Fig. 3 Normalized electrical rectification data of a QCL with π-shape electrode at different bias-currents between 0.9 Ith and 1.6 Ith. Inset: The experimental setup for a microwave rectification measurement.
To characterize the high frequency modulation performance of the devices, we applied a microwave rectification technology [11]. The inset of Figure 3 shows a diagram of the experimental setup. In this measurement, a microwave signal and a direct current (DC) bias are injected into the laser through a bias-T. The laser is DC biased at different currents, while the RF power is kept at 10 dBm. An1.024 kHz amplitude modulation is added to the RF generator by an additional arbitrary function generator while sweeping from 100 MHz to 15 GHz in steps of 100 MHz. Figure 3 shows the normalized electrical rectification curves for a QCL with π-shape metal contact electrode measured at widespread driving currents ranging from 0.9 Ith (360 mA) to 1.6 Ith (640 mA). Further increase in modulation bandwidth with the increasing DC current is observed. It is also in good agreement with literature [6-8], where similar results were obtained above lasing threshold.
Figure 4 shows the comparison results of the normalized electrical rectification data and corresponding fitting results at 1.6 Ith of a conventional QCL and a π-shape QCL. As can be seen, the -3 dB bandwidth of the conventional QCL is 870 MHz and the π-shape electrode QCL is 4.5 GHz. By fitting the measured value with the theoretical curve (dashed line) of the rectification model, the corresponding capacitance value can be extracted. It can be obtained that the parasitic capacitance of the conventional structure QCL is 36.6 pF, while the parasitic capacitance of the π-shape electrode QCL is 7.1 pF. The test results show that after integrating a π-shape electrode, the parasitic capacitance of the device is reduced effectively, resulting in an increase of the -3 dB modulation bandwidth from 870 MHz to 4.5 GHz.