Spectral line shape of resonant four-wave mixing induced by broad-bandwidth lasers.

Abstract

We present a theoretical and experimental study of the line shape of resonant four-wave mixing induced by broad-bandwidth laser radiation that revises the theory of Meacher, Smith, Ewart, and Cooper (MSEC) [Phys. Rev. A 46, 2718 (1992)]. We adopt the same method as MSEC but correct for an invalid integral used to average over the distribution of atomic velocities. The revised theory predicts a Voigt line shape composed of a homogeneous, Lorentzian component, defined by the collisional rate \ensuremath{\Gamma}, and an inhomogeneous, Doppler component, which is a squared Gaussian. The width of the inhomogeneous component is reduced by a factor of \ensuremath{\surd}2 compared to the simple Doppler width predicted by MSEC. In the limit of dominant Doppler broadening, the width of the homogeneous component is predicted to be 4\ensuremath{\Gamma}, whereas in the limit of dominant homogeneous broadening, the predicted width is 2\ensuremath{\Gamma}. An experimental measurement is reported of the line shape of the four-wave-mixing signal using a broad-bandwidth, ``modeless,'' laser resonant with the ${\mathit{Q}}_{1}$(6) line of the A${\mathrm{}}^{2}$\ensuremath{\Sigma}-X${\mathrm{}}^{2}$\ensuremath{\Pi}(0,0) system of the hydroxyl radical. The measured widths of the Voigt components were found to be consistent with the predictions of the revised theory. \textcopyright{} 1996 The American Physical Society.