The other, which faces a specialised type of solar cell made of a material called indium gallium arsenide, is sculpted into a structure called a photonic crystal that causes it to emit infra-red radiation selectively at the frequency best absorbed by the cell.

ECONOMIST: Solar power

It transforms only around 0.1% of the infra-red light thrown at it into electricity (compared with 20% for an ordinary solar cell).

ECONOMIST: Monitor