Realignment-enhanced coherent anti-Stokes Raman scattering and three-dimensional imaging in anisotropic fluids

Abstract

We apply coherent anti-Stokes Raman Scattering (CARS) microscopy to characterize director structures in liquid crystals. We demonstrate that the polarized CARS signal in these anisotropic fluids strongly depends on alignment of chemical bonds/molecules with respect to the collinear polarizations of Stokes and pump/probe excitation beams. This dependence allows for the visualization of the bond/molecular orientations via polarized detection of the CARS signal and thus for CARS polarization microscopy of liquid crystal director fields, as we demonstrate using structures in different phases. On the other hand, laser-induced director realignment at powers above a well-defined threshold provides the capability for all-optical CARS signal enhancement in liquid crystals. Since liquid crystalline alignment can be controlled by electric and magnetic fields, this demonstrates the feasibility of CARS signal modulation by applying external fields.

Optical Microscopy of Soft Matter Systems

“…These imaging techniques are based on nonlinear light-matter interactions that result in either emission, such as multiphoton excitation fluorescence/luminescence, or scattering, such as second harmonic generation (SHG) [30], sum frequency generation (SFG) [31], third harmonic generation (THG) [32], and coherent Raman scattering, namely, coherent anti-Stokes Raman scattering (CARS) [8][9][10]33] and stimulated Raman scattering (SRS) [34,35].…”

“…An example of multiphoton excitation fluorescence polarizing microscopy in Figure 10.12 shows a three-photon excitation fluorescence image of a colloidal particle in a smectic liquid crystal [10] (note that the liquid crystal molecules in this particular case serve as fluorophores themselves). The intensity of detected NLO polarizing microscopy signals depends on the angle between the polarization of excitation pulses and the liquid crystal director n(r) as ∼ cos 2m for the detection with no polarizer and as ∼ cos 2(m+1) for imaging with the polarizer in the detection channel collinear with the polarization of the excitation beam, where m is the order of the nonlinear process (e.g., m = 2 for two-photon excitation and m = 3 for three-photon excitation [8][9][10]). …”

“…10.14c) has a polarization control based on a twisted nematic achromatic polarization rotator for excitation and epi-detection of CARS signals and an optional rotating polarizer in the forward-detection channel. Since CARS is a three-photon process, the dependence of the CARS signal on the angle between the polarization of excitation light and the director of the sample is ∝ cos 6 with no polarizer in the detection channel and ∝ cos 8 with a polarizer collinear to excitation polarization in the detection [8][9][10]. Similar to other NLO polarizing microscopy techniques, CARS polarizing microscopy exhibits a stronger sensitivity to spatial variations of the director n(r) as compared with the single-photon FCPM imaging.…”

“…We note that this chapter gives an overview of what can be done in the studies of soft matter systems with different optical microscopy modes available. For more comprehensive and detailed description of microscopy techniques and sample studies in different imaging modes as well as the physics behind different studies of soft matter phenomena, readers can refer to the original works [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The introduction into imaging basics and a detailed description of optical imaging techniques can be found in recent textbooks and reviews [15][16][17][18][19][20][21][22].…”

Optically generated adaptive localized structures in confined chiral liquid crystals doped with fullerene

“…The decrease of the threshold laser power needed to induce the T3-1s with a 1064 nm beam is due to the enhancement of the optical reorientation of n͑r͒ enabled by fullerene doping, similar to that in dyedoped LCs. 12,13 However, these Torons do not reappear after turning off U, in contrast to the ones generated by blue light ͓Figs. 2͑d͒-2͑i͔͒ because, due to the low absorption, infrared beam does not deposit fullerene.…”