Nonlinear optical response of truly chiral phonons: Light-induced phonon angular momentum, Peltier effect, and orbital current

The nonlinear optical responses of chiral phonons to terahertz and infrared light are studied using the nonlinear response theory. We show that the photo-induced angular momentum increases with the square of the chiral-phonon relaxation time $\tau$, giving a significantly larger angular momentum compared to ordinary phonons. We also find that the photo-induced Peltier effect by chiral phonons occurs through a mechanism distinct from those proposed recently; the induced energy current scales $\propto\tau^2$, giving a larger energy current in the clean limit. We prove a linear relation between the generated angular momentum and the energy current. Lastly, we show that the orbital current, an analog of the spin current, occurs through a nonlinear response. These findings demonstrate the unique properties and functionalities of chiral phonons.