Superluminal terahertz pulses

In femtosecond terahertz-pulse (T-ray) imaging of metal structures with dim ensions of the order of the wavelength, it is observed that the T rays prop agate faster than the vacuum speed of light. In the case of apertures this can be understood as a waveguide effect in which superluminal velocities ar e expected close to the cutoff frequency. However, the effect is also obser ved close to knife edges and in propagation past thin metal wires. (C) 1999 Optical Society of America.