Giant modal gain, amplified surface plasmon-polariton propagation, and slowing down of energy velocity in a metal-semiconductor-metal structure

We investigate surface plasmon-polariton SPP propagation in a metal-semiconductor-metal structure where semiconductor is highly excited to have an optical gain. We show that near the SPP resonance, the imaginary part of the propagation wave vector changes from positive to hugely negative, corresponding to an amplified SPP propagation. The SPP mode experiences an unexpected giant modal gain that is 1000 times of material gain in the excited semiconductor, a phenomenon not known to exist in any other system. We show that the physical origin of such giant gain is the slowing down of average energy propagation in the structure.