may be explained considering that Bloch components from the first band and those from the second band are roughly out of phase at y = w [see Figs. 4(b) and (d)] so that they interfere destructively in Eq. (5). Additionally, the central peak is unaltered in practical terms, giving Δ$_{y}$ = 54 nm. In the numerical simulations given above we have shown that beamsizes along the x -axis are larger than the diffraction limit λ$_{d}$/ 2 attained by quasi-stationary Bessel beams propa- gating in the medium of dielectric constant ϵ$_{d}$ , whereas Δ$_{y}$ is clearly subwavelength. Control over the wave pattern and thus over its FWHM in the x direction is exercised by the spec- trum of spatial frequencies k$_{x}$ : the higher bandwidth the lower Δ$_{x}$. From k 2 x ≤ β 2 max − β 2 estimating the spectral domain of the wavefield we conclude that decreasing β leads to a widening of the spectrum. However, if β < β$_{c}$ the plane-wave components in the dielec- tric material propagate homogeneously so that they are not necessarily coupled resonantly around the metal-dielectric surfaces. In principle, this effect might modify significantly the localization of the field in the direction of the periodicity. Let us clarify these aspects. The contour plot shown in Fig. 8 corresponds to a diffraction-free beam of propagation constant β = 10 . 157 µ m − 1 ( βp = 4 . 571) coinciding with the lower limit of k$_{‖}$ for the third band (see Fig. 2). The transverse frequency γp = 7 . 446 guarantees that all the three bands of allowed frequencies are excited; this is a high value since if it were generated in a uniform medium with ¯ ϵ$_{d}$ = 2 . 89 would give a Bessel beam of width Δ$_{x}$ = 136 nm. Fig. 8 shows that the intensity | h$_{x}$ | 2 is distributed mainly around the interface y = 0. A narrow peak on the focus is produced exhibiting a width Δ$_{x}$ = 132 nm, which is slightly lower than that just given above. Also strong sidelobes arise in the vicinity of focus. This suggests that bandgaps inherent in a photonic crystal provide a mechanism to achieve a superresolving effect going beyond the diffraction-induced Bessel limit. The origin of this superresolving effect is not the excitation of SPPs but an enhancement of high spatial frequencies [39]. On other hand, the field distribution in the y -axis demonstrates a subwavelength focus of FWHM Δ$_{y}$ = 44 nm. Sidelobes on the interfaces are accompanied with others peaks in the middle of the dielectric slabs. This is not surprising since Bloch components of the third 15 V. HYBRID CONSTRUCTION OF DIFFRACTION-FREE WAVES