We present a large area (1 cm2) nanoimprinted metamaterial comprising a fishnet structure and its Babinet complement, which shows giant cross polarization. When illuminated with s-polarized light, the reflected beam can be p-polarized up to 96%, depending on the azimuthal orientation of the sample. This experimental result is close to the result of numerical simulations, which predict 98.7% of cross-polarization. It is further shown, that 95-100% cross polarization is only achieved in the case when the fishnet is combined with its Babinet complement. Each structure alone (either an ordinary fishnet or a plane with metallic rectangles only) shows substantially less polarization conversion.
© 2015 Optical Society of America
Long before the term “metamaterial” was coined, it had already been observed that sub-wavelength sized meander-type metallic lines can lead to a substantial polarization conversion for microwave radiation . This allowed producing ultrathin polarizers, optically active elements (rotators for linear polarization) and also converters from linear to elliptic or circular polarization. Several geometries of – meanwhile called – metamaterials have been suggested to carry out such action in the GHz and THz range, including anisotropic (but a-chiral) materials [2–7] or chiral materials [8–15].
A notable, quite early report (1990) on polarization conversion in the visible range was published by Bryan-Brown and associates, who achieved up to 66% of polarization conversion simply by rotating an ordinary metallic grating (as used, for instance, in a spectrometer) such that the grooves of the grating were no longer parallel or perpendicular to the plane of incidence . With increasing manufacturing capabilities on the micro- and nano-scale, optically active metamaterials with more advanced geometries became accessible in the IR and visible range of the spectrum. Again, the publications can be sorted into those reporting on anisotropic (but a-chiral) structures [17–23] or on structures that show a handedness [24–30]. Further, it was shown that noble metal nanorods in aqueous solution are apt to convert the polarization of visible light in the scattered beam .
Recently, the ‘guinea pig’ of metamaterials, metallic split ring resonators, have been theoretically compared with their Babinet complements (split rings “out of air”, cut out of thin metallic films) [32,33], and their use as frequency selective polarizers has been suggested . The real thriving thing is, however, a combination of a metamaterial design with its Babinet complement in one and the same metamaterial structure. For instance, Navarro-Cia and associates have combined alternating layers of metallic double-split ring resonators and their Babinet complements in order to create a metamaterial with ultralow group velocity in the GHz range . Hannam and associates have calculated the optical activity of a Swiss cross structure combined with its Babinet complement  and predicted a polarization rotation of up to 20°, and Zhu et al. showed similar theoretical results in the optical range . Experimental verification in the GHz range was published very recently . Reports on experimental results on metamaterials comprising the positive structure and simultaneously the negative Babinet complement for applications in the visible spectral range are scarce. In one report, a metamaterial was combined (only) with its “semi”- Babinet complement (i.e. a complement which misses some features), which easily allows for chiral structures and which shows some circular dichroism . Full Babinet-complemented structures were shown to act as ultrathin polarizers with a extinction ratio better than 1:1500 in the IR , or were used as ultrathin coloration coating for plastic consumer products , or as fluorescence enhancers [41,42]. However, in none of those publications cross polarization in general or polarization rotation in particular was reported for visible light.
In this contribution, we show that giant cross polarization can be achieved in the visible spectral range by combining a silver (Ag) fishnet structure with its Babinet complement. When the fishnet (of rectangular holes) is rotated by about 56° with respect to its intrinsic coordinate system, the reflected light comprises of up to 96.5% of p-polarized light when the illumination is s-polarized. This experimental result is nicely confirmed by numerical simulations, which predict 98.7% of cross-polarized light. These results hold for a double-fishnet structure, combined with its Babinet complement. Each constituent alone shows substantially less cross polarization in the reflected beam, confirming the intrinsic necessity of the combination of a double layer fishnet with its complement. Numerical simulations also predict that a single-layer fishnet combined with its complement should show up to 99.6% of cross polarization. In a sense, this would be the most straight forward Babinet-complemented structure: a metallic film with holes combined with “holes in air, filled with metal”. However, we were not able to achieve such high polarization conversion in experiments. Only 70.2% were achievable in this case. This could be explained by the fact that a single layer of Ag is less stable, specifically against oxidation and sulfidation. Nevertheless, the results show a promising approach towards ultrathin polarization-rotating devices for micro-optical systems. Further, we have produced the cm2 large samples using nanoimprint lithography and subsequent evaporation of the metal. Both techniques are comparatively cheap and certainly further up-scalable, so that low cost, ultrathin polarization converters of large size will become feasible using nanoimprint lithography.
