Ⅲ. Plasmon-enhanced Infrared Spectroscopy Based on Metamaterial Absorbers with Dielectric
3.5 Experimental Results
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absorption difference spectrum is calculated and extracted, as the difference between the absorption laterally shifted of the bare MA structures and absorption of the MA coated with ODT in order to match absorption peaks. The peak-to-peak values of the absorption difference for the MA with nanopedestal at two IR fingerprint wavelengths were 4.2 and 3.7 times higher than those of the control structure at 3427 nm, 3509 nm, respectively. The enhanced SEIRA signals from the nanopedestal structure are caused by strong coupling rate, originating in improved the integrated nearfield intensities from effective sensing area induced in surface engineering. We introduce an analytic framework of the temporal coupled mode theory (TCMT) to quantitatively describe the enhanced SEIRA sensing signal from both MA structures. In this optically coupled system, the coupling system between vibrational modes of ODT molecules and the plasmonic resonance can be modeled as TCMT framework. In this absorber structure, since the transmitted light can be almost totally suppressed by thick bottom layer as an optical mirror, we consider it as a single port resonator model shown in Figure 3.5.2.
Figure 3.5.2. Conceptual schematic of the TCMT based coupling modes between the vibrational modes of ODT molecules and the plasmonic MAs as the single port resonator system.
S
+S
-A Q P
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The dielectric function of the ODT molecule was characterized by using a Lorentz oscillator model, in which we iteratively fitted two fingerprint oscillators and two absorption loss rates of the ODT molecules,66 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, respectively, in the TCMT analysis. In the TCMT, the MA structures are regarded as an optical resonator with plasmonic mode amplitude A coupled to the incoming (S+) and outcoming (S-) traveling EM waves through a single port. By additional resonators of ODT vibrational strength with strong vibrational mode amplitude P and weak vibrational mode amplitude Q corresponding to the asymmetric and symmetric CH2 stretching vibrations, their interactions can be analyzed by the following coupled mode equations.3
𝑑𝐴
𝑑𝑡 = 𝑗𝜔0𝐴 − (𝛾𝑎+ 𝛾𝑟)𝐴 + 𝑗𝜇1𝑃 + 𝑗𝜇2𝑄 + √2𝛾𝑟𝑆+
(3.1) 𝑑𝑃
𝑑𝑡 = 𝑗𝜔1𝑃 − 𝛾1𝑃 + 𝑗𝜇1𝐴 𝑎𝑛𝑑 𝑑𝑄
𝑑𝑡 = 𝑗𝜔2𝑄 − 𝛾2𝑄 + 𝑗𝜇2𝐴
(3.2)
𝑆−= −𝑆++ √2𝛾𝛾𝐴
(3.3)
𝐴𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 = 4𝛾𝛾(𝛾𝑎+ 𝛾𝜇1+ 𝛾𝜇2)
(𝜔 − 𝜔0− 𝜔𝜇1− 𝜔𝜇2)2+ (𝛾𝛾+ 𝛾𝑎+ 𝛾𝜇1+ 𝛾𝜇2)2
(3.4) Where γa and γb are the absorption loss rate, and radiation loss rate of the ODT-coated plasmonic MA, respectively, γ1 and γ2 are the absorption loss rates for asymmetric and symmetric vibrations of the ODT, respectively. The S+, S- are amplitude of incident and reflected electric fields, respectively. The μ1 and μ2 are the coupling rates between plasmonic resonance and vibrations of molecules at asymmetric and symmetric vibrations, respectively, which mean direct energy exchange rates. The ω0 is the resonance frequency of the MA coated with ODT. The effective loss rate and effective frequency shifts are defined as γμi = μi2γi / [(ω - ωi)2 + γi2], and ωμi = μi2(ω - ωi) / [(ω - ωi)2 + γi2], respectively. The i = 1, 2 of subscripts indicates parameter notation at ODT vibrational wavelengths at 3427 nm (i = 1) and 3509 nm (i = 2), respectively.
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Table 3.5.1. Extracted two coupling rates between the two infrared vibrations of the ODT monolayer and the plasmon resonance of the MA structure through TCMT analysis.
As shown in Table 3.5.1, the coupling rates, µi extracted from the measured absorption spectra revealed that the MA array with nanopedestal etched for 8 min showed values 1.73 and 1.75 times higher than those of the control MA array for the asymmetric and symmetric CH2 stretching vibrations of ODT, respectively. The enhanced coupling rates of the ODT-coated MAs with nanopedestal system show that strong coupling between the plasmonic resonance and molecular vibrations reflects the integrated effects of improving sensitive detection, resulting in enhancing SERIA sensing signals. We note that the absorption difference SEIRA sensing signals of the ODT-coated MA structure could also be affected by the ratio of absorption loss rate and radiation loss rate of resonator as discussed,3 and the two MA structures used in this research was designed and fabricated with similar ratio for fair comparison as shown in Table 3.5.2.
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Table 3.5.2. Fitting parameters of TCMT analysis for ODT-coated control MA and MA with nanopedestal, respectively.
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Eventually, SEIRA enhancement factor (EF) for two metamaterial absorbers is calculated by comparing our baseline-corrected SEIRA sensing signal by using asymmetric least squares smoothing (AsLSS) algorithm with the ODT absorption extracted from infrared reflection absorption spectroscopy (IRRAS) measurement.4 The SEIRA EF for the two MA structures is described as following relation.
𝐸𝐹 =𝐼𝑆𝐸𝐼𝑅𝐴
𝐼𝐼𝑅𝑅𝐴𝑆× 𝐴𝑢𝑛𝑖𝑡 𝐴𝑎𝑛𝑡𝑒𝑛𝑛𝑎
(3.5) Where Aunit is the unit cell area, Aantenna is the revealed area of Au surfaces, ISEIRA is the peak-to-peak value of the AsLSS baseline-corrected vibrational signal, and Iref is the reference data of the ODT from IRRAS measurement. As a result, as shown in Figure 3.5.3, EF of 700 and 290 were obtained in the MA with nanopedestal and the MA with dielectric film, respectively. The EF is low compared to other previous studies reported so far, but our structures could actually offer a very high sensing signal based on the extended sensing area and high value of the integrated nearfield intensities from the strongly confined nearfield enhancement induced in easy-to-control vertical gap through undercut etching process. By optimizing the structure to achieve more enhanced coupling mode between molecular vibrations and plasmonic resonances, the sensitivity of molecule detection can be further enhanced.
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Figure 3.5.3. Measured reflection spectra and these asymmetric least square smoothing (AsLSS) baselines for the control MA (a) and the MA with nanopedestal (b), respectively. Extracted vibrational signals of a monolayer of ODT molecules for the control MA (c) and the MA with nanopedestal (d), respectively.
3000 3200 3400 3600 3800 4000
0.0 0.2 0.4 0.6 0.8 1.0
Reflection
Wavelength (nm)
Measurement AsLSS Baseline
3000 3200 3400 3600 3800 4000
0.0 0.2 0.4 0.6 0.8 1.0
Reflection
Wavelength (nm)
Measurement AsLSS Baseline
3200 3300 3400 3500 3600 3700
0.95 1.00 1.05 1.10 1.15 1.20 1.25
Vibrational Signal
Wavelength [nm]
3200 3300 3400 3500 3600 3700
0.95 1.00 1.05 1.10 1.15 1.20 1.25
Vibrational Signal
Wavelength [nm]
(a) (b)
(c) (d)
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