Letter pubs.acs.org/NanoLett
Electromagnetic Field Enhancement and Spectrum Shaping through Plasmonically Integrated Optical Vortices Wonmi Ahn,† Svetlana V. Boriskina,† Yan Hong, and Björn M. Reinhard* Department of Chemistry and The Photonics Center, Boston University, Boston, Massachusetts 02215, United States S Supporting Information *
ABSTRACT: We introduce a new design approach for surfaceenhanced Raman spectroscopy (SERS) substrates that is based on molding the optical powerflow through a sequence of coupled nanoscale optical vortices “pinned” to rationally designed plasmonic nanostructures, referred to as Vortex Nanogear Transmissions (VNTs). We fabricated VNTs composed of Au nanodiscs by electron beam lithography on quartz substrates and characterized their nearand far-field responses through combination of computational electromagnetism, and elastic and inelastic scattering spectroscopy. Pronounced dips in the far-field scattering spectra of VNTs provide experimental evidence for an efficient light trapping and circulation within the nanostructures. Furthermore, we demonstrate that VNT integration into periodic arrays of Au nanoparticles facilitates the generation of high E-field enhancements in the VNTs at multiple defined wavelengths. We show that spectrum shaping in nested VNT structures is achieved through an electromagnetic feedmechanism driven by the coherent multiple scattering in the plasmonic arrays and that this process can be rationally controlled by tuning the array period. The ability to generate high E-field enhancements at predefined locations and frequencies makes nested VNTs interesting substrates for challenging SERS applications. KEYWORDS: Nanoplasmonics, nano-optics, phase singularities, optical vortices, vortex nanogear transmissions, surface enhanced Raman spectroscopy
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the E-field needs to be simultaneously enhanced at both the pump wavelength and the Stokes-shifted Raman wavelength.18,23,27−31 Other applications, such as on-chip photonbased information processing or light harvesting, require a polarization control of the near- and far-field responses of the nanostructures and the ability to control the emission directionality of quantum emitters located in their vicinity.32,33 The rational fabrication of noble metal nanostructures for these desired applications requires appropriate design strategies to predict geometries that generate specific near- and far-field properties. The conventional approach to achieve giant E-field enhancements in gap antennas of spheres, rods, and nanoprisms suffers from the short dephasing times of LSPs due to high dissipative and radiative losses in metal NPs. Previously explored strategies for creating slowly dephasing (i.e., narrowline width) resonances in the spectra of plasmonic structures include long-range in-phase radiative coupling19,22,34−38 and Fano-resonance engineering through near-field dipole coupling.39−45 The former approach also yields enhancement of the near-field intensity,22,35,46−48 which is crucial for plasmonenhanced spectroscopies, but leads to tens-of-wavelengths-long
oble metal nanoparticles (NPs) and their assemblies are now frequently used to enhance,1−4 localize,5,6 or 7−11 electromagnetic radiation. All of these functionalities guide are enabled by a strong coupling of the incident light to collective oscillations of conduction band electrons in the NPs, known as localized surface plasmons (LSPs). These charge oscillations in the metal achieve a charge pile-up on the surface of NPs and result in the generation of secondary dipole fields around the NPs. In closely spaced NPs, the charge oscillations couple and induce the localization of charges of opposite signs on the two sides of the gap separating the particles. These quasi-static near-field interactions dominate the electromagnetic interparticle coupling for separations below S0 = 1/k0, where k0 is the free-space wavenumber at the resonance wavelength. The mode localization in strongly coupled nanoparticles results in the formation of nanoscale volumes of giant E-field enhancement, so-called hot-spots.12−18 Particles that are separated by more than S0 can interact through diffractive coupling,19−21 and it has been demonstrated that near- and far-field interactions can synergistically combine to create a cascade multiscale E-field enhancement.22−25 For many applications the generation of large E-fields is a prerequisite but not a sufficient condition and additional requirements need to be met. To boost the signal intensity in surface enhanced Raman spectroscopy (SERS),26 for instance, © 2011 American Chemical Society
Received: September 27, 2011 Revised: December 9, 2011 Published: December 15, 2011 219
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is the first practical realization of the theoretical VNT concept. Furthermore, we demonstrate that both the bandwidth and efficiency of the SERS signal enhancement can be increased through integration of the VNTs into a two-dimensional array of radiatively coupled NPs with a properly tuned period.22 We characterize the elastic and inelastic scattering spectra of the fabricated nested VNT arrays and show through combination of state-of-the-art computational electromagnetics and experimental spectroscopy that they provide new opportunities for E-field enhancement and optical spectrum shaping. Results and Discussion. Design of Nested VNTs for Applications in SERS. One example of a possible VNT nanostructure design is a linear periodic chain of Au nanosphere dimers (Figure 1a) coupled via subwavelength
