Atomic systems, such as atoms, ions, and molecules, exist only in discrete energy states. The change from one energy state to another is called a transition, and it is accompanied by the emission or absorption of photons.
The Bohr frequency relation gives the wavelength of the absorbed or emitted radiation:
E2 − E1 = hν21
where E2 and E1 are two discrete energy levels, ν21 is the frequency, and h is Planck's constant. The energy gap of this atomic system corresponds to an electromagnetic wave with a frequency ν21, which is able to interact with the atomic system. For the purposes of the following analysis, solid materials are now considered approximately as an aggregate of very many of the same atomic systems. In thermal equilibrium, the number of particles in the lower energy state of the material is much higher than that in the upper energy state, and electromagnetic waves interact with it to cause atoms and molecules to rise from a low energy level to a high energy level, that is, absorption.
Laser operation requires changing the energy balance of the laser material so that the energy is stored in the atoms, ions, or molecules of the material. The particles in the laser material are transitioned from a low-energy state to a high-energy state by an external pump source, i.e., the pump radiation causes a "particle number reversal". When an electromagnetic wave of moderate frequency is incident on the "inverted" laser material, the incident photons will cause the atoms at the higher energy level to fall to the lower energy level and emit additional photons, forming light wave amplification. Eventually, the energy is taken out of the atomic system and fed to the radiated field. The release of stored energy by interacting with electromagnetic waves is based on stimulated or induced radiation.
In short, when a material is stimulated in such a way that its atoms (molecules) are more distributed at higher energy levels than at lower energy levels, the material is able to amplify radiation at a frequency corresponding to the energy level difference. The word "laser" in English is the abbreviation of "stimulated radiation light amplification".
Quantum mechanical studies of the interaction between radiation and matter show that stimulated radiation and excitation fields are practically indistinguishable, that is, stimulated radiation and excitation fields are identical in terms of directionality, polarization, phase and spectral characteristics. These factors determine the extremely high coherence of stimulated radiation, which is the basic characteristic of laser radiation.
In solid-state lasers, energy levels and associated transition frequencies are caused by the various quantum energy levels or permissible electron quantum states in the electron orbits around the nucleus. In addition to this electronic transition, polyatomic molecules in gases also have energy levels formed by the vibration and rotation of the molecule as a whole.
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