A simple and effective polarization-insensitive triple narrow-band plasmonic perfect absorber is proposed and investigated numerically. This device can serve as an ultra-sensitive refractive index sensor from the near-infrared to the visible region. The triple narrow-band perfect absorber consists of a structured metal film constructed with an assembly of vertical-square-split-ring (VSSR) resonators. The triple narrow-band perfect absorption is due to the hybrid modes between the surface plasmon polaritons and guided modes with different order. Furthermore, the absorption peak shows a highly sensitive response to the change of refractive index in the surrounding medium. A careful design for the perfect absorption based refractive index sensor can yield a sensitivity of 1194, 816, and 473 nm/refractive index unit (RIU), respectively. Due to the high sensing performance, the triple narrow-band perfect absorber provides great potential for applications in enhanced sensing and spectroscopy.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Recent years, extraordinary optical properties in plasmonic nanostructures and metamaterial structures have been drawn increasingly attention for their potential and fascinating applications in thermal emitters , surface-enhanced spectroscopy , detectors and sensors [3,4], and solar cells . These optical properties and applications greatly benefit from intrinsic propagating or localized surface plasmon polaritons resonances arising from the coherent conduction electron oscillation in metallic nanostructures. The surface plasmon polaritons resonances can be excited when plasmonic nanostructures are exposed to light from the near-infrared to the ultraviolet, and then couple light strongly and greatly to enhance the light-matter interactions. The spectral and spatial properties of surface plasmon polaritons usually depend strongly on the size, shape and materials of the nanostructures and the surrounding dielectric medium [6–9]. The strong dependence of the surface plasmon polaritons generation on the refractive index of the adjacent dielectric and metal material makes them highly suitable candidate for refractive index sensing or detecting. Generally, the refractive index sensors are widely known as surface plasmon resonance sensors when working at the visible and near-infrared due to the surface plasmon resonance operation mechanism. Thus, the nanostructure can be served as refractive index sensor or detector of the flammable gas, toxic liquid, and biomolecule [4,7–9]. The intrinsic ohmic losses are usually inevitable in the nanostructure, which can be used to construct resonant plasmonic perfect absorbers by proper design of the unit-cell structure at optical frequency region [10–29]. The plasmonic perfect absorbers have been rapidly developed due to their promising applications such as thermal emitting [1,10], solar cells [5,11], electromagnetic (EM) energy harvesting [12,17], and sensing or detection [17–20,29–39]. The plasmonic perfect absorber can be generally categorized into broadband and narrowband types. Broadband perfect absorbers are applied widely in thermal emitting [1,10], and solar cells [5,11], as well as energy harvesting [12,17]. While narrow-band perfect absorbers generally could be applied for sensing or detection [4,13,17–20,29–39]. For sensing applications, surrounding dielectric medium can be measured on top of the periodic nanostructure surface, and the medium is usually liquid or gas [4,13,29–39].
So far, numerous plasmonic sensors based on the various perfect absorption nanostructures for refractive index sensing have been reported and obtained significant progresses [17–20,29–39]. Generally, typical plasmonic perfect absorber structures for refractive index sensing applications can be classified into two configurations [17–20,29–39]: tri-layer metal-dielectric-metal and bi-layer metal-dielectric structures. The incident light energy could be efficiently confined and absorbed with near unity in the slit or gap of the nanostructure by manipulating geometrical parameters and effective impedance. Among these tri-layer perfect absorbers [13–20,33–36], the intermediate dielectric layer provides space to accommodate the enhanced EM field originated from Fabry-Perot or surface plasmon polaritons resonances [13–20]. Although the tri-layer metal-dielectric-metal perfect absorbers can achieve the multi-band refractive index sensing application due to the multiple resonance modes, there are still some limitations for the complex geometries and low sensitivity value [13–20,32–37]. In addition, It is difficult to bond with the sensing medium for all above mentioned metal-dielectric-metal structure perfect absorbers, and therefore disadvantageous for the sensing of refractive index changes of the analyte (such as gas and liquid) [32–37]. For the bi-layer dielectric-metal perfect absorbers [23–29,38,39], the metallic nanostructure placed on the dielectric layer directly can reinforce absorption due to the EM interference effect or propagating surface plasmon resonance. Although the bi-layer dielectric-metal structure perfect absorbers are easily filled with analyte, the most of which are typically optimized only for single or double narrow-band, thus limiting their potential applications in spectroscopic, sensing and detection [9,29,38,39]. The most previous proposed sensors based on perfect absorption for refractive index sensing were worked in terahertz and infrared region, and few operation in the visible regime, which would hamper in many practical sensing applications, such as biology and chemistry. Thus, the triple-band or multi-band perfect absorber with excellent absorbability and sensitivity is still highly desirable operated from infrared to visible region.
