Talk:Terahertz time-domain spectroscopy

Coherent measurement

"Because the measurement technique is coherent, it naturally rejects incoherent radiation. Additionally, because the time slice of the measurement is extremely narrow, the noise contribution to the measurement is extremely low."

I think this remark about the measurement being coherent needs explaining, as it stands, it seems meaningless. How is the measurement coherent? — Preceding unsigned comment added by 130.246.148.59 (talk) 18:10, 7 January 2021 (UTC)

Discussion of the two edits made on 21 Jan.

The first edit was for the generation discussion only. The major aspect of the change was to remove the word "spark" from the discussion. A spark is conduction through an ionized gas (plasma). There are definetly no sparks in THz generation. The EM emitted from a spark gap is a good analogy for the generation method, but it is important to stress that no spark ever occurs.

I have not registered as a contributor yet (not certain how) but I am an R&D scientist building state-of-the-art THz-TDS systems. Additionally, my Ph.D. research group studied spark and arc analytical instrumentation so I am also familar with that technolgy. FYI, what is the difference between a spark and an arc? An arc is caused by the simple overvoltage of a gap. Thus, there is no precise temporal control of when the arc will occur. A spark is generated by a high voltage switch. Thus, there is very precise temporal control.

The second edit fixed some minor problems with the Genreation section and added the Detection section. This is my first pass of the Detection section so it may not be as clean as it could be.

I plan on contributing to the pulse shape / frequency content section soon.

-JSW —The preceding unsigned comment was added by 141.214.17.5 (talk • contribs) 06:34, 22 January 2006 (UTC)

You missed one thing: the short duration of the generated terahertz pulse does not require a rapid carrier trapping (or recombination) time in the substrate. A rapid increase of the photocurrent is sufficient; a rapid decrease also works but is not crucial. In fact, most practitioners of THz-TDS use semi-insulating gallium arsenide (GaAs) substrates for THz generation. In that material, the photocurrent persists for nanoseconds after optical excitation, and yet the generated THz pulse is still short. —The preceding unsigned comment was added by 70.240.180.76 (talk • contribs) 00:40, 24 January 2006 (UTC)

Yes, that is correct. I thought it might be too involved to explain so succiently. Good edit.

-JSW —The preceding unsigned comment was added by 208.178.99.146 (talk • contribs) 20:20, 24 January 2006 (UTC)

nonpolar?

The introduction says one of the advantages of the form of spectroscopy is that the radiation is "nonpolar". This term requires some elaboration. It's not clear to me what is meant, and I am more familiar with TDS than the typical reader. Spiel496 18:50, 6 March 2006 (UTC)

I would have to consider that to be an error. Which I am about to fix. There is no meaning in describing electromagnetic radiation as 'nonpolar'. —The preceding unsigned comment was added by 70.240.204.99 (talk • contribs) 03:38, 15 March 2006 (UTC)

Thz Spectroscopy =/= TDTS

Can I ask why is the Terahertz Spectroscopy completelly redirected to TDTS?!

That's ridiculous! THz Spectroscopy can be

• time domain (TDTS)
• frequency domain (FDTS)

We us broadband THz sources for time domain measuremnets (by optical rectification, photoconductive witches,...). Measurements are usually slow, lower resolution, but wide spectra. We us monochromatic (continuous wave, CW) THz sources for frequency domain measuremnets (by frequency matching - +tunable,...). Measurements are quick, high resolution in the frequency domain, but usually narrow spectra. —The preceding unsigned comment was added by 86.49.61.227 (talk • contribs) 19:57, 17 November 2006 (UTC)

Please do create new articles on Terahertz spectroscopy and Terahertz frequency domain spectroscopy as necessary! Melchoir 19:24, 18 November 2006 (UTC)

Thanks. OK, I'll do it. I don't know how to create and set them "as necessary", but they'll be created, some basic info will be added. Milan Berta 19:59, 18 November 2006 (UTC)

Removal of Surface Emission Generation

I removed the recently added reference to the 'Surface Emission' generation technique because, as far as I know, this technique is not commonly applied to spectroscopic applications. If you could provide some references to its application, I would be happy to look through them and reconsider its inclusion. ronningt (talk) 15:02, 19 July 2008 (UTC)

