The nonlinear Fano effect

Fano effect goes nonlinear The Fano effect is a spectral distortion arising from quantum-mechanical resonance or interference between two competing optical pathways. It is ubiquitous in optical spectroscopy of atoms, solids and semiconductor heterostructures. It indicates how discrete energy states, for example of an atom, are coupled to the continuum of states in its environment. The effect has been widely studied, but usually in a linear regime at low excitation power. A new study uses semiconductor quantum dots to explore the physics of the nonlinear Fano regime. Clear Fano resonances are observed, and they can be tuned by changing the device design or with applied voltages. In the nonlinear regime, visibility of Fano interferences increases dramatically, which could be used as a sensitive probe of the degree of coupling between discrete states and the continuum, which is relevant for example for qubits where coupling to the environment needs to be kept to a minimum. Clear Fano resonances that can be tuned by changing the device design or with applied voltages are observed. In the nonlinear regime, the visibility of the Fano interferences increases dramatically, which could be used as a sensitive probe of the degree of coupling between discrete states and the continuum, which is relevant for example for qubits where coupling to the environment needs to be kept to a minimum. The Fano effect 1 is ubiquitous in the spectroscopy of, for instance, atoms 1 , 2 , bulk solids 3 , 4 and semiconductor heterostructures 5 , 6 , 7 . It arises when quantum interference takes place between two competing optical pathways, one connecting the energy ground state and an excited discrete state, the other connecting the ground state with a continuum of energy states. The nature of the interference changes rapidly as a function of energy, giving rise to characteristically asymmetric lineshapes. The Fano effect is particularly important in the interpretation of electronic transport 5 , 6 and optical spectra 7 , 8 in semiconductors. Whereas Fano’s original theory 1 applies to the linear regime at low power, at higher power a laser field strongly admixes the states and the physics becomes rich, leading, for example, to a remarkable interplay of coherent nonlinear transitions 9 . Despite the general importance of Fano physics, this nonlinear regime has received very little attention experimentally, presumably because the classic autoionization processes 2 , the original test-bed