2. Experimental methods
The sample is a three dimensional structure on a silicon substrate, prepared by UV nanoimprint lithography. It is periodic in both lateral directions, x and y, with pitch sizes of 600 nm and 500 nm, respectively. The details about the imprint master and the fabrication process are described elsewhere . After imprint and etching of the residual photoresist layer, there was a 420 nm by 230 nm by 160 nm (dimensions in x, y, and z, respectively) cubic plateau of photoresist located in the center of each unit cell (UV-Cur21 on top of a thin LOR 1A layer, both from Micro Resist Technology GmbH, light blue in Fig. 1(a,b)). A double layer of silver, separated by SiO2 (Ag/SiO2/Ag with thicknesses of 40 nm/20 nm/40 nm, respectively) was deposited on the sample surface by electron beam sputtering. The structure of one unit cell is schematically illustrated in Fig. 1(a). It can be seen that the Ag/SiO2/Ag layers consist of two parts in space. The lower part is directly in contact with the substrate, forming a traditional double layer fishnet structure . The second part (in rectangular shape) is placed on the plateau of the photoresist, and forms a paired-layer structure . Obviously, the paired-layers of all unit cells form an upper plane which is the Babinet complement of the double layer fishnet underneath. Figure 1(c) shows an SEM micrograph of the structure (the whole nanoimprinted structure measures 1 cm × 1 cm). For reference measurements, the upper layer can be removed by a lift-off of the photoresist plateaus. This leaves the double layer fishnet only as depicted in Fig. 1(d). Mono-layer fishnet structures with monolayers on the plateaus as shown in Fig. 1(b) have also been produced by evaporating a 40 nm thick Ag film. Again, the upper layer of metallic rectangular monolayers and the lower single layer fishnet with rectangular holes are Babinet complements.
Co- and cross-polarized reflectivities of the sample were measured at an incident angle of 45° on a lab-built setup (other angles of incidence give similar results). Randomly polarized light in the wavelength range 400 – 1000 nm from a halogen lamp passed through a polarizer to become s-polarized. The light beam was then slightly focused onto the sample surface to form a light spot with ~3 mm diameter. Such weak focusing onto the 1 cm by 1 cm nanoimprinted arrays facilitated a quasi-plane-wave measurement with minimal light focusing. The reflected light was collected by a lens and subsequently collimated onto the fiber of a fiber-based spectrometer (QuestTM X, B&W Tek Inc.). A second polarizer was inserted in between the lens and the collimator, to selectively allow either p- or s-polarized reflected light to pass. During the measurement, the sample was rotated with respect to the z-axis by different azimuthal angles. The p- and s- polarized intensities of the reflected beam Irp and Irs are connected with the incident intensity of the s-polarized beam Iis (provided ) by
Occasionally, the samples were additionally characterized on a commercial ellipsometer (J. A. Woollam) in order to double check the experiments carried out on the home built reflectometer. The results from both setups agree quantitatively very well. Numerical simulations were carried out with the commercial Maxwell-solver RSoft DiffractMOD, using the first five harmonics and the materials parameters (dielectric constants) for Ag and Si as included in the software package. The refractive index of the photoresist was assumed to be n = 1.49.
