Ultrafast single-nanowire multi-terahertz spectroscopy with sub-cycle temporal resolution

TL;DR

This paper introduces a novel ultrafast terahertz spectroscopy technique combining near-field microscopy and sub-cycle detection to study single nanowires at the nanoscale with femtosecond resolution.

Contribution

It extends broadband THz spectroscopy to the sub-nanoparticle scale, enabling spatially and temporally resolved measurements of individual nanostructures.

Findings

Observed ultrafast formation of a local carrier depletion layer in a single InAs nanowire

Achieved sub-50 fs temporal resolution in nanoscale THz measurements

Demonstrated spatially resolved dielectric function dynamics at the surface of a nanowire

Abstract

Phase-locked ultrashort pulses in the rich terahertz (THz) spectral range have provided key insights into phenomena as diverse as quantum confinement, first-order phase transitions, high-temperature superconductivity, and carrier transport in nanomaterials. Ultrabroadband electro-optic sampling of few-cycle field transients can even reveal novel dynamics that occur faster than a single oscillation cycle of light. However, conventional THz 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 THz spectroscopy (20 - 50 THz) 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). 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 ($<$50 fs) formation of a local carrier depletion layer.