1. Introduction

Near-field enhancement through coupling of light to plasmonic nanostructures has been one of the subjects of intensive research in the last few years [1–4]. Large electromagnetic field enhancement in the vicinity of plasmonic nanostructures are used in a variety of applications such as nanolasers [5], heat-assisted magnetic recording [6], surface-enhanced Raman scattering (SERS) [7], light trapping [8], photo-thermal effect [9], surface enhanced infrared absorption [1], nonlinear optics [10], optical switching [4]. Plasmonic nanostructures such as nanoantennas have demonstrated the capability of near-field enhancement by the nanoscale confinement of light due to excitation of the highly localized surface plasmon mode [1–3,11–13]. Small gaps between the plasmonic nanoantennas act as an electromagnetically ultra-intense hot-spots [14,15]. In a conventional plasmonic nanoantenna, the maximum enhancement of the electric fields (|E|/|E₀|) in the vicinity of nanoantennas have been reported to be a few hundred. Researchers have enhanced the optical near-fields in the vicinity of nanoantennas by employing hybrid configurations in which different photonic structures are coupled to nanoantennas [16–18] such as photonic crystal resonators [17], photonic crystal membranes [18], and tapered optical fibers [16].

On the other hand, beaming light from a metallic subwavelength aperture surrounded by periodic circular grooves (BES) has been studied extensively due to its high transmission through the subwavelength hole beyond the diffraction limit [19–22]. In a BES, the periodic circular grooves (present around the subwavelength aperture) on the input side of the plasmonic film acts as an antenna for collection of the incident optical radiation and for coupling it into a surface wave, which consists of a surface plasmon polariton and a surface scattered wave – with the surface plasmon polariton being the main constituent of the surface wave. Depending on the groove period, the surface waves launched at the different grooves add up in phase leading to a surface wave propagating towards the central aperture. This surface wave subsequently reconverts to a propagating mode at the central aperture. Depending on BES geometrical parameters (such as the distance between the innermost grating and the rim of the aperture, the groove period, and groove depth), constructive interference can occur between the optical wave generated from the surface wave and the direct transmission component at the subwavelength aperture, leading to enhanced transmission through the aperture as well as near-field enhancement [23,24].

In this paper, we demonstrate that a hybrid plasmonic structure consisting of a plasmonic nanograting – such as a subwavelength aperture in a plasmonic thin film surrounded by concentric periodic grooves (BES), a two-dimensional nanohole array in a plasmonic thin film, or a nanoline array in a plasmonic thin film – coupled to a plasmonic nanoantenna on its top surface [see Figs. 1(a) and 1(b)] shows a huge near-field enhancement in the transmission mode. We observe that the electric field enhancement for a plasmonic nanograting coupled to a plasmonic nanoantenna is significantly higher than that for only a plasmonic nanograting or only a plasmonic nanoantenna.

 

Fig. 1. (a) A Three-dimensional schematic of the hybrid plasmonic nanostructure consisting of a plasmonic nanoantenna coupled to a plasmonic bull’s eye structure (BES). (b) Cross-sectional view and (c) Top view of the bull’s eye structure. The diameter of the central SiO₂ filled aperture, the thickness of the Ag metal film, and the groove period are denoted as ‘D’, ‘H’ and ‘P’, respectively. The Au plasmonic nanoantennas have a length ‘L’, height ‘H₂’ and gap ‘G’ between the nanoantennas. (d) The electric field enhancement spectrum of the hybrid nanostructure calculated at the point monitor placed at the center of the gap between the two arms of the bowtie nanoantenna. (e) The spatial distribution (in xy plane) of the simulated |E|/|E₀| in the vicinity of the bowtie nanoantenna present on top of the BES, calculated at the resonance wavelength. For (d) and (e), the following optimized geometrical parameters are chosen for the bull’s eye structure — the diameter of the central SiO₂ filled aperture (D) is 260 nm, the thickness of the Ag metal plate (H) is 170 nm, the groove period (P) is 850 nm, and there are a total of 15 circular grooves. The width (W) and the depth (H₁) of each groove are 200 nm and 60 nm, respectively. We have chosen the height (H₂), Gap (G), and length (L) of the each arm of the gold bowtie nanoantenna as 10 nm, 2.5 nm, and 64 nm, respectively. The two triangles forming the bowtie nanoantennas are equilateral triangles. All fields were calculated for normal incidence of light and the polarization was taken to be in the direction parallel to nanoantenna axis.

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In the case of BES-based nanogratings studied in this paper, the nano-aperture in the plasmonic thin film is filled with SiO₂ such that the plasmonic nanoantennas can be developed on the surface of the SiO₂ film. The presence of the circular grooves around the nano-aperture in the plasmonic thin films can result in enhanced transmission through the aperture as well as near-field enhancement. Positioning a nanoantenna on the top surface of the SiO₂ aperture, and shining light from bottom of the substrate, results in coupling of light to the nanoantenna which gets confined in sub-nanometer gap of nanoantenna at the resonance wavelength. While Fig. 1(a) shows the 3D view of the proposed high field enhancement hybrid nanostructure, Figs. 1(b) and 1(c) show the cross-sectional view and the front view of the BES, respectively. Employing the proposed hybrid nanostructure consisting of a BES coupled to a plasmonic nanoantenna, an electric field enhancement of more than 9000 is achieved for a 2.5 nm gap between the two arms of a bowtie nanoantenna, of more than 4000 for a 5 nm gap and of more than 1000 for 10 nm gap. To the best of our knowledge, ultra-high electric field enhancements of this magnitude using the hybrid nanostructures being proposed in this paper – plasmonic nanoantennas coupled to plasmonic nanogratings – have not been reported previously. Moreover, there is no previous report of calculations of EM field enhancements in the transmission mode – evaluated in the vicinity of plasmonic nanoantennas coupled to plasmonic nanogratings. A comparison of the near-field enhancements in the vicinity of plasmonic nanoantennas coupled to different types of plasmonic nanogratings – two-dimensional nanohole arrays in plasmonic thin films, nanoline arrays in plasmonic thin films, or subwavelength apertures in plasmonic thin films surrounded by concentric periodic grooves – has also not been reported in previous literature. Moreover, in this paper we provide a comparison of the electric field enhancements in the vicinity of different types of plasmonic nanoantennas coupled to plasmonic nanogratings. In this paper, the different parameters impacting the electric field enhancement such as the aperture diameter and the number of rings have been optimized to achieve the highest possible near-field enhancement in the nanoantennas coupled to the BESs.

