The development of long-distance communication with low power consumption and high resolution requires the electromagnetic wave frequency ranging from GHz to THz, such as from 5G to 6G (G for generation) in our daily communications. However, the THz E&M wave and material interactions are still in its infancy. THz technology may serve as a nonionizing and non-destructive way for multiple applications including: information and communications technology, non-destructive evaluation, security check, quality control of food and agricultural products, environmental monitoring, and ultrafast computing... Motivated by its advances, we derived the thermodynamic theory and formula to understand how THz optics interacts with ferroelectric/ferroelastic materials, and predict an ultrafast phase transition in experimentally fabricated group-IV monochalcogenides, through computing the optical responses of electrons and phonons. Direction-dependent electron energy loss spectrum and second harmonic generation signals are also calculated. One could refer to npj 2D Materials & Applications, 2021, 5, 16 (https://www.nature.com/articles/s41699-020-00189-7) for our collabroated work with Prof. Shunhong Zhang from USTC.