Conventional time domain spectroscopy uses a translation slide to adjust the delay between a split ultrafast laser beam arriving at a sample. The first beam causes excitation of a sample into a non-equilibrium state, followed by the delayed pulse, that probes the sample response. By using the translation slide the delay can be lengthened by increasing the distance one arm of the split has to travel, thus analysing the sample response over different time delays.
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 severe disadvantage of the conventional approach is the fact that a physical mass needs to be moved to accomplish the time-delay, significantly limiting the scanning speed. In addition, translation stages suffer from residual manufacturing tolerances and alignment error impeding precise calibration of potentially causing measurement artifacts.
Asynchronous optical sampling measures the same sample characteristics but uses two ultrafast lasers with offset repetition rates. The time separation between excitation and probe pulse is scanned automatically at a constant rate given by the offset frequency. This has the benefit of offering high speed analysis with a 1 nanosecond time window with a 45 femtosecond resolution in just a few milliseconds. Additionally, the stability of the system is improved as there are no moving mechanics to affect pointing stability or spot sizes.
Laser Quantum offer a number of products for ASOPS:
Read more about the advancement of this equipment here: Ultrafast optical sampling oscilloscope measures at femtosecond time scales
|Scientific paper - Two-colour high-speed asynchronous optical sampling||1213KB|
|Scientific paper - Femtosecond time-resolved optical pump-probe spectroscopy||205KB|
|Scientific paper - Ultrafast time-domain spectroscopy based on ASOPS||0KB|
|Whitepaper - Sub-nm layer measurements using ASOPS and laser induced ultrasound||484KB|
|Whitepaper - Ultrafast optical sampling oscilloscope||406KB|
|Whitepaper - Introduction into high speed ASOPS||276KB|
|Scientific paper - Terahertz cyclotron resonance spectroscopy of an AlGaN-GaN heterostructure||1782KB|
|Scientific paper - Picosecond Photoacoustic Metrology of SiO2 and LiNbO3 Laser Systems||756KB|