Due to the extremely high field enhancement in the vicinity of the nanoantennas, the proposed hybrid nanostructure can be employed for several applications such as extremely sensitive chemical and biological sensing based on SERS and plasmon-enhanced fluorescence (PEF). In the case of SERS, the Raman signals from molecules lying in the vicinity of plasmonic nanostructures are significantly enhanced due to electromagnetic enhancement (occurring due to excitation of surface plasmons in the metallic nanostructures or thin films) as well as chemical enhancement [7]. The electromagnetic enhancement of SERS signals is given by the expression (|E_loc(ω_L)|²/|E₀(ω_L)|²)*(|E_loc(ω_S)|²/|E₀(ω_S)|²) which is approximately equal to (|E_loc|⁴/|E₀|⁴) as the difference in the frequency of the incident photons (ω_L) and that of the scattered photons (ω_S) is small. Here E_loc is the local electric field strength at the molecule and E₀ is the electric field strength of the incident light. In the case of PEF, the incident field leads to both the excited electronic state of the fluorophore molecules as well as excitation of surface plasmons in the metallic nanostructures or metallic thin films in the vicinity of the molecules (but spaced from the molecules just enough to avoid quenching). In PEF, the emission wavelength is independent of the incident (excitation) wavelength, and the enhancement of the fluorescence signals depends on the enhancement of the incident field in the vicinity of the plasmonic nanostructures and is proportional to (|E_loc|²/|E₀|²) [25,26], where E_loc is the local electric field strength and E₀ is the electric field strength of the incident light. Moreover, the structures being proposed by us are highly suitable for nonlinear optical phenomena such as second harmonic generation and four-wave mixing [27,28] that require high optical electric fields. Other possible applications are optical trapping, near-field scanning optical microscopy, data storage, etc.

The outline of the paper as follows: We first studied electric field enhancements around an isolated bull's eye structure, around an isolated bowtie nanoantenna, and around the hybrid nanostructure consisting of a plasmonic nanoantenna coupled to a BES, as shown in Fig. 2. Corresponding spatial distributions of the electric field enhancements were calculated, as shown in Fig. 3. Figure 4 presents a comparison of the near-field enhancements in the vicinity of plasmonic bowtie nanoantennas coupled to different types of plasmonic nanogratings – two-dimensional nanohole arrays in plasmonic thin films, nanoline arrays in plasmonic thin films, or subwavelength apertures in plasmonic thin films surrounded by concentric periodic grooves. Thereafter, we optimized the geometrical parameters of the BES and the nanoantennas, and analyzed the effects of varying the different geometrical parameters on electric field enhancements, as shown in Figs. 5 and 6. Subsequently, we studied the electric field enhancements of light passing through different configurations of the plasmonic thin films – such as a plasmonic thin film with a single aperture only, a plasmonic thin film with circular grooves at the input side only and an aperture at the center of the grooves, and a plasmonic thin film with circular grooves on both the sides and an aperture at the center of the grooves, as shown in Fig. 7. Moreover, we provide a comparison of the electric field enhancements in the vicinity of different types of plasmonic nanoantennas coupled to plasmonic nanogratings in Fig. 8. Finally, the electric field enhancement in a hybrid nanostructure consisting of a BES and a gold bowtie nanoaperture has been studied and the electric field enhancement spectra have been shown in Fig. 9.

