Superluminal propagation of terahertz pulses in sub-wavelength structures

Summary form only given. In femtosecond terahertz pulse (T-ray) imaging of metal structures with dimensions on the order of the wavelength, it is observed that the pulses propagate faster than the vacuum speed of light. In the case of apertures, this can be understood as a waveguide effect in which superluminal velocities are expected close to the cutoff frequency. However, the effect is also observed close to knife-edges and in propagation past thin metal wires Here an alternative method of achieving sub-wavelength spatial resolution in THz imaging is demonstrated. An optical beam is focused to a small spot in an optical rectification crystal resulting in T-rays emerging in a beam, which in the nearfield region has a sub-wavelength diameter.