designs with low spatial density of electromagnetic hot-spots and high scattering losses. The latter approach can yield miniaturized nanostructures featuring tunable narrow dips in their scattering spectra. While these spectral dips are useful for increasing the spectral resolution of refractive index nanosensors,42 they do not necessarily overlap with the intensity peaks in the nanostructure’s near-field spectrum.49 To address some of the challenges associated with engineering nanoplasmonic structures with predetermined near- and farfield responses, we explore in this study a novel concept for optical power capture and transfer on the nanoscale, which is based on light recirculation through optical vortices. Different from the conventional approach to nanoplasmonic structures design, which seeks to optimize the constructive interference of the E-fields generated by individual oscillating nanoparticle dipoles, our design approach seeks to generate a landscape of nanoscale optical vortices that efficiently circulate and route the optical powerflow through noble metal nanostructures. Central to the understanding of this approach is the concept of the time-averaged Poynting vector S = 1/2 Re[E × H*], which describes the vector field of the optical powerflow in and around plasmonic nanostructures (E and H are electric and magnetic field vectors, respectively). The optical powerflow density and direction are determined by the phase landscape of the electromagnetic field. At points of destructive interference (zero intensity in the interference field) the phase of the electromagnetic field is undefined and the whole range of phase values (from 0 to 2π) coexists. These locations are referred to as phase-singularities. By the definition of the Poynting vector, the powerflow always occurs in the direction of the phase change, and thus the phase singularities represent centers of local circulating optical power flow, or simply optical vortices.50−52 Unlike previous studies of optical vortices in propagating and scattered electromagnetic fields, this work aims at “pinning” optical vortices to rationally engineered plasmonic nanostructures in order to facilitate and tailor the interaction of the recirculated light with collective oscillations of conduction band electrons in NPs. The important property of optical vortices is that they can be formed in free space50,51,53,54 because phase singularities in the electromagnetic field are characterized by zero absorption. Consequently, optical vortices enable to route the optical power around the NPs and, thus, through the ambient medium rather than through the NP volume. This redirection of the light flow is associated with a significant increase in optical plasmon dephasing times and enhancement of light interactions with the environment. We have previously demonstrated the feasibility of the proposed rational design approach for plasmonic54 and optoplasmonic53 networks through rigorous electromagnetic calculations. Furthermore, we have shown that an intuitive understanding of the working principle of the coupled vortex nanostructures can be obtained by invoking an analogy to hydrodynamic transmissions.54 The individual optical vortices can be pictured as gears made of light, which, if arranged in a transmission-like sequence, can route the optical powerflow through the plasmonic nanostructure. Because of this illustrative analogy we refer in the following to nanostructures that sustain coupled optical vortices as “Vortex Nanogear Transmissions” (VNTs). The aim of this work is to design and implement a VNTbased planar SERS substrate that generates high E-field enhancements at two (or more) discrete wavelengths (pump and emission wavelengths). To the best of our knowledge, this
Figure 1. Dimer-chain Vortex Nanogear Transmission. (a) Schematic of the VNT structure composed of five identical Au nanosphere dimers. The NP diameters d are 170 nm, dimer gaps wx are 20 nm, and interdimer separations wy vary between 25 and 45 nm. (b) Far-field scattering (Qscat) and absorption (Qabs) efficiencies of the VNT under normal illumination by a plane wave polarized along the dimers axes and (c) the corresponding E-field intensity enhancement measured in the gap of the central dimer. The intensities calculated on a surface of a single 170 nm nanosphere (dash gray) and in the gap of an isolated dimer (solid gray) are also shown for comparison. The spectral position of the VNT trapped-mode resonance can be tuned by varying wy, shown for 25 nm (green), 35 nm (blue), and 45 nm (red) width. (d) Poynting vector intensity distribution and a vector powerflow map in the plane cutting through the centers of the dimer gaps. Dashed circles show the projections of the particles on the VNT central plane and orange arrows schematically highlight the powerflow pattern through the VNT.
