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Difference field propagating through the QCL above threshold. The delay stage used has only 25 mm of travel distance, so five separate scans were stitched together. The top-right inset shows a basic schematic of the emitter and QCL. The bottom-left inset shows a power spectrum (Fourier transform magnitude-squared) of the difference field. The bottom-right inset shows a zoomed-in view of the 16th echo and the corresponding time-domain oscillations.
(a) Measured gain at different biases (dots) with double-Lorentzian fits (lines). Peaks are labeled with the corresponding transitions. The lasing spectrum of a similar device is shown for reference. (b) Band diagram of the device measured. Starting from the left injection barrier, layer thicknesses in angstrom are 51/82/31/68/42/161/37/93, with the bold fonts representing barriers. The 161 Å well is bulk-doped to , resulting in a sheet density per module.
(a) Gain at 2.2 THz, gain at 1.25 THz, light output, and current density as functions of QCL voltage bias. The three dotted vertical lines indicate the onset of lasing, the onset of NDR, and the cessation of lasing. (b) Gain, light output, and voltage as functions of QCL current bias. Dotted vertical line indicates the onset of lasing. (NDR regime has been suppressed for clarity).
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