## 2 Photon Crystal Design  
The following structure (see Figure 1) has been designed and prepared to realize LRSPP propagation along silver-ZnS/air interface: SiO\({}_{2}\)/(HL)\({}^{19}\)/Al\({}_{2}\)O\({}_{3}\)/ZnS(1)/Ag/ZnS(2)/air, where SiO\({}_{2}\) are standard 2.3 mm thick fused silica plate, (HL)\({}^{19}\) are 19 pairs of tantalum pentoxide and silicon dioxide layers (that is transparent dielectrics having high and low refraction indices which form a 1D Photonic Crystal) with the thicknesses respectively 96.6 and 134.4 nm, Al\({}_{2}\)O\({}_{3}\) layer has a thickness of 20 nm, ZnS layers 1 and 2 have the thicknesses respectively 52 and 20 nm, Ag is 12.5 nm-thick silver layer. SiO\({}_{2}\)/(HL)\({}^{19}\) Photonic Crystal on silica substrate has been prepared by magnetron sputtering in one coating run, and afterward all other layers have been deposited on top of this finished structure also in one (but another) coating run. In the Introduction, we already mentioned the usefulness of the exploitation of non-oxide materials together with metal nanofilms [25; 26]. (Shortcomings related with the use of broadly accepted oxide materials for such purposes, e.g., an easy oxidation of ultrathin metal layers, strong tendency of such layers to form nanoparticle islands or agglomerates in the absence of any special \"wetting layers\" or the like, are discussed in the same references and elsewhere). Through extensive experimentation with ultrathin (sub-15 nm) Ag layers deposited by either thermal evaporation or sputtering, we have found that one possible solution to improve the thin layer morphology (preventing island-type growth) was to carefully select the underlying layer chemistry, and established that ZnS sublayers (substrate layers for Ag) led consistently to improving the layer morphology. In particular, the near IR reflection/transmission (R/T) spectra of custom-designed low-emissivity multilayer coatings employing them have been systematically studied. It is well-known that when the morphology of layers is compromised (island growth with holes in-between takes place), this unavoidably leads to the insufficient reflection of near-IR light by these layers and the coatings based on them. Only if the R/T spectra of multilayer coatings employing at least two Ag layers of thickness below 15 nm are fitted very closely with the spectra predicted using the data pertinent for idealized Ag layers (bulk \(n\) and \(k\) data), we could conclude that the morphology of growth is close to an ideal one. Exactly such a situation takes place in practice, which allows us to claim the quality of ultrathin silver layers when ZnS substrate layer was used. A thickness of 12.5 nm for the silver layer has been selected as the minimal thickness for which, according to our experience, high-quality of the film is achieved. For the principles of optimal PC design see [27]. The thicknesses of H and L layers forming PC were selected following the procedure optimizing the extinction per unit length (for a given wavelength and external media, of course), the thickness of the ZnS(2) layer of \(d_{ZnS2}=20\) nm was selected as \"the smallest sufficient\" to protect the silver layer from atmosphere. Al\({}_{2}\)O\({}_{3}\) layer (of again the smallest reasonable thickness) was used to improve the quality and adhesion of the first ZnS(1) layer. Finally, we optimize the thickness of this same first internal ZnS(1) layer to achieve the most favorable conditions for LRSPP propagation, and optimize the number of pairs of layers in PC to have the maximal amplification of the light wave on the silver-ZnS interface. An essential difference of the structure at hand in comparison with the earlier used PC supporting LRSPP propagation [8; 9; 10; 11; 13] is the appearance of one additional protective ZnS(2) layer which necessitates the following modifications.