Fig. 1. Conduction band diagram and the relevant wave functions of the active core under an applied electric field of 60 kV/cm.

Fig. 2. EL spectrum measured at a sub-threshold current.

Fig. 3. Concept design of superposed DFB grating dual-wavelength QCL. Inset: SEM image of the dual-period superposed gratings.

Fig. 4. Coupling coefficient (κ) varies with the grating duty cycle (σ) for the two wavelengths. The insets show the calculated modal profiles of λ₁ emission with σ = 0.5 (right) and λ₂ emission with σ = 0.4 (left), respectively.

Fig. 5. (a) Optical spectra measured at various currents at 20°C in CW mode. (b) Single-mode spectra of the 7.61 µm and 7.06 µm emission at the currents of 1.2I_th and 1.7I_th, respectively, with SMSR above 25 dB.

Fig. 6. (a) Light-current-voltage (L-I-V) curves of the dual-wavelength QCL measured at 10°C and 20°C in CW mode. The three colored areas under the 20°C L-I curve correspond to three different injection current ranges with different lasing modes. (b) Temperature-dependent spectra of single-mode λ₁ (upper panel) and single-mode λ₂ (lower panel) measured from 10°C to 40°C; the insets show the linear tuning of the peak wavenumber with temperature.