The theory of surface plasmon polariton (SPP) amplification in the metal-insulator-metal structure is developed
from first principles. Basing on the dyadic Green’s function of the structure, an exact analytical expression for
the gain coefficient is derived. It is shown that the amplification can occur without a population inversion and
can reach giant (∼ 106 cm−1) values. The physical origin of this effect is associated with a positive feedback
provided by the reflecting waveguide walls that does not require an external pumping. It is also predicted that the
same physical phenomenon can lead to SPP generation and the corresponding threshold condition is obtained.
It is stressed that such an effect does not imply stimulated emission in the structure and is principally different