Ultrafast field-resolved multi-terahertz nano-spectroscopy

The broad, multi-terahertz spectral window ranging from 10 to 100 THz (1 THz = 1 · 10 Hz) contains a plethora of fundamental excitations in solid state systems, including plasmons, phonons, magnons and interlevel transitions in excitons [1]. Field-resolved detection of phase-stable multi-terahertz probe pulses by electro-optic sampling (EOS) provides a direct measure of the polarization response of such phenomena. It thus reveals their evolution with a temporal resolution much faster than a single oscillation cycle of the multi-terahertz probe [2]. However, the diffraction limit restricts the spatial resolution of conventional, far-field multi-terahertz spectroscopy to half the probe wavelength (µm - mm). Consequently, in ensembles of nano-sized objects like nanowires or quantum dots, the measured dielectric function is averaged and important information can be obscured completely. Yet, accessing the optical properties of these nanostructures is crucial for understanding how their properties differ from those of a bulk material. Scattering-type near-field scanning optical microscopy (s-NSOM) can be used to overcome the diffraction limit [3].