Magnetic Metamaterials at Telecommunication and Visible Frequencies

Arrays of gold split rings with a 50-nm minimum feature size and with an $LC$ resonance at 200 THz frequency ($1.5\mu \mathrm{m}$ wavelength) are fabricated. For normal-incidence conditions, they exhibit a pronounced fundamental magnetic mode, arising from a coupling via the electric component of the incident light. For oblique incidence, a coupling via the magnetic component is demonstrated as well. Moreover, we identify a novel higher-order magnetic resonance at around 370 THz (800 nm wavelength) that evolves out of the Mie resonance for oblique incidence. Comparison with theory delivers good agreement and also shows that the structures allow for a negative magnetic permeability.

Figure 1

Electron micrograph of a split ring array with a total size of $(100\mu \mathrm{m}{)}^2$. The lower right-hand side inset shows the dimensions of an individual split ring. The corresponding measured normal-incidence transmission and reflection spectra for horizontal and vertical polarization are shown in (b) and (c), respectively. For (b), one can couple to the fundamental magnetic mode at $1.5\mu \mathrm{m}$ wavelength via the electric-field component of the incident light; for (c), one cannot.

Figure 2

Measured transmission spectra taken for oblique incidence for the configurations shown as insets (where $\alpha =60°$). In (a), coupling to the fundamental magnetic mode at $1.5\mu \mathrm{m}$ wavelength is only possible via the magnetic-field component of the incident light; for (b), both electric and magnetic field can couple. Note the small but significant feature in (a) for 60° around $1.5\mu \mathrm{m}$ wavelength. The lower gray area in (a) is the transmission into the linear polarization orthogonal to the incident one for $\alpha =60°$. We argue that this observable can be viewed as a fingerprint of magnetic resonances under these conditions.

Figure 3

Calculated transmission spectra for different angles on the same scale as the experiment (Fig. 2). The polarization geometries in (a) and (b) are identical to those in Figs. 2a, 2b, respectively. The insets schematically show the current distributions at the frequencies and angles marked by the dots.