2. Numerical modeling using the FDTD

We have studied the electromagnetic field enhancements in the vicinity of the proposed hybrid nanostructure as well as the plasmon resonances of the nanostructure by employing finite difference time domain (FDTD) numerical modeling. The nanoantennas on the top surface of BESs, as well as the nanoantennas coupled to a two-dimensional nanohole array in a plasmonic thin film or to a nanoline array in a plasmonic thin film, are simulated for determining the electric field enhancement as a function of wavelength (i.e. electric field spectra) using 3D FDTD method. The FDTD method solves coupled Maxwell’s equations based on Yee’s algorithm (which solves both electric and magnetic fields simultaneously, rather than electric or magnetic field alone) in time and spatial domain on each grid cell by converting the continuous-time differential equations to difference equations [29]. We have calculated the SERS enhancement (∼|E|⁴/|E₀|⁴), the intensity enhancement (|E|²/|E₀|²) and field enhancement (|E|/|E₀|), where |E| is the magnitude of the electric field and |E₀| is the magnitude of the incident electric field. In this study, we used Lumerical FDTD Solutions as a tool for analyzing the fields. The perfectly matched layer (PML) boundary condition was used in all three (x, y and z) directions. We carried out grid size convergence test and ensured that the grid size was fine enough such that with a further reduction of grid size, the FDTD simulation results remained almost the same. Variable grid sizes were employed in the FDTD simulations – while the grid size was taken to be 0.5 nm in the region where the nanoantenna was present, it was taken to be 10 nm in all other regions of the simulation regions. The Lorentz-Deybe model [30] is used as the dispersion model for different plasmonic metals employed in this paper. The unit magnitude plane wave nature source has been taken for calculating all the fields. The BES structure being studied in this paper consists of a central cylindrical aperture filled with SiO₂ material and surrounded by circular concentric grooves, present on both sides of the Ag metal film as shown in Figs. 1(a)–1(c). The plasmonic nanoantenna structure is present on top of the SiO₂ layer and in the center of the circular concentric grooves. The geometrical parameters of the bowtie nanoantenna and the BES are shown in Figs. 1(a) and 1(b), respectively. The fields are calculated at a distance of 3 nm above the surface of the aperture in the case of the isolated bull’s eye structure. In other cases, such as the isolated bowtie nanoantenna and the bowtie nanoantenna with BES, the fields are calculated at the center of the gap of the nanoantenna. The polarization of the incident light is taken parallel to the nanoantenna axis for all calculations described in this paper.

3. Results and discussions

In order to study the hybrid nanostructure consisting of the bull's eye structure coupled to a plasmonic bowtie nanoantenna, we first studied the EM fields around an isolated bull's eye structure as well as those around an isolated bowtie nanoantenna. Figure 2 shows the electric field enhancement in the vicinity of a plasmonic bowtie nanoantenna (BNA) as well as that of a plasmonic BNA on the top surface of the bull's eye structure (BES). The electric field enhancement of an isolated BES is shown in Fig. 2(a). The maximum field enhancement |E|/|E₀| of ∼ 30 is achieved in an isolated bull's eye structure at the resonance wavelength of 896 nm. In the case of the isolated bowtie nanoantenna [as shown in Fig. 2(b)], the maximum field enhancement is 364 for a 3 nm gap (G) between the two arms of the bowtie nanoantenna. As shown in Fig. 2(c), the electric field enhancement for the hybrid nanostructure (gold bowtie nanoantenna integrated with BES) is 6560 at a resonance wavelength of 900 nm, for a 3 nm gap between the two arms of the bowtie nanoantenna. Due to the extraordinary transmission of light in the bull's eye structure and the high coupling between the bull's eye structure and bowtie nanoantenna, the maximum field enhancement is increased by more than 18 times compared to that of an isolated bowtie nanoantenna. The calculated intensity enhancement (|E|²/|E₀|²) and SERS EM enhancement factor (|E|⁴/|E₀|⁴) are shown in Fig. 2(d). It can be observed that a SERS EM enhancement factor greater than 1.5 × 10¹⁵ can be obtained at the center of the gap of the bowtie nanoantenna coupled to a BES.

Figure 3 shows the spatial distribution of the electric field enhancement calculated at the resonance wavelength of an isolated BES [Figs. 3(a) and 3(b)] and of the hybrid nanostructure consisting of a BES coupled to a plasmonic nanoantenna [Figs. 3(c) and 3(d)]. It can be observed that the electric field is highly confined at the center of the aperture [see Figs. 3(a) and 3(b)] due to focusing of the surface plasmon polaritons in a bull's eye structure by the circular grooves. Figures 3(c) and 3(d) show the top view and the cross-sectional view, respectively, of the spatial distribution of the electric field enhancement of a bowtie nanoantenna present in the hybrid nanostructure (consisting of a BES coupled to a plasmonic nanoantenna). It clearly shows the highly localized field at the center of the gap of the bowtie nanoantenna resulting from the coupling of light energy from the BES to the bowtie nanoantenna.

 

Fig. 2. Electric field enhancement spectra for: (a) an isolated BES calculated at a distance of 3 nm above the surface (point A), (b) an isolated bowtie nanoantenna calculated at the center of the gap of the bowtie nanoantenna (point B) and, (c) a hybrid nanostructure consisting of a bowtie nanoantenna coupled to a BES calculated at the center of the gap of the bowtie nanoantenna (point C). The electric field enhancement spectra for only the bowtie nanoantenna and for only bull’s eye structure is also plotted for comparison. (d) The electric field intensity enhancement factor (red) and the SERS EM enhancement factor (blue) of the hybrid nanostructure consisting of a bowtie nanoantenna coupled to a BES as a function of wavelength. The following optimized geometrical parameters are chosen for the bull’s eye structure — the diameter of the central SiO₂ filled aperture (D) is 260 nm, the thickness of the Ag metal film (H) is 170 nm, the groove period (P) is 850 nm, and there are a total of 15 circular grooves around the nanoantenna structure. The width (W) and the depth (H₁) of each groove are 200 nm and 60 nm, respectively. The length (L), gap (G), and height (H₂) of the bowtie nanoantenna are 68 nm, 3 nm, and 10 nm, respectively. The two triangles forming the bowtie nanoantennas are equilateral triangles. All fields were calculated for normal incidence of light and the polarization was taken to be in the direction parallel to nanoantenna axis.

