Plasmon-Enhanced Infrared Spectroscopy Based on Metamaterial

Publication Date (Web): August 20, 2018 ... near-field intensities forming on the top and bottom of nanoantennas that further enhance molecular sensin...
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Plasmon-Enhanced Infrared Spectroscopy Based on Metamaterial Absorbers with Dielectric Nanopedestals Inyong Hwang,† Jaeyeon Yu,† Jihye Lee,‡ Jun-Hyuk Choi,‡ Dae-Geun Choi,‡ Sohee Jeon,‡ Jongwon Lee,*,† and Joo-Yun Jung*,‡ †

School of Electrical and Computer Engineering, Ulsan National Institute of Science and Technology, Ulsan, 44919, Korea Nano-convergence Mechanical Systems Research Division, Korea Institute of Machinery and Materials, Daejeon, 305-343, Korea

‡

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S Supporting Information *

ABSTRACT: In this study, we introduce a sensing platform based on plasmonic metamaterial absorbers (MAs) with dielectric nanopedestals for the ultrasensitive detection of monolayer biomolecules. MAs with nanopedestals, which allow access to extremely high near-field intensities forming on the top and bottom of nanoantennas that further enhance molecular sensing signals, consist of cross-shaped gold (Au) nanoantennas on dielectric nanopedestals and an Au layer as a mirror layer. The sensing characteristics of the MAs with nanopedestals are compared with control MAs using an unetched dielectric spacer. To provide strong coupling between the molecular vibrations of the target 1-octacanethiol (ODT) molecules and plasmonic resonance from the MAs, the dimensions of the cross-antennas are properly designed and the MAs fabricated by using nanoimprint lithography. Temporal coupled-mode theory is used to analyze the experimentally measured absorption spectra containing the enhanced vibrational signatures of molecules of the ODT monolayer. Based on MAs with nanopedestals, the molecular sensing signal is enhanced over four times compared with that of the control MAs. The proposed structure based on MAs with nanopedestals may provide a promising sensing platform for future applications of ultrasensitive biological and chemical sensing and detection. KEYWORDS: metamaterials, plasmonics, surface-enhanced infrared absorption spectroscopy, infrared vibrations

M

their chemical bonds, composition, and molecular configuration, which allows for label-free identification.18 Fourier transform infrared spectroscopy through infrared fingerprint vibrations is used in such fields as pharmacy, safety, food, and general substance identification.18 However, owing to the small molecular absorption cross-section in the infrared range, the direct detection of minute amounts of analyte molecules or thin samples, such as monolayers, is limited. To address this limitation, surface-enhanced infrared absorption spectroscopy (SEIRA) or surface-enhanced Raman spectroscopy (SERS) have been proposed to directly enhance infrared fingerprint vibrations and verified by employing a variety of plasmonic structures that enable strong coupling between the induced electromagnetic fields and the fingerprint vibrations of the biomolecules of interest.19−22 For applications of SEIRA, a number of metallic structures and geometries; such as metallic nanorods, split-ring resonators, fan-shaped nanoantennas, and nanoantennas on nanopedestals and MAs, have been investigated in the recent past.23−29 Approaches that use metallic nanoantennas on pedestals, where the cross-section of

etamaterials have garnered widespread interest in recent years due to their ability to control, enhance, and manipulate light at subwavelength scales by means of properly designed metallic nanostructures. This property leads to interesting applications, such as super-resolution imaging,1−3 negative refraction,1,2 nonlinear optics with significantly relaxed phase matching constraints,4−6 anomalous reflection and refraction,7 and metamaterial absorbers (MAs).6,8−10 Of applications based on metamaterials, MAs have attracted considerable interest owing to their unique ability to independently control macroscopic optical properties of refractive index and characteristic impedance.11−14 MAs usually consist of metallic antennas on top of a metallic layer as a mirror layer separated by a thin dielectric spacer layer, in which all incident radiation at a particular wavelength is absorbed by the excitation of surface plasmon polaritons. The metallodielectric subwavelength structures can be designed in a variety of forms, such as cylinders, crosses, and trapezoids.9 Recently, the metamaterial perfect absorbers in particular have been employed to achieve the highly sensitive label-free detection of biomolecules.15−17 In a different context, biomolecule detection using infrared fingerprint vibration has exhibited significant potential as the vibrational modes of the biomolecules are directly linked to © XXXX American Chemical Society

