Time-resolved coherent anti-Stokes Raman scattering microscopy: Imaging based on Raman free induction decay

Harvard University, Department of Chemistry and Chemical Biology, 12 Oxford Street, Cambridge, Massachusetts 02138
Applied Physics Letters (Impact Factor: 3.3). 04/2002; 80(9):1505 - 1507. DOI: 10.1063/1.1456262
Source: IEEE Xplore


A time-resolved coherent anti-Stokes Raman scattering (CARS) microscope allows three-dimensional imaging based on Raman free induction decay of molecular vibration with no requirement for labeling of the sample with natural or artificial fluorophores. A major benefit of the technique is the capability to completely remove nonresonant coherent background signal from the sample and the solvent, and thus increasing the detection sensitivity of CARS microscopy significantly. © 2002 American Institute of Physics.

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    ABSTRACT: We demonstrate a system for the phase-resolved epi-detection of coherent anti-Stokes Raman scattering (CARS) signals in highly scattering and/or thick samples. With this setup, we measure the complex vibrational responses of multiple components in a thick, highly-scattering pharmaceutical tablet in real time and verify that the epi- and forward-detected information are in very good agreement.
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    • "Yet the logic of the coherent accumulation, which leads to the high selectivity is identical, and the theoretical techniques used for both analytic treatment and numerical simulation of the molecular dynamics are equivalent. The proposed coherent Raman oscillator is inherently different from other common methods of coherent Raman spectroscopy [14] [15], such as Raman fluorescence spectroscopy, stimulated Raman spectroscopy (SRS) [16] [17] and coherent anti-Stokes Raman spectroscopy (CARS) [18] [19] [20] (where just recently important contributions were reported using two frequency combs [21, 22]). All those well-established methods measure molecular vibrational levels in the ground electronic potential, and therefore, tune the pump field away from all absorption bands to ensure a purely virtual Raman transition between ground levels. "
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    ABSTRACT: Free induction decay is the coherent emission of light that follows the excitation of a medium by a short pulse. During the coherence time of the medium ($T_2$), all atoms/molecules oscillate 'in unison', forming a macroscopic dipole that emits light as a large coherent antenna, 'broadcasting' information on the quantum state of the atoms/molecules and its dynamical evolution. We present an optical oscillator, where the coherent dipole emission from a dynamical wave-packet, is amplified beyond the lasing threshold. By placing a molecular medium in an optical cavity that is synchronously pumped by a frequency comb laser, emission from the excitation of one pump pulse can return to the medium with subsequent pump pulses, allowing stimulated amplification. When threshold is crossed, a broadband coherent oscillation is achieved, bearing information on the coherent wave-packet dynamics inside the medium. We analyze theoretically this coherent Raman oscillator and simulate thoroughly it's dynamics under most realistic conditions for a model system of Alkali dimers ($Li_2, K_2$) in a hot gas cell ($100-300^{\circ}C$), showing that the oscillation condition is well within reach. If realized, this coherent Raman oscillator can open avenues for precise measurement of vibrational dynamics in molecules.
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    • "This non-resonant background exists even in pure substances because multiple pathways exist to the CARS wavelength. Many approaches have been developed to reduce this background, such as polarisation control (Oudar et al., 1979; Yuratich and Hanna, 1977), time-delay (Volkmer et al., 2002; von Vacano and Motzkus, 2006), frequency modulation (Ganikhanov et al., 2006) and interferometric detection (Cheng, 2007; Evans et al., 2004; Müller and Zumbusch, 2007). The last has the advantage that the amplitude and phase are detected, where the amplitude is linear in the number (density) of the molecules. "
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