Terahertz frequency-domain modulation spectroscopy system based on self-heterodyne technique

We demonstrate a frequency-domain modulation spectroscopy system in the terahertz (THz) range based on the self-heterodyne technique to enhance the sensitivity of the small spectral change measurement. To evaluate the effectiveness of our system, we measured absorption of the THz wave caused by photo-induced free carriers generated in a high-resistive (4000 Ωcm) Si substrate. The 400-μm-thick Si substrate was excited by a 775-nm laser light, which was chopped at 1.7 kHz. By applying a tandem demodulation lock-in amplifier tuned at the modulation frequency of 1.7 kHz, we confirmed that the sensitivity of the amplitude absorption measurement was enhanced at least 14 dB. Our new system will pave the way for the sensitive measurement of THz spectral changes induced by photoexcitation in chemical, material, and biomedical sciences.