We introduce a new scheme for the detection of terahertz pulses based on the frequency selective optoelectronic downconversion of its individual modes with a continous-wave (CW) ErAs:InGaAs photoconductive antenna (PCA) driven by a comb-based CW photonic signal. The detection scheme can be used as metrology tool for the analysis of the fundamental resolution and stability limits of terahertz pulses and the mode-locked-lasers (MLLs) that drives them, as well as an ultra-high-resolution measurement technique for terahertz components or gas spectroscopy. We demonstrate both applications by measuring the linewidth of two frequency components of the particular terahertz pulse analyzed here (one at 75 GHz and one at 340 GHz) and by measuring a very narrowband filter between 70 and 80 GHz. The main advantage of this technique with respect to other terahertz pulse detection schemes is its capability of performing ultra-high-resolution measurements without the need of unpractically long scanning ranges or synchronization of two MLLs.