Received: May 24, 2018 Published: August 20, 2018 A

DOI: 10.1021/acsphotonics.8b00702 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. Schematic unit cells for the control MA structure (a) and MA with the nanopedestal structure (b). Structural dimensions of the control MA structure are P1 = 1200 nm, L1 = 930 nm, and W1 = 200 nm, and for the MA with the nanopedestal structure are P2 = 1300 nm, L2 = 1100 nm, W2 = 200 nm, and U = 80 nm. The schematic cross-section of the ODT-coated control MA structure (c) and MA with the nanopedestal structure (d) for the plane indicated in (a) and (b).

for the nanopatterning of large areas and cost-effective fabrication.34−36 To form MAs with nanopedestals, we used isotropic dry-etching, and precisely controlled the vertical and lateral size of the nanopedestal through the etching time. Unlike the conventional plasmonic structures with several tens of nanometer size gap, which is made by using electron-beam lithography, our structure utilizes vertical gap formed by undercut etching of SiO2 dielectric spacer; thus, it is easy to control the size of the vertical gap according to the SiO2 layer thickness and large size of array of plasmonic nanostructures with several tens of nanometer size gap can be fabricated through the NIL process. Using the MAs with nanopedestals, we successfully detected the monolayer of molecules of 1octadecanethiol (ODT), with a SEIRA detection signal nearly four times higher than that from control MAs consisting of Au cross nanoantennas on an unetched SiO2 dielectric spacer on an Au layer. The MAs with nanopedestals proposed in this study can be applied to other antenna geometries designed for ultrasensitive biological and chemical sensing and detection.

the dielectric pedestal beneath the metallic antenna is slightly smaller than the size of the antenna, provide an increased effective sensing area as they allow access to the bottoms of the top metallic antennas. The increased effective sensing area leads to more enhanced SEIRA signals than detected by metallic antennas directly patterned on the dielectric substrate.25,27,28 Other approaches using MAs for applications of SEIRA have shown remarkable sensitivity, as the MAs provide a unique sensing platform with tailored absorption properties and strong field enhancement.16,29−33 These metallic structures used for SEIRA application are mostly fabricated by conventional lithographic tools, such as electronbeam lithography, because of typical requirement of nanometer-sized gap between plasmonic resonators to induce high near-field enhancement, thus, large area patterning is challenging and even costly. In this study, we propose a method for the detection of ultrasensitive biomolecules based on MAs with dielectric nanopedestals that features high near-field enhancement, the maximum field overlap, and an increased coupling rate with analyte molecules. MAs with nanopedestal or similar structures were previously proposed for the potential application of wavelength-selective uncooled infrared sensors,10 but the structures have not yet been applied to the SEIRA application. We show that MAs with nanopedestals may provide the sensing capability needed to detect molecules of the analyte monolayer. The structure consists of an Au cross nanoantenna on an etched SiO2 dielectric spacer as a nanopedestal, the cross-section of which is smaller than that of the cross nanoantenna, and a bottom Au layer. The accessible sensing area of the structure increases as the cross-section of the nanopedestal beneath the cross nanoantenna decreases. The cross-shaped Au nanoantenna structure relaxes polarization sensitivity, and the structure may be used for the detection of analyte molecules illuminated by unpolarized light. The plasmonic resonance wavelength of the structure can be easily tuned to the wavelength of the analyte fingerprint vibrations by adjusting the length of the Au cross antenna. We fabricated the structure using nanoimprint lithography (NIL), which allows

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RESULTS AND DISCUSSION Figure 1a,b shows a schematic of the structure of the control MA and the MA with the nanopedestal structure, respectively. The two structures consisted of a 150 nm thick Au mirror layer, a 30 nm thick top Au cross-shaped nanoantenna, and a 50 nm thick SiO2 dielectric spacer between the Au layers. For the MA with the nanopedestal structure, isotropic undercut etching was formed at a lateral etching depth of up to 80 nm to reveal the bottom surface of the top Au nanoantenna. For monolayer detection, ODT was used as the target molecule. It is well-known that ODT molecules exhibit infrared fingerprint vibrations at wavelengths of 3509 and 3427 nm owing to symmetric and asymmetric CH 2 stretching vibrations, respectively, and ODT-coated samples can be prepared on the Au layer in the form of a self-assembled monolayer (SAM).37 The schematic cross-section at the planes of the two structures with ODT SAM on the Au layer, indicated in Figure 1a,b, are shown in Figure 1c,d. We note that for the MA with the nanopedestal structure, the ODT monolayer is formed B

