1. Introduction

Glancing angle deposition (GLAD) is a low-cost method for fabricating large-area nanostructured thin films (NTFs). By arranging the substrate tilted at an angle with respect to the vapor flux during deposition, the vapor flux arrive on the substrate to form random nucleation site. During the initial stage of deposition, nucleation sites form on a smooth substrate to prevent the flux reaching onto their shadow areas. The shadowing effect lead to the columns grow toward the incident deposition flux. The columnar structures can be slanted posts, vertical posts, helices, zig-zags, and square spirals by employing substrate rotation [1–4]. To achieve perfect light extinction, GLAD has been applied to control the refractive index of a dielectric NTF by altering its porosity. A five-layered obliquely deposited structure exhibits a graded refractive index profile, providing broadband and wide-angle antireflection [5]. Such a profile, predicted by Dobrowolski [6], requires a thick index-matching NTF film on the top to provide low reflectivity at long wavelengths and large angles of incidence. For instance, an SiO₂ NTF was deposited at a deposition angle of 87° between the vapor flux and the surface normal; it had a refractive index of approximately 1.05, which is close to that of air.

Recently, various ultra-thin metal nanostructures have been fabricated to perform localized surface plasmonic resonance (LSPR) under illumination and thus to absorb light [7]. Electromagnetic waves that propagate through the subwavelength structure of a composite induce local plasmonic resonance. Highly efficient light absorbers with thicknesses of less than 500 nm have been developed over the past ten years, such structures as metallic gratings, nanoparticles sub-wavelength slits, and metallic trapezoidal arrays [8–14]. However, the extinctance of most plasmonic structures cannot be maintained 80% over all visible wavelengths and both p and s polarization states. Plasmonic structures with three-dimensional metal-dielectric structures have exhibited near-perfect extinction recently [15]. However, the broadband reflectance over a wide range of angles was not as low as aforementioned five-layered dielectric structure. The plasmonic structures fabricated by GLAD techniques including nanoparticles and tilted nanorods with high extinction exhibit strong sensitivity in biosensing. The high extinction of silver nanoparticles can be utilized for detect the binding event of biomolecules to the nanoparticle surface [16]. On the other hand, Ag nanorod arrays with high extinction have been used as high sensitive substrates in SERS (surface-enhanced Raman scattering) measurement [17]. In a recent work, a one-turn silver nanohelix array was firstly deposited using GLAD on a transparent glass substrate and then one-turn aluminum nanohelixes were deposited on the tops of silver nanohelixes to form an aluminum-silver nanohelix array with a thickness around 470 nm [18]. The aluminum nanohelixes exhibited index matching to reduce the reflectance and hot spots corresponding to LSPR support strong light extinction. In this work, an index-matching SiO₂ NTF is deposited on the top of a plasmonic nanohelix array to enhance its antireflective property for high extinction.

This work has two parts. Firstly, a silver nanohelix array with pitch length around 131 nm were fabricated using GLAD. The silver nanohelixes were capped with two-turn SiO₂ nanohelixes to form a SiO₂-Ag nanohelix array for index matching. The s-polarized and p-polarized extinctance spectra of the SiO₂-Ag nanohelix array were measured. Secondly, the aforementioned aluminum-silver nanohelix array was fabricated and then a two-turn SiO₂ nanohelix array was obliquely deposited upon it to form an SiO₂-Al-Ag nanohelix array on a BK7 glass substrate. The extinctance spectra of the Al-Ag nanohelix array and SiO₂-Al-Ag nanohelix array at various wavelengths and angles of incidence were measured to demonstrate that the SiO₂ nanohelixes enhance extinctance.

