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Electric field enhancement and far-field radiation pattern of the nanoantenna with concentric rings.

Chen SW, Huang YH, Chao BK, Hsueh CH, Li JH - Nanoscale Res Lett (2014)

Bottom Line: The directivity of a dipole antenna can be improved by directivity-enhanced Raman scattering structure, which is a combination of a dipole antenna and a ring reflector layer on a ground plane.The measured results show that the structure with concentric rings can have stronger SERS signals.The proposed structure can be useful for several nanoantenna applications, such as sensing or detecting.

View Article: PubMed Central - PubMed

Affiliation: Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei, 10617, Taiwan, shihwenc@gmail.com.

ABSTRACT
The optical antennas have the potential in various applications because of their field enhancement and directivity control. The directivity of a dipole antenna can be improved by directivity-enhanced Raman scattering structure, which is a combination of a dipole antenna and a ring reflector layer on a ground plane. The concentric rings can collect the light into the center hole. Depending upon the geometry of the antenna inside the hole, different electric field enhancements can be achieved. In this paper, we propose to combine the concentric rings with the directivity-enhanced Raman scattering structure in order to study its electric field enhancement and the far-field radiation pattern by finite-difference time-domain simulations. Compared with the structure without the concentric rings over the ground plane, it is found that our proposed structure can obtain stronger electric field enhancements and narrower radiation beams because the gold rings can help to couple the light into the nanoantenna and they also scatter light into the far field and modify the far-field radiation pattern. The designed structures were fabricated and the chemical molecules of thiophenol were attached on the structures for surface-enhanced Raman scattering (SERS) measurements. The measured results show that the structure with concentric rings can have stronger SERS signals. The effects of the dielectric layer thickness in our proposed structure on the near-field enhancements and far-field radiation are also investigated. The proposed structure can be useful for several nanoantenna applications, such as sensing or detecting.

No MeSH data available.


Schematic diagrams of studied structures. (a) the DERS structure and (b) the structure with concentric rings. The upper subfigure is the top view and the lower subfigure is the cross-section view. The dipole antenna in the center of the upper subfigure is 130-nm long, 45-nm wide, and 50-nm high with a 20-nm gap. The corrugated structure is placed on TiO2 spacer over the gold ground with a glass substrate. The diameter of the center groove is fixed at 500 nm in the DERS structure, and it is indicted as D in the structure-added concentric rings. The periodicity and width of the grating rings in (b) are denoted as P and W. The thickness of TiO2 spacer is 40 nm in the DERS structure in (a), and it is indicated as H in the structure with concentric rings in (b). The thickness of the gold layer atop the glass is 150 nm.
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Fig1: Schematic diagrams of studied structures. (a) the DERS structure and (b) the structure with concentric rings. The upper subfigure is the top view and the lower subfigure is the cross-section view. The dipole antenna in the center of the upper subfigure is 130-nm long, 45-nm wide, and 50-nm high with a 20-nm gap. The corrugated structure is placed on TiO2 spacer over the gold ground with a glass substrate. The diameter of the center groove is fixed at 500 nm in the DERS structure, and it is indicted as D in the structure-added concentric rings. The periodicity and width of the grating rings in (b) are denoted as P and W. The thickness of TiO2 spacer is 40 nm in the DERS structure in (a), and it is indicated as H in the structure with concentric rings in (b). The thickness of the gold layer atop the glass is 150 nm.

