Enhancement of the linear electro-optic effect by high pressure

It is challenging to design electro-optic (EO) crystals with both a large bandgap and a high EO coefficient. We propose that some semiconductors synthesized under a high pressure environment can present both a large bandgap and a high linear EO coefficient because of the enhanced strength of the valence bond. The electronic band structures and linear EO coefficients of the $II\mathrm{Si}{\mathrm{N}}_{2}$ ($II=\mathrm{Mg}$, Sr, Ba) compounds are studied using first-principles calculation. Our calculated results predict that both $\mathrm{SrSi}{\mathrm{N}}_{2}$ and $\mathrm{BaSi}{\mathrm{N}}_{2}$, under a high-pressure condition, will crystalize into an orthorhombic structure with the same space group of $\mathrm{MgSi}{\mathrm{N}}_{2}$ under ambient pressure. The calculated bandgaps are 5.48, 4.41, and 3.57 eV, respectively, for $\mathrm{MgSi}{\mathrm{N}}_{2}, \mathrm{SrSi}{\mathrm{N}}_{2}$, and $\mathrm{BaSi}{\mathrm{N}}_{2}$, while the calculated linear EO coefficients are 1.55, 12.09, and 21.60 pm/V at a fiber communication wavelength of 1550 nm. The increasing linear EO coefficients can be interpreted by the enhanced bond strength and a reduced bandgap. This work indicates that it is feasible to enhance the linear EO coefficient with high pressure.