Optical probing of composite fermions in a two-dimensional electron gas

In a high magnetic field, electrons confined to two dimensions form highly correlated states driven entirely by electron-electron interactions. Transport and cyclotron-resonance experiments on these fractional quantum Hall effect states, and the associated fractionally charged excitations, suggest the existence of composite fermions-electrons with two flux quanta attached. Using optical spectroscopy, we show that the two flux quanta in a composite fermion interacting with an exciton (a bound state of an electron and a hole) lead to filling-factor-dependent features in the optical emission spectrum, which are symmetric around filling factor v = 1/2, and fractionally charged excitations lead to fractionally charged excitons. In the vicinity of the incompressible v = 1/3 state we observe a doublet structure in the emission line, corresponding to excitations of the incompressible fluid. At filling factors v > 1/3, corresponding to the transition to a compressible metallic state, a new emission line appears, which we attribute to the fractionally charged quasi-exciton.