Distributing the Optical Near‐Field for Efficient Field‐Enhancements in Nanostructures

Abstract

Circularly polarized light imparts a sense of rotation on the electron density in ring-shaped gold nanostructures. As a consequence, the near-field enhancement becomes homogeneous on the surface of the nanostructures, thereby increasing the opportunity for interaction with molecules. This type of nanostructured samples can find a broad range of applications in chemical processes where the interaction between molecules and local field enhancements play an important role. When molecules approach a nanostructured metal surface that is illuminated by visible light, these molecules experience a very intricate electric field. On the one hand, the field results from light itself, which is incident on the nanostructures, scattered and diffracted from them or reflected from their interfaces. On the other hand, light's electromagnetic oscillations drive the electron density within the nanostructures and, in turn, these variations of electron density constitute the source of a very inhomogeneous surface electric field–the local field.1 Molecules placed in the local field experience electric forces along the field lines, which become very crowded in two sorts of regions on the nanostructures. First, in the sharp regions, such as corners or tips, where the field lines are pulled together by geometric constraints. And second, in the regions of highest electron density, where the electron charges constitute strong sources for the local field. These regions of crowded field lines, at the nanostructured metal surface, correspond to local field enhancements and are commonly referred to as “hotspots”. One of the reasons why such hotspots are very important, is because of what can happen to molecules within them. Within hotspots, molecules can experience chemical transformations,2 photochemical reactions,3 catalytic reactions4, 5 and extreme enlargement of their optical properties.6, 7 A particularly spectacular example is surface enhanced Raman scattering,8 whereby the Raman signal can be increased up to fourteen orders of magnitude.9, 10 Unfortunately though, hotspots suffer from two limiting factors–they can become too hot and they are only spots. Indeed, in an attempt to achieve ever greater enlargement of the optical response of molecules, the incident light intensity can be increased. Besides the desired increase of the local field intensity however, an undesired increase of nanostructure heating occurs. The heating is due to the local electric currents that are associated with the oscillations of the electron density, according to Joule's effect. It has thus been shown that, in the hotspots, laser-induced heating can melt the nanostructures and even give rise to hydrodynamic processes.11, 12 Therefore, there is an upper limit to how much the local field can be enhanced within a hotspot without inducing irreversible damage to the surface of the sample. In order to further benefit from enlarged optical response of molecules, a better choice might be to nanoengineer larger spots. Recently, there has been considerable efforts towards engineering local field enhancements. Hotspots can thus be obtained at randomly roughened surfaces,13, 14 through plasmon excitations,15 due to nanostructuring,16 or by making use of antenna designs.17 In all of these cases, local field enhancement occurs only within a small region of the sample surface. From a practical point of view, it would be very interesting to expand the local field enhancements to the whole sample surface, thereby increasing the opportunity for interaction with molecules. Because it originates from the inhomogeneity of electron density in the nanostructures, such an expanded field can still be called “local” in nature, however it would no longer be localized in small spots. To avoid any confusion, from now on, we shall designate this field as the “near-field”. As we shall prove in this work, an effective approach to obtain near-field enhancements over the entire sample surface is to drive the electron surface currents along ring-shaped nanostructures. Here we demonstrate that circularly polarized light imparts a sense of rotation on the electron density in ring-shaped gold nanostructures. As a consequence, the near-field enhancement becomes homogeneous on the surface of the nanostructures, thereby increasing the opportunity for interaction with molecules, see Figures 1a and 1b. Interestingly, for linearly polarized light oriented along the Y and X directions, a pattern of pronounced hotspots indicates strong coupling between the nanostructures along these directions. Upon rotating the direction of linearly polarized light, the pattern of hotspots does not always follow the direction of polarization but instead appears “pinned” by the strong coupling along the Y and X directions. Consequently, while for circularly polarized light the near-field is homogeneous, for randomly distributed linearly polarized light the near-field is inhomogeneous. This difference reflects the difference in electron