Lasers

Abstract

This section gives a survey on the laser, its mechanism, properties, and present status, dealing with these subjects to the extent that they are relevant to laser-produced plasmas. The basic physical process involved is the stimulated emission of radiation. Consider an atom, molecule, or a solid, in which electrons can occupy two energy levels E₁ and E₂. We have E = E₂ − E₁ = ћω, where ћ is Planck’s constant divided by 2π, and ω is the frequency of the light quantum (photon) which is emitted when the electron undergoes an optical transition from the upper level E₂ to the lower level E₁. Such an optical transition can occur spontaneously within a time τ, the spontaneous lifetime of the electron in the excited state E₂. For atoms and molecules these lifetimes are in the order of 10⁻⁸ seconds. Besides this spontaneous emission, the time in which the photon is emitted can be decreased by another photon of the same frequency ω which passes the excited atom and stimulates deexcitation by pulling out the photon from the atom. The two resulting photons have the same frequency and the same phase which results in a high coherence of the radiation. Stimulated emission was described in 1917 by Einstein in an ingenious derivation from Planck’s law of quantized radiation.