where ρ , 0 ρ are the resistivities at a certain temperature and at the initial measured temperature, respectively. a E , B k and T are the activation energy, Boltzmann constant and temperature (K), respectively. Figure 2b presents the temperature-dependent resistivity of the as-deposited and annealed a-Si films. As seen, the resistivities of both the as-deposited and the annealed a-Si films decreases rapidly when the temperature increases. The resistance changes are plotted in where R and 0 R are the resistances at temperature T and at the initial temperature, respectively, are inversely proportional to the increase of temperature (Figure 2c). The activation energies ( a E ) of the two a-Si films were then obtained from the slope of ( ) 0 ln R R against the temperature changes, and the TCR values ( α ) were finally calculated using the following relation [18]: 1 E d ρ  Although the TCR of the asdeposited a-Si film~1.5% (K −1 ) is lower compared to that of the 500 • C annealed a-Si film~4% (K −1 ), however, the value is rather good for bolometers. To be compatible with the fabrication of the elaborate MEMS-based devices, thermal annealing is not desirable. We also investigated a-Si 1-x Ge x alloys for bolometers with x variated from 0.3-0.7 (Figure 2e,f). As seen that the performance of the a-SiGe alloys are not much improved or even become worse compared to that of the pure a-Si film. Therefore, we chose the as-deposited a-Si for our MEMS-based wavelength-selective bolometers.

Fabrication of MEMS-Based Wavelength-Selective Bolometers
The MEMS-based wavelength-selective bolometers were then fabricated using several steps of lithography and following the structural parameters optimized by numerical simulations. The fabrication details are shown in Figure 3a-f. The devices were processed on a 3-inch double sides polished Si wafer with Si 3 N 4 /SiO 2 layered films on both sides (Figure 3a). An array of bolometers with different sizes (0.2 × 0.2 mm 2 , 0.5 × 0.5 mm 2 , 1 × 1 mm 2 and 2 × 2 mm 2 ) and with different wavelength resonances were arranged on the 3-inch wafer. Accordingly, the patterned a-Si films array and their lateral Pt electrodes were fabricated using two steps of lithography combined with sputtering deposition of a-Si and electron beam deposition of Pt (Figure 3b). Subsequently, a patterned 100-nm-thick Si 3 N 4 layer was conformally deposited on each a-Si bolometer to electrically insulate the bolometer with the Au film of the absorber layer (Figure 3c). Then the wavelength-selective absorber on each bolometer were fabricated depending on the targeted resonance (Figure 3d,e). To process the membrane, a Si 3 N 4 mask for the anisotropic wet-etching of Si on the back side of each bolometer was formed using RIE (CHF 3 gas) followed by a photoresist mask. Finally, the membrane-supported Si bolometers array was achieved by applying an anisotropic wet-etching at the back side of the Si wafer using a hot KOH solution (8 mg/l, 80 • C) (Figure 3f).   (Figure 3a). An array of bolometers with different sizes (0.2 × 0.2 mm 2 , 0.5 × 0.5 mm 2 , 1 × 1 mm 2 and 2 × 2 mm 2 ) and with different wavelength resonances were arranged on the 3-inch wafer. Accordingly, the patterned a-Si films array and their lateral Pt electrodes were fabricated using two steps of lithography combined with sputtering deposition of a-Si and electron beam deposition of Pt (Figure 3b). Subsequently, a patterned 100-nmthick Si3N4 layer was conformally deposited on each a-Si bolometer to electrically insulate the bolometer with the Au film of the absorber layer (Figure 3c). Then the wavelength-selective absorber on each bolometer were fabricated depending on the targeted resonance (Figure 3d,e). To process the membrane, a Si3N4 mask for the anisotropic wet-etching of Si on the back side of each bolometer was formed using RIE (CHF3 gas) followed by a photoresist mask. Finally, the membrane-supported Si bolometers array was achieved by applying an anisotropic wet-etching at the back side of the Si wafer using a hot KOH solution (8 mg/l, 80 °C) (Figure 3f).

