Tunable Fano quantum-interference dynamics using a topological phase transition in (Bi1-xInx)2Se3

Asymmetric Fano resonance arises from quantum interference between discrete and continuum states. The characteristic asymmetry has attracted strong interests in understanding light-induced optoelectronic responses and corresponding applications. In conventional solids, however, the tunability of Fano resonance is generally limited by a material's intrinsic property. Topological insulators are unique states of matter embodying both conducting Dirac surface and underlying bulk. If it is possible to manipulate the two coexisting states, then it should form an ideal laboratory for realizing a tunable topological Fano system. Here, with the recently discovered topological phase transition in (Bi1-xInx)(2)Se-3, we report tunable Fano interference phenomena. By engineering the spatial overlap between surface Dirac electrons (continuous terahertz transitions) and bulk phonon (discrete mode at similar to 2 terahertz), we continuously tune, abruptly switch, and dynamically modulate the Fano resonance. Eliminating the topological surface via decreasing spin-orbit coupling-that is, across topological and nontopological phases, we find that the asymmetric Fano spectra return to the symmetric profile. Laser-excited ultrafast terahertz spectroscopy reveals that the controlled spatial overlap is responsible for the picosecond tunability of the Fano resonance, suggesting potentials toward optically controllable topological Fano systems.