  We propose a new approach leading to giant gain enhancement. It is based on
unconventional slow wave resonance associated to a degenerate band edge (DBE)
in the dispersion diagram for a special class of photonic crystals supporting
two modes at each frequency. We show that the gain enhancement in a Fabry-Perot
cavity (FPC) when operating at the DBE is several orders of magnitude stronger
when compared to a cavity of the same length made of a standard photonic
crystal with a regular band edge (RBE). The giant gain condition is explained
by a significant increase in the photon lifetime and in the local density of
states. We have demonstrated the existence of DBE operated special cavities
that provide for superior gain conditions for solid-state lasers, quantum
cascade lasers, traveling wave tubes, and distributed solid state amplifiers.
We also report the possibility to achieve low-threshold lasing in FPC with DBE
compared to RBE-based lasers.
