Propagation of single-cycle terahertz pulses through and past wavelength-sized metal structures has been studied experimentally. In waveguides close to cutoff, it is found that the phase velocity can become superluminal and even negative. Multiple reflections of evanescent waves inside the waveguide are found to be the cause of a negative phase velocity below the cutoff frequency. The centroid delay of terahertz pulses propagating past a thin metal wire is found to be advanced or delayed depending on the polarization with respect to the wire. In all cases of superluminal propagation described here, the principle of causality is preserved. In a restricted sense, exchange of information faster than the speed of light is found possible, however, the principle of causality ensures that information cannot advance by more than the inverse bandwidth of the signal. This eliminates causal-loop paradoxes and ensures that faster-than-light communication is not practical.
- single-cycle terahertz pulses