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The material is
usually not excited by external energy, all the electronics are set when
the home in the ground state energy level A, the energy excited by the
outside world, the part of the electronics will go up to the level B,
high-energy level, then they will soon be in the form of fluorescence
transitions , fading into the second-level C material. Being at the
highest level of electronic stimulation to stay a very short time, about
eight in 10 of the negative side seconds. However, some substances when
the electronic transition to the second energy level can stay a long
time. Chromium ions, such as ruby will be able to stay a few
milliseconds, so there remains the formation of a stable state
(metastable state).
Electrons from the metastable state further to the low-energy-level
transition, the generated photons. If the photons in the optical
resonant cavity (both ends of a cavity mirror), the reflection back of
the same nature of the transition will be induced to produce the same
frequency, with the phase of photons. The two-photon-induced effects
will renewed, and so it will continue to generate strong enough on the
same frequency, with the phase of light from the optical resonant cavity
has a semi-permeable mirror reflection of that shot to go, and this is
stimulated emission.
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