Ultrafast time domain or pump probe spectroscopy is a technique with an expanding application range, where the sample is excited by a pulse of light then analysed by a second arriving a short time later. The sample response (e.g. reflectivity, absorption, Raman scattering, luminescence, THz electric field) is monitored over time to evaluate transient phenomena in solid, liquid or gaseous samples.
The conventional method of adjusting the time-delay between the pump and probe pulses involve a linear translation stage that changes the path length travelled by the pump/or probe pulse. However, a significant disadvantage of such conventional systems is the inherently low acquisition rate for a full transient. A 1ns long transient necessary for a 1GHz resolution requires a delay stage travel distance of 15cm. The time required for accelerating and decelerating the stage between 10,000 or more data points and to efficiently average out technical laser noise leads to total acquisition times in the range of a few minutes.
Asynchronous Optical Sampling (ASOPS) is a time-domain spectroscopy technique first introduced with picosecond lasers in the late eighties that employs no mechanical delay stage and thus eliminates the problems outlined above. Laser Quantum’s implementation of a high-speed ASOPS uses two Taccor femtosecond lasers whose repetition rates of 1 GHz are linked with an offset Delta fR of a few kilohertz by the TL-1000-ASOPS timing unit. As a result of the offset, the delay between pairs of pulses from the lasers is repetitively ramped between zero and the inverse laser repetition rate at a scan rate equal to Delta fR. The key advantage of Laser Quantums’s high-speed ASOPS lies in the scan rates of several kilohertz at a time-delay resolution of < 60fs, a performance that is impossible with conventional systems. It allows completing the acquisition of a time-domain trace before technical noise of common femtosecond lasers with significant Fourier content in the acoustic regime up to 1kHz can affect the signal. This permits measurements at the shot-noise limit without the use of lock-in amplifiers or other noise suppression measures. Ultrafast spectroscopy of previously impossible processes e.g. Protein folding or studies in rapidly changing conditions are now possible.