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(Color online) (a) Schematic of the experimental setup (see text for description). The QCL is driven at in continuous wave at and . The output power of the rf generator is . Two sets of sidebands are generated (green and red arrows). The first set (green) at is the result of the parametric mixing of the near-IR and terahertz waves. The second set (red) is around each of the two terahertz sidebands, and is produced by the laser amplitude modulation in the gigahertz range. (b) Measured upconverted QC laser spectrum with the setup displayed in the top panel. In this case, the laser is modulated at , as shown by the separation between the generated sidebands in the inset on the right.
(Color online) Upconverted terahertz QCL spectra in scale obtained by increasing the modulation frequency from 0 (bottom trace) to (top trace). The rf power is of . Spectra are offset vertically to help readability, and are shown as displayed on an optical spectrum analyzer by zooming on the right hand side terahertz sideband of Fig. 1(b). Note that without modulation (bottom trace), the QCL is single mode.
(Color online) Histogram of the number of sidebands as a function of the modulation frequency, obtained from the traces of Fig. 2.
(Color online) (a) Schematic of the microwave rectification setup. The terahertz QC laser is dc biased at . The amplitude of the rf generator is modulated at , while is swept from in steps of . The rectified voltage, , is measured across the laser with the help of a lock-in amplifier. The lumped element model of the QC laser including the wire bond inductance is also shown. (b) Total amplitude of the modulated current, , as a function of the modulation frequency, obtained from the measured rectified voltage . The thick black and red curves correspond to rf powers of 5 and , respectively. Also shown are the computed traces based on the lumped element model (including the measured frequency response of the bias T and rf cable attenuation) for different values of (Ref. 16). The horizontal red dashed line corresponds to the power attenuation for .
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