Liquid crystals lasers

FBAs are used to brighten not only textile materials but also paper, leather and plastics. They are important constituents of household detergent formulations. More specialised areas of application include lasers, liquid crystals and biological stains. By far the most important uses for FBAs, however, are in applications to textiles and paper. Much of what follows will be concerned with these two categories. 

In some absorbing (at the 5145 A line of an Ar laser) liquid crystals [4-4 -bis(heptyloxy )azoxybertzene], a study ° has shown peculiar polarization dependences on the optical fields. These effects are undoubtedly related to the electronic stmctures of these liquid crystals and their changes following photoabsorption, but the exact mechanisms remain to be ascertained. 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

First, the recording layer which contains photochromic spirobenzothiopyran in liquid-crystal polymer or polymer such as vinyl chloride-vinylidene chloride copolymer, is made colored by UV irradiation. In the recording (writing) process, a colorless recording dot in the recording layer is formed by semiconductor laser beams (789 nm, 15-20 mW).100 This process is essentially thermal decoloration of the photomerocyanine form by laser beam. 

Holographic optical elements can also be made by the preparation of polymer-dispersed liquid crystals using twin lasers in transmission holographic photopolymerisation (see section 5.4.2). They have also been made using photorefractive composites of polymer dispersed liquid crystals (see section 5.6.3). 

Experiments were performed using a titanium sapphire laser oscillator capable of producing pulses with bandwidths up to 80 nm FWHM. The output of the oscillator was evaluated to make sure there were no changes in the spectrum across the beam and was compressed with a double prism pair arrangement. The pulse shaper uses prisms as the dispersive elements, two cylindrical concave mirrors, and a spatial light modulator (CRI Inc. SLM-256), composed of two 128-pixel liquid crystal masks in series. The SLM was placed at the Fourier plane [5]. After compression and pulse shaping, 200 pJ pulses were used to interrogate the samples. 

In order to develop the dyes for these fields, characteristics of known dyes have been re-examined, and some anlhraquinone dyes have heen found usable. One example of use is in thermal-transfer recording where the sublimation properties of disperse dyes are applied. Anthraquinune compounds have also been found to be useful dichroic dyes for gtiest-hosl liquid crystal displays when the substituents are properly selected to have high order parameters. These dichroic dyes can be used for polarizer films of LCD systems as well. Anlhraquinone derivatives that absorb in the near-infrared region have also been discovered, which may be applicable in semiconductor laser recording. 

The laser recorded information is preserved for a long time if the sample is cooled below Tg. From this viewpoint LC polymers differ usefully from low-molecular liquid crystals. The information storage time in the devices based on low-molecular liquid crystals do not usually exceed a few days. The described effects demonstrate the capabilities for the control of structural and optical properties of LC polymeric materials. 

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