Stable isotope ratio analysis at trace concentrations using degenerate four-wave mixing with a circularly polarized pulsed probe beam.

Stable isotope analysis based on vectorial optical-phase conjugation by resonant degenerate four-wave mixing (D4WM) is reported by using a D4WM method with vertically polarized pump beams and a circularly polarized probe beam. Since the polarization of the signal beam is different from that of the pump beams, the background radiation is suppressed more effectively. Excellent sensitivity, high spectral resolution, and efficient optical detection make this an effective and unusually convenient nonlinear spectrometric method for the analysis of trace amounts of stable isotopes. Using an excimer-pumped pulsed dye laser, the fine structures of lithium are examined. A detection limit of 2.5 ng/mL lithium is observed while a Doppler-free resolution is maintained by using transient "coherent-grating" based D4WM spectroscopy.