THz spectroscopy setup
THz radiation was generated via two DFB diode lasers mixed in an InGaAs photodiode (TeraScan 1550 from TOPTICA). The diverging beam was collimated by a parabolic mirror with 2“ diameter and 3“ focal length. The collimated beam was transported through an air passage of ~30 cm leading to water absorption lines in the spectrum. To focus on the detector element with a size of 2x2 mm, an identical parabolic mirror was used. Detector and emitter were placed in the focus point of the respective parabolic mirror (see. Fig. 1).
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The frequency of the emitter was adjusted by tuning the wavelengths of the two lasers. For each frequency step the modulated power was measured with a lock-in amplifier at f = 1 kHz. Fig. 2 displays the measured spectral response. The sharp absorptions peaks are characteristic water absorption lines. The width of the red curve is determined by standing waves in the setup.
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To obtain an absolute power calibration of the MPY-RS2 detector below 1 THz, the mismatch of detector element size with the transform limited THz spot size was corrected. Fig. 3 shows the spectrally flat detector response with responsivity of ~20 kV/W. The raw data (gray) was smoothed to filter standing waves in the experimental setup.
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