The detection of minuscule changes in the local refractive index by localized surface plasmon resonances (LSPRs), carried by metal nanostructures, has been used successfully in applications such as real-time and label-free detection of molecular binding events. However, localized plasmons demonstrate 1–2 orders of magnitude lower figure of merit (FOM) compared with their propagating counterparts. Here, we propose and experimentally demonstrate a high-performance refractive index sensor based on a structure of double-layered metal grating (DMG) with an FOM and FOM* reaching 38 and 40 respectively under normal incidence. Such a high FOM and FOM* arise from a result of a sharp fano resonance, which is caused by the coherent interference between the LSPR from the individual top gold stripes and Wood's anomaly (WA). Moreover, a small conformal decay length of ~68 nm is determined in DMG, indicating that the DMG is a promising candidate for label-free biomedical sensing.
© 2015 Optical Society of America
Arising from the coupling of light with collective oscillations of the electrons at the surface of a metal film or metallic nanostructure, surface plasmons (SPs) can tightly concentrate optical field into a nanoscale spatial region . This remarkable feature endows plasmonic structures an extreme sensitivity to the refractive index of the surrounding medium with the penetration depth of the evanescent field . Currently, commercially available refractive index sensors are dominated by the conventional propagating surface plasmon resonance (PSPR)-based systems (usually consist of a continuous gold film and a prism ), because they can provide an extremely high index sensitivity and figure of merit (FOM). However, PSPR-based approach suffers from the two main limitations. First, these systems require some complex and expensive equipments to couple and monitor light, which hinders its integration into a cost-effective and portable device for the rapid detection of analytes in the fields of point-of-care (POC) testing and personal medicine. Second, for PSPRs, their decay lengths are roughly of the order of half of the resonance wavelength (that is a few hundred nanometers), meaning a fact that the PSPR-based sensors aren’t quite suitable for monitoring the local refractive index changes at the nanometer level caused by small analytes. In contrast, localized surface plasmonn resonances (LSPRs), carried by metal nanoparticles and nanostructures, possess a remarkably smaller (at least an order of magnitude) decay lengths  compared to those of PSPR, thereby enabling them much more matching the requirements called for by the modern refractometer biosensing applications. Unfortunately, due to a broad full width at half maximum (FWHM) arising from strong radiative damping, the nanoparticle-based sensing platforms are known to present FOM values (0.8–5.4)  which are at least an order of magnitude smaller than those of PSPR ones (~108) [5–7]. Motivated by recent progress in the design and fabrication of more complicated metallic nanostructures, such as metamaterials  and periodic arrays of nanoholes [5,9] or nanoparticles [10,11], researchers have achieved some advances in improving the performance of nanoplasmonic sensors to a comparable level to the conventional PSPR-based ones [12–17]. These well-engineered nanostructures are designed to suppress the radiative loss of LSPRs by using Fano resonances, namely coupling them with the other resonant systems with a narrow linewidth, such as plasmonic subradiative modes [18,19], photonic cavity modes [20,21] and diffractive modes [10,22,23]. The FOMs of LSPR-based sensors can be further improved, for instance, by using phase-  and wavevector-interrogation [25,26] schemes. Despite the aforementioned advances made on improving the performance of the nanoplasmonic sensors, most of them are still restricted in widespread practical use because of their complex geometries and/or special detection manners. For example, a coupling element and/or an oblique incident detection system are still usually required to achieve the optimal sensing performance for the metamaterials of nanorod array  and plasmonic crystals [10,22].
Understanding these problems in sensing applications of existing plasmonics structures, we proposed and fabricated a simple structure of double-layered metal grating (DMG) by interference lithography (IL) and metal deposition, which is composed of a dielectric grating sandwiched between two gold gratings (Fig. 1(a)). Arising from the interplay between Wood’s anomaly (WA) and the LSPR of the top gold stripe, a sharp coupled resonance is created and experimentally demonstrates an FOM of 38 and FOM* of 40 under normal illumination, which is superior to most of nanoplasmonic sensors using Fano resonances under normal incident detection systems [27–30]. Moreover, at the sensing resonance, our structure presents a tightly confined electric field spatial distribution, leading to a small decay length of around 68 nm. These properties suggest that our sensor offers great potential in realistic biosensing applications.
