Sensitive Detection Methods Based on Nonlinear Optical Phase Conjugation by Degenerate Four-Wave Mixing in the Liquid Phase.

William M. Tong

Degenerate four-wave mixing (D4WM) is presented as a simple nonlinear laser method with spectral resolution high enough to resolve isotope/hyperfine lines and detection sensitivity sufficient for sub-ppb or sub-attomole detection in various atomizers and detection cells. Two counterpropagating pump beams and a probe beam are mixed inside the nonlinear medium that consists of analyte atomic or molecular species, and the phase conjugate signal beam is generated as the wavefront-reversed replica of the probe beam. Optical alignment is relatively simple, and optical signal collection is highly efficient since the signal is a laser beam. Continuous sample introduction is possible using an air-acetylene flame as the atomizer. When a relatively low-pressure demountable discharge cell is used as the atomizer, Lorentzian (pressure) broadening is also minimized in addition to cancellation of Doppler broadening. Degenerate four-wave mixing is also effective in detecting trace amounts of analytes in the condensed phase at room temperature using continuously flowing capillary cells that are suitable for interfacing to liquid chromatography and capillary electrophoresis. In addition to pulsed lasers, such as excimer- and Nd:YAG-pumped dye lasers, one can also use relatively low-power continuous-wave lasers such as argon-ion lasers or single-frequency ring dye lasers.

David M. Pepper

M. Ducloy

Fan Rong-Wei, Deying Chen, Fang Tao +3 more

In this paper, frequency-doubled Nd:YAG laser's phase conjugation has been investigated experimentally using resonant degenerate four-wave mixing (DFWM). The iodine solution was used as resonant medium. In this way, the energy of the DFWM phase conjugation beam arrived at 43μJ/pulse while the total pump beam's energy reached 8mJ/pulse. The relation between the pulse energy of the DFWM phase conjugation beam and that of the pump beam was investigated. We also measured the optical field distributions of pump beam and DFWM phase conjugation beam, from which it can be noticed that the DFWM phase conjugation can improve the laser beam quality. At the same time, we observed the influence of the pump beam's disturbance on the DFWM signal.

Concetto R. Giuliano

Bao Xue Chen, Shunsuke Kobayashi

A novel analytical consideration is presented for dealing with phase conjugation by degenerate four-TE wave mixing in a planar waveguide bounded by a nonlinear cladding, in which both phase-conjugate power coupling and intensity-dependent field distributions are discussed. New wave solutions for signal and phase-conjugation reflections are obtained with anomalous power-dependent field distributions, and are used to specific numerical example based on a nematic liquid crystal.