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

4 E. DelRe, M. Segev, D. Christodoulides, B. Crosignani, and G. Salamo
An optimal arrangement was identified in Stontium-Barium-Niobate (SBN)
doped with rhodium impurities. Characterized by an r33 220pm/V ,for
an extraordinary c polarized beam launch, index modulations of the order
of |∆n| (1/2)n 3 r33E0 ∼ 2 − 5 · 10 −4 for achievable fields of the order of
E0 ∼ 1-3kV/cm could be reached, a nonlinearity sufficient to support the
formation of a 10 µm wide soliton.
Fig. 2. The original soliton observation schematic, as reported in [7]. The extraordinary
launch beam is focused in proximity of the input facet. Transient dynamics
were detected recording the output intensity transmitted through an appropriate
aperture positioned before the output detector.
The first pioneering experiments, reported in [7] were carried out with
the set-up indicated in Fig.(2). In relating the first results, we more often
delve upon details of transient - as opposed to steady-state - effects, and
miss the main and revolutionary point: where no previous phenomenology
even hinted at self-focusing, these first experiments indicated, unmistakably,
that a visible continuous-wave beam propagating in a biased sample would
actually self-trap, the ensuing spatial soliton being readily accessible to direct
observation (see Fig.(1)). Although the initial conjecture would later
not prove to be greatly adherent with first findings, it did express the main
physical idea supporting the phenomena. These pioneering results established
that very narrow beams launched in a properly-biased photorefractive crystal
would self-trap, and propagate in a robust fashion, undistorted by fanning
and other noise sources in the crystal or even fairly large deviations from
optimal launch conditions [8]. For example, it was established that a 15 µm
sized continuous-wave 457nm µW beam would not suffer fanning and selftrap
for external fields from 400-500 V/cm. This observation led to a rapid
series of predictions and experiments, which now form the phenomenological
basis of photorefractive spatial solitons [8, 9, 10].
As commonly occurs when scientific progress is in action, those first experiments
posed more questions than answers. First, the results indicated a transient
self-trapping process (see insert in Fig.(2)), during a time window of
several hundreds of milliseconds, not characterized by stringent existence con-