1. Introduction

As known, the surface plasmons (SPs) and Fano resonance have been extensively studied over the last decade [1,2]. SPs demonstrate a resonant oscillation of conduction electrons at the interface between the negative and positive permittivity material stimulated by incident light. Because of relatively intense local light-field enhancement, metallic nano-particles impassion great interest in various applications such as surface-enhanced Raman scattering [3–7], plasmon-enhanced fluorescence [8,9], plasmon enhancing effectiveness of hot-electron photodetection [10,11], near-field lightwave focusing [12,13], and nano-photolithography [14,15]. Recently, 2D nano-materials have been widely employed in optoelectronics [16–19], optical modulator [20–22], biosensors [23], due to their attractive performances including tunable direct band gaps, relatively wide interlayer distances, high mobility and remarkable mechanical strength. It should be noted that the local light-field can be remarkable elevated by the coupling between the stimulated surface plasmon modes, and improve the capability of the plasmonic device. So far, the plasmonic coupling effect has commonly been investigated according to different purpose mentioned above. Prashant K et al. find a near-field coupling between noble metal nano-particles [24], Xiaolei Wen et al. design a quasi-3D micro-system composed of Ag nano-cubes and Ag nano-hole arrays [25]. Hong Wei et al. fabricate a coupling architecture of gold nano-particles and nano-holes according to a particular chemical modification by putting gold nano-particles into the nano-holes pre-formed in a gold film [26]. As demonstrated, the featured micro-nano-structures can be utilized to greatly enhance the localized light-fields originated from incident lightwaves with suitable frequency. However, most of the coupled micro-nano-structures are mainly fabricated based on low precision manufacturing process, such as chemical modification, and thus being very difficult to be repeated with a high precision.

Generally, Fano resonance can be found in atomic physics, nano-photonics, electronic and magnetic metamaterials. As demonstrated, the typical Fano resonance line-shaped pattern can be formed through a relatively strong interference between two scattering modes, one due to scattering within continuous states and the second to an excitation of a discrete state. Currently, Fano resonance is already applied to various fields such as light switching, nano-focusing, and absorbers. Maowen Song et al. propose a periodically repeated ring-disk complementary structure for breaking the near-field diffraction limit via plasmonic Fano resonance [27]. Z.L. Sámson et al. demonstrate an innovative concept for achieving nano-scale electro-optic switching over a planar metamaterial based on controlled surface plasmonic effect [28].

In this paper, a metasurface constructed by an arrayed nano-ridge-aperture coupled with a central nano-cylinder (NRANC), is explored. The tapered structure can be utilized to concentrate the light energy, localize the incident light-field and remarkably enhance the amplitude of the SPs along the propagation direction [11]. The localized surface plasmons (LSPs) excited on the NRANC also induce a featured electrical dipolar distribution. LSPs are non-propagating excitation of the conduction electrons of metallic nano-structures. The dipolar radiation would enhance the transmittance so as to modulate the optical characters of the NRANC. By adding a nano-cylinder on the center of the nano-ridge-aperture, the coupling of the LSPs mode on the nano-tips and nano-cylinder would enhance a strength of the electrical dipolar. Varying the radius of the nano-cylinder, the transmittance peak of the NRANC could be regulated from 0.8 µm to 1.6 µm and the transmittance rate adjusted from 0.13 to 0.04. Surface plasmon polaritons (SPPs) are typical electromagnetic excitation propagating at the interface between a dielectric and a conductor, and evanescently confined in the perpendicular direction. The SPPs aroused from the periodic structure would interference with the LSPs excited on the NRANC. And the Fano resonance can be clearly observed over the NRANCs. By varying the incident angle of illumination lasers, the Fano resonance can be adjusted so as to generate a transmittance peak shift. When the incidence angle changed from 0° to 50°, the ∼633 nm mode can be adjusted from 0.56 to 0.8 µm and also the ∼1.2 µm mode regulated from 1.1 to 1.58µm.

