SENSITIVE DETECTION OF NITRIC-OXIDE USING SEEDED PARAMETRIC 4-WAVE-MIXING

A sensitive near-resonant four-wave mixing technique based on two-phot on parametric four-wave mixing has been developed. Seeded parametric f our-wave mixing requires only a single laser as an additional phase ma tched ''seeder'' field is generated via parametric four-wave mixing of the pump beam in a high gain cell. The seeder field travels collinear ly with the pump beam providing efficient nondegenerate four-wave mixi ng in a second medium. This simple arrangement facilitates the detecti on of complex molecular spectra by simply scanning the pump laser. See ded parametric four-wave mixing is demonstrated in both a low pressure cell and an air/acetylene flame with detection of the two-photon C (2 ) Pi(upsilon'=0)<--X (2) Pi(upsilon ''=0) spectrum of nitric oxide. Fr om the cell data a detection limit of 10(12) molecules/cm(3) is establ ished. A theoretical model of seeded parametric four-wave mixing is de veloped from existing parametric four-wave mixing theory. The addition of the seeder field significantly modifies the parametric four-wave m ixing behaviour such that in the small signal regime, the signal inten sity can readily be made to scale as the cube of the laser pump power while the density dependence follows a more familiar square law depend ence, In general, we find excellent agreement between theory and exper iment. Limitations to the process result from an ac Stark shift of the two-photon resonance in the high pressure seeder cell caused by the g eneration of a strong seeder field, as well as a reduction in phase ma tching efficiency due to the presence of certain buffer species. Vario us optimizations are suggested which should overcome these limitations , providing even greater detection sensitivity. (C) 1998 American Inst itute of Physics, [S0021-9606(98)01014-9].