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Communication Dans Un Congrès Année : 2010

Background-free nonlinear Raman microscopy and spectroscopy at interfaces

David Gachet
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Sophie Brustlein
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Hervé Rigneault
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Résumé

Gaussian focused beams exhibit specific properties that drive the emitted fields in nonlinear optical microscopy. We discuss here how the basic field and object symmetry properties can be favorably used to obtain background-free images and spectra in third-order nonlinear optical processes. We concentrate here on coherent anti-Stokes Raman scattering (CARS), the non-linear analogue of Raman scattering. In CARS microscopy, the vibrational related (“resonant”) signal is often overwhelmed by an electronic related (“nonresonant”) background. Various strategies have been implemented to circumvent the undesirable effects of this nonresonant background: picosecond pulse excitation, time resolved, polarization sensitive and Epi detections, signal processing, interferometry, spectral or spatial phase control. Among them, the efficient background suppression methods always require complex technical implementation (interferometry, phase mask...). We revisit two-wave interference experiments and suggest new simple methods to obtain background-free images and spectra in CARS microscopy in a collinear configuration when the resonant medium is forming an arbitrary interface with a nonresonant medium. In the case of an interface that is perpendicular to the optical axis (transverse interface), this is done by the subtraction of two CARS signals generated at the interface where the roles of the resonant and nonresonant medium are inverted. The case of an interface that is parallel to the optical axis (axial interface) is reminiscent of the Young's double slit experiment. We show that the CARS signal is here angularly shifted and that a background-free signal is obtained after the subtraction of two CARS signals emitted in symmetrical directions. Those simple operations cancel the nonresonant background detected in the forward direction and use the nonresonant medium as a local oscillator with which the resonant signal is measured in heterodyne detection. They highlight “chemical” interfaces between two chemically different media, in the vein of methods demonstrated by Krishnamachari and Potma. Such schemes are useful when the sample's nonresonant background is stronger than the resonant signal as usually found in CARS microscopy and spectroscopy. Experimental demonstrations have been conducted on test samples and living cells. The schemes we demonstrate here are simple to implement and are fully compatible with commercial CARS microscopes. In addition, they are not limited to nonlinear processes and apply to any process exhibiting both coherent and resonant features.
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Dates et versions

hal-00490359 , version 1 (08-06-2010)

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  • HAL Id : hal-00490359 , version 1

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David Gachet, Sophie Brustlein, Franck Billard, Hervé Rigneault. Background-free nonlinear Raman microscopy and spectroscopy at interfaces. AMARIS'10 - Advances in Molecular Nonlinear Optics, May 2010, Cachan, France. ⟨hal-00490359⟩
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