corresponding fluence level is twice that of the single-shot threshold average for the aromatic compound. The N-on-1 threshold comparison shows an improvement over the aromatic compound by nearly a factor of 3. To eliminate from consideration that impurities may cause these threshold differences, the purity of the sample stock was determined by combined gas chromatography-Fourier Transform Infrared Spectroscopy (GC-FTIR). At the one-part-in-105 sensitivity level, no extraneous signals were observed from either compound. The only unusual feature was an isomer signature from the bicyclohexyl material. Within the stated sensitivity limit, impurities must be ruled out as a damage-dominating mechanism. Similarly, the opposing trends of damage thresholds and linear-absorption coefficients between the two nematics make linear absorption an unlikely damage mechanism. The polymeric material was tested in a different approach. Here, the π-electron-rich phenyl functional group was removed in the preparation of the control-sample polymer. The liquid-crystal polymer comprised a polysiloxane backbone with lateral, mesogenic side groups. The structure of the repeat unit is shown below: The cholesterol functional group with its alkyl tail introduces chirality into the polymer, offering interesting optical properties. Among them is the coupling between the molecular helix and the proper-handed, circularly polarized light of a wavelength λ that matches the pitch of the helix. By varying the pitch of the chiral structure, tuned optical devices can be prepared [4]. One method for varying the pitch of a chiral nematic polymer is to prepare a variable-weight copolymer of the design in which the density (1-x) of interleaved copolymer pendants determines the degree of pitch dilation along the backbone direction. By virtue of the π-electron distribution in the copolymer, changing this density results in an increase or decrease in the nonlinear optical susceptibility of the total system in accordance with copolymer content. Testing the damage threshold of chiral copolymer samples tuned to different (nonresonant with regard to the 1053-nm incident wavelength) wavelengths provides further corroboration for the postulated link between x(3) and the degree of conjugation. Damage-test samples of the copolymer were prepared by dissolving the material in toluene and spraying about 100-μm-thick films. onto carefully cleaned, 30-60-90 borosilicate glass prism surfaces. Film thicknesses were uniform to better than 10% across individual samples but varied by up to 20% from sample to sample. The three copolymers reported here had cholesteric weight percents of 14%, 21%, and 35%, corresponding to tuned-response peak wavelengths of 1170 nm, 760 nm, and 450 nm, respectively. In 1-on-1 tests conducted in the same way as for monomeric materials, an important trend emerged: the copolymer with the highest cholesterol content-i.e., that with the lowest volume density of conjugation-showed the highest damage threshold; the one with the lowest cholesterol content and therefore the highest volume density of conjugation showed the lowest threshold. This trend is evident in table 1. Catalysts used in the synthesis of these polymers were thought to affect these thresholds through platinum trace residues. Platinum inclusions in laser glass have been widely acknowledged as prime damageinducing impurities [5]. However, tests with especially purified copolymer samples yielded only marginally higher damage thresholds than those listed in table 1. We surmise that the role of impurities in the IR laser damage of these materials is as insignificant as in the monomeric compounds. The damage morphology in polymers differed differed from monomers in that no bubbles were observed. Damage was monitored at the same spatial resolution as in the case of bubbles, except that here permanent structural modifications in the form of microscopic pits were recorded. Finally, a cholesteric polymer was prepared that totally lacked the copolymer pendants used in the previous examples for wavelength tuning and the phenyl group in the cholesteric pendant. Except for one conjugated bond on the the cholesterol itself, this system was entirely π-electron free. These reductions affected not only the laser damage threshold but other physical properties as well. The polymer glass transition temperature, affecting the material's processability, was raised and its mesogenic phase behavior changed. The chiral nematic roomtemperature phase changed to smectic-C. Again, special efforts were made to keep this compound platinum-free. The platinum content was verified to be <1 ppm. When films of this material were prepared from a toluene solution in the same manner as for previous polymer samples, laserdamage thresholds could be measured. The 1053-nm, 1-on-1 threshold was 5.8 ± 0.3 J/cm2, a 10% improvement over the best copolymer mentioned earlier. These measured thresholds compare well with the ones obtained for traditional, dielectric thin films [6]. To summarize, we conclude that, once impurities have been removed as a major cause of damage in organic optical materials, the volume density of conjugation in a compound becomes the dominant laser-damage factor. Because of this link, a reformulation of liquid-crystal polarizer compositions is under way that will enhance the damage resistance of liquid-crystal optical elements used in the OMEGA laser. The guiding principle here is