Valley-resolved optical spectroscopy and coherent excitation of quantum Hall edge states in graphene

We show that chiral edge states in graphene under quantum Hall effect conditions can be selectively probed and excited by terahertz or infrared radiation with single-quasiparticle sensitivity without affecting bulk states. Moreover, valley-selective excitation of edge states is possible with high fidelity. The underlying physical mechanism is the inevitable violation of adiabaticity and inversion symmetry breaking for electron states near the edge. This leads to the formation of Landau-level-specific and valley-specific absorbance spectral peaks that are spectrally well separated from each other and from absorption by the bulk states, and have different polarization selection rules. Furthermore, inversion symmetry breaking enables coherent driving of chiral edge photocurrents due to second-order nonlinear optical rectification which becomes allowed in the electric-dipole approximation.