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

22 E. DelRe, M. Segev, D. Christodoulides, B. Crosignani, and G. Salamo
Fig. 15. Observations of dark photovoltaic solitons, from [83]. (a) linearly diffracting
intensity distribution of a dark notch; (b) self-trapped intensity distribution,
for various propagation distances in the sample.
Fig. 16. Self-trapping of a photovoltaic vortex, from [46]. (a) output intensity
distribution before self-focusing begins, and phase pattern; (b) output intensity
distribution trapped by the photovoltaic field, and phase pattern.
(and lower) power levels. As will be discussed in the section on applications,
photorefractive solitons, and the waveguides they induce, combine properties
that suggest interesting applications ranging from reconfigurable directional
couplers, beam splitters, waveguide switching devices, tunable waveguides for
second harmonic generation, and highly efficient optical parametric oscillators
in soliton-induced waveguides. In general, however, the formation time
of solitons in most photorefractive materials is rather long, except when very
high intensities are used [32]. This is because the photorefractive nonlinearity
relies on charge separation, for which the response time is the dielectric
relaxation time, i.e., inversely proportional to the product of the mobility
and the optical intensity, and the mobility in photorefractive oxides is low.
In principle, however, photorefractive semiconductors, (e.g., InP, CdZnTe),
have a high mobility and could offer formation times a thousand fold faster
than in the other photorefractives. However, the electrooptic effects in these
semiconductors are tiny, which implies that solitons that are as narrow as 20
optical wavelengths necessitate very large applied fields, making solitons in