Optically tunable Fano resonances in carbon nanotubes

The Fano effects are ubiquitous in spectroscopy study of atoms, semiconductor nanostructures, photonics, and strongly correlated systems. It arises when quantum interference takes place between two competing optical pathways linking the discrete and continuum states. Fano resonance plays a crucial role in defining the optical transitions, scattering, transport and heat dissipation in low-dimensional systems. Most studies of Fano interference predominantly addressed linear regime at low-excitation intensities, and largely neglect the nonlinear effects that emerge at higher excitation levels. Notably, strong optical driving field can generate remarkably complex photon-dressed quantum states, modifying the amplitude and energy of excitation channels central to the quantum interference. Here, we use semiconducting single-wall carbon nanotubes (SWCNTs) as a model system to demonstrate continuous manipulation of many-body Fano resonances at the ultrafast timescale. Specifically, by tuning the quantum pathways of phonon Raman scattering across the exciton resonance using negatively detuned laser pulses, the Fano lineshapes transition continuously from antiresonance to dispersive features and to symmetric Lorentzian. We show that such nonlinear Fano effects depends sensitively on the driving energy and intensity, leading to ultrafast switching between asymmetric Fano lineshapes and symmetric Lorentzian profiles. Our study opens up new pathways for coherent control of novel many-body exciton-phonon quantum interference and detection of extremely weak coupling between discrete and continuum quantum states in a rich variety of low-dimensional systems.