  A scheme for active second harmonics generation is suggested. The system
comprises $N$ three-level atoms in ladder configuration, situated into resonant
cavity. It is found that the system can lase in either superradiant or
subradiant regime, depending on the number of atoms $N$. When N passes some
critical value the transition from the super to subradiance occurs in a
phase-transition-like manner. Stability study of the steady state supports this
conclusion.
