laser (light amplification by stimulated emission of radiation) A device that produces coherent light beam. The word "coherent" means that the light photons are all in same phase and are monochromatic.
The ordinary light is produced when an excited atom makes a spontaneous transition to the ground state. Since such transitions in different atoms are completely random the emitted photons have random phases.
In certain atoms (or molecules), there exist states that have considerably greater lifetime compared to other excited states. These states are called metastable states. Fig. l1 shows such a system. The state E1 is a metastable state, and states E0 and E2 are ground state and an excited state respectively. If the system is irradiated with em radiation of frequency, n such that,
hn = E2- E1
then electrons will be raised to the state E2. From E2, the electrons make transitions to the ground state E0 or metastable state E1 with emission of photons. As the state E1 is metastable, the number of atoms in this state becomes larger than that in the ground state, if the atoms are continuously irradiated. This situation is called "population inversion". Now a photon with frequency n ¢ , given by the equation,
h n ¢ = E1-E0
produces stimulated emission of photon by inducing transition from metastable to ground state when it encounters another atom in the metastable state. The photons produced by stimulated emission are all in phase and have the same frequency. The population of photons is multiplied by reflecting the light beam back and forth through the lasing medium using mirrors.
Einstein (in 1916) proposed the idea of stimulated emission. This prediction attracted little attention until 1954, when Townes and coworkers developed laser in molecular system of ammonia that gives radiation in the microwave region (called maser). The first laser was constructed by Miaman in 1960 using ruby crystal (Al2O3 with 0.05% Cr2O3). The process of population inversion is achieved by xenon flash tube lamp, which wounds a cylindrical ruby crystal, several cm long and few mm in diameter. The ends of the ruby crystal are polished flat, normal to the axis, to act as cavity mirrors.
More common laser in the laboratory is the He-Ne laser. The He atoms which are 10 times more than Ne atoms, are raised to the excited states He2 and He3 (see fig. l2) by collision with electrons accelerated in electric field. These are metastable states. These excited He atoms interact with Ne atoms and excite them to metastable states Ne4 and Ne6. Laser transition takes place from these states to the states labelled Ne3 and Ne5 respectively. One of the light wavelength is re enforced by choosing mirrors with high reflectivity for the particular wavelength desired.
Lasers are finding wide applications because of their special characteristics of coherence, monochromaticity, directionality and great intensity of radiation (see also fusion, holography).