Anomalous magneto-plasmonic spectra: evidence for stimulated emission or superradiance?

Previous studies on electron-hole magneto-plasmas have been limited to relatively low laser intensity and/or low magnetic fields. Here, we extend this regime by probing the emission characteristics of dense magneto-plasmas in high magnetic fields (25 T) and at carrier densities approaching 10$^{13}$/cm$^{2}$. Using a 150 fs, 775 nm Ti:sapphire chirped pulse amplifier and optical parametric amplifier, we have performed intensity and magnetic field dependent magneto-photoluminescence (MPL) measurements on the heavy hole exciton in 8 nm In$_{0.2}$Ga$_{0.8}$As multiple quantum well (QW) samples separated by 15 nm GaAs barriers. Above a threshold intensity, the emission from higher-lying Landau levels (LL's) exhibit anomalous features appearing asymmetrically on the high-energy side of the peaks. These narrow features dominate the spectrum at high excitation power. The line width of the feature is significantly narrower than the lowest LL, implying a different physical origin than simple radiative recombination. In addition, the appearance of the peaks correlates with a threshold magnetic field value of approximately 13 T. An examination of the wavelength dependence of the MPL spectra as well as a line-shape analysis suggest that the inter-LL emission is a stimulated process, arising from the high electron hole densities.