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Figure 2. (a−d) Cross-sectional view ((a) control MA, (c) MA with nanopedestal) and top view ((b) control MA, (d) MA with nanopedestal) of the calculated near-field enhancement for the control MA structure at the asymmetric vibration wavelength of 3427 nm of the ODT. The crosssectional planes are indicated in Figure 1a,b and the top view was monitored at the bottom of the top Au cross-antenna. Measurement (black) and simulated (red) absorption spectra of the control MA (e) and MA with the nanopedestal (f) structure.

around the revealed surface of the top Au nanoantenna and the bottom Au surface, as shown in Figure 1d. The top Au cross nanoantennas of the two structures were designed separately to exhibit plasmonic resonance near the two wavelengths of the ODT fingerprint vibrations. The dimensions of the cross nanoantenna of the two structures are provided in caption of Figure 1. To support plasmonic resonance near wavelengths of the ODT fingerprint vibrations for the two MA structures, the top cross nanoantenna of the MA with the nanopedestal structure requires longer antenna length than that of the control MA structure owing to the smaller effective refractive index of the partially removed SiO2 layer. The longer antenna length required for the MA with the nanopedestal structure increases the effective sensing area, which is crucial for applications of SEIRA. A set of numerical simulations was carried out using the commercial finite-difference time domain (FDTD) simulations to design an MA with a nanopedestal structure and the control MA structure (see Methods for details of the simulation). Figure 2a−d shows the crosssectional view and the top view of the near-field intensities induced in both structures at a wavelength of 3427 nm

(asymmetric vibration wavelength of ODT molecules), where the top view of near-field intensities was monitored beneath the top Au cross nanoantenna with illumination provided by xpolarized light. According to the results of the simulation, the maximum local near-field enhancement over 70 was induced at the bottom end of the corners of the top cross nanoantenna, and the normalized field strength of the MA with the nanopedestal structure was slightly higher than that of the control MA structure. The MA with the nanopedestal structure, having larger structural dimensions than the control MA structure, allowed for ODT molecules to access the bottom surface of the cross nanoantenna. These structural properties led to an increased effective sensing area, which resulted in stronger mode coupling between vibrations of ODT molecules in the plasmonic mode of the MA structure. In applications of SEIRA, the near-field intensities and accessible sensing area of the structure play a major role in boosting sensing signals of the molecules. For a quantitative comparison of both structures, we calculated the sensing area and integrated near-field intensities over the region occupied by the ODT monolayer (ODT monolayer thickness ∼ 2.8 nm) C

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absorption spectrum than the structure of the control MA owing to a higher sensitivity to the refractive index. To increase the effective sensing area of the structure of the MA with the nanopedestal, we maximized the depth of the undercut etching of the SiO2 nanopedestal to 80 nm along all lateral directions and 50 nm along the vertical direction. We note that SEIRA absorption signal based on MAs is affected not only by the near-field enhancement, but also by the coupling condition.38 Under critical coupling conditions, where perfect absorption occurs, the SEIRA absorption signal may be lower than undercoupling or critical-coupling conditions. Taking this into account, we intentionally designed the plasmonic structures away from perfect absorption, operating in under-coupling regime where radiation damping rate is less than absorption damping rate. In device fabrication, the NIL process was used to form a 700 μm × 700 μm two-dimensional array of the MA with a nanopedestal structure and the control MA structure. The fabrication process is shown in Figure 3a, and the details of device fabrication are provided in Methods. Figure 3b,c shows scanning electron microscope images of the fabricated control MA array and the MA with the nanopedestal array, respectively. For the MA with the nanopedestal structure, we removed the SiO2 layer by ∼80 nm along all lateral directions and 50 nm along the vertical direction. As the partial removal of the SiO2 layer reduces the effective refractive index of the spacer layer, the isotropic etching process gave rise to blueshifted absorption spectra. To form the MA with the

coated on the Au layer at a wavelength of 3427 nm. According to the calculations shown in Table 1, the MA with the Table 1. Sensing Area and Integrated near-Field Intensity of the Control MA Structure and MA with the Nanopedestal Structure (for the Latter, The Undercut Etch Depth was 80 nm) case control MA structure MA with nanopedestal structure (U = 80 nm)

sensing area (μm2)

integrated near-field intensity V ∫ 0 ODTI/I0dV (108 × nm3)