2. Design and fabrication
The DMGs sustain a rich source of resonances including LSPRs from the individual units and propagating modes such as Wood’s anomalies and surface plasmon plaritons (SPPs), which enables it possible to investigate the interference between these different modes based on the lateral or vertical coupling between the neighboring units. Some intriguing optical properties, such as SPP unidirectional coupling , negative refraction , Fano resonance  and vertical far-field coupling , have been extensively explored in the DMGs. For sensing purpose, an extremely sharp resonance with strong local field accessible for analytes is crucial. It is therefore significantly important to optimize the electric field profile and tailor the Fano-type line shape in the DMGs by adjusting the structure parameters.
According to the aforementioned consideration, we first examined the influence of the top gold stripe width on the optical properties of the DMGs. Figure 1(a) shows a schematic of a DMG composed of a photoresist grating sandwiched by the upper and lower gold gratings. According to the SEM image of the fabricated structure shown in Fig. 2(d), the top gold stripes are slightly larger than the supporting photoresist ones in model. For an ideal DMG, both gold and the photoresist stripes are rectangular in cross section. In this case, a DMG can be fully characterized by the following four parameters: lattice constant a, dielectric grating height h, width of the top gold stripe w and thickness of the deposited gold layer t. Figure 1(b) depicts simulated width-modulated reflectance spectra of the DMG in water. The sizes of the DMGs are a = 600 nm, h = 465 nm, t = 100 nm, and w is swept from 140 nm to 460 nm. In this work, all simulations are performed by the finite-difference time-domain (FDTD) method (FDTD solutions, Lumerical Solutions). The dielectric function of bulk gold in the near-infrared region was described according to the Drude model with the plasma frequency ωpl = 1.37 × 1016 rad/s and the collision frequency γ = 1.2 × 1014 /s. The refractive indices of photoresist, PMMA and quartz were set to be 1.61, 1.49 and 1.48, respectively. As shown in Fig. 1(b), three resonance modes are located in the near-infrared region, marked as D1, D2 and D3, respectively. In the same figure, the three black dashed lines, which are located at 800 nm, 888 nm and 928 nm, denote the WA(1)water, WA(1)sub and SPP(−1)Au/PMMA, respectively. The wavelengths of these modes are calculated by the following equations [35–37]:Equation (1) is the Bragg coupling condition, where k0 is the wavevector of the incident light, and kmode is the wavevector of a specific mode (SPP or WA) of the structure. i is the grating order for the reciprocal lattice vector Gx (|Gx| = 2π/a, a is the lattice constant), θ is the incidence angle from air. Equations (2)-(3) describe the wavevectors of the SPP and WA modes, where εm and εd are the dielectric constants of the metal and the surrounding medium, respectively. In the narrow-width regime (w < 200 nm), D1 acts as an LSPR-like mode, revealed by the electric field distribution (point I, Fig. 1(d)), with the strongest field being tightly localized at the four edges of the top gold stripes. Due to the overlapping with the WA(1)water, this LSPR mode shows an asymmetric line shape with a steep side in the left. D1, therefore, is here considered as a coupled LSPR-WA(1)water mode. As the width of the top gold stripe is increased, D1 is shifted to a longer wavelength and eventually overlaps with WA(1)sub (Fig. 1(b)). The coupling between the LSPR and WA(1)sub also leads to a Fano-type asymmetric line shape with a steep side in the right, which is similar to LSPR-WA(1)water mode. Interfered by WA(1)sub, the weaker coupling efficiency of incident light to the LSPR of the top gold stripe is observed, in accompany with a fraction of optical energy being transferred into the substrate (w = 400 nm, point IV, Fig. 1(d)). As a result, the LSPR-WA(1)sub is not considered to be suitable for index sensing. For D2, corresponding to SPP(−1)Au/PMMA (pointII, Fig. 1(d)), its positions are almost stable against the change of the top gold stripe width (Fig. 1(b)). Note that D2 disappears as the width of the top gold stripe reaches around 220 nm. It is due to the fact that the gap between the adjacent bottom gold stripes becomes too large to support SPPs efficiently. While in the broad-width regime (w > 300 nm), D2 recurs as SPP(−1) at the bottom surface of the top gold stripe, indicated by the involved field distribution (point V, Fig. 1(d)). Similar to D1, D3 also exhibits a clear red-shift as the width of the top gold stripe is increased. The electric field at the point III is mainly distributed in the region sandwiched between the upper and lower gold gratings, indicating that D3 could be the Fabry-Pérot (FP) type cavity mode arising from the interaction between upper gold and bottom gold. D3 is weakened and eventually disappears (point VI, Fig. 1(d)) as the width of the top gold stripe is broadened. In order to further confirm the origin of D3, the simulated height-modulated reflectance spectra are plotted in Fig. 1(c). A clear periodic modulation of the height of the photoresist grating on the wavelength of D3 is observed, which is the typical characteristic of FP modes. Therefore, we ascertain that D3 is an FP mode. In addition, Fig. 1(c) also shows a coupling between SPP and FP mode in height modulation.
