Two types of phonon modes in crystalline structure

(Top) acoustic phonons, where the movement of atoms is in phase

(Bottom) optical phonons, where the movement of atoms is out-of-phase. Optical phonons can be excited by electromagnetic waves.

https://www.researchgate.net/figure/Two-types-of-phonon-modes-in-crystalline-structure-up-acoustic-phonons-where-the_fig16_333337278