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

5.2 Pump Probe Measurement 
Multiple quantum well (MQW) structures have applications in modulators, 
high-speed photodetectors and all-optical switching elements. The potential for high 
operating speed is essential. As MQW related devices are scaled to high optical 
powers and speeds, it is becoming increasingly important to understand the dynamics 
of photo-generated carriers in the active region. Determining the carrier transit time 
will help us know the fundamental operating speed limit. Because the measurements 
described above were probably limited by the measurement technique, rather than the 
freq (100.0MHz to 70.00GHz)
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intrinsic device properties, a measurement method that determines the intrinsic 
behavior of the device was needed. 
5.2.1 Method 
Optical pump–probe measurements can be used to obtain information on ultrafast 
phenomena. The general principle is the following. A sample is pumped by a very 
short pump pulse, which generates some distribution of carriers (or other 
modification) in the sample. After an adjustable time delay (controlled with an optical 
delay line), a probe pulse is incident on the sample, and its transmission or reflection is 
measured. By monitoring the probe signal as a function of the time delay, it is possible 
to obtain information on the decay of the pump-generated excitation or on other 
processes initiated by the pump pulses. The probe signal is typically averaged over 
many pulses, and a fast photodetector is not required. The temporal resolution is 
fundamentally limited only by the pulse duration. The pump-probe technique is 
normally used to determine the carrier lifetime and optical nonlinearity of a 
semiconductor material. [92-97]. 
Figure 5.9: Pump probe measurement of reflectivity change 
The wavelengths of pump and probe beam do not need to be identical. A so-called 
two-color pump–probe measurement, based on two synchronized sources of short 
pulses (e.g. a laser and an optical parametric oscillator, or two parametric oscillators 
pumped with the same laser) has additional capabilities in ultrafast spectroscopy. 
Pump–probe measurements can be used, for example, to monitor the recovery of a 

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saturable absorber after its excitation, the speed of diffusion of photoexcited carriers, 
or the melting of a sample after being hit by an intense pulse.