A research team led by Kaihui Liu at Peking University, together with Zhipei Sun from Aalto University, has demonstrated a new way to endow optical fibres with powerful nonlinear optical functions—long considered difficult to achieve in fully fibre-based systems.

The study, published in Nature Materials, introduces a fibre-end integration strategy that attaches ultrathin van der Waals crystals directly onto the flat end face of an optical fibre. By carefully stacking and rotating layers of rhombohedral boron nitride (rBN), the researchers created a compact nonlinear optical crystal that works seamlessly inside an all-fibre architecture.

Why this matters

Optical fibres are the backbone of modern photonics, underpinning telecommunications, fibre lasers, and sensing technologies. They are robust, compact, and highly stable. However, standard silica fibres lack second-order optical nonlinearity—a key ingredient for functions such as frequency doubling, quantum light generation, and fast electro-optic modulation.

Until now, adding these capabilities typically required bulky external crystals and free-space optical components, undermining the advantages of fibre systems.

A twist that makes the difference

The team solved this problem by exploiting a unique property of two-dimensional materials. When  rBN flakes are stacked with precisely chosen rotation angles, they introduce a controllable geometric phase. This “twist-based” phase matching allows light inside the fibre to efficiently interact with the crystal—even under simple linear polarization conditions commonly used in fibre systems.

Using this approach, the researchers achieved:

• Highly efficient second-harmonic generation (SHG), converting infrared light to visible light with record performance for fibre-integrated van der Waals devices.

• Photon-pair generation via spontaneous parametric down-conversion (SPDC), a core process for quantum optics and secure communications.

• An all-fibre mode-locked frequency-doubling laser, built by integrating the nonlinear rBN crystal together with a graphene saturable absorber on the same fibre end face.

All of these functions operate within a fully sealed, alignment-free fibre system.

Figure | Fibre-integrated nonlinear s crystal and all-fibre frequency-doubling laser.