Page 347 - Electrical Properties of Materials

Applications 329

atom is stationary, these two forces cancel each other. When the atom moves,
the apparent frequencies of the two waves are Doppler shifted. The counter-
propagating wave gets closer to resonance, and the copropagating wave gets
farther away. The one that is closer to resonance exerts a higher force upon the
atom, and hence the net effect is to slow down the atom. Atoms being slowed
down means that the temperature has declined. In real life the cooling must,
of course, be done in three dimensions, necessitating three counterpropagating
laser waves, but the principle is the same. The set of atoms cooled down is
∗
called optical molasses. For sodium atoms and using a particular resonance, ∗ Molasses (used as a singular noun) is a
the lowest temperature achievable was 240 μK, with theory and experiments sticky sugary substance associated with
cold temperatures. The saying is ‘slow as
in good agreement.
molasses in January’.
12.13.11 Optical radar

One of the obvious device applications is in producing a radar. If it can be
done with microwaves, why not with lasers? The wavelength is much smaller,
so we may end up with higher accuracy in a smaller package. This is indeed the
case; some of the optical radars may weigh less than 20 kg and can recognize
a moving car as a car and not a blotch on a screen. They can also determine
the position of objects (e.g. clouds, layers of air turbulence, agents of pollution)
which do not give sufficient reflection at microwave frequencies. The two lasers
used most often are YAG lasers at 1.06 μm and CO 2 lasers at 10.6 μm, the
latter has the merit of being able to penetrate fog, haze, and smoke. Optical
radars are best known under the name of Lidar (light detection and ranging) but
also as Ladar (laser detection and ranging) and Oadar (optical aids to detection
and ranging).

12.13.12 Optical discs
Once the laser was invented, applications for data storage came immedi-
ately to mind. Nowadays we can hardly imagine life without CDs and DVDs.
They will be discussed among optical memories in somewhat more detail in
Appendix VI, Section A6.7.

12.13.13 Medical applications
Medical applications are growing fast, particularly in the United States, where
the medical profession is much less conservative than in Europe. The essential
property of lasers that becomes useful is that the radiated energy can be con-
centrated on a small spot and that different tissues have different absorptions.
A uniquely useful application is, for example, the reattachment of the human
retina by providing the right amount of heat at the right place. Surgeons may
use higher laser energy to vaporize tissue (a useful way to get rid of malig-
nant tumours) or lower laser energy to coagulate tissue, that is stop bleeding. It
is actually possible to make bloodless cuts without causing pain. The number
of various medical applications is high (I understand in ophthalmology alone,
as many as forty different problems are treated by lasers) but not very wide-
spread as yet. I want to finish the list by mentioning one more, rather bizarre,
application practised by some gynaecologists; to open up the Fallopian tube.