Abstract
This review summarizes recent developments of saturable absorbers (SAs) based on 2D materials for nonlinear optical absorption and ultrafast pulsed laser generation. Apart from graphene, various 2D materials such as topological insulators and transition metal dichalcogenides are investigated for SA applications and their important potential as passive mode-lockers for femtosecond laser production are extensively investigated. By selecting appropriate 2D materials, a wide spectral range of passively mode-locked pulsed lasers are obtained, covering visible, midinfrared and a terahertz region. A set of different approaches is used for fabricating SA modules of fiber laser photonics, which include sandwiching, side-polishing and tapering methods. Noticeably, through systematic studies, it is demonstrated that layer-stacking seldom deteriorates the SA performance of 2D materials in the evanescent regime, although their ultrathin nature may improve the efficiency in a transmission mode like sandwich-type SAs. The direction for designing new SAs is presented based on material characterization.
Highlights
Saturable absorption is one of nonlinear optical phenomena in which optical absorption decreases as incident light intensity increases, which can be explained by excited electron dynamics and Pauli’s blocking principle
As the saturable absorption process repeats by saturable absorbers (SAs), eventually lasers with a significantly high intensity would only survive in the cavity and they would correspond to highly mode-locked pulsed lasers, which is referred as passive mode-locking
Through extensive investigation on mode-locked fiber lasers performed over recent decades, it was discovered that five different types of pulses can appear in the fiber cavity via mode-locking, depending on the interplay between the Kerr effect and group velocity dispersion, which are the conventional solution [68,69], stretched pulse [70], similariton [71], dissipative soliton [72,73] and dissipative soliton resonance [74]
Summary
Along with ever-growing laser technologies, nonlinear optics has occupied one of the most important areas in modern photonics [1]. After the discovery of graphene, many new kinds of 2D nanomaterials have been developed and intensively investigated for their applications to photonic engineering in parallel with studies using conventional nanomaterials [7,8,9,10,11] These post-graphene 2D materials are transition metal dichalcogenides (TMDCs), topological insulators (TIs), phosphorene, MXenes and so on [12,13,14,15,16,17,18,19,20]. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations This field presented many noteworthy results, from which compact, high-performance photonic devices were successfully fabricated for optical switching and ultrafast pulsed laser production. We summarize the results and present the perspective of 2D material SAs in laser technologies
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