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Two-dimensional materials have gained enormous attention in the scientific community recently for their unconventional properties and characteristics which find applications in a large number of industries.
2D materials are materials which ideally consist of a single layer of atoms, therefore giving them a sheet-like structure. These materials are significantly different from their bulk counterparts in terms of their physical and chemical properties. These 2D materials became a focus of a new branch of material research after the discovery of graphene which was isolated from graphite in 2004. Phosphorene which is a 2D counterpart to bulk black phosphorus (BP). It is a semiconducting material with its bandgap dependent on the thickness of its layer. Black phosphorus has applications in electronics and optoelectronics and by isolating a monolayer of this material opens up new pathways for its applications by tuning the band gap that can be extended from 0.3 eV to 2 eV. Thus by adjusting the thickness which is essentially the number of layers of the black phosphorus affects the field-effect mobility and current on/off ratio which is a critical property for its applicability in optoelectronic devices. A few ways to extract phosphorene include mechanical cleavage, physical exfoliation and liquid phase exfoliation.
Ultrafast Fiber Lasers
Ultrafast fiber lasers have been in existence for quite some time now and have a wide range of scientific and industrial utilities. This is driven by continuous development of saturable absorbers along with low-loss new gain fiber medium, enabling optimum ultrafast pulse generations. The function of a saturable absorber is to act as a switch in a laser cavity which in simple words is the reduction of optical absorption which increases in the incident light intensity, resulting in the production of ultrashort pulses using either mode locking or Q-switching techniques. In past various saturable absorbers have been employed such as semiconductor saturable absorber mirrors (SESAMs) and nonlinear polarization evolution (NPE) for generation of ultrafast optical pulses but they possess inherent limitations such as narrow operating bandwidth, the complexity of fabrication and sensitivity to environmental fluctuations. Thus there is a great need for the development of alternative materials which can provide optimum nonlinear properties to be used as saturable absorbers in ultrafast lasers.
Advantages of Phosphorene
Having highly tunable bandgap which is dependent on the thickness of phosphorene enables its use for the development of ultrafast lasers. Phosphorene shows nonlinear absorption in the ultrafast regime. This optically saturable absorption can be used to design optical fiber for ultrafast lasers. The first demonstration of nonlinear optical properties of exfoliated layers of black phosphorus was done by Chen et. al. at Shenzhen University in China. They demonstrated both Q-switched and mode-locked performance of phosphorene based ultrafast lasers, which later could be used in the wavelength range of 1-3 μm thus proving the candidacy of phosphorene as a viable broadband saturable absorber material. This is particularly of great interest in photonics since it bridges the present gap between the zero bandgap graphene and relatively large bandgap transition-metal dichalcogenides (TMDs).
Apart from using the only phosphorene a heterojunction between various 2D materials might hold the key to get the perfect saturable absorbers. Liu. et. al. demonstrated graphene/phosphorene nano-heterojunctions for the development of ultrafast lasers. Their findings indicate that the combined advantages of ultrafast relaxation, a broadband response in graphene, and a strong light-matter interaction in phosphorene combine together by employing this heterojunction and thus by integrating them to erbium-doped fiber laser they demonstrated the generation of a stable ultrashort pulse of the pulse duration of 148 femtoseconds. The fabrication of 2D nano-heterojunctions is yet another approach that can be employed for efficient, scalable and tunable materials for ultrafast photonics.
In conclusions, phosphorene has opened up new pathways for the development of tunable saturable absorbers for the development of ultrafast lasers. It has shown enormous promise for the development of these lasers and further research is required for industrial realization by scaling up the phosphorene synthesis techniques and thus provide a cost-effective and highly efficient substitute for currently employed SESAMs.
Sources and Further Reading