RESONANT AND SURFACE-POLARITONS IN QUANTUM-WELLS

We show how the breaking of the translational invariance in a quantum well modifies the concept of polariton with respect to that defined for bulk material. Polaritons in quantum wells result from the combination of the exciton states with the radiation field. They are here obtained as the solutions of Maxwell equations with retardation, provided an appropriate nonlocal response function is used for the electric susceptibility, and Maxwell boundary conditions are imposed. We find two types of polaritons depending on the values of the in-plane wavevector k-parallel-to: those at k-parallel-to < omega/v (where v = c/n is the velocity of light in the sample) are resonant with the radiation field in the barrier and those at k-parallel-to > omega/v cannot be coupled to waves in the barrier. In both cases explicit expressions are given for radiative shifts and radiative broadenings as functions of k-parallel-to. Numerical results are obtained for GaAs-Ga(1-x)Al(x) As and for CuCl quantum wells and ne experiments are suggested. The existence of resonant and surface polaritons justifies an interpretation of the temperature dependence of the radiative lifetime suggested by the same authors. It also decreases the radiative efficiency in the direction perpendicular to the planes and increases the radiative efficiency parallel to the planes with increasing temperature.