  In this Letter, we study theoretically reflectance of a monolayer comprizing
regularly spaced quantum $\Lambda$-emitters. Due to high density of the latter,
the monolayer almost totally reflects the incident field in the vicinity of the
system's collective (excitonic) resonance. The emitter self-action through the
secondary field provides a positive feedback, interplay of which with the
inherent nonlinearity of an emitter itself, results in an exotic behavior of
the system reflectance, including bistability, self-oscillations, and chaotic
dynamics. All these features might be of interest for nanophotonic applications
