The process of emission refers to the release of energy in the form of a photon following the spontaneous decay of an excited atom into a lower energy level. Emission can be characterized based on whether the atom is stimulated by incoming photons of a specific frequency, which is otherwise known as stimulated emission, to reduce its energy level.
The energy that is lost following any type of emission can be transferred to an electromagnetic field to create a new photon, and whether this photon is created with a phase, frequency, polarization and direction of travel, or not, will indicate whether the emission was stimulated or spontaneous, respectively. Stimulated emission is also termed as laser emission, whereas spontaneous emission can also be referred to as general emission.
Stimulated (Laser) Emission
The term ‘laser’ itself is an acronym for light amplification by stimulated emission of radiation. Laser emission, or stimulated emission, refers to the specific activation of an atom or ion by a laser that will lead to its decay to a lower energy level. The energy that is released during this process will cause the excited electrons to move from the lower energy orbit to a higher energy orbit that surrounds to nucleus.
Once the excited electrons return to their ground state, particles of light, which are otherwise referred to as photons, are emitted at the same wavelength. Additionally, the emitted photons are coherent, signifying that each crest and trough of the light waves are consistent with each other.
Laser emission will only occur in atoms that exhibit an optical frequency, or wavelength, of a specific gain bandwidth, as lasers typically operate at wavelengths in which the gain medium is at its highest gain. The usefulness of understanding laser emission is directly attributed to the wide range of products that currently utilize laser atoms during surgical procedures, precision cutting tools, CDROM readers, scanning products at commercial stores and holograms, to name a few. In these applications, laser atoms are maintained in the excited state by continuously pumping the laser to maintain the atoms in a continuous excited state.
Spontaneous (General) Emission
As compared to stimulated, or laser emission, spontaneous emission arises, as its name indicates, spontaneously, rather than as a result of an excitation that is produced from a laser ion in a gain medium. Once the atom, or other quantum mechanical entitiy such as a molecule or subatomic particle, becomes excited and reaches a higher-energy level, it can either spontaneously return to its ground state, or release its excess energy in the form of a photon that is carried in a spontaneous direction.
The specific properties of the atom, as well as the structure of the surrounding medium that is inducing the excitation of the atom, are two of the primary determining factors used to establish the rate of the spontaneous emission, and subsequently the overall lifetime of the excited level of the atom.
As a quantum effect, spontaneous emission is a useful energy property that can be suppressed or modified by manipulating the placement of an atom or ion within a microcavity structure to alter the overall structure of the optical field of the emission. The photons that are produced by spontaneous emission of randomly excited atoms are referred to as luminescence.