Photorefractive Solitons (Chapter in Springer book ... - Tripod

2 E. DelRe, M. Segev, D. Christodoulides, B. Crosignani, and G. Salamo
tive solitons have evolved into a major subject contributing to research at
the cutting edge of nonlinear science. The intrinsic complexity of the photorefractive
light-induced index-change, driven by several charge transport
mechanisms, utilizing linear and quadratic electro-optic effects, and having
a polarization-dependent tensorial behavior, contributes in giving rise to a
rich phenomenology. Much is understood now about the formation processes
of the various types of photorefractive solitons, and many of the parameters
can be controlled individually: at the same time numerous questions
are still open. Also important, research on solitons in photorefractives has
benefited not only the soliton community, but has also introduced a number
of new ingredients to photorefractive studies at large. For example, understanding
the propagation dynamics of beams in photorefractives (rather than
two- and four-wave-mixing), including the formation of self-oscillators (e.g.,
the so-called ”double phase conjugator”) has considerably benefited from the
understanding gained in photorefrative soliton research. Likewise, exploring
spatially-localized effects that emerge and find their full realization directly
within the sample, such as instabilities and spontaneous pattern formation,
with and without a cavity, is now nicely understood through the intimate
connection between solitons and modulation instability [4]. These distinguish
soliton phenomenology from holographic and wave-mixing schemes, which, by
nature, deal principally with the manipulation of light as a device to an effect
which occurs outside the crystal. In this Chapter, we attempt to provide an
updated overview on the fascinating phenomenon of photorefractive solitons,
which continuously brings in new surprises, new effects, new excitement, to
the photorefractives community, to the much broader nonlinear optics community,
and to soliton science at large.
2 The discovery of solitons in photorefractives
In the wake of renewed interest in soliton propagation, triggered by studies
on temporal solitons in an ever more competitive fiber optic network, the beginning
of the 1990s see an intense effort aimed at finding accessible physical
systems in which to experimentally investigate spatial solitons. Conventional
Kerr-type nonlinear schemes presented crippling limitations connected to the
extremely high optical intensities involved, and suffering from the fundamental
constraints associated with the instabilities and catastrophic collapse of
Kerr solitons in bulk media. Motivated by the strong nonlinear response of
photorefractive crystals, even at low optical intensities, M. Segev, B. Crosignani,
and A. Yariv, were able to formulate in 1992 the first photorefractionbased
self-trapping mechanism. This embryonic idea sets the beginning of the
field, and, indeed, of our description [5].
A spatial soliton is a beam which, by virtue of a robust balance between
diffraction and nonlinearity, does not change its shape during propagation. A
direct observation of a spatial soliton in photorefractives is shown in Fig.(1).