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

We performed density functional theory calculations for the topological insulators (TIs) of Bi
₂
Se
₃
and CoSb
₃
(skutterudite) in a comparison with Bi
₂
Te
₃
and In
₂
Co
₄
Sb
₁₂
(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 Bi
₂
Se
₃
and Bi
₂
Te
₃
, 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 Bi
₂
Se
₃
surface in both the conduction and valence bands, whereas this is not evident for Bi
₂
Te
₃
. Assuming the importance of linear dispersion for a fast electronic relaxation, and considering the decent oxidation resistance of the Bi
₂
Se
₃
surface, we suggest that Bi
₂
Se
₃
is superior to Bi
₂
Te
₃
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 CoSb
₃
-TI-based saturable absorber.