Analysis of terahertz waveforms measured by photoconductive and electrooptic sampling

Terahertz (THz) waveform measurements by photoconductive (PC) sampling and free-space electrooptic sampling (FS-EOS) are analyzed and quantitatively compared. Our data suggest that a short dipole antenna used in a PC receiver contributes a flat frequency response when used without a substrate lens and a jω response when used with a substrate lens, for the specific THz frequency range and optical system investigated in our experiments. These findings are explained using results from basic antenna theory. Experiments testing our theory for a variety of THz waveforms (obtained by using different THz emitters and simple as well as shaped optical excitation pulses) and for different carrier lifetimes are also presented. Finally, we demonstrate near-field effects in the PC sampling measurements of broad-band THz waveforms and explore the evolution of THz radiation from the near field into the far field