Ultrafast Control of Coherent Acoustic Lattice Dynamics in the Transition Metal Dichalcogenide Alloy WSSe

Coherent acoustic phonons (CAPs)—-vibrational modes prepared in a coherent state that propagate as long-wavelength strain waves—can dynamically modulate crystal structure and, in some cases, symmetry, offering unique opportunities for controlling material properties. We investigate CAP generation in exfoliated multilayer flakes of the alloy tungsten sulfide selenide (mathematical equation, hereafter WSSe). Using high-fluence 400 nm excitation together with ultrafast transient-reflection spectroscopy, we track the coupled carrier-lattice response, revealing dynamics consistent with a sequence of rapid carrier thermalization and exciton formation, phonon recycling, and photoinduced stress from prompt deformation potential and slower thermoelastic contributions that combined drive a coherent oscillation at 27 GHz. The fractional amplitude of the oscillatory component attributed to an acoustic mode in a coherent state is substantially larger in WSSe than in the parent crystals WS2 and WSe2, where the coherent contribution represents only a minor perturbation superimposed on a dominant monotonic background. This pronounced enhancement indicates that the alloy does not behave as a simple interpolation between the binary compounds, but instead exhibits an emergent optical-acoustic response linked to chalcogen mixing. Finally, by implementing a two-pulse excitation scheme, we demonstrate optical control of the CAP phase and amplitude, highlighting the potential of TMDC alloys to support dynamic modulation of optomechanical and acoustic responses for advanced device engineering.