Everything Science Grade 12

CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER 16.5

ground

state

excited

state

ground

state

excited

state

laser

photon

Stimulated Emission

2 laser

photons

first step second step

Figure 16.9: Stimulatedemission is also a two step process, as shown here. First, a laser
photon encounters an atom that has been raisedto an excited state, just like in the case
of spontaneous emission. The photon then causes the atom to decay to its ground state
and emit another photon identical to the incoming photon. This is thesecond step in
the creation of a laser beam. It happens many,many times as the laserphotons pass
through the optical cavity until the laser beam builds up to full strength.

DEFINITION: Population inversion

Population inversion is when more atoms are in an excited state than in

their ground state. It isa necessary condition tosustain a laser beam,

so that there are enoughexcited atoms that can be stimulated to emit

more photons.

ground state

metastable

state

ground state

metastable

state

������ ��

� � � ������ ��

� � �

usually most electrons are in most electrons in excitedmetastable

the ground state state = population inversion

Therefore, materials used to make laser light must must have metastable states which
can allow population inversion to occur whenan external energy source is applied.
Some substances whichare used to make lasersare listed in table 16.3.You can see
that gases (such as Helium-Neon mixture), liquids (such as dyes), andsolids (such as
the precious stone ruby)are all used to make lasers.