Spectral Narrowing Accompanies Enhanced Spatial Resolution in Saturated Coherent Anti-Stokes Raman Scattering (CARS): Comparisons of Experiment and Theory

Abstract

We demonstrate theoretically and confirm experimentally the mechanism by which spectral narrowing accompanies enhanced spatial resolution in a saturated coherent anti-Stokes Raman scattering (CARS) signal that is demodulated at the third harmonic (3f) of the pump modulation frequency (f). Under these modulation conditions, theory predicts a narrowing of the full width at half-maximum (FWHM) of the CARS spectrum by a factor of 2.0 with respect to that of the spectrum obtained by demodulation at the fundamental frequency. Theory also predicts an improvement of spatial resolution by a factor of 1.7. Experimentally, narrowing of the FWHM of the CARS spectrum of 1,4-bis((E)-2-methylstyryl) benzene (MSB) crystals by a factor of 2.5 is observed upon saturation. Further experimental confirmation is provided from investigating diamond particles, for which spectral narrowing was enhanced by a factor of 2.8 and spatial resolution was enhanced by a factor of 2. Details of the mechanism and execution of the saturated CARS experiment are elucidated and limits to its applicability are suggested, one of which is the conclusion that the saturation approach is not suitable for extraction of harmonics beyond 3f. In this work, we have developed a more comprehensive understanding of the correlation between the observed experimental results and experimental factors than has been previously reported. Disciplines Disciplines Physical Chemistry Comments Comments

Localization of Nonblinking Point Sources Using Higher-Order-Mode Detection and Optical Heterodyning: Developing a Strategy for Extending the Scope of Molecular, Super-resolution Imaging

“…Several attempts − have been made to improve the spatial resolution in vibrational imaging. These include STED-like approaches ,, and exploiting the nonlinear saturation of the population difference of vibrational states at high laser intensities. , We have observed that given the high laser intensities required for saturated CARS measurements and the propensity for sample damage, an improvement of a factor of “only” ∼2 could be achieved for the spatial and spectral resolution, with respect to the diffraction limit . We concluded that improvements in the spatial resolution of nonblinking probes must be accompanied not only by a novel experimental design but also by novel methods of data analysis.…”

“…25,26 We have observed that given the high laser intensities required for saturated CARS measurements and the propensity for sample damage, an improvement of a factor of "only" ∼2 could be achieved for the spatial and spectral resolution, with respect to the diffraction limit. 26 We concluded that improvements in the spatial resolution of nonblinking probes must be accompanied not only by a novel experimental design but also by novel methods of data analysis.…”

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“…SAX microscopy, pioneered by Fujita and coworkers, [5a,b] is a simple approach to sub‐diffraction imaging using saturation of the relevant transition for resolution improvement. SAX imaging has been successfully implemented in fluorescence as well as Raman‐based methods [5a,9] . A high intensity modulated excitation source is tightly focused on the sample.…”