In this study, we introduce a sensing platform based on a plasmonic metasurface absorber (MA) with a vertical nanogap for the ultra- sensitive detection of monolayer molecules. The vertical nanogap of the MA, where the extremely high near-field is uniformly dis- tributed and exposed to the external environment, is formed by an under-cut structure between a metallic cross nanoantenna and themirror layer. The accessible sensing area and the enhanced near-field of the MA further enhance the sensitivity of surface-enhancedinfrared absorption for the target molecule of 1-octadecanethiol. To provide strong coupling between the molecular vibrations and plas- monic resonance, the design parameters of the MA with a vertical nanogap are numerically designed

Hwang, Inyong

Jung, Joo-Yun

Lee, Jongwon

Journal of Sensor Science and Technology

ScholarWorks@UNIST

275 J. Sens. Sci. Technol. Vol. 27, No. 5, 2018Journal of Sensor Science and TechnologyVol. 27, No. 5 (2018) pp. 275-279http://dx.doi.org/10.5369/JSST.2018.27.5.275pISSN 1225-5475/eISSN 2093-7563Plasmon-enhanced Infrared Spectroscopy Based on Metasurface Absorberwith Vertical NanogapInyong Hwang1, Jongwon Lee1, and Joo-Yun Jung2,+AbstractIn this study, we introduce a sensing platform based on a plasmonic metasurface absorber (MA) with a vertical nanogap for the ultra-sensitive detection of monolayer molecules. The vertical nanogap of the MA, where the extremely high near-field is uniformly dis-tributed and exposed to the external environment, is formed by an under-cut structure between a metallic cross nanoantenna and themirror layer. The accessible sensing area and the enhanced near-field of the MA further enhance the sensitivity of surface-enhancedinfrared absorption for the target molecule of 1-octadecanethiol. To provide strong coupling between the molecular vibrations and plas-monic resonance, the design parameters of the MA with a vertical nanogap are numerically designed.Keywords: Metasurfaces, plasmonics, Surface-enhanced infrared absorption, nanogap1. INTRODUCTION Infrared (IR) absorption spectroscopy is a powerful and label-free optical detection method to determine structural properties ofvarious molecules via their IR absorption fingerprint vibrations[1]. IR absorption spectroscopy through IR fingerprint vibrationsis used in such fields as pharmacy, safety, food, and generalsubstance identification. However, for molecules with small IRabsorption cross-sections such as minute amounts of analytemolecules or very thin samples such as monolayers, conventionalIR absorption cannot provide good sensitivity. To address thischallenge, surface-enhanced infrared absorption (SEIRA)spectroscopy has been proposed to enhance the IR fingerprintvibrations of molecules in the vicinity of the plasmonic metallicsurface through coupling between the fingerprint vibrations ofmolecules and the enhanced and confined electromagnetic near-fields called hot spots using localized surface plasmon resonance.For applications of SEIRA, numerous plasmonic metallicnanostructures, such as nanodisk, nanosized gap, nanoantennas,metamaterial, and metasurface absorber (MA) have beeninvestigated recently [2-8].Metasurfaces are a type of thin metamaterial structurecomprising an array of sub-wavelength-sized resonators and canproduce unique spectral responses through the abrupt changes inthe phase and amplitude of the incident electromagnetic wave atthe surface of the metal layer from microwave to opticalwavelengths [9-12]. The ability of metasurfaces to control andenhance electromagnetic waves at sub-wavelength scale leads tointeresting applications, such as hologram [10], lens [11],anomalous reflection and refraction [13], and absorbers [14].Particularly, metasurfaces have been used as a component ofnarrowband [15], broadband [16], or tunable so-called perfectabsorbers [17]. MAs consist of metallic resonators on top of ametallic mirror layer separated by a thin dielectric spacer