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Transparency of truly bulk GaN substrates obtained by ammonothermal method was measured in the mid infrared spectral range by Fourier spectroscopy. The same measurements were performed for GaN templates grown by meatlorganic vapour phase epitaxy on sapphire and SiC substrates. It has been clearly observed that truly bulk GaN substrates are transparent up to ∼7 µm whereas GaN templates grown on sapphire and SiC are transparent only up to ∼6.5 and ∼5.5 µm, respectively, due to non-transparency of sapphire and SiC in this spectral range. It has been shown that the transparency 'cut off' at ∼7 µm for GaN crystals results from the absorption of light by the second harmonic of optical phonons, which is very significant due to the strong electron-phonon coupling in this material. Also it has been clearly presented that the absorption 'cut-off' in the infrared spectral region can be easily tuned by increasing of free carrier concentration in GaN. It was observed that the infrared transparency can be shifted from ∼7 to ∼2 µm or even shorter wavelengths when the electron concentration is increased up to ∼1019 cm-3. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]