Controlling inelastic light scattering quantum pathways in graphene

Inelastic light scattering spectroscopy has, since its first discovery(1,2), been an indispensable tool in physical science for probing elementary excitations, such as phonons(3), magnons(4) and plasmons(5) in both bulk and nanoscale materials. In the quantum mechanical picture of inelastic light scattering, incident photons first excite a set of intermediate electronic states, which then generate crystal elementary excitations and radiate energy-shifted photons(6). The intermediate electronic excitations therefore have a crucial role as quantum pathways in inelastic light scattering, and this is exemplified by resonant Raman scattering(6) and Raman interference(7,8). The ability to control these excitation pathways can open up new opportunities to probe, manipulate and utilize inelastic light scattering. Here we achieve excitation pathway control in graphene with electrostatic doping. Our study reveals quantum interference between different Raman pathways in graphene: when some of the pathways are blocked, the one-phonon Raman intensity does not diminish, as commonly expected, but increases dramatically. This discovery sheds new light on the understanding of resonance Raman scattering in graphene. In addition, we demonstrate hot-electron luminescence9 in graphene as the Fermi energy approaches half the laser excitation energy. This hot luminescence, which is another form of inelastic light scattering, results from excited-state relaxation channels that become available only in heavily doped graphene.