In this work, a triple narrow-band plasmonic perfect absorber based on a vertical-square-split-ring (VSSR) resonator structure working from near-infrared to visible region was studied. The absorbance is over 98% at three different resonance frequencies under normal incidence. An underlying physical mechanism of the observed perfect absorption is investigated through the analyses of electric and magnetic field distributions. Furthermore, the sensitivity of the designed sensor based on plasmonic perfect absorber can reach high sensitivities of about 1194, 816 and 473 nm/refractive index unit (RIU), respectively, which are much larger than previous reports [4,7,8,18–20,31–38]. In addition, the most sensitive region inside the nanostructure of the perfect absorber also can be easily accessible to gas or liquid sensing target. Thus, the design strategy offers a promising simple and efficient approach to obtain the desired multispectral sensor.
2. Structure design and simulations
Based on our previous research, the plasmonic vertical-split-ring nanostructure could achieve the dual-band perfect absorption in infrared region, which is mainly attributed to the guide mode excitations with different order . Thus, the more plasmonic resonance modes may be excited by the appropriate geometric parameter of the vertical-split-ring nanostructure. The multiple narrow-band plasmonic perfect absorber based on vertical-split-ring nanostructure was designed. The design schematic of the proposed plasmonic perfect absorber is depicted in Figs. 1(a-d). The elementary building block of the unit-cell of the perfect absorber only consists of all metal nanostructure based on VSSRs placed on a glass substrate, which is periodically arranged in the x- and y-axis directions, as shown in Fig. 1(a). Distinct from the previous metal-dielectric-metal structure, the incident light is propagating along the openings plane of the VSSR in z-axis direction. In this configuration, the excitations of electric and magnetic dipoles in the VSSR can be obtained by magnetic and electric fields of incident light, respectively. Particularly, as shown in Figs. 1(b-d), the unit-cell structure designed here can be seen as a resonant cavity, which can support different response modes in optical frequency range. The VSSR can concentrate light and cause electric and magnetic field enhancement significantly in the gap or slit at resonances. Compared with the tri-layer metal-dielectric-metal structure, the gas or liquid sample is easily diffused over the surface empty area of the periodic nanostructure, which can be served as a sensor. The incident light propagating along the z-axis direction with polarization along the x-axis direction is normally incident to the periodic nanostructure surface of the proposed perfect absorber.
To study its efficiency and obtain insight into the physics mechanism of the observed perfect absorption, three-dimensional (3D) finite element method (FEM) simulations was performed for the proposed nanostructure. In the simulation, the periodic boundary conditions both in the x-axis and y-axis directions are applied for the transverse boundaries to replicate an infinite array of the VSSRs. The nanostructured metal film compound of VSSRs array adhered on a 50 nm thickness continuous gold film which is arranged on the glass substrate with a periodicity of 500 nm, as illustrated in Fig. 1(a). The geometric parameter of unit-cell structure of the proposed absorber is optimized by the FEM simulation. The optimization of the proposed absorber unit-cell structure is based on the trial and error approach, where we aim to maintain the absorbance spectrum over 90% in the optical frequency region from 150 to 550 THz. Finally, the optimized geometrical parameters of the proposed perfect absorber are given as: px = py = 500 nm, l = 400 nm, g = w = 20 nm, h = 500 nm, ts = 100 μm. Here, the gold is selected as the material of the periodic metallic nanostructure, and the relative permittivity of the gold in the optical frequency is described by Drude model detailed in [40,41]. The corresponding relative permittivity of the gold is shown in Fig. 1(e). It can be seen that the real part of the permittivity (ɛ’) of the gold is below zero in our interested frequency range (150–550 THz), indicating that the gold nanostructure can support surface plasmon polariton resonances. In addition, the imaginary part of the permittivity (ɛ’’) of the gold is greater than zero significantly in this frequency range, indicating that the intrinsic losses are inevitable in gold nanostructure and finally enhanced perfect absorption for the incident light. The thickness of the continuous gold film is much larger than the typical skin depth in the optical frequency regime, thus the transmission can be blocked. Thus, in our design, the absorbance can be defined as A(ω) = 1 - R(ω), where R(ω) represents the reﬂectance as functions of frequency ω.
