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

12 E. DelRe, M. Segev, D. Christodoulides, B. Crosignani, and G. Salamo
Fig. 6. Soliton bright and dark existence curve, as reported in [19].
beam intensity and background illumination are such as to project the two
conditions on the very same point on the (u0, ∆ξ) parameter space. Note
that this powerful device breaks down as soon as we consider the o(η) terms,
or even simple corrections in the actual nonlinearity, such as those deriving
from dielectric nonlinearity [31].
In order to test the validity of the treatment, experiments have been carried
out with the aim of detecting for what conditions of launch a 1+1D
steady-state soliton would form [19, 32, 33], and some examples of results are
contained in Fig.(7) for bright solitons and Fig.(8) for dark.
Indeed other experiments have led to much the same conclusions, for
which the qualitative agreement is full, but for some regions of parameters,
where the quantitative comparison is weaker. At present it is believed that
the discrepancy is due more to the presence of extraneous effects than to
a deficiency in the model. For example, it has been observed that some of
the background illumination, which is made to propagate along an ordinary
mode, in order to not undergo substantial evolution, is actually trapped by
the soliton through the finite r13 coefficient. Another source of uncertainty
is connected to the difficulty in establishing the precise value of rij for the
given sample: this can depend on the level of purity of the poling, on the
presence of considerable clamping and on temperature.
Concerning more fundamental aspects of the model, we note that although
the results shown in Fig.(7) support the approximation contained in Eq.(4),
the actual beam evolution shows a clear and reproducible self-bending effect