which in principle might lead to a singular behavior of v$_{0}$ . In this case we also have B$_{0}$ = 0. Eq. (21) represents the condition for the existence of a surface resonance in a metal- dielectric interface [23]. Therefore k$_{yd}$ would be purely imaginary, which is associated with the existence of evanescent waves in the dielectric films. In addition M$_{0}$ would be a diagonal matrix with leading to M$_{0}$ = I . The latter particular case might bring new insights onto our basic analysis however it deserves a thorough study out of here. The periodic function h$_{K}$ is depicted in Fig. 4 for the multilayered setup with parameters given in the caption of Fig. 2 and at different values of k$_{‖}$ . In all cases, the complex amplitude | h$_{K}$ | is an even function whereas its argument is an odd function respect to the center of the metallic film, as shown in Eq. (10), provided it is set off to zero. Peaks are formed at metal-dielectric interfaces and valleys at the center of the slabs; this rule holds in the purely-evanescent regime when k$_{‖}$ > β$_{c}$ , however the maximum of intensity is reached at the center of the dielectrics if k$_{‖}$ belongs to the third band [see Fig. 4(e)]. On other hand we observe that phases of the wavefunction h$_{K}$ have a characteristic variation at different spectral bands. For instance, phases are ∼ π rad at the right side of the central metallic slab y = w in the top band; however, this phase turns to ∼ 0 in the second band. We show below that this is of relevance in the formation of an on-axis focus. Let us establish some favourable conditions for the formation of a focus along the z -axis. Using ( x, y ) = (0 , 0) in Eq. (5) we obtain the wavefield amplitude h$_{x}$ = ∑ K ∫ a$_{K}$dk$_{x}$ at the origin as a summation of the amplitudes a$_{K}$ corresponding to different Bloch modes. This may be interpreted as an interference of Bloch-type individuals. If the phase of their amplitudes are manipulated in order to have the same value leading to in-phase waves, the oscillatory superposition yields the highest intensity achievable. Excepting a few particular 9 III. FOCUS GENERATION ( − ϵ$_{m}$ ) ϵ$_{d}$ = p − w w , (23) ± K = w p k$_{ym}$ − ( 1 − w p ) k$_{yd}$, (22) providing the eigenvector v$_{0}$ as either the unit vector [1 , 0] or [0 , 1]. Moreover, Im( K ) = 0 except when K vanishes, a case that occurs if additionally