Tunneling of single-cycle terahertz pulses through waveguides

Propagation of single-cycle terahertz pulses through and past wavelength-si zed metal structures has been studied experimentally. In waveguides close t o cutoff, it is found that the phase velocity can become superluminal and e ven negative. Multiple reflections of evanescent waves inside the waveguide are found to be the cause of a negative phase velocity below the cutoff fr equency. The centroid delay of terahertz pulses propagating past a thin met al wire is found to be advanced or delayed depending on the polarization wi th respect to the wire. In all cases of superluminal propagation described here, the principle of causality is preserved. In a restricted sense, excha nge of information faster than the speed of light is found possible, howeve r, the principle of causality ensures that information cannot advance by mo re than the inverse bandwidth of the signal. This eliminates causal-loop pa radoxes and ensures that faster-than-light communication is not practical. (C) 2000 Elsevier Science B.V. All rights reserved.