gaps.54 This structural motif has been rationally designed to generate a resonant electromagnetic response at a defined wavelength using rigorous generalized Mie theory (GMT)55−57 calculations (see Methods in the Supporting Information). GMT algorithms are computationally efficient and have been 220
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Figure 2. Nested VNT arrays with a tailored near-field response. (a) SEM image of a representative nested VNT. (b) Calculated spectrum of the near-field intensity generated in the center of the optimized nested VNT structure under the x-polarized plane wave illumination (d = 170 nm, wx = 20 nm, wy = 25 nm, array period: 610 nm). Corresponding spectra of an Au monomer (green) and dimer (blue) are shown for comparison. Pump laser wavelength (785 nm) and the target Raman line (1077 cm−1 band of pMA) are marked with vertical dotted lines. (c−e) Spatial distributions of the near-field intensity (left) and the real part of the principal field component, Re(Ex) (bottom right) in the VNT calculated at the wavelengths corresponding to the three resonant peaks in (b). The resonance wavelength is increasing from left to right.
what the need for very short interparticle separations when compared with high-performance nanodimer antennas. Visualization of the optical powerflow through the nanostructure (Figure 1d) reveals formation of coupled optical vortices threaded through the dimer gaps. In essence, the nanostructure in Figure 1a acts as a “mirrorless resonator” capable of efficient trapping of incoming light. The resulting reduction of scattered losses is manifested by a dip in the scattering spectrum shown in Figure 1b. A reduced radiative decay of the collective resonance leads to a prolonged interaction of the trapped light with Au NPs, which in turn, is accompanied by an increase of the absorption efficiency at the resonance wavelength (Figure 1b). The simple VNT shown in Figure 1a generates high E-field intensity enhancement only in a narrow frequency band around its resonance wavelength. Efficient SERS substrates require, however, high E-field intensity enhancements spatially colocalized at multiple wavelengths (the pump and the Stokes shifted Raman band). Although this limitation can, in principle, be overcome by inducing multiple peaks in the E-field intensity spectrum by increasing the morphological complexity of the VNTs, we focus in this manuscript on an alternative approach, which allows us to retain the geometrically simple model VNTs from Figure 1a. As we will demonstrate in the following, the integration of VNTs into regular arrays of Au NPs facilitates the generation of high E-field intensities in the VNTs in tunable wavelength bands. SEM images of an exemplary “nested VNT” structure, which contains a central belt of VNTs embedded into two adjacent NP arrays, are shown in Figure 2a, and their experimental scattering spectra will be shown below. The two
demonstrated to facilitate accurate calculations of the morphology-dependent near- and far-field properties of plasmonic nanostructures.22,23,49,57,58 The planar VNT in Figure 1a is characterized by two spatial axes. The x-axis points along the dimer axes in the VNT, whereas the y-axis runs parallel to the pentamer axis of the VNT made of five dimers. Structural parameters that describe the VNT include the diameter of the NPs (d) and the edge-toedge NP separations along the x-axis (wx) and along the y-axis (wy). The illumination of a (d = 170 nm, wx = 20 nm) VNT by a plane wave that is linearly polarized along the x-axis and whose wave vector (k) points perpendicular to the VNT plane results in the excitation of a collective resonance. This resonance is characterized by a narrow dip in the scattering efficiency, whose spectral position is tunable through wy (Figure 1b). As shown in Figure 1c, the dip in the scattering efficiency is accompanied by a strong resonant enhancement of the nearfield intensity as evaluated in the gap of the central dimer in the VNT.54 For comparison, we included the near-field intensity enhancements on a single 170 nm nanosphere (dash gray) and in the gap of an isolated (d = 170 nm, wx = 20 nm) dimer (solid gray). Because of the strong quasi-static coupling of the LSPs in dimers of NPs with a narrow gap, dimers can provide high Efield intensity enhancements and are, therefore, frequently used as building blocks for engineered plasmonic arrays in diverse applications.38,48,59,61 Our simulations indicate that VNTs facilitate E-field intensity enhancements that are significantly higher than those of dimers with identical geometric parameters. The VNT approach, consequently, relaxes some221
