High speed, low driving voltage vertical cavity germanium-silicon modulators for optical

 1460nm

T
(t
=
0
)/
T
Bias (V)
Figure 5.11: Relative transmission variation of the modulator, immediately after perturbation from the 
pump pulse, vs the bias voltage for various pump wavelengths. 
When the pump wavelength is fixed at 1.4µm, the modulator exhibits a significant 
response for bias voltages larger than 4V, and the recovery time decreases with 
increasing bias. The minimum value of the recovery time obtained was 20ps at 11V. 
At this particular wavelength, photo-carriers (generated by the pump pulse) reduce 
transmission of the absorber. 
For a wavelength of 1.43µm, the response is similar. In this case, the transmission 
initially increases before recovering to its initial value. For voltages between 4 and 8 V, 
the recovery time decreases with increasing voltage. For the higher bias voltages we 
observe a smaller response and overshoot of the transmission change. This can be 
explained by the large QCSE and detuning pump and probe wavelength with the 
modulator absorption peak. 
0
40
80
0
4V
5V
6V
6.5V
7V
7.5V
8V
8.5V
9V
10V

T
 (
n
o
t 
s
c
a
le
d
)
Time (ps)
Reverse bias
0
40
80
4.53999E-5
1.2341E-4
3.35463E-4
6V
6.5V
7V
7.5V
fit 

=44ps
fit 

=30ps

T
 (
n
o
t 
s
c
a
le
d
)
Time (ps)
Reverse bias

---

 
 
 
82 
(a) (b) 
Figure 5.12: (a) Differential transmission spectroscopy at 1.43μm (b) Fitting for lifetime at 6V and 6.5V 
Fig 5.12 (a) shows the differential transmission spectroscopy at 1.43μm, it can be 
seen that the lifetime of the carriers decreases as the voltage/electric field intensity 
increases. The signal is very weak when the voltage is lower than 4V. This is because 
the modulator material structure is not yet optimized. There are intrinsic spacers, a 
thick buffer layer and too many quantum wells in the structure, leading to low electric 
field on each quantum well. An improved device design including better selection of 
parameters such as thickness and number of quantum wells can enable better 
measurement results. Fig 5.12 (b) shows the fitting curve of the carrier recovery time. 
The fitting curves show that under 6V, the carrier lifetime is 44 ps and under 6.5 V it 
reduces to 30ps. It can be seen on the plot that at 11V the lifetime is under 10ps, 
indicating >100GHz modulation capacity in the Ge/SiGe quantum well materials 
system.