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In this work, we study the absorption properties of AlN in the range of 1.5 -5.5 eV, as well as the meta-stable change in absorption induced by ultraviolet(UV) irradiation (photochromism), and the restoration of the initial stateunder the action of irradiation of 2 - 4 eV or elevated temperatures. UVirradiation results in a decrease of the absorption coefficient from 110cm$^{-1}$ to 55 cm$^{-1}$ at 4.7 eV while in the visible range the absorptioncoefficient increases from values below 5 cm$^{-1}$ to ~35 cm$^{-1}$.Measurements with two linear polarizations, parallel and perpendicular to theoptical axis, provide determination of several different absorption bands at2.6, 2.8, 3.4, 4.0, 4.5, and 4.8 eV. The bands at 2.6 eV and 3.4 eV identifythe defect levels near to the valence band, while the band peaking at 2.8 eV isrelated to the conduction band. The photochromism allows controlling theabsorption of light in two related spectral ranges because the decrease of UVabsorption and increase of the visible absorption are related to switching thecharge state of the same defects.

Photochromism and influence of point defect charge states on optical absorption in aluminum nitride (AlN)