Effect of chirp in diffraction of short electromagnetic pulses through subwavelength apertures

Summary form only given. We demonstrate experimentally that the temporal advancement of THz pulses in diffraction by a subwavelength aperture is due to the intrinsic negative chirp. The effect is clearly observed only for sufficiently small apertures, when the bandwidth of the transmitted pulse exceeds the bandwidth on the incident pulse. The temporal advancement can be removed by eliminating the chirp of the THz pulse. For pulses generated by transient current in low temperature GaAs switches, the chirp is eliminated when long (1 ps) optical pulses are used to excite the photoconductor.