Page 353 - Electrical Properties of Materials

The atom laser 335

than the initial state, as shown in Fig. 12.29(b). In this case, the process is
known as Stokes scattering; the photon loses energy, and is scattered with a

lower frequency f = f – E/h, where E is the difference in energy between
the initial and de-excited states. The molecule then decays from the de-excited
state to the initial state by phonon relaxation. Alternatively, the de-excited state
may have lower vibrational energy, as shown in Fig. 12.29(c). In this case, the
process is known as anti-Stokes scattering; the photon gains energy, and is

scattered with a higher frequency f = f + E/h.
At room temperature, there will be a range of initial states, populated ac-
cording to the normal Boltzmann distribution. It will therefore be possible for a
stream of photons of the same frequency f to excite molecules from these states
to a range of excited states, and for the molecules then to relax back to a further
range of de-excited states. As a result, the scattered radiation will in general
consist of a number of different frequencies, known as the Raman spectrum.
However, these can only consist of discrete frequencies f ± E 1 /h, f ± E 2 /h,
f ± E 3 /h, ..., where E 1 , E 2 , E 3 ...are the energy differences between
initial and de-excited vibrational states. The lines in the Raman spectrum there-
fore always exist in pairs. However, their intensities will differ, since these must
depend on the initial population distribution. In thermodynamic equilibrium,
the uppermost state will have a smaller population, so the anti-Stokes line will
always be weaker than the corresponding Stokes line. Because the set of val- hf hf S
ues of E is unique to a particular molecule, Raman scattering can be used for S
materials analysis. In this case, it is known as Raman spectroscopy. hf hf
The Raman effect may also be used in a more complex process known P S
as stimulated Raman scattering (SRS). The material is simply pumped at a
De-excited
frequency f P , as shown in Fig. 12.30. If the pumping is strong enough, the pop- state
ulation in the excited state may become inverted. In this case the arrival of a ΔE
signal photon at the frequency f S = f P – E/h may stimulate a downward trans-
ition, generating a second, similar photon. A signal beam at f S will then grow
as it propagates through the material, leading to travelling wave amplification. Fig. 12.30
Because the gain is small, Raman amplifiers typically involve high pump Stimulated Raman scattering in the
powers and long path lengths. However, suitably long paths can easily be Raman amplifier.
created in optical fibre. For silica glass, the Raman shift is a few terahertz,
allowing amplification of signals at around 1580 nm wavelength to be carried
out using a co-propagating pump at 1480 nm, with a total usable bandwidth
of about 48 nm. As a result, the effect allows amplification to be carried
out within a standard transmission fibre, increasing the distance before signal
regeneration is needed in a fibre communication system.

12.14 The atom laser
It would be quite legitimate to ask why we need another section on atom lasers,
when so much has already been said about the various energy states of atoms
and how leaping from one energy state to another one may lead to laser action.
The atom laser is only called a laser. It is not a proper laser in the sense that
it has nothing to do with Light Amplification by Stimulated Emission of Ra-
diation. A less often used alternative name, matter wave laser, however, gives
away the secret. It is concerned with coherent matter waves in much the same
way as ordinary lasers are concerned with coherent electromagnetic waves.