Considering the key technological processes include the common Electron Beam Lithography and Inductively Coupled Plasma Etching, the constructed metasurface can be fabricated conveniently with a sub-10 nm resolution so as to exhibit a repeated manufacturing with a very high location and repetition precision, which means a practical application prospect. The near-field lightwave distributions over an air-metal interface of the metasurface are acquired by performing scanning near-field optical microscopy measurements. A typical subwavelength near-field light spot as small as 96 nm at x-direction of the tested metasurface has been observed and thus demonstrate an effectiveness of generating a desired nano-scale SPPs distribution through a special metasurface.

2. Simulation

The layout of the NRANCs leading to a special metasurface and their featured characters are depicted in Fig. 1. Figure 1(a) shows the structural diagrammatic of the NRANCs, the NRANCs is composed of a ridge-aperture with a cylinder in the center. The ridge-aperture is composed of four isosceles triangles. The base length of each isosceles triangle is 200 nm and its height is 350 nm. The radius of the cylinder is 100 nm, thus the distance between the center of the circle and an apex of one nano-tip is $100\sqrt 2 $nm. As shown in Fig. 1(b), the NRANC will be imprinted onto the surface of an aluminum film pre-deposited over a common silicon substrate. The thickness t of aluminum film will be controlled at 100 nm.

 

Fig. 1. The layout of a basic NRANC which consists of a nano-ridge-aperture coupled with a central nano-cylinder for constructing a special metasurface by orderly arranging plenty of NRANC. (a) Several key geometric parameters including the side length of the central nano-square being 200 nm and the total length of the maximum aperture size being 900 nm. (b) Schematic of a single NRANC formed in a metal film pre-deposited over a common silicon substrate. (c) A laser beam with a needed polarization is incident normally upon the metasurface constructed. The side views of the structure: (d) a single NRANC and (e) a nano-ridge-aperture and (f) a nano-cylinder.

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First, the coupling effect of the LSPs between the nano-ridge-aperture and the nano-cylinder is analyzed. The finite-difference time-domain (FDTD) solutions are used to simulate the near-field electric-component distribution and also calculate the local light-field (electric-field) enhancement factor on the top surface of the metal film. The typical simulation results are demonstrated by Fig. 2. The numerical simulations are performed on isolated structure including the nano-ridge-aperture, the nano-cylinder and the NRANC. The boundary conditions are set as perfectly matched layer on both the x, y, z-direction. A laser beam of 633 nm is firstly conducted to present a needed polarization of 135° from x-axis and then normally incident upon the constructed metasurface along the z-direction, as indicated by a black dash-dot line in Fig. 1(c). It should be noted that the graphs are already rotated 45°.

 

Fig. 2. Near-field lightwave intensity distribution of a single NRANC with different structural parameter configuration. (a) The typical lightwave intensity distribution corresponding to a nano-ridge-aperture without shaping a coupling nano-cavity with the central nano-cylinder. (b) The similar case corresponding to a nano-cylinder. (c)∼(f) A nano-complex constructed by practically coupling a nano-ridge-aperture with a central nano-cylinder but having different gap size through arranging different nano-cylinder radius of (c)100 nm, (d)130 nm, (e)135 nm, and (f) $100\sqrt 2 $nm, respectively. (g) The relationship between the maxima of the near-field lightwave intensity over the NRANC with the radius of the nano-cylinder r.

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As shown by Figs. 2(a) and 2(b), both the nano-ridge-aperture and the nano-cylinder already present a remarkable local surface electric-field enhancement related closely with a relatively strong free electron gathering or compressing over the metallic nano-apex. Around the apex of a planar metallic nano-tip, a relatively bright convex tip-shaped linear electric-field distribution nearing apexes, which will be gradually weaken as being away from the apexes, can be observed, as indicated by both the pale blue tine-shaped short line spots in Fig. 2(a). The similar situation can also be observed to the nano-cylinder, where both relatively bright arc-shaped linear electric-fields distributed over the outer-edges of two opposite sides of a single nano-cylinder, as indicated in Fig. 2(b). But the local electric-field enhancement is weaker than that of nano-ridge-aperture because the surface state density of an arc-shaped nano-cylinder for gathering surface free electron is considerably lower than that at the relatively sharp apex of a nano-tip. According to the indicator of the color-bar set, the maximum value of the square of the electric-field amplitude (|E|²) corresponding to the nano-cylinder is ∼28 V²/m², and that of the nano-tip ∼116 V²/m² so as to exhibit a more than four times increment.

