Masers

Microwave Amplification by Stimulated Emission of Radiation (MASER) devices work by inducing atoms into an excited energy state. As Einstein predicted, once in that state they can amplify radiation at a frequency particular to the element or molecule used as the masing medium. Technically the Soviets came first.
 Charles Hard Townes, Nikolay Basov, and Alexander Prokhorov United States and Russia 1953 Masers

Microwave Amplification by Stimulated Emission of Radiation (MASER) devices work by inducing atoms into an excited energy state. As Einstein predicted, once in that state they can amplify radiation at a frequency particular to the element or molecule used as the masing medium. Technically the Soviets came first.