Leaky-wave antennas with anisotropic metamaterials

Leaky wave antennas can offer high directivity and frequency scanning [I]. An isotropic dielectric slab with refractive index n > 1 does not radiate effectively unless spatial variations are introduced to excite fast-wave space harmonics [I]. To realize uniform leaky wave structures, potentially in the optical domain, a metamaterial slab with n >C 1 has been proposed to achieve a narrow radiation beam [2, 3]. This approach suffers the limitation of large transverse slab dimensions, because the operating wavelength inside the slab is much greater than the free space wavelength, due to the small value of n [4]. Low refractive index metamaterials that require small material permeabilities face challenges of implementation [3]. In addition, radiation occurs in a narrow angular range between broadside and the critical angle sin(n), which limits beam scanning. Finally, we note that while a microstrip transmission line loaded with series capacitors and shunt inductors has been studied to realize metamaterial leaky-wave antennas in the microwave frequency regime [5], identifying the optical counterparts is difficult. To overcome these limitations, we present a forward/backward radiating leaky waveguide at optical wavelengths based on nonmagnetic uniaxially anisotropic metamaterials having permittivity tensors in which one component is different from the others in sign [6]. The resulting hyperbolic dispersion relation enables radiation from backfire (-90°) to endfire (90°), and the slab thickness can be small relative to the free space wavelength.