High-resolution molecular spectroscopy with 2.5 terahertz quantum cascade lasers

Quantum cascade lasers operating at about 2.5 terahertz have been implemented in a spectrometer for high resolution molecular spectroscopy. Linewidth, frequency tunability and frequency stability of the lasers were investigated by mixing their radiation with radiation from a 2.5 terahertz gas laser. Two first laser characteristics were found to be sufficient for Doppler-limited spectroscopy. For a demonstration of the spectrometer, absorption at rotational transitions of methanol was measured. The cascade lasers served as radiation source. A gas laser was used to determine the absolute frequency simultaneously with the detection of absorption signal by mixing the gas laser radiation with a part of that from cascade lasers. Amplitude as well as frequency modulation of the output power of the cascade lasers was used. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519 terahertz were determined.