According to simulations, the coupling behaviors between two LSPs modes will be remarkably influenced by the gap size or the separation extent between cylinder and the nanotips of the nano-ridge-aperture. The gap size is closely related to the radius of the nano-cylinder r. Under the condition of the structural parameters of the nano-ridge-aperture being fixed, the larger the r, the smaller the gap size of the nano-subassembly is. Currently, the distance between the nano-cylinder and the convex apexes in the nano-ridge-aperture is mainly considered. The typical near-field lightwave intensity distributions corresponding to a nano-ridge-aperture coupled tightly with a nano-cylinder but having different radius, are shown in Figs. 2(c)–2(f). Because a basic NRANC is constructed by a nano-cylinder at the center of a nano-ridge-aperture, whether an effectively structural coupling over the nano-tip and the nano-cylinder is realized or not, there is an apparent local electric-field enhancement being already generated on both the nano-tips and nano-cylinder. Usually, the maxima of |E|² between the coupled nano-structures is much larger than that of the isolated nano-structures because of a strong surface light-field interference resonance of two modes of LSPs stimulated in the nano-cavity constructed. With the increase of the radius of the nano-cylinder r, the gap size will be decreased gradually and thus lead to a remarkable enhancement of the spectral lightwave intensity in the nano-cavity with a suitable gap configuration, which can also attribute to an intense compressing of the electric component distributed in each metallic gap of a single NRANC, as shown in Fig. 2(g).

As shown by Figs.. 2(c) and 2(d), the maxima of |E|² corresponding to a NRANC with a central nano-cylinder's radius of less than 100 nm and thus the gap size being more than 40 nm, is similar with that demonstrated by Figs.. 2(a) and 2(b). No obvious enhancement can be observed and the local light-field distributed in the gap is still weak, which means that no functioned coupling is realized. When the gap size is decreased to be less than 40 nm, the maximum value of |E|² will present a sharp enhancement as decreasing the distance, because the interference resonance condition of LSPs stimulated by the incident lightwaves of 633nm wavelength in the nano-cavity is satisfied. When the radius of the nano-cylinder reaches 130 nm and thus the gap size is only ∼11nm, as show by Fig. 2(d), the lightwaves localized in the nano-cavity is highly concentrated or compressed and then the maxima of |E|² almost 1.4×10³ V²/m².

Considering the layout designed, the stimulated LSPs are characterized by two coupled light-spots distributed at both the opposite apexes of a nano-ridge-aperture and the opposite small outer-edges of a nano-cylinder, which are all correlated closely with an induced electric-dipole located at the apexes of a nano-ridge-aperture or the small outer-edges of a nano-cylinder, as shown in Fig. 2(g). Especially, the surface charges induced over both the nano-ridge-aperture and the nano-cylinder will be tightly coupled together to shape a dual-electric-dipole architecture, so as to construct a close correlation between surface LSPs stimulated over spatially adjacent nano-facets, which also means a strongly spatial compression enhancement of the surface LSPs by a special nano-cavity along the nano-scaled depth direction. So, we can guess that there maybe exists a stronger interference between the induced dipole distributed over the opposite nano-scaled facets in each gap between the a nano-ridge-aperture and the central nano-cylinder with a relatively large diameter. As the increasing of the radius of the nano-cylinder or the shortening of the gap size, the size of the light-spots restrained in the nano-cavity demonstrates a remarkably shortening trend such as from typical 2×122 nm×12 nm to 15 nm×9 nm, and therefore a sharply increase of the light-spot intensity from 116 V²/m² to 1.4×10³ V²/m². Continuously decreasing the nano-cavity depth or even to zero, the constructed light-spots will undergo a variance of gradually becoming dim and finally disappeared. According to the Maxwell theory, an intense compressed light-spot also means a strong electric-dipole distribution over both the apex of the nano-ridge-aperture and the small outer-edge of the nano-cylinder.

