Combining materials with ferromagnetic and plasmonic properties has recently become an active area of research.1–12 The generation of so-called magnetoplasmonic materials allows one to study a wide variety of physical phenomena in which both characters (magnetic and plasmonic) are intertwined, i.e., it is possible to control one by acting on the other and vice versa.13 For example, the plasmonic characteristics of different kinds of magnetoplasmonic structures can be tuned by the action of an external magnetic field, basically by its capability of acting on the off diagonal elements of the dielectric tensor of the system, which, for the ferromagnetic material case, are sizable.5, 10, 12, 14, 15 On the other hand, it is also possible to enhance the magneto-optical (MO) activity in continuous films and nanostructured magnetoplasmonic systems upon excitation of their characteristic surface plasmon resonances.1, 2, 5–7, 9, 11, 12, 16, 17 In this second case, since the MO activity is proportional to the electromagnetic (EM) field in the MO active material,2, 17–19 the physical mechanism lies on the light harvesting properties of the plasmonic system, which effectively concentrates the EM field in the MO active material region.
From an applied point of view, materials with increased MO activity are relevant in the context of telecommunications, where they could be used to construct ultrafast optical switches and integrated optical isolators/circulators elements; they also offer a potential for the development of high-speed MO spatial light modulators for high density holographic data storage and 3D displays and they could serve for magnetic field sensing as well.20 Obtaining these materials is therefore pertinent for several application niches, and magnetoplasmonic systems constitute a very promising choice. Different kinds of magnetoplasmonic materials have been proposed so far, with the plasmonic material being a transition metal (Au, Ag, etc.) and the ferromagnetic component either a metal (Fe, Co, Ni)5, 6, 9–11, 13–16, 21, 22 or an oxide (Fe or Co oxides or garnets).7, 12, 23 In these magnetoplasmonic materials the enhancement of the MO activity comes along with an increase of the optical absorption, which limits their use in transmission related devices. The current situation still allows for further improvement, either by additionally increasing the MO activity or by the reduction of the optical absorption.
Here we present our approach to reach this objective by performing a strong EM field redistribution at the nanoscale in metal-dielectric magnetoplasmonic materials, which takes place by inserting dielectric (SiO2) layers into Au/Co/Au nanodisks in adequate positions. This dielectric layer insertion gives rise to hybrid plasmonic modes with a new distribution of the EM field.24–26 We will show how by selecting the position of the dielectric layer inside the structure we have an effective way to increase the EM field in the MO active components (Co), while reducing it in the other non-MO active, lossy elements, providing a system with enhanced MO activity and moderate absorption.
Three structures, Au/Co/SiO2/Au, Au/SiO2/Co/Au, and Au/Co/Au nanodisks, as sketched in Figure1a, were fabricated by means of colloidal lithography and evaporation (see Experimental Section). The thickness of each layer was 15 nm for Au, 10 nm for Co, and 20 nm for SiO2. An identical amount of Au and Co was used in the Au/Co/Au structure in order to compare the modification induced in the MO response by the insertion of the SiO2 layer. Figure 1b shows a representative atomic force microscopy (AFM) image of one of the fabricated systems. An ensemble of nanodisks randomly distributed over the surface is clearly observable, with an average density around 7.5 × 108 disks cm−2 and an average interdisk distance of 350 nm. This warrants the lack of electromagnetic interdisk interaction. The actual shape of the nanodisks is that of truncated cones25, 26 and approximate values of the upper and lower nanodisk diameters are 70 nm and 110 nm respectively, with slightly larger values for the fully metallic nanodisks.
In Figure2a we show the extinction spectrum at normal incidence for the Au/Co/SiO2/Au nanodisk structure (solid line). The spectrum has a clear extinction peak around 750 nm, the position which from now on we denote as the HWP (high wavelength peak), and a shoulder around 600 nm (LWP, low wavelength peak), which is consistent with the results obtained in Au/SiO2/Au nanodisks,24 with some small differences due to the presence of Co, which damps and blueshifts the resonances.13 The presence of Co in the structure also makes it exhibit magneto-optical activity, whose spectral dependence in the same wavelength range is shown in Figure 2c. Here we depict, as a continuous line, the measured intensity of the magneto-optical activity IΦ for this structure, defined as the modulus () of the complex Kerr rotation of the structure, Φ (Φ = θ + iϕ), where θ is the Kerr rotation and ϕ is the Kerr ellipticity. As it can be observed, two features are present: one in the HWP region and another one in the LWP region. For comparison, the extinction spectrum and the MO activity for the Au/Co/Au nanodisks sample are also plotted in Figures 2a,c as dashed lines. As can be observed, this sample has only one peak located between the two features of the Au/Co/SiO2/Au nanodisk structure.27 This is related to the localized surface plasmon resonance (LSPR) of the Au/Co/Au nanodisks, which induces an enhancement in the MO activity.6 The effect of inserting a SiO2 layer into the Au/Co/Au nanodisk is to split this resonance into two resonances, originating from the localized plasmons of the top (Au/Co) and bottom (Au) metallic nanodisks separated by the SiO2 layer. In this structure, one sees a correspondence between optical extinction and MO activity: the higher the extinction, the stronger the MO activity, which is consistent with previous studies in this kind of systems.6, 7, 11
On the other hand, if the SiO2 layer is placed in between the top Au layer and the Co layer, i.e., if the nanodisk structure is Au/SiO2/Co/Au, a drastic change in the optical response is observed with respect to the previous case. As can be seen in Figure3a, the spectrum appears reversed, exhibiting a peak in the LWP region with only a shoulder on the region of the HWP. The complex Kerr rotation spectrum (Figure 3c) is, however, very similar to the spectrum of the preceding sample, with a peak in the MO activity in the spectral region of the HWP and a shoulder in the LWP region. Remarkably, this implies that in this specific structure we obtain high MO activity for a configuration corresponding to low optical extinction in the HWP region, contradicting the naïve rule of thumb that a larger absorption provides a larger MO activity.