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Fig. 3. The spatial distribution of the electric field enhancement (|E|/|E₀|) in the xy and xz planes, in the vicinity of: (a), (b) an isolated bull’s eye structure at the resonance wavelength for an isolated BES and (c), (d) a bowtie nanoantenna of the hybrid nanostructure at the resonance wavelength for the hybrid nanostructure consisting of a BES and a bowtie nanoantenna. All fields were calculated for normal incidence of light and the polarization was taken to be in the direction parallel to nanoantenna axis. The following optimized geometrical parameters are chosen for the bull’s eye structure – the diameter of the central SiO₂ filled aperture (D) is 260 nm; the thickness of the Ag metal film (H) is 170 nm; the groove period (P) is 850 nm and there are a total of 15 circular grooves around the nanoantenna structure. The width (W) and the depth (H₁) of each groove are 200 nm and 60 nm, respectively. The two triangles forming the bowtie nanoantennas are equilateral triangles. For (c) and (d), we have chosen the height (H₂), Gap (G), and length (L) of each arm of the gold bowtie nanoantenna as 10 nm, 3 nm, and 64 nm, respectively.

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We also calculated the values of electric field enhancement in the vicinity of a bowtie nanoantenna coupled with two other common nanograting structures exhibiting EOT – a two-dimensional nanohole array in a plasmonic thin film and a nanoline array in a plasmonic thin film. Figures 4(a)–4(c) show schematics of all three nanograting structures exhibiting EOT – a two-dimensional nanohole array in a plasmonic thin film [Fig. 4(a)], a nanoline array in a plasmonic thin film [Fig. 4(b)], and a subwavelength aperture in a plasmonic thin film surrounded by concentric periodic grooves [Fig. 4(c)]. Figure 4(d) shows a comparison of the electric field enhancement in a bowtie nanoantenna on top of a bull’s eye structure (red), on top of a nanoline array in a plasmonic thin film (green), and on top of a nanohole array in a plasmonic thin film (blue). The plasmonic nanoantenna coupled to a bull’s eye structure shows the highest field confinement and near-field enhancement when compared to a plasmonic nanoantenna on top of a nanoline array in a plasmonic thin film or on top of a nanohole array in a plasmonic thin film because of the focusing of the surface plasmon polaritons in a bull's eye structure towards the center of the circular grooves which leads to a higher near-field enhancement in the center of a plasmonic nanoantenna placed just above the bull's eye structure. The physical phenomena behind both the EOT structures (i.e., BES and nanoline array) are almost same. The periodic nanoline grooves (in the nanoline array) and the periodic circular grooves (in a BES) on the input side of the plasmonic film collect and couple the incident optical radiation into surface waves, which consists of a surface plasmon polariton and a surface scattered wave. Depending on the groove period, the surface waves launched at the different grooves add up in phase leading to a surface wave propagating towards the central aperture. This surface wave subsequently reconverts to a propagating mode at the central aperture. Constructive interference can occur between the optical wave generated from the surface wave and the direct transmission component at the subwavelength aperture, leading to enhanced transmission through the aperture as well as near-field enhancement. But, the near field enhancement in BES is approximately two times higher compared to the near field enhancement of nanoline array structure, as shown in Fig. 4(d). In the case of a BES, propagating surface waves are directed toward the central aperture due to its circular ring structure, thereby leading to focusing of energy at the central aperture. But in case of a nanoline array, the surface waves propagate toward the aperture from two sides, thereby leading to less focusing of the energy at the aperture. As a result, the field enhancement of nanoantenna coupled with nanoline array is less than the nanoantenna coupled with BES.

 

Fig. 4. Schematics showing bowtie nanoantenna structures developed on top of different nanograting structures exhibiting EOT: (a) a two-dimensional nanohole array in a plasmonic thin film, (b) a nanoline array in a plasmonic thin film and (c) bull’s eye structure. (d) The E-field enhancement spectra for all three cases calculated at point A (in the center of the plasmonic nanoantennas). All fields were calculated for normal incidence of light and the polarization was taken to be in the direction parallel to nanoantenna axis. The nanohole array substrate consists of a central SiO₂ filled hole, with a diameter D₁= 300 nm and periodicity P₁= 840 nm, surrounded by air holes of diameter D₂= 200 nm. The following optimized geometrical parameters are chosen for nanoline substrate – the diameter of the SiO₂ filled hole (D₃) is 280 nm, the nanoline period is (P₂) is 850 nm, the width of the air slit (w₂) is 150 nm, and the length of the nanoline (L) is 4400 nm. The thicknesses of the plasmonic thin film were taken to be the same for all three nanograting structures exhibiting EOT. The length (L), gap (G), and height (H₂) of the bowtie nanoantenna are 68 nm, 3 nm, and 10 nm, respectively. For the case of the bull’s eye structure, the number of circular grooves was taken to be 15. The two triangles forming the bowtie nanoantennas are equilateral triangles. The following optimized geometrical parameters are chosen for the bull’s eye structure – the diameter of the central SiO₂ filled aperture (D) is 260 nm, the thickness of the Ag metal film (H) is 170 nm, the groove period (P) is 850 nm, and there are a total of 15 circular grooves around the nanoantenna structure. The width (W) and the depth (H₁) of each groove are 200 nm and 60 nm, respectively.

