  Nonlinear optics is of crucial importance in several fields of science and
technology with applications in frequency conversion, entangled-photon
generation, self-referencing of frequency combs, crystal characterization,
sensing, and ultra-short light pulse generation and characterization. In recent
years, layered materials and related heterostructures have attracted huge
attention in this field, due to their huge nonlinear optical susceptibilities,
their ease of integration on photonic platforms, and their 2D nature which
relaxes the phase-matching constraints and thus offers a practically unlimited
bandwidth for parametric nonlinear processes. In this review the most recent
advances in this field, highlighting their importance and impact both for
fundamental and technological aspects, are reported and explained, and an
outlook on future research directions for nonlinear optics with atomically thin
materials is provided.
