An atom in an excited state can emit a photon to get to a lower state. Given time, it will do this by spontaneous emission. However, if a photon comes by with the right energy, it will be stimulated to emit the photon sooner.
This photon emitted by stimulated emission looks exactly like the incoming photon that caused its emission; the two photons are coherent--they are "in phase" as well as having exactly the same frequency.
Ordinarily most atoms are in their ground states and fewer and fewer atoms will be in higher and higher excited states. If photons with energy equal to the difference in two energy states interact with atoms most of the photons will be absorbed and used to excite the atoms. Few will interact to cause stimulated emission simply because there are so few atoms in excited states. However, if conditions can be arranged so that there are more atoms in a higher energy state than in a lower energy state, then stimulated emission--or "lasing"--will occur. This condition is known as a population inversion.
Chromium atoms are the "active" laser material in a ruby laser.
Cr atoms have a metastable, excited state at 1.8 eV above the ground state.
This allows a population inversion to occur which is necessary for lasing.
Perhaps the most common laser, at least for use in an undergraduate lab or classroom, is a Neon laser (also called a Helium&endash;Neon laser). Helium and neon gas at low pressure are contained in a cylinder with mirrored ends. An electric discharge is created through this helium&endash;neon mixture to excite the atoms. Helium has an excited state at 20.61 eV which is very close to neon's excited state of 20.66 eV; these energy levels are shownhere. Electron collisions can raise helium atoms to their 20.61 eV excited state. Additional thermal motion of the helium and neon atoms allow such an excited helium atom to collide or interact with neon atoms and put these neon atoms into their 20.66 eV excited states. Neon has another excited state, 1.96 eV below this (18.70 eV above the ground state). With the help of the transfer of energy from helium to neon, there can be a population inversion between these two excited states of neon. Note that most neon atoms may still be in their ground states; the population inversion does not have to involve the ground state. As long as there are more neon atoms excited with 20.66 eV than with 18.70 eV there is a population inversion and the probability of stimulated emission is greater than the probability of absorption--and lasing occurs.
Electrons from an electric discharge can collide with and excite the helium and neon atoms. By including some additional thermal energy, an excited helium atom may transfer its energy to a neon atom. This allows a population inversion with more neon atoms excited to 20.66 eV than to 18.70 eV.
(c) Doug Davis, 1997; all rights reserved