Molecular Spectroscopy with terahertz quantum cascade lasers

We have implemented quantum cascade lasers (QCLs) operating at about 2.5 THz in a spectrometer for high resolution molecular spectroscopy. One QCL has a Fabry-Perot resonator while the other is a distributed feedback laser. Linewidth and frequency tunability of both QCLs were investigated by mixing the radiation from the QCL with that from a 2.5 THz gas laser. Both were found sufficient for Doppler limited spectroscopy. Rotational transitions of methanol were detected in absorption using a QCL as radiation source. Amplitude as well as frequency modulation of the output power of the QCL were used. The absolute frequency was determined simultaneously with the absorption signal by mixing a small part of the radiation from the QCL with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519 THz were measured in order to demonstrate the performance of the spectrometer.