We describe measurements of the angular radiation patterns from lens-coupled terahertz antennas fabricated on photoconductive substrates. These measurements were performed with a novel terahertz (THz) time-domain spectrometer in which the femtosecond optical pulses used to gate the emitter and receiver antennas were delivered by optical fiber. We used this system to perform a comparison between the two substrate-lens designs commonly used in THz time-domain spectrometers. We measured both E-plane and H-plane emission patterns for a 90° bow-tie antenna. By comparing these experimental results with simulations based on Fresnel–Kirchoff diffraction, we find that the choice of substrate-lens design is important in determining not only the directivity of the emitted beam but also the spectral bandwidth. These results emphasize the significance of this crucial component in the design of broadband THz spectrometers.
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