When the radius of the nano-cylinder reaches $100\sqrt 2 $nm, as shown by Fig. 2(f), the nano-cylinder is already stuck by four nano-apexes so as to form four concave nano-tips at the junctions between each nano-apex and the nano-cylinder, where relatively weak surface lightwave gathering can also be observed, as indicated by the maxima of |E|² of ∼900 V²/m², and then present a relatively broad surface distribution almost uniformly covering the whole facets of each nano-tip and partial nano-cylinder connected directly with the nano-tips, which is completely different with that shown by Figs. 2(c)–2(e). The relationship between the maxima of the near-field lightwave intensity over the NRANC with the radius of the nano-cylinderr is also given by Fig. 2(g). As shown, the maxima of |E|² increases slowly when the radius of the nano-cylinder is less than a typical value of ∼100nm, and then present a dramatically rising at ∼130nm, and thus drop sharply when the radius of the nano-cylinder is more than the value above.

The resonance wavelength of LSPs of the NRANC can be adjusted through varying its geometry for shaping a needed metasurface, and the optical characteristic including transmittance and reflectance can be modulated. As designed, the radius of the nano-cylinder is chosen between 0–100$\sqrt 2 $ nm, where 0 nm means that no nano-cylinder in the structure. To characterize the NRANC, the field distribution of the NRANC are also analyzed carefully, as shown in Fig. 3. The transmission spectra of an aluminum NRANC with different radius of the nano-cylinder chosen in the range above, as a function of incident wavelength, are given by Fig. 3(a). Generally, the transmission spectra present a trend of the peak splitting from a single wide peak indicated by both curves colored with blue and orange corresponding to 0 and 30 nm radius of the nano-cylinder, to gradually remarkable dual-peak and then flatting as the increasing of the radius of the nano-cylinder in the wavelength range from 0.4 µm to 2 µm. According to Fig. 3(a), there is only one peak of 823 nm on the transmission spectra, when the radius of the nano-cylinder is 30 nm.

 

Fig. 3. (a) The transmission spectra of a single NRANC with different nano-cylinder's radius. Light-field or electric-field intensity and its direction vector distribution on the xz plane or surface of the NRANC with a radius of the nano-cylinder being 30 nm corresponding to an incident lightwave with a central wavelength of 823 nm: (b) |E|; (c) |E_x|; (d) phase(E_x); (e) |E_z|. Light-field or electric-field intensity and its direction vector distribution on the xz plane or surface of the NRANC with a radius of the nano-cylinder being 70 nm corresponding to an incident lightwave with a central wavelength of 674 nm: (f) |E|; (g) |E_x|; (h) phase(E_x); (i) |E_z|. Light-field or electric-field intensity and its direction vector distribution on the xz plane or surface of the NRANC with a radius of the nano-cylinder being 70 nm corresponding to an incident lightwave with a central wavelength of 1.214 µm: (j) |E|; (k) |E_x|; (l) phase(E_x); (m) |E_z|.

