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(a) Scanning electron microscope (SEM) image of a ring-QCL. (b) Close-ups of devices with six different grating duty cycles (see right upper corner of each image). The values are the percentage that is covered by Ti/Au.
(a) Single-mode emission was achieved at all GDCs. The lowest SMSR amounts to 11 dB (14% GDC). (b) Logarithmic plot of the recorded spectrum of the 72% GDC device that shows the highest SMSR that is almost 27 dB. Measurements were performed in pulsed operation (100 ns/5 kHz) at room temperature and a driving current level of 0.4 A.
The collected optical power versus current density characteristic of the device with a 72% GDC. The upper axis shows the corresponding current values. The inset is a plot of slope efficiencies and Jth values for all reported GDCs.
Substrate to surface emission ratio (Psub/Psurf) of all realized GDCs. The solid line depicts the ratio of the measured values, whereas the dashed line is corrected for the absorption of the substrate and the transmission at the substrate-air interface. The inset illustrates the measured peak power levels for surface (solid, squares) and substrate emission (solid, dots) and for the substrate emission regarding absorption and reflection (dashed, circles).
Simulation results for an infinitely long ridge resonator with the same heterostructure and DFB grating parameters as the experiment. The cavity losses towards substrate αsub and surface αsurf (blue and red, dots and squares, respectively) as well as the ratio αsub/αsurf (squares, green) are plotted. The inset shows the calculated confinement factor in the growth direction Γz over the realized GDC values.
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