3.1.2 Spontaneous and Stimulated Emission
Light is emitted by atoms (molecules) of the light source material that can be a gas, a liquid, or a solid. Atoms can be in different excited states when electrons possess energy according to their position in relation to the nucleus of an atom. The closer the electron is to the nucleus, the lower is the energy. When the energy of an electron changes, it must do so in certain definite steps, and not in a continuous manner. The positions in which electrons may be found according to their energy are called energy levels and sublevels. These levels are counted by their steps outward, and the numbers allotted to them are their quantum numbers. Excitation of atoms (molecules) can be provided by different ways: by heating, by electrical discharge, or by optical pumping.
An excited atom (molecule) is able to relax with time. In other words, the excited state has a lifetime that refers to the time the atom (molecule) stays in its excited state before emitting a photon spontaneously (spontaneous emission) or Lose energy nonradiatively (by collisions with the other atoms). Thus, the lifetime is related to the rates of excited state decay, to the facility of the relaxation pathway, radiative and nonradiative. If the rate of spontaneous emission or any of the other rates are fast, the lifetime is short (for commonly used fluorescent compounds typical excited state decay times are within the range 1 ns-1 ms). An atomic optical transition is typically an electronic transition, where energy is given out as electromagnetic radiation in the optical range. Direction of spontaneously emitted photons is random, and the frequency (wavelength) is also random in the limits of the bandwidth of luminescence of the excited transition. As a result, most spontaneous emitting light sources have an isotropic direction of emission and a wide range of frequencies (polychromatic).
Intensive stimulated emission of light by a group of atoms (molecules) is possible when the higher energy levels of these atoms are populated more intensively than the lower ones (inversed population). Such inversion of population can be done by different methods including two component gas systems with coinciding energetic levels with different relaxation times or optical pumping. Stimulated emission is characterized by the generation of a new photon which is identical to the excitation photon that initially interacted with the atom. As a result, we receive two photons with the same wavelength, phase, and direction of propagation, instead of one. Stimulation emission is the basic concept for lasing. Stimulated emission in an active medium with inversed population is developed as a photon avalanche with identical directions of propagation, frequencies, and polarizations. A laser is an active medium with inversed population that is placed between two paralleled mirrors. During lasing, the photon avalanche is propagating between two paralleled mirrors and is amplified during each time of intersection with the active medium. As a result, the laser beam is formed with a very low divergence and single wavelength (monochromatic beam).