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Figures 3(b)–3(e) show the typical distribution characters of the |E| and its components of E_x and E_z over the xz cross-plane along a central line of the two opposite metal nano-apexes of the NRANC, respectively. The component of E_y is not given here because of being much weaker than both the E_x and E_z. Arrows shows the electric-field density, where its length and direction indicate the electric-field strength and the direction. As demonstrated in Fig. 3(b), there are light spots on both the top and bottom apexes of each nano-tip and the opposite outer-edges of the nano-cylinder. The maximum intensity of the converged light-spots with a rough value of 6.3 V/m, which are located at the top apexes of two opposite nano-tips, is much higher than others with a similar scale of ∼3 V/m. As shown in Figs.. 3(c) and 3(e), the E_x-distribution can be used to show the lightwave propagation behavior, and the E_z-distribution is closely related with the induced surface charges by surface lightwaves stimulated over the surface of the NRANC. Figure. 3(d) shows the featured distribution of phase (E_x) and therefore presents the wavefront transformation details over both the air-metal and the metal-silicon interfaces. The color shows the value of the phase (E_x), for example, the red means the phase being positive and the maximum labeled by the crimson being π (3.14) and the blue being negative and the minimum labeled by the dark blue being -π (−3.14). According to the layered lightwave morphology on the air side, the constructed light-fields can be viewed as a superposition of the incident lightwaves and their reflective components. So, a similar plane wave can be used to describe the light-fields formed. At the position where z is equal to 0.9 and indicated by the dark blue, the phase is about −3.14. As z decreased, the phase is gradually increased, and then at the position of z being 0.5, the phase approaches the value 0. The distance between two z positions is ∼0.4 µm, which is also the separation distance between the central light intensity distribution of adjacent interference fringes. In other words, the half-wavelength of the lightwaves processed by the NRANC is ∼0.4 µm, which means a method of measuring the wavelength of incident lightwaves. On the silicon side, the outgoing light-fields are constructed by diffractive lightwaves away from the NRANC, the distance between a position where the phase is 3.14 and then an adjacent position with a phase of zero is equal to the half-wavelength of lightwaves in silicon wafer. It should be noted that the E_x will decay fast along the depth direction of each nano-cavity of the NRANCs, when the lightwaves travel through the nano-cavity. At the position of the z being 0.04, the E_x has already decayed to be less than 1, which is much lower than the value of ∼2.5 at the position of z being 0.3. The total transmission field can be viewed as diffraction light-field with a divergence angle of ∼75°, which can be more clearly identified by the opposite oblique wavefront boundary as demonstrated in Fig. 3(d).

The relatively high electric-field strength around the metallic nano-tips and two opposite outer-edges of a metallic nano-cylinder related with the E_z distribution, as shown in Fig. 3(e), is due to the induced electrical dipolar. Considering the case that incident lightwaves travel through the nano-cavity and then reach the Al-Si interface will demonstrate a relatively strong attenuation, both the bottom dipoles distributed over the Al-Si interface is much weaker than that over the top convex tips. The magnitude of E_z on the top nano-tips and the bottom concave tip on the Al-Si interface are almost ∼5.2 V/m and ∼2.5 V/m. It is obvious that the electric-dipoles on the Al-Si interface will also radiate lightwaves so as to enhance the transmission intensity through the NRANC. At the position of x being 0, the Ez indicated by a black dim linear spot extended up and down, is very small. Two couples of charges of different polarity of the electric-dipoles, can be roughly viewed at the boundary of the black dim linear spot above. So, the electric-field in both air and silicon media appears as a superposition of the traveling lightwaves and the dipole radiation.