Diagrams

I think that we really need to show time-domain traces and spectra for a pulse transmitted through a sample, compared with a reference pulse from an empty TDS system. This will make it much easier to explain how materials can be characterised. Papa November (talk) 11:06, 17 April 2011 (UTC)

Analysis

I also think we need to include a bit of maths here... an explanation of how to calculate the refractive index/thickness, Fresnel reflectivity and Beer's Law absorption coefficient from the Fourier-transformed traces. Papa November (talk) 09:02, 27 July 2011 (UTC)

Safety

Can someone with domain knowledge speak to the information presented in this paper? http://arxiv.org/pdf/0910.5294v1.pdf — Preceding unsigned comment added by 2600:1004:B018:B0FA:2487:4132:C486:2AEB (talk) 17:57, 23 September 2015 (UTC)

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Assessment comment

The comment(s) below were originally left at Talk:Terahertz time-domain spectroscopy/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

The comment about THz being non-damaging to human tissue is misleading. Nonionizing radiations have thresholds above which they are not safe. See the Terahertz radiation article for a comment about safety. Montyhp (talk) 03:38, 14 October 2009 (UTC)

Last edited at 03:38, 14 October 2009 (UTC). Substituted at 07:48, 30 April 2016 (UTC)

"Advantages of THz radiation" section

This section contains a number of misleadingly or falsely claimed "advantages" over other wavelength ranges for spectroscopy.

• "many materials are transparent to THz" - that's a disadvantage for spectroscopists, we need wavelength ranges where the light is absorbed by the sample.
• "THz radiation is safe for biological tissues because it is non-ionizing" - not an advantage over other spectral ranges, since the same is true for the far/mid/near IR, VIS and near UV spectral ranges
• "images formed with THz radiation have relatively good resolution" - the THz range has the worst imaging resolution of all, since the wavelength is longer than in any other spectral range used for spectroscopy, other than magnetic resonance techniques.
• "many interesting materials have unique spectral fingerprints in the THz range, which means that THz radiation can be used to identify them" - that is true for very many more interesting materials in the IR spectral range. Also, the "many" materials claimed to be transparent in the THz range clearly do not have spectral signatures here - the author needs to clarify what they mean by "many".
• "Since many materials are transparent to THz radiation, these items of interest can be observed through visually opaque intervening layers, such as packaging and clothing." This is presumably what was meant by the previous claim of transparency as an advantage, in which case the previous claim is an unnecessary duplication. In any case, this cannot be claimed as an advantage of THz over other spectral ranges, since near IR measurements through packaging are already routine for example in the pharmaceutical industry.
• "The ultrashort width of the THz radiation pulses allows ..." - an explanation is needed of why the same would not also apply to ultrashort pulses in any other spectral range
• "THz measurements are non-contact" - the same is true of spectroscopy in all other spectral ranges, unless attenuated total reflection techniques are being used.
  

David Moss, Karlsruhe Institute of Technology, 30th November 2016 — Preceding unsigned comment added by 2A00:1398:9:FB01:DD7:5946:1217:5989 (talk) 11:02, 30 November 2016 (UTC)

Removed Paragraph From "Explanation"

I removed this paragraph in favor of a more brief explanation:

For detection, the electric field of the terahertz pulse is sampled and digitized, conceptually similar to the way an audio card transforms the electrical voltage levels of an audio signal into numbers describing the audio waveform. In THz-TDS, the electric field of the THz pulse interacts in the detector with a much shorter laser pulse (e.g. 0.1 picoseconds) in a way that produces an electrical signal proportional to the electric field of the THz pulse at the time the laser pulse gates the detector on. By repeating this procedure and varying the timing of the gating laser pulse, it is possible to scan the THz pulse and reconstruct its electric field as a function of time. Subsequently, a Fourier transform is performed to extract the frequency spectrum from the time-domain data.

I don't know that there's advantage to saying the electric field is digitized. These measurements could be done from LT-GaAs analog. Also, I think the audio reference is too specific and only helps those with knowledge of audio cards. It's not true that the THz electric field interacts with the laser pulse. With LT-GaAs, the THz electric field interacts with the charge carriers created by the ultrashort pulse. The final two sentences are covered by the (new) preceding paragraph. — Preceding unsigned comment added by Mwarren14 (talk • contribs) 15:43, 20 December 2017 (UTC)