Figure 2. Details of the laser tuning setup.

(a) Schematic of the dual-grating Littrow-type external cavity used in this work. The back facets of the laser chips are coated with a two-layer anti-reflection coating (AR) as described in ref. 16. The collimating lens is an AR-coated aspheric lens with a focal length of 1.87 mm and a numerical aperture of 0.85. The non-polarizing beam splitter is made of ZnSe and transmits (reflects) approximately 50% of the incident light. Two gold-coated 150 grooves per mm diffraction gratings are used to provide frequency-tunable feedback for the two mid-IR pump frequencies (ω₁ and ω₂) independently. (b) Mid-IR emission spectra of a THz DFG-QCL in the setup depicted in (a) for several different grating positions. In this example one diffraction grating tunes mid-IR pump frequency ω₂ from 980 cm⁻¹ to 1054 cm⁻¹ and the other diffraction grating tunes mid-IR pump frequency ω₁ from 1091 cm⁻¹ to 1170 cm⁻¹. The electroluminescence spectrum of the laser material is shown with a dashed line. (c) Selected THz emission spectra of the device recorded for the same diffraction grating positions as in (b).