Abstract
The exact solution is obtained for describing the external field effects in the optically induced molecular reorientation and bistability in a homeotropic nematic liquid crystal. With a low external field applied transverse or parallel to the laser propagation direction, optical bistability can always be suppressed or enhanced and hence can be seen in all existing nematics at a low laser power (typically < 1 kW/cm2). The fields corresponding to the tricritical point are obtained.