William M. Tong

Sensitive sub-Doppler multiwave-mixing spectroscopy for flame and graphite furnace atomizers

Resonant degenerate four-wave mixing is presented as an unusually sensitive nonlinear laser method that yields Doppler-free spectral resolution at trace-concentration levels even when using low laser power levels. Using a non-planar four-wave mixing optical setup, one can extract the signal beam from the input beams more easily, and hence, suppress the background noise more effectively and improve signal-to- noise ratios. Optical alignment is simple and convenient for this multiphoton setup. Sub-Doppler spectral resolution allows reliable measurement of many isotope and hyperfine lines using room-pressure flame atomizers, low-pressure discharge atomizers or room-pressure graphite furnace atomizers. While maintaining sub-Doppler spectral resolution, four-wave mixing still yields parts-per-trillion level detection sensitivity using these popular analytical atomizers. While flame atomizers offer convenient and fast sample introduction, low- pressure discharge atomizers offer better spectral resolution (i.e., sub-Doppler plus sub-Lorentzian), and graphite furnace atomizers yield lower atomizer background noise. Since laser power requirements are low (e.g., mW for CW lasers and nJ for pulsed lasers), many compact lasers (e.g., solid-state lasers) could be used in this simple yet sensitive nonlinear laser method.

Optical nonlinearity and multiplex holographic storage in azo side-chain liquid crystalline polymer

Optical nonlinearity and multiplex holographic storage using azo side-chain polymer are studied by using degenerate four- wave mixing and polarized pump-probe laser method. The He-Ne excitation laser wavelength is located at the tail end of the absorption peak of the sample. This material shows high grating diffraction efficiency and promises information storage capability using weak absorption. The diffraction efficiency of the grating is up to the order of 10-2. The effect of input beam polarization planes on grating is studied. The diffraction grating efficiency is lowest when the polarization of the redout beam is orthogonal to that of both writing beams. An angle multiplex holographic storage study is also performed in the sample. Information can be stored for a long time at room temperature. The information readout of this multiplex holographic storage system can be controlled by the polarization state of the readout beam. Dependence of grating diffraction efficiency on the sample temperature is studied. It is demonstrated that the higher the temperature under the melting temperature of the sample, the higher the diffraction efficiency and the faster the relaxation time of the grating. The grating is completely erased by increasing the temperature over the melting temperature of the sample. Reading the grating with a circularly polarized light does not erase the grating. When two recording beams are turned off, a large part of the grating remains for a long time and only a small part of the grating degrades. The results show that the azo side-chain polymer is a good information storage material. The anisotropic optical nonlinear refractive index of the sample is also measured. The relationship of the nonlinear refractive index of the samples, the different ratios of azo functional groups in the polymers, and the temperature of the sample are also studied. The highest nonlinear refractive index is up to the order of 10-2. The optical nonlinearity of the samples increase with increasing sample temperature. No anisotropic nonlinearity appears when the temperature of the sample is over the melting temperature of the sample. The anisotropic nonlinearity levels of three samples with different azo side-group ratios in the polymer are compared.

Sensitive sub-Doppler nonlinear spectroscopy for hyperfine-structure analysis using simple atomizers

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 un usually 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.

Sensitive on-column absorbance detection of native molecules

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.

Excited-state optical storage study in a dye-doped film using four-wave mixing spectroscopy

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 photo-induced excited-state population grating and trans-cis isomer grating.

Sensitive capillary-based on-column detection method by laser wave mixing

Laser wave-mixing spectroscopy is presented as a simple, sensitive, on-column detection method for capillary electrophoresis. The use of a single focusing lens to focus and mix two input beams significantly simplifies the optical alignment requirement of this nonlinear laser method. High signal collection efficiency allows excellent detection sensitivity for both fluorescing and non-fluorescing analytes, since the signal is a sharp coherent beam. This laser detection method can be conveniently interfaced to capillary-based separation systems since it offers small detector probe volumes, efficient use of short excitation or absorption path lengths, efficient use of low laser power levels available from small, compact, inexpensive laser sources, and inherently narrower peak widths (i.e., squared Gaussian). Potential applications include sensitive detection of biomolecules, either labeled or in their native forms, using appropriate excitation wavelengths. Furthermore, laser wave-mixing detection can be used also in the indirect detection mode where the solvent or the buffer system yields a baseline signal, and non-absorbing analytes can be measured indirectly in the form of negative peaks.

