  Topological modes (TMs) are typically localized at boundaries, interfaces and
dislocations, and exponentially decay into the bulk of a large enough lattice.
Recently, the non-Hermitian skin effect has been leveraged to delocalize the
wavefunctions of TMs from the boundary and thus to increase the capacity of TMs
dramatically. Here, we explore the capability of nonlinearity in designing and
reconfiguring the wavefunctions of TMs. With growing intensity, wavefunctions
of these in-gap nonlinear TMs undergo an initial deviation from exponential
decay, gradually merge into arbitrarily designable plateaus, then encompass the
entire nonlinear domain, and eventually concentrate at the nonlinear boundary.
Intriguingly, such extended nonlinear TMs are still robust against defects and
disorders, and stable in dynamics under external excitation. Advancing the
conceptual understanding of the nonlinear TMs, our results open new avenues for
increasing the capacity of TMs and developing compact and reconfigurable
topological devices.