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arrays contain 24 × 49 nanodiscs each and enclose a central belt of 48 individual VNTs. The fundamental idea underlying the nested VNT design is to use the adjacent NP arrays as collector panels to focus incident light of a tunable wavelength range into the central VNT belt and, thus, to achieve a broadening of the bandwidth of the E-field intensity spectrum in the VNTs. This approach utilizes the fact that upon illumination of the nested VNT with light linearly polarized along the x-axis the NPs in the collector arrays reradiate light along the y-axis into the array plane.22,34 The directed emission of optical energy from the collector arrays toward the center of the structure represents a feedmechanism for E-field intensity enhancement in the central VNT belt within a frequency range defined by the periodicity of the NP array. The simulations performed for a nested VNT (d = 170 nm, wx = 20 nm, wy = 25 nm) in Figure 2b illustrate the broadening of the E-field intensity spectrum that stems from the integration of the VNTs into NP arrays. In the calculations, we used the nested array geometry “cropped” to 3 periods (center-to-center separation: 610 nm) in length along the x-axis and 20 periods in y-direction. This configuration retains all the spectral features of the larger nested array investigated experimentally, yet significantly reduces the required computational effort. The spectrum of the nested VNTs, evaluated at the center of the VNT belt, is significantly broader and more complex than the spectra of individual VNTs shown in Figure 1c. In particular, the nested VNT E-field intensity spectrum contains three pronounced near-field resonances at 797, 823, and 868 nm. The E-field intensity distribution maps in Figure 2c−e give insight into the origins of the three peaks observed in the Efield intensity spectrum. The most red-shifted peak at 868 nm corresponds to the excitation of the “trapped-mode” resonance of the isolated VNTs (see a blue spectrum in Figure 1c for comparison) and features pronounced hot-spots only in the VNT belt area. The intensity distributions corresponding to the two peaks appearing on the shorter-wavelength side of the VNT resonance reveal that in these cases the hot-spots in the VNT belt are fed by the energy guided to the belt from the surrounding periodic NP arrays. Coupling between the LSP resonances of the dimers in the VNT belt and the photonicplasmonic resonances of the periodic array results in the formation of collective VNT-array modes. This mode hybridization60 is accompanied by a frequency splitting, as evidenced by the appearance of two rather than one array-induced peaks in the nested VNT E-field intensity spectrum at 797 at 823 nm, respectively (Figure 2b). Spatial maps of the real part of the major E-field component (Ex) in the VNT belt are shown next to the intensity maps in Figure 2c−e to highlight the differences in the near-field phase distributions at different peak intensity wavelengths. The precise resonance wavelengths of the collective VNTarray photonic-plasmonic modes depend on the size of the NPs, the number of dimers per VNT, the interparticle separations in the VNTs, and the grating period of the collection arrays. All of these parameters can be controlled with conventional nanofabrication approaches enabling a rational spectral shaping in nested VNTs. Figure 2c−e shows that the Efield intensity maps of the VNT-array modes and the VNT trapped mode spatially overlap in the gaps between the NPs of the individual VNTs. The spatial colocalization of the near-field associated with the three resonant peaks in the E-field intensity spectrum of the nested VNT, which is in combination with the
tunability of the spectral position of the individual resonances, is of high value for the optimization of SERS substrates.23,31 If one of the resonances is tuned to the laser pump and another to the Raman emission wavelength of the analyte of interest, the nested VNTs are expected to provide strong SERS signal enhancements in the focal points of the VNT structure. Of relevance to this work is the 785 nm wavelength of the excitation laser and the 1077 cm−1 vibrational band (C−S stretch) of the analyte para-mercaptoaniline (pMA) (indicated by dotted lines in Figure 2b). In Figure 3, we investigate the influence of the collector NP array morphology on the collective resonant response that
Figure 3. Tuning the nested VNT spectral response by varying the period of the collector array. (a) A schematic of a linear NP chain with period da and the plane wave illumination conditions. (b) Calculated spectra of the near-field intensity generated in the center of an isolated VNT with d = 170 nm, wx = 20 nm, wy = 25 nm (purple) and in the center of the linear periodic NP chain for different da values (as indicated in the legend). (c) Corresponding E-field intensity spectra of the nested VNT arrays with the collector array of varying period da (same legend as in (b)). Pump wavelength and the Raman line are marked with vertical dotted lines. (d) Poynting vector intensity distribution and a vector powerflow map in the linear periodic chain at the wavelength of its photonic-plasmonic resonance (plotted in the plane cutting through the NP centers). Orange arrows schematically highlight the powerflow enhancement along the chain.