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In our study, we first analyzed the effects of the different geometrical parameters of an isolated bull's eye structure (BES) on the electric field enhancement in the center of the BES. Then the geometrical parameters of a plasmonic bowtie nanoantenna integrated with the BES were varied, and their effect on the electric field enhancement observed. The electric field enhancements of other plasmonic nanoantennas such as a ring bowtie nanoantenna, rod antenna nanoantenna, and disc nanoantenna were also compared with that of bowtie nanoantenna. Firstly, the effect of the variation of the aperture’s diameter on electric field enhancement was studied. Figure 5(a) shows the electric field enhancement (|E|/|E₀|) spectra for a BES structure, calculated at a distance of 3 nm above the surface of the aperture, as a function of the diameter of the aperture. One can observe from Fig. 5(a) that the electric field enhancement (at the resonance wavelength) is highest for the aperture diameter of 260 nm. For aperture diameters less than 260 nm, the electric field enhancement decreases with decreasing aperture diameters due to less transmission through the aperture, as the cutoff function of the aperture becomes significant [31]. When the aperture diameter is increased beyond 260 nm, the transmission intensity reduces due to the destructive interference between the optical wave generated from the surface wave and the direct transmission component at the subwavelength aperture [23]. Figure 5(b) shows the variation of the electric field enhancement – calculated at a distance of 3 nm above the surface of the aperture at the respective plasmon resonance wavelength – with a variation of number of grooves around the central aperture. The inset shows the corresponding electric field enhancement spectra for the different number of grooves. It can be observed from Fig. 5(b) that the strength of the surface plasmon polaritons depends on the number of circular grooves. The electric field enhancement gradually increases with an increase in the number of grooves, reaching a saturation value of ∼ 32 when the number of grooves is 18. The surface plasmon propagation length decides the saturation limit of the number of grooves. The effective propagation length is reduced in corrugated structure due to scattering losses [32] and it increases with the period of the structure [31]. Therefore, the number of grooves needed for achieving the maximum electric field enhancement (the saturation level) is different for different periods.

 

Fig. 5. Effect of the different geometrical parameters of an isolated bull’s eye structure (containing no plasmonic nanoantenna): (a) Effect of varying the aperture diameter ‘D’ on the electric field enhancement (|E|/|E₀|) spectra calculated at a distance of 3 nm above the surface of the bull’s eye structure when the number of circular grooves, the groove periodicity, and the thickness of the plasmonic (Ag) thin film were taken to be 15, 850 nm, and 170 nm, respectively. (b) Effect of varying the number of rings ‘N’ on the electric field enhancement (|E|/|E₀|) calculated at a distance of 3 nm above the surface of the bull’s eye structure when the aperture diameter, the groove periodicity, and the thickness of the plasmonic (Ag) thin film were taken to be 260, 850 nm, and 170 nm, respectively. (c) Electric-field enhancement (red) and the corresponding resonance wavelength (green) for different periods of the circular grooves of the bull’s eye structure, when the number of circular grooves, the aperture diameter, and the thickness of the plasmonic (Ag) thin film were taken to be 15, 260 nm, and 170 nm, respectively. (d) Color map showing the variation of |E|/|E₀| for different thicknesses of the plasmonic thin film in the bull’s eye structure and for different wavelengths of the incident light, when the aperture diameter, the number of circular grooves, and the thickness of the plasmonic (Ag) thin film were taken to be 260, 15, and 170 nm, respectively. All fields were calculated for normal incidence of light and the polarization was taken to be in the direction parallel to nanoantenna axis.

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As can be seen in Fig. 5(c), the maximum |E|/|E₀| occurs at a groove period of 850 nm. At this periodicity of the gratings, the incident light is effectively coupled to a surface wave which consists of a surface plasmon polariton and a surface scattered wave. Depending on the groove period, the surface waves launched at the different grooves add up in phase leading to a surface wave propagating towards the central aperture. This surface wave subsequently reconverts to a propagating mode at the central aperture. Depending on BES geometrical parameters (such as the distance between the innermost grating and the rim of the aperture, the groove period, and groove depth), constructive interference can occur between the optical wave generated from the surface wave and the direct transmission component at the subwavelength aperture, leading to an enhanced near-field enhancement at the resonance wavelength. The magnitude of maximum electric field enhancement and the corresponding resonance wavelength are ∼ 31 and 896 nm respectively. A strong peak of electric field enhancement occurs around the wavelength slightly larger than the period of the groove, which is consistent with the previous work [31,33].

We then analyzed the effect of varying the thickness (H) of the plasmonic thin film from 140 nm to 230 nm in the steps of 10 nm. It can be observed from Fig. 5(d) that the electric field enhancement reaches a maximum value when the thickness of the plasmonic thin film lies between 170 nm and 200 nm. The electric field enhancement decreases for larger thicknesses of the Ag thin film as the transmission intensity is an exponential function of the substrate thickness [21]. We have taken the thickness of the Ag metal film to be 170 nm so that the film thickness is not less than the penetration depth of silver but is thin enough to exhibit extraordinary transmission. The electric field enhancement reduces when the thickness of the Ag thin film smaller than 170 nm. As there are periodic grooves of 60 nm height on both sides of the Ag film, the thickness of the Ag film is 50 nm in regions where the grooves are present. If we further reduce the thickness of the metal film (the height of the gratings kept at 60 nm), then the thickness of the Ag film (in regions where the grooves are present) becomes thinner than the penetration depth of silver [34].