layer.Near-unity absorption of MAs at a particular wavelength producesa confined and enhanced electromagnetic near-field between themetallic resonators and the metal mirror owing to the excitation oflocalized surface plasmon resonance. To enhance the sensitivity of SEIRA, an important strategy is tomaximize the hot spot overlap with the target molecule, whichindicates an increased effective sensing area. Approaches that usenanoantennas on dielectric pedestals, where the cross-section ofthe dielectric pedestals underneath the nanoantenna is smaller thanthe size of the nanoantenna, provide an increased effective sensingarea as they allow access to the bottoms of nanoantennas [4, 5].In this study, a method for the detection of ultrasensitivemolecules based on MAs with vertical nanogaps is numerically1School of Electrical and Computer Engineering, Ulsan National Institute ofScience and Technology, Ulsan, 44919, Korea2Nano-convergence Mechanical Systems Research Division, Korea Institute Machinery and Materials, Daejeon, 305-343, Korea+Corresponding author: jjy2121@kimm.re.kr(Received: Aug. 6, 2018, Accepted: Aug. 25, 2018)This is an Open Access article distributed under the terms of the CreativeCommons Attribution Non-Commercial License(http://creativecommons.org/licenses/bync/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.Inyong Hwang, Jongwon Lee, and Joo-Yun JungJ. Sens. Sci. Technol. Vol. 27, No. 5, 2018 276investigated. A vertical nanogap, where the hot spot is uniformlydistributed and exposed to the external environment, is formed byan under-cut structure between a metallic cross nanoantenna andthe mirror layer. The accessible sensing area of the MAs increasesas the cross-section of the dielectric layer beneath the crossnanoantenna decreases. The plasmonic resonance wavelength ofthe MA can be tuned to the wavelength of the target molecularfingerprint vibrations by adjusting the design parameters of theMA. Using the MAs with vertical nanogaps, a monolayer ofmolecules of 1-octadecanethiol (ODT) is detected with a highlyenhanced SEIRA signal.2. RESULTS AND DISCUSSIONS2.1 Design and simulationsFigure 1 (a) and (b) show the schematics of a single unit of theMA and the proposed MA with a vertical nanogap, respectively.The two MAs consist of an array of top gold cross nanoantenna,an Au mirror of thickness 150 nm, and an SiO2 dielectric spacerlayer between the Au layers. The design parameters of the MAsare the dimensions of the cross nanoantenna (length L, width W,and thickness t1), array period (P), and dimensions of the SiO2dielectric layer (thickness t2, lateral under-cut depth ΔX and ΔY,and vertical under-cut depth ΔZ). For monolayer SEIRAdetection, we chose ODT as the target molecule. An ODTmolecule shows IR fingerprint vibrations at the wavelengths of3509 nm and 3427 nm owing to symmetric and asymmetricstretching vibrations of the CH2 and CH3 groups, and ODT-coatedsamples can be prepared on the Au film in the form of a self-assembled monolayer [18]. ODT is a monolayer of a carbon thiolchain with 18 carbon atoms and forms a conformal coatedhomogeneous layer with a thickness of 2.8 nm on the Au surface[19]. Figure 1 (c) and (d) show the schematic cross-section at theplanes of two MAs with ODT coated on the Au layer. The ODTmonolayer for MA with a vertical nanogap is formed around therevealed surface of the top Au cross nanoantenna and the surfaceof the Au mirror layer as shown in Fig. 1 (d). Simulatedabsorption spectral responses and electric near-fields distributionsof MAs were calculated using the commercial finite-differencetime domain (FDTD) software Lumerical FDTD-Solutions ver.8.19. The normal incident plane wave with the electric fieldpolarization along the y-direction excited a single unit of MAs asshown in Fig. 1 (a) and (b) with a periodic boundary conditionapplied in the x-y plane. The perfectly matched layer boundarycondition along the z-direction was used. The complex refractiveindex of the