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The use of polarized light as an approach to further control the extraordinary transmission (EOT) through nanostructured metallic films has recently gained attention. In this work, it is shown that aperture shape and orientation not only determine the intensity of the polarized light emitted, corroborating the previous work of others, but also can be used to spectrally tune the relative peak intensity of surface plasmon polaritons modes. The high extinction ratio of high aspect ratio apertures lends itself to the creation of micron-sized structures that emit at different wavelengths depending upon the orientation of linearly polarized incident light. This has many potential applications including the prospect of color shifting pixels for high definition television (HDTV) and thin film electroluminescent (TFEL) devices as well as novel polarization mode dispersion control components.
©2006 Optical Society of America
1. Introduction
Recent years have witnessed significant advances in the understanding of the extraordinary transmission of light through nanostructured metal films (so called EOT films). Optical transmission greater than that predicted by standard aperture theory1 have been well documented2 as have the effects of varying the geometry and symmetry.3 Surface plasmon polariton (SPP) modes4,5 coupled with Wood’s anomaly6-8 are widely thought to account for this extraordinary transmission3,9-11 and the observed spectral maxima and minima, respectively. However it should be noted that the diffraction of evanescent waves have also been proposed to account for the enhanced transmission.12
An area that has been of increasing interest, both theoretically and experimentally, is the effect of hole shape on transmission, and how the hole shape can be used to affect or control the polarization of the transmitted light. Previously, it was shown that SPP travel in the same direction as the polarization of the incident light with the divergence of the SPP proportional to cos2ϕ, where ϕ is the half angle of the total divergence with respect to the orientation of the polarized light within the plane of the metal/dielectric interface.13,14 Altewisher, et al., looked at the polarized light transmission characteristics of an array of subwavelength holes and found that the SPP modes mediating the transmission of light were themselves polarized collinear with the input beam.15 Further, this polarization was maintained upon subsequent light emission. The arrays used in that study were cylindrical apertures. It has been shown by Gordon, et al. that the departure from apertures of high symmetry can lead to preferentially polarized transmission.16 Most recently, Degiron, et al., have shown that the shape of a single hole can have polarization dependent transmission due to the excitation of localized SP modes.17 In this Letter, we use Gordon’s findings to design arrays of polarizing apertures that can be fabricated over one another to create both polarization-dependent transmission as well as polarization dependent coloration (e.g., spectral filtering).
2. Procedure and Experimental Setup
Silver films were prepared on silica substrates using e-beam deposition at a pressure of 5.5 × 10-6 torr to a thickness of 150 nm. Figure 1 shows two arrays that are have a square crystal structure and a periodicity of 530 nm. In Fig. 1(a), the apertures are oriented such that the p polarized light is in the [0,1] direction (as defined in the Figure, [x,y]), and Fig. 1(b) shows a structure with the apertures rotated 45° (p polarized light in the [1,1] direction). The aspect ratio, r, defined as the ratio of x to y dimensions, was varied between 7 and 1, where r = 1 is a square, with one side of the aperture held constant at 187.5 nm. Spectroscopic measurements were performed on a Zeiss Axioplan 2 optical microscope coupled with an Ocean Optics HR2000CG-UV-NIR fiber spectrometer.
Spectral measurements were performed at various degrees of polarization of the incident beam. Figure 1c shows the expected cos2θ dependence (Malus’ Law) of transmittance through the two polarizers as well as the expected shift resulting from a rotation in the orientation of the apertures from 0° to 45°.16 The data points in Fig. 1(c) are the intensities of the [0,1] transmission maxima which corresponds to 680 nm.
The observed polarization dependence and the capability of having two peaks based on the [1,0] and [0,1] modes leads to the concept of patterning two polarizing array structures within the same area of a silver film. This is accomplished by creating two distinct binary bitmap images, referred to as primary arrays that are structures of high aspect ratio apertures similar to those shown in Fig. 1. One of the primary array bitmap images is rotated 90° and placed directly on top of the remaining primary array bitmap image which forms the new complex color-switching array. A FIB milled color-switching array is shown in Fig. 2.
