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We demonstrate that azimuthally polarized micro-ring THz quantum-cascade lasers can be coupled to hollow metallic waveguides with efficiencies ≥96% and perfectly matched with the TE 01 waveguide mode, giving attenuation losses < 3dB/m. The use of Ag coated-flexible pipe waveguides allows propagation of either vertically polarized or azimuthally polarized QCL beams with propagation losses as low as 2.1–4.4 dB/m and bending losses lower than 1.2 dB.

Experimental evidence of a physical phenomenon characteristic of semiconductor lasers, i.e. the cooling of the electrons above the threshold for stimulated emission, is reported. By using a terahertz quantum-cascade laser as a prototype structure we show that this effect is directly related with the internal quantum efficiency (η int ).

A detailed time resolved experimental investigation of the thermal parameters causing the temperature performance degradation of THz QCLs will be presented. Possible technological solutions aimed to the extension of the operating temperatures of THz QCLs will be discussed.

We have measured the electronic temperature of the upper laser level (TE u) during continuous-wave operation in THz quantum-cascade lasers and found an experimental correlation with the occurrence of stimulated emission and the internal quantum efficiency.

The wall-plug efficiency and the thermal resistance of bound-to-continuum THz quantum-cascade lasers are extracted from the analysis of micro-probe photoluminescence spectra, via the direct measurement of the lattice temperature as a function of the electrical-power.

We present a comparative Monte-Carlo analysis of a set of GaAs-based THz quantum cascade lasers, allowing us to identify the effects limiting the device performance in the threshold region. Reasonable agreement with experiment is found.

We report the first measurements of the conduction subband electronic temperatures and local lattice temperature in THz quantum cascade lasers based on different active region and optical waveguide configurations both below and well above the laser threshold. These results have been achieved by using a non-invasive microprobe photoluminesce technique during device continuous wave operation.

The electronic temperatures, the lattice temperature and the electron-lattice energy relaxation rate in bound-to-continuum THz quantum-cascade lasers are extracted from the analysis of micro-probe photoluminescence spectra and compared with results for resonant-phonon structures.