Thanks to possessing a narrow LSPR-WA(1)water mode with a strongly localized electric field, the DMG with the narrower width of top gold stripes would be preferential for refractive index sensing. We fabricated the DMGs via a combination of interference lithography and metal deposition. A schematic diagram illustrating the fabrication procedure of DMG structure is given in Fig. 2(a). First, a PMMA (Sigma-Aldrich) thin film with thickness of ~50 nm was spin-coated on a cleaned quartz slide as an adherent, ensuring the firm attachment of the photoresist grating. After that, a photoresist film was spun onto the PMMA layer, and the photoresist grating pattern was then formed by two-beam interference lithography (Fig. 2(b)). Finally, the DMG was obtained after sequential electron beam evaporation of 5-nm nickel and 100-nm gold on the photoresist grating. The overall fabrication procedure is very simple and cost-effective, which is beneficial for practical sensing applications. Figures 2(c) and 2(d) show the top-view and cross-sectional scanning electron microscopy (SEM) images of a DMG. Gold stripes seem to be rounded in cross section, and both the top and bottom of each photoresist stripe are somewhat larger than the middle. For the DMGs, the lattice constant is determined by the angle between the two incident laser beams, and the height of dielectric grating can be varied by adjusting the thickness of the photoresist film. In addition, the width of the gold stripe depends on both the exposure dose and developing condition, and their thickness can also be precisely controlled by electron beam evaporation.
3. Results and discussion
We measured the zero-order reflectance spectrum of a DMG (a = 600 nm, h = 465 nm, w = 210 nm and t = 100 nm) at normal incidence in air with a p-polarized incident wave (the electric field is perpendicular to the gold stripes), as shown in Fig. 3(a) by the black curve. It is noted that all zero-order reflectance spectra were taken on an ultraviolet/visible/near-infrared spectrometer (Lambda 950, PerkinElmer) under p-polarized incident waves. The corresponding simulated spectrum, marked by the red curve, is also plotted in Fig. 3(a). The measured and simulated results agree well with each other in the prediction of the peak and dip positions. In air, there are three reflectance dips (marked as D1, D2 and D3) corresponding to the LSPR, SPP and FP modes, respectively. When water is applied onto the sample surface, D1 is shifted to a longer wavelength of 814 nm and shows a narrow (~18 nm) asymmetric line shape with a steep left drop edge (Fig. 3(b)). In contrast, as an SPP mode at the gold/PMMA interface, D2 is insensitive to the change of surrounding medium. For D3, a clear red-shift from 787 to 1033 nm occurs. It is understood that the change of effective refractive index will lead to a modification of the resonance wavelength of FP mode. It is noted that, an unexpected small dip between D1 and D2 appears in the measured reflectance spectrum in water. It is believed that this dip is caused by the small displacement between the upper and lower gold gratings  owing to a slight deviation of the deposited direction in metal deposition (not strictly normal to the sample surface). To present this dip in simulation, we modified the model of the DMG via shifting the top gold stripes to the right by a distance of ~25 nm (See the insets of Fig. 3). As expected, this dip caused by the geometric symmetry breaking emerges in the simulated reflectance spectrum, as shown by the red curve in Fig. 3(b).
