If an atom or ion with laser activity is in an excited state – quantum mechanical level (e.g. by an optical pump). It may spontaneously decay to a lower energy level after some time, releasing energy in the form of photons, emitted in a random spatial direction, a process known as spontaneous emission. If the incident photon has the appropriate photon energy (or optical frequency), the photon emission may also be excited by the incident photon, which is known as stimulated radiation. In this case, the photon is emitted in the mode of the incident photon.
Schematic diagram of stimulated radiation
The incident photon excites an excited atom or ion, causing it to transition from the excited state to the ground state.
The physics of stimulated radiation can be described in the context of quantum optics. There are also semi-classical descriptions (dealing with the interaction of oscillating dipoles with electromagnetic fields), and the original idea of stimulated radiation was published by Einstein before quantum mechanics was fully developed.
The amplification effect of stimulated radiation can be reduced or completely suppressed in the medium because too many laser active atoms are in the lower state of the laser transition because these atoms absorb photons, thus attenuating the light. In a simple two-stage system, laser amplification requires a so-called particle number reversal.
The rate of the excitation radiation process of an excited atom can be calculated as the product of the so-called emission cross section and the photon flux density (the number of photons per unit area and time). Such a term is often used for rate equation modeling. Photon flux density can be calculated as the intensity of light divided by the energy of the photon.
In lasers well above the threshold, stimulated emission is superior to spontaneous emission, and power efficiency can be high. In order to satisfy this condition, the incident light intensity must be higher than the saturation intensity.
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