Rami Arieli: "The
Laser Adventure" Chapter 2.9 page 1
2.9 Stimulated (Forced) Transitions(*)
The strength of an optical signal
(The number of photons) is described
which means the ratio of measured power over a specific surface area (since
power is a measure of the amount of energy
over time, than Intensity is a measure of the amount of energy over time
over surface area).
An Optical signal
is an oscillating electromagnetic field, and an atom
can be described as an electric dipole.
Energy density (n(t))
= The number of photons per unit volume = The energy of the electromagnetic
radiation in units of hn or h(bar)w.
When an optical signal of the right frequency (hn
equal to the energy difference between the energy levels E2-E1)
is approaching an atom, both the atoms at the lower energy level (E1)
and the atoms at upper energy level (E2) will start to oscillate.
That is why there are two forced processes: absorption
and stimulated emission.