density behavior, depending on the nature of light polarization. The work presented here is intended as a proof of principle; by varying the dimensions and the shape of the metallic rings, further device optimization can be obtained. As a guideline for such device optimization, we propose a quantitative figure or merit for homogeneity of surface near-fields. This type of nanostructured samples can find a broad range of applications in chemical processes where the interaction between molecules and local field enhancements play an important role. In order to maximize the useful sample surface, we avoided using circular rings, which do not pack very efficiently, and instead preferred square-rings. Increasing the interaction cross-section between molecules and enhanced optical near-field in nanostructures. In (a), in response to linearly polarized incident light (red arrow), local field enhancements of the electric field form hotspots on the surface of square-ring gold nanostructures. For circularly polarized incident light (red oriented circle), the electric local field enhancements are homogeneous on the entire surface of the nanostructures. In (b), the second harmonic generation matches the pattern of electric local field enhancements for both linearly and circularly polarized light. In (c), scanning electron microscopy image of the fabricated gold nanostructures. The square-ring shaped nanostructures are made of gold (30 nm thick) grown on top of SiO2 (100 nm)/Si(001) substrate, and are produced by electron beam lithography. More details on the deposition and on the rest of the sample preparation can be found in Ref. 18. The outer side of each square measures 1000 nm, the width of the metallic stripes is 200 nm, and all the nanostructures are 200 nm apart from each other, as shown in Figure 1c. The total area of the sample is 2.5 by 2.5 mm. When these structures are illuminated at the wavelength of 800 nm, a plasmon resonance is excited. The presence of the resonance was evidenced by numerically simulating the reflection and transmission spectra of the nanostructures, see Figure 2a. Moreover, for the purpose of completeness, the same nanostructures were deposited on a transparent substrate (glass), so that reflection and transmission spectra could also be experimentally verified, see Figure 2b. It should be noted that the small difference between the experimentally and computationally determined spectra is due to the fact that we did not include the substrate in the numerical simulations. As Figure 2 illustrates, this difference increases with the wavelength due to the frequency dispersion of both the index of refraction and of the absorption coefficient of the substrate. Despite the small difference in peak positions, the numerical simulation are in excellent agreement with the experimental measurements. At resonance, reflection from different interfaces, scattering, diffraction and various near-field enhancements can be observed. All of these phenomena are rigorously taken into account in numerical solvers of Maxwell equations, which are therefore suitable for mapping with high resolution the electromagnetic fields. For linearly polarized incident light, reflection and transmission spectra from the square-shaped gold nanostructures were numerically simulated and experimentally measured as is it shown in (a) and (b), respectively. In order to accurately map the near-field enhancements at the surface of the gold nanostructures, we perform two independent numerical simulations. The first one, performed with RSoft's DiffractMOD,19 evaluates the electric fields in air, 1 nm above the gold surface. The second one, performed with MAGMAS,20, 21 evaluates the electric currents inside the gold layer, 1 nm below the surface. Both simulations assume monochromatic wave excitation under normal incidence (the wavelength of the incoming light is 800 nm) and periodic boundary conditions, which significantly reduces the simulation time. Figures 3a and 3b show the electric field intensity for vertically and horizontally polarized light, respectively. In both cases, intense hotspots can be observed, corresponding to near-field enhancements on the opposing sides of the square-rings that are perpendicular to the direction of polarization. Because, in the case of circularly polarized light, both vertical and horizontal field components are present in equal amplitudes (yet out-of-phase by π/2), one might initially expect circularly polarized light to simply produce a superposition of the field profiles in Figure 3a and 3b, i.e. four hotspots, one on each side of the square-rings. Figures 3c and 3d show, however, that this is not the case. Our result indicates that the phase between the vertical and horizontal field components of circularly polarized light plays a crucial role in the generation of the near-field. Numerical simulations of the electric fields and currents indicate the manner in which charges are driven in response to the polarization state of light. From (a) to (d), numerical simulations of the electric (near)fields in a plane immediately