The terahertz and mid-infrared spectral domain host a multitude of interesting low-energy elementary excitations, such as phonons, plasmons and magnons. Ultrafast optical spectroscopy has provided key insights into the dynamics of these collective excitations. Unfortunately, the spatial resolution of such (far-field) studies is intrinsically limited to the scale of the probing wavelength by diffraction. Thus, the optical response cannot resolve individual nano-objects, confined polariton waves, or local surface effects.
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