Four-wave mixing with non-resonant pump and resonant probe for OH detection in flames

A new type of two-color four-wave mixing spectroscopy is theoretically and experimentally demonstrated. By employing a non-resonant pumping and resonant probing scheme, OH radicals are detected in a laminar, atmospheric pressure methane/air flame as a proof-of-principle experiment. Two 532 nm laser beams from a multi-mode Nd:YAG laser are utilized to create an electrostrictive grating in the flame. A probe beam in resonance with the Q(1)( 5) rotational line belonging to the A(2)Sigma-X-2 Pi(1,0) band at 283 nm is employed to achieve species selective detection of OH radicals. In detail, in the present paper the coherence effects of the pump beams are investigated by changing the temporal delay between the two pump beams. A signal enhanced by a factor of 3 is obtained in the coherence range. Furthermore, the dependence of the signal intensity on the laser pulse energy of the pump and probe beams is studied in detail. At high pulse energies even for the pumping process a certain saturation behavior is observed. In general, the demonstrated four-wave mixing scheme is estimated to be superior in terms of the saturation limitation in comparison to the widely applied degenerate four-wave mixing, but still providing a high sensitivity and species selectivity due to the strong resonant signal enhancement.