Energy Gaps of the Fractional Quantum Hall Effect Measured by Magneto-Optics

We have studied the magnetic-field dependence of the spectral position of the luminescence line due to radiative recombination of two-dimensional electrons in the extreme quantum limit. We verify the recent theoretical prediction that downward cusps occur in the spectral position of the line at fractional filling factors. From the strength of these cusps we derive values for the Fractional Quantum Hall Effect energy gaps and their dependence on the magnetic field.