Wavelength Tunability
The resonance spectra of the bolometers array were firstly characterized using an FTIR. As we discussed, the resonance of the bolometers can be readily tuned just by changing the size of the square resonator. Here we designed and fabricated a series of MEMS-based quad-wavelength bolometers chips. Figure 4a-d, from top to bottom, presents SEM images, simulated and measured absorptivities

Wavelength Tunability
The resonance spectra of the bolometers array were firstly characterized using an FTIR. As we discussed, the resonance of the bolometers can be readily tuned just by changing the size of the square resonator. Here we designed and fabricated a series of MEMS-based quad-wavelength bolometers chips. Figure 4a-d, from top to bottom, presents SEM images, simulated and measured absorptivities of the quad-wavelength bolometers. Four absorbers of the device have the same periodicity of 1.2 µm and the same insulator thickness of 0.045 µm but different resonators of 0.59 µm (Figure 4a), 0.64 µm (Figure 4b), 0.72 µm (Figure 4c) and 0.77 µm (Figure 4d) aiming at four different wavelengths of 3.11 µm 3.39 µm, 3.73 µm and 3.96 µm (middle panels of Figure 4). As seen in bottom panels of Figure 4, the fabricated MEMS-based quad-wavelength bolometers chip exhibits near-unity absorption resonances (>0.89) at four different wavelengths which are the same as the pre-designed resonances, evidencing the proper model and precise fabrication process used in this work. The resonances of the MEMS-based quad-wavelength bolometers presented here were chosen in the MIR atmospheric window; they can be also extended to the shorter or longer wavelength regions depending on the specific practical spectroscopic applications.

Wavelength-Selective Responsivity Measurement
To elucidate the spectral response characteristic of the proposed MEMS-based wavelengthselective bolometers, we performed the wavelength-dependent responsivity measurement. The measurement setup is illustrated in Figure 5a. In this measurement, a pulsed tunable IR laser (pulse width of 104 fs, repetition rate of 1 kHz) was used as the excitation source. The IR response of the sensor was measured via a read-out circuit, and then acquired using a source meter collected to a PC. The laser power at each wavelength was above the sensor's threshold. The spectral response of the sensor was finally normalized to the power spectrum of the laser.

Wavelength-Selective Responsivity Measurement
To elucidate the spectral response characteristic of the proposed MEMS-based wavelength-selective bolometers, we performed the wavelength-dependent responsivity measurement. The measurement setup is illustrated in Figure 5a. In this measurement, a pulsed tunable IR laser (pulse width of 104 fs, repetition rate of 1 kHz) was used as the excitation source. The IR response of the sensor was measured via a read-out circuit, and then acquired using a source meter collected to a PC. The laser power at each wavelength was above the sensor's threshold. The spectral response of the sensor was finally normalized to the power spectrum of the laser.  Figure 5b shows the measured absorptivity spectrum of a typical MEMS-based wavelengthselective bolometer having resonance at 3.73 µm. As we have discussed earlier, the resonantly absorbed IR light at the absorber is converted to heat, then conductively transfers to the bolometer to enable the change of the resistance. The resistance change can be measured as the IR response by applying a current through two Pt electrodes of the bolometer. Figure 5c displays the measured responsivity spectrum of the fabricated device. The measured spectrum responsivity curve exhibits a clear resonance. More interestingly, the resonance peak of the responsivity is the same as the absorptivity peak of the device (i.e., 3.73 µm), proving that the conceptual design for MEMS-based wavelength-selective bolometers is realizable. It should be noted that the measurement was done at a high frequency (1 kHz), because of which the responsivity was just a few mV/W. In the real applications, the device is expected to operate at the lower frequency such as a few hertz, such that the responsivity can be one to two orders higher compared to the value measured at the kilohertz frequency. In addition, the fabricated bolometer presented here have the size of a few hundred microns to millimeter scales; it can be scaled down to a few ten of microns, then the thermal isolation can be further improved for the higher responsivity and response time. The device is also compatible with the bridge-supported sensor technology which is accessible to the read-out integrated circuit (ROIC) for practical applications in IR imaging and thermography.

