Exciton-polariton eigenmodes in light-coupled In0.04Ga0.96As/GaAs semiconductor multiple-quantum-well periodic structures

Features of exciton-polariton eigenmodes in a series of light-coupled In0.04Ga0.96As/GaAs semiconductor multiple quantum wells with varying number of quantum wells N from 1 to 100, and with various periodicities (Bragg, near-Bragg, and anti-Bragg), are studied in linear measurements of reflection, transmission, and absorption. At Bragg periodicity (period d = lambda(x)/2), a photonic band-gap mode grows in amplitude and increases linearly in linewidth with increasing N. The N times increased radiative damping rate is seen to arise from the light character of the eigenmode being swept out of a photonic band-gap structure. The slope of linewidth versus N gives the radiative linewidth of the exciton. Away from Bragg periodicity two branches of energy levels can he resolved in absorption, corresponding to the N exciton-polariton normal modes in the multiple-quantum-well structure. Signatures of individual modes becoming optically active are observed in the rich structure of reflection spectra for changing quantum-well periodicity. Antireflection coating of the samples is shown to be an effective way of thus isolating the multiple-quantum-well response.