Strong Polarization in the Optical Transmission through Elliptical Nanohole Arrays

Strong polarization dependence is observed in the optical transmission through nanohole arrays in metals. It is shown that the degree of polarization is determined by the ellipticity and orientation of the holes; the polarization axis lies perpendicular to the broad edge of the ellipse. Furthermore, the depolarization ratio shows a squared dependence on the aspect ratio of the holes, which is discussed in terms of coupling into and out of the surface plasmon modes. The observed results will be useful for tailoring the polarization behavior of metallic nanophotonic elements in many applications, including surface plasmon enhanced optical sensing and ultrafast optical switching.

Figure 1

Scanning electron micrographs of square nanohole arrays in gold. (a) Nearly circular holes. (b) Elliptical holes, 0.6 aspect ratio and major axis at $-12°$ to the $[1,0]$. (c) Elliptical holes, 0.6 aspect ratio and major axis at $33°$ to the $[1,0]$ axis. (d) An expanded view of (c) showing the full $16.1\mu \mathrm{m}$ wide array of 529 holes (holes spaced by 704 nm).

Figure 2

Transmission spectrum through elliptical nanohole array for two orthogonal linear polarizations, with a 0.3 aspect ratio between the minor and major axes of the ellipse. The $p$ polarization is parallel to the $[0,1]$ direction.

Figure 3

Polarization dependence of transmission at the $(0,1)$ resonance peak, normalized to the maximum. Transmission shows cosine dependence when the major axis of the ellipse is oriented both at $0°$ and $33°$ to the $[1,0]$ axis of the array. The $p$ polarization, along the $[0,1]$ direction of the lattice, corresponds to a polarization angle of zero degrees. The maximum transmission occurs for polarization perpendicular to the broad side of the ellipse.

Figure 4

The depolarization ratio of the transmitted light as a function of the aspect ratio of the holes. Solid line shows the $y=x^2$ curve.

Figure 5

(a) Elliptical holes approaching limiting case of slits, which only show enhanced transmission for $p$ polarized light. (b) Enhanced SP excitation perpendicular to major axis of ellipse leads to preferential excitation of the $(0,1)$ resonance with respect to the $(1,0)$ resonance. (c) Coupling both into and out of the SP mode is required to obtain enhanced transmission from periodic array of holes, which results in a squared dependence of the enhanced transmission on the coupling strength.