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Understanding the effects of dielectric medium, substrate, and depth on electric fields and SERS of quasi-3D plasmonic nanostructures

Jiajie Xu, Pavel Kvasnička, Matthew Idso, Roger W. Jordan, Heng Gong, Jiří Homola, and Qiuming Yu

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Abstract

The local electric field distribution and the effect of surface-enhanced Raman spectroscopy (SERS) were investigated on the quasi-3D (Q3D) plasmonic nanostructures formed by gold nanohole and nanodisc array layers physically separated by a dielectric medium. The local electric fields at the top gold nanoholes and bottom gold nanodiscs as a function of the dielectric medium, substrate, and depth of Q3D plasmonic nanostructures upon the irradiation of a 785 nm laser were calculated using the three-dimensional finite-difference time-domain (3D-FDTD) method. The intensity of the maximum local electric fields was shown to oscillate with the depth and the stronger local electric fields occurring at the top or bottom gold layer strongly depend on the dielectric medium, substrate, and depth of the nanostructure. This phenomenon was determined to be related to the Fabry-Pérot interference effect and the interaction of localized surface plasmons (LSPs). The enhancement factors (EFs) of SERS obtained from the 3D-FDTD simulations were compared to those calculated from the SERS experiments conducted on the Q3D plasmonic nanostructures fabricated on silicon and ITO coated glass substrates with different depths. The same trend was obtained from both methods. The capabilities of tuning not only the intensity but also the location of the maximum local electric fields by varying the depth, dielectric medium, and substrate make Q3D plasmonic nanostructures well suited for highly sensitive and reproducible SERS detection and analysis.

©2011 Optical Society of America

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Figures (7)
Fig. 1
Fig. 1 Schematics of the Q3D gold nanostructure arrays in side-views (a - d) and 3D-perpective (e). Nanostructure arrays were fabricated on (a) Si, ITO, or glass substrate using PMMA resist; (b) ITO coated glass substrate with PMMA resist; (c) homogenous PDMS or PU; and (d) homogeneous Si. All the Q3D nanostructures have the same diameter (400 nm) and the edge-to-edge spacing (100 nm) but varied depth.

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Fig. 2
Fig. 2 (a) Comparison of reflectance and transmittance spectra obtained from FDTD simulations of the whole hole-disc array structure (blue lines) and those calculated from Eq. (1) with reflection and transmission coefficients of the free-standing disc and hole layer obtained from FDTD simulations (red lines). (b) Dependence of the excitation field Eexc at the top holes plane and at the bottom discs plane on the depth of the structure in the Fabry-Pérot thin-film interference model.

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Fig. 3
Fig. 3 The depth dependence of the fourth power of the maximum local electric field |Etop|4 and |Ebottom|4 obtained from 3D-FDTD simulations for the Q3D built in PMMA resist on the substrates of (a) Si, (b) ITO, (c) glass, and (d) ITO coated glass. The blue, red and green lines are the maximum local electric fields of the top gold nanohole layer, bottom gold nanodisc and the summation of the top and bottom fields, respectively. The diameter and the spacing were maintained 400 and 100 nm, respectively, for all arrays.

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Fig. 4
Fig. 4 The 3D-FDTD calculated the square root of the electric field intensities (|E|2) along the x-z plane and the x-y plane at the Au/air interfaces of the top gold nanoholes and the bottom gold nanodiscs of Q3D on Si and ITO coated glass substrates. The depth was varied from 200 to 600 nm while the diameter and the spacing were maintained 400 and 100 nm, respectively. The incident light is 785 nm with the polarization in the x-axis and the amplitude 1 V/m.

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Fig. 5
Fig. 5 The depth dependence of the fourth power of the maximum local electric field |Eloc_max/E0|4 obtained from the 3D-FDTD simulations for the Q3D made in homogeneous (a) PDMS, (b) PU and (c) Si media. The blue, red, and green lines are the maximum local electric fields of top gold nanohole layer, bottom gold nanodisc and the summation of the top and bottom, respectively. The diameter and spacing were maintained 400 and 100 nm, respectively.

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Fig. 6
Fig. 6 Comparison of the experimental and FDTD simulated EFs of Q3D with different depth fabricated using PMMA resist via EBL on (a) Si and (b) ITO coated glass substrates, respectively. The insets are the SEM images of one nanostructure array showing the diameter of 400 nm and the spacing of 100 nm, which were maintained the same for all arrays.

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Fig. 7
Fig. 7 SERS spectra of 4-MP adsorbed on Q3D plasmonic nanostructures with the depth of 300 nm (a) and 370 nm (b) fabricated on Si (blue lines) and ITO coated glass (red lines) substrates via EBL using PMMA resist. The insets are the depth profiles of the Q3D plasmonic nanostructure arrays measured by AFM.

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Tables (1)

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Table 1 The optical properties (refractive index, absorption and extinction coefficient) at 785 nm of all materials involved in this study [16].

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Equations (2)

Equations on this page are rendered with MathJax. Learn more.

r= r 1 + r 2 t 1 2 e 2ikd 1− r 1 r 2 e 2ikd t= t 1 t 2 e ikd 1− r 1 r 2 e ikd
p=λ/(2n)
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