Indirect Optical Methods Autocorrelation and Up-conversion

We have seen that the limitations of the time characteristics of electronic devices requires the use of optical delays between the pump and probe pulses in ps flash photolysis. There are also indirect ways of using optical properties to measure the kinetics of laser pulses and of fluorescence, known as autocorrelation and up-conversion . These rely on the non-linear properties of certain materials or chemical systems, i.e. they are based on fast biphotonic processes. 

2 Up-conversion. This is similar to autocorrelation, but the laser pulse is mixed with the luminescence light of a sample in an SHG crystal to generate light of frequency vL + vE. The time between the laser pulse (vL) and the luminescence emission (vE) is varied by means of an optical delay path, and the light of added frequency is detected by a usual PM tube via optical filters. 

There are many variations on these basic systems of autocorrelation and up-conversion. In the former the SHG crystal can be replaced by a solution of molecules which can be excited only by light of frequency 2v anthracene for example cannot be excited directly by a laser light of 532 nm, but will fluoresce when excited by two photons. 

---