Ultrafast Nonequilibrium Carrier Dynamics in Topological Insulator Bi₂Se₃ Probed by Terahertz Spectroscopy at Room Temperature

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents a systematic investigation of ultrafast nonequilibrium carrier dynamics in Bi₂Se₃ thin films using optical pump-terahertz probe (OPTP) spectroscopy. The authors clearly demonstrate a pump-induced sign reversal of the transient THz conductivity (Δσ) from positive to negative with increasing pump-probe delay time, and attribute this behavior to the interplay between bulk carriers and Dirac surface states. By combining time-resolved THz transmission measurements with frequency-resolved conductivity analysis and Drude-Smith model fitting, the work quantitatively extracts key transport parameters and distinguishes the respective contributions from bulk and surface states. The results provide valuable insights into carrier relaxation pathways and the role of enhanced scattering in surface states, making this work a meaningful contribution to the field of ultrafast spectroscopy and topological materials.

Overall, I believe this manuscript is suitable for publication after minor revisions. The following points should be addressed to further improve the clarity and completeness of the work:

1. The authors refer to the THz transmission signal ΔE/E₀ obtained from OPTP measurements; however, the manuscript does not explicitly present the reference THz waveform E₀. It would be helpful to include the corresponding TDS signal (E₀) or provide its characteristic parameters for completeness.
2. In Figure 2, the transition of Δσ from positive to negative is a key result of the study. It is recommended that the authors explicitly indicate the transition time (e.g., by adding a marker or guide line) in the figure to enhance clarity.
3. The manuscript employs the Tinkham thin-film approximation to extract transient photoconductivity and assumes a refractive index of n = 3.09 for the Al₂O₃ substrate in the THz range. However, the origin of this value is not specified. The authors should clarify whether this refractive index is obtained from their own calculations or taken from other literatures.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

In this work the authors used optical pump - THz probe (OPTP) 
spectroscopy to study canonical topological insulator Bi2Se3.

Ever since Bi2Se3 was predicted to be a topological insulator,
almost 20 years ago, it has been a subject of numerous experimental
and theoretical studies. A variety of different experimental 
techniques have been applied to it, including numerous spectroscopies: 
time-domain, THz, infrared, microwave, optical, etc. It appears 
that the authors are unfamiliar with most of these studies, 
as they don't cite or mention them in their paper. Some of those 
previous studies have reported much more detail and much more
thorough results on Bi2Se3. In that sense, I don't see anything
new or original in the present manuscript that would be worth 
publishing. The most serious shortcoming of the work is that
measurements were reported only at room temperature. Some
of the most interesting and important effects are observed
at low temperatures. 

For all these reasons, I cannot recommend publication of the
present manuscript. 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Authors studied photoinduced ultrafast dynamics of Bi2Se3 films using OPTP. However, the methods and  samples repeated, as reported previously. The sign change of the dynamics ΔE(t) from first negative to positive latter was also reported and interpreted well in APL 106, 011901 (2015); The only advace may be the separation of negative and positive components in the dynamics by Eq. (2) .

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

In the first round of refereeing I pointed out some deficiencies 
of the manuscript which precluded me from recommended publication. 
The authors have now resubmitted the manuscript with a number of 
changes. They also addressed the issues I raised in their 
response letter. 

I am still not convinced that there is anything novel in this 
manuscript that deserves publication. According to the authors, 
their main goal was to study "systematic disentanglement of 
bulk and surface carrier contributions". This problems has been 
studied numerous times before and the results (much better then 
presented here) have been reported. Those previous studies were 
done at low temperatures, where the "disentanglement" is easier 
to achieve.