3. Results and discussion
Figure 2 presents the simulated reflectance (R(ω)) and absorbance (A(ω)) of triple narrow-band perfect absorber. Sharp triple narrow-band reflection dips are observed clearly in Figs. 2(b-d), which is corresponding to three different resonance points, f1 = 239.2 THz, f2 = 312.7 THz, and f3 = 476.2 THz, respectively. At resonances, the reflectance is decreased to 0.6%, 1.7% and 0.2%, and the corresponding absorbance is up to 99.4%, 98.3% and 99.8%, respectively. Obviously, the designed perfect absorber is polarization insensitive for both TE and TM modes under normal incidence due to its four-fold rotation symmetry of the unit-cell structure. Similarly to previous most studies, the FWHM (full-width at half maximum) and Q-factor of the proposed perfect absorber were defined to further illustrate the narrow-band properties [34–39]. The FWHM is the resonance bandwidth and the Q-factor refers to the ratio between the center frequency and FWHM bandwidth of a resonance .
As shown in Figs. 2(b-d), the frequency (wavelength) FWHM of three resonances is about 14.3 THz (73.8 nm), 9.4 THz (28.2 nm), and 17.1 THz (22.8 nm), respectively. Thus, the corresponding Q-factors are about 16.7, 22.3, and 27.8, respectively. Obviously, these novel optical features not only show a perfect ultra-narrow absorption but also present triple narrow absorption bands. In addition, these obtained sharp resonances for the proposed perfect absorber can be believed to produce potential applications in refractive index sensing since the narrow bandwidth could highly improve the sensitivity [17–20]. Thus, when the refractive index values of surrounding environment of the perfect absorber is changed slightly, the ability of sensing or detecting the shift of resonance frequency can be enhanced by the narrow FWHM and the higher Q-factor. The triple narrow-band perfect absorption originates from the enhanced EM field coupling and confinement in the nanostructure due to the excitation of different plasmon resonances modes.
In order to explore the resonant modes of the triple narrow-band perfect absorption, the electric and magnetic field distributions in unit-cell nanostructure of periodic VSSR arrays have been carried out. Figure 3 present the induced electric and magnetic field (Ex and Hy) distributions at resonance frequencies of f1 = 239.2 THz, f2 = 312.7 THz, and f3 = 476.2 THz, respectively. In comparison, the VSSR has a clear advantage in that it couples directly with not only the electric field but also the magnetic field under normal incidence [25–27]. It can be observed that the localized electric and magnetic fields (Ex and Hy) are strongly concentrated and enhanced in the middle air gaps of the both sides (left and right sides) of the unit-cell structure along x-axis and y-axis direction, respectively. Due to the strong enhancement of the EM field in the VSSRs, the incident light energy can be efficiently confined in the VSSRs spacer with no reflection. It can be inferred that such enhanced electric and magnetic field distributions patterns at the air gaps mean the different guided mode excitations. It can be found that the perfect standing wave along the z-direction and surface plasmon polariton resonances mode have been excited at different resonance frequencies. These spatial field features confirms existence of the resonances formation of guided modes and surface plasmon polaritons modes by the excitation of strong near-field coupling between the air gap and paired metallic nanostructures.
