Coherent Phonon Lab Tour

Pump & probe technique is analogous to stroboscope, which uses a a short pulsed flashlight and a camera with no fast time resolution. It utilizes two short pulses, either of electromagnetic field (e.g. light) or particles (e.g. electrons), depending on the target of the experiment.

A pump pulse initiates the phenomenon we would like to study -- collective atomic motion in a solid, for example. A probe pulse arrives later (or earlier) with respect to the pump and cut out the phenomenon at the time of its arrival, and the result is recorded. This procedure is repeated many times at a variety of timings when the probe arrives. Finally, the results are sorted according to the timing, and we obtain the time-evolution of our target phenomena.

The problem is how to know the exact timing of the probe pulse arrival on femtosecond time scale. If we use light of the same color (wavelength) for both pump and probe, this is easily done by splitting a single laser beam (of femtosecond pulses) into two using a half-mirror-like beam splitter. After the split, the pump and probe pulses travel different paths so that they arrive at the target at different times defined by the path length difference.

Temporal synchronization in a femtosecond precision is demanding if we use electronic circuits. Control of spatial position in 0.3 micrometer precision is much easier, and in the pump-probe scheme this is equivalent of having a control of 1 femtosecond. (Remember that 1 femtosecond is 0.3 light micrometer!)