0.4396 0.8704

1.81471 5.74132

nanopedestal structure provided a sensing area 2 times larger and integrated near-field intensities 3.2 times larger than those of the control MA structure owing to the larger structural dimensions of the cross nanoantenna, and the additional nearfield area formed at the bottom surface of the top cross nanoantenna and the surface of the bottom layer of the Au mirror. Figure 2e,f shows the simulated and experimentally measured absorption spectra of the bare control MA array and the bare MA with the nanopedestal array, respectively. The plasmonic resonances of the MA with the nanopedestal and the control MA were at 3400 and 3500 nm, respectively. The MA with the nanopedestal structure was designed to have its absorption peak at a shorter wavelength compared with the control MA structure, as the coating of the ODT with the MA with nanopedestal structure gives rise to a higher red-shifted

Figure 3. (a) Fabrication of the MA. SEM images of the control MA array (b) and MA with the nanopedestal array (c). Simulated (d) and experimental measurements (e) of the absorption spectrum of the MA with the nanopedestal array for different isotropic etch depths and etching times. D

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Figure 4. Experimental absorption spectrum (black curve) of the ODT-coated control MA array (a), the ODT-coated MA with nanopedestal array (b), and TCMT fittings of the absorption spectrum for the two structures (red curve). (c) Experimental absorption difference spectrum of the control MA array (black curve) and MA with the nanopedestal array (red curve). The green and blue lines indicate the two ODT vibrational wavelengths.

absorption difference spectrum, the absorption spectrum of the bare MA structure was laterally shifted to match the absorption peak to the ODT-coated MA structure to extract only the absorption difference caused by ODT fingerprint vibrations. As shown in Figure 4c, the peak-to-peak values of the differences in terms of absorption of the MA with the nanopedestal structure at the two ODT vibrational wavelengths were 4.2 and 3.7 times higher than the control MA structure at wavelengths of 3427 and 3509 nm, respectively. We attribute the enhanced SEIRA signal to the enhanced coupling rate of the ODTcoated MA with the nanopedestal structure, originating in the increased effective sensing area and integrated near-field intensities induced in the structure. To quantitatively examine the enhanced SEIRA signal generated in the MA with the nanopedestal structure, a framework of the temporal coupled-mode theory (TCMT) for the MA structure was used by adding two resonators describing the two ODT vibrational absorptions. The dielectric constant of the ODT was calculated using a Lorentz model, where we fitted two oscillators and two damping frequencies of the ODT molecules, γ1 = 2.5 × 1012 rad/s at a frequency of ω1 = 5.50 × 1014 rad/s, and γ2 = 1.8 × 1012 rad/s at a frequency of ω2 = 5.37 × 1014 rad/s, in the TCMT modeling. The coupled mode equations used for this derivation are provided in the Supporting Information. The absorption of the ODT-coated MA structure can be described by the following equation:38

nanopedestal structure supporting plasmonic resonance near the vibrational wavelengths of the ODT, we conducted isotropic etching with a two-minute step-up to eight minutes. Figure 3d,e shows the simulated and measured absorption spectra of the MA with nanopedestal arrays of different etch depths and isotropic dry etching times, respectively. The absorption peak of the unetched MA structure designed to construct cross antennas on the nanopedestal was initially located at a wavelength of 4300 nm, and as the etching time increased, the absorption peak shifted to a shorter wavelength. Finally, after etching for eight minutes, the absorption peak of the structure reached a wavelength of 3400 nm. From the data and SEM images shown in Figure 3, an etch depth of 80 nm of the MA with the nanopedestal structure was confirmed. We noted that isotropic etching exceeding 10 min caused a collapse of the top cross-antennas, and the absorption peak became severely distorted for the overetched array, as shown in Figure 3e. For the experimental demonstration of the detection of the ODT monolayer, the fabricated samples were immersed in a solution of 1 mmol ODT (CAS Number 2885−00−9) dissolved in absolute ethanol for 24 h to form a uniform coating on the Au cross-antenna. The samples were then washed with ethanol to remove unwanted ODT molecules that were not formed as a SAM, and the samples were dried by being blown with N2. The absorption spectrum of the MAs coated with the ODT monolayer was measured using a Fourier transform infrared spectrometer (Bruker, Vertex70) equipped with a nitrogen-cooled HgCdTe photodetector and a midinfrared optical microscope (Hyperion 3000). The absorption spectrum was recorded at a spectral resolution of 2 cm−1 with 256 scans. Figure 4a,b shows the absorption spectra of the control MA array and the MA with the nanopedestal array and coated by the ODT monolayer. For direct comparison with the control MA structure, the MA etched for eight minutes with the nanopedestal structure was used for SEIRA measurements, as the plasmonic resonances of the two ODT-coated MA structures were formed at nearly the same wavelength. The experimentally measured absorption spectra of the two samples contained vibrational signatures of ODT molecules at the two ODT vibrational wavelengths, where the absorption spectrum of the MA with the nanopedestal structure is more severely distorted. To better illustrate the SEIRA detection signal, the absorption difference spectrum (ΔA = Abare,shifted − AODT‑coated) was used as shown in Figure 4c. In the extraction of the

