The pump-probe technique is a widely used method for studying the ultrafast dynamics (fs–ns) of photoexcited carriers in materials. The basic principle is as follows: a pulse train from an ultrafast (mode-locked) laser is split into two beams:
- Pump beam: A high-fluence (or high-intensity) beam used to photoexcite the material, generating photocarriers in a non-equilibrium state.
- Probe beam: A low-fluence beam with an adjustable time delay, controlled by a motorized delay stage, that spatially overlaps with the pump beam on the sample.
The pump-induced change in the transmission or reflection of the probe beam is measured as a function of the time delay between pump and probe pulses. This change in the material’s optical constants provides information about the recombination and relaxation dynamics of photoexcited carriers.
In practice, the pump beam is modulated (using a mechanical chopper, acousto-optic modulator, etc.), and the change in probe intensity is detected with a photodetector. The output voltage is then demodulated by a lock-in amplifier synchronized to the pump modulation frequency. The most common setup is a one-color (or degenerate) pump-probe, where pump and probe beams share the same wavelength (e.g., 800 nm – 800 nm). Different wavelengths for pump and probe beams (two-color pump-probe) are also used to excite carriers and probe their states at different energies (e.g., 400 nm – 800 nm, 800 nm – THz).
Additionally, instead of a single-wavelength probe, a spectrally resolved broad continuum can be used in transient absorption spectroscopy, offering deeper insights than the two-color scheme.