  The control of Mott phase is intertwined with the spatial reorganization of
the electronic states. Out-of-equilibrium driving forces typically lead to
electronic patterns that are absent at equilibrium, whose nature is however
often elusive. Here, we unveil a nanoscale pattern formation in the
Ca$_2$RuO$_4$ Mott insulator. We demonstrate how an applied electric field
spatially reconstructs the insulating phase that, uniquely after switching off
the electric field, exhibits nanoscale stripe domains. The stripe pattern has
regions with inequivalent octahedral distortions that we directly observe
through high-resolution scanning transmission electron microscopy. The
nanotexture depends on the orientation of the electric field, it is
non-volatile and rewritable. We theoretically simulate the charge and orbital
reconstruction induced by a quench dynamics of the applied electric field
providing clear-cut mechanisms for the stripe phase formation. Our results open
the path for the design of non-volatile electronics based on voltage-controlled
nanometric phases.
