  Chip-based terahertz (THz) devices are emerging as versatile tools for
manipulating mm-wave frequencies in the context of integrated high-speed
communication technologies for potential sixth-generation (6G) wireless
applications. The characterization of THz devices is typically performed using
far-field techniques that provide limited information about the underlying
physical mechanisms producing them. As the library of chip-based
functionalities expands, e.g., for tailoring the emission and directional
propagation properties of THz antennas and waveguides, novel characterization
techniques will likely be beneficial for observing subtle effects that are
sensitive to a device's structural parameters. Here we present near-field
measurements showing the emission properties of a broadband THz emitter placed
in the vicinity of a photonic crystal (PHC) slab. These experiments reveal
long-predicted emission properties, but which to our knowledge have yet to be
experimentally observed at THz frequencies. We demonstrate three distinct
effects between 0.3-0.5 THz: (i) field suppression at frequencies corresponding
to quasi-TE bandgaps (ii) a frequency-dependent directed emission along two
distinct pathways for two neighboring frequencies, resulting in a local field
concentration; (iii) a re-direction of the emission, achieved by rotating the
PHC with respect to the dipole orientation. Simulations reveal that the
observed behavior can be predicted from the underlying band structure. Our
results highlight the opportunities that PHCs can potentially provide for
alignment-free, chip-based 6G technologies. Our experimental technique extends
the applicability realms of THz spectroscopy and will find use for
characterizing the THz modes supported by true samples, whose inherent
imperfections cannot realistically be accounted for by simulations,
particularly in highly dispersive frequency bands.
