  MXenes are two-dimensional materials with a rich set of remarkable chemical
and electromagnetic properties, the latter including saturable absorption and
intense surface plasmon resonances. To fully harness the functionality of
MXenes for applications in optics, electronics and sensing, it is important to
understand the interaction of light with MXenes on atomic and femtosecond
dimensions. Here, we use ultrafast electron diffraction and high-resolution
electron microscopy to investigate the laser-induced structural dynamics of
Ti3C2Tx nanosheets. We find an exceptionally fast lattice response with an
electron-phonon coupling time of 230 femtoseconds. Repetitive femtosecond laser
excitation transforms Ti3C2Tx through a structural transition into a
metamaterial with deeply sub-wavelength nanoripples that are aligned with the
laser polarization. By a further laser illumination, the material is reversibly
photo-switchable between a flat and rippled morphology. The resulting
nanostructured MXene metamaterial with directional nanoripples is expected to
exhibit an anisotropic optical and electronic response as well as an enhanced
chemical activity that can be switched on and off by light.
