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Presented is a new means of synthesising thin, planar low or zero index metamaterials (LIM/ZIM) via frequency selective surfaces (FSS), the parameters of which are optimised using a genetic algorithm. FSS-based LIM/ZIM possess advantages such as light weight, ease of fabrication, low loss, and are readily scalable in frequency.
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Introduction: This Letter presents a novel method for the design of low or zero index of refraction metamaterials (LIM/ZIM), namely the use of a genetically-engineered frequency selective surface (FSS) to realise a thin, planar structure that has very low loss. Previously, such materials were realised using metallic spheres, cylinders, or sheets of conducting grids placed in a host dielectric medium [1, 2]. However, we will show here that conventional planar FSS technology provides an ideal means of synthesising LIM/ZIM, especially when combined with a robust optimisation technique such as the genetic algorithm.
Theoretical background: According to Snell's law of refraction,
... (1)
a ray originating inside a low index material (of refractive index n1 < 1) will, upon traversing the interface at an angle y1 with respect to the normal, be refracted into free space (n2 = 1) only very slightly; i.e. for a wide range of incident angles, the refracted rays will still be nearly parallel upon exiting the LIM=ZIM (θ2 will be very small). Because of this property, LIM=ZIM have been proposed as focusing lenses or superstrates for increasing the directivity of small antennas [2].
Since metamaterials may also be realised by means of a properlydesigned frequency selective surface [3, 4], the approach taken here...