Optical switching device: Fiber-optic hydrogen sensors

A hydrogen permeable optical reflective layer (4) of a transition metal is deposited on transition metal (hydride) i:::: layer (3) which can switch from a black absorbing state. A hydrogen permeable catalytic layer (5) of a transition metal is deposited 0 on top of the reflective layer (4). Ti and/or Pd may be used as transition metal(s) in all of the three layers (3,4,5). Co-sputtering may 0 be used to deposit a transition metal (hydride) switching layer (3) with a maximum thickness of 100 nm on a substrate (2) which M can be of any material. The thickness of the optical reflective layer (4), which is larger than the thickness of the switching layer 0 (3), is more than 10 nm (but preferably 50-200 nm) so that there is (no or) little transmission. The thickness of the catalytic layer > (5) is about 10 nm. If a detector (11) is included one can produce a hydrogen sensor. Alternatively, one can produce a temperature ~ controlled solar energy converter (17) by including a fluid heater (18).