Potential Applications
Here we discuss some potential applications of the proposed MEMS-based wavelength-selective bolometer. As we mentioned earlier, the proposed device is devoted to serve as a new platform for IR spectroscopic sensors such as portable NDIR sensors, chemical IR imaging or IR remote sensing systems wherein the specific resonances are designed at vibrations of the targeted gases and molecules. The proposed wavelength-selective bolometer can be also applied for compact IR spectrometers, true temperature and emissivity measurements as well as multi-wavelength thermography by integrating multiple resonant sensors in a single chip.  Figure 5b shows the measured absorptivity spectrum of a typical MEMS-based wavelengthselective bolometer having resonance at 3.73 µm. As we have discussed earlier, the resonantly absorbed IR light at the absorber is converted to heat, then conductively transfers to the bolometer to enable the change of the resistance. The resistance change can be measured as the IR response by applying a current through two Pt electrodes of the bolometer. Figure 5c displays the measured responsivity spectrum of the fabricated device. The measured spectrum responsivity curve exhibits a clear resonance. More interestingly, the resonance peak of the responsivity is the same as the absorptivity peak of the device (i.e., 3.73 µm), proving that the conceptual design for MEMS-based wavelength-selective bolometers is realizable. It should be noted that the measurement was done at a high frequency (1 kHz), because of which the responsivity was just a few mV/W. In the real applications, the device is expected to operate at the lower frequency such as a few hertz, such that the responsivity can be one to two orders higher compared to the value measured at the kilohertz frequency. In addition, the fabricated bolometer presented here have the size of a few hundred microns to millimeter scales; it can be scaled down to a few ten of microns, then the thermal isolation can be further improved for the higher responsivity and response time. The device is also compatible with the bridge-supported sensor technology which is accessible to the read-out integrated circuit (ROIC) for practical applications in IR imaging and thermography.

Potential Applications
Here we discuss some potential applications of the proposed MEMS-based wavelength-selective bolometer. As we mentioned earlier, the proposed device is devoted to serve as a new platform for IR spectroscopic sensors such as portable NDIR sensors, chemical IR imaging or IR remote sensing systems wherein the specific resonances are designed at vibrations of the targeted gases and molecules. The proposed wavelength-selective bolometer can be also applied for compact IR spectrometers, true temperature and emissivity measurements as well as multi-wavelength thermography by integrating multiple resonant sensors in a single chip.

Conclusions
We have successfully demonstrated a design for the MEMS-based wavelength-selective bolometers. The device used a-Si film deposited by DC sputtering at room temperature as the bolometer material. The fabricated a-Si film showed reasonably high TCR values above 1.5% (K −1 ). A patterned metalinsulator-metal absorber structure was used as the efficient IR light-to-heat transducer for the wavelength-selective bolometers. The proposed bolometers revealed a facile resonance tunability just by changing the size of the patterned square metallic resonators arranged on top of each sensor. We also provided a detailed fabrication procedure to realize the proposed devices. As a proof of concept, we fabricated a set of MEMS-based quad-wavelength wavelength-selective bolometers having resonances in the MIR atmospheric window region. The fabricated devices exhibited the same resonances with the pre-designed devices. More interestingly, the measured spectral responsivity curve showed a clear wavelength-selective response following the absorptivity resonance, which proves the feasibility of the conceptual design of the MEMS-based wavelength-selective bolometers presented in this work. Although this work indicated resonances of bolometers in the MIR atmospheric window region, other resonant wavelengths and different number of resonances can be arranged in a single MEMS-based IR bolometers array chip depending on the specific practical applications such as NDIR sensors, IR spectroscopic sensing or imaging devices.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2072-666X/10/6/416/s1, Figure S1: Effects of the resonator thickness, insulator thickness, periodicity and polarization on optical spectra of the proposed wavelength selective bolometers; Figure S2: Simulated spectra of the proposed device with resonance in the near IR and long-wavelength IR regions. Funding: This work was partially supported by JSPS KAKENHI (16F16315, JP16H06364, 16H03820), and CREST "Phase Interface Science for Highly Efficient Energy Utilization" (JPMJCR13C3) from Japan Science and Technology Agency.