Low Threshold and Robust Ultrafast Pulses from Freestanding‐Growth 2D Quaternary BiCuSeO

Abstract Saturable absorber is a key component in ultrafast pulsed laser systems, which are pivotal in ultrafast physics, high‐speed communication, etc. 2D materials appear to be superior candidates as saturable absorbers for compact pulsed fiber lasers in recent years due to their wide band response and ease of integration. However, the integration of mode‐locked devices inevitably involves contamination or polymer in the interface between the layered saturable absorber and the fiber, which contributes to a high start‐up threshold for the ultrafast pulsed lasers. This study develops an on‐fiber femtosecond laser with 2D narrow‐gap semiconducting BiCuSeO as a saturable absorber by a fully dry integration process thanks to the controllable growth of freestanding quaternary BiCuSeO nanoflakes. The corresponding thermodynamic growth mechanism is systemically revealed by electron microscopy and bind‐energy calculation. An electrostatic‐force‐assisted transfer method is developed to avoid damage or chemical contamination. The BiCuSeO‐based femtosecond laser shows decent output performance (pulse width, signal‐noise ratio and repetition rate are 395 fs, 61 dB, 17.1 MHz) with an ultra‐low threshold of 30 mW. The strategy of controllable growth of free‐standing 2D material facilitates a clean interface between the saturable absorber and fibers, which is favorable for high‐performance ultrafast photonic devices.