Inhomogeneous excitation-regulated coherent strain wave in 2H-MoTe2 revealed by ultrafast electron microscopy

The field of optical detection and manipulation of transient states with ultrafast lasers is rapidly growing and shows great promise. In this field, new states can be generated through photodoping or optical-induced strain. However, traditional ultrafast measurements have been plagued by the challenge of inhomogeneous excitation, which is difficult to avoid. Nevertheless, recent developments suggest that this situation might change and open up more possibilities for the ultrafast manipulation of materials. One such possibility involves introducing nonuniform order parameter fields, like carrier density gradients. In this study, we demonstrate the potential to manipulate the coherent photoacoustic wave in 2H-MoTe2 by adjusting the homogeneity of laser excitation, which is closely linked to the thickness of the flake. Our ultrafast electron diffraction experiments reveal that, as the flake thickness increases, the photoacoustic wave transforms from a standing wave with breathing motions to a solitonlike traveling wave. This transformation is characterized by distinct diffraction intensity oscillations. For flake thicknesses below the critical value of approximately 40 nm, homogeneous excitation results in out of phase intensity oscillations of the Friedel pairs, and only a fundamental frequency component is detected. On the other hand, in thicker flakes, inhomogeneous excitation leads to in-phase intensity oscillations and the emergence of a second harmonic component. Overall, our findings highlight the unique aspects of inhomogeneous excitation and its implications for the optical control of exotic properties in strain-related transient states.