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.
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.
Nonlinear spectroscopic method based on degenerate four-wave mixing allows unusually sensitive measurement of isotopes and hyperfine profiles with sub-Doppler spectral resolution. Compared to other isotope capable methods including mass spectrometry, wave mixing offers a simple, more cost effective approach with minimal sample preparation steps. Since no two hyperfine profiles are alike, hyperfine-based isotope analysis offers unambiguous isotope information. Unlike mass-based methods such as mass spectrometry, frequency-based wave-mixing spectroscopy offers more information rich spectra. A non-planar 3-D wave-mixing optical setup generates a coherent signal beam in its own space and propagation direction. Hence, virtually 100% of the signal can be collected and detected conveniently. Using compact solid-state lasers and popular atomizers, a wave-mixing setup is relatively compact. Sub-Doppler spectral resolution can be further enhanced by using simple low-pressure atomizers. Ultrasensitive fingerprinting of important isotopes promises many potential applications.
Sub-Doppler laser wave-mixing spectroscopy is presented as a sensitive and high-resolution optical method for measuring isotopes and hyperfine structures. By fingerprinting isotopes at sensitive levels, one can track biohazardous pollutants in various environmental samples since isotope and hyperfine profiles are unique and characteristic of specific locations and sources including those for heavy metal contaminants and chemical runoff. We present an unusually sensitive optical absorption method that offers isotope specificity and resolution at excellent detection sensitivity levels. Sub-Doppler laser wave-mixing spectroscopy is a resonant nonlinear optical technique which uses three intersecting laser beams to produce a signal beam that has all the coherent properties of the original input laser beams. This laser-like signal beam can be efficiently directed, collected, filtered and detected. The use of counter-propagating input beams minimizes Doppler broadening and the resulting sub-Doppler spectral resolution is suitable for isotope and hyperfine splitting measurements. This relatively simple absorption-based method offers better sensitivity and selectivity levels and it requires minimum sample preparation steps.
Degenerate four-wave mixing (DFWM) from laser-populated excited states, i.e., two-step DFWM (2S-DFWM), has been performed to investigate the possibility of increasing signal quality (i.e., strength or signal-to-noise ratio) when species with low transition probabilities or far-UV transitions are to be detected or when large beam areas are used. Gold atoms, aspirated into an air-acetylene flame, were chosen as a suitable species for this investigation. The 2S-DFWM signal strength was found to be comparable to the ordinary (one-step) DFWM signals for moderately high UV-light intensities but substantially better for low UV-light intensities. This finding implies that DFWM detection of species with lower transition probabilities in the first step as compared with gold (<10(-8) s(-1)) can benefit from the 2S-DFWM technique when moderate or low UV-light intensities are available. Additional possible advantages of using 2S-DFWM are also discussed.
The use of picosecond lasers for degenerate four-wave mixing (DFWM) diagnostics of atmospheric pressure flames has been suggested as a means of overcoming the complicated collision-rate dependence of the DFWM signal in the nanosecond-laser pulse-length regime. In this paper DFWM spectroscopy in the regime in which the laser pulse length (τL) is less than the characteristic collisional time (τC) is investigated theoretically. DFWM signal levels for the phase-conjugate geometry (counterpropagating pump beams) are calculated by integration of the time-dependent density-matrix equations at numerous grid points along the phase-matching axis and by summing of the polarization contribution from each of these grid points. Both purely homogeneously broadened resonances and resonances that are both collision broadened and Doppler broadened are considered. DFWM signal generation for these two types of resonances in the fully transient regime (τL≪τC) is strikingly different. For a homogeneously broadened resonance, the DFWM signal persists long after the laser beam has passed through the medium. The decay rate of the DFWM signal is determined by the collision rate in the medium, and the integrated DFWM signal is still dependent on the collision rate even in this fully transient regime. However, when the Doppler broadening of the resonance is much greater than the collisional broadening, the DFWM signal decays with a fall time close to that of the laser pulse. For τL≪τC, we find that it is the laser spectral width, rather than the collisional width, that dictates the range of velocity groups with which the pulses effectively interact. The signal decays with a fall time close to that of the laser pulse because the excited velocity groups radiate out of phase with one another. In a medium in which the Doppler width is much greater than the collisional width, the DFWM reflectivity is independent of collision rate for τL≪τC, but the signal level is dependent on the Doppler width. Finally, we investigate the effect of different laser pulse lengths on reflectivity saturation curves for resonances with different levels of Doppler broadening.
In this article, we analyse a new sensitive spectroscopic technique in which a sample cell is irradiated by a two-frequency pump beam (ω + Δ ± δ/2) and a counter-propagating probe beam (ω). The phase-conjugate fields re-emitted at frequencies ω ± δ, via four-wave mixing processes, are detected through their heterodyne beating with the probe field. The properties of the heterodyne beat signal are studied by means of a perturbation calculation of the nonlinear response of the medium, carried out to the third order in the incident fields. In resonant gas media, the emission lineshape exhibits a Doppler-free doublet structure [frequency splitting 3 δ/2 for single-photon resonances, and δ/2 for two-photon transitions]. By using a phase-sensitive detection of the beat signal, one can separately analyse real (dispersion) or imaginary parts (absorption) of the nonlinear susceptibility. For single-photon transitions, the signal phase shift is related to the atomic levels lifetime and allows one to study the relaxation dynamics in gas phase (like quenching collisions, velocity diffusion, etc...). This is particularly interesting for cross-over resonances, in which only the common level contributes. In three-level systems, one also predicts a selective resonant enhancement of the phase-conjugate emission through Raman-type two-photon processes. Most of these predictions have been experimentally verified.
Phase conjugation of a cw hydrogen fluoride (HF) laser beam has been investigated experimentally and theoretically using resonant degenerate four-wave mixing (DFWM) in an HF gas absorption cell. A single-line single-mode HF laser, operating on P1(8), P1(9),or P1(10) transitions, provided total (forward plus backward) pump beam intensities of 130, 600, and 400 W/sq. cm., respectively, in the HF absorption cell. A probe beam intensity of the order of 10 W/sq. cm. was also provided. Conjugate reflection of the probe beam was investigated as a function of gas cell pressure and as a function of frequency detuning from line center. The variation of reflectivity with gas cell pressure at fixed laser beam intensity indicated a peak reflectivity of the order of .0001 at pressures from 2 to 4 Torr. Measurement of conjugate beam intensity, as a function of frequency detuning from the line center,provided a signal with a full width at half maximum (FWHM) of about 30 MHz. The latter is of the order of the homogeneous width, for HF at the test pressure, and may be compared to the Doppler width delta nu sub d = 300 MHz for the medium. An Analytical model is presented which provides theoretical estimatesmore » for the variation of reflectivity with pressure. Effects of diffusion, thermal conduction, pump depletion, and a nonuniform (Gaussian) profile are considered. Theoretical estimates for peak reflectivity agree with experiments to within a factor of about 2.« less