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(a) Schematic of the experimental setup where a coplanar probe connected to a network analyzer is positioned over the THz QCL contacts. The QCL is 2.65 (1.06) mm-long and is driven at 4 K. [(b) and (c)] computed two-dimensional mode intensity profiles along the section of the waveguide at 2.3 THz (b) and 20 GHz (c). is the -component of the electric field, and the -plane is the plane of the laser facet. The light orange horizontal lines represent the top metal contact and the ground plane.
Measured (black solid line) and simulated (red dashed line) plots in the 0.1–55 GHz range for a 2.65 mm-long (top panel) and a 1.06 mm-long (bottom panel) ridge, driven at current densities of 140 and respectively. Inset: Voltage/current characteristic of the 2.65 mm-long ridge at 4 K. The vertical dashed line indicates the laser threshold .
Microwave attenuation (black curve, left axis) and effective index (red curve, right axis) derived from the distributed parameters. As shown by the dashed lines the attenuation is given by the sum of two contributions: (i) a virtually frequency independent component due to the shunt conductance (dashed-dotted line) and (ii) a -dependent contribution stemming from the series resistance (dotted line).
Spectra in scale from a 4 mm-long, 2.3 THz, MMW QCL, amplitude modulated at frequencies between 18 and 21.5 GHz, with an RF power of 13 dBm. Without modulation the QCL is single mode. The THz spectra were up-converted at and measured with a high resolution optical spectrum analyzer using the technique described in Refs. 4 and 14. Inset: Spectra in linear scale of a 1.5 mm-long, 2.3 THz, MMW QCL, amplitude modulated at frequencies between 24.0 and 24.7 GHz, with an RF power of 20 dBm. Spectra were collected with a Fourier transform spectrometer with a resolution of .
Measured plots in the 0.1–55 GHz range for the 2.65 mm-long device at current densities of 250 mA (below threshold) and 330 mA (above threshold), respectively. Inset: Computed group effective index at 2.3 THz (, blue line) and effective index at 20 GHz (, red line) as a function of the ridge width.
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