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.