Abstract: An alkali metal vapor enclosure comprising: an internal surface comprising nanostructures; a transmissive portion to enable the internal surface to be illuminated; wherein the enclosure contains atoms of an alkali metal which are in a vapor state and/or adsorbed onto the internal surface; and wherein illumination of the internal surface via the transmissive portion with light having a frequency to cause a temperature rise in the nanostructures by exciting an enhanced optical extinction mechanism in the nanostructures, causes an increase in the density of the alkali metal vapor.