3. Results and discussion
First, we show the results for the reflection of a sample containing only the double layer fishnet but no paired layers, as shown in Fig. 1(d), i.e. the plateaus of photoresist and paired layers were removed by lift-off. The angle of incidence was fixed at 45° and the structure was rotated around the azimuth (0° denoting the situation where the incoming s-polarized electric field is parallel to the short axis of the holes of the fishnet). Three major dispersive modes are observed in the experimental graphs of |rps|2 and |rss|2 shown in Fig. 2(a) and Fig. 2(b), respectively: two fairly straight modes intersecting with each other at the point (56°, 755 nm), together with a semi-circular mode for wavelengths shorter than 700 nm. These modes are verified by the numerical simulations shown in Figs. 2(d) and 2(e). These modes do also appear in the monolayer sample (vide infra Fig. 3) and, of course modified, for a quadratic pitch (vide infra Fig. 8(d)-8(f)), so that we conclude that their dispersion is determined by the geometry of the grating modes rather than by plasmonic resonances. It is seen in Fig. 2(a) and (d) that the largest values of cross-polarization |rps|2 occur above (at longer wavelengths) the intersecting points of the SPP modes, especially above the (56°, 755 nm) point. It is seen in Fig. 2(b) that the co-polarized reflection |rss|2 is strong (50 – 90%) at almost every azimuthal angle and wavelength. In contrast, the cross-polarized reflection |rps|2 is weak, less than 1% in the experiment (see Fig. 2(a)) and less than 4.2% in the numerical simulations (see Fig. 2(d)).
The facts, that the experimentally measured cross polarization is smaller than the calculated one and that the peaks of the calculated cross polarization are sharper compared to the measured peaks could be due to fabricational imperfections. However, we want to note that the nanoimprinted samples are on large scale (cm2-scale) and hence macroscopically useful compared to other metamaterials frequently prepared with more accurate e-beam lithography, but on a 100 fold smaller area. Hence, the quite good quantitative agreement of the experimental and numerical results proves that nanoimprint lithography is apt to produce fairly homogeneous metamaterials on a cm2-scale.
The rather weak cross polarization shown in Fig. 2(a) also leads to a small polarization conversion C of 1.2% or 6.4% in case of the experimental or numerical results which are shown in Figs. 2(c) and 2(f), respectively. If we assume the holes in the fishnet being filled with photoresist, a polarization conversion of 12% is found numerically (data not shown). It is noted that a similar range of cross polarization has been reported previously from a similar fishnet structure with square holes of smaller size .
The reflectance into the cross polarized mode can be dramatically enhanced when the double layer fishnet is combined with its Babinet compliment, the paired layer structure from Fig. 1(a). Figure 3 shows the reflectivities of the sample containing both the fishnet part and the paired-layer part. The incident angle is again adjusted to 45°. It is seen that the locations of the major dispersive features are similar as in Fig. 2, so we conclude that they are determined by the periodicities in the x-y plane. In sharp contrast to the fishnet-only sample from Fig. 2, the intensity of the cross-polarization near the (56°, 755 nm) point is now drastically increased by more than one order of magnitude, with the maximum value of |rps|2 being 29.90% (see Fig. 3(a)). On another hand, an overall decrease of the co-polarized reflectivity |rss|2 is observed, with minima along the dispersive features, see Fig. 3(b). At some points, the content of the cross polarization C within the reflected beam reaches 96.5% in the experiment and almost 99% in the calculations, see Figs. 3(c) and 3(f), respectively. These high values of cross polarization content C in the reflected beam are reached on a diagonal from (30°, 600 nm) to (60°, 800 nm). We note in passing that in principle, it is possible to determine also the degree of ellipticity by determining the phase of the reflected light using a rotation compensator, however, in case of 96% of cross polarization content we can safely say already from intensity measurements that the reflected light must be fairly linearly polarized and hence postpone phase sensitive measurements for the time being.