Abs =

4γr(γa + γμ1 + γμ2) (ω − ω0 − ωμ1 − ωμ2)2 + (γr + γa + γμ1 + γμ2)2 (1)

where γr and γa are the absorption and radiation damping rates of the MA, respectively, ω0 is the resonant frequency of the ODT-coated MA structure, and γμi = μ2i γi/[(ω − ωi)2 + γ2i ] and ωμi = μ2i (ω − ωi)/[(ω − ωi)2 + γ2i ] are the effective damping constants and the modified vibrational frequency of the ODT molecules for asymmetric (i = 1) and symmetric (i = 2) vibrational modes, respectively. μi is the coupling rate corresponding to direct energy exchange between the resonator mode of the MA, and the asymmetric (i = 1) and symmetric (i = 2) vibrational modes of the ODT. A higher μi generated larger absorption modulation from the ODT molecules. In this calculation, the ODT-coated resonant frequency (ω0) obtained from the bare MA structure was adjusted to match the TCMT absorption spectra using the E

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structures shown in Figure 1a,b are used by applying periodic boundary conditions along the x- and y-directions, and the perfectly matched layer (PML) boundary condition along the z-direction. The Au layers were modeled using the Drude model with plasma frequency ωp = 1.378 × 1016 rad/s and collision frequency Γ = 1.224 × 1014 rad/s. For the SiO2 layer, the built-in dielectric constant in the Lumerical FDTD was used. To calculate the integrated near-field intensity, an additional mechanical refinement of 2 nm along all direction was used. Sample Fabrication. For NIL, two silicon (Si) masters with cross-shaped hole arrays of different sizes for MAs with the nanopedestals and the control MAs were made by using standard electron-beam lithography (EBL) and reactive ion etching (RIE). We first prepared a polyurethane-acrylate (PUA) mold film with the cross-shaped nanoantenna arrays replicated using the Si master. We proceeded with the deposition of a 10/150 nm thick chromium (Cr)/Au bottom layer on a silicon wafer using an electron-beam evaporator, and the SiO2 layer was deposited on the bottom Au layer by plasma-enhanced chemical vapor deposition (PECVD). Prior to the NIL process, the bilayer resistance was spin coated. The lift-off resistance was LOR1A (MicroChem), and was used as an undercut profile-patterned layer for the lift-off process, followed by a UV curable silicon-based resistance, LV400 (Chemoptics). The cross-shaped hole pattern arrays were transferred by the UV NIL process at a pressure of three bar under UV exposure for 95 s. We removed the LV400 residue using RIE with a mixture of CF4 and O2 at a power of 100 W, and the form of the undercut profile-patterned LOR layer using oxygen RIE at a power of 50 W. A thin layer of Cr (3 nm) as an adhesion layer and an Au layer of 30 nm were deposited using the electron-beam evaporator, and the lift-off process was used to form the cross-shaped nanoantenna. Finally, the SiO2 nanopedestal structure was formed by using a plasma isotropic dry etching process with a mixture of CF4 (30 sccm) and O2 (10 sccm) at a power of 300 W for a precisely controlled etching duration.

