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. Several successful attempts have been made at using terahertz pulses for imaging. A near-field method for improving the spatial resolution in THz imaging has been reported in which a metal aperture was used to improve the resolution to 140 /spl mu/m. Here an alternative method of achieving sub-wavelength spatial resolution in THz imaging is demonstrated. An optical beam is focussed to a small spot in an optical rectification crystal resulting in T-rays emerging in a beam, which in the near-field region has a sub-wavelength diameter. If the sample is placed directly on the generation crystal, it can be imaged in the far infrared with /spl lambda//4.3 resolution. This experimental setup was used to image metal structures with dimensions of the order of the THz wavelengths.