High-efficiency second-order nonlinear processes in an optical microfibre assisted by few-layer GaSe.

Non-linear optics: All-fibre optical wavelength converter An optical fibre coated with gallium selenide nanoflakes could convert the wavelength of an incident light efficiently via the second-order nonlinear optical effect. This wavelength-converting optical fibre, developed by Xuetao Gan and colleagues at China's Northwestern Polytechnical University, could be incorporated into existing optical fibre-based infrastructure to provide a wide range of applications, including in telecommunications, signal processing and multi-parameter sensing. The researchers found that, with the assistance of gallium selenide nanoflakes, the efficiency of wavelength conversion in the coated microfibre was more than four orders of magnitude higher than that obtained in a bare microfibre. The all-fibre wavelength converter allows the easy operation with a sub-milliwatt continuous-wave laser over a broad wavelength range, and has the considerable stability and repeatability. The centrosymmetric nature of silica fibre precludes the realisation of second-order nonlinear processes in optical fibre systems. Recently, the integration of 2D materials with optical fibres has opened up a great opportunity to develop all-fibre active devices. Here, we demonstrate high-efficiency second-order nonlinear frequency conversions in an optical microfibre assisted with few-layer gallium selenide (GaSe) nanoflakes. Attributed to the strong evanescent field of the microfibre and ultrahigh second-order nonlinearity of the GaSe nanoflakes, second harmonic generation (SHG) and sum-frequency generation (SFG) are effectively achieved with only sub-milliwatt continuous-wave (CW) lasers in the wavelength range of 1500-1620 nm, covering the C and L telecom bands. The SHG intensity from the microfibre is enhanced by more than four orders of magnitude with the assistance of the GaSe nanoflakes on fibre nonlinear processes. Moreover, in the SFG process, the intensity transfer between different frequencies can be effectively manipulated by changing the wavelengths and powers of two pump lasers. The realised strong second-order nonlinearity in the GaSe-integrated microfibre might expand the applications of all-fibre devices in all-optical signal processing and new light source generation at awkward wavelengths.