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| Nunes, J.A.; Tong, W.G. Optical fiber-based wave mixing as a convenient and sensitive laser analytical tool for condensed-phase analytes. Applied Spectroscopy, vol.52, (no.5), Soc. Appl. Spectrosc, May 1998. p.763-9. 30 references. Language: English. Pub type: Experimental Abstract: A fiber-optic degenerate four-wave mixing (D4WM) probe for the measurement of small absorptions in liquid-phase samples is described. Laser D4WM is a nonlinear laser spectroscopic technique that has proven to be highly sensitive for the detection of trace analytes in condensed-phase media. A significant improvement in the forward-scattering optical arrangement of D4WM is demonstrated by using optical fibers for both laser light input and output. There is considerable flexibility inherent in the design since the system may be used in three configurations: (1) the simplest case of transmitting the signal radiation by optical fiber to the detection electronics, (2) the case of guiding: the excitation beams to the analyte by polarization-maintaining optical fibers, and (3) the combination of both. The optical fiber-based D4WM system is shown to be an effective and sensitive laser analytical spectroscopic method for trace analysis, offering advantages such as detection in very small probe volumes, remote and in situ analysis, and convenient and efficient optical alignment enhancements obtained by the use of optical fibers. |
| Neyer, David W.; Rahn, Larry
A.; Chandler, David W.; Nunes, Jon A.; Tong, William G. Circular dichroism spectroscopy using coherent laser-induced thermal gratings. Journal of the American Chemical Society v119, n35 (Sept 3, 1997):8293 (8 pages). Abstract: The detection and real-time measurement of circular dichroism (CD) in liquid samples was demonstrated using a new pulsed four-wave mixing technique. The technique uses interference from two laser beams with polarizations controlled and modulated using a photoelastic modulator. An internal heterodyne process was used and interference from coherent thermal gratings was found to enhance a weak circular dichroism signal. |
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Nunes, JA; Tong, WG; Chandler,
DW; Rahn, LA.
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| Wu, Z; Tong, WG. Absorbance detection of amino acids by laser wave mixing in microbore liquid chromatography. Journal of Chromatography. a, 1998 May 1, 805(1-2):63-9. (UI: 98282637) Language: English Abstract: Nonlinear optical phase conjugation by degenerate four-wave mixing is demonstrated as a sensitive "absorbance" detection method for microbore high-performance liquid chromatography. An argon ion laser operating at the 488-nm line is used as the excitation light source to generate the wave-mixing signal for dabsyl-labeled amino acids. Advantages of the nonlinear laser detection method include: virtually 100% optical signal collection efficiency, generation of the signal in the form of a coherent laser beam, signal measurement against a virtually dark background, reliable detection of small absorbance values, excellent detection sensitivity for both fluorescing and non-fluorescing analytes, relatively simple one-color one-laser optical setup, and low power or energy requirements for continuous-wave or pulsed lasers. Using our one-laser one-color nonlinear laser detector for "absorbance" measurements in liquid chromatography, we report a crude preliminary "injected" detection limit of 780 fmol for glycine. |
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Bao, Xinxian; Li, Chunfei;
Tian, Yanqing; Tong, William G. |
| Bao, Xinxian; Li, Chunfei;
Tong, William G. Third-order nonlinearity and polarization holographic storage in acceptor-donor azo dye-doped PMMA films SPIE-Int. Soc. Opt. Eng, Vol. 3473, p. 143-150, Third-Order Nonlinear Optical Materials, Mark G. Kuzyk; Ed. Publication Date: 10/1998 Abstract: Third-order nonlinearity and polarization holographic storage in acceptor-donor azo dye-doped PMMA films are studied by using nonresonant degenerate four-wave mixing with a He-Ne laser. The samples exhibit reasonably large third-order susceptibility and strong polarization sensitivity, and the highest third-order nonlinear susceptibility is up to the order of 10(superscript -3) esu. The relationship between optical nonlinearity and 'push' and 'pull' electron ability in polar azo molecules is studied, and further effect from a push- electron group attached to azobenzene ring on third-order nonlinearity of the samples is explored. Third-order nonlinearity of the sample is found to be proportional to the push-pull electron ability of polar azo molecules. Attaching a side group with push-electron ability to the azobenzene ring increases third-order nonlinearity (dipole moment) of the material. Polarization holographic storage properties in azo dye-doped PMMA films are also studied. The samples show strong polarization sensitivity and real-time information storage properties. The polarization hologram in the sample results from a birefringent grating formed by photoinduced reorientation of polar azo molecules. The optical phase conjugation signal consists of both fast and slow parts when two recording beams are turned off. They respond to the processes of four-wave mixing and holographic grating. It is demonstrated that nonresonant nonlinearity of samples mainly results from photoinduced anisotropy of polar azo chromophores. The advantages of nonresonant nonlinearity and its applications are discussed. |
| Mickadeit, F.; Kemp, H.; Schafer,
J.; Tong, W.G. Sensitive sub-Doppler nonlinear spectroscopy for hyperfine structure analysis using simple atomizers. SPIE-Int. Soc. Opt. Eng, 1998, 3270, p.168-73. 25 references. Pub type: Experimental Abstract: Laser wave-mixing spectroscopy is presented as a sub-Doppler method that offers not only high spectral resolution, but also excellent detection sensitivity. It offers spectral resolution suitable for hyperfine structure analysis and isotope ratio measurements. In a non-planar backward-scattering four-wave mixing optical configuration, two of the three input beams counter propagate and the Doppler broadening is minimized, and hence, spectral resolution is enhanced. Since the signal is a coherent beam, optical collection is efficient and signal detection is convenient. This simple multi-photon nonlinear laser method offers unusually sensitive detection limits that are suitable for trace-concentration isotope analysis using a few different types of simple analytical atomizers. Reliable measurement of hyperfine structures allows effective determination of isotope ratios for chemical analysis. |
