Topological Insulators for Mode-Locking of 2-μm Fiber Lasers

We performed density functional theory calculations for the topological insulators (TIs) of Bi₂Se₃ and CoSb₃ (skutterudite) in a comparison with Bi₂Te₃ and In₂Co₄Sb₁₂ (filled skutterudite) to explore whether the former TIs can serve as saturable-absorption materials for laser mode-locking at the wavelengths of 2 μm and beyond. The calculated electronic-band structures and optical-absorption spectra indicate the potential of these TIs in terms of midinfrared saturable-absorption applications. Especially, in consideration of the film and bulk forms of Bi₂Se₃ and Bi₂Te₃, we investigated the electronic and/or optical properties of the metallic surface states and the semiconducting interior states, respectively. They exhibited an excellent broadband operability regardless of the state, and particularly, we noted a linearly dispersive relation around the Fermi levels of all of the electrons of the Bi₂Se₃ surface in both the conduction and valence bands, whereas this is not evident for Bi₂Te₃. Assuming the importance of linear dispersion for a fast electronic relaxation, and considering the decent oxidation resistance of the Bi₂Se₃ surface, we suggest that Bi₂Se₃ is superior to Bi₂Te₃ for midinfrared mode-locking. A review of the experimental studies regarding TI-based 2-μm saturable absorbers that have been performed so far is presented, and the discussion and rationalization of the possible universality of TIs regarding mid-to far-infrared passive mode lockers results in the suggestion of a new CoSb₃-TI-based saturable absorber.