2. Experiments and results

2.1 Ag and SiO₂-Ag nanohelix

The formation of nanohelix relies on the matching between deposition rate and spin rate. In our previous work [19,20], various Ag, Al, and Au nanohelix arrays were deposited using different spin rates with respect to a constant deposition rate to derive an optimum condition for a spiral-like nanohelix structure. Once the spin rate departed from the optimum rate, the NTF would be changed to screw-like nanohelix array or upright nanopillar array. The metal nanohelixes here were fabricated to exhibit spiral like morphology. The SiO₂ nanohelixes were screw-like upon the Ag nanohelixes individually to avoid overlap with adjacent nanohelixes. In this work, nanohelixes were deposited on BK7 glass substrates in an electron evaporation system; during this process, the substrate normal was tilted at an angle of 86° to the direction of incidence of the vapor. The center of the substrate and the evaporation source were vertically separated by 290 mm. Liquid nitrogen was passed through a loop under the substrate to cool the substrate holder to −140°C. A background pressure of $4\times {10}^{-6}$ torr was achieved by pumping before evaporation. For the Ag nanohelixes, the deposition rate was maintained at 0.3 nm/s. To match the refractive index of air, the SiO₂ nanohelixes were grown upon the silver nanohelixes. The deposition environment was the same as used previously. The silver nanohelixes were deposited on a BK7 substrate at a substrate spin rate of 0.017 rpm. Then the SiO₂ nanohelixes were grown on the silver nanohelixes at deposition rate of 0.3 nm/s and a substrate spin rate of 0.029 rpm to form a two-turn SiO₂ nanohelix array. The morphologies of the Ag and SiO₂-Ag nanohelix arrays are characterized by scanning electron microscopy (SEM), as shown in Fig. 1. Table 1 presents the average pitch lengths (P), radii of curvature (Rc), arm widths (W) and spin rates (ω) of the SiO₂ and Ag nanohelixes. From the top view of each sample, the average distance between adjacent nanohelixes is 230nm. The pitch length, 209 nm, of the SiO₂ nanohelix array is larger than that of the Ag nanohelix array. The mean radius of curvature, 92 nm, and arm width, 46 nm, of the SiO₂ nanohelix array are less than those of the Ag nanohelix array.

 

Fig. 1 Top-view and cross-section SEM images of (a) Ag nanohelix array, (b) SiO₂-Ag nanohelix array and (c) schematic drawing of nanohelix with the definition of polarizations.

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Table 1. Average pitch lengths (P), radii of curvature (Rc), arm widths (W) and spin rates (ω) of SiO₂ and Ag nanohelixes.

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The reflectance (R) and transmittance (T) of both Ag and SiO₂-Ag nanohelix arrays were measured by attaching the samples to an integrating sphere to collect both diffuse and specular light intensities in Hitachi 4150 spectrophotometer. The module of direct light detection for Hitachi 4150 spectrophotometer was also used to measure the specular R and T of both Ag and SiO₂-Ag nanohelix arrays. Extinctance (E) is defined as 1-R-T. The plane of incidence in optical measurement is defined such that the initially growing rods of each sample lying on the y-z plane, as shown in Fig. 1(c). Figure 2(a) shows the measured s-polarized and p-polarized T, R, and E of the Ag nanohelix array as functions wavelength from 300 nm to 1000 nm. The subscript s and p of T, R, and E represent the s-polarization and p- polarization, respectively. Figure 3 shows the measured spectra of specular and diffuse extinctance. The average difference of extinctance between both cases is 5.72%. For the s-polarization and p- polarization, the spectra are similar. The transmittances have minimum values of T_s = 7.77% and T_p = 8.02% at wavelengths of $\text{λ}=498\text{nm}$ and $\text{λ}=512$ nm, respectively. The transmittances increase to T_s = 36.90% and T_p = 37.38% at a wavelength of $\text{λ}=1000\text{nm}$v. The difference between p-polarized and s-polarized transmittances is less than 0.51% over the wavelength range. The reflectance has a minimum close to zero at wavelengths of 431 nm and 433 nm for s- and p-polarization, respectively. The peak values of reflectance R_s = 50.65% and R_p = 41.55% occur at wavelengths of 678 nm and 669 nm, respectively. Hence, the extinctances reach its maximum values of E_s = 87.38% and E_p = 87.89% at wavelengths of 435 nm and 440 nm, respectively. The high extinctance at wavelengths around 430 nm is caused by the transverse plasmonic mode resonance [21] that occurs when the direction of the oscillating electric field is perpendicular to the metal rod. Figure 2(b) shows the spectra of R, T, and E for the SiO₂-Ag nanohelix array. The dips of reflectance are shifted to wavelengths below 400 nm. The capping SiO₂ nanohelixes reduce the reflectance at wavelengths above 468 nm. The difference in transmittance between both polarization states is less than 0.79% over the whole wavelength range. The extinctance decreases from 86.01% at $\text{λ}=421\text{nm}$ to 83.16% at $\text{λ}=470\text{nm}$ for p-polarization. The SiO₂ nanohelix array is a layer with an equivalent refractive index closed to that of air. There is no index matching effect between the glass and silver. Owing to the light coupling effect that is caused by the SiO₂ nanohelixes, the extinctances of both polarization states are higher than those of the aforementioned Ag nanohelix array at wavelengths above 470 nm. The average non-polarized extinctance from $\text{λ}=470\text{nm}$ to $\text{λ}=1000\text{nm}$ is 50.26%, which exceeds that of 27.87% for an Ag nanohelix array. The extinction peak was shifted from 435 nm to 377nm nm as the silver nanohelixes were capped with SiO₂ nanohelixes. The plasmonic resonant wavelength of a nanostructure is shifted due to the change of surrounding material [16].

