Angela Pizzuto, Enrique Castro-Camus and Daniel M. Mittleman
Journal of Infrared, Millimeter, and Terahertz Waves 44, 769 (2023)
Scattering-type scanning near-field optical microscopy (s-SNOM) has become a powerful tool for subwavelength-resolved imaging and optical spectroscopy. The technique is particularly useful in the terahertz or other long-wavelength regimes, where the diffraction limit prohibits resolving even micron-sized objects. This approach can also drastically improve the spatial resolution of nonlinear measurements such as laser terahertz emission microscopy (LTEM), in which a broadband THz pulse is generated after photoexcitation by a near-infrared (NIR) pulse. However, in all prior near-field LTEM experiments, the pump spot has been localized to the scattering probe, to couple to the generated THz radiation at the center of the illuminated region. Here, we demonstrate the first nonlocal near-field LTEM experiments, in which an ultrafast NIR pulse photoexcites a sample at a location that is laterally shifted from the location of the near-field probe which couples the THz signal to the far field. Increasing lateral shifts of the pump spot produce larger time delays in the arrival of the emitted broadband THz pulse, consistent with drift of the subsurface dipole between the pump location and probe tip. Monte Carlo simulations corroborate the time shift for the dipole formation, which in turn produces the THz emission with a relative delay. The simulation results show excellent agreement with experiments. This nonlocal s-SNOM approach to LTEM offers a new opportunity for studying lateral transport on the nanoscale and may be particularly useful in anisotropic materials.