These results indicate that first, the insertion of the SiO2 layer within the nanodisk structure induces strong modifications in their extinction properties, and second, the specific position of this layer allows control of the spectral region where the optical extinction is maximized or minimized. Regarding the MO activity for both nanodisks samples, it is larger than that of the fully metallic structure with an equivalent amount of Au and Co in the nanodisks and, moreover, a larger extinction does not necessarily implies a larger MO activity.
In order to obtain further insight of the physical mechanism underlying these effects, we performed numerical simulations (see details in the Experimental Section). The results for the extinction and MO activity are depicted in Figure 2b,d, and Figure 3b,d, showing an excellent agreement with the experiments, therefore confirming the validity of the used modeling to describe the experimental structures. Since both optical absorption and MO activity are ruled by the magnitude of the EM field in the optically absorbing and MO active components of the systems respectively,2, 17–19 in Figure4 we also show the spatial distribution of the intensity of the EM field (namely the square of the modulus of the electric field) along the Y–Z plane for both metal-dielectric structures at the LWP (left) and the HWP (right) spectral regions. As a reference, the EM field distribution of similar Au/Co/Au nanodisks can be found in ref. 17. Regarding the Au/Co/SiO2/Au nanodisk structure, we can see that the amount of EM field for both spectral regions is larger in the bottom Au disk. Additionally, comparing both spectral regions, the EM field is larger for the HWP region than for the LWP region. This explains the larger optical absorption obtained in the HWP region for this structure. On the other hand, for the Au/SiO2/Co/Au nanodisk structure, the EM field for both LWP and HWP regions is larger near the top Au disk. Comparing both spectral regions, the EM field now is larger in the LWP region than in the HWP one. Again, the larger EM field concentration in the absorbing components of the structure in the LWP in this specific case allows one to understand the larger optical extinction observed in this spectral region. On the other hand, the values of the total intensity of the EM field inside the Co layer (related to the MO activity) are very similar for the two structures and the two spectral regions, which explains the similar MO activity observed for the two samples.28 These findings indicate that, while the EM field in the whole structure governs the extinction response of the system, the value in the Co layer is the main responsible of the MO activity and therefore both magnitudes can be decoupled providing us a way to design magnetoplasmonic systems with low absorption and high magneto-optical activity.
For transmission related applications, it is convenient to simultaneously compare the MO activity and optical extinction, by defining a relevant figure of merit, which can be the ratio between the MO activity and the optical extinction, MO/Ext. This magnitude takes into account on one hand the pure MO activity of the structure and on the other its optical losses. In Figure5 we present the spectral dependence of this ratio, obtained from the experimental data, for the two metal- dielectric structures, with the corresponding equivalent structure without the SiO2 spacer. Regarding the Au/Co/SiO2/Au structure, two clear peaks are observed around LWP and HWP. The HWP peak is due to the characteristic large MO activity of the structure in this spectral region, while the LWP is due (in spite of the lower MO activity) to the low optical extinction at this wavelength. On the other hand, for the Au/SiO2/Co/Au structure, mainly a single peak, more intense than those obtained for the other structure, is observed in the HWP region, which is due to the simultaneous large MO activity and low optical absorption characteristics for this wavelength. The influence of inserting a dielectric layer in this figure of merit can be more clearly seen if we compare it with that for the fully metallic structure, which exhibits lower values than those for the metal-dielectric structures, and a mainly featureless spectral behavior. In this case, the weak maximum in the HWP region is simply due to the low extinction values of the fully metallic system in this region, but with no relevant MO activity being present.
For a comparison of these metal-dielectric magnetoplasmonic nanodisks with other MO materials relevant in transmission related applications, we have calculated the modulus of Faraday rotation divided by the transmission. In the spectral range where the metal-dielectric structures have low extinction this ratio is of the same magnitude than the calculated for a Bi-substitute YFe2O5 garnet structure29 and higher than the calculated one for materials made from Fe or Co oxide nanoparticles embedded in a dielectric matrix.30, 31 This opens the door to the use of magnetoplasmonic materials in such devices. Moreover, since the wavelength position of the resonances obtained in these metal-dielectric nanodisks can be controlled by simply modifying their structural parameters (diameter, layer thickness), it will be possible to tune the spectral position at which the optical absorption is reduced and the MO activity maximized.
In conclusion, simultaneous large MO activity and low optical losses are obtained in Au/Co/Au magnetoplasmonic nanodisks in which a SiO2 dielectric layer is inserted at specific positions within the nanostructure. These effects are particularly important at wavelengths where characteristic resonant modes of the nanodisks are excited. The main effect of inserting the dielectric layer in the nanodisks is a strong redistribution of the electromagnetic (EM) field inside the structure upon plasmon resonance excitation, increasing the EM field in the MO active layer (Co) with its simultaneous reduction in the other absorbing but non-MO-active components of the system. The polarization conversion versus optical extinction figure of merit of these novel systems could be spectrally tuned by an adequate selection of the internal parameters of the nanodisks.