Coherent control through phonon anharmonicity.

Anharmonic lattice vibrations play a key role in many physical phenomena. The most direct evidence for anharmonicity is a measurement of the oscillation frequency as a function of the amplitude. For Raman-active lattice vibrations, this is not a trivial task, and anharmonicity is typically probed indirectly. Thus, direct measurement and control of the anharmonicity of a single Raman mode is still lacking. We show that ultrafast double pump-probe spectroscopy could be used to directly observe frequency shifts of Raman phonons as a function of the oscillation amplitude and disentangle the coherent contributions from quasi-harmonic sources such as temperature and changes to the carrier density in SnTe and SnSe. Moreover, we show that coherent displacive phononic excitation in tandem with electron-phonon coupling is a pathway to control phonon anharmonicity dynamically. Finally, our methodology could be used to isolate the basic mechanisms driving nonlinear optical phenomena based on their unique timestamps.