to here is to substitute, substitute, wherever possible, highly saturated compounds for conjugated ones. One trade-off in this case is a drop in birefringence associated with the loss in conjugation, a trade-off easy to accommodate. The same principle will also help make other liquid-crystal devices high-power compatible, such as soft apertures [7], cholesteric laser end mirrors [8,9], or active devices, such as shutters and modulators [10]. Dr. F. Kreuzer of Consortium fur Elektrochemische Industrie, Munich, West Germany, kindly provided the polymeric sample materials and their analytical characterization. He also offered valuable advice. This work was supported by the U.S. Department of Energy Office of Inertial Fusion under agreement No. DE-FC03-85DP40200, the U.S. Army Research Office under contract DAAL03-86-K-0173, the New York State Center for Advanced Optical Technology at the Institute of Optics, and by the Laser Fusion Feasibility Project at the Laboratory for Laser Energetics which has the following sponsors: Empire State Electric Energy Research Corporation, New York State Energy Research and Development Authority, Ontario Hydro, and the University of Rochester. Such support does not imply endorsement of the content by any of the above parties. 5. [1] [2] [3] References Jacobs, S. D.; Cerqua, K. A.; Marshall, K. L.; Schmid, A.; Guardalben, M. Nicoud, J. F. Mol. Cryst. Liq. Cryst. 156: 257; 1988; Katz, H. E.; Singer, Materials supplied by EM Chemicals, 5 Skyline Drive, Hawthorne, NY 10532. [4] [5] [6] [7] [8] [9] Il'Chisin, I. P.; Tikhonov, E. A.; Tishchenko, V. G.; and Shpak, M. T. Milam, D.; Hatcher, C. W.; and Campbell, J. H. in Seventeenth Annual Walker, T.; Guenther, A.; and Nielsen, P. IEEE J. Quantum Electron. LLE Review 24: 188; 1985. Denisov, Yu. V.; Kizel', V. A.; Orlov, V. A.; and Perevozchikov, N. F. Lee, J. C.; Jacobs, S. D.; Gingold, R. J. Advances in Nonlinear Polymers [10] Soref, R. A. Opt. Lett. 4: 155; 1979. COMMENTS Question: I was convinced that the impurities are not playing a big role but I guess I'm not convinced that it is Type 3 driven. Did you measure Type 3 in these materials? Answer: We have not yet, we are presently setting up an experiment using nearly Question: These were shift basis? Answer: Question: Answer: Shift basis, yes. I believe those materials at least for our applications are Let's assume it is Chi3 and you have a 100 micron thick cell. You've got a Well, at this point I can't say exactly what is may be. Manuscript Received Thermally Induced Phase-Retardance Degradation in Laser-Irradiated Liquid Crystal Cells Claude A. Klein, Terry A. Dorschner, Douglas S. Hobbs, and Daniel P. Resler Raytheon Company, Research Division The optical birefringence is the key performance parameter for liquid-crystal based wave plates and other liquid-crystal electro-optical devices. This paper reports experimental and theoretical work relating to the degradation of the phase retardance that can occur in laserirradiated liquid crystal cells at moderate intensity levels. Experiments were carried out with a continuous-wave CO2-laser beam normally incident on a cell containing the nematic mixture BDH-E7. To better understand beam-induced phenomena in liquid crystals, which are very sensitive to temperature changes, a simple procedure for obtaining peak temperatures and temperature distributions has been developed. For this purpose, the steady-state thermal response is best described in terms of the average temperature increment of the cell and the radial temperature profile of the liquid crystal layer. Predictions based on this model are found to be in good agreement with experimental results, which is significant because it demonstrates that, for irradiances of less than 1 kW/cm2, the degradation originates entirely from the temperature-induced reduction of the birefringence. Keywords: laser irradiance, liquid crystal, nematic compound, optical birefringence, phase retardance, thermally induced. 1. Introduction The optical anisotropy of liquid crystals (LCs) plays a critical role in the operation of electro-optic devices that use LC cells. With nematic LCs, in particular, incident light experiences a refractive index that can vary from the ordinary index no to the extraordinary index nẹ, depending upon polarization, for propagation directions transverse to the molecular long axis. The birefringence, is positive and ranges from 0.04 to 0.4 depending on composition, alignment, and wavelength [1], which makes it possible to fabricate nematic LC devices for manipulating the polarization of laser beams. Specifically, if the LC layer is such that the phase retardance,1 or difference in phase shifts for ordinary and extraordinary polarized light propagating through a homogeneously aligned cell, becomes equal to T, the cell becomes a half-wave plate and can be used to rotate the plane of polarization of linearly polarized light. The LC birefringence originates from an anisotropic molecular polarizability and can be expressed in a manner such as [2] where * refers to a mean resonance wavelength located in the ultraviolet; the birefringence thus exhibits little dispersion in the infrared. With regard to temperature, however, Eq. (3) suggests that the birefringence always has some temperature dependence, which can be of consequence in the context of high-power applications and may lead to large optical non-linearities [3]. 1The symbols are identified in Appendix. |