  The genesis of lasing, as an evolution of the laser hybrid eigenstates
comprised of electromagnetic modes and atomic polarization, is considered. It
is shown that the start of coherent generation at the laser threshold is
preceded by the formation of a special hybrid state at the lasing
pre-threshold. This special state is characterized by an enhanced coupling
among excited atoms and electromagnetic modes. This leads to an increase in the
rate of stimulated emission in the special state and, ultimately, to lasing. At
the lasing pre-threshold, the transformation of hybrid eigenstates has the
features of an exceptional point (EP) observed in non-Hermitian systems. The
special state is formed when eigenfrequencies of two hybrid states coalesce or
come close to each other. Below the pre-threshold, lifetimes of all hybrid
states grow with increasing pump rate. When the pump rate crosses the
pre-threshold, resonance trapping occurs with the lifetime of the special state
continuing to increase while the lifetimes of all other eigenstates begin to
decrease. Consequently, the latter eigenstates do not participate in the
lasing. Thus, above the pre-threshold, a laser transitions into the single-mode
regime.

Optics;Other Condensed Matter