Au layer was described by the Drude model with theplasma frequency ωp = 1.378×1016 rad/s and the collisionfrequency Γ = 1.224×1014 rad/s. The refractive index of SiO2 wasobtained from the Lumerical Palik database and fitted within thesimulation wavelength range. To model the ODT IR fingerprintvibrations, the IR refractive index of ODT was used from theliterature [20].2.2 Simulated results and discussionsFigure 2 (a) shows the simulated absorption spectral response ofMA as shown in Fig. 1 (a). The simulated MA has the designparameters of P = 1100 nm, L = 900 nm, W = 200 nm, t1 = 50nm, and t2 = 40 nm, and the plasmonic resonance of the MA is ata wavelength of 3570 nm with 92% absorption. Figure 2 (b)shows the simulated absorption spectral responses of the MA witha vertical nanogap for different under-cut depths. The simulatedMA with a vertical nanogap has the design parameters of P =1300 nm, L = 1070 nm, W = 200 nm, t1 = 50 nm, and t2 = 40 nm,and the absorption resonance peak of the unetched MA is initiallylocated at a wavelength of 4200 nm. As the under-cut depthincreases, the absorption resonance peak shifts to a shorterwavelength owing to the reduction in the effective refractive indexof the dielectric spacer layer. The plasmonic resonance of the MAwith a vertical nanogap, whose dimensions are ΔX = ΔY = 70 nmand ΔZ = 40 nm, is at a wavelength of 3380 nm. Owing to ahigher sensitivity to the external refractive index caused by theincrease in the effective sensing area of the MA with a verticalnanogap, the coating of the ODT on the MA with a verticalnanogap produces a higher red-shift of resonance wavelength thanthe coating of the ODT on the MA. Therefore, the MA with avertical nanogap is designed to have its absorption resonance peakFig. 1. (a) Schematics of a single unit of MA and (b) MA with a ver-tical nanogap. (c) The schematic cross-sections of the ODT-coated MA and (d) MA with a vertical nanogap for the planeindicated in (a) and (b)I 2 IPlasmon-enhanced Infrared Spectroscopy Based on Metasurface Absorber with Vertical Nanogap277 J. Sens. Sci. Technol. Vol. 27, No. 5, 2018at a shorter wavelength compared with that of the MA.Figure 3 (a) and (b) show the cross-sectional views of theelectric near-field distributions of the MA as shown in Fig. 1 (a)and the MA with a vertical nanogap as shown in Fig. 1 (b),respectively. From the results of near-field calculation, most of theenhanced near-fields are confined at the bottom of the Au crossnanoantenna, and the MA as well as the MA with a verticalnanogap illustrate a maximum field enhancement |E|/|E0| ofapproximately 100. The formation of the vertical nanogap of theMA by the under-cut structure increases the accessible sensingarea beneath the Au cross nanoantenna. In addition, with thedecrease in the thickness of the vertical nanogap (20 nm for Fig.3 (c) and 10 nm for Fig. 3 (d)) as shown in Fig. 3 (c) and (d)), theenhanced near-field is confined and accumulated in the verticalnanogap and the enhanced near-field is extended along the side ofbottom of the cross nanoantenna. This near-field distribution ofthe vertical nanogap suggests that the target molecular largespatial overlapping with the near-field can enhance the plasmonic-molecular coupling and increase the sensitivity of the SEIRAsignal.Figure 4 shows the simulated absorption spectral responses ofthe MA and the MAs with vertical nanogaps coated by the ODTmonolayer. The black solid curves in Fig. 4 indicate the absorptionspectral responses of the MA and the MAs with vertical nanogapsbefore the ODT monolayers are coated on them. The formation ofthe vertical nanogap of the MA and the decrease in the thicknessof the vertical nanogap increase the sensitivity to the externalrefractive index. The ODT coating resulted in a resonancewavelength shift of 26 nm, 109 nm, 198 nm, and 423 nm for theMA as shown in Fig. 4 (a), the MA with a vertical nanogap ofthickness 40 nm as shown in Fig. 4 (b), the MA with a verticalnanogap of thickness 20 nm as shown in Fig. 4 (c), and the MAFig. 