Ways to control the emission of light

The next important step forward was taken by Einstein in 1916 in his famous work entitled The emission and absorption of radiation by the quantum theory (Einstein 1916), wherein he introduced two types of quantmn transition of an atom or molecule between discrete quantum states, namely  

An induced (stimulated) transition of an atom between energy states. This type of transition is due to radiation acting on the atom, and its probability is proportional 

The idea of the possibility of light amplification by the stimulated emission of radiation in a medium with an inverted population had opened a little the way to the production of flows of identical photons, but subject to one condition a flow of equally identical photons had to be fed to the input of the amplifying medium. Otherwise, all the imperfections of the light beam at the input of the amplifying medium would be repeated in an amplified form at its output. All common light sources emit chaotic radiation, from which it is impossible to isolate a flow of identical photons of any intensity for their subsequent amplification. It took one more decisive idea, namely, to place the amplifying medium in a resonant cavity wherein the photon loss factor was lower than the gain factor, that is, to establish a positive resonance 

Idea of population inversion, hght amplification (1930sV 

Microwave oscillators allowed the start of systematic research on the absorption spectra of molecules in the microwave band, and microwave spectroscopy made its appearance. It was perhaps here that the first s3mthesis took place of the concepts of quantum mechanics, which clearly explained microwave molecular spectra, and the concepts of experimental radio engineering. 

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