Broadband Terahertz Near-Field Excitation and Detection of Silicon Photonic Crystal Modes

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