To express the confinement of the optical field, referring to the calculation reported in Ref. [
38] can be done. In this publication, Almeida and co-workers defined the ratio
η of optical power within the slot over the total power within a unit cell, and the optical intensity
I as
η divided by the cross-section of the slot, i.e.,
Ws·
h with
h the height of the slab. In case of standard slot waveguides, taken as a function of the slot width
Ws, the ratio saturates around 30% for
Ws higher than 200 nm, and
I decreases from 30 to 3 µm
-2 when
Ws ranges from 10 to 200 nm. In that case, the confinement is done only by the index contrast and the group index remains around 3-4. In order to apply such factors in our case, we also needed to take into account the increase of electromagnetic density due to the spatial compression of waves related to slow light propagation. Therefore, we expressed the optical density as the product of the group index and the previous optical intensity. In case of the new slot PhC waveguides we proposed, we found that the fraction of the optical power at
k=0.43 within the comb is near 25%. Considering the group index around 140, an optical density as high as 540 µm
-2 is thus obtained. This optical density represent an enhancement by a factor 7 if compared with typical values around 70 µm
-2 just reported in previous works for classical TIR slot waveguides with narrow slots of 50 nm width, and a factor 40 if compared with more feasible slot width around 100 nm, whose optical density is around 12 µm
-2. We believe that such waveguides combining a Bragg-like corrugated slit and slow wave phenomena can be used in future devices relying on non-linear optics or bio-sensors enhanced by large group light propagation.