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Although it took some time to establish the word, photonics is both
widely accepted and used throughout the world and a major area of
activity concerns nonlinear materials. In these the nonlinearity
mainly arises from second-order or third-order nonlinear optical
processes. A restriction is that second-order processes only occur
in media that do not possess a centre of symmetry. Optical fibres,
on the other hand, being made of silica glass, created by fusing
SiO molecules, are made of material with a centre of z symmetry, so
the bulk of all processes are governed by third-order nonlinearity.
Indeed, optical fibre nonlinearities have been extensively studied
for the last thirty years and can be truly hailed as a success
story of nonlinear optics. In fact, the fabrication ofsuch fibres,
and the exploitation oftheir nonlinearity, is in an advanced stage
- not least being their capacity to sustain envelope solitons. What
then ofsecond-order nonlinearity? This is also well-known for its
connection to second-harmonic generation. It is an immediate
concern, however, to understand how waves can mix and conserve both
energy and momentum ofthe photons involved. The problem is that the
wave vectors cannot be made to match without a great deal of
effort, or at least some clever arrangement has to be made - a
special geometry, or crystal arrangement. The whole business is
called phase matching and an inspection ofthe state-of-the-art
today, reveals the subject to be in an advanced state."
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