We can see in Fig. 3(a) that the field is mainly confined at the central region of the aperture surface of the BES at the resonance wavelength. Therefore, we placed a gold bowtie nanoantenna on the top and the center of the SiO₂ filled aperture of the BES. To achieve the highest possible field enhancement (in the vicinity of the plasmonic nanoantenna coupled to a BES) and to ensure that the resonance wavelength of the proposed nanostructure is same as BES, we optimized the parameters of a nanoantenna coupled to a BES such as the thickness (H₂) and gap between the nanoantenna arms (G) for a given length of the nanoantenna. Figure 6(a) shows the variation of the electric field enhancement as the thickness of the isolated gold bowtie nanoantennas was varied. One can observe that the electric field enhancement increases with a decrease in the thickness of the nanoantennas because thinner thickness ensures stronger near-field coupling at the bowtie tip [35,36]. In this case, the maximum field enhancement occurs at a thickness of 10 nm, as shown in Fig. 6(a). The effect of varying the gap ‘G’ between the two arms of the bowtie nanoantenna on the electric field enhancement spectra was also evaluated. On varying the gap ‘G’ between the two arms of the nanoantenna and keeping all the other parameters constant, we observe [see Fig. 6(b)] that there is a substantial increase in the value of electric field enhancement of the proposed hybrid nanostructure as the gap is reduced from 10 nm to 2.5 nm. This is because on reducing the gap size, there is a stronger confinement of the plasmonic mode [36,37].

 

Fig. 6. E-field enhancement profile for different (a) heights (H) and, (b) gaps (G) of the bowtie nanoantenna on top of the bull’s eye structure of the hybrid nanostructure. The field is calculated at the center of the bowtie nanoantenna. The length of each arm of the gold bowtie nanoantenna was taken to be 64 nm in all the simulations. All fields were calculated for normal incidence of light and the polarization was taken to be in the direction parallel to nanoantenna axis. In (a), the gap (G) between the two arms of the bowtie nanoantenna was kept at 3 nm as the height (H₂) of the nanoantenna arms was varied. On the other hand, in (b), the height (H₂) of the nanoantenna arms was kept at 10 nm as the gap (G) between the two arms of the bowtie nanoantenna was varied. The two triangles forming the bowtie nanoantennas are equilateral triangles. The following optimized geometrical parameters are chosen for the bull’s eye structure – the diameter of the central SiO₂ filled aperture (D) is 260 nm, the thickness of the Ag metal film (H) is 170 nm, the groove period (P) is 850 nm, and there are a total of 15 circular grooves around the nanoantenna structure. The width (W) and the depth (H₁) of each groove are 200 nm and 60 nm, respectively.

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We studied the electric field enhancement of light passing through a different configurations of the plasmonic thin films: (a) a plasmonic thin film with a single aperture only, (b) a plasmonic thin film with circular grooves at the input side only and an aperture in the center of the grooves, and (c) a plasmonic thin film with circular grooves on both the sides and an aperture in the center of the grooves. Figure 7(a) shows the schematic of all the three configurations. We have taken the geometrical parameters of these structures to be based on the optimizations shown in Fig. 5: the diameter of the central SiO₂ filled aperture (D) being 260 nm, the thickness of the Ag metal film (H) being 170 nm, and when the circular grooves are present, the groove period (P) being 850 nm, and the number of grooves being 15. The dimensions of the central aperture and the grooves, as well as the number of grooves, were taken to be the same for the two configurations having the grooves around the aperture, with the grooves being present only at input side or being present on both sides. Figure 7(b) shows a comparison of the electric field enhancement spectra (|E|/|E₀|) for all the three configurations, the electric field enhancements being calculated at a distance of 3 nm above the center of the top surface of the SiO₂ filled aperture. One can observe from Fig. 7(b) that there is a significant electric field enhancement at the resonance wavelength in the two configurations having circular grooves around the aperture (with the grooves being present only at input side or being present on both the sides) as compared to the configuration consisting of a single aperture having no circular grooves. The electric field enhancement occurs in both the configurations having the grooves around the aperture due to the excitation of SPPs at the resonance wavelength, λ_spp=2π/Re (k_spp); where k_spp is the wavenumber of the surface plasmon polariton and is defined by k_spp= (2π/λ₀) [ε_mε_d / (ε_m+ε_d)]^1/2; λ₀ is the incident wavelength, and ε_m and ε_d are the effective permittivity of plasmonic thin film and dielectric media next to the plasmonic thin film, respectively [21,33].

 

Fig. 7. Effect of circular nanogratings on electric field enhancement spectra just above a subwavelength aperture in a plasmonic thin film for three different configurations: (I) no circular grooves present, (II) circular grooves are only present at the input side, and (III) circular grooves are present on both sides of the metallic subwavelength aperture. (a) Schematic of all three configurations. (b) A comparison of the electric field enhancement (|E|/|E₀|) spectra for these three cases, and the corresponding spatial distribution of the electric field enhancement calculated at resonance wavelength. All fields were calculated for normal incidence of light and the polarization was taken to be in the direction parallel to nanoantenna axis. The following optimized geometrical parameters are chosen for the bull’s eye structure – the diameter of the central SiO₂ filled aperture (D) is 260 nm, the thickness of the Ag metal film (H) is 170 nm, the groove period (P) is 850 nm, and there are a total of 15 circular grooves around the nanoantenna structure. The width (W) and the depth (H₁) of each groove are 200 nm and 60 nm, respectively.

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On the other hand, the transmission of light through single apertures having no grooves around the apertures does not involve coupling of light into SPPs in the grooves but is rather based on localized SPs and there is very weak coupling [38]. This leads to a very low value of electric field enhancement for the configuration having only a single aperture. Moreover, the enhancement is greater in the case of the configuration having grooves on both sides. It has to be noted that the maximum electric field enhancement for the configuration having grooves on both the sides is not very much larger than the electric field enhancement for the configuration when the grooves are only on the input side, showing that the contribution of the exit side grooves on the value of the electric field enhancement is minimal [21]. The corresponding spatial distributions of all three configurations at their respective resonance wavelengths are also shown on the right of Fig. 7(b). It can be clearly seen from the spatial distributions that the grooves on the output side help to reduce the reflection from the bull's eye structure. Thus, the grooves on the output side have some contribution (though a weak contribution) towards the enhancement of the electric field [21].