accompanies the integration of VNTs into periodic collector arrays through GMT simulations. The photonic-plasmonic mode of the NP arrays can be progressively red shifted by increasing the array period (an extract of the array is indicated by the individual line in Figure 3a) to achieve a better overlap with the trapped-mode resonance of the VNT belt (Figure 3b). The E-field intensity spectra (evaluated on the surface of the central NP) shown in Figure 3b are calculated for a 20-periodlong linear chain of Au NPs, which corresponds to the length of the nested VNT structure used in the calculations in Figure 2b. 222
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indices of the substrate (glass, nr ≈ 1.5) and superstrate (air, nr ≈ 1.0),61 and assuming an angle of incidence of φinc = 55°. Figure 4a,b compares the measured and simulated scattering spectra of the VNT belt containing 12 individual VNTs with an
We mention here in passing that an increase of the periodic NP array size in both directions enables to boost the intensity of the hot-spots generated on the NPs in a systematic fashion. While Figure 3b compares the spectra of noninteracting VNTs and NP arrays, Figure 3c shows the E-field intensity spectra of the nested VNTs with identical geometrical parameters. As discussed above, the E-field intensity spectrum of the VNT is found to change considerably upon integration with its exact shape depending on the periodicity of the NP array. If the individual resonances start to overlap (see red curve in Figure 3c), the highest peak E-field intensity value results. However, the spectral positions of the resonances can be separated and shifted to optimize the overlap with both the pump laser wavelength and the target Raman line (in this example, the 1077 cm−1 vibrational band of pMA) by adjusting the periodicity of the NP arrays. The additional VNT-array resonances in the E-field intensity spectrum of the nested VNT that are missing in the isolated VNT spectrum arise from an optical powerflow from the NP arrays into the VNT. To visualize this feed mechanism, we plot the vector map of the optical powerflow along the periodic NP chain illuminated at the resonance wavelength of its photonic−plasmonic mode in Figure 3d. The enhanced in-plane powerflow in the direction perpendicular to the E-field polarization is clearly observed. Since the collective VNT-array resonances are driven by the collector arrays through directed light emission, the E-field intensity spectra in Figures 2b and 3c, as well as the spatial distributions in Figure 2c−e are strongly polarization dependent. Only for light polarized along the VNT x-axis (i.e., parallel to the dimer axis) the NP arrays radiate toward the VNT belt (compare with Figure 3d) and can sustain the high E-field enhancement in center of the structure. If the polarization direction is rotated by 90°, the NPs radiate toward the sides and cannot drive the coupled modes in the VNT belt. The response of the individual VNTs is also strongly polarization dependent. An efficient light circulation in the VNTs through coupled vortices is only possible for light polarization along the VNT x-axis. The predicted polarization dependencies are conveniently tested experimentally and can be used for an experimental validation of the electromagnetic coupling mechanisms in the investigated VNT structures. Far-Field Optical Characterization of Nested VNTs. In the first step of a systematic characterization of the far-field responses of nested VNTs, we investigated the scattering spectra of the individual components: the collector array and the VNT belt. The experimental spectra were obtained through the dark-field illumination of the structures through a high numerical aperture (NA = 1.2−1.4) oil condenser, corresponding to an incidence angle φinc between 53 and 72°. The samples were illuminated from the substrate side and the scattered light was detected using a 60× (NA = 0.70) objective. The collected light was dispersed and analyzed using a 300 mm focal length imaging spectrometer. Polarization resolved scattering spectra were acquired by placing a polarizer in the beam-path of the collected light. The intepretation of the recorded far-field scattering spectra was aided through GMT simulations. To simulate the polarization resolved dark-field scattering spectra of nested VNT structures, we averaged over k vectors in the x− z and y−z planes with E-field vectors pointing along the x- and y-axis, respectively (see Figure S1 in the Supporting Information). All simulations were performed with an effective refractive index of nr = 1.25, which averages the refractive
Figure 4. Far-field characterization of VNTs. (a) Experimental scattering spectra of a VNT belt with d = 139 nm; wx = 30 nm; wy = 46 nm illuminated by light polarized along the x-axis (red) and y-axis (blue). Inset: A SEM image of an individual VNT belt used in far-field characterization (Scale bar = 200 nm). The scattering spectra were obtained under dark-field illumination through a high numerical aperture (NA = 1.2−1.4) oil condenser (corresponding to an incidence angle φinc between 53 and 72°) and using a 60× (NA = 0.70) objective to collect the scattered light. (b) Corresponding GMTsimulated spectra of an isolated VNT with the same parameters as in (a) illuminated by a plane wave incident at φinc = 55° and averaged over the in-plane k-vector orientation (see Supporting Information).
