Water vapor absorption in THz range

In summary, THz-TDS is a spectroscopic technique that uses radiation in the THz range and consists of an emitter and detector. By comparing a reference signal with a sample signal, spectroscopic properties can be obtained. The fluctuations in the time domain after the main pulse can have two explanations - re-emitted radiation by water molecules or missing signal components at certain frequencies. However, it is more likely that the fluctuations are due to a deconvolution error in the frequency domain to time domain conversion. The use of a Gaussian window in the time domain can also contribute to response smoothing at high delay.
  • #1
Mikhail_MR
17
0
Hi everyone!

Terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique that is using the radiation in the THz range. https://en.wikipedia.org/wiki/Terahertz_time-domain_spectroscopy

A THz-TDS setup consists of an emitter and detector. The emitter creates a short broadband THz-pulse which propagates through a sample. The detector records this short pulse in the time domain (figure 1a in https://royalsocietypublishing.org/doi/full/10.1098/rspa.2007.0294). If we compare a reference signal (without sample) with a sample signal, we can obtain spectroscopic properties of a sample. We can also transform the signal into the frequency domain using the Fourier transform (figure 1b).

When a THz pulse propagates through the air, a part of it is absorbed by water vapor. It results in sharp absorption peaks in the frequency domain as it can be seen in figure 1b (solid line shows a signal with water vapor, the dashed line shows a signal without water vapor between emitter and detector). Sharp peaks in the frequency domain are translated to monochromatic oscillations (fluctuations) in the time domain.

The signal is recorded in the time domain. What is the physical reason for these fluctuations after the main pulse? I can think of two explanations, but I have arguments for and against both of them.

  1. The absorbed radiation is re-emitted by water molecules after some time. Since water molecules absorb the radiation only in specific frequency regions, the re-emitted radiation consists of few sine oscillations. But how can I assume that the radiation is re-emitted towards the detector and not in any other direction? Can I somehow estimate the part that is radiated towards the detector?
  2. The fluctuations are due to missing signal components at these frequencies. But why do the fluctuations after the main pulse stop after some time? Please take a look at figure 6a in the same paper. I would expect them to have the same amplitude until the end of the signal.
I would like to know that is the physical reason for the fluctuations after the main pulse in the time domain.

I appreciate any help.

Kind regards
 
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  • #2
I think the explanation (2) is closer to reality - frequency domain to time domain deconvolution error is to blame. The paper you references also use Gaussian window in time domain, therefore the response is shaped by window (resulting in response smoothing at high delay), not by a physical process.
 
  • #3
Thank you for your response. Could you please explain what deconvolution you have in mind. The figure 6a is a little bit misleading. The solid line shows a signal before Gaussian window was applied. As you can see, the fluctuations become less after some time.
 

1. What is water vapor absorption in the THz range?

Water vapor absorption in the THz range refers to the phenomenon in which water molecules in the Earth's atmosphere absorb electromagnetic radiation in the terahertz frequency range (0.1-10 THz). This absorption is caused by the rotational and vibrational modes of the water molecule, which interact with the THz radiation and cause it to be absorbed.

2. Why is water vapor absorption in the THz range important?

Water vapor absorption in the THz range is important because it affects the transmission of THz radiation through the atmosphere. This can impact various applications that use THz radiation, such as remote sensing, security screening, and communication systems. Understanding and characterizing water vapor absorption in this range is crucial for the development and improvement of these applications.

3. How does water vapor concentration affect absorption in the THz range?

The absorption of THz radiation by water vapor is directly proportional to the concentration of water molecules in the atmosphere. This means that as the water vapor concentration increases, so does the absorption of THz radiation. This is why areas with high levels of humidity tend to have higher levels of water vapor absorption in the THz range.

4. Can water vapor absorption in the THz range be controlled or minimized?

Water vapor absorption in the THz range cannot be completely eliminated, but it can be minimized through various methods. One approach is to use frequency tuning, where the THz radiation is tuned to a frequency range with lower water vapor absorption. Another method is to use dehumidifiers or other techniques to reduce the water vapor concentration in the atmosphere.

5. How is water vapor absorption in the THz range measured or studied?

Water vapor absorption in the THz range is typically measured using spectrometers or other instruments that can detect and analyze THz radiation. These measurements are then compared to theoretical models and simulations to better understand the mechanisms of water vapor absorption in this range. Additionally, field studies and experiments are also conducted to gather data on water vapor absorption in different environments and conditions.

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