Asynchronous ultrafast pump-probe experiments: Towards high speed ultrafast imaging with ultrahigh spectral resolution

Femtosecond pump-probe experiments allow the generation/detection of acoustic waves in the GHz-THz frequency range for investigating matter properties at sub-micron scales. To circumvent the main drawback of these experiments, namely their limited acquisition speed due to the use of mechanical delay line, asynchronous optical sampling (ASOPS) has emerged in the 80´s. The key feature of this method consists in using two different lasers for pump and probe pulses. A slight difference between their repetition rates creates the pump-probe delay, the mechanical delay line being then no longer needed: The acquisition rate is considerably sped up. Pump-probe delays from zero to the inverse of the repetition rate are scanned during one beating period (in the millisecond range) between pump and probe asynchronous pulse trains. The strong potential of this technique has been demonstrated in the ultrafast acoustics community with 1 GHz repetition rate lasers.