Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution

The authors demonstrate ultrabroadband time-resolved THz spectroscopy on a single InAs nanowire with 10 nm spatial resolution and sub-100 fs time resolution. Phase-locked ultrashort pulses in the rich terahertz spectral range 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 have provided key insights into phenomena as diverse as quantum confinement 7 , first-order phase transitions 8 , 12 , high-temperature superconductivity 11 and carrier transport in nanomaterials 1 , 6 , 13 , 14 , 15 . Ultrabroadband electro-optic sampling of few-cycle field transients 1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light 4 , 8 , 10 . However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM) 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 . We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (