Mid-infrared Molecular Refractive Index and Vibrational Modes Sensing Tingting Wu 1
1, 2
1
, Yu Luo , Lei Wei
1,2
School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 2 CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Singapore
[email protected]
We utilize tunable graphene plasmons to study nanometricsize molecular refractive index and vibrational modes sensing by a graphene integrated silicon grating. The great electric field enhancement and wide tunability of the localized graphene plasmons enable the proposed device accurate label-free identification of the molecular refractive index and the midinfrared vibrational modes by detecting the resonant wavelength and the absorption spectrum before and after the protein layer formation. Our calculation results pave the way for the further development of novel cost-effective biosensors by an active plasmonic device. Keywords—mid-infrared plasmons; vibrational modes
biosensor;
graphene
tunable
I. INTRODUCTION Graphene has been demonstrated as a promising alternative to noble metals for highly integrated active plasmonic devices at infrared and THz ranges [1-9]. Compared with metals, graphene plasmons possess long lifetimes, significant electric field confinement and enhancement, and extraordinary external tunability. Active plasmons allow external control on the incident light confined around graphene with deeply subwavelength dimensions. The graphene plasmonic waves have extremely high field confinement resulting in strong lightmatter interactions [10, 11]. Molecules structural vibrational modes produce specific infrared spectral features which can be used to resolve the corresponding chemical identity. The interaction between the infrared light and the nanometric-size molecules is normally very weak. Significant field localization and enhancement produced by graphene plasmons can greatly increase the overlap between the light and the molecules, leading to superior detection sensitivity in the molecular refractive index and vibrational fingerprints simultaneously [12, 13]. We employ tunable graphene plasmons for biosensing via detecting the plasmonic resonant wavelength and the absorption spectrum before and after protein immobilization. The resonant wavelength shift is used to characterize the refractive index. The absorption variation is utilized to identify the vibrational modes. Our device consists of a graphene layer integrated above a patterned silicon grating substrate. The results show that the device resonant wavelength and the absorption over a broadband spectral
range as a function of Fermi level is sufficient to provide accurate molecular refractive index and chemical identification. II. WAVEGUIDE STRUCTRUE AND GUIDED MODE PROFILES Figure 1(a) shows the schematic of graphene plasmonic biosensor with a continuous graphene layer immobilized on top of a grating comprising of a periodically patterned narrow air slots array in a silicon layer. The grating is used to compensate the wavevector mismatch to stimulate graphene plasmons. For normal-incidence, when the grating period matches the plasmonic wave period, the graphene plasmons are excited. One advantage of the graphene integrated silicon grating structures is that the graphene layer is continuous. Finitedifference time-domain (FDTD) calculations are performed on a unit cell with periodic boundary conditions. Graphene is modeled as a two-dimensional surface with complex conductivity from Kubo formula [14]. The whole device is surrounded by air.
Figure 1. (a) Geometry of the graphene plasmonic biosensor. The graphene integrated above a silicon grating is direct contact with the nanometric molecule layer. (b) The near-field Ex and Ez profile at the plasmonic resonant frequency. Calculated (c) transmission and (d) reflection spectra without molecule layer at different graphene Fermi levels.
We use recombinant protein bilayer (A/G and goat antimouse immunoglobulin G) with two vibrational modes at 5.995 µm and 6.527 µm to act as the sensing analyte. The protein layer is modeled as an 8-nm thick layer having Lorentzian complex permittivity with parameters extracted from [15]. The two mid-IR molecule vibration modes can lie
in the plasmonic resonance region where doped graphene plasmonic sensor with arranged silicon grating periodic size P in the range of tens of nanometers supports intense graphene plasmons for attainable Fermi levels less than 1 eV. With different grating periods and graphene Fermi levels, the system working at resonant wavelength from mid-infrared to THz can be built. We focus on the silicon grating waveguide with P=40 nm, W= 16 nm, t=80 nm as shown in Fig. 1 where the wavelength tuning range can sweep across the target vibrational fingerprints. P is the periodic, W is the silicon channel width, and t is the silicon grating thickness. The dielectric function of graphene is modeled as 1 + 4π iσ (ω ) ωt , with σ (ω ) being the frequency-dependent surface conductivity. We assume carrier mobility of the graphene to be μ = 10000 cm 2 (Vs) . The localized plasmon frequencies scale with EF and can be tuned by controlling the Fermi energy level through varying the charge carriers of the system. From Figs. 1(c) and 1(d), we can find that the resonant plasmonic wavelength can be tuned continuously over a wide wavelength range from 6.99 µm to 5.33 µm by a small change in the graphene Fermi level from 0.3 eV to 0.5 eV. The excited localized plasmonic mode with high confinement of the structure is associated with the guided-wave resonance. When the incident light couples to the graphene plasmonic mode, the resonance leads to a sharp notch on the transmission spectra. The resonance spectral width decreases inversely with the Fermi level. The local electric field intensity at graphene plasmons is shown in Fig. 1(b).
III. REFRACTIVE INDEX AND VIBRATIONAL MODES SENSING
Figure 2. Calculated graphene absorption spectrum with and without the biomolecule under different Fermi levels.
Fig. 3(a) depicts the resonant wavelength values before and after protein layer formation and the corresponding wavelength red shift values at different graphene Fermi levels (EF). To demonstrate the vibrational modes in protein, we provide the extinction spectra in Fig. 3(b). Extinction spectra (1-A1/A0) are calculated by normalizing the absorption spectrum with protein (A1) by that of condition without molecular (A0). The emergence of two extinction dips at EF=0.38 eV (corresponds to resonant wavelength of 6.468 µm), and EF=0.44 eV (corresponds to resonant wavelength of 5.896 µm), correspond to the two protein vibrational modes (6.527 µm and 5.995 µm), respectively. The extinction dips result from that the graphene plasmons resonance couples to the molecular vibrations [16]. The important effect is that there is a one-to-one correlation between the molecular vibrational modes and the graphene Fermi level where the plasmon resonant band overlaps with the vibrational modes.
The localized graphene plasmons depend on the surrounding environment refractive index. The extinction spectrum of the sensor is presented in Fig. 2 before and after protein layer formation, showing radical changes upon protein immobilization at different Fermi level conditions. With the protein formation, the first prominent phenomenon is the red shift of the plasmonic resonances as a consequence of the variation in the refractive index at the sensor upper surface. Wavelength shift due to the protein at plasmonic resonance exceeding 400 nm at Fermi level of 0.3 eV. The greatly improved limit of detection on the protein refractive index is simply induced by the great increment in the filed intensity.
Figure 3. (a)/(c) The plasmonic resonant wavelength with and without biomolecules and the corresponding wavelength shift under different Fermi levels for the molecule contact the graphene and 1 nm above the graphene film. (b)/(d) Absorption before (black curve) and after (red curve) protein formation and extinction spectra (blue curve) for Fermi energy level from 0.3 eV to 0.5 eV with protein immobilized directly upon/1 nm above the graphene. Extinction is calculated as the relative difference in absorption between regions with (A1) and without (A0) molecule.
Therefore, the resonant wavelength shift is adequate to detect the molecular refractive index and the extinction spectrum variation is used to detect the vibrational modes of the nanometric-size protein layer in the graphene integrated
silicon grating plasmonic device. One is more interested in detecting a thin molecule layer placed over some distances from the graphene film in practical applications [17]. Therefore, we consider monolayer protein molecules separated 1 nm above from the graphene for reference. This leads to resonant wavelength red shift from 186 nm to 93 nm when the Fermi energy level is tuned from 0.3 eV to 0.5 eV, as shown in Fig. 3(c). From Fig. 3(d), two extinction dips at Fermi level of 0.36 eV (corresponds to resonant wavelength of 6.468 µm) and 0.41 eV (corresponds to resonant wavelength of 6.026 µm) correspond to the two protein vibrational modes, respectively.
IV. CONCLUSION The above results demonstrate that the graphene integrated silicon grating biosensor combines the superior molecular refractive index detecting, with unique molecular chemical identification at mid-infrared spectroscopy, together with the extra degree of freedom enabled by the graphene electrooptical tunability. Clearly, the dynamic tunability and the large field enhancement provided by graphene are the distinctive advantages of the device for surface plasmons enhanced midIR biosensor over wide wavelength range with a single device. Graphene plasmons therefore opens more opportunities by exploiting its large electro-optical tunability, and in particular, the realization of label-free chemical identification.
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ACKNOWLEDGMENT Singapore Ministry of Education Academic Research Fund Tier 2 (MOE2015-T2-1-066, MOE2015-T2-2-010, MOE2015T2-1-145); NRF-CRP grant (NRF2015NRF-CRP002-008); Nanyang Technological University (Startup grant: Lei Wei, Yu Luo).
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