above the air/gold interface. From (e) to (h), numerical simulations of the electric currents in a plane immediately below the air/gold interface. In each case, the direction of polarization for the incident light is indicated with a white arrow or oriented circle. The color-scales are linear. The behavior associated with circularly polarized light is clearly not a direct superposition of the patterns for horizontal and vertical linearly polarized light that are shown above. Indeed, for circularly polarized light, Figure 3c and 3d demonstrate that instead of exhibiting hotspots, the electric field intensities are spatially much more homogeneously distributed than in Figures 3a and 3b. A more careful examination of Figure 3c reveals that the near-field enhancement pattern has a sense of rotation that is mirrored in Figure 3d, even though the nanostructures are clearly not chiral. Consequently, it is the chirality of light that is imparted onto the near-fields of the nanostructures. In order to better understand this physical mechanism, we can examine the distribution of the electric currents, which are induced by light at the surface of the nanostructures. Upon examining the distribution of electric currents at 1 nm below the surface of the nanostructures, the similarities with the electric near-fields become obvious. First, for linearly polarized light, there are hotspots on the square-ring sides that are perpendicular to the direction of polarization, see Figures 3e and 3f. And second, for circularly polarized light, there is a sense of rotation in the pattern, which can be related to the direction of circularly polarized light. Henceforth, Figure 3 clearly illustrates the agreement between two complementary theoretical approaches. This is a strong indication for the validity of the results, however for the sake of scientific rigor, an experimental confirmation is necessary. The experimental study is performed with second harmonic generation (SHG) microscopy, a technique whose usefulness for visualizing field enhancements in nanostructures has been demonstrated in several works already.22-27 We use ultrafast pulsed lasers (∼100 fs, 82 MHz), which are capable of delivering very high electric field intensities to the electron population in the nanostructures. When the intense electric field of light becomes comparable with the electric near-fields, the oscillations of the electron density are driven in the nonlinear (anharmonic) regime. This nonlinear regime can be described by a mathematical expansion in series, corresponding to higher order frequency terms. The second harmonic generation technique consists in measuring the term in the nonlinear expansion series that is associated with the double frequency of the illumination. An intuitively very appealing explanation for this technique is to consider that, in response to the electric field of light, most electrons within the nanostructures are driven in the linear regime. However, in the hotspots, due to near-field enhancements, the electrons are driven in the nonlinear regime. As a consequence, second harmonic signal is generated precisely from the regions of near-field enhancements. In essence, the technique offers built-in light sources for the near-field enhancement signal, at a frequency that is easily separable from any noise at the illumination frequency, such as scattering, diffraction or reflection. Therefore, SHG microscopy can accurately map near-field enhancements in nanostructures, see Figure 4. Second harmonic generation (SHG) matches the electric local field enhancements. In (a) and (b), SHG microscopy pictures for vertical and horizontal linearly polarized light, respectively, reveals hotspots that are mostly paired along the direction of polarization. In (c) and (d), SHG microscopy for right- and left-hand circularly polarized light shows a homogeneous SHG response from the surface of the square-rings. In (e) and (f), in response to linearly polarized incident light at + and –45°, respectively, the SHG signal is inhomogeneous. In each case, brighter and darker stripes can be observed along the direction of polarization. For clarity, the border between some of the stripes was highlighted with white lines. In all panels, the white arrows and oriented circles indicate the direction of polarization. Moreover, for clarity, the geometry of 9 nanostructures has been highlighted with white lines. Figure 4a, presents a SHG micrograph from the top left corner of the nanostructure array, for incident light linearly polarized along the vertical direction, as indicated by the white arrow. An array of hotspots can be observed. Most of them occur in pairs, which are oriented along the direction of polarization. Upon superimposing the layout of the nanostructures on the SHG micrograph, it becomes clear that the location of these hotspots corresponds to the sides of the square-ring nanostructures, which are perpendicular to the polarization. The same trend can be observed for horizontally polarized light, as can be seen in Figure 4b. It should also be noted that the paired