Optical fiber-based wave-mixing probe

A fiber-based wave-mixing probe is presented as a simple spectroscopic measurement tool with minimum optical alignment requirements for trace-concentration analytes. For a multi-photon nonlinear laser-induced grating method to become widely accessible, the optical setup must be compact, easy to align and portable. We demonstrate a significant improvement in the forward-scattering (self-diffraction) wave-mixing optical setup using optical fibers for both laser input and signal output interfaces. There is considerable flexibility inherent in the design, since the wave-mixing probe can be used in multiple configurations. Wave-mixing spectroscopy is presented as an effective and sensitive analytical method for trace analysis, offering advantages such as detection in very small sample volumes, remote and in-situ analyses, and convenient and efficient alignment enhancements obtained by the use of optical fibers.

Sensitive absorbance measurement for gas-phase analytes based on multiwave mixing spectroscopy

Multi-wave mixing spectroscopy is presented as a simple and sensitive laser method for elemental analysis at trace- concentration levels with sub-Doppler spectral resolution. Since the signal is a coherent beam, virtually 100% of the generated signal can be directed into a photodetector. The nonlinear signal has a cubic dependence on excitation intensity, and hence, laser power requirements are low, and mW-level continuous-wave lasers and nJ-level pulsed dye lasers can be used for this multi-photon spectroscopic setup. The optical alignment and beam quality requirements are relatively less demanding compared to other multi-photon methods, and hence, compact inexpensive lasers, including laser diodes, can be used. Since the coherence time and coherence length requirements are also relatively low, many types of laser sources can be used quite conveniently in this nonlinear spectroscopic setup. Parts-per-trillion level detection sensitivity can be obtained for a wide range of fluorescing and non-fluorescing analytes and matrices using various analytical atomizers. The graphite furnace atomizer offers many advantages, including convenient sample introduction for solid, liquid or gas analytes, high atomization temperature, clean atomization environment, and minimum source and chemical interferences, resulting in lower atomizer background noise. Taking advantage of the unique features of this multi-wave mixing optical method and those of a graphite furnace atomizer, one can obtain both excellent spectral resolution and detection sensitivity.

Sensitive wave-mixing detectors for capillary electrophoresis and liquid chromatography

A sensitive absorbance measurement method based on multi- wave mixing optical method is demonstrated as an effective detector for capillary electrophoresis or liquid chromatography. The use of a single focusing lens to focus and mix the two input beams both simplifies the optical setup and utilizes the laser power very efficiently inside a small probe volume. Hence, inexpensive low-power (mW level) lasers can be used in this nonlinear multi-photon detection method, including low-power argon ion lasers, He-Ne lasers and laser diodes. The resulting small (pL range) probe volume (laser beam overlap zone) allows convenient interfacing of this detector to capillary-based electrophoresis or chromatography separation systems. Excellent signal collection efficiency (virtually 100%) for the collimated coherent signal beam, allows detection sensitivity levels similar to those of fluorescence-based methods, and yet wave-mixing detectors are applicable for both fluorescing and non-fluorescing flowing analytes. The coherent signal beam is generated by thermally-induced refractive-index spatial gratings formed by the two input beams, and hence, the signal strength also depends on some solvent properties. The wave-mixing detector is demonstrated to be effective for trace analysis, offering advantages such as detection in very small sample volumes, remote and in situ analysis, and convenient as well as efficient alignment enhancements obtained by the introduction of optical fibers into the detector optical setup.

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

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.