Fig. 3

The dispersion curve of \(\mathrm{{S}}{\mathrm{{i}}_\mathrm{{3}}}{\mathrm{{N}}_\mathrm{{4}}}\) defined by Eq. (2). The range of the thick red solid line is investigated in this work. The thin black dashed lines are all scaling lines, and their values are \({\nu _1}\) = 54.5 THz (\({\lambda _\mathrm{{1}}}\) = 5.50 \({\upmu \textrm{m}}\)), \({\nu _2} = 173\) THz (\({\lambda _2} = 1.73\) \({\upmu \textrm{m}}\)), \({\nu _3}=600\) THz (\({\lambda _\mathrm{{3}}} = 0.50\) \({\upmu \textrm{m}}\)), and \({\nu _4} = 968\) THz (\({\lambda _\mathrm{{4}}} = 0.31\) \({\upmu \textrm{m}}\)).