Fig. 1. Experiments on three different samples: (a) small SRRs with a fundamental magnetic resonance centered at 1.5μm wavelength, (b) a slightly detuned structure, and (c) large SRRs with a higher-order resonance around 1.5μm. For each case, the two linear polarizations are shown (horizontal on top, vertical at bottom). The different columns (from left to right) show electron micrographs, measured linear transmittance spectra, measured SHG signal strength for excitation centered at 1.5μm wavelength, and corresponding THG signal strength. The arrows indicate the incident linear polarization (black), the measured linear polarization of the SHG (red), and that of the THG (green) – if sufficiently large. For clarity, the nonlinear signals are normalized to (a), horizontal incident polarization.

Fig. 2. Experiments on control samples: (a) similar (but not identical) to Fig. 1(a), (b) metamaterial composed of single cut wires, and (c) metamaterial composed of “T”-structures. The representation is as in Fig. 1.

Fig. 3. Oblique-incidence transmittance spectra of two samples. Row (a) corresponds to the sample in Fig. 1(a), rows (b) and (c) to that in Fig. 1(c). The angle of incidence with respect to the surface normal is indicated by the color, positive angles (solid curves) and negative angles (dots) are depicted. The left column corresponds to p-polarization, the right column to s-polarization. The polarization geometries (for positive angles) are also illustrated by the insets.

Fig. 4. SHG signal strength obtained from SRRs excited under oblique incidence as a function of the angle of incidence α with respect to the surface normal. The samples and geometries directly correspond to those shown in Fig. 3. For convenience, the excitation geometries are again illustrated here. All SHG signal strengths are normalized to that for normal incidence in Fig. 1(a), horizontal incident linear polarization. Note the different SHG signal scales.