 

Fig. 2 Measured s-polarized and p- polarized spectra of transmittance, reflectance and extinctance for (a) Ag nanohelix array, (b) SiO₂-Ag nanohelix array.

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Fig. 3 The diffuse and specular extinctance spectra for (a) Ag nanohelix array, (b) SiO2-Ag nanohelix array.

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2.2 Al-Ag nanohelix array and SiO₂-Al-Ag nanohelix array

Based on measurements of the Ag and SiO₂-Ag helix array, the SiO₂ nanohelix array is an index matching layer that reduces the reflection of the nanostructured film. SiO₂ nanohelixes are grown upon an Al-Ag helix array that has been demonstrated to be a strong light absorber, which increases extinction. During fabrication, electron beam evaporation was used to perform GLAD; Ag nanohelices and Al nanohelix were fabricated by the same deposition with a deposition angle of 86° from the direction of incidence of the vapor. During the deposition, the pressure in the vacuum chamber was maintained at around $4\times {10}^{-6}$torr, and the temperature of the substrate holder was at −140°C. The Ag and Al nanohelix arrays were deposited at substrate spin rates of 0.017 rpm. The two-turn SiO₂ nanohelixes were deposited on the Al-Ag nanohelix array at a deposition rate of 0.3 nm/s and a spin rate of 0.029 rpm. Figure 4 shows the morphologies of the Al-Ag and SiO₂-Al-Ag nanohelix arrays. Table 2 presents the P, Rc, W and ω of SiO₂, Al and Ag nanohelixes, respectively. From the top view of each sample, the average distance between adjacent nanohelixes is 359nm. The mean pitch angle, 50°, and pitch length, 217 nm, of the Al nanohelix array are larger than those of the Ag nanohelix array. The mean radius of curvature, 96 nm, and arm width, 42 nm, of the Al nanohelix array were less than those of the Ag nanohelix array, so most of the helices are close but separate from each other. The mean pitch length, 212 nm, of the SiO₂ nanohelixes that were grown upon the Al nanohelixes is less than that of the Al nanohelixes. The mean radius of curvature, 86 nm, and arm width, 47 nm, of the SiO₂ nanohelixes were also smaller than those of the bottom Ag nanohelixes. Therefore, the SiO₂ nanohelixes with a small radius of curvature on the Al-Ag nanohelixes supported the distribution of individual SiO₂-Al-Ag nanohelixes on the glass surface.

 

Fig. 4 Top-view and cross-section SEM images of (a) Al-Ag nanohelix array, (b) SiO₂-Al-Ag nanohelix array.

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Table 2. P, Rc, W and ω of SiO₂, Al and Ag nanohelixes.