2. (a) Simulated absorption spectral response of the MA and (b)the MA with a vertical nanogap for different under-cutdepths.Fig. 3. (a) Cross-sectional views of the simulated electric near-fielddistributions at the plasmonic resonances for MA, (b) MAwith a vertical nanogap of thickness 40 nm, (c) MA with avertical nanogap of thickness 20 nm, and (d) MA with a ver-tical nanogap of thickness 10 nm. Fig. 4. (a) Simulated absorption spectral responses of the ODT-coated MA, (b) the ODT-coated MA with a vertical nanogapof thickness 40 m, (c) the ODT-coated MA with a verticalnanogap of thickness 20 m, and (d) the ODT-coated MA witha vertical nanogap of thickness 10 m.I 3 IInyong Hwang, Jongwon Lee, and Joo-Yun JungJ. Sens. Sci. Technol. Vol. 27, No. 5, 2018 278with a vertical nanogap of thickness 10 nm as shown in Fig. 4 (d),respectively. Therefore, the MA with a vertical nanogap isdesigned to have its absorption resonance peak at a shorterwavelength compared with that of the MA, and the absorptionresonances of the four ODT-coated MAs shift toward longerwavelengths and are formed at nearly the same wavelength. Thesimulated absorption spectral responses of the MAs with thevertical nanogaps except the MA shown in Fig. 4 (a) havevibrational signatures of ODT molecules at the two ODTvibrational wavelengths, where the absorption spectral response ofthe MA with a vertical nanogap of thickness 10 nm is moreseverely distorted. To better illustrate the SEIRA detection signal, the absorptiondifference spectral responses (ΔA = AFitting - AODT coating) are shownin Fig. 5. The absorption difference spectral response for the MA(black solid curve) does not contain the vibrational signatures ofODT molecules at the two ODT vibrational wavelengths. Thepeak-to-peak values of the differences in terms of absorption ofthe MA with a vertical nanogap of thickness 40 nm (red dashedcurve) at the two ODT vibrational wavelengths are 11.3 % and5.6 %. The blue dash-dotted curve in Fig. 5 shows that the peak-to-peak values of the differences in terms of absorption of the MAwith a vertical nanogap of thickness 10 nm at the two ODTvibrational wavelengths are 36.2 % and 20.8 %. The enhancedSEIRA detection signal originates in the increased effectivesensing area and integrated near-field distribution in the verticalnanogap of MA.3. CONCLUSIONSThe monolayer detection of ODT molecules using an MA witha vertical nanogap was numerically demonstrated. The simulatedresults showed that creating a vertical nanogap of MA anddecreasing the thickness of the vertical nanogap enhanced theplasmonic-molecular coupling and increased the sensitivity of theSEIRA signal. For the label-free optical sensing applications ofthe future, an MA with a vertical nanogap can provide a sensingplatform to achieve ultrasensitive detection of a monolayer ofmolecules.ACKNOWLEDGMENTThis work was supported by the Center for Advanced Meta-Materials (CAMM), funded by the Ministry of Science, ICT, andFuture Planning as a Global Frontier Project (CAMM-No.2014M3A6B3063707) and the Korea Institute of Machinery andMaterials under Grant NK 211D.REFERENCES[1] P. R. Griffiths, J. A. De Haseth, and J. D. Winefordner, Fou-rier Transform Infrared Spectrometry, John Wiley & Sons,Hoboken, NJ, 2007.[2] F. Neubrech, A. Pucci, T. W. Cornelius, S. Karim, A. Gar-cia-Etxarri, and J. Aizpurua, “Resonant plasmonic andvibrational coupling in a tailored nanoantenna for infrareddetection,” Phys. Rev. Lett., Vol. 101, No. 15, pp. 157403-157406, 2008.[3] E. Cubukcu, S. Zhang, Y. S. Park, G. Bartal, and X. 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Plasmon-enhanced Infrared Spectroscopy Based on Metasurface Absorber with Vertical Nanogap

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Plasmon-enhanced Infrared Spectroscopy Based on Metasurface Absorber with Vertical Nanogap

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