Maser in the context of James P. Gordon

Atomic, molecular, and optical physics (AMO) is the study of matter–matter and light–matter interactions, at the scale of one or a few atoms and energy scales around several electron volts. The three areas are closely interrelated. AMO theory includes classical, semi-classical and quantum treatments. Typically, the theory and applications of emission, absorption, scattering of electromagnetic radiation (light) from excited atoms and molecules, analysis of spectroscopy, generation of lasers and masers, and the optical properties of matter in general, fall into these categories.

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a new photon with a frequency, polarization, and direction of travel that are all identical to the photons of the incident wave. This is in contrast to spontaneous emission, which occurs at a characteristic rate for each of the atoms/oscillators in the upper energy state regardless of the external electromagnetic field.

According to the American Physical Society, the first person to correctly predict the phenomenon of stimulated emission was Albert Einstein in a series of papers starting in 1916, culminating in what is now called the Einstein B Coefficient. Einstein's work became the theoretical foundation of the maser and the laser. The process is identical in form to atomic absorption in which the energy of an absorbed photon causes an identical but opposite atomic transition: from the lower level to a higher energy level. In normal media at thermal equilibrium, absorption exceeds stimulated emission because there are more electrons in the lower energy states than in the higher energy states. However, when a population inversion is present, the rate of stimulated emission exceeds that of absorption, and a net optical amplification can be achieved. Such a gain medium, along with an optical resonator, is at the heart of a laser or maser.Lacking a feedback mechanism, laser amplifiers and superluminescent sources also function on the basis of stimulated emission.

Charles Hard Townes (July 28, 1915 – January 27, 2015) was an American physicist. Townes worked on the theory and application of the maser, for which he obtained the fundamental patent, and other work in quantum electronics associated with both maser and laser devices. He shared the 1964 Nobel Prize in Physics with Nikolay Basov and Alexander Prokhorov. Townes was an adviser to the United States Government, meeting every US president from Harry S. Truman (1945) to Bill Clinton (1999).

He directed the US government's Science and Technology Advisory Committee for the Apollo lunar landing program. After becoming a professor of the University of California, Berkeley in 1967, he began an astrophysical program that produced several important discoveries, for example, the black hole at the center of the Milky Way galaxy.

Arthur Leonard Schawlow (May 5, 1921 – April 28, 1999) was an American physicist who, along with Charles Townes, developed the theoretical basis for laser science. His central insight was the use of two mirrors as the resonant cavity to take maser action from microwaves to visible wavelengths. He shared the 1981 Nobel Prize in Physics with Nicolaas Bloembergen and Kai Siegbahn for his work using lasers to determine atomic energy levels with great precision.

Nikolay Gennadiyevich Basov (Russian: Никола́й Генна́диевич Ба́сов; 14 December 1922 – 1 July 2001) was a Russian Soviet physicist and educator. For his fundamental work in the field of quantum electronics that led to the development of laser and maser, Basov shared the 1964 Nobel Prize in Physics with Alexander Prokhorov and Charles Hard Townes.

Alexander Mikhailovich Prokhorov (born Alexander Michael Prochoroff, Russian: Алекса́ндр Миха́йлович Про́хоров; 11 July 1916 – 8 January 2002) was an Australian-born Soviet-Russian physicist and researcher whose work focused on quantum electronics. His most famous and well-known works were on optics and electromagnetic research. He was jointly awarded the Nobel Prize in Physics in 1964 with Charles Hard Townes and Nikolay Basov for his fundamental work that led to the development of the laser and the maser.

Solar radio emission refers to radio waves that are naturally produced by the Sun, primarily from the lower and upper layers of the atmosphere called the chromosphere and corona, respectively. The Sun produces radio emissions through four known mechanisms, each of which operates primarily by converting the energy of moving electrons into electromagnetic radiation. The four emission mechanisms are thermal bremsstrahlung (braking) emission, gyromagnetic emission, plasma emission, and electron-cyclotron maser emission. The first two are incoherent mechanisms, which means that they are the summation of radiation generated independently by many individual particles. These mechanisms are primarily responsible for the persistent "background" emissions that slowly vary as structures in the atmosphere evolve. The latter two processes are coherent mechanisms, which refers to special cases where radiation is efficiently produced at a particular set of frequencies. Coherent mechanisms can produce much larger brightness temperatures (intensities) and are primarily responsible for the intense spikes of radiation called solar radio bursts, which are byproducts of the same processes that lead to other forms of solar activity like solar flares and coronal mass ejections.

For an electromagnetic wave, the coherence time is the time over which a propagating wave (especially a laser or maser beam) may be considered coherent, meaning that its phase is, on average, predictable.

In long-distance transmission systems, the coherence time may be reduced by propagation factors such as dispersion, scattering, and diffraction.