Coherent detection of few-cycle terahertz pulses with a minimum number of optical elements

In this presentation, we describe a more robust way to read out the degree of focusing of the probe pulse in terahertz-induced lensing and hence to detect the momentary field of the terahertz pulse. For this we use probe pulses at a wavelength lambda of 1.5 mum and a standard silicon photodiode. We show that the relative change of the photodiode signal is proportional to the terahertz electric field and that the sensitivity is the same as when the central intensity is measured. However, if the active area of the photodiode is larger than the probe beam, a small relative mispositioning does not degrade the sensitivity.