We also calculated the field enhancement in three other common nanoantenna structures, a ring bowtie nanoantenna, a rod nanoantenna and a disc nanoantenna. Figure 8(a) shows the three dimensional schematic of a ring bowtie plasmonic nanoantenna on top surface of the BES. Schematics of the different plasmonic nanoantennas that were studied in this paper is shown in Figs. 8(b)–8(e). The electric field enhancement spectra of the isolated nanoantennas are shown in Fig. 8(f), which shows that the maximum electric field enhancement in the middle of the nanoantennas is obtained in the case of bowtie nanoantennas. It can be observed that a rod nanoantenna shows a greater enhancement than a disc nanoantenna, which can be attributed to the larger concentration of EM fields in nanorods as compared with nanodiscs [4]. Figure 8(g) shows the comparison of the electric field enhancement spectra calculated at the center of the gap for a disc nanoantenna (green), a bowtie nanoantenna (red), a rod nanoantenna (black) and a ring bowtie nanoantenna (pink) when these nanoantennas are present on top of the BES. It can be calculated from Figs. 8(f) and 8(g) that the optical field enhancement factors of the nanoantennas coupled to the BESs are 18 times, 19 times, 22 times, and 20 times the values of the electric field enhancements for the isolated nanoantennas, for the bowtie nanoantennas, the rod nanoantennas, the disc nanoantennas, and the ring bowtie nanoantennas, respectively. It can be observed that a bowtie nanoantenna coupled to a BES exhibits the largest electric field enhancement (|E|/|E₀|) of ∼ 6800 at a wavelength of 902 nm which is substantially greater than the electric field enhancement values for the other nanoantennas coupled to the BES structures.

 

Fig. 8. (a) Schematic of a plasmonic nanoantenna on top of the bull’s eye structure. Illustrations of different geometries of plasmonic nanoantennas: (b) Disc nanoantenna, (c) Rod nanoantenna, (d) Bowtie nanoantenna and (e) Ring bowtie nanoantenna. Electric field enhancement spectra for: (f) the isolated plasmonic nanoantennas and (g) the plasmonic nanoantennas on top of the bull’s eye structure. All fields were calculated for normal incidence of light and the polarization was taken to be in the direction parallel to nanoantenna axis. The gap size and the height for all the nanoantenna structures were kept at 3 nm and 10 nm, respectively. The length of each arm of the bowtie nanoantenna as well as of the ring bowtie nanoantenna was taken to be 68 nm. The two triangles forming the bowtie nanoantennas as well as the ring bowtie nanoantennas are equilateral triangles. In the case of a ring bowtie nanoantenna, the width of the ring was taken as 5 nm. The diameter of the disc antenna was taken to be 70 nm. The length of the rod nanoantenna was taken to be 64 nm. The following optimized geometrical parameters are chosen for the bull’s eye structure – the diameter of the central SiO₂ filled aperture (D) is 260 nm, the thickness of the Ag metal film (H) is 170 nm, the groove period (P) is 850 nm, and there are a total of 15 circular grooves around the nanoantenna structure. The width (W) and the depth (H₁) of each groove are 200 nm and 60 nm, respectively.

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We have also calculated the electric field enhancement in a hybrid structure consisting of a bowtie nanoaperture coupled to a BES in order to compare the results with those of a hybrid structure consisting of a plasmonic nanoantenna coupled to a BES. Figure 9(a) shows the schematic of the hybrid nanostructure consisting of a bowtie nanoaperture and a BES. In this case, we have taken the normal incidence and transverse polarization (i.e. polarization in the direction perpendicular to the bowtie nanoaperture axis) of the incident light. The geometrical parameters of the BES structure are same as in the previous structures. We have optimized the geometrical parameters of the nanoaperture to match the resonance wavelength of both the nanoresonators. The following parameters of the nanoaperture have been taken – the height (H₃), Gap (G₃), and length (L₃) of the bowtie nanoaperture as 20 nm, 10 nm, and 83 nm, respectively. As shown in Fig. 9(b), the maximum field enhancement is ∼ 84 for a 10 nm gap (G) of an isolated bowtie nanoaperture. The electric field enhancement for the hybrid nanostructure is ∼ 1264 at a resonance wavelength of 900 nm, for a 10 nm gap of the bowtie nanoaperture. Due to the extraordinary transmission of light in the bull's eye structure and the high coupling to the bowtie nanoaperture, the maximum field enhancement is increased by more than 15 times compared to that of an isolated bowtie nanoaperture. Thus, the coupling efficiency from BES to the bowtie nanoaperture is almost as high as that in the case of bowtie nanoantenna structures.

 

Fig. 9. (a) A three-dimensional schematic of the hybrid plasmonic nanostructure consisting of a plasmonic bowtie nanoaperture coupled to a plasmonic bull’s eye structure (BES). (b) The electric field enhancement spectra for the isolated bowtie nanoaperture and for the hybrid plasmonic nanostructure. The following optimized geometrical parameters are chosen for the bull’s eye structure – the diameter of the central SiO₂ filled aperture (D) is 260 nm, the thickness of the Ag metal plate (H) is 170 nm, the groove period (P) is 850 nm, and there are a total of 15 circular grooves. The width (W) and the depth (H₁) of each groove are 200 nm and 60 nm, respectively. We have chosen the height (H₃), Gap (G₃), and length (L₃) of the gold bowtie nanoaperture as 20 nm, 10 nm, and 83 nm, respectively. All fields were calculated for normal incidence of light and the polarization was taken to be in the direction perpendicular to the bowtie nanoaperture axis.

