The concept behind the development of the laser was proposed by Albert Einstein in his 1917 paper Zur Quantentheorie der Strahlung (On the Quantum Theory of Radiation
Heitler, The Quantum Theory of Radiation
, Oxford University Press, 1950.
The Quantum Theory of Radiation
. Third Edition, Oxford, Clarendon Press, 1954, Appendix, Sec.3, p.409.
Einstein established the theoretical possibility of lasers in a 1916 paper, "On the quantum theory of radiation
." He showed that light exists as photons, bundles of wave energy, and that the energy of the photon corresponds to the frequency of the waves in the bundle.
The quantum theory of radiation
. Oxford, Clarendon Press, 1954, Ch.