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.
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.
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.
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.
Abstract Resonance scattering techniques are very useful for high-resolution atomic spectroscopy. The applicability of these techniques has been much extended, particularly through the rapid development of tunable-laser technology. The use of a narrow-band tunable laser, acting on a collimated atomic beam, gives a direct method enabling, for example, hyperfine structure and isotope shift studies. The intensity of lasers allows stepwise excitations to be performed, and with the two-photon absorption technique, Doppler-free measurements on thermal gases are also possible. By using pulsed lasers, time-resolved measurements yielding radiative life-times and structural information can be performed. The basic resonance scattering methods can be combined with radiofrequency and coherence techniques to yield a resolution, limited only by the uncertainty relation. Optical double resonance and level-crossing techniques, not requiring a narrow-band light source, have been extensively used. Several examples of the application of resonance scattering methods are given.
Up to now, Doppler-free nonlinear spectroscopy has been applied to atoms inside a gas medium (volume spectroscopy). We have extended these methods to atoms in the vicinity of a surface. The basic principle consists of monitoring reflectivity changes produced when a laser beam, incident on a glass–vapor interface, is brought into resonance with an atomic transition. For Doppler-free spectroscopy, two counterpropagating beams are used to operate velocity selection. If these two beams are under normal incidence in the glass—velocity selection perpendicular to the surface—one observes saturated dispersion (selective reflection sensitive to the real index). For incidence angles in the glass (prism) larger than the critical angle (total internal reflection), a standing evanescent wave is produced in the gas (velocity selection along the surface), and saturated absorption is detected. Experimental demonstration was performed on the Na resonance line; high sensitivity (10−4 relative reflectivity change) and spectral resolution as good as 20 MHz were obtained. The main limitation to better resolution comes from the short atomic transit time in the evanescent wave (typical extension ~λ). Relevant applications of these techniques should also concern the diagnostics of optically thick media, as analysis of interaction between gas-phase atoms and the surface, notably atom–wall collisions, and modification of the spontaneous emission characteristics.
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.
Sub-Doppler resonances in caesium vapours are studied in a laser field produced by counterpropagating two-frequency light beams with mutually orthogonal linear polarisations. The beams are in resonance with optical transitions in the D1 line, the frequency difference of the field spectral components being equal to the hyperfine ground-state splitting in the Cs atom (≈9.2 GHz). It has already been shown that in this configuration, the hypercontrast effect can be observed for sub-Doppler resonances, which makes this configuration promising for the employment in newgeneration miniature optical frequency standards. In the present work, two different two-frequency configurations are compared with each other and with the single-frequency configuration widely used in practice for observing saturated absorption resonances. The parameters of nonlinear resonances are measured at various temperatures of caesium vapours and at different optical field intensities. The results of the investigations performed make it possible to find an optimal two-frequency scheme for exciting nonlinear resonances and to estimate a potential of the scheme for its applications in quantum metrology.