average diameter (d) of 139 nm, a height of 70 nm, wx = 30 nm, wy = 46 nm, and the edge-to-edge separation between VNTs is of 556 nm. The spectrum of the emitted light with a linear polarization along the VNT x-axis (red) shows a pronounced dip at 745 nm in the scattering intensity, which is absent in the symmetric spectrum obtained with perpendicular light polarization (blue). The dip in the scattering spectrum indicates an efficient light circulation in the VNTs, which minimizes radiative losses (compare with Figure 1b). A system of coupled optical vortices in the VNTs facilitates an effective storage of the incident optical energy in the E-field of the VNTs at this frequency. Although the exact shape of the scattering spectrum depends on the angle of incidence, the characteristic spectral feature of VNTs remains pronounced for the incident light polarized along the x-axis (see Figure S2 in the Supporting Information). The shape and polarization properties of the simulated spectra in Figure 4b, which reproduce the dip in the scattering intensity, are in excellent agreement with the experimental spectra (Figure 4a). In the next step, we recorded the scattering spectra of the combined nested VNT structures (d = 138 nm, wx = 26 nm, wy = 35 nm, the center-to-center separation between NPs in a collector array = 552 nm). In these experiments, we used a slit in the entrance port (opening ∼1.0 μm) to confine the active area from which light was collected to the center of the nested VNT where the VNT belt is located (solid box in Figure 5a). In a second set of experiments, we recorded spectra from an equally sized area in an adjacent collector array (dashed box in Figure 5a). Figure 5b,c shows representative polarization resolved dark-field scattering spectra from the VNT belt and collector array areas, respectively; the spectrum for light polarized along the VNT x-axis is plotted in red and the spectrum for light polarized along the VNT y-axis is plotted in 223
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Figure 5. Far-field characterization of nested VNT arrays. (a) A SEM image of a nested VNT array with d = 138 nm; wx = 26 nm; wy = 35 nm in the VNT belt; center-to-center distance between NPs in collector arrays = 552 nm. Light was collected from the center of the nested VNT where the VNT belt is located (solid box) and from an adjacent collector array (dashed box). Corresponding experimental scattering spectra of the VNT belt (b) and collector arrays (c) in the nested VNTs. Red spectra were obtained for light polarized along the x-axis while blue spectra correspond to light polarized along the y-axis.
Figure 6. Scattering images of periodic collector arrays (a,c) and nested VNTs (b,d) under whitelight excitation. Excitation light was polarized along the VNT y-axis (top, blue) and VNT x-axis (bottom, red). Scattering intensities along the indicated lines are included next to the scattering images.
blue. As anticipated, the scattering spectra of the nested VNTs are strongly polarization dependent (Figure 5b). The experimental spectrum for light polarized along the VNT xaxis is significantly sharper (full width at half-maximum, fwhm =
124 nm) than the spectrum with perpendicular light polarization (fwhm = 132 nm). Furthermore, the spectrum for light polarized along the VNT x-axis peaks at a resonance wavelength of λres = 683 nm and is overall blue shifted relative 224
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We incubated the nested VNT arrays investigated in this work with a saturated solution of pMA in ethanol for 0.5 h, washed away excess pMA with copious amounts of ethanol and then dry-blowed the sample in a N2 stream. The SERS spectra were acquired with a 40× air objective using a home-built Raman microscope, described in ref 23. We again used a slit with variable opening in the entrance port of the spectrometer to confine the active area (25.1 × 2.4 μm2) on the sample to the center of the nested VNT arrays containing the VNT belt. After a spectrum was acquired, we translated the sample and recorded SERS spectra of the collector NP array. Subsequently, we measured SERS spectra of isolated VNTs that had the same size and geometric parameters as the nested VNTs but were missing the collector NP arrays. These VNT reference structures were fabricated on the same chip as the nested VNTs to minimize any systematic differences between the measurements. The opening of the spectrometer entrance slit was kept constant to ensure an identical active area for all SERS measurements. All spectra were background corrected by subtraction of a spectrum recorded from an adjacent area on the chip that did not contain any Au NPs and baseline corrected to account for the quartz vibrational bands at 810 and 1158 cm−1. Figure 7a shows representative SERS spectra for a nested VNT (red), an isolated VNT belt without collector NP array (blue), and an isolated collector array (green) obtained with unpolarized 785 nm laser light. The nested VNT shows significantly higher SERS intensities than both the isolated VNT belt and the NP array. For the collector NP array the signal intensity is very weak and for the