Fig. 1. (a). Calculated waveguide losses for different wavelengths in a metal-metal waveguide assuming a “lossless” active region. (b) Calculated waveguide losses in a metal-metal waveguide assuming a realistic active region design with an average doping of 5×10¹⁵ cm^-3. We note that long-wavelength, λ>200 µm, QCLs typically use lower doped active regions.

Fig. 2. Calculated temperature dependence of waveguide losses for 100 µm wavelength in a metal-metal waveguide assuming a “lossless” active region. Optical constants of metals were estimated using Eq. (1). The data for temperatures below 80 K is very sensitive to the purity of metals and is not shown.

Fig. 3. Conduction band diagrams of (a) three- and (b) four-quantum-well resonant-phonon active region designs, reported in Refs. [16] and [4] respectively. A single quantum-cascade stage is marked by a box. Both structures utilized the GaAs/Al_0.15Ga_0.85As material system. The layer sequences, starting from the injection barrier, are 48/96/20/74/42/161 Å for (a) and 49/79/25/66/41/156/33/90 Å for (b). Laser transitions are shown with arrows. Also shown are the transition dipole moments and emission energies for the laser transitions, calculated for single isolated modules of the structures. The four-quantum-well resonant-phonon active region design is shown for reference only; it is not used in our experiments.

Fig. 4. (a). Current density-voltage characteristic and an emission spectrum (inset) of a representative device processed with a gold metal-metal waveguide. Devices processed with a copper metal-metal waveguides displayed similar current density-voltage characteristics and emission spectra. (b) Light intensity-current density (LI) characteristics of the best-performing device with a gold metal-metal waveguide, 1.3mm-long and 150µm-wide. Inset: the LI characteristics of the device close to the maximum operating temperature of 168 K. The data are not corrected for an estimated 10% power collection efficiency.

Fig. 5. (a). The LI characteristics of a 1.4mm-long and 125µm-wide device with a copper metal-metal waveguide. Inset: the LI characteristics of the device close to the maximum operating temperature of 178 K. The data are not corrected for an estimated 10% power collection efficiency. Dips in the LI characteristics at current density ~1150 A/cm² are due to some of the laser emission lines coincide with atmospheric absorption lines. (b) Threshold current density as a function of temperature for the device in (a). Inset: the LI characteristics of another device with a copper metal-metal waveguide, 1.6mm-long and 100µm-wide, close to its maximum operating temperature of 177 K.