Light can be strongly confined in subwavelength spatial regions through the interaction with plasmons, the collective electronic modes appearing in metals and semiconductors. This confinement, which is particularly important in the terahertz spectral region, amplifies light-matter interaction and provides a powerful mechanism for efficiently generating nonlinear optical phenomena. These effects are particularly relevant in graphene and topological insulators, where massless Dirac fermions show a naturally nonlinear optical behaviour in the terahertz range. We have shown that the Dirac plasmon resonance in Bi2Se3 topological insulators can be tuned over one octave by employing intense broadband terahertz radiation delivered by the TeraFERMI beamline at FERMI@Elettra. This paves the way towards tunable terahertz nonlinear devices based on topological insulators, with potential applications in opto-electronics, communication, and sensing technologies.
Image: Plasmons are collective oscillations of electrons that can be directly excited by electromagnetic radiation in the presence of an extra momentum (red arrow). This is achieved in the present experiment, through ribbon arrays fabricated onto the surface of topological insulator Bi2Se3 films, excited after illumination with sub-ps, half-cycle THz pulses produced at the FERMI free-electron laser.