  Current carrying chiral edge states in quantum Hall systems have fascinating
properties that are usually studied by electron spectroscopy and
interferometry. Here we demonstrate that electron occupation, current, and
electron coherence in chiral edge states can be selectively probed and
controlled by low-energy electromagnetic radiation in the microwave to infrared
range without affecting electron states in the bulk or destroying quantum Hall
effect conditions in the bulk of the sample. Both linear and nonlinear optical
control is possible due to inevitable violation of adiabaticity and inversion
symmetry breaking for electron states near the edge. This opens up new pathways
for frequency- and polarization-selective spectroscopy and control of
individual edge states.
