In modern condensed-matter physics, a central goal is to discover materials with novel properties that can advance everyday technologies. However, searching for new functionalities solely under equilibrium conditions has limited the scope of exploration. Recently, probing the dynamics of materials driven far from equilibrium with ultrashort laser pulses has emerged as a powerful approach, adding time as a control parameter for interrogating material properties and creating new phases. In this talk, I will introduce our recent ultrafast scattering experiments on spontaneously organized, symmetry-broken “electronic crystals” in solids known as charge density waves (CDWs), which form periodic structures distinct from the underlying lattice. Understanding the dynamic properties of CDWs is essential because they often intertwine with other exotic phases of matter such as superconductivity and magnetism, and this entanglement may enable the discovery of new states of matter under photoexcitation. Moreover, their strong tunability by ultrafast light holds promise for high-speed, nonvolatile memory devices.

Among prototypical CDW materials, EuTe4 stands out for exceptional properties, including record-breaking thermal hysteresis [1,2] and an intrinsic moiré CDW superstructure [3], both of which originate from the coexistence of incommensurate CDWs localized on different Te layers. Our ultrafast optical investigations of the CDW phases in EuTe4 highlight three key results: (i) the creation of shear-type topological defects arising from joint commensuration between the coexisting CDWs [4], (ii) bidirectional optical control of these CDWs via their phase competition [5,6], and (iii) sublattice-resolved detection of coherent CDW-amplitude oscillations [7]. Together, these studies reveal a large, unique sub-picosecond photo-response of CDWs, thereby establishing EuTe4 as an ideal platform for all-optical exploration of moiré physics without the need for artificial stacking.