The pump-probe technique is one of the most common methods to access information on the ultrafast dynamics (fs-ns) of photoexcited carriers in a material. The main idea is the following: The beam composed of a pulse train delivered by an ultrafast (mode-locked) laser is split in two:

(1) a pump beam with a high optical fluence (or intensity) that is used to photoexcite excite the material generating photocarriers to a non-equilibrium state

(2)  a probe beam with a weak optical fluence and an adjustable time delay (controlled by a motorized delay stage) that spatially overlaps with the pump beam on the sample.

One measures the pump-induced change in the transmission or reflection of the probe beam as a function of the relative delay between the pump and probe pulses. The change in the optical constants induced by the pump contains information on the recombination and relaxation of photoexcited carriers in the material. In practice, the pump beam is modulated (mechanical chopper, acousto-optic modulator, etc…) and the change in the probe intensity is measured with a photodetector which output voltage is demodulated by a lock-in amplifier synchronized with the pump modulation frequency. The most usual technique is the one-color pump-probe (or degenerate pump-probe) for which pump and probe are at the same wavelength (ex: 800 nm – 800 nm). Different wavelengths can also be used for the pump and probe (two-color pump-probe) in order to excite carriers and probe their state at a different energy (ex: 400 nm – 800 nm, 800 nm – THz, etc…). Instead of using a single wavelength probe, a spectrally resolved broad continuum can also be used, this is the transient absorption spectroscopy technique which  provide more insights than the two-color scheme.

Example of typical pump-probe. (left) A typical decay curve showing the relative change in transmission of the probe vs time delay. (right) Schematic of pump-probe in reflection.

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