isolated VNT belt the signal intensity is almost negligible. The filling fraction for the nested VNT spectrum is somewhat higher than that of the isolated VNT belt, because in this case some NPs of the collector arrays will also be located in the active area from which the spectrum is recorded. Given the very weak signal intensity in case of the collector NP array, the contribution from these NPs, however, cannot account for the dramatic signal intensity difference between nested and isolated VNT. Instead, we ascribe the significantly higher SERS signal intensity of the nested VNT to the existence of synergistic electromagnetic interactions between the VNT belt and the collector arrays that boost the SERS signal enhancement beyond the values of the individual, isolated components. Since the SERS spectra of both the nested and isolated VNTs are dominated by the VNT belt, their relative signal intensities quantify the magnitude of this synergistic effect. For the 1077 cm−1 band, we observe the maximum gain in signal enhancement by 9.4. This experimental value is in good agreement with the calculated value of ∼7 based on GMT simulations. The calculated enhancement factors for the 1077 cm−1 Raman band of pMA are 4.7 × 105 and 6.8 × 104 in case of the nested and isolated VNTs, respectively. The existence of a strong synergistic SERS signal enhancement in nested VNTs is consistent with their electromagnetic design. One of the resonances of the nested VNT structure was designed to peak close to the 785 nm pump wavelength, whereas the trapped-mode VNT resonance was designed to overlap with the 1077 cm−1 band of pMA. VNT structures that are lacking the collector NP arrays interact only weakly with the pump laser and, consequently, achieve much lower SERS signal enhancements than the nested VNTs. We validated that this observation is reproducible through repeated (up to 5 times)
to the spectrum of perpendicular light polarization (λres = 719 nm). Similar to our observation for the isolated VNT belt, only the spectrum of light polarized along the VNT x-axis shows a dip in the scattering intensity at 777 nm. This dip is again an indication of an efficient transfer of optical energy into the Efield sustained by the VNTs. Different than for the VNTs, the scattering spectra of the collector NP arrays overlap for the two orthogonal light polarizations (Figure 5c) and show no polarization dependency or systematic differences in fwhm and peak resonance wavelength. The scattering spectra of the NP arrays are symmetric and void of any dips in scattering intensity. The polarization dependent scattering properties of nested VNTs are further illustrated by comparing the scattering images of nested VNTs and regular collector arrays acquired with orthogonal orientations of a polarizer in the beam path of the scattered light (Figure 6). Whereas the periodic collector arrays appear as homogeneous areas for light polarized along the y- or x-axis (Figure 6a,c, respectively), the nested VNT arrays show polarization dependent differences in the spatial intensity distributions. For y-polarized light the central VNT belt in Figure 6b has a lower scattering intensity than the surrounding collector arrays and is discernible as a black horizontal streak in the center of the nested VNT arrays. For x-polarized light the scattering intensity at the VNT belt is, however, higher than in the neighboring collector arrays and creates a bright streak in the center of the nested VNT structure in Figure 6d. This observation is consistent with the NP arrays’ role as collector panels that guide light from surrounding areas into the central VNT belt. The NPs in the arrays reradiate the incident light preferentially into directions perpendicular to the E-field associated with the excited LSP modes. In the case of xpolarized light, this mechanism results in a light concentration in the center of the nested VNT structure where it creates the observed high scattering intensity in the VNT belt. Validating E-field Enhancement and Electromagnetic Feed-Mechanism in Nested VNT Arrays through SERS. So far, our experimental characterization of the nested VNTs and their individual components was limited to far-field measurements. To further characterize the predicted near-field response of nested VNTs, we will in the next step augment our far-field studies with SERS measurements. The SERS signal enhancement of a molecular vibration scales as the product of the Efield intensity enhancement at the pump and emission wavelength and is therefore a sensitive probe to monitor the E-field enhancement through VNTs at specific wavelengths.28 We focus in this section on a (d = 169 nm, wx = 21 nm, wy = 28 nm) nested VNT with 24 × 49 NP collector arrays (centerto-center separation: 619 nm) as SERS substrate, since this array period provided the overall strongest signal intensities for the 1077 cm−1 band of the analyte pMA with the chosen VNT configuration. The center-to-center separation of the NPs in the collector arrays in this nested VNT is slightly longer (619 vs 610 nm) than the optimum separation predicted by the GMT simulations (Figure 3c). We emphasize, however, that the simulations were performed for VNTs built from spheres (instead of nanodiscs) embedded in a homogeneous medium. Although the simulations provide a qualitatively correct description of the morphology dependence of the electromagnetic response, the predicted structural parameters are approximate and can be expected to require minor adjustments to optimize the nested VNT performance. 225
