Unraveling phase transformation with phononic hyperbolicity using off-resonant terahertz light

Noncontacting and nondestructive control of geometric phase in conventional semiconductors plays a pivotal role in various applications. In the current work, we present a theoretical and computational investigation of terahertz (THz) light-induced phase transformation of conventional binary semiconducting compounds among different structures such as rock salt, zinc-blende, wurtzite, and hexagonal phases. Using MgS and MgSe as prototypical examples, we perform anharmonic phonon-mediated calculations and reveal large contrasting lattice-contributed dielectric susceptibility in the THz regime. We then construct a THz-induced phase diagram under intermediate temperature and reveal rock salt to hexagonal and then wurtzite structure transformations with increasing light intensity. This does not require a high-temperature environment as observed in traditional experiments. The low energy barrier suggests that the phase transition kinetics can be fast, and the stable room temperature phonon dispersions guarantee their nonvolatile nature. Furthermore, we disclose the phononic hyperbolicity with strong anisotropic THz susceptibility components, which serves as a natural hyperbolic material with a negative refractive index. Our work suggests the potential to realize metastable hidden phases using noninvasive THz irradiation, which expands the conventional pressure-temperature ($P$--$T$) phase diagram by adding light as an additional control factor.