  A spectral singularity is a mathematical notion with an intriguing physical
realization in terms of certain zero-width resonances. In optics it manifests
as lasing at the threshold gain. We explore the application of their
recently-developed nonlinear generalization in the study of the effect of a
nonlinearity on the lasing threshold condition for an infinite planar slab of
gain medium. In particular, for a Kerr nonlinearity, we derive an explicit
expression for the intensity of the emitted waves from the slab and discuss the
implications of our results for the time-reversed system that acts as a
coherent perfect absorber.