measured data. The calculated absorption spectra from eq 1 yields excellent agreement with the experimental results shown in Figure 4a,b by the dashed red curves. The extracted coupling rates of the ODT-coated MA structures for the two ODT vibrational wavelengths are shown in Table 2. The Table 2. Extracted Coupling Rates between the Plasmon Mode of the MA Structure and the Two ODT Vibrations Using TCMT Modeling case control MA MA with nanopedestal (U = 80 nm)

μ1 (rad/s; λ = 3427 nm)

μ2 (rad/s; λ = 3509 nm)

2.6 × 1012 4.5 × 1012

1.6 × 1012 2.8 × 1012

coupling rates extracted from the measured data revealed that the MA etched for eight minutes with the nanopedestal array showed values 1.73 and 1.75 times higher for the asymmetric and symmetric vibrational modes of the ODT, respectively, than the control MA structure. The increased coupling rates of the ODT-coated MA with the nanopedestal system revealed that stronger interaction between plasmon resonance and molecular vibrations played a key role in achieving ultrasensitive detection, resulting in enhanced SEIRA detection signals. We note that the signal strength of the absorption difference of the ODT-coated MA structure could also be affected by the intrinsic and extrinsic loss rates of the resonator, as discussed in ref 38, and the two MA structures used in this study had similar loss rates, as shown in Table S1 in the Supporting Information. Finally, SEIRA enhancement factors (EF) for each structure were calculated by comparing our baseline-corrected SEIRA detection signal (see Supporting Information) with the ODT absorption extracted from infrared reflection absorption spectroscopy (IRRAS) measurement.28 As a result, we obtained EF of 700 and 290 for the MA with nanopedestal and the control MA structure, respectively. Although the SEIRA EF is low compared to other studies reported so far, our structure can provide a very high detection signal in practice based on the large sensing area and strong field enhancement induced in easy-to-control vertical gap. By optimizing the structure to achieve more enhanced mode coupling between plasmons and molecular vibrations, the sensitivity can be further improved. In conclusion, we experimentally showed the monolayer detection of ODT molecules using an MA with a nanopedestal structure and a control MA structure. The experimental data were examined by using a TCMT framework. The experimental results and an analytic model of the absorption spectra for the ODT-MA systems revealed that the SEIRA signal was significantly enhanced for ODT-coated MA with the nanopedestal structure compared with the control MA through the enhanced coupling rate between ODT vibrational modes and the plasmonic resonant mode. Our structuring and fabrication processes can be applied to other resonator geometries. For low-cost sensing applications of the future, it may provide a sensing platform to achieve ultrasensitive detection of layers of biomolecules of nanometer sizes.

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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.8b00702.

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Additional experimental details and supporting figure (PDF).

AUTHOR INFORMATION

Corresponding Authors

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

Jongwon Lee: 0000-0002-3878-0650 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the 2015 Research Fund (No. 1.150141.01) of UNIST (Ulsan National Institute of Science and Technology) (J.L.), the Basic Science Research Programs through the National Research Foundation of Korea (NRF)

METHODS Numerical Calculations. FDTD calculations were performed using a commercial software package (Lumerical FDTD Solutions, ver. 8.19). In device simulation, the unit cell F

