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What does determine the plasma frequency? The formula is:

$$\omega_p² = \frac{4 \pi N e²}{m}$$

and for frequencies above the plasma frequency the material is transparent. So for indium tin oxide (ITO) the frequency of the visible light is above the plasma frequency, which makes ITO transparent in the visible spectrum.

But why? Can the plasma frequency be tuned to a specific value by tuning the charge carrier density N, or is it a fixed value for different materials?

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    $\begingroup$ Indeed, in semiconductors one can change the plasma frequency through doping, and this changes the reflectivity vs wavelength behavior. $\endgroup$ – Jon Custer Jan 29 '19 at 20:06

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