  Three-dimensional topological insulators possess various interesting
properties that are promising for various modern applications, including in the
recently emerging fields of ultrafast THz photonics and spintronics. Their
gapless spin-momentum-locked topological surface states with the presence of
chiral spin structure are relevant for the development of light
helicity-sensitive THz emitters and detectors. In this paper, we report
femtosecond excitation pulse wavelength and helicity-dependent response of the
three-dimensional topological insulators for an enhanced broadband THz pulse
emission. Specifically, the excitation wavelength has been varied in a large
range from near UV to near IR and two model systems of Bi2Te3 and Bi2Se3 single
crystals have been studied. It was observed that the photoexcitation at shorter
wavelengths enhances the THz emission from both the surface and bulk states.
Primarily, circular photogalvanic effect is responsible for the surface
contribution, while the prominent contribution in the bulk is from photon-drag
effect. The wavelength-dependence of the THz emission is explained by
accounting for the free carrier absorption of THz radiation within the material
accurately.
