The pump-probe technique is one of the most common methods to obtain information on the ultrafast electron dynamics (sub-ps to ns) in a medium. The main idea is the following. The beam containing the pulses delivered by an ultrafast (mode-locked) laser is split in two:
(1) a pump beam with a high optical fluence (or power density) that is used to excite the sample 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 overlaps 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 time delay, i.e. as a function of the time delay between the arrival of the pump and probe pulses. This measurement contains information on the relaxation of the electrons in the medium. 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…). These wavelengths can be generated, e.g. via nonlinear crystals or optical parametric amplifiers/ oscillators.
Page last updated on September 3, 2017 (JM/CP).