  The collective modes of helical electron gases interacting with light have
been studied in an extended random phase approximation. By separating two kinds
of electron density oscillations, the complicate operator dynamics coupling
electrons and photons can be simplified and solved. The inverse operator
transformation that interprets electron oscillations and photons with quasi
particles has been developed to study the interaction between surface plasma
polaritons (SPPs) and emitters. Besides the ordinary interaction induced by
electric field, we find an additional term which plays important roles at small
distance arising from electron exchanging effect.
