Phase-matching-free intracavity second harmonic generation from a WSe₂ monolayer in a passively mode-locked picosecond fiber laser

We demonstrate phase-matching-free intra-cavity SHG in a passively mode-locked, Yb³⁺ doped, picosecond fiber laser from a monolayer TMD. This shows the feasibility of harvesting the ultra-high non-linearity of non-centrosymmetric monolayers in an all-fiber integrated laser system.

A monolayer WSe₂ flake is exfoliated and transferred onto the tip of a dome-shaped angle-polished passive fiber. Hyperspectral PL mapping on the fiber confirms the core is fully covered by the monolayer. In order to incorporate it directly into the laser cavity, it is sandwiched between a passive fiber and the Yb-doped gain fiber. The quadratic power dependence of the SHG signal is clearly indicative of second order nonlinear phenomena. In comparison, a linear relation in the absence of the monolayer indicates that the SHG response originates from the monolayer but not interfaces or non-centrosymmetric features in the laser cavity.

We report SHG generation inside a fiber laser cavity by transferring a WSe₂ monolayer onto a fiber core while preserving stable mode-locking and output power. Such a fully integrated fiber laser system can provide second harmonics at relatively low peak powers (30-50 W). This opens up the pathway for fully fiber-integrated Carrier Envelope Offset (CEO) stabilization and self-referencing f-2f interferometry in frequency combs.