bei48482_FM

148 Chapter Four

Figure 4.26The principle of the laser.

hv′

hv′

hv′

hv′

E 2

Excited state

E 0

Ground state

E 1

Metastable

state

E 2

E 1

hv

′′

hv

′′

hv

′′

E 2

E 1

E 0 E 0

E 2

E 1

E 0

hv

hv

hv

hv

hv

hv

Atoms in ground state

are pumped to state E 2

by photons of

hv′ = E 2 – E 0 (or

by collisions).

Rapid transition to

metastable state E 1

by spontaneous

emission of photons

of hv′′ = E 2 – E 1 (or

in some other way).

Metastable states

occupied in many

atoms.

Stimulated emission occurs when

photons of hv = E 1 – E 0 are

incident, with the secondary

photons themselves inducing

further transitions to produce

an avalanche of coherent

photons.

Figure 4.27The ruby laser. In order for stimulated emission to exceed stimulated absorption, more than half the Cr3+ions in the ruby

rod must be in the metastable state. This laser produces a pulse of red light after each flash of the lamp.

Radiationless transition

Laser transition

694.3 nm

Optical pumping

550 nm

Ground state

1.79 eV

2.25 eV

Metastable state

Cr3+ ion

Xenon flash lamp

Ruby rod

Partly transparent

mirror

Mirror

of atoms, the more upward transitions there will be from the ground state to the

metastable state. However, at the same time the pumping will stimulate downward

transitions from the metastable state to the ground state. When half the atoms are in

each state, the rate of stimulated emissions will equal the rate of stimulated absorp-

tions, so the assembly cannot ever have more than half its atoms in the metastable

state. In this situation laser amplification cannot occur. A population inversion is only

possible when the stimulated absorptions are to a higher energy level than the

metastable one from which the stimulated emission takes place, which prevents the

pumping from depopulating the metastable state.

In a three-level laser, more than half the atoms must be in the metastable state for

stimulated induced emission to predominate. This is not the case for a four-level laser.

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