Mentions: To study the near-field enhancement and the far-field radiation for the combinations of the concentric rings and the DERS structure, the parameters of the DERS structure studied in a previous work by A. Ahmed and R. Gordon[12] are adopted in our simulations. The geometry of the DERS structure is shown in Figure 1a, where the upper subfigure is the top view and the lower subfigure is the cross-section view. According to Chon et al.[5], it was designed for enhancing Raman signals with excitation laser wavelength of λ = 785 nm and Stokes shifted scattered radiation at λ = 880 nm. All parameters of the structure are set to achieve the strong surface-enhanced Raman scattering signals at the mean wavelength of λ = 840 nm. The dipole antenna in the center of the upper subfigure in Figure 1a is 130-nm long, 45-nm wide, and 50-nm high with a 20-nm gap, and the diameter of the central groove is 500 nm as denoted in the lower subfigure in Figure 1a, where the thicknesses of TiO2 spacer and Au ground plane are 40 and 150 nm, respectively. The combination of the DERS and the concentric rings structure we proposed is plotted in Figure 1b which contains several periodic gold rings and has the dipole antenna of the same size as the DERS structure. The periodicity and the width of the gold rings are denoted as P and W, respectively. The diameter of the center groove is defined as D. The height of the gold rings is 50 nm and the thicknesses of TiO2 spacer is indicated as H. The structure in Figure 1b can be considered as adding the periodic gold rings with the flexibility of the diameter of the center groove to the structure shown in Figure 1a. It can also be viewed as the structure of putting a nanoantenna in the center of the concentric rings. It has been shown that the resonance depends on the period of grating, and the maximum intensity can be obtained when W = P/2[19]. Hence, the geometric parameters of our proposed structure shown in Figure 1b are set as D = P and W = P/2. We assume that there are three gold rings for simplification and simulate several cases with different periodicities of rings in our study. The Lumerical FDTD Solutions[30], a commercial software based on the finite-difference time-domain (FDTD) method, is used to simulate the near-field intensities and far-field radiations of the structures. For the near-field calculations, the incident wave is x polarized with the wavelength from 500 to 950 nm illuminated in the - z direction in Figure 1. Because the structure is symmetric, the boundary conditions at y = 0 and x = 0 are set as symmetric and anti-symmetric boundary conditions, respectively. The boundary conditions at the other sides are set as perfectly matched layer (PML) boundary conditions. Thus, the simulation domain 5,000 × 5,000 × 2,000 nm is reduced to a quarter region. A uniform mesh of 2 × 2 × 2 nm is adopted in the region of the dipole antenna 130 × 45 × 50 nm, and the non-uniform meshes are applied in the rest regions to save computation time with sufficient numerical accuracy. The time step, 3.7936 attoseconds for this mesh case, is to satisfy the numerical stability. It is worth studying the far-field radiation patterns as the nanoantenna is excited. Because the large field enhancements occurred in the gap region of the nanoantenna, an electric dipole source is placed at the feed gap of the dipole antenna to simulate the far-field radiations. The refractive indexes of gold, TiO2, and glass are adopted from the experiment data[31–33].Figure 1


Electric field enhancement and far-field radiation pattern of the nanoantenna with concentric rings.

Chen SW, Huang YH, Chao BK, Hsueh CH, Li JH - Nanoscale Res Lett (2014)

