In this presentation we discuss application of broadband terahertz (THz) time-domain spectroscopy for probing subwavelength
(micrometer) size objects. The problem of weak coupling between THz waves and sub-wavelength objects,
which limits the use of THz spectroscopy to large samples, is mitigated by employing surface plasmon waves. In one
implementation, THz surface waves, excited on a broad-band planar THz bow-tie antenna, are used to enhance the
interaction with a small particle placed on the antenna surface. The surface field distribution near the particle is mapped
with an integrated sub-wavelength aperture THz near-field probe. We demonstrate that imaging and probing of the subwavelength size dielectric particles (TiO<sub>2</sub> and SrTiO<sub>3</sub>) can be realized using the enhanced THz field between the antenna and the probe. We also discuss THz wave confinement using two sharp metallic needles. We demonstrate that in the near-field region of the needle tips, the electric field of THz pulses is concentrated to a volume smaller than (10 μm)<sup>3</sup> without limiting the THz pulse bandwidth. Application of both methods for high spatial resolution imaging and spectroscopy will be discussed.
Visualization of terahertz (THz) plasmons with local probes allows studying ultra-fast plasmonic phenomena in the time
domain. We demonstrate that the integrated sub-wavelength aperture near-field probe can be used to map THz surface
plasmon waves in space and time with high resolution. Using experimental near-field observations of plasmon waves
formed on a metallic surface by tightly focused THz pulses and of standing plasmon waves in THz antennas, we show
that this probe detects the spatial derivative of the electric field rather than the plasmon field itself. The understanding of
the coupling mechanism provides a framework for interpretation of near-field images.