Giant Enhancement of Optical Second Harmonic Generation in Hollow-Core Fiber Integrated with GaSe Nanoflakes

Abstract

All-fiber functional devices are superior to conventional optical crystals for next-generation integrated optics owing to their natural compatibility with optical fiber systems. Nonlinear optical fiber devices play an important role in frequency conversion and optical parametric amplification. However, optical fibers are unsuitable for all-optical systems owing to the intrinsic properties of pure quartz. Optical second harmonic generation (SHG), which is significant in practical optical applications, is theoretically forbidden in traditional centrosymmetric non-crystalline fused silica fibers. Consequently, generating giant second-order optical processes in optical fibers remains challenging. Many studies have attempted to artificially break the centrosymmetry of fused silica fibers using various poling techniques, such as thermal or electric field poling, which can enhance the second-order nonlinear optical susceptibility. However, these methods require difficult and complicated fabrication processes, and the corresponding hybrid optical fibers exhibit an inefficient harmonic generation process, which greatly increases the cost and limits the development of all-fiber nonlinear functionalization. Therefore, there is an urgent need for new fabrication methods and technical means for functionalizing optical fiber devices that can improve the second-order nonlinear effect while remaining simple and practical. Herein, we propose an improved solution-filling method that can effectively deposit highly nonlinear GaSe nanoflakes directly on the inner walls of hollowcore fibers (HCF) with a length of up to half a meter. In addition, the as-fabricated hollow-core fiber integrated with GaSe nanoflakes (GaSe-HCF) is used to demonstrate that the second-order nonlinear effect of the optical fiber is enhanced by the ultrahigh nonlinear effect of the GaSe materials. Compared to previously reported MoS2-embedded hollow-core fibers (MoS2-HCF) and conventional optical fibers, the SHG of the GaSe-HCF is three and two orders of magnitude stronger than that of bare HCF and MoS2-HCF, respectively. A GaSe-HCF with a length of up to half a meter was successfully prepared using the new filling method and exhibited good expansibility. The pressure process was exploited by adding a short length