As illustrated in the Fig. 3, it can be found that the zeroth-order, first-order, and second-order guided mode excitations are located at 239.2 THz, 312.7 THz, and 476.2 THz, respectively. The spatial field features above are caused by the following reason: the incident light at the air-VSSR interface propagates and damps through the air gaps, and then the residual energy at air-gold interfaces of the bottom has been reflected . On the other hand, at the third resonance frequency of 476.2 THz, the propagating and localized surface plasmon polaritons modes are also excited beyond the guided mode. It is well known that the surface plasmon polariton is formed by the interaction between external photons and free electrons in the interface of the dielectric and gold [44–46]. Rely on the gold VSSR nanostructure, the resonance modes mentioned above can be well excited under normal incident lights. The spatial field profiles caused by the hybrid mode excitations occur in the nanostructure, and the intrinsic loss usually takes place in bulk gold materials. Thus, the losses caused by the different mode excitation could be ascribed to the gold nanostructure array since the loss dielectric nature of gold material at optical frequencies, which can be confirmed by the power loss density of the unit-cell of the proposed perfect absorber.
It is also necessary to explore the distributions of power loss density in the unit-cell structure since it can provide detail information about where and how the perfect absorption happens. In order to reveal the detail information about perfect EM absorption in proposed periodic VSSR structure arrays, the distributions of power loss density in unit-cell nanostructure have been thoroughly investigated. Figure 4 presents the x-z plane and 3D distributions of power loss density in the unit-cell structure at different resonance frequencies. At the first resonance frequency of 239.2 THz (as shown in Figs. 4(a)), it is clearly that the power loss density is mainly distributed on the upper and down air gap edges of the x-z plane of the VSSR nanostructure at resonance, which is consistent with the distributions of the electric fields (see Figs. 3(a)). However, at the second and third resonance frequencies of 312.7 THz and 476.2 THz, as shown in Figs. 4(b) and 4(c), the distributions of power loss density become more complex. In these cases, the power loss densities are distributed both on the vicinity of x-z plane and y-z plane of VSSR structure at 312.7 THz and 476.2 THz, due to the combination of the guided mode with different order in the air gaps and the surface plasmon polaritons mode on the air-gold substrate, three near-unity absorption are obtained in this metallic nanostructure [25,43–46]. This result suggests a new approach to obtain a multispectral response by integrating different resonance modes in a single nanostructure which provides potential applications in plasmonic optical devices including sensors [24,28].
According to above results and discussions, the designed triple narrow-band perfect absorber could be functioned a high sensitive refractive index sensor. Thus, the refractive index sensing properties of the proposed perfect absorber have been further investigated. As shown inset of Fig. 5(b), we present the schematic of the unit-cell of the perfect absorption-based sensor, which is not the practical configuration as refractive index sensor. Generally, the simple change of refractive index is precisely evaluated by ellipsometry, and sensors are expected to detect some specific targets. However, in this work, we just present a possible example, which will open a new route for multiple narrow-band perfect absorbers applications toward sensing of biomedical and chemical molecules with different mass concentrations as well as detecting chemical reactions in a nano-environment [28,33]. For the refractive index sensing application, we used air as the reference media, and the aqueous glucose solution as the measured media, both of which are surrounding the surface of the proposed metallic nanostructure perfect absorber. Figure 5(a) presents the 3D array of the proposed perfect absorber covered with analyte. It should be noticed that the all analyte is covered all the surface of the proposed perfect absorber as shown in Fig. 5(a). When refractive index value of the surrounding medium (from air to aqueous glucose solution) is varied over 1.00 to 1.05 in intervals of 0.01 (as shown in Fig. 5(b)), obvious red-shifts of the three absorption peaks are observed. It indicates that the resonance frequency of the triple narrow-band perfect absorber is highly dependent on the refractive index variations introduced by the surrounding analyte. For example, it is observed that a red-shift is up to 14.5 nm for the first resonance frequency when refractive index value increases from 1.00 to 1.01. In addition, the absorbance magnitudes of the perfect absorption-based sensor are nearly unchanged (over 98%) while the refractive index value of the surrounding analyte sensing. It can be expected that the proposed sensor could provide ultra-high refractive index sensitivity if a high-resolution spectrometer is used in practical application.
