Significance Since the discovery of graphene in 2004, new monoelement based two-dimensional opto-electronic nanomaterials have received a great attention owing to the novel and excellent optoelectronic, electronic and mechanical features. Especially in recent years, it has stimulated the researchers to investigate the group-VA nanomaterials because of the excellent physiochemical properties. As a transition metal in group-VA, bismuth-based nanomaterials have been one of the research hot spots in the fields of materials and optics, which show great potential in the applications of electronics, opto-electronics and nonlinear optics, etc.
Progress Generally speaking, the optical and electronic properties of the bismuth based nanomaterials (including bismuthene) are related to the band structures and the carrier mobility in the nanomaterial systems. Normally, the design, fabrication and characterization have important impacts on the optical and electronic applications of the bismuth based nanomaterials. It is no doubt that the theoretical simulation, experimental study and the final application of the bismuth based nanomaterials have attracted a lot of attentions. Therefore, we briefly introduce the theoretical and experimental studies in terms of the simulation, fabrication, characterization and nonlinear optical applications in the recent years in this review. Theoretically, the first-principle simulation is the most powerful tool to understand the stability, band structures and carrier mobility. Up to date, both Heyd-Scuseria-Ernzerhof (HSE) hybrid-density functional theory (DFT) calculation and Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional calculation are used to simulate the monolayer bismuthene bandstructures and densities of states no matter taking the spin-orbit coupling (SOC) effect into account or not. Experimentally, different fabrication strategies of the bismuth based nanomaterials are reported. In 1960s, the physically sputtering method was taken to make the high quality bismuth ultrathin film in the micro-nano scale. With the development of the technology, molecular beam epitaxy (MBE) technology, ultrahigh vacuum (UHV) evaporation technology and the liquid-phase exfoliation (LPE) technology were proposed to obtain the few-layered bismuth ultrathin membranes. It was found that with different substrates, owing to the different lattice mismatching, the initial disordered wetting layer owing to the bismuth substrate interaction at the interface was observed, following by the stack of the bismuth layers until the normal lattice formation of the bismuth film. Obviously, the bismuth-based nanomaterials have the different topological insulating features. Thus the bismuth based nanomaterials are expected to possess a large nonlinear optical susceptibility like other topological insulators. Since 2017, researchers have investigated the nonlinear optical properties of few-layered bismuth based nanomaterials with different Z-scan and spatial phase modulation measurements. Currently, with the bismuthene in the fiber evanescent field as the saturable absorber, the minimum pulse duration of as short as 200 fs was realized in an Er-doped fiber at 1561 nm. While for the Yb or Tm doped fiber lasers, the mode-locking pulse duration was much longer. For the Q-switched operation, the shortest pulse durations at 1, 1.3 and 2 μm were measured as 150, 155 and 440 ns, respectively. However, in the mid-infrared optical region, no matter the gain medium was a bulk crystal (Er∶SrF2) or an Er∶ZBLAN gain fiber, the pulse duration was as long as ~ 1 μs. In addition, owing to the thermo-optical effect of bismuth, the bismuth nanomaterials could be used to realize the all-optical modulation with another manipulating beam. The recent investigations confirmed that the bismuth nanomaterials, synthesized by a lot of methods with different structures and thicknesses, possessed the excellent nonlinear optical features so that they could be used to produce the stable pulses in the optical region from the near-infrared to the mid-infrared.
Conclusion and Prospect In conclusion, we briefly review the recent progress in the preparation, characterization and the nonlinear optical properties of bismuth-based nanomaterials, and highlight the research of the optical absorption features and the applications in the laser pulse generation. We believe that with the continuous in-depth research on preparation technology and properties of bismuthene nanomaterials, bismuthene materials will inevitably exert greater advantages in the fields of optoelectronic information technology and material science and technology. However, the future research of bismuth nanomaterials should be emphasized in these three aspects:
1) In terms of theoretical research, the electronic band structure of bismuth nanomaterials has been basically clear, but there are few studies on photogenerated carriers and their optical properties, therefore, the theoretical model and micro-mechanism still need further research;
2) Although the preparation process of bismuth nanomaterials grown on silicon has been relatively mature, the application of bismuth nanomaterials in silicon-based photonics is still in urgent need of development;
3) The bismuth nanomaterials exhibit good nonlinear optical absorption characteristics and have achieved mode-locked fiber laser output. However, solid-state mode-locked lasers have not been reported, and the mode-locked pulse width in the mid-infrared band is as high as nanosecond. Obviously, researchers need to further study the effect of the anisotropy and topological properties of bismuthene materials on the nonlinear optical properties, and further optimize the nonlinear optical properties of bismuthene nanomaterials to achieve the breakthrough of the above-mentioned mode-locked laser.
