Experimental demonstration of the saturation and weakening of the resonant response of the SRR and the CSRR

Summary form only given. In 1999, J. B. Pendry and co-workers proposed the Split Ring Resonator (SRR) for designing magnetic materials at high frequencies from non magnetic metal (J. B. Pendry et al., IEEE Trans. MTT 47, 2075, 1999). This possibility was experimentally demonstrated in microwaves (D. R. Smith et al., Phys. Rev. Lett. 84, 4184, 2000) and millimeter waves (T. J. Yen et al., Science 303, 1494, 2004). However, when the particle is scaled down enough to resonate in the optical range, the resonance frequency achieves a saturation level and the magnetic response becomes very weak (J. Zhou et al., Phys. Rev. Lett. 95, 223902, 2005). This phenomenon was only demonstrated through numerical simulations, but there has not been yet an experimental probe. Separately, Zentgraf et al. experimentally demonstrated that the Babinet´s principle is almost valid for the SRR and its complementary screen, the so called Complementary-SRR (CSRR), in spite of bad conductivity of metals and big thickness of the screens (T. Zentgraf et al., Phys. Rev. B 76, 033407, 2007). Now, in this paper, we experimentally study the saturation and weakening phenomenon for both SRR and CSRR. Figure 1 shows the unit cell of a 2D array of square SRRs, being yellow for silver and blue for the substrate. The CSRR looks the same but with yellow and blue colors interchanged. The samples were manufactured by ion milling over silver lying on an Infrasil substrate. The geometry was checked by S EM microscopy. Transmission and reflection coefficients were obtained by FTIR. Table 1 shows the resonance frequencies for geometries gradually scaling from period a = 2000 nm down to a = 500 nm. For instance, the fundamental resonance of the SRR was found at 38.3 THz for a = 2000 and 73.9 THz for a = 500 nm. Thus, the resonance frequency grew up by a factor of 1.9, while the geometry was reduced by a bigger factor of 4. It is clearly evidencing that the beginning of the saturation. Also we observed the weaken- ng of the resonant response for smaller particles. This phenomenon will be critical for application of metamaterials in optics.