Pump & probe technique uses two short pulses --- either of light, electric field or particle beams.

A pump pulse initiates the phenomenon whose time-evolution (like motion of an atom) we would like to know.  A probe pulse usually comes later with respect to the pump, and “illuminate” the atom for a short while so that its position at the time is recorded on the detector.  The technique is analogous to stroboscope, which uses a a short pulsed flashlight (probe) and a camera (detector) with much slower time-resolution.

To record the motion precisely, we must know not only the exact position but the exact arrival time of the probe pulse with respect to the pump.  The big problem in femtosecond pump & probe measurements is the synchronization between the two pulses.  This is easily circumvented by splitting a single femtosecond pulse into two using a beam splitter (like a half mirror).  After the beam splitter the pump and probe pulses travel different paths, so that they arrive at different time -- defined by the difference in the path length -- at the target.

In our laboratory, we use two (usually identical) femtosecond light pulses as pump and probe.  The pump pulse is focused onto a solid sample surface, and excite electron-hole pairs and coherent phonons.  The probe pulse is also focused on the same spot on the sample, and its reflection from the sample or transmission through the sample is detected.  This procedure is repeated at different path lengths (or delay time) between the pump and probe pulses until we obtain the time evolution of the reflectivity or transmittance.

<PREVIOUS PAGE