In order to verify that it is actually the joint action of the fishnet and its Babinet compliment, we also carried out calculations of the paired layer grating without the fishnet. Unfortunately, we have no stamp available to produce such a large area structure with nanoimprint lithography, but so far the numerical simulations and the experimental results were very similar in case of the pure fishnet sample shown in Fig. 2 and the coupled structure shown in Fig. 3, so that we feel confident that we can draw valid conclusions form the simulations alone. Figure 4 shows the numerical results.
This time, a distinct feature for |rps|2 appears in the longer wavelength region 900-1000 nm around 60° azimuth, while the region near the (56°, 755 nm) point shows values smaller than half of those for the combined structure, see Fig. 3(d). Further, Fig. 4(c) shows that the maximal content of cross-polarized intensity C is below 70%. Hence, we conclude that the giant content of cross-polarized light of 96 to 99% (see Figs. 3(c) and 3(f), respectively) can indeed be achieved only by the combined action of the fishnet and its Babinet complement, the paired layer structure.
Figure 5 compares the amount of the cross polarized light in case of the combined structure in Fig. 5(a), actually a reprint of Fig. 3(f), and the simple sum of the relative cross-polarized contents of the “double fishnet only” and the “double paired layer only” structures shown in Fig. 5(b). Both figures are distinctly different both qualitatively and quantitatively, further corroborating the interpretation that only the electromagnetically coupled action of the fishnet and the paired layer structure gives rise to the giant polarization conversion.
Of course, the question arises whether it is necessary for giant cross polarization in the reflected light to apply a double layer fishnet structure, combined with paired layers as sketched in Fig. 1(a), or whether it would be sufficient to combine a monolayer fishnet with a single layer of Ag on top of the plateau of photoresist (see Fig. 1(b)). The experimental result and the simulations for such a structure are shown in Fig. 6.
While the numerically predicted content of cross polarized light shown in Fig. 6(f) goes up to 99.6%, slightly more than what has been calculated for the double layer fishnet / double paired layer structure (c.f. Fig. 3(f)), we could achieve only 70% of cross-polarization in the experiments as shown in Fig. 6(c). This could be due to some degradation of the monolayer silver film due to oxidation or sulfidation, which should degrade the sample response less in the case of double layers where on the one hand the lower Ag layer is protected from the ambient anyway, but where much of the dispersive reflectivity might also be governed by metal/dielectric/metal waveguide modes [46–48]. For those, the inner surfaces of the two metallic layers are crucial, which do both not face the ambient. More research is needed to unravel the details, which are however beyond the scope of the current contribution.
Up to now, we have considered only s-polarized light in the incoming beam. The left column in Fig. 7 shows numerical simulations for p-polarized incoming light. As expected, the numerical results for |rps|2 (c.f. Figure 3(d)) and |rsp|2 (c.f. Figure 7(a)) are identical within numerical discretization error. However, the results for |rss|2 and |rpp|2 (see Figs. 3(e) and 7(b), respectively) are somewhat different. Nevertheless, a giant cross polarization content C of close to 100% is again achieved also for p-polarized light.
Strictly speaking, the two layers in the samples shown in Figs. 1(a) and 1(b) (the (double)fishnet and the (double) metallic rectangles are not “true” Babinet complements. Although the roles of the metal and the dielectric are inverted, the holes of the lower fishnet structures are filled with photoresist, while the dielectric in the upper plane is air. Further, the lower fishnet connects to a substrate, while the top layer of rectangles connects to air. For practical reasons, it is of course impossible to put a “true” Babinet complemented structure freely hanging in air, but one can of course simulate such a structure. Hence we replaced the substrate as well as the photoresist with air in the simulations, shown in the right column of Fig. 7. Again, a giant content of cross polarized light is achieved (this time again calculated for s-polarized incoming light, see Fig. 7(f)) and overall the structure behaves very much the same as the “imperfect” Babinet complement discussed experimentally and numerically in Fig. 3. Hence, we conclude that the overall behavior is governed by the fact that in the Babinet complements the metal (negative real part of the dielectric function) is exchanged with some dielectric (positive real part), while the absolute amount of the dielectric function of the dielectric has lesser influence.