To calibrate the sensing performance of the DMG, we recorded the reflectance spectra (Fig. 4(a)) under normal excitation at room temperature when the DMG was immersed in a series of glycerol–water solutions with varying concentrations (0, 17, 32, 47 and 60 wt% in glycerol) and thus the corresponding refractive indices (1.333, 1.354, 1.373, 1,394 and 1.413). The spectral positions of these resonances versus the refractive indices are plotted in Fig. 4(b), and the bulk index sensitivities of 525, 325 and 559 nm/RIU are obtained by linear fittings for D1, D2 and D3, respectively. The FWHMs of D1, D2 and D3 are in the ranges of 13.8–17.6, 12.2–23.5 and 62.0–97.8 nm, respectively. Therefore, the maximum values of experimental FOMs of D1, D2 and D3 modes are determined to be 38, 27 and 9. Figures 4(d) and 4(e) show the corresponding simulated reflectance spectra and index sensitivities, displaying an excellent agreement with the measured ones.
In a real POC testing, however, it is much more convenient to monitor the changes in light intensity at a fixed wavelength instead of the peak/dip shifts induced by changes in refractive index of the surrounding medium. Therefore, an alternative figure of merit, termed FOM* was proposed , which is defined as:Eq. (4), the FOM* as a function of the wavelength has been calculated by extracting the data from Fig. 4(a), as shown in Fig. 4(c). In accordance with an increase of the reflectance intensity from 15.3% to 27% when the refractive index of surrounding medium is changed from 1.354 to 1.373, an FOM* of 40 is obtained at 823 nm for the LSPR-WA(1) coupled mode. Note that, an even higher FOM* of ~100 is obtained for this mode in the corresponding simulation (Fig. 4(f)). This deviation probably originates from the fact that the real geometry of the DMG and the practical configuration of reflectance measurement don’t perfectly match the ideal situations in simulation. For example, the surface scattering and grain boundary effects  in our gold film and a beam divergence of ~3° on our spectrophotometer can both affect the strength and line shape of the coupled LSPR-WA mode. Such a high FOM* is attributed to a combination of the narrow line shape with a steep left drop edge and a high sensitivity, as a result of the coupling between the LSPR of the top gold stripe and WA in water. This FOM* is greatly improved in comparison with that of a state-of-the-art FOM* achieved in gold nanorods  and plasmonic nanocavities with metal-dielectric-metal optical antennas (~21) .
For the real biosensing, only a fraction of electric field located at the region of the nanostructure surface which is occupied by the adsorbates can make the contribution to the actual spectral shifts. Therefore, the highly localized sensing volume is of great importance to bimolecular detection. To quantitatively describe how fast the electric field decays away from the surface, decay length ld has been extensively used [40–42]. Generally, the decay length ld can be fitted using the following exponential equation:Eq. (5). In order to more accurately evaluate the role of the local field distribution in the realistic biosensing, we provided another decay length of the DMGs by using the method in ref . For this method, an equation has been proposed to correlate the plasmonic spectral shift Δλ to the thickness of the adsorbate d [40–42]:Eq. (6) with the variable d, a conformal decay length ld of ~68 nm is extracted. This decay length is smaller than the reported plasmonic crystal with strong coupling [42,43], suggesting that our DMG can provide great potential in biosensing, suggests that our DMG can provide great potential in biosensing.
In summary, by means of a rational design and a simple and high-throughput nanofabrication process, we have proposed and experimentally demonstrated the double-layered metal gratings for sensing. The gold stripes supported on the photoresist ones hold a strong local electric field, coupled with WA via grating diffraction under normal incidence. The narrow and asymmetric spectral line-shape caused by the Fano resonance enables the DMG to possess an FOM and FOM* as high as 38 and 40, respectively. In addition, we obtain a small conformal ld of ~68 nm for the DMG, which is much smaller than the previously reported strongly coupled systems. The DMG with these advantages will be found broad applications in biomedical sensing.
The authors acknowledge the financial support from the National NaturalScience foundation of China (11374376, 11174374), the Key project of DEGP (No. 2012CXZD0001), and Innovative Talents Training Program for Doctoral Students of Sun Yat-sen University.
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