## Harmonic Generation, Induced Nonlinearity and Optical
Bistability in Nonlinear Composites

####
Ohad Levy and David J. Bergman,
School of Physics and Astronomy,
Raymond and Beverly Sackler Faculty of Exact Sciences\\
Tel Aviv University, Tel Aviv 69978, Israel;
David G. Stroud, Department of Physics, Ohio State University, Columbus, OH
43210, USA

We investigate the dielectric response of composite materials containing a
quadratically
nonlinear component. The bulk effective second order
nonlinearity coefficients of a few simple microgeometries are
calculated, and
found to diverge in the vicinity of a quasistatic resonance
of the composite. It is shown that
second and third harmonic generation (SHG and THG)
can be much enhanced in such composites, compared to bulk samples
of the nonlinear component. A new induced cubic nonlinearity (ICN),
which also
diverges near a resonance, is generated in the composite, even though
none
of its components possesses it intrinsically.
This ICN may be much larger than the
effective nonlinearity of a
composite with the same microgeometry and a cubic nonlinear component.
Finally, such composites are shown to exhibit optical bistability.
Such bistability is shown to be theoretically possible far away
from a quasistatic resonance, and
even when all the components have real, positive dielectric constants.
This is in contrast
to bistability in composites containing a cubic nonlinear
component,
in which at least one metallic component and
a close approach to a resonance are needed. However, tuning to the
vicinity
of a resonance is still needed in order to obtain bistability at
reasonable levels of the applied
field. Thresholds in the order of 10^4 W/cm^2 are predicted
for a particular layered microgeometry with three components.
PACS numbers: 42.70.Nq, 42.65.Pc, 78.66.Sq