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Figure 5 shows p-polarized and s-polarized extinctances versus wavelength from 400 nm to 2000 nm and angle of incidence from 0° to 70°. The plane of incidence for the measurement is the x-z plane. The s-polarized extinctance decreases as the angle of incidence increases. At wavelengths of 400 nm, 1200 nm and 1800 nm, the extinctance is less than 70% at angles of incidence that exceed 65°, 56° and 63°, respectively. The s-polarized extinctance, averaged over the entire ranges of wavelengths and angles of incidence, is 85.26%. Generally, the p-polarized extinctance exceeds the s-polarized extinctance. The p-polarized extinctance exceeds 90% over wavelengths from 400 nm to 650 nm and angles of incidence from 0° to 65°. The p-polarized extinctance exceeds 84% over the whole spectrum in Fig. 5(b). The p-polarized extinctance, averaged over the entire ranges of wavelengths and angles of incidence, is 91.38%. For the Al-Ag nanohelixes, a previous simulation [18] indicates that the field enhancement induced by the LSPR is strong within the narrow gaps among Al and Ag helixes. The hot spot distribution within these metal helices support multiple resonance for different wavelengths, polarization states and angles of incidence.

 

Fig. 5 Measured (a) s-polarized and (b) p- polarized extinctance as functions of wavelength and angle of incidence for Al-Ag nanohelix array. Al-Ag nanohelix array is shown with plane of incidence.

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Figure 6 plots the measured s-polarized and p-polarized E values of the SiO₂-Al-Ag nanohelix. The s-polarized extinctance exceeds 90% over wavelengths from 400 nm to 800 nm and angles of incidence from 0° to 55°. The s-polarized extinctance exceeds 82% over the whole spectrum in Fig. 6(a). The p-polarized extinctance exceeds 92% over wavelengths from 400 nm to 1000 nm and angles of incidence from 0° to 70°. The p-polarized extinctance exceeds 89% over the whole spectrum in Fig. 6(b). The p-polarized extinctance, averaged over the ranges of wavelengths and angles of incidence, exceeds 94%. Figure 7 plots the extinctance averaged over all wavelengths as a function of the angle of incidence. The extinctance of the SiO₂-Al-Ag nanohelix array exceeds the average s-polarized extinctance of the Al-Ag nanohelix array, as shown in Fig. 7(a), especially at angles of over 40°. For p-polarization, the extinctance of the SiO₂-Al-Ag nanohelix array is especially increased at angles of incidence of greater than 50°, as shown in Fig. 7(b). The capped SiO₂ nanohelix array acts as an index-matching layer that couples incident light into the plasmonic structure over a wide range of angle.

 

Fig. 6 Measured (a) s-polarized and (b) p- polarized extinctance as functions of wavelength and angle of incidence for SiO₂-Al-Ag nanohelix array. SiO₂-Al-Ag nanohelix array is shown with plane of incidence.

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Fig. 7 Average extinctance of Al-Ag nanohelix and SiO₂-Al-Ag nanohelix at wavelengths from 400 nm to 2000 nm for (a) s-polarization and (b) p-polarization.

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3. Conclusion

In conclusion, an SiO₂ nanohelix array was successfully capped onto a metal nanohelix array to function as an index-matching layer. As predicted, in an all-dielectric nanostructured layer that provides perfect antireflection, SiO₂ nanohelixes upon Ag nanohelixes shift the localized plasmonic mode and reduce the reflection at wavelengths of greater than 472 nm. However, although the Al-Ag nanohelix has been proposed to be a highly efficient light absorber, the SiO₂ nanohelixes upon the Al-Ag nanohelixes form an overall structure with a closer approach to perfect antireflection as ultra-low reflection over a broad band and wide range of angle. Perfect antireflection of an all-dielectric layered structure is successfully realized in a plasmonic structure. The method herein can be used to increase the efficiencies of various light absorbers that comprise metal nanostructures by depositing low-index dielectric nanostructured thin films on them. It will be applied in the near future in solar cells, multifunctional sensors, thermal emitters, and photothermal therapy, which benefit from the improved efficiency of light harvesting.