One sentence from reference say: “ Unlike nonlinear crystals, a lack of phonon modes in plasma prevents THz absorption, resulting in broadband THz radiation with no spectral gap”。 Can we understand that this plasma does not absorb terahertz waves? But， we also often see literature mentioning the absorption of terahertz waves by plasma. So what is the relationship between the nature of the plasma and the absorption of terahertz waves?
The answer is 'yes' and the process is called free carrier absorption. The effect is used all the time to measure electron densities in various plasma conditions using lasers with wavelengths from the near infrared to the millimeter. Either direct attenuation or phase shift have been used depending on wavelength and plasma characteristic.
It mostly depends on the density of the carriers. You could use Drude model to simulate the optical response of a film under different density. The carriers' effect would influence the film's optical state (transparent-loss increasing-loss decreasing-reflectant) with the increase of the density.
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