When the radius of the nano-cylinder is between 40 nm to $100\sqrt 2 $nm, two peaks on the transmission spectra are firstly formed and then gradually flatted as the radius of the nano-cylinder being increased. The typical cases of the |E| and its components of E_x and E_z distribution corresponding to the NRANC with a radius of the nano-cylinder being 70 nm, are presented in Figs.. 3(f)–3(m). According to the Fig. 3(a), there are two transmission peaks at 674 nm and 1.214 µm, respectively. Being different from the NRANC with a radius of the nano-cylinder being 30 nm, the distance between the nano-tips of the nano-ridge-aperture and the adjacent outer-edges of the nano-cylinder is short, as shown on Figs.. 3(g) and 3(k), the converging light-spots on the nano-tips and the outer-edges of the nano-cylinder can be effectively coupled together, so as to enhance the light-field intensity. With respect to the peak of 674 nm, the intensity of the light-spot on the convex nano-tips over the top surface is almost 8.4 V/m. To the E_x distribution on the xz cross plane displayed in Figs.. 3(g) and 3(h), the lightwaves couldn’t travel through the nanostructure to reach the Al-Si interface. And the distribution of the Ez on Fig. 3(i) shows that induced surface charges mainly distribute on the incident side of the metal film. To 1.214 µm wavelength, the infrared lightwaves can travel through the narrow nano-ridge-aperture, and the intensity of induced electric-dipole on Al-Si interface is relatively stronger. To the electric-field shown in Fig. 3(j), the electric-field intensity on the top surface of the metal nanostructure is much weaker than that on Al-Si interface, which are already mainly compressed around the composite outer-sides of the central nano-cylinder, so the 1.214 µm manifests a special kind of the bottom electric-dipolar radiation mode. The intensity of the light-spot over the edges of the nano-ridge-aperture on the Al-Si interface is almost 3.3 V/m.

On the nano-tips, there is a large number of surface state, which corresponds to the surface energy level in the forbidden band. At the interface between the silicon and aluminum, a Schottky barrier is created, the height of the barrier is typically lower than the energy bandgap of silicon. For the NRANC with a radius of the nano-cylinder being 70 nm, the excitation wavelength of 674 nm is corresponding to the aluminum nano-tip surface energy mode on the air side, the excitation wavelength of 1.214 µm is corresponding to the aluminum nano-cylinder surface energy mode on the silicon side. When the radius of the nano-cylinder is small, there is only one resonance mode in the wavelength range of 0.4–2 µm. When the coupling effect occurs, there will be some new energy bands, so the resonance mode on the edge of the nano-cylinder can be found.

When the radius of nano-cylinder in NRANC is between 0–30 nm, there is only one LSPs mode (top dipolar mode). When the radius of nano-cylinder in NRANC is between 40-$100\sqrt 2 $nm, the bottom dipolar mode is generated. As the increase of the radius of the nano-cylinder, the bottom dipolar mode is red-shifted which means the resonance wavelength shifts to longer wavelength direction, and the resonance wavelength of the bottom dipolar mode is changed from 0.8 µm to 1.6 µm, and the transmittance rate adjusted from 0.13 to 0.04.

Next, the interference between LSPs mode caused by the nano-structure and surface plasmon polaritons (SPPs) mode out of the periodic arrays is study. Set the radius of the nano-cylinder as 60 nm. The simulation about the transmittance and the near-field lightwave distribution on the upper surface of the metal film is done. The numerical simulations are performed on the periodic array based on the Periodic Boundary Conditions with a period of 1.1 µm on both x- and y-direction. The excitation condition is obtained by folding the dispersion curves of the SPPs in the first Brillouin zone, following [29]:

A kind of metasurface can be constructed by orderly arranging a large number of the NRANC, as shown by Fig. 3. According to the transmission spectra demonstrated in Fig. 4(a), there is an extreme transmittance value at 670 nm. To the metasurface composed of a periodically arranging NRANC, the SPP mode on the silicon side can be excited and then the resonance wavelength can also be easily calculated by Equ.1. For the integers of m = 4 and n = 4, the resonance or stimulating wavelength of SPP is 780 nm and the wavelength of SPP over the air-metal interface is 194 nm. As a narrow mode of SPP, it will interfere with the broad continuum mode of the LSPs provided by each isolated NRANC and thus result in a sharper Fano resonance. It can be seen that a Fano shape line is already generated near 780 nm in the transmittance spectrum. Figures. 4(b) and 4(c) demonstrate the near-field lightwave intensity distribution of an isolated aluminum NRANC and an arrayed aluminum NRANC. Both nano-tips in an isolated aluminum NRANC like two nano-antenna to resonantly absorb and compress incident lightwaves onto their apex zone, and the intensity over the nano-antenna arms is gradually weakened from the maxima of |E|² of ∼73 V²/m² to ∼17 V²/m². By the way, several wave stripe patterns can also be observed on the NRANC array. According to rough calculation, the strip pitch equals to 2λ_spp over a periodic metasurface formed. The SPPs excited on the NRANC array modulate the near-field distribution, which shows a higher intensity on the edge of the triangle. In Fig. 4(c), the strip shows an obvious excitation of SPP. The edge of the triangle is between two dark lines. So, the intensity on the edge of the triangle demonstrates a high value and a wave-shaped profile. Because a part of the incident energy is redistributed at the edge of the triangle, the light energy distributed on the vertex of the triangle is low than that on the island-shaped structure.

