Coherent anti-Stokes Raman scattering polarized microscopy of three-dimensional director structures in liquid crystals

Abstract

We demonstrate three-dimensional vibrational imaging of director structures in liquid crystals using coherent anti-Stokes Raman scattering (CARS) polarized microscopy. Spatial mapping of the structures is based on sensitivity of a polarized CARS signal to orientation of anisotropic molecules in liquid crystals. As an example, we study structures in a smectic material and demonstrate that single-scan CARS and two-photon fluorescence images of molecular orientation patterns are consistent with each other and with the structure model. Long-range orientational order is an important property of liquid crystals (LCs) [1]. Local average molecular orientations are described by the director n n− ≡ , which is an optical axis in the uniaxial LC materials. Non-invasive imaging of the three-dimensional (3-D) spatial patterns of ) ( r n r is important for fundamental LC research and for a broad range of technological applications. Fluorescence Confocal Polarizing Microscopy (FCPM) [2] visualizes 3-D director fields by taking advantage of (a) doping anisometric dyes that homogeneously distribute and align in the LC and (b) polarized excitation and fluorescence detection. In this approach, the absorption/emission transition dipoles of elongated dye molecules follow ) ( r n r and the polarized confocal imaging visualizes the 3-D director patterns [2]. However, this approach requires doping a specially selected dye that at small concentrations would provide strong contrast without affecting the LC properties. The labeling-free technique of interest is confocal Raman microscopy [3,4], which utilizes a Raman vibration and the signal dependence on orientations of molecular bonds. To study 3-D molecular orientation patterns, several techniques employ nonlinear processes, such as second harmonic generation (SHG) [5], third harmonic generation (THG) [6], and two-photon fluorescence (TPF) [7]. Coherent anti-Stokes Raman scattering (CARS) microscopy has been used for visualizing chemical composition in biological and lyotropic systems [8,9]. We report 3-D imaging of LC director structures using CARS polarized microscopy (CARSPM), which does not require doping the LC with dyes and offers chemical selectivity and bond-orientation specificity superior to that of other techniques [2][3][4][5][6][7]; it also provides 5 10 times faster imaging (stronger signal) than confocal Raman microscopy [3,4]. We demonstrate that CARSPM is a viable technique for mapping of 3-D patterns of molecular orientations and LC director dynamics.In CARS microscopy, a pump wave is used as a probe and the signal is derived by the 3-rd order nonlinear polarization

Optical nanotomography of anisotropic fluids

“…With improved signal-to-noise ratio and a more robust optical model, a fully 3D director profile reconstruction will be possible. Comparing ONT with other imaging techniques such as coherent anti-Stokes Raman scattering 20,21 and fluorescence confocal polarizing microscopy 22,23 , ONT's strength lies in its ultrahigh resolution within ∼500 nm of the surface, the non-necessity of doping the sample with a dye and the ability to study high-birefringence materials. Of course, ONT has two potential drawbacks: it requires a free upper surface and the fibre can be invasive, depending on the material being studied.…”

Direct mapping of local director field of nematic liquid crystals at the nanoscale

“…The crude 2-nitro terephthaloyl dichloride was obtained as an orange oil and used in the next step without further purification. 2-Nitro, 1,4-benzenedicarboxylic acid, 1,4-bis(4-carboxyphenyl) ester (2). A total of 3.45 g 4-hydroxybenzoic acid (25 mmol) was gradually dissolved into 2.8 g KOH (50 mmol) aqueous solution (100 mL), and the solution was cooled in an ice water bath.…”

“…The manipulation of these nanometer-size molecules into coherent, centimeter-scale structures is now routine. Although we have become adept at deducing textures indirectly through optical microscopies (1)(2)(3)(4)(5), probing the molecular-scale organization requires either a glassy material or rapid cooling of samples, providing metastable states that can be directly visualized through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) (6)(7)(8). Although these techniques are effective to study a variety of more complex LC phases, including smectic LCs (6,7,9), cholesteric and blue phases (10), and biological LC polymers (11,12), nonglassy, low molecular weight nematic LCs (NLCs) reorient during fast freezing.…”

Active Shape-Morphing Elastomeric Colloids in Short-Pitch Cholesteric Liquid Crystals

“…Linear polarization of the laser beam was controlled by rotating a λ/2-wave plate. LCE particles were also studied using broadband coherent anti-Stokes Raman scattering polarizing microscopy (CARS-PM) [30,31] via probing the polarized CARS-PM signals from the aromatic C=C stretching within 1400-1600 cm -1 .…”