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confirmation of the electromagnetic feed-mechanism of the Efield enhancement in the VNT belt through the NP collector array. We conclude that the experimental SERS studies are consistent with the feed-mechanisms of the E-field intensity in the central VNT belt through the collector array as predicted by our GMT simulations. The primary goal of this exploratory study was to experimentally validate the electromagnetic design approach of nested VNTs. In the future, the performance of these electromagnetic materials can be systematically enhanced for SERS applications through optimization of the configuration of the embedded VNTs. Conclusions. Vortex Nanogear Transmissions are a new design approach for plasmonic nanostructures that tries to mold the internal powerflow around singular nodes of the Poynting vector within the structures. We demonstrated that an efficient light circulation within VNTs achieves long dissipation times and generates high E-field enhancements. We designed and fabricated VNTs, and their measured far-field scattering spectra contain a characteristic intensity dip, which indicates an efficient circulation of the electromagnetic energy within the structure. We verified that the E-field enhancement generated by VNTs can be further increased through integration of VNTs into regular nanoparticle arrays. The latter serve as collector panels for the incident light, which they then focus into the central VNT belt where it further enhances the local E-field intensity.22,62 These nested VNTs exhibit distinct maxima in their near-field spectra, which are amenable to a rational spectral tuning through the configuration of contained VNTs and the morphology of the collector arrays. In this work, the geometrical parameters of the nested VNTs were chosen to provide high E-field enhancements at both the 785 nm laser pump wavelength and the 1077 cm−1 shifted Raman band of the small molecule para-mercaptoaniline, which we used as analyte for surface enhanced Raman spectroscopy (SERS). We characterized the nested VNT structures through elastic and inelastic scattering spectroscopy and confirmed that the E-field intensity enhancement provided by the feed-mechanisms through the collector arrays can be modulated through control of the light polarization.
Figure 7. Validation of electromagnetic feed-mechanism through collector NP arrays in nested VNTs using SERS. (a) Experimental SERS spectra of pMA from a nested VNT (red), an isolated VNT belt without collector NP array (blue), and an isolated collector array (green) obtained with unpolarized 785 nm laser light. (b) SERS spectra of a nested VNT array for laser excitation polarized along the VNT x- (red) and y-axis (blue). Insets show histograms of the SERS intensities of the 1077 cm−1 band from up to 5 measurements with error bars (absolute errors).
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SERS measurements on the prepared sample (inset of Figure 7a). On the basis of our electromagnetic simulations and the performed experimental far-field scattering studies of the nested VNT structures, we anticipate that the E-field intensity feedmechanisms of the central VNT belt through the collector array are strongly polarization dependent. To experimentally verify the possibility to modulate the E-field enhancement in the VNT belt by switching the energy transfer from the collector array on and off through control of the polarization of the incident light, we recorded SERS spectra for different polarization of the incident 785 nm laser light. Figure 7b compares the SERS spectra recorded from a VNT belt in a nested VNT structure obtained with the excitation light polarization pointing along the VNT x-axis (red, feedmechanism on) and along the VNT y-axis (blue, feed mechanism off). We emphasize that we measured the laser power for both laser polarizations and found it to be slightly lower in case of the x-polarized light. Nevertheless, we observed SERS signal enhancements at 1077 cm−1 that are higher by a factor of 2.3 for x-polarized excitation light. The observation of higher SERS signal intensities for an excitation laser polarization pointing along the VNT x-axis is an independent
ASSOCIATED CONTENT
S Supporting Information *
Figures S1 and S2 and Methods. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
These authors contributed equally to this work.
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ACKNOWLEDGMENTS This work was supported by the National Institutes of Health through Grant 5R01CA138509-03 and the National Science Foundation through Grants CBET-0853798 and CBET0953121.
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REFERENCES
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