Abstract: An optically switchable device (1) has a heterostructure (2) defining a first potential well (3) separated by a barrier layer (4) from a second potential well (5) which provides an electron energy level (e.sub.5) which is lower in electron energy than the lowest electron energy level (e.sub.3) of the first potential well (3). The barrier layer (4) provides an intermediate electron energy level (e.sub.4) and is sufficiently thick to inhibit tunnelling of holes from the first (3) to the second (5) potential well. The barrier layer (4) thus confines holes of electron-hole pairs generated in the first potential well (3) by an incident optical beam to the first potential well (3) while facilitating transfer of the electrons from the first potential well (3) to the second potential well (5) via the intermediate electron energy level (e.sub.4) provided by the barrier layer (4).