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(22) Chirumamilla, M.; Toma, A.; Gopalakrishnan, A.; Das, G.; Zaccaria, R. P.; Krahne, R.; Rondanina, E.; Leoncini, M.; Liberale, C.; De Angelis, F.; Di Fabrizio, E. 3D Nanostar Dimers with a Sub-10-nm Gap for Single-/Few- Molecule Surface-Enhanced Raman Scattering. Adv. Mater. 2014, 26, 2353−2358. (23) Neubrech, F.; Pucci, A.; Cornelius, T. W.; Karim, S.; GarciaEtxarri, A.; Aizpurua, J. Resonant Plasmonic and Vibrational Coupling in a Tailored Nanoantenna for Infrared Detection. Phys. Rev. Lett. 2008, 101, 157403. (24) Cubukcu, E.; Zhang, S.; Park, Y. S.; Bartal, G.; Zhang, X. Split ring resonator sensors for infrared detection of single molecular monolayers. Appl. Phys. Lett. 2009, 95, 043113. (25) Cetin, A. E.; Etezadi, D.; Altug, H. Accessible Nearfields by Nanoantennas on Nanopedestals for Ultrasensitive Vibrational Spectroscopy. Adv. Opt. Mater. 2014, 2, 866−872. (26) Brown, L. V.; Yang, X.; Zhao, K.; Zheng, B. Y.; Nordlander, P.; Halas, N. J. Fan-Shaped Gold Nanoantennas above Reflective Substrates for Surface-Enhanced Infrared Absorption (SEIRA). Nano Lett. 2015, 15, 1272−1280. (27) Huck, C.; Toma, A.; Neubrech, F.; Chirumamilla, M.; Vogt, J.; De Angelis, F.; Pucci, A. Gold Nanoantennas on a Pedestal for Plasmonic Enhancement in the Infrared. ACS Photonics 2015, 2, 497− 505. (28) Huck, C.; Vogt, J.; Sendner, M.; Hengstler, D.; Neubrech, F.; Pucci, A. Plasmonic Enhancement of Infrared Vibrational Signals: Nanoslits versus Nanorods. ACS Photonics 2015, 2, 1489−1497. (29) Chen, X. S.; Ciraci, C.; Smith, D. R.; Oh, S. H. NanogapEnhanced Infrared Spectroscopy with Template-Stripped Wafer-Scale Arrays of Buried Plasmonic Cavities. Nano Lett. 2015, 15, 107−113. (30) Chen, K.; Dao, T. D.; Ishii, S.; Aono, M.; Nagao, T. Infrared Aluminum Metamaterial Perfect Absorbers for Plasmon-Enhanced Infrared Spectroscopy. Adv. Funct. Mater. 2015, 25, 6637−6643. (31) Ishikawa, A.; Tanaka, T. Metamaterial Absorbers for Infrared Detection of Molecular Self-Assembled Monolayers. Sci. Rep. 2015, 5, 12570. (32) Cetin, A. E.; Korkmaz, S.; Durmaz, H.; Aslan, E.; Kaya, S.; Paiella, R.; Turkmen, M. Quantification of Multiple Molecular Fingerprints by Dual-Resonant Perfect Absorber. Adv. Opt. Mater. 2016, 4, 1274−1280. (33) Le, T. H. H.; Tanaka, T. Plasmonics-Nanofluidics Hydrid Metamaterial: An Ultrasensitive Platform for Infrared Absorption Spectroscopy and Quantitative Measurement of Molecules. ACS Nano 2017, 11, 9780−9788. (34) Sung, S.; Kim, C. H.; Lee, J.; Jung, J. Y.; Jeong, J. H.; Choi, J. H.; Lee, E. S. Advanced Metal Lift-offs and Nanoimprint for Plasmonic Metal Patterns. Int. J. Pr Eng. Man-Gt 2014, 1, 25−30. (35) Wang, C.; Zhang, Q.; Song, Y.; Chou, S. Y. Plasmonic BarCoupled Dots-on-Pillar Cavity Antenna with Dual Resonances for Infrared Absorption and Sensing: Performance and Nanoimprint Fabrication. ACS Nano 2014, 8, 2618−2624. (36) Rumler, M.; Foerthner, M.; Baier, L.; Evanschitzky, P.; Becker, M.; Rommel, M.; Frey, L. Large area manufacturing of plasmonic colour filters using substrate conformal imprint lithography. Nano Futures 2017, 1, 015002. (37) Popenoe, D. D. Theoretical and Experimental Methods for in Situ Infrared Spectroelectrochemistry of Organic Monomolecular Films; Iowa State University, 1992. (38) Adato, R.; Artar, A.; Erramilli, S.; Altug, H. Engineered Absorption Enhancement and Induced Transparency in Coupled Molecular and Plasmonic Resonator Systems. Nano Lett. 2013, 13, 2584−2591.

Grant No. 2016R1C1B2009604 (J.L.), and the Center for Advanced Meta-Materials (CAMM), funded by the Ministry of Science, ICT, and Future Planning as a Global Frontier Project (CAMM-No. 2014M3A6B3063707). This work was also supported by the Korea Institute of Machinery and Materials under Grant NK 211D.

