We present a new technique for direct conversion of thermal energy into coherent radiation. The near-surface layer of evaporated excited atoms of rare earth metals is proposed to be used as a converter. There is an inverted population in this layer. Operation principles of two level lasers on the transitions to the ground state using Eu are considered. The fact of detachment of Sm atoms in the excited $4f^{N-1}5d6s^2$ state during the thermal evaporation process has been proved experimentally.

• Received 3 July 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.123902