Mode loss spectra in THz quantum-cascade lasers with gold- and silver-based double metal waveguides

Spectra of the waveguide loss coefficient for THz radiation of a quantum-cascade laser with a gold- and silver-based double metal waveguide (DMW) are calculated based on the measurements of resistivity of the metals for different temperatures. It is shown that, taking into account the absorption of THz radiation by free carriers and optical phonons, the spectrum of total mode losses has a broad minimum in the range of 3–6 THz, which shifts to higher frequencies with increasing temperature. The minimum losses in the Au-based waveguide increase from 8 to 27 cm^-1 as temperature increases from 100 to 300 K. The use of an Ag-based DMW makes it possible to decrease the losses by 2–4 cm^-1 compared to the Au-based DMW.