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In order to fabricate the bull’s eye structures (BESs), nanoline arrays, and nanohole arrays described in this paper, FIB milling (using a gallium ion beam [39,40]) can be employed. Moreover, electron beam lithography (e.g. using a negative photoresist [41]) or helium ion lithography [42] followed by metal deposition and lift-off can also be employed for fabrication the bull’s eye structures (BESs), nanoline arrays, and nanohole arrays described in this paper. In order to fabricate the plasmonic nanoantennas in the middle of the plasmonic nanogratings structures such as the BESs, helium ion beam milling [39,43] or transmission electron beam ablation lithography [44] can be employed. In particular, multibeam ion microscope (such as Zeiss ORION NanoFab [42]) equipped with a gallium ion beam column and a helium ion beam column can be employed for the fabrication of the hybrid plasmonic structures – consisting of plasmonic nanoantennas coupled to bull's eye structures – being described in this paper. A silver film layer (H = 170 nm) can be deposited on a freely suspended silicon nitride membrane (200 nm thick) on silicon. The gallium focused ion beam milling can be employed to form the larger ring shaped grooves (∼ 200 nm groove size) for form the BESs on top surface of the silver film. The central hole can also be milled using the gallium focused ion beam milling. This can be followed by helium ion lithography (by employing a positive resist such as ZEP520A) and electron beam deposition to deposit silica (SiO₂) inside the central hole followed by deposition of a thin layer (10 nm) of gold on top of the SiO₂. This can be followed by lift-off process to leave a circular region of gold (10 nm thick) on top of the silica (SiO₂) film filling up the central hole inside the silver layer. In order to fabricate the plasmonic nanoantennas in the middle of the BESs (on top of the silica layer), helium ion beam milling can be employed to carve out the nanoantenna structures from the gold layer. After this, the silicon nitride membrane can be turned upside (using a spacer layer such that the nanoantennas and the rings don't touch any surface) and gallium focused ion beam milling employed to first remove the silicon nitride film layer to form a freely suspended silver film [19] and then to form the ring shaped grooves on the back side of the silver film.

4. Conclusions

In this paper, we have designed a hybrid plasmonic nanostructure – consisting of a gold nanoantenna present on top of a plasmonic nanograting exhibiting EOT (such as a BES) and evaluated in the transmission mode – for achieving ultra-large near-field enhancement in the near-infrared optical regime. We have demonstrated that extremely large electric field enhancements in the vicinity of plasmonic nanoantennas can be achieved at resonant wavelengths when the plasmonic nanoantennas are effectively coupled to light transmitted through plasmonic nanogratings exhibiting EOT. We employed FDTD simulations to demonstrate extremely large electric field enhancements for four different nanoantenna geometries – bowtie, ring bowtie, rod, and disc and provided a comparison of the electric field enhancement factors of these different nanoantenna geometries. The highest electric field enhancement (|E|/|E₀|) of ∼ 9000 was achieved for a bowtie nanoantenna having a gap of 2.5 nm between the two arms of a bowtie nanoantenna, the nanoantenna being present on top of the BES, which is 21 times more than that of an isolated bowtie nanoantenna. Employing the proposed hybrid nanostructure consisting of a BES coupled to a plasmonic nanoantenna, an electric field enhancement (|E|/|E₀|) of more than 4000 is achieved for a 5 nm gap between the two arms of a bowtie nanoantenna, and of more than 1000 for a 10 nm gap. These values of electric field enhancement are substantially higher than what have been reported previously. When the hybrid nanostructure proposed in this paper (bowtie nanoantenna coupled to a BES) is applied to surface-enhanced Raman scattering, a SERS EM enhancement factor greater than 1 × 10¹² is obtained for a 10 nm gap between the two arms of the bowtie nanoantenna and greater than 1.5 × 10¹⁵ is obtained for a 3 nm gap between the two arms of the bowtie nanoantenna coupled to a BES. These values of electric SERS EM enhancement are substantially higher than what have been reported in previous literature. In the case of other nanoantenna geometries, the electric field enhancements in the hybrid nanostructures (consisting of both the nanoantenna and the BES) were found to be ∼ 20 times more than those in isolated bowtie nanoantennas. In this paper, we have demonstrated the effect of changing the geometrical parameters of BES and of the nanoantenna on the electric field enhancement. Moreover, the tuning of the resonance wavelength of the BES, as well as that of the combined nanostructure consisting of the BES and the nanoantenna, was demonstrated over a large wavelength range from the visible to the near-infrared regime (between 500 nm to 1200 nm) by varying the periodicity of the nanorings of the BES. A comparison of the near-field enhancements in the vicinity of plasmonic nanoantennas coupled to different types of plasmonic nanogratings – two-dimensional nanohole arrays in plasmonic thin films, nanoline arrays in plasmonic thin films, or subwavelength apertures in plasmonic thin films surrounded by concentric periodic grooves – was also presented in this paper. It was observed that a plasmonic nanoantenna coupled to a bull’s eye structure shows the highest field confinement and near-field enhancement when compared to a plasmonic nanoantenna on top of a nanoline array in a plasmonic thin film or on top of a nanohole array in a plasmonic thin film.