Direct Interbranch Relaxation of Polaritons in a Microcavity with Embedded CdSe/(Cd,Mg)Se Quantum Wells

In polariton microcavities, the upper polariton branch is often hardly observable in reflectivity or photoluminescence measurements, which hampers reliable determination of the vacuum Rabi splitting value. We studied the photoluminescence excitation of a semiconductor microcavity with eight embedded CdSe/(Cd,Mg)Se quantum wells and observed a maximum in the emission from the lower polariton branch when the energy of the excitation was tuned to that of the upper polariton branch. This indicates that, in microcavities, where the upper polariton branch is hardly observable but interbranch polariton relaxation is efficient, the Rabi splitting value can be determined based on the energy difference between the absorption and emission maxima in photoluminescence excitation measurements. The Rabi splitting value of the studied sample as determined by reflectivity measurements was 18 ± 2 meV. Strong coupling conditions allow us to observe lasing with a relatively low threshold of 0.24 kW cm−2.