The THz sum-frequency counterparts of stimulated Raman scattering

Coherently excited phonons are unique tools to control material properties, drive phase transitions or even access hidden phases on ultrafast time scales. The increasing availability of high-field THz and mid-infrared sources facilitates targeting of specific resonantly driven phonon modes, while leaving the material in its electronic ground state. Nevertheless, this direct excitation is restricted to IR-active modes, whereas purely Ramanactive modes are symmetry protected against even intense resonant THz fields. Thus, for Raman-active modes, nonlinear excitation mechanisms must be employed, which are either mediated through modulation of the electric polarizability or through anharmonicities of the crystal lattice. Respectively, these difference-frequency processes are conventional impulsive stimulated Raman scattering (ISRS) and, more recently, ionic Raman scattering (IRS), which both lack the precise selectivity of resonant excitation. Here, we present the THz sum-frequency counterparts of these two mechanisms, which are more selective, nonimpulsive, and provide direct control over the phonon phase. We demonstrate THz sum-frequency excitation of the archetypal Raman-active phonon in diamond. This two-photon absorption process, the upconversion counter part of ISRS, directly imprints the carrier-envelope phase of the light field onto the coherent phonon's phase. Additionally, our theoretical formalism based on first-principles calculations in combination with phenomenological modeling predicts an efficient sum-frequency counterpart for IRS, which was subsequently confirmed by other experimental research groups. In summary, we complete the map of photonic and ionic Raman excitation mechanisms with their sumfrequency counterparts, providing a comprehensive guide for selective excitation of coherent phonons and other Raman-active modes by strong THz fields.