Functionalized Optical Micro/Nanowires And Their Lasing Applications

Optical micro/nanowires, including optical microfibers, semiconductor nanowires, and metal nanowires etc., have become one of the frontier research directions in fields of fiber optics and micro/nanophotonics. As novel integrated coherent sources, lasers based on quantum-dots-/rare-earth-ions-/dye-doped microfibers and semiconductor nanowires have intrigued great research interests and found wide applications in optical communication, sensing and signal processing. To realize versatile functionalities of optical micro/nanowires in lasing applications, we propose two kinds of functionalized hybridized nanowires (including hybrid semiconductor-metal nanowires and graphene-coated microfibers), to realize sub-diffraction-limit lasing beams and sub-picosecond lasing pulses, respectively.In the first part of the work, owing to the strong optical confinement (λ/10～λ/20) of metal nanowires and high optical gain (e.g.,3000 cm’1) of semiconductor nanowires, we proposed a longitudinally hybrid photon-plasmon nanowire laser, which can provide spatially-separated surface plasmon cavity mode, and sub-diffraction-limit beam size (～λ/10) as well. The lasing wavelength is centered at-723 nm, with a line-width of 0.38 nm and a threshold of～60 μJ/cm2. Also, the hybrid photon-plasmon nanowire laser can be modulated by solely modifying the propagation loss of the plasmon modes in the Ag nanowire. For example, we realize lasing switching by coupling a second nanowire to the Ag nanowire (equivalent to introducing an extra loss to the original cavity). These results show great potential in ultrafast modulation for plasmonic lasers.In the second part of the work, we propose an effective approach to transferring CVD graphene layers onto microfibers. Based on the saturable absorption (SA) of graphene, we use the graphene-coated microfiber to realize a low-insertion-loss, and low-threshold saturable absorber. For example, by coating a 20-μm-length graphene layer onto 1-μm-diameter microfiber, we realize a saturable absorber with a net transmittance of 73%(i.e., the transmittance of the microfiber without graphene is 90%), and a threshold of 40 MW/cm2. In addition, we transfer a 15-μm-length multi-layer graphene flake onto microfiber and use it to realize all-optical modulation with modulation depth of 12%. Also, by using a 1-μm-diameter microfiber with 20-μm-length monolayer graphene flake, we achieve a passively mode-locked fiber laser with a repetition rate of 8.6 MHz and a pulse duration of 970 fs. These results lay the foundation for all-microfiber mode-locked lasers, and provide a new approach to ultrafast modulation for microfiber lasers.