Schematic diagrams of studied structures. (a) the DERS structure and (b) the structure with concentric rings. The upper subfigure is the top view and the lower subfigure is the cross-section view. The dipole antenna in the center of the upper subfigure is 130-nm long, 45-nm wide, and 50-nm high with a 20-nm gap. The corrugated structure is placed on TiO2 spacer over the gold ground with a glass substrate. The diameter of the center groove is fixed at 500 nm in the DERS structure, and it is indicted as D in the structure-added concentric rings. The periodicity and width of the grating rings in (b) are denoted as P and W. The thickness of TiO2 spacer is 40 nm in the DERS structure in (a), and it is indicated as H in the structure with concentric rings in (b). The thickness of the gold layer atop the glass is 150 nm.
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Fig1: Schematic diagrams of studied structures. (a) the DERS structure and (b) the structure with concentric rings. The upper subfigure is the top view and the lower subfigure is the cross-section view. The dipole antenna in the center of the upper subfigure is 130-nm long, 45-nm wide, and 50-nm high with a 20-nm gap. The corrugated structure is placed on TiO2 spacer over the gold ground with a glass substrate. The diameter of the center groove is fixed at 500 nm in the DERS structure, and it is indicted as D in the structure-added concentric rings. The periodicity and width of the grating rings in (b) are denoted as P and W. The thickness of TiO2 spacer is 40 nm in the DERS structure in (a), and it is indicated as H in the structure with concentric rings in (b). The thickness of the gold layer atop the glass is 150 nm.
Mentions: To study the near-field enhancement and the far-field radiation for the combinations of the concentric rings and the DERS structure, the parameters of the DERS structure studied in a previous work by A. Ahmed and R. Gordon[12] are adopted in our simulations. The geometry of the DERS structure is shown in Figure 1a, where the upper subfigure is the top view and the lower subfigure is the cross-section view. According to Chon et al.[5], it was designed for enhancing Raman signals with excitation laser wavelength of λ = 785 nm and Stokes shifted scattered radiation at λ = 880 nm. All parameters of the structure are set to achieve the strong surface-enhanced Raman scattering signals at the mean wavelength of λ = 840 nm. The dipole antenna in the center of the upper subfigure in Figure 1a is 130-nm long, 45-nm wide, and 50-nm high with a 20-nm gap, and the diameter of the central groove is 500 nm as denoted in the lower subfigure in Figure 1a, where the thicknesses of TiO2 spacer and Au ground plane are 40 and 150 nm, respectively. The combination of the DERS and the concentric rings structure we proposed is plotted in Figure 1b which contains several periodic gold rings and has the dipole antenna of the same size as the DERS structure. The periodicity and the width of the gold rings are denoted as P and W, respectively. The diameter of the center groove is defined as D. The height of the gold rings is 50 nm and the thicknesses of TiO2 spacer is indicated as H. The structure in Figure 1b can be considered as adding the periodic gold rings with the flexibility of the diameter of the center groove to the structure shown in Figure 1a. It can also be viewed as the structure of putting a nanoantenna in the center of the concentric rings. It has been shown that the resonance depends on the period of grating, and the maximum intensity can be obtained when W = P/2[19]. Hence, the geometric parameters of our proposed structure shown in Figure 1b are set as D = P and W = P/2. We assume that there are three gold rings for simplification and simulate several cases with different periodicities of rings in our study. The Lumerical FDTD Solutions[30], a commercial software based on the finite-difference time-domain (FDTD) method, is used to simulate the near-field intensities and far-field radiations of the structures. For the near-field calculations, the incident wave is x polarized with the wavelength from 500 to 950 nm illuminated in the - z direction in Figure 1. Because the structure is symmetric, the boundary conditions at y = 0 and x = 0 are set as symmetric and anti-symmetric boundary conditions, respectively. The boundary conditions at the other sides are set as perfectly matched layer (PML) boundary conditions. Thus, the simulation domain 5,000 × 5,000 × 2,000 nm is reduced to a quarter region. A uniform mesh of 2 × 2 × 2 nm is adopted in the region of the dipole antenna 130 × 45 × 50 nm, and the non-uniform meshes are applied in the rest regions to save computation time with sufficient numerical accuracy. The time step, 3.7936 attoseconds for this mesh case, is to satisfy the numerical stability. It is worth studying the far-field radiation patterns as the nanoantenna is excited. Because the large field enhancements occurred in the gap region of the nanoantenna, an electric dipole source is placed at the feed gap of the dipole antenna to simulate the far-field radiations. The refractive indexes of gold, TiO2, and glass are adopted from the experiment data[31–33].Figure 1

Bottom Line: The directivity of a dipole antenna can be improved by directivity-enhanced Raman scattering structure, which is a combination of a dipole antenna and a ring reflector layer on a ground plane.The measured results show that the structure with concentric rings can have stronger SERS signals.The proposed structure can be useful for several nanoantenna applications, such as sensing or detecting.

View Article: PubMed Central - PubMed

Affiliation: Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei, 10617, Taiwan, shihwenc@gmail.com.

ABSTRACT
The optical antennas have the potential in various applications because of their field enhancement and directivity control. The directivity of a dipole antenna can be improved by directivity-enhanced Raman scattering structure, which is a combination of a dipole antenna and a ring reflector layer on a ground plane. The concentric rings can collect the light into the center hole. Depending upon the geometry of the antenna inside the hole, different electric field enhancements can be achieved. In this paper, we propose to combine the concentric rings with the directivity-enhanced Raman scattering structure in order to study its electric field enhancement and the far-field radiation pattern by finite-difference time-domain simulations. Compared with the structure without the concentric rings over the ground plane, it is found that our proposed structure can obtain stronger electric field enhancements and narrower radiation beams because the gold rings can help to couple the light into the nanoantenna and they also scatter light into the far field and modify the far-field radiation pattern. The designed structures were fabricated and the chemical molecules of thiophenol were attached on the structures for surface-enhanced Raman scattering (SERS) measurements. The measured results show that the structure with concentric rings can have stronger SERS signals. The effects of the dielectric layer thickness in our proposed structure on the near-field enhancements and far-field radiation are also investigated. The proposed structure can be useful for several nanoantenna applications, such as sensing or detecting.

No MeSH data available.