Page 325 - Electrical Properties of Materials

Semiconductor lasers 307

the so-called stagnation region. Then most of the energy is associated with the
random translation and rotation of the gas molecules and only about one-tenth
of the energy is associated with vibration. Next, the gas is expanded through a
supersonic nozzle, causing the translational and rotational energies to change
into the directed kinetic energy of the flow. The vibrational energy would disap-
pear entirely if it remained in equilibrium with the decreasing gas temperature.
But the vibrational relaxation times are long in comparison with the expan-
sion time, hence the population of the vibrational levels remains practically
unchanged. At the same time, the lower level population diminishes rapidly
with the expansion, leading to significant population inversion after a few cen-
timetres downstream. For CO 2 gas the emission wavelength is again 10.6 μm,
using other gases the typical range is from 8 to 14 μm, although oscillations
may be achieved at much shorter wavelengths, as well.
The advantage of gas-dynamic lasers is the potential for high average
powers because waste energy can be removed quickly by high-speed flow.

12.6.5 Excimer lasers
Excimers are molecules which happen to be bound in an excited state and not in
the ground state, so their operation differs somewhat from the general scheme.
Their main representatives are the rare gas halides like KrF or XeCl. They need
powerful pumps in the form of discharges, optical excitation, or high-current,
high-voltage electron beams. Their advantages are high efficiency and high
pulse energy in a part of the spectrum (in the ultraviolet down to wavelengths
of about 100 nm) which was inaccessible before. Most of them are inherently
broadband and offer the further advantage of tuneability.

12.6.6 Chemical lasers
As the name suggests, the population inversion comes about as a result of
chemical reactions. The classification is not quite clear. Some of the excimer
lasers relying on chemical reactive collisional processes could also be included
within this category. The clearest examples are those when two commercially
∗ The advent of chemical lasers raises
available bottled gases are let together, and monochromatic light emission is an intriguing problem I have often
brought about by the chemical reaction. ∗ asked myself about. What path would
technology have followed if electricity
12.7 Semiconductor lasers had never been discovered? The ques-
tion may be posed because electricity
12.7.1 Fundamentals and technology developed separately, the
former being a purely scientific pastime
We shall dwell on semiconductor lasers a little longer because they are in a until the fourth decade of the nineteenth
century. Had scientists been less inter-
quite special category. For us they are important for the reason that we have
ested in electricity or had they been
already invested much effort in understanding semiconductors, so that any re- just a bit lazier, it is quite conceivable
turn on that investment is welcome. There are, though, some more compelling that the social need for fast communic-
reasons as well. ations (following the invention of the
locomotive) would have been satisfied
1. They are of interest because the technology and properties of semicon- by systems based on modulated light. In
the search for better light sources, the
ductors are better known than those of practically any other family of
chemical laser could then have been in-
materials. vented by the joint efforts of chemists
2. Laser action is due to injection of charged carriers, so semiconductor and communication engineers a century
lasers are eminently suitable for electronic control. ago.