  Spectral singularities are among generic mathematical features of complex
scattering potentials. Physically they correspond to scattering states that
behave like zero-width resonances. For a simple optical system, we show that a
spectral singularity appears whenever the gain coefficient coincides with its
threshold value and other parameters of the system are selected properly. We
explore a concrete realization of spectral singularities for a typical
semiconductor gain medium and propose a method of constructing a tunable laser
that operates at threshold gain.
