Low-power compact laser-based nonlinear degenerate four-wave mixing detection for flowing liquids

Abstract

Forward-scattering degenerate four-wave mixing is presented as a sensitive nonlinear laser-based absorbance detection method for room- temperature condensed-phase analytes using compact low-power lasers. In the liquid phase, the signal is generated mainly by the formation of spatial gratings due to thermally-induced refractive index change, resulting from constructive interference between the input beams. This nonlinear laser method offers convenient and efficient optical signal detection since the signal is a coherent beam and it can be collected and measured virtually against a dark background. Since only two input beams are used, the optical alignment is simple compared to other multiphoton methods. The use of a single lens for all the input beams provides tighter focusing and higher wave-mixing efficiency and maximizes photon density available at the sample cell. Hence, laser power requirements are unusually low (<10 mW), allowing the use of portable, low-cost lasers such as He-Ne lasers and diode lasers. Since only a single laser is required, the overall optical setup in this one- color one-laser method can fit in a simple compact package with minimum laser and optics requirements. The detection sensitivity approaches those of laser fluorescence methods, yet this compact nonlinear absorbance detector can detect both fluorescing and nonfluorescing analytes.