The spectra of mode loss in terahertz quantum cascade laser (THz QCL) with double metal waveguide (DMW) based on Au, Cu and Ag have been analysed. On the basis of measurements of the resistivity of DMW claddings for the temperature range from 4.2 to 300 K, the loss coefficients of THz radiation at Au-, Cu-, Ag-claddings are calculated. We show that the Ag-based DMW allows to reduce the losses by 1.3–7.0 cm<sup>-1</sup> in comparison with Au-based DMW at room temperature. Our calculations show that the Ag-based DMW has slightly lower losses (~1 cm<sup>-1</sup>) than Cu-based DMW at cryogenic temperatures (below 100 K). The use of Cu-based DMW allows to reduce the loss coefficient in comparison with Ag-based DMW at higher temperatures (above 100 K). Temperature dependence of threshold gain for THz QCL with Au-, Cu-, Ag-based DMW are calculated. Taking into account the absorption of THz radiation by free carriers and optical phonons, the spectrum of total mode loss of THz QCL with different thickness of n+-GaAs top contact layer are analyses.
We have designed and fabricated terahertz quantum cascade lasers (THz QCLs) with double metal waveguide (DMW) based on three and four-quantum well GaAs/Al<sub>0.15</sub>Ga<sub>0.85</sub>As active module with resonant-phonon depopulation scheme. Three-well and four-well THz QCLs have a lasing frequencies of 3.2 THz and 2.3 THz, respectively. We investigate the dependence of threshold current and lasing output power on temperature for fabricated THz QCL. We propose to use DMW based on silver (Ag) for reducing the losses of the waveguide. The spectra of the loss coefficient of the DMW based on Au and Ag are calculated. It is shown, that the use of Ag-based DMW allows to reduce losses by 2-4 cm<sup>–1</sup> in comparison with Au-based DMW. Taking into account the absorption of THz radiation by free carriers and optical phonons, the spectrum of total mode losses has a wide minimum in the region of 3-6 THz, which shifts to the highfrequency region of the spectrum with increasing temperature. The postgrowth processing for THz QCL with Ag-Ag DMW are studied.