Basic Principles
Despite its complex quantum-mechanical structure, an atom can be conveniently modeled as a nucleus (containing
protons and neutrons) surrounded by a cloud of electrons. Associated with the atom or molecule of any given material
is a specific set of quantized (discrete) energy states (orbits) that the electrons can occupy. Supply of energy (in
the form of heat, exposure to intense light, or other means) by an external source can cause an electron to move
from a lower energy state to a higher energy (excited) state. Exciting the atoms is called pumping because it leads
to increasing the population of electrons in higher states [Fig. TF15-2(a)]. Spontaneous emission of a photon (light
energy) occurs when the electron in the excited state moves to a lower state [Fig.TF15-2(b)], and stimulated emission
[Fig. TF15-2(c)] happens when an emitted photon “entices” an electron in an excited state of another atom to move to
a lower state, thereby emitting a second photon of identical energy, wavelength, and wavefront (phase).