Читать книгу Introduction To Modern Planar Transmission Lines - Anand K. Verma - Страница 242

In the case of a DNG‐based matched flat superlens, the image is reconstructed at the second focus by amplification of the evanescent electric fields. Thus, the DNG slab provides an opportunity to design subdiffraction lens, i.e. practically a lens without any diffraction limit. The refractive index of the DNG, matched to air medium, is . The longitudinal wavevector k_x, given by equation (5.5.28), is rewritten for the backward wave supporting DNG slab as follows [J.2, J.13, J.22]:

(5.5.36)

The electric fields of both the propagating and evanescent waves, inside the DNG slab, are obtained using the above equations with equation (5.5.29a,b):

(5.5.37)

Equation (5.5.37a) shows that the propagating waves acquire the leading phase (β_xd) at the end of the DNG slab of thickness x = d. The evanescent waves are amplified exponentially by α_xd at the end of the slab. Figure (5.12c) shows such behavior of the DNG based superlens. At the output of the DNG slab, the wave is still evanescent waves in the near‐field region; although at the image plane its magnitude level is the same as that of at the object plane. So at the second focus, the image has high resolution beyond the diffraction limit. It is noted that the evanescent waves still propagate at the interface along the y‐axis in the (y − z)‐plane as the surface waves. The surface wave is bounded in the x‐direction. The amplification of the evanescent wave is also treated through the concept of coupled quasi‐particles called plasmon‐polariton, especially in the optical frequency range [J.13, B.11].