We explore a novel phase matching scheme for gas-phase rotational coherent anti-Stokes Raman spectroscopy (CARS). The scheme significantly simplifies the employment of the technique in general. Two laser beams, one broadband and one narrowband, are crossed at arbitrary angle and the generated rotational CARS signal, copropagating with the probe beam, is isolated using a polarization gating technique. The effect of phase-vector mismatch for various experimental implementations was measured experimentally and compared to calculations. The spatial resolution of the current technique is improved by more than an order of magnitude over standard gas-phase CARS experimental arrangements, providing an interaction length of less than 50 μm when desired. Both the pump and Stokes photons originate from the broadband pulse, and are therefore automatically overlapped temporally and spatially. Significantly improved signal levels are achieved because of both the ease of alignment and the higher pulse energy available to the pump and Stokes fields. We demonstrate the technique for single-laser-shot 1D rotational CARS signal generation over approximately a 1 cm field in a flame.
Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a powerful tool for chemical sensing, molecular dynamics measurements, and rovibrational spectroscopy since its development over 30 years ago, finding use in fields of study as diverse as combustion diagnostics, cell biology, plasma physics, and the standoff detection of explosives. The capability for acquiring resolved CARS spectra in multiple spatial dimensions within a single laser shot has been a long-standing goal for the study of dynamical processes, but has proven elusive because of both phase-matching and detection considerations. Here, by combining new phase matching and detection schemes with the high efficiency of femtosecond excitation of Raman coherences, we introduce a technique for single-shot two-dimensional (2D) spatial measurements of gas phase CARS spectra. We demonstrate a spectrometer enabling both 2D plane imaging and spectroscopy simultaneously, and present the instantaneous measurement of 15,000 spatially correlated rotational CARS spectra in N2 and air over a 2D field of 40 mm(2).
Purely rotational spectral signals of coherent anti-Stokes Raman scattering (CARS) from nitrogen molecules are studied as a function of the vibration-rotation interaction that weakens the rigid rotor approximation under which the dominant terms of the Raman cross section are calculated. The effect of the vibration-rotation interaction is quantified by means of the Herman-Wallis (HW) factor, and different approaches to its determination are evaluated in terms of their relative contribution to the CARS intensity and thermometric measurements made in a fuel-rich hydrocarbon flame. Known HW factors are contrasted with more complete expressions of recent derivation, and it is found that relative line strength adjustments amount to about a few percent. Such differences result in temperature corrections of less than 1%. This value should be considered for the definition of the ideal thermometric accuracy of the technique but it is of minor importance in comparison with other sources of uncertainty (e.g. Raman line widths) that emerge from the complexity typical of reactive gas mixtures.
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