New light-matter phase: Asymmetric nonlinear self-consistent grating in a low-Q CW superradiant laser with symmetric Fabry-Perot cavity

We carry out numerical modelling of the Maxwell-Bloch equations for a CW superradiant lasing of a homogeneous two-level active medium in a symmetric low-Q Fabry-Perot cavity and find a remarkable phase transition. Namely, at a pumping level n > n ( a ) exceeding by several times the lasing threshold, n ( s ) , a stationary (or nonstationary multimode) lasing of more or less symmetric counter-propagating electromagnetic waves is replaced with a steady-state monochromatic (or weakly modulated) lasing of asymmetric counter-propagating waves with different amplitudes a + , a − . The spontaneous transition to a new field structure is accompanied by formation of a highly asymmetric strongly nonlinear half-wavelength grating of the self-consistent polarization and population inversion of active medium n z [1] , [2] . Many similar numerical runs with noise taking into account show that in a rather wide range of laser parameters the grating appears with equal probabilities 50:50 at one and the same distance from left or right ends of an active sample and has always one and the same asymmetric profile n z ( ζ ) with maximum at symmetric points with respect to the cavity center ζ = 0. As a rule, a formation of the steady-state grating takes a long transient period of time and includes generation of several superradiant pulses (see Fig. 1a ).