 

Fig. 4. (a) The transmission spectra (blue solid line) of an isolated aluminum NRANC and the transmission spectra (red dash-dot line) of an arrayed aluminum NRANC metasurface. (b) The near-field lightwave intensity distribution on the isolated aluminum NRANC at 780 nm normal incidence. (c) The near-field lightwave intensity distribution on an arrayed aluminum NRANC metasurface.

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In general, a variance of the lightwave incidence condition will induce the wavelength shift of SPPs, so as to allow a controlled coupling between the SPP mode and the localized mode at specific angles. It should be noted that the change of the lightwave incidence condition will lead to a shift of the phase and the amplitude of the LSPs mode and SPPs mode, and further the interference profile between both modes above. Figure. 5 shows the variance of the transmission spectra at different lightwave incident angle θ for an arrayed NRANC with a 60-nm radius of the central nano-cylinder and a 1.1µm period of the nano-structure constructed. A metasurface composed of an arrayed NRANC above is illuminated by a transverse magnetic (TM) lightwaves. The incident direction and the polarization state of the incident beams are shown in Fig. 5(a). Figure. 5(b) shows a typical nano-focusing of the near-field lightwaves over an apex of one nano-tip. As illustrated in Fig. 5(c), a ∼600 nm wavelength of resonance mode and a ∼1.2 µm wavelength of resonance mode generate a remarkable red shift phenomenon with increasing the incident angle of illumination beams, as indicated by two dashed lines. The ∼633 nm mode can be adjusted from 0.56 to 0.8 µm and also the ∼1.2 µm mode can be regulated from 1.1 to 1.58µm.

 

Fig. 5. (a) Schematic of a 633 nm beams incident upon a metasurface at 45°. (b) Simulation of the near-field lightwave intensity distribution over a single aluminum NRANC lead to a metasurface above. (c) Transmission spectra at different incident angle and demonstrating a remarkable red shift corresponding to a ∼600 nm wavelength of resonance mode and a ∼1.2 µm wavelength of resonance mode.

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3. Experiments

In order to verify the theoretical analysis, an arrayed NRANC on an aluminum film is fabricated. The samples are fabricated on a single-side polished 2-inch n-type (ρ∼10 Ω•cm) 500 µm thick silicon wafer with {100} crystallographic plane orientation. First, a 100nm thick aluminum layer is deposited over the silicon wafer by Electron Beam Evaporation. Next, an imprinted mask is formed by Electron Beam Lithography (JEOL, JBX 6300FS), Finally, a ridge-aperture and a central nano-cylinder based on am aluminum film is defined by Inductively Coupled Plasma etching so as to construct a basic NRANC and further a special metasurface based on periodically arranging the NRANC with a period of 1.1µm along the x- and y-direction, respectively. The Scanning Electron Microscope photographs and several typical near-field optical characteristics are shown in Fig. 6. The near-field optical properties can be observed by a Scattering Scanning Near-field Optical Microscope (NeaSNOM, Neaspec GmbH Co.). The incident direction and polarization state of laser beams are the same with that in simulations shown in Fig. 5(a). On the near-field optical measurement, the arrow NCR probe is used to focus the incident light and scatter it. Recording the scattered light and get the information of the near-field lightwaves. According to the SEM image shown on Fig. 6(a), the roughness of the aluminum is quite large. The aluminum particle obviously influence the lightwave distribution near it, as seen by Fig. 6(c) with some small and dense light-spot noises on the aluminum film. As demonstrated, the bright light-spot distribution is attributed to the light modulating of the NRANC, such as the light-spot in the lower right corner of the NRANC. According to the AFM image shown in Fig. 6(d), the exact thickness of the aluminum is ∼ 92 nm. It is different from the aluminum thickness in the simulation (100 nm). And it would arise a difference between simulating near-field light distribution and experimental results.

