Coherent phonon control beyond amplitude saturation (2508.16422v1)

Abstract: Nonlinearity is often associated with amplification, instability, or emergent behavior - where small inputs produce disproportionately large outputs. Yet in many physical systems, nonlinearities act as limiting mechanisms, suppressing the response below linear expectations, as exemplified by the breakdown of Hooke's law at large forces. These limiting effects can become significant in the optical control of quantum materials, where large-amplitude lattice motion is essential for reaching states far from equilibrium. Displacive excitation of coherent phonons, perhaps the most widely used control strategy, assumes linearity between the driving force on the lattice and the optically excited carrier density. However, the breakdown of this assumption remains largely unexplored, and strategies are needed to overcome the resulting nonlinear bottlenecks. Here, we show that sequential optical excitation overcomes the nonlinear amplitude saturation of a key phonon mode in the van der Waals ferroelectric Td-WTe$_2$. Using time-resolved second harmonic generation, we track the response of the interlayer sliding mode governing ferroelectricity to both single- and double-pulse excitation. With increasing optical excitation, the phonon amplitude saturates and declines - a limitation we trace to band-specific electron-phonon coupling that weakens the driving force on the nuclei. Sequential in-phase excitation of coherent sliding motion, which outlasts the electronic excitation, avoids population of counteracting electronic states. This enables higher phonon amplitudes for the same optical energy input. We exploit the enhanced lattice response to perform high-amplitude vibrational spectroscopy in the excited state, revealing a novel form of anharmonic phonon coupling that emerges exclusively under nonequilibrium conditions, and previously elusive coherent vibrational modulations of electronic states.