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

We performed density functional theory calculations for the topological insulators (TIs) of Bi2Se3and CoSb3(skutterudite) in a comparison with Bi2Te3and 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 Bi2Se3and 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 Bi2Se3surface 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 Bi2Se3surface, we suggest that Bi2Se3is superior to Bi2Te3for 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.