  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 state
under the action of irradiation of 2 - 4 eV or elevated temperatures. UV
irradiation results in a decrease of the absorption coefficient from 110
cm$^{-1}$ to 55 cm$^{-1}$ at 4.7 eV while in the visible range the absorption
coefficient increases from values below 5 cm$^{-1}$ to ~35 cm$^{-1}$.
Measurements with two linear polarizations, parallel and perpendicular to the
optical axis, provide determination of several different absorption bands at
2.6, 2.8, 3.4, 4.0, 4.5, and 4.8 eV. The bands at 2.6 eV and 3.4 eV identify
the defect levels near to the valence band, while the band peaking at 2.8 eV is
related to the conduction band. The photochromism allows controlling the
absorption of light in two related spectral ranges because the decrease of UV
absorption and increase of the visible absorption are related to switching the
charge state of the same defects.
