Laser recording

In order to develop the dyes for these fields, characteristics of known dyes have been re-examined, and some anthraquinone dyes have been found usable. One example of use is in thermal-transfer recording where the sublimation properties of disperse dyes are appHed. Anthraquinone compounds have also been found to be usehil dichroic dyes for guest-host Hquid 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. Anthraquinone derivatives that absorb in the near-infrared region have also been discovered, which may be appHcable in semiconductor laser recording. 

Nowadays, polymeric photoconductors may be used in electrophotography, microfilms, photothermoplastic recording, spatial light modulators, and nonlinear elements. The combination of photosensitivity with high quality electrical and mechanical properties permits the use of such materials in optoelectronics, holography, laser recording and information processes. The applications of the various types of polymers were reported in the final parts of the relevant items in the earlier sections. Here, we will briefly analyze the common features of photoconductive polymer applications. The separate questions of each type have been dealt with in some books and papers [3, 11, 14, 329]. 

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. 

Optical information storage, which has been a dream since the discovery of the laser, is now becoming a commercial reality. Read-only consumer products (video and digital audio disks) have provided a solid technological base for the development and introduction of the more sophisticated write-once and erasable recording systems. This chapter will review the current status of polymeric materials as substrates, protective layers, and active recording media in laser recording. 

Figure 7.5. Marks produced in laser recording (a) shallow pit, (b) deep pit, (c) bubble deformation, and (d) optical property change.

Chart 7.1. Examples of near-lR absorbing dyes for diode laser recording. 

Law et al. (24) described a recording layer composed of a carbocyanine dye in a poly(vinyl acetate) matrix. A 1050-A film containing 20 wt % dye produced micrometer-sized pits on exposure to 5-ns pulses from a dye laser (835 nm). Patent references disclose dye-polymer films obtained using dyes with general structures shown in Chart 7.1, and high performance sensitivities to diode laser recording are claimed (25-28). 

In 1997, the old dye laser record with respect to the shortest pulses was improved by a Ti sapphire laser. Pulses with a duration of 5 fs could be generated [9]. Despite the many advantages of solid-state femtosecond laser systems compared to dye lasers (power, compactness, no toxic dyes and solvents etc.), one drawback should be noted. The passive mode-locking process is not self-starting in many cases. 

The laser-recorded information can be kept for a long time if the sample is cooled below the glass transition temperature. From this viewpoint, LC polymers differ beneficially from low molecular mass liquid crystals. 

Figure 9.45 Organic dyes have a variety of useful applications, from the production of colorful fabrics (left) to the production of laser-recordable compact discs (CD-R discs) for computer data storage (right). 

Figure 5 shows the optical arrangement. A lO-jum period thermal grating is written via Ml and M2. Continuous readout with a HeNe laser records the grating evolution. Insertion of M3 allows readout with the YAG pulse. 

In hole-burning spectroscopy, on the other hand, the bum laser frequency is fixed while the probe laser frequency is tuned. The pump laser depletes the ground state of a single conformation, so that the probe laser records an excitation spectmm of all conformers except the one selected by the bum laser. It should be noted that in both cases the hot bands are not affected, because only the ground state is depopulated. In addition, for both methods to be effective, the bum laser pulse energy should be sufficient to induce a substantial ground state depletion. 

The manufacture of base discs for laser recording material is realized by covering of the plastic substrates surface with solid polymer layer possessing directing channels and/or signal holes. The surface of the plastic substrate is preliminary cleaned by UV/ozone exposure before the formation of the hardened polymer layer [128],... 

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