 

Fig. 6. SEM, AFM and near-field lightwave intensity distribution of the NRANC metasurface. (a) SEM images of a metasurface sample, (b) near-field lightwave intensity distribution on the sample, (c) near-field lightwave intensity distribution along red dashed line. The white dashed lines are the outlines of the nano-apertures. The red dotted line is the trendline of the electric intensity distribution curve. (d) AFM image of the metasurface sample.

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From the near-field lightwave intensity distribution, we can see that a nano-focusing in the visible range can be effectively realized by an aluminum NRANC metasurface. Figure. 6(b) demonstrates a 4×4 and a 2×2 near-field optical image. Comparing with the simulation result shown in Fig. 5(b), there is a difference. The field enhancement in the experimental SNOM figure appears outside the structure marked by dashed line, while it is in the gap for the simulations, i.e. inside the structure. The above appearance is mainly due to the difference of the principle for conducting simulations and SNOM tests. As demonstrated, the simulations exhibit the electrical field distribution, but the scattering typed SNOM records the field component out-of-plane field. As shown in Fig. 3, the main field component in the gap is in a plane, however, the scattering typed SNOM is mainly sensitive to the out-of-plane field component pointing along the tip axis, so as to display a little variance of the light enhanced position between the simulations and experimental data. There are light spots at the same position on each element in the ridge aperture, and there is a difference in strength via 0.75 µV to 1.11 µV. The near-field optical image of one element of the NRANC array is shown on Fig. 6(c). The reason of the intensity of the light spot near the lower left nano-tip being very high can be attributed to the coupling effect between the nano-tips and the central nano-cylinder. And the Fano resonant arising from the coupling between the LSPs and SPPs also enhance the near-field light intensity. The full width at half maximum (FWHM) of the near-field light spot of ∼96 nm at x-direction can be estimated. Considering the wavelength of incident laser beams is 633 nm, the light-spot size is one order of magnitude smaller than the wavelength, so as to greatly break through the near-field diffraction limit. The peak near-field lightwave intensity from the metasurface is measured to be as high as 1.11 µV. In conclusion, the experiment results are fit well with simulations, so as to verify theoretical prediction. The measurements about the spectral transmittance are being conducted.

4. Conclusion

In summary, this paper investigates a metasurface constructed by an arrayed nano-ridge-aperture with a central nano-cylinder (NRANC), where LSPs and Fano resonances are simulated. A nano-distance effect between the nano-apex and the nano-cylinder for tuning the coupled SPPs and LSPs modes is discussed carefully. With the distance decreased, the coupling effect between two LSPs modes is enhanced and the maxima of the near-field lightwave density is increased sharply. By arraying the nanostructure, the SPPs mode is stimulated, and it couples with LSPs mode so as to induce a Fano resonance. By adjusting the incidence angle of illuminating lasers being, a transmission shift indicating a local variance of the Fano resonance. This paper give a method to design the metasurface with typical transmittance characteristic. By using nanostructure, under the illumination, there are typical dipole mode excited on the metal surface, and the dipolar radiation could influence the transmittance characteristic of the metal. And the near-field electrical distribution shows the induced electrical dipole. Through performing the near-field measurements, a typical hot-spot with a full width at half maximum (FWHM) being ∼96 nm of the local TM-lightwaves at x-direction is already observed at 45° incidence of 633 nm lasers. The method developed highlights several potential applications such as surface enhanced Roman spectroscopy, ray absorbing material, and low-cost nanolithography.