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

34 E. DelRe, M. Segev, D. Christodoulides, B. Crosignani, and G. Salamo
component. It carries and conserves angular momentum, although its constituents
exchange angular momentum as they propagate. The soliton turns
out to be generically robust, so much that even during inelastic collisions
with other composite solitons, the collision products are predicted to be also
composite rotating propeller solitons [115].
The relatively ease with which vector and composite solitons are generated
in photorefractives, by employing the mutual incoherence technique,
has also led to a series of experimental efforts demonstrating interactioncollisions
between vector solitons. It turns out that temporal optical vector
solitons, and non-optical vector solitons are very difficult to generate, hence
the collision experiments with optical spatial vector soliton were truly pioneering.
For example, it has been predicted, more than two decades ago, that
collisions between Manakov-like vector solitons give rise to a symmetric exchange
of energy between the soliton constituents. This elegant phenomenon
was observed only recently, with photorefractive vector (Manakov-like) solitons
[116, 117]. The energy-exchanges between the soliton components (which
have nothing to do with photorefractive two-wave-mixing) have direct implications
in a new form of reversible computing, in which a ”state” is coded
as the ratio between the soliton components [118]. In this scheme, computation
is performed through the energy-exchange interactions (in which the
”states” change) during collisions between vector solitons. The experiments
have shown that indeed, not only such symmetric energy exchanges do occur,
but also information can be transferred through a series of cascaded collisions
between vector solitons [116, 117].
Finally, photorefractives were also the means for experimental studies
of interaction-collisions between multi-mode solitons, in which shape transformations
were observed [119]. These were the first ever experiments with
collisions of multi-mode solitons.
The general ideas behind multi-component vector solitons proved invaluable
for later developments and in particular to the area of incoherent solitons
discussed in the next section.
10 Incoherent solitons: self-trapping of
weakly-correlated wavepackets
Until 1996, the commonly held belief was that all soliton structures should be
inherently coherent entities. In that year however, an experiment carried out
at Princeton demonstrated beyond doubt that self-trapping of a partially
spatially-incoherent light beam [42] is in fact possible, if the nonlinearity
has a non-instantaneous temporal response. In that experiment, the optical
beam was quasi-monochromatic, but partially spatially-incoherent and the
nonlinear medium was photorefractive, with a response much slower than
the characteristic time of the phase fluctuations in the incoherent beam. The
resultant self-trapped beam is now commonly referred to as an ”incoherent