In this review we describe the challenges and opportunities for creating magnetically active metamaterials in the optical part of the spectrum. Several techniques for extracting the effective parameters of plasmonic metamaterials are introduced and compared. The emphasis is on the periodic metamaterials whose unit cell is much smaller than the optical wavelength. The conceptual differences between microwave and optical metamaterials are demonstrated. We show that concepts for microwave metamaterials can be extended to the optical domain in a rather limited way. Whenever a metamaterial that exhibits magnetic response in the microwave part of the spectrum is scaled down in size to exhibit similar electromagnetic behavior in the optical part of the spectrum, plasmonic effects play a major role. Specifically, we demonstrate that for every unit cell's geometry of a magnetically active metamaterial, there exists the shortest wavelength Î»res beyond which the magnetic response disappears. In general, the more elaborate is the unit cell (e.g., split rings with narrow inter-ring gaps, etc.), the longer is the Î»res.
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