Page 346 - Electrical Properties of Materials

328 Lasers

12.13.4 Study of rapid events
With the aid of picosecond and sub-picosecond light pulses, a large num-
ber of rapidly occurring phenomena may be studied in physics, chemistry,
and biology. The usual technique is to generate a phenomenon by a strong
pulse and probe it by another time-delayed pulse. A field in which these tech-
niques have been successfully utilized is the creation and decay of excitons in
a semiconductor crystal.

12.13.5 Plasma diagnostics
Many interesting properties of plasmas may be deduced by their scatter of laser
light.

12.13.6 Plasma heating
A plasma may be heated to high temperatures by absorbing energy from
powerful lasers.

12.13.7 Acoustics
Properties of high-frequency (in the GHz range) acoustic waves in solids may
be studied by interacting them with laser light.

12.13.8 Genetics
Chromosomes may be destroyed selectively by illuminating single cells with
focused laser beams.

12.13.9 Metrology
The velocity of light may be determined from the relationship, c = νλ,by
measuring the frequency and wavelength of certain laser oscillations. The laser
is stabilized by locking it to a molecular absorption line, and its frequency
is measured by comparing it with an accurately known frequency, which is
multiplied up from the microwave into the optical range. The wavelength is
measured independently by interferometric methods. The accuracy with which
we know the velocity of light was improved this way by a factor of a hundred.

12.13.10 Manipulation of atoms by light
There are many ways of doing so, all very interesting but leading too far away
from our central direction. It is, however, definitely worthwhile to look at least
at one of those interactions, responsible for cooling.
From what we have done so far, it is easy to deduce that lasers can heat
materials. But cool them? How is that possible? In fact, if we take Doppler
∗
∗ Not so simple in the general case. cooling as an example the principles are quite simple. Let us imagine a 1D
The 1997 Nobel Prize was awarded to gas in which atoms move with random velocities, and assume the existence of
Steven Chu, Claude Cohen-Tannoudji, two counterpropagating laser waves of the same intensity and same frequency.
and William D. Phillips for development
of methods to cool and trap atoms with The frequency is chosen so that it is a little below an atomic resonant frequency.
laser light. Both beams exert a force upon the atoms due to their radiation pressure. If the