PHYSICS

emission of radiation. It is important to note that the instant at
which emission occurs, and the path of the resulting photon
varies from excited particle to excited particle since spontaneous
emission is a random process. The spontaneous radiation is
incoherent monochromatic radiation as it is produced by one of
the particles differs in direction and phase from that that produced
by anotherthe second particle (fig. 21.18).
Fig. 21.19. Diagram representing the stimulated emission of a photon
by an incoming photon
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1
E III
--E I .
-­ ,
I
--E·I
-­ ...,.... y , ..
IE,. r-; I
I,., I
II ".. /
I
...LE,
1
,.I
....,­ -
I
==~'
I ,"\ ,.,
I ',/ \.i~
..L - " E T
2
2
hv s E; -E,
--
-­ -+­
W WE, I ,,, "\
I • \ I \ ,+
-+­ --I... \.l ,I
~
Y\r
Fig. 21.18. Diagram representing the spontaneous emission of a photon
by an atom
Stimulated Emission. This process is the basis of laser behavior.
Here, the excited laser particles are struck by photons produced
by spontaneous emission. Collisions of this type cause the excited
particles to relax immediately to the lower energy state and to
simultaneously emit a photon of exactly the same energy as the
photon that stimulated the process. The emitted photon travels in
exactly the same direction and is precisely in phase with the
photon that caused the emission (fig. 21.19). Therefore, the stimulated
emission is totally coherent with the incoming radiation.
Absorption. This process competes with stimulated emission: an
E: II
- - II __E,
-r -­ '
- -E I
hv=E,. -E,
E,.·
~/~
I \ 1 ,I
-E,
....!.... ' oJ
3
-.­
3
jJ\[
incident photon can cause atomic transition either upward
(stimulated absorption) or downward (stimulated emission).
Population Inversion. In order to have light amplification in a
laser, it is necessary for the number of photons produced by
stimulated emission to exceed the number lost by absorption. This
condition will prevail only when the number of particles at the
higher energy state exceeds the number in the lower; in other
words, a population inversion from the normal distribution of the
energy states must exist. Fig. 21.20 contrasts the effect of incoming
radiation on a non-inversed population with that on an inverted
one.
E
1
• •
E 1 •• • •
E
o
••
••••••Pumpin g
Eo •• ¢::J
Fig. 21.20. Inversion of
population: a - population
of the levels before pumping;
b - population of the levels
after application of pumping
1
~ [~ I ~
V­__~I
3
Ruby rod
Flashlamp
Silver
reflectors
Fig. 21.21. Components of laser:
1 ­ source of pumping; 2 - active
medium; 3 - optical resonator
made from two mirrors
Components of a Laser. A laser consists of the active medium
(e.g., a solid crystal, a gas, a solution of an organic dye, a
semiconductor), which is placed in an optical resonator (made
from two mirrors) with a pumping source (fig. 21.21).
Properties of Laser Radiation. The main properties of laser radiation
are monochromaticity, coherence, directionality, and
brightness. Some types of lasers have the potential to change the
frequency (or wavelength) of the radiation. Other laser devices
produce ultra-short pulses of radiation.
Monochromaticity indicates a high spectral purity of the radiation.
Monochromatic radiation typically has a narrow spectral
interval and is characterized by predominately a single frequency
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