We report results obtained with a scanning near-field optical microscope which was developed, recently. The scanning head of the device is mounted on an inverted polarizing microscope. The sample is illuminated through a microfabricated tip which is mounted on a single-mode optical wave-guiding fiber. The light transmitted through the sample is collected in the far field by a microscope lens. The distance between the tip and the sample is controlled by shear force detection, using a nonconventional setup. We applied this new equipment successfully in order to investigate birefringent and dichroic liquid crystal films, using polarization modulation or fluorescence detection, respectively. An optical resolution down to ≈200 nm was obtained on the sample of a cholesteric liquid crystal for a wavelength of 488 nm.
Photorefractive liquid crystal-polymer dispersion with different morphologyThe photorefractive effect in a photoconducting polymer containing small liquid crystal droplets is studied. Two beam coupling experiments indicate a high optical gain at moderate field strength ͑Ͼ50 cm −1 at 10 V/µm͒. The writing of photorefractive gratings is reversible. The dynamic behavior can be described by a two step process of the grating formation, the faster time constants being of the order of 100 ms. Modulation of the external voltage indicates that the reorientation of the director in the liquid crystals droplets is faster than the rise and decay times of the space charge field.
A photorefractive system composed of liquid crystal droplets dispersed in a photoconducting polymer is characterized by means of two-beam coupling. The amplitudes and phases of the spatial variations of the refractive index and the absorption coefficient are measured using the moving grating technique. Dynamic measurements indicate that the phase of the refractive index modulation is nearly constant while the amplitude increases gradually. The maximum value of the internal space charge field can be estimated from the amplitude of the phase grating and is found to be of the order of one-hundredth of the external bias field.
Beyond conventional electrooptic applications, liquid crystals become increasingly important in the topical field of organic electronics. Here, some fundamental findings are reviewed and two examples for the combination of liquid crystals and organic semiconductors are described in more detail. The first part of the paper describes the electroluminescent properties of very thin layers of aligned p-(phenylene vinylene) oligomers, which are embedded in a stack of thin organic semiconductor layers and sandwiched between two electrodes. Both the wavelength of the emitted luminescence (varying from green to red) and the dichroic ratio increase with increasing length of the aromatic backbone of the molecules. High brightness and low threshold voltages could be achieved. The photorefractive system described in the second part of the paper consists of small droplets of a low molar mass liquid crystal, which are dispersed in a photoconducting polymer. Two-beam coupling experiments indicate a high performance at reasonable external voltages. From dynamic diffraction measurements, the amplitude of the internal space charge field can be estimated.
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