Laser analytical spectrometry based on optical phase conjugation by degenerate four-wave mixing in a flowing liquid analyte cell

Abstract

Nonlinear laser spectroscopy based on optical phase conjugation by degenerate four-wave mixing in an absorbing liquid analyte solution is reported as a sensitive analytical technique using a relatively low-power continuous-wave argon ion laser as the excitation source. This novel laser method provides excellent detection sensitivity since the analytical signal is a wavefront-reversed replica of the probe beam. Optical signal detection is convenient and efficient since the signal is a visible coherent laser beam. Important characteristics of this nonlinear laser method include cubic dependence of signal on laser power and quadratic dependence of signal on concentration. Excellent sensitivity, small detection volume, and convenient sample introduction offer many potential applications in trace-level condensed-phase analysis of continuously flowing systems. A preliminary detection limit of 2.9 X 10(-18) mol of eosin B in a simple flow cell is reported.

Trace-concentration detection of cobalt in a liquid flow cell by degenerate four-wave mixing using low-power off-resonant laser excitation

“…Although the D4WM signal is inversely proportional to analyte fluorescence efficiency as described in eq 1, we still observed reasonably good detection limits (attomole) for highly fluorescing analytes as reported previously (19).…”

“…Since the signal is generated mostly by E¡Ep grating, angle 0 should be kept small and more laser intensity should be allocated for Et and £p beams instead of the Eb beam. As described in our previous reports (19,22), a D4WM setup with stronger E{ and Ep beams and a weaker Eb beam offers stronger signal and better S/N even when the total power of all input beams remains the same. Allocating more power to Ef and £p beams (i.e., grating writing beams) enhances the formation of the EfEp grating, which is the wide-period grating that contributes more in signal generation than the Ef/Ep grating.…”

“…It is important to use an appropriate response time for the detector electronics when modulating at higher frequency. For instance, the result of the signal dependence on modulation frequency reported for eosin B in ref 19 was collected using a slow detection response time (i.e., lock-in amplifier time constant, etc.). When detection response time is sufficiently fast, eosin B yields similar results as shown in Figure 2, with a wider range of usable modulation frequency.…”

“…As shown in Figure 1, the D4WM experimental setup is similar to that used in our previous report for eosin B (19). The 514-nm line of an argon-ion laser (Coherent, Palo Alto, CA, Model Innova 90-6) is used as the source for all three input beams.…”

“…We have also reported an effective signal-to-noise enhancement method employing polarization-modulated detection schemes in D4WM experiments (17,18). We also demonstrated phase conjugation by D4WM in the liquid phase for eosin B, a molecule with relatively high fluorescence quantum yield, using a low-power argon-ion laser (19). The nonlinear signal in the liquid phase can be observed by using a relatively low * Author to whom correspondence should be addressed.…”