  It is known that the doping of liquid crystal with dyes usually changes the
physical properties of the host, like the transition temperatures and the
optical absorption among others. In this work we report a study of the
nonlinear optical refraction of a dye doped sample of the E7 thermotropic
liquid crystal by the Z-scan technique. It was found that the nonlinear
refraction of the sample is higher than the undoped one, diverging at the
clearing point. Close to the N-I transition, the nonlinear birefringence is
characterized by a critical exponent that seams to confirm the tricritical
hypothesis of the nature of the N-I phase transition, being independent of the
doping.
