Lasers pp 3-8 | Cite as


  • K. Thyagarajan
  • Ajoy Ghatak
Part of the Graduate Texts in Physics book series (GTP)


An atomic system is characterized by discrete energy states, and usually the atoms exist in the lowest energy state, which is normally referred to as the ground state. An atom in a lower energy state may be excited to a higher energy state through a variety of processes. One of the important processes of excitation is through collisions with other particles. The excitation can also occur through the absorption of electromagnetic radiation of proper frequencies; such a process is known as stimulated absorption or simply as absorption. On the other hand, when the atom is in the excited state, it can make a transition to a lower energy state through the emission of electromagnetic radiation; however, in contrast to the absorption process, the emission process can occur in two different ways.


Spontaneous Emission Stimulate Raman Scattering Population Inversion Laser Medium Lower Energy State 
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© Springer Science+Business Media, LLC 2011

Authors and Affiliations

  1. 1.Department of PhysicsIndian Institute of TechnologyNew DelhiIndia

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