As shown in Figs. 5(c-e), both high modulation depth and relative narrow FWHM can be kept in the refractive index ranges, which are two crucial factors for high sensing performance. Thus, for its narrow bandwidth and large modulation depth, the proposed triple narrow-band perfect absorber might be used to detect poisonous material and flammable gases [4,18,47–49]. To evaluate the refractive index sensing performance of triple narrow-band plasmonic perfect absorption-based sensor, the sensitivity (S =λ /Δn) and figure of merit (FOM = S ∕ FWHM) has been further proposed [4,31]. The S is defined as the resonance wavelength shift Δλ caused by a certain refractive index change Δn in the environment, which is corresponding to the shift wavelength per refractive index unit (nm / RIU). The FOM is defined as the S divided by the FWHM of a resonance, since the absolute wavelength and the bandwidth of the resonance are crucial factors for a sensor [4,31]. The relations between Δλ and Δn has been illustrated in Fig. 6(a). Based on the fitting results, it can be easily found that the wavelength shift Δλ is linearity to refractive index variation Δn. As shown in Figs. 6(b-d), the periodic VSSR nanostructure arrays show ultra-high bulk refractive index sensitivities of about 1194, 816 and 473 nm ∕ RIU for the resonance wavelength λ1, λ2, and λ3, respectively, which is much better than previous designed sensors operated in optical frequency region [4,7,8,18–20,31–38]. It can be inferred that the strongly enhanced EM field and different resonance mode excitations in the single metallic nanostructure should be response to the high refractive index sensitivity of the dielectric material. The wavelength FWHM of the designed perfect absorber structure surrounding air is about 73.8 nm, 28.2 nm, and 22.8 nm, respectively. Thus, the corresponding FOM is about 16.17, 28.94, and 20.74, respectively, which is much remarkable compared with previous designs [7,19,20,31,35,37]. These excellent characteristics further confirm that the proposed triple narrow-band plasmonic perfect absorber could be a highly promising device for detecting refractive index changes of poisonous material and flammable gases.
Due to the very high-aspect ratio (h/g in Fig. 1) of 25, which could cause some challenges to practically fabricate the periodic VSSR structure arrays with current fabrication technologies. The stimulated-emission-depletion-inspired direct laser writing (STED-DLW) technique have been proposed to be a suitable method to achieve the prototype of the high performance sensor which is already apllied in the fabrication of helical metamaterials [50,51]. Using this approach, an axial and lateral resolution below the diffraction limit can be achieved . To avoid multiple inversion steps during fabrication, we could firstly fabricate polymer arrays of the hollow shells of the desired gold structures via STED-DLW . Then, a polymer floor is additionally written in between the shell structures in order to cap the thin conductive, but transparent ITO layer on the substrate. In this process, Gold is deposited inside the polymer templates up to the desired height by controlling the deposition current and time during a subsequent electrochemical-deposition step . Furthermore, the mechanical stability of the proposed nanostructure also could need to be considered in practical fabrication.
In conclusion, a simple and effective design of the triple narrow-band plasmonic perfect absorber constructed with an assembly of the periodic VSRR structure array has been demonstrated numerically and theoretically. The numerical simulation result indicates that the perfect absorber can achieve absorbance of 99.4%, 98.3% and 99.8% at 239.2 THz, 312.7 THz, and 476.2 THz, respectively. The distributions of the enhanced electric and magnetic fields and power loss density reveal that the triple near unity absorption is due to the combination of the guide mode with different order and the surface plasmon polaritons mode. The further simulation results indicate that the design has a highly sensitive response to the refractive index change in the surrounding analyte. This triple narrow-band plasmonic perfect absorption-based refractive index sensor can achieve a sensitivity of 1194, 816 and 473 nm ∕ RIU, respectively. Compared with previous work [4,7,19,20,31,35,37], it can be inferred that the periodic VSSR structure arrays with better S and FOM are capable to be applied in high-quality multispectral sensors for biotechnology, gas detection, medical diagnostics and spatial biosensing.
National Natural Science Foundation of China (NSFC) (61605147, 61701185, 61801186); Natural Science Foundation of Hubei Province (2017CFB588); Hubei Provincial Department of Education (D20181107).
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