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REFERENCES

(1) Pendry, J. B. Negative refraction makes a perfect lens. Phys. Rev. Lett. 2000, 85, 3966−3969. (2) Smith, D. R.; Pendry, J. B.; Wiltshire, M. C. K. Metamaterials and negative refractive index. Science 2004, 305, 788−792. (3) Zhang, X.; Liu, Z. W. Superlenses to overcome the diffraction limit. Nat. Mater. 2008, 7, 435−441. (4) Klein, M. W.; Enkrich, C.; Wegener, M.; Linden, S. Secondharmonic generation from magnetic metamaterials. Science 2006, 313, 502−504. (5) Lee, J.; Tymchenko, M.; Argyropoulos, C.; Chen, P. Y.; Lu, F.; Demmerle, F.; Boehm, G.; Amann, M. C.; Alu, A.; Belkin, M. A. Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions. Nature 2014, 511, 65−69. (6) Lassiter, J. B.; Chen, X. S.; Liu, X. J.; Ciraci, C.; Hoang, T. B.; Larouche, S.; Oh, S. H.; Mikkelsen, M. H.; Smith, D. R. ThirdHarmonic Generation Enhancement by Film-Coupled Plasmonic Stripe Resonators. ACS Photonics 2014, 1, 1212−1217. (7) Yu, N. F.; Genevet, P.; Kats, M. A.; Aieta, F.; Tetienne, J. P.; Capasso, F.; Gaburro, Z. Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction. Science 2011, 334, 333−337. (8) Landy, N. I.; Sajuyigbe, S.; Mock, J. J.; Smith, D. R.; Padilla, W. J. Perfect metamaterial absorber. Phys. Rev. Lett. 2008, 100, 207402. (9) Watts, C. M.; Liu, X. L.; Padilla, W. J. Metamaterial Electromagnetic Wave Absorbers. Adv. Mater. 2012, 24, Op98− Op120. (10) Ogawa, S.; Fujisawa, D.; Hata, H.; Uetsuki, M.; Misaki, K.; Kimata, M. Mushroom plasmonic metamaterial infrared absorbers. Appl. Phys. Lett. 2015, 106, 041105. (11) Cai, W. S.; Chettiar, U. K.; Kildishev, A. V.; Shalaev, V. M. Optical cloaking with metamaterials. Nat. Photonics 2007, 1, 224− 227. (12) Choi, M.; Lee, S. H.; Kim, Y.; Kang, S. B.; Shin, J.; Kwak, M. H.; Kang, K. Y.; Lee, Y. H.; Park, N.; Min, B. A terahertz metamaterial with unnaturally high refractive index. Nature 2011, 470, 369−373. (13) Moitra, P.; Yang, Y. M.; Anderson, Z.; Kravchenko, I. I.; Briggs, D. P.; Valentine, J. Realization of an all-dielectric zero-index optical metamaterial. Nat. Photonics 2013, 7, 791−795. (14) Suchowski, H.; O’Brien, K.; Wong, Z. J.; Salandrino, A.; Yin, X. B.; Zhang, X. Phase Mismatch-Free Nonlinear Propagation in Optical Zero-Index Materials. Science 2013, 342, 1223−1226. (15) Liu, N.; Mesch, M.; Weiss, T.; Hentschel, M.; Giessen, H. Infrared Perfect Absorber and Its Application As Plasmonic Sensor. Nano Lett. 2010, 10, 2342−2348. (16) Chen, K.; Adato, R.; Altug, H. Dual-Band Perfect Absorber for Multispectral Plasmon-Enhanced Infrared Spectroscopy. ACS Nano 2012, 6, 7998−8006. (17) Li, Y. Q.; Su, L.; Shou, C.; Yu, C. M.; Deng, J. J.; Fang, Y. Surface-enhanced molecular spectroscopy (SEMS) based on perfectabsorber metamaterials in the mid-infrared. Sci. Rep. 2013, 3, 2865. (18) John, M.; Chalmers, P. R. G. Handbook of Vibrational Spectroscopy; John Wiley & Sons Ltd.: U.K., 2003. (19) Hartstein, A.; Kirtley, J. R.; Tsang, J. C. Enhancement of the Infrared-Absorption from Molecular Monolayers with Thin Metal Overlayers. Phys. Rev. Lett. 1980, 45, 201−204. (20) Kawata, S. Near-Field Optics and Surface Plasmon Polaritons; Springer: Heidelberg, 2001. (21) Aroca, R. F.; Ross, D. J.; Domingo, C. Surface-enhanced infrared spectroscopy. Appl. Spectrosc. 2004, 58, 324a−338a. G

DOI: 10.1021/acsphotonics.8b00702 ACS Photonics XXXX, XXX, XXX−XXX