New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Optics regained
revealing individual atoms under rather special conditions have been achieved, but there is still
some way to go before useful images are readily available.
We are thus left with the awkward situation that we can encode information at the right level
of detail using X-rays, neutrons or electrons, but we cannot focus an image experimentally. Nor
can we satisfactorily record the phases (as with a hologram) and so we must solve the problem of
interpreting the encoded pattern (as for example figure 5). The whole history of X-ray diffraction
from 19 12 to the present date is concerned with finding ways of sorting out the encoded patterns
without a lens. The success has been almost unbelievable and structures such as large proteins
have been worked out in detail by these indirect techniques.
Thus in present-day optics we group together all the various imaging systems together with
X-ray, electron and neutron diffraction and see them all as aspects of the same basic physical
principle of encoding and decoding information using radiation as the investigating medium.
Lasers
It is clear from the last section that the laser has had an enormous influence on the developments
in Optics and therefore is worthy of a separate section.
As with so many discoveries, one can find the seeds in scientific work of a much earlier period.
Einstein, in fact, as long ago as 19 16 suggested that the phenomenon of stimulated emission of
radiation should exist but it was not until about forty years later that stimulated emission was
demonstrated; the first lasers operating in the visible light region appeared in 1960.
Until the discovery of laser action, there were only three essentially different ways of producing
electromagnetic radiation. The thermal radiation from hot bodies was one possibility (e.g.
infra-red sources, tungsten filament lamps, etc). The spontaneous emission of radiation from
excited atoms or molecules was a second (e.g. sodium or mercury-vapour lamps, characteristic
X-rays, etc). The radiation produced by the acceleration or deceleration of charges (classically)
was the third (e.g. radio and micro-wave sources, ‘white’ X-radiation, etc).
The exploitation of stimulated emission is a remarkable story. The basic idea is simple. An
atom which is in an excited state with an energy E, can be stimulated to decay to a lower state
El by the action of a photon of frequency U such that hu = E2 - El . We could write the equation
for stimulated emission as
E2 + (hu) +- El i- 2 (ku)
But the truly remarkable features of the phenomenon are, (a) that the new photon has the same
frequency as the one causing stimulation, (b) that the new photon’travels in the same direction as
the original one, (c) that the new photon is in phase with the original one, (d) that the new
photon has the same polarization as the original one and (e) that the instantaneous rate at which
the process of production of new photons occurs is proportional to the density of existing
photons of the same frequency.
Feature (e) thus means that the process has in-built positive feed-back which results in an
‘avalanche’ effect; feature (b) means that all the new photons are travelling in the same direction
and hence the result is enormous power at one single frequency travelling in one single direction.
The earliest maser (microwave amplification by stimulated emission of radiation) used the two
lowest energy levels of an ammonia molecule. The first laser (light replaces microwave in the
acronym) used ruby, which is aluminium oxide with some of the aluminium atoms replaced by
241