Laser
Stimulated Emission
The laser is an abbreviation of "Light Amplification by Stimulated Emission of Radiation". It explains "stimulated emission" that exists etymologically of the laser here. The stimulated emission is foundation of the principle of the laser.
The particle that is the foundation of the material is called an atom. The center of the atom is an atomic nucleus, and the electron turns round the surroundings. The place where the electron can exist is limited to the orbit.
The atom has some orbits. An orbit near the atomic nucleus is steady. A far orbit is unstable. Therefore, the electron has the character to try to stay on in an inside orbit as much as possible. The state that the electron is installed on an inside orbit is called a ground state.
The electron jumps to an outside orbit when the atom gets energy (heat, light, and electrical discharge, etc.) from the outside. This jump is called excitation. excited state maintains the energy obtained from the outside. Therefore, excited state is larger than ground states energy.
The electron jumps down at once in the orbit in the origin and returns to the ground state because excited state is very unstable. Excited state naturally returns to the ground state even if it leaves it. In this process, the maintained energy is discharged as light. This is called a spontaneous emission.
Just before the spontaneous emission, the light of the spontaneous emitted radiation is incidence and from another atom. In this case, an interesting phenomenon happens. Excited state returns to the ground state by stimulating the spontaneous emission light that came from another atom. At this time, the maintained energy is discharged as light. This is called a stimulated emission.
There are two kinds of processes where excited state returns to the ground state as follows. -It naturally returns to the ground state even if it leaves it. At this time,the atom spontaneous emission light. -The spontaneous emission light of other atoms is received, and it returns to the ground state compulsorily. At this time, the atom stimulated emission light.
Light is a particle and the wave at the same time. In this case, light is interpreted as the particle. One of the light recognized as a particle is called a photon.
The photon of the spontaneous emission light stimulates other atoms, and the photon of the stimulated emission light generates it. The number of photons comes to have amplified to twice. The stimulated emission light has the same wavelength and phase as the spontaneous emission light.
In a word, both are indistinguishable. The doubled photon is coherent light.
The excited atom is stimulated by the photon one after another, and coherent light is increased. If this is repeated, it becomes laser light.
The stimulated emission is a phenomenon forecast by Einstein in 1917. It is at 1950's that the idea of generating the laser light by using this stimulated emission was proposed.