Nonlinear phase-matched van der Waals crystals integrated on optical fibres.

High optical nonlinearity can enable classical and quantum functionalities in all-fibre laser systems. However, despite long-standing efforts to exploit second-order optical nonlinearity in conventional all-fibre systems, nonlinear optical conversion efficiencies remain modest. Here we demonstrate all-fibre integration of twist-phase-matched rhombohedral boron nitride (rBN) flakes on the end facet of optical fibres for second-harmonic generation (SHG) and spontaneous parametric downconversion (SPDC). We provide local and global optimization of the interflake twist angles for phase-matching design, achieving an SHG conversion efficiency of ~4.1% and an SPDC coincidence rate of ~90 in van der Waals crystals integrated on optical fibre devices. Finally, we design an all-fibre frequency-doubling ultrafast laser by integrating a multifunctional nonlinear crystal of a graphene/rBN heterostructure to simultaneously generate mode-locked pulses and intracavity SHG emission. This work establishes a route for developing high-efficiency, second-order nonlinear functionalities, such as optical parametric oscillators, optical modulators and entangled photon sources, in all-fibre lasers.