Optical nanotomography of anisotropic fluids

Abstract

The physical properties of anisotropic fluids can be manipulated on very short length scales of 100 nm or less by appropriate treatment of the confining substrate(s) 1,2 . This facilitates the use of ordered fluids in a variety of scientific endeavours and applications. Although future advances will require a complete understanding of their structure at the nanoscale level, high-resolution three-dimensional optical imaging of the fluid's molecular orientation profile is beyond the reach of extant techniques 3 . Here, we report a powerful imaging approach based on the collection of transmitted light in the far field that is emitted from a light source with a subwavelength aperture. We acquire high-resolution images by raster-scanning, at multiple heights, an optical fibre immersed inside a thin layer of anisotropic fluid, thereby facilitating the visualization of the fluid's structure with a resolvable volume ∼1/500 of that attainable by current methods. This novel technique offers the intriguing possibility of three-dimensional nanoscale reconstruction of a variety of soft materials, here the first direct visualization and measurement of the liquid-crystal molecular orientation relaxation length.The idea of using a subwavelength metal-coated fibre aperture to investigate a surface with high resolution was proposed initially by Synge 4 ; this has evolved into the technique of near-field optical microscopy 5 for two-dimensional (2D) imaging. Here, we present an entirely new imaging approach that involves the use of polarized light, emitted from a tapered optical fibre immersed in an anisotropic medium, in the far field to investigate molecular orientation in three dimensions at nanoscale levels. As there are no significant scattering sources due to dielectric inhomogeneities, the near-field light does not scatter in the customary manner, but instead decays exponentially with distance and is not detected downstream. Instead, the light that reaches the detector from the fibre aperture consists of small-wave-vector Fourier components, and is retarded by a phase δ as it propagates through the continuous birefringent fluid medium. By carrying out in-plane (xy) scans inside the sample at a series of heights z i above the surface, we obtain intensity matrix slices, from which we extract information about the fluid's local optical properties. The initial image (i = 1) is acquired by raster-scanning the fibre at height z 1 = h 1 (Fig.

Thermotropic nematic order upon nanocapillary filling

“…Phenomenological considerations [1] and computer simulations on liquid crystals in thin film and nanopore confinement [13][14][15][16][17][18] indicate, however, pronounced heterogeneities, in particular interface-induced molecular layering and orientational immobility in the pore wall proximity. Despite recent experimental advancements in optical techniques directly probing orientational order parameter profiles in the proximity of planar, solid walls [19][20][21], achieving the spatial resolution necessary to rigorously explore such inhomogeneities in nanometer-sized capillaries remains still experimentally extremely demanding.…”

Optical Imaging of Liquid Crystals at the Nanoscale

“…[71] We begin by assuming that the director in the nematic phase is rendered non-uniform either by a patterned surface treatment resulting in a non-uniform easy axis, or by an externally applied field. Let us examine how NSOM can be applied to create/analyze images in three dimensions.…”

Liquid crystal quenched orientational disorder at an AFM-scribed alignment surface

“…39 This approach facilitates imaging with resolution of 60 Â 60 Â 2 nm along the x, y, and z-axes for a ber aperture located a distance 2 < z < 500 nm above the surface. 39 This approach facilitates imaging with resolution of 60 Â 60 Â 2 nm along the x, y, and z-axes for a ber aperture located a distance 2 < z < 500 nm above the surface.…”

“…39 We then performed a Berreman 4 Â 4 optical calculation 40 for light propagation through a cell with a director prole 4(x, y, z) described by our calculation, dividing the liquid crystal into thin layers with both electric and magnetic eld boundary conditions applied at the interface of each slab, thus accounting for multiple reections. 39 We then performed a Berreman 4 Â 4 optical calculation 40 for light propagation through a cell with a director prole 4(x, y, z) described by our calculation, dividing the liquid crystal into thin layers with both electric and magnetic eld boundary conditions applied at the interface of each slab, thus accounting for multiple reections.…”