It is shown that thin layers of palladium coated magnesium lanthanide alloys reversibly go through an optical transition by variation of the hydrogen concentration, just as has recently been shown for pure lanthanides. With these layers optical switches can be constructed that exhibit three different optical states: a color-neutral transparent state at high hydrogen concentration, a nontransparent dark absorbing state at intermediate hydrogen pressures, and a highly reflective metallic state at low hydrogen pressures. The ratio in transmission between the transparent state and the reflecting state is more than 1000.

Switchable mirrors 1±3 made of thin ®lms of the hydrides of yttrium (YH x ), lanthanum (LaH x ) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, a-YH x,0.23 is a shiny, hexagonally close-packed metal, b-YH 26d is a face-centred cubic metal with a blue tint in re¯ection and a small transparency window at red wavelengths, whereas hexagonally close-packed g-YH x.2.85 is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of realtime visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium ®lms in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission pro®les along the length of the ®lm vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled. We can also induce electromigration of hydrogen, which diffuses towards the anode when a current¯ows through the ®lm. Consequently, hydrogen in insulating YH 3-d behaves as a negative ion, in agreement with recent strong-electron-correlation theories 4,5 . This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.Diffusion of hydrogen in metals has attracted considerable attention 6±8

The optical transmission of the recently discovered switchable yttrium hydride films is determined spectroscopically as a function of hydrogen content. This is done during electrochemical loading of Pd-capped Y film electrodes, thereby continuously changing the hydrogen concentration. The effect of the Pd cap layer on the film transmission is determined from measurements on a series of films with varying Pd layer thickness. The results are in good agreement with transmission measurements of in situ gas phase loaded, uncapped Y films. Both data sets can be consistently described with simple optical decay lengths such as 277.8 nm for YH 3Ϫ␦ and 15.1 nm for Pd at បϭ1.96 eV. The hydrogen concentration dependence of the optical transmission is discussed and compared with previous optical measurements on bulk samples and band-structure calculations.

The optical effect upon insertion of hydrogen into Pd-coated magnesium lanthanide switchable mirrors is investigated in terms of the changes of their complex refractive indices. A significant change in the optical constants of LnMg layers is seen between the as-deposited state and the dehydrided state after one cycle. Furthermore, the optical effect of switching the Pd cap layer to a PdH cap layer was determined. It is shown that the Pd layer mainly limits the visible transmittance of the hydrided stack to about 35%–40%. Whereas the extinction coefficient of dehydrided LnMg layers at 550 nm is between 2.2 and 3.1, it is as low as 10−4 in the transparent state. This is of great promise to applications requiring large optical contrast (e.g., optical switches).