Black phosphorus saturable absorber for ultrafast mode-locked pulse laser via evanescent field interaction

Black phosphorus, or BP, has found a lot of applications in recent years including photonics. The most recent studies have shown that the material has an excellent optical nonlinearity useful in many areas, one of which is in saturable absorption for passive mode-locking. A direct interaction scheme for mode-locking, however, has a potential to optically cause permanent damage to the already delicate material. Evanescent field interaction scheme has already been proven to be a useful method to prevent such danger for other 2-dimensional nanomaterials. In this report, we have utilized the evanescent field interaction to demonstrate that the optical nonlinear characteristics of BP is sufficiently strong to use in such an indirect interaction method. The successful demonstration of the passive mode-locking operation has generated pulses with the pulse duration, repetition rate, and time bandwidth product of 2.18 ps, 15.59 MHz, and 0.336, respectively. A black phosphorus (BP) D-shaped fiber saturable absorber is constructed to employ evanescent field interaction to use the nonlinear optical absorption of BP for passive mode-locking operation in C-band. The constructed device exhibited light modulation depth of ∼3.31% with saturation peak power density of 12.5 MW/cm², and generated mode-locked pulses with duration of ∼2.18 ps with repetition rate of 15.59 MHz.