Interestingly, the reversed structure, where the double layer fishnet faces the air, but the paired layers face the silicon substrate shows only a medium cross polarization content of 41% as shown in Figs. 8(a)-8(c). Again, only simulations are possible to asses this geometry as the nanoimprint technique is not capable to fabricate such a structure easily.
Finally, we numerically considered a double layer fishnet coupled to its Babinet-complement, which comprises quadratic holes and quadratic paired layers, respectively. In this case, the holes and the paired layers measure 311 nm × 311 nm, and the periodicity is 547.7 nm in both directions so that the area of a unit cell is the same for the quadratic structure and the rectangular structure discussed before. The results are shown in Figs. 8(d)-8(f).
An important difference between a fishnet with a square unit cell and a fishnet with a rectangular unit cell is the azimuthal periodicity. As expected, the reflectivities are now symmetric around the 45° azimuth, see Fig. 8. However, exactly at 45°, there is no cross polarization observable at any wavelength in Fig. 8(d) because an additional symmetry is available in the quadratic structure which is not available in the rectangular structure discussed before.
We have shown that a large area (1 cm2) nanoimprinted metamaterial comprising a double layer fishnet and its Babinet complement, a plane of paired metallic layers, can show giant cross polarization in reflection. While the incoming beam was s-polarized, the reflected beam was p-polarized to more than 96%. This experimental result is close to the result of numerical simulations, which predict 98.7% of cross-polarization. To the best of our knowledge, this is the first time that such a high polarization conversion has been reported in a reflected beam in the visible range of the electromagnetic spectrum. We note in passing, that the fact that experimental outcome and numerical simulations are close for the double-layer structure shows the superior quality of the large-area nanoimprinted metamaterial. We have further shown that such high polarization conversion is only achieved in the case when a double layer fishnet is combined with its Babinet complement. Each structure alone shows substantially less polarization conversion (A few % only in case of the double-layer fishnet and less than 70% for the pure paired layer structure). Numerical simulations show that a metamaterial comprising a monolayer-fishnet and its Babinet complement shows close to 100% cross polarization at certain azimuthal angles. This indicates that the quality of being a Babinet-complemented structure is most important for polarization conversion, but not the fact that double layer fishnets comprise optical magnetic resonances. Though, we could not experimentally reproduce this high value and only achieved 70% in this case. Obviously, the double layer leads to more stable structures at present, but we do not want to exclude that improved sample preparation (e.g. by stabilizing the surface of the silver films by hydrogenation ) may lead to Ag monolayer samples that meet the numerically calculated polarization conversion efficiency of almost 100% in the future. Further theoretical work on the details and background of all the resonances found in the azimuthal dispersion plots is certainly needed, but beyond the scope of the current manuscript. First attempts to handle similar structures theoretically have been carried out recently [33,35].
This work proves that large-scale, nanoimprinted metamaterials, showing giant cross polarization in the reflected light, are technically feasible and show high quality optical responses. Specifically, this work may trigger further research towards large area, ultrathin polarization converters for use in micron scale and highly integrated optical systems. Further, as the content of cross polarization depends critically on the azimuthal orientation of the sample, one could think of a rotation-to-polarization converter, for instance for an optical feed-back loop to determine a rotation angle, or generally in micro-opto-electro-mechanical systems (MOEMS) in the wider range.
We would like to cordially thank Heidi Piglmayer-Brezina for e-beam sputtering and taking the SEM images. This work was financially supported by the European Research Council (ERC Starting Grant 257158 ‘Active NP’) and partially by the Austrian Klima- und Energiefonds (SolarTrap, Grant 843929).
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