  The unique surface edge states make topological insulators a primary focus
among different applications. In this article, we synthesized a large single
crystal of Niobium(Nb)-doped Bi2Se3 topological insulator (TI) with a formula
Nb0.25Bi2Se3. The single crystal has characterized by using various techniques
such as Powder X-ray Diffractometer (PXRD), DC magnetization measurements,
Raman, and Ultrafast transient absorption spectroscopy (TRUS). There are (00l)
reflections in the PXRD, and Superconductivity ingrown crystal is evident from
clearly visible diamagnetic transition at 2.5K in both FC and ZFC measurements.
The Raman spectroscopy is used to find the different vibrational modes in the
sample. Further, the sample is excited by a pump of 1.90 eV, and a kinetic
decay profile at 1.38 eV is considered for terahertz analysis. The differential
decay profile has different vibrations, and these oscillations have analyzed in
terms of terahertz. This article not only provides evidence of terahertz
generation in Nb-doped sample along with undoped sample but also show that the
dopant atom changes the dynamics of charge carriers and thereby the shift in
the Terahertz frequency response. In conclusion, a suitable dopant can be used
as a processor for the tunability of terahertz frequency in TI.