hotspots are significantly brighter than the singles ones, which are situated at the edge of the array. This difference in intensity indicates that near-field coupling (most likely of antenna type) takes place between the nanostructures, reinforcing the near-field enhancements. These experimental observations of hotspots are in perfect agreement with the theoretical simulations for linearly polarized light. What about circularly polarized light? Figure 4c shows a SHG micrograph from the same regions of the sample, at the same illumination intensity, but, this time, upon illumination with circularly polarized light. Contrary to the case of linearly polarized illumination, here, no hotspots can be observed. Instead, the SHG signal is rather homogeneous throughout the entire array, the observable small variations being attributable to fabrication defects of the array. Within this array, all individual square-rings are clearly distinguishable. Note that in Figure 4a and the sides of the on which there is no field enhancement are not at Because all in Figure were obtained for the same we can that the why the entire are in Figures 4c and is because of a homogeneous field enhancement throughout the nanostructures. Moreover, in both Figures 4c and the SHG signal is from the of the array. It that, upon illumination with circularly polarized light, coupling is also place between the nanostructures. Because these nanostructures are 200 nm this be a direct coupling and is more likely of antenna the of 200 nm is comparable to the resolution limit of the SHG the SHG signal is simply a superposition of the four hotspots for vertical horizontal linearly polarized light, which appears homogeneous due to optical resolution The to this important is to the results for circularly polarized light with for randomly oriented linearly polarized light, with to polarization along and Figures and 3d SHG for linearly polarized light along and –45°, respectively. As in the case of circularly polarized light, the nanostructures are clearly but there is an important Indeed, as it is by the white lines the nanostructured array, in the case of linearly polarized light at the SHG signal is no longer Figures 4a, and could the that the pattern of hotspots in these nanostructures always the direction of linearly polarized in this is from being the case. For all polarization the pattern of hotspots appears “pinned” by the strong coupling along the Y and X directions, see the complementary Consequently, circularly polarized light is in order to achieve homogeneous field enhancements. This strong experimental in with both of numerical indicate that circularly polarized light is capable of the electron density along ring-shaped nanostructures. we this a proof however, there are two that to be the of the SHG First, we that the SHG signal in Figure is due to electric field enhancements, but what about the This is an important we found that SHG from hotspots that to both electric and field Numerical simulations of the fields at the surface of the square-rings are shown in Figure More Figures and demonstrate that, for linearly polarized light, the field hotspots are situated on the sides of the square-rings that are to the direction of polarization. In other in these nanostructures, electric and field enhancements are clearly distinguishable. A of these fields with Figure 4a and shows that the SHG originates from the electric and not the field enhancements. The same is by examining the location of the field enhancements observed in the case of circularly polarized light, see Figures and of a rather homogeneous field enhancements, as it is the case for electric these pronounced are not present in the SHG signal for circularly polarized light. The of the near-field enhancements are clearly from the electric In (a) and (b), for vertically and horizontally linearly polarized incident light, respectively, the are situated to the direction of polarization. In (c) and (d), for right- and left-hand circularly polarized light, respectively, the field patterns a superposition of the patterns associated with vertical and horizontal linearly polarized light. In (e) and (f), scanning electron microscopy of the nanostructures illumination with intensity above the damage In both the direction of linearly polarized light is indicated with a white arrow and the regions indicate the location of highest field enhancements. in Figures 4a and the nanostructures are not visible and, in order to the of the SHG hotspots, we the layout of the nanostructured array on top of the is this In order to precisely the SHG hotspots, we increased the to 1 i.e. above the damage of these nanostructures. The results for vertical and horizontal linearly polarized light can be seen in Figures and respectively. These present scanning electron microscopy of the nanostructures, obtained illumination. In each case, can be observed on some of the square The regions correspond to the same regions, which are indicated by superposition of the layout in Figure 4a and 4b. 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