| Berniolles, S.; Hongjing Kan;
Lu Dai; Knittle, J.; Tong, W.G. Sensitive on-column absorbance detection of native molecules. SPIE-Int. Soc. Opt. Eng, 1998. 3270, p.200-6. 27 references. Pub type: Experimental Abstract: Laser wave mixing is presented as a sensitive detection method for absorbance measurements in flowing liquid analytes. Wave mixing is an unusually sensitive multi-photon nonlinear optical method since the analytical signal is generated as a coherent laser beam. Since the bright signal is visible to the naked eye, optical alignment is convenient. For liquid analytes in continuously flowing cells, we have demonstrated excellent detection sensitivity levels using various wave-mixing optical configurations and laser sources. Since it is an optical absorption method, laser wave-mixing detection offers excellent detection sensitivity for both fluorescing and non-fluorescing analytes. Hence, one does not have to label non-fluorescing analytes with tags in order to obtain good detection sensitivity in wave-mixing detection methods. Sensitivity detection of analytes in their native form offers many obvious advantages especially when interfaced to popular capillary separation methods. Since the analyte laser probe volume is very small, wave-mixing detection is suitable for on-column detection in various capillary electrophoresis or micro liquid chromatography systems. |
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Wu, Z; Tong, W G. |
| Xinxian Bao; Chunfei Li; Hongjing
Kan; Lu Dai; Tong, W.G. Excited-state optical storage study in a dye-doped film using four-wave mixing spectroscopy. SPIE-Int. Soc. Opt. Eng, Vol.2998, 1997. p.343-7. 8 references. Pub type: Experimental Abstract: Four-wave mixing spectroscopy is presented as a convenient and effective optical method for the study of excited-state optical storage properties and nonlinear mechanisms in a dye-doped polymer film. The dynamic processes of optical storage properties and the efficiency of the diffraction grating are discussed. Backward-scattering four-wave mixing and forward-scattering four-wave mixing optical configurations are presented. A simple energy-level model is used to explain the excited-state optical storage mechanism, resulting from a photoinduced excited-state population grating and a trans-cis isomer grating. |
| Berniolles, S.; Nunes, J.A.;
Tong, W.G. Low-power compact laser-based nonlinear degenerate four-wave mixing detection for flowing liquids. SPIE-Int. Soc. Opt. Eng, Vol.2546, 1995. p.145-51. 15 references. Pub type: Practical; Experimental 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 multi-photon 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 sir the sample cell. Hence, laser power requirement 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 non-fluorescing analytes. |
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Berniolles, S; Wu, Z; Tong,
W G.
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WU ZQ; LIU JY; TONG WG.
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| Nunes, Jon A.; Tong, William
G. Sensitive circular dichroism spectroscopy based on nonlinear degenerate four-wave mixing. Analytical Chemistry v65, n21 (Nov 1, 1993):2990 (5 pages). Abstract: Degenerate four-wave mixing is demonstrated as an effective and sensitive laser analytical spectroscopic method for circular dichroism measurements. A forward-scattering degenerate four-wave mixing optical setup is used to obtain simple optical alignment, highly efficient wave mixing, and very effective use of low laser power. This nonlinear laser-based circular dichroism method offers many advantages, including easy and efficient optical signal collection, use of very short analyte path lengths (e.g., 0.1 mm), and excellent detection sensitivity that is comparable or better than conventional laser-based or non-laser-based circular dichroism methods. Using an analyte path length of only 0.1 mm, and a probe volume of 98 pL, a circular dichroism mass detection limit of 0.68 pg or 2.8 fmol is reported for (+)Co(en)33+. COPYRIGHT American Chemical Society 1993 |
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Wu, Z; Tong, WG. |
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WU ZQ; TONG WG.
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| Wu, ZQ; Tong, WG. Trace-concentration detection of cobalt in a liquid flow cell by degenerate four-wave mixing using low-power off-resonant laser excitation. Analytical Chemistry, 1991 Sep 15, 63(18):1943-7. (UI: 92088136) Language: English; Pub type: JOURNAL ARTICLE Abstract: Optical phase conjugation by degenerate four-wave mixing (D4WM) in an absorbing metal-ion solution using a low-power argon-ion laser as the excitation source is demonstrated. This nonlinear laser technique can be used as a sensitive analytical spectroscopic method for trace-concentration measurement of metal ions in a small-volume continuously flowing analyte cell. Several important characteristics are discussed, including the effects of solvent properties, excitation wave-length, laser intensity, and analyte absorptivity on signal intensity. Detection of 0.26 ng (4.4 pmol) of cobalt inside the laser probe volume of 0.14 microL is reported using an excitation wavelength that is 136 nm away from the maximum absorption wavelength of the analyte solution. The minimum absorbance measured in our D4WM experiment is 2.0 X 10(-5) without complex formation for cobalt. The D4WM detection sensitivity, in terms of the concentration-absorptivity product, is 4.05 X 10(-4) cm-1 for cobalt(II) in ethanol. Our preliminary detection sensitivity compares favorably with other laser-based spectrometric methods. This nonlinear laser technique is applicable to both fluorescing and nonfluorescing analytes. |
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ANDREWS JM; WEED KM; TONG
WG.
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Wu, ZQ; Tong, WG. |
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LUENA GA; TONG WG.
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| Wu, ZQ; Tong, WG. Laser analytical spectrometry based on optical phase conjugation by degenerate four-wave mixing in a flowing liquid analyte cell. Analytical Chemistry, 1989 May 1, 61(9):998-1001. (UI: 89271064) Language: English; Pub type: JOURNAL ARTICLE 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. |
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