Novel non-linear optical techniques for diagnostics: laser-induced gratings and two-colour four-wave mixing

Electrostriction, population transfer caused by absorption of laser energy and subsequent thermalization are the most important mechanisms responsible for the formation of laser-induced gratings. Applying time resolved light diffraction from electrostrictive gratings allows for the measurements of t hermodynamic properties like temperature, gas composition, and flow velocit y. Thermal gratings can be employed as a very sensitive spectroscopic tool. The Two-Colour Four-Wave Mixing (TCFWM) signal formed by a non-resonant pum p laser and a resonant probe laser in an acetylene/air diffusion flame comp rises of contributions due to soot and C-2, the latter is generated by vapo rising the soot. Profile measurements across the flame have shown that the signal composition changes within the flame. Intermediate level labelling by Two-Colour Resonant Four-Wave Mixing Spectr oscopy (TC-RFWM) is applied to reduce the spectral complexity of molecular spectra. In addition, we observe extra resonances forbidden by a convention al three-level scheme in OH- and NH-containing flames, A detailed investiga tion shows that the newly observed TC-RFWM resonances are induced by collis ional energy transfer within the population, alignment and orientation grat ings formed in both electronic states that are coupled to the laser fields. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevie r SAS.