Fig. 1. Scanning electron micrographs of polarizing films with aspect ratios of 2.4 with (a) apertures aligned perpendicular to the square array and (b) apertures rotated 45° with respect to the square array. (c) The normalized transmission of films 1a (closed triangles) and 1b (open squares) are plotted against the angle of incident polarization. Solid and dashed lines are cos2ϕ curves drawn as a guide to the eye and to compare the experimental results with Malus’ Law.
Fig. 2. A color-switching array composed of two primary arrays, ao equal to 375nm in the x-and 500 nm in the y- direction.
3. Results
The primary array whose major axis is normal to a given linearly polarized component will transmit most efficiently. As the polarization is rotated, the primary array that is the dominant transmitter changes, as does the transmission spectrum, and in this way color-tunable structures are made. The transmission exhibits the same cos2θ dependence with polarization for transmission spectra, as predicted, with a 90° phase shift between the primary arrays. Since the spectrum of the transmitted light depends on the periodicity of the aperture, the approach developed here can be extended to cover a broad spectral range of polarization-controlled filters. Figure 3 compares the transmission of the primary arrays to the transmission of a color-switching array with the requisite polarization. It can be seen that the spectra of the primary and color-switching arrays match in there spectral position but the amplitudes are not always unchanged, specifically the [1,1] SPP mode appear often to have decrease in amplitude. This can be attributed to interference effects from the apertures which are at 90° to the polarization. For the most part, two primary arrays of any periodicities can be combined to form color-switching arrays. This allows for a priori design of color-switching films from a palette of primary arrays. Multiple configurations can be made to yield a large combination of color switching arrays; however, it should be noted that one cannot ignore that there will be contributions which result from two primary arrays being combined.
Fig. 3. Transmission spectra for primary (solid lines) and color-switching arrays (dashed lines). Color-switching films composed of primary arrays with ao equal to 375 and 500 nm.
For a complete characterization of these color-switching arrays at normal incidence, a polarization plot was constructed. This type of plot shows the transmission spectra as a grey-scale image for all polarizations. As can be seen in Fig. 4, the structure shifts from emitting in the red to emitting in the green as the polarization of the incident light is oriented with the aperture’s major axis. In comparison a polarization plot for a rectangular array of apertures does not show the polarization dependence that is seen with the high aspect ratio apertures.
Fig. 4. Plot of a color-switching film with ao equal to 375 nm in the x- and 500 nm in the y-direction where white and black corresponds to transmission maxima and minima, respectively. A transmission optical micrograph of the 32 by 32µm structure is inset for both polarizations.
4. Conclusions
We have shown that the orientation of a high aspect ratio aperture not only controls the intensity of the overall transmission from a nanostructured metallic film due to an individual apertures polarization dependence but also that individual SPP modes can be excited with greater efficiency. This is in contrast to a rectangular array of isotropic apertures which do not show the same degree of polarization sensitivity. With this in mind, structures have been designed that give a greater range of tunability to the intensity and position of peaks. Highly anisotropic apertures can be switched on and off depending on the polarization of the incident light which allows for a single area to have multiple color emission capabilities. This type of technology could be quite useful for polarization control in telecommunication systems as well as for thin film display applications. However, the complete characterization of these effects which includes new aperture geometry due to the superposition of two apertures, super-periods, as well as possible interference effects due to apertures scattering plasmons which are out of phase with one another still needs to be addressed.
Acknowledgments
The authors gratefully acknowledge support from the National Science Foundation (grant DGE-0234619 to JRD) and the Defense Advanced Research Projects Agency (grants N66001-03-1-8900 and N66001-04-1-8933 to JB). We would also like to thank Professors S. Foulger and G. Chumanov (Clemson University) for useful discussions and the use of equipment.
References and Links
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