Local nonlinearity engineering of evanescentfield-interaction fiber devices embedding in black phosphorus quantum dots

Tapered fiber ( TF) and D-shaped fiber (DF) are two types of widely investigated devices in facilitating evanescent-field interactions with external materials. Although they have been found to be particularly useful in various ultrafast regimes, to date there is still no clear or systematic investigation on their local nonlinearities as well as the exerted influences on ultrafast behaviors. Herein, we present such thorough investigation through local nonlinearity engineering on TF and then in contrast with a DF as a reference. Optically deposited black phosphorus quantum dots (BPQDs) are used for saturable absorption. The nanometer-scale extremely small sizes of the BPQDs helpfully eliminate size-induced uncertainties or distortions during both device fabrication and the latter light-matter interaction. For the TF, in the experiment, it is found that the local nonlinear effect starts to be rather appreciable as the TF shrinks to a moderate thickness. Remarkably in comparison, the local nonlinearity of the DF itself can even be neglected reasonably, but after coating with BPQDs, it possesses a much larger modulation depth than any of the used BPQDscoated TFs with different thicknesses/lengths. Further, we theoretically analyze the related locally nonlinear effects and reveal, for the first time, the direct origin of saturable absorption with evanescent-field-based general structures.

Automated summary

Tapered fiber and D-shaped fiber are two widely investigated devices for evanescent-field interactions with external materials. In this study, the local nonlinearities of these fibers were thoroughly investigated. It was found that the local nonlinear effect in the tapered fiber becomes appreciable as it shrinks, while the D-shaped fiber itself has negligible local nonlinearity. However, after coating with black phosphorus quantum dots, the D-shaped fiber exhibits a larger modulation depth compared to any of the coated tapered fibers of different thicknesses/lengths. The study also provides a theoretical analysis on the origin of saturable absorption with evanescent-field-based structures.