Optical Observation of Fermi Energy Discontinuities and Optical Anomalies in the Fractional Quantum Hall Regime.

We present magneto-transmission and emission spectra of n-type multiple quantum wells (MQWs). At even integral filing factors, new peaks appear on the low energy side of the absorption spectra, due to passage of the Fermi level from one Landau level to the next. At nu ~~ 1 a striking anomaly is observed in both emission and absorption, manifest by a discontinuous jump in the lowest energy transition for T 10K. In the regime of the fractional quantum Hall effect at nu = 2/3, a high energy peak appears in the emission for T 2K, while the absorption remains unchanged.