Terahertz time-domain stectroscopy using a power detector

Summary form only given. Terahertz time-domain spectroscopy (THz-TDS) is nowadays commonly used to measure the absorption coefficient and refractive index of materials over a very broad spectral band (typically from 0.1 up to several THz). Usual TDS set-ups include ultra-fast emitter and receiver, like photo-conductive antennas or non-linear crystals. Indeed, their ultra fast response time allows one to generate and/or detect sub-picosecond electromagnetic waveforms, whose spectra spread up to the THz range. Nevertheless, such devices require a whole optical bench to manage a pulsed laser beam in both space and equivalent time. On the contrary, power detectors (e.g. bolometers, Golay cells, pyroelectric detectors...) are more convenient to use, allowing an easy mapping the spatial distribution of THz beams. However, the temporal resolution of these detector remains poor (millisecond at best) as they integrate the incoming THz power. Therefore the THz pulse transient evolution and thus the THz E-field phase are lost. Here we describe a THz Michelson interferometer in which the receiver is a pyroelectric detector (Gentec THz 9B-MT, 200-ms rise time). The interferometer permits us to retrieve the phase of the signal. Transient autocorrelation curves of a picosecond pulse have been recorded and are in a good agreement with analytic expectations. The modulus and phase of the THz signal is obtained by Fourier-transforming the recorded waveforms. 2D-mappings of the THz beam power, of the transient signals and of the frequency spatial distribution have been experimentally achieved. The influence of the impinging THz polarization on the measurement dynamics has also been studied. Finally, we employed the set-up to perform broadband spectroscopy of samples.