Ultrafast time domain spectroscopy is a technique for monitoring sample activity on picosecond to nanosecond timescales. The sample is excited using a pulse of light, and its subsequent photo-excited dynamics are monitored via a second pulse that has been delayed by adjusting the length of its journey to the sample. The delay can be scanned with femtosecond precision and using multiple pulses and several time delays the sample dynamics can be monitored with respect to time after the arrival of the excitation pulse.
The sample response is often analysed in terms of reflectivity, transmission or other electro-optic phenomena. Ultrafast time-domain spectroscopy is used to evaluate carrier relaxation, lattice vibrations, and other collective dynamic phenomena (polaritons, plasmons, superconductivity, phase transitions etc) in solids, semiconductors and gases. Typical experimental realisations include: Terahertz (THz) time domain spectroscopy, ultrafast pump probe spectroscopy, semiconductor and wafer inspection based on picosecond ultrasound, time resolved photoelectron spectroscopy and rapid scanning optical coherence tomography.
A particularly attractive method for ultrafast time domain spectroscopy pioneered by Laser Quantum is asynchronous optical sampling. Rather than the conventional approach of using a mechanical delay stage, two offset-locked femtosecond lasers are used to rapidly scan the delay between pump and probe pulses. High-speed ASOPS out-performs classical approaches by orders of magnitude in measurement speed and noise performance.
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