Pump–probe measurements can be used to obtain information on ultrafast phenomena, typically on picosecond or femtosecond time scales. The general principle is the following:
- The investigated object is hit by some pump pulse, which generates some kind of excitation (or other modification) in the sample.
- After an adjustable time delay, a typically weaker probe pulse hits the sample, and its transmittance or reflectance is measured. That reveals to which extent the sample is still affected by the pump pulse at that time.
- By monitoring the probe signal as a function of the time delay, it is possible to obtain information on the decay of the generated excitation, or on other processes initiated by the pump pulses.
The pump and probe pulses are in most cases derived from a common source, e.g. from a mode-locked laser and a beam splitter. The relative timing is controlled with an optical delay line. Note that a fast photodetector is not required; the temporal resolution is fundamentally limited only by the pulse duration of pump and probe pulses.
In some cases, the wavelengths of pump and probe beam are not 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 laser spectroscopy. One may then perform pump–probe measurements with different pump and signal wavelengths. It is vital in such cases to ensure tight synchronization of the different laser sources with a very low relative timing jitter, as the jitter could otherwise spoil the attainable temporal resolution.
The probe signal is often quite weak. It may be averaged over many pulses in order to obtain a better signal-to-noise ratio, assuming that the behavior of the tested object is fully reproducible. That is often the case if the applied pulse intensities are not too high.
Examples for Applications of Pump–Probe Measurements
Characterization of Saturable Absorbers
Pump–probe measurements can be used, for example, to monitor the recovery of a saturable absorber (e.g. a SESAM) after its excitation. For SESAMs, one often observes a rather fast partial recovery, followed by a slower complete recovery of the saturable loss (see Figure 1).
The speed of a photodiode can depend on various factors, one of them being the diffusion of photoexcited carriers. For investigating that, one may also very the exact position where the probe pulses are applied.
Laser Material Processing
In laser material processing, e.g. involving laser ablation, the occurring processes may be rather complicated. To some extent, they can be investigated with pump–probe measurements. For example, one may find out during which time interval after an intense pulse the material is found in liquid form.
Pump–probe methods are also used in various methods of time-resolved spectroscopy. Obviously, the addition of the temporal dimension to the spectral one greatly expands the opportunities for investigating systems.
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See also: time-resolved spectroscopy, ultrafast optics, saturable absorbers, semiconductor saturable absorber mirrors, optical sampling, synchronization of lasers, laser spectroscopy
and other articles in the categories optical metrology, methods