Direct imaging of molecular symmetry by coherent anti-stokes Raman scattering

Abstract

Nonlinear optical methods, such as coherent anti-Stokes Raman scattering and stimulated Raman scattering, are able to perform label-free imaging, with chemical bonds specificity. Here we demonstrate that the use of circularly polarized light allows to retrieve not only the chemical nature but also the symmetry of the probed sample, in a single measurement. Our symmetry-resolved scheme offers simple access to the local organization of vibrational bonds and as a result provides enhanced image contrast for anisotropic samples, as well as an improved chemical selectivity. We quantify the local organization of vibrational bonds on crystalline and biological samples, thus providing information not accessible by spontaneous Raman and stimulated Raman scattering techniques. This work stands for a symmetry-resolved contrast in vibrational microscopy, with potential application in biological diagnostic.

High-speed polarization-resolved coherent Raman scattering imaging

“…2(a)], revealing the outside ring of the MLV spherical shape where molecular bonds are oriented over an isotropic background, where no polarization dependence is expected. This structural contrast capability is very similar to that found by symmetry filtering using circular polarizations, as presented recently [27]. Here, however, the averaged orientation φ 2 can be additionally obtained from the phase of the SRS modulated signal [ Fig.…”

Compact and versatile laser system for polarization-sensitive stimulated Raman spectroscopy

“…Coherent Raman techniques can be divided into two categories, coherent anti-Stokes Raman scattering (CARS) [6][7][8][9][10] and stimulated Raman scattering (SRS) [11][12][13][14][15][16], where Raman signals are detected from the inelastic scattering of light to higher energies (CARS) or to lower energies (SRS). CARS signals contain a nonreso- * Corresponding author: hk@dfm.dk nant background arising from nonresonant contributions to the nonlinear optical response, which alters the Raman spectra.…”

“…Many of the laser systems for CARS and SRS are expensive and bulky laser systems and often based on femtosecond pulses [8,17]. Pulses from the lasers require temporal synchronization [10,11,14], thus making the CARS and SRS techniques both complex and cumbersome to operate.…”

“…Polarization-sensitive Raman spectroscopy has been used for assigning and characterizing vibrational modes via the measurement of their depolarization ratio [19] or optical activity [6,[20][21][22][23]. Initially performed by spontaneous Raman scattering, polarization-sensitive measurements have recently been carried out with coherent Raman scattering, leading to a significant improvement in speed and sensitivity [6,10,18,19,24]. Information about the molecular structure is very valuable in, for example, synthetic chemistry or polymorph analysis.…”

Excitation parameters optimized for coherent anti-Stokes Raman scattering imaging of myelinated tissue

“…CARS can, therefore, be used to probe the nanostructural orientation of vibrational bonds within ordered samples. [1][2][3] CARS requires two laser sources, termed "pump" (ω p ) and "Stokes" (ω s ), which are tuned such that the energy difference between the frequencies of the two beams matches the potential energy in the vibrational resonance of a molecule. A photon from a third beam, the "probe" (ω pr ), then interrogates the excited vibrational mode, generating a blueshifted photon, the "anti-Stokes" (ω as ).…”