Terahertz Birefringence and Dichroism of KTA Crystal

Round 1

Reviewer 1 Report

This paper investigated refraction indices and absorption spectra of KTA crystal in the main transparency window and THz frequency range. A commercial THz spectrometer (TeraView, UK) is used to determine for the first time all three components of the  index and the absorption in the 0.3-2.1 THz spectral range. Dispersions of all refractive index components were approximated in the form of Sellmeier equations for the first time. It is found that in the spectral region below 0.5 THz, the crystal can be considered as almost uniaxial. The phase-matching angles for DFG generation along XY plane during the interaction of a pulse centered at 1.0642 µm and a pulse tunable from 0.3 µm to 1.1 µm range is presented. The collinear phase-matching conditions for DFG are also given. At room temperature, the largest absorption coefficient component a_z at 1 THz is about 7 cm^-1 and the other two components are below 1 cm^-1. It is much lower than that for widely used LiNbO₃ crystal (respectively, 21 cm^-1 and 7 cm^-1). Again, absorption coefficients are rapidly decreasing in the longer wavelength range that render  KTA crystals as one of the most prospective for long wavelength THz wave generation under intense pump.  The data can be useful for researchers in related fields and I recommend to publish it in Crystals journal. Nevertheless, as it is mentioned in the manuscript that one of the advantages of KTA crystals is the value of nonlinearity, it would be helpful to see numerical comparison of the KTA nonlinearity with those of other standard crystals.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors present experimental results for the three-axis dependent absorption coefficient of KTA nonlinear crystal using Time-domain Spectroscopy in the Terahertz range. Using a method described in Ref.9 they compute the respective refractive index. They conclude that KTA has a large birefringence of Δn_x-z≈0.76 below 1THz, which is attributed to a strong absorption of THz waves for the z-axis around 1.23THz. The authors discuss the possibility to achieve phase matching between THz and NIR pump for DFG and SFG. The results are important for the THz community, which can be used to design new THz generation and detection systems. Before publication I do have a few questions:

• It is not clear to me what the benefits over ohter NL crystals like ZnTe or LiNbO are. The author should emphasize the advantages of using KTA over other nonlinear crystal. (better loss, phase matching ability, etc.)
• It is well known that one can achieve phase matching with ZnTe using a pump at around 780nm. What improvement is expected with KTA from Fig4?
• In the introduction, the author mention that the another report (ref. 8) is not accurate and that it will be shown later. There is not reference to it later in the text. From the reference I can see that the data is different. The author should have a short discussion why that is. Give a possible explanation why the difference. Especially if mentioned in the introduction.
• How did the authors reference their methods? Did you use a reference sample to calibrate the system?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Huang et al. characterized the dielectric function of KTA over a broad frequency range and suggest it could be used for THz generation. The work is relevant, the figures are clear, and a properly revised version should be accepted.

However, I found the current description of the results poor in some parts, especially about the THz generation. In particular the following must be improved:

• Several quantities in Fig.4 and in the corresponding text are undefined, e.g., what is kOPO, s, f, VZ angle? 
• The authors talk about DFG, but only one pump wavelength is given, it is unclear to me whether this is optical rectification instead

Minor points:

• please check the spelling through the text. Many articles are missing and there are typos here and there (e.g. principle should be principal in the abstract, I suppose)
• "high" damage threshold of 1 GW/cm2, this is far lower than other crystals, please remove "high" or specify which crystals you are referring to
• "Moreover, as will be shown later, the result carried out in [8] was not accurate". I found no indication later in the text discussing this point. I also think that this sentence might sound offensive to the authors of [8]. Please revise it.
• "We assume that absorption coefficient of the samples is below 0.1 cm-1" and "Both curves fall to immeasurably low levels below 0.5 THz". Can the authors estimate a lower limit for the detectable absorption?

Author Response

Please see the attachment.

Author Response File: Author Response.docx