Spectral characterization of terahertz quantum cascade lasers by heterodyne and homodyne mixing

We have studied the current- and temperature-driven frequency tunability of quantum cascade lasers operating at 2.5 THz. The design of the active medium follows the so-called bound-to-continuum approach. One laser has a Fabry-Perot type resonator. It operates on several modes simultaneously. The other has a distributed feedback structure on top of the active medium which leads to single mode emission. High-resolution spectra of the laser emission by heterodyne mixing with the radiation from a THz gas laser were obtained. In the case of the Fabry-Perot laser, homodyne mixing of its modes was performed too. The emission frequency increases with current for both lasers and all modes. The analysis of the spectra shows up to 2% variation of the difference frequency due to changes of the laser current. The origins of these shifts are discussed.