Population Inversion - Spectra-Physics Pulseo User Manual

Pulseo High Peak Power, Diode Pumped, Q-Switched, 355 nm Laser System

Population Inversion

B-2
Figure B-1: Electrons occupy distinct orbitals that are defined as the
probability of finding an electron at a given position. The shape of the
orbital is determined by the radial and angular dependence of this
probability.
Likewise, when an atom excited to
. The atom may decay spontaneously, emitting a photon with energy
E – E
2 1
and frequency
hν
Spontaneous decay can also occur without emission of a photon, the lost
energy taking another form, e.g., transfer of kinetic energy by collision
with another atom. An atom excited to
by interacting with a photon of frequency
E
1
of a pair of photons that are identical to the incident one in phase, fre-
quency, and direction. This is known as stimulated emission. By contrast,
spontaneous emission produces photons that have no directional or phase
relationship with one another.
A laser is designed to use absorption and both spontaneous and stimulated
emission to create conditions favorable to light amplification. The follow-
ing paragraphs describe these conditions.
The net absorption at a given frequency is the difference between the rates
of emission and absorption at that frequency. It can be shown that the rate
of excitation from
E
1
the lower level ( ) and the transition probability. Similarly, the rate of
N
1
stimulated emission is proportional to the population of the upper level (
and the transition probability. Moreover, the transition probability depends
on the flux of the incident wave and a characteristic of the transition called
its "cross section." The absorption coefficient depends only on the differ-
ence between the populations involved,
dent wave.
decays to
E
2
E E
–
2 1
ν
= ----------------- -
h
can also be stimulated to decay to
E
2
to is proportional to both the number of atoms in
E
2
and
N
1
, it loses energy equal to
E
1
ν
, emitting energy in the form
, and the flux of the inci-
N
2
[2]
)
N
2