The invention relates to an optical switching device comprising a substrate and a switching film which comprises a hydride of a trivalent metal, which hydride can be reversibly switched between a low-hydrogen, mirror-like composition and a high-hydrogen transparent composition by an exchange of hydrogen. The invention additionally relates to the use of such a switching device.
In international patent application WO 98/10329 (PHN 15969) filed by Applicants, a switching device is disclosed in which a film of a hydride of a trivalent metal, such as gadolinium hydride GdH.sub.x, can be reversibly switched between a low-hydrogen (x&lt;.apprxeq.2) composition and a high-hydrogen (x&gt;.apprxeq.2) composition by an exchange of hydrogen. Both compositions have different optical properties. At a low hydrogen content (x&lt;.apprxeq.2), the film has a metallic mirror-like or reflective character and is non-transparent. At a high hydrogen content (x&gt;.apprxeq.2), the film is semiconductive, transparent and yellow in transmission. When the switching film in the low-hydrogen state is exposed to hydrogen, the mirror-like state is converted to the hydrogen-rich transparent state. The transparent film can be converted. back to the mirror-like state by heating and/or evacuation of hydrogen. The switching operation can be carried out at room temperature. Said patent application also discloses the use of a hydride of an alloy of the trivalent metal and magnesium. The presence of magnesium in the alloy increases the transmission of the switching film in the transparent state and decreases; the transmission in the non-transparent state, resulting in an improvement of the contrast, which is the ratio between the transmission in of the transparent and the non-transparent state of the switching film. The addition of magnesium makes the switching film colourless in the transparent state. Moreover, the switching speed from the mirror-like state to the transparent state is increased.
The reflectivity of a GdH.sub.x switching film in the mirror-like (low-hydrogen) state is about 15 to 20% in the visible wavelength range. The reflectivity of a (Gd.sub.0.3 Mg.sub.0.7)H.sub.x switching film is about 50 to 70% in the same state. For some applications it is desirable that the reflectivity is increased to a level which, preferably, is comparable to that of a metal reflector like aluminium, which has a reflectivity &gt;90%.