Observation of the transition from the near-field to the far-field region for broadband terahertz radiation

Summary form only given. In the antenna theory, it has long been well known that the space surrounding an antenna can be divided into the near- and far-field region. But in the terahertz (THz) optoelectronics community, this had not received much attention until recently when Budiarto et al. (1998) observed the near-field effect in the measurement of THz radiation from a large aperture photoconductive transmitter. In their experiment, due to the very large aperture size (3 cm), all the measurements were limited to the near-field region, where the waveforms keep changing as they propagate away from the emitter. In the work, we observed the transition of the broadband THz radiation from the near-field regime to the far-field regime where the waveform does not depend on the distance between the emitter and detector and where the amplitude of the field follows the 1/r rule. We also observed that the transition distance depends on the aperture size of both the transmitter and receiver. We believe that these observations help to elucidate the nature of the broadband electromagnetic wave propagation and the transmitter/receiver system.