To quantitatively evidence our interpretation regarding the additional enhancement mechanism described above, we calculated the phase of the different frequency components at a particular position. Figure 4 shows the unwrapped phase of the electric field in the y- direction at the longitudinal position equal to 10.55a with respect to the front face of the structure. Moreover, the normalized intensity spectrum at the same spatial position is shown in Fig. 4. It follows that for the frequencies, at which the intensities are locally maximum, the phases are integer multiples of π. This is in accord with our abovementioned interpretation of the large enhancement factors, since a constructive interference between a specific path requires also the same amount of phase shift44.Figure 4Accumulated phase and intensity enhancement relation. Superimposed phase and normalized intensity spectrum. The unwrapped phases are calculated with respect to the phase at the entrance of the structure. The gray cross markers indicate the locations of the intensity peaks.