Читать книгу Introduction To Modern Planar Transmission Lines - Anand K. Verma - Страница 240

The focal length (f) of a thin lens is related to its radius of curvature (R) by the expression f = R/(n − 1), where n is the refractive index of a lens. For n = +1, the lens does not refract, and the focusing of the EM‐wave at the image plane does not occur. However, for a DNG lens with n = −1, refraction occurs. Veselago [J.3] has shown that due to negative refraction in a DNG slab, even a slab acts as a flat lens. Pendry has further shown that such a lens is a perfect lens in the near‐field region as it enables recovery of the decaying evanescent field at the image plane [J.2]. Such recovery of the evanescent waves is not possible with a normal lens, irrespective of the size of its aperture. The perfect lens, also called the superlens, has a subwavelength resolution, breaking the diffraction limit barrier. The superlens creates an image in the near‐field very close to the lens, so it is difficult to use it in practice. However, anisotropic hyperbolic DNG lens creates a perfect image in the far‐field region, by converting the evanescent waves into the propagating waves. Such a lens is called the hyperlens. The proof of concept has been demonstrated experimentally for both the superlens and hyperlens in optical and microwave range. The proper functioning of these lenses is limited by the losses associated with a DNG medium. A brief theory of three lenses is presented in this section.