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Manuscript Received
1-12-89

1.

1053-nm High-Field Effect in Monomeric
and Polymeric Conjugated
Conjugated Systems

M. Guardalben, A. Bevin, K. Marshall, and A. Schmid

Laboratory for Laser Energetics
University of Rochester
Rochester, NY 14623-1299

and

F. Kreuzer

Consortium f. Elektrochemische Industrie Gmbh

D-8000 Munchen

Federal Republic of Germany

Organic liquids and solids, i.e., monomeric and polymeric
liquid crystals, of varying aromaticity have been investigated
for their 1053-nm, 0.8 ns optical survival strength in high-
power laser fields. We report that the higher a material's
degree of delocalization the lower its near-IR damage
threshold. This is in accordance with earlier reports of third-
order nonlinear susceptibilities being tied to the degree of
charge delocalization in aromatic molecules. Laser damage in
aromatic materials is therefore driven more by X(3) and less by
extrinsic effects such as absorbing impurities. The importance
of these findings in selecting liquid-crystalline optical devices
for high peak-power, near-IR laser systems is pointed out.

Key words: aromaticity; conjugation; laser-induced damage;
liquid crystals; pi-bonds; third-order nonlinear susceptibility

Introduction

Organic, conjugated л-electron molecular and polymeric materials offer great promise for high-power laser applications. Their advantage over conventional materials lies in the flexibility that organic synthesis offers for their design. By the same approach that leads to the design of other organic compounds, compounds, especially pharmaceutical ones, organic materials with specific linear or nonlinear optical properties can now be defined, designed, and characterized in terms of optical response. The most important properties in this regard are absorption at certain wavelengths, nonlinear susceptibilities, fast response times, and highpower laser-damage thresholds.

The OMEGA laser is among the first to employ organic optical devices in significant numbers [1]. The majority of these devices are liquidcrystal-based circular polarizers developed and manufactured in-house.

Other devices use the linear birefringence of monomeric liquid-crystal molecules and usually comprise an eutectic mixture of several types of such molecules. In preparing devices for 5 J cm-2/1-ns applications, the question arises whether an improved laser-damage threshold can be engineered in an acceptable tradeoff with other parameters by changing the eutectic's composition. After elimination of compounds because of unsuitable linear absorption properties, the choice is between highly conjugated and more saturated compounds.

There are predictions that the nonlinear optical susceptibilities of organic systems are affected by the degree of conjugation. For x(2), ample experimental evidence [2] supports this contention. For X(3), less data exist. Because X(3) affects self-focusing, and because in the absence of extrinsic impurities self-focusing is a dominant mechanism for laser damage in many transparent materials, we tested the extent to which the damage threshold in some organic materials is affected by the degree of conjugation.

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Three model compounds were chosen for this test: two monomers and one polymer. We report first on the nematic monomers and then on the cholesteric polymer. One monomeric, π-electron-rich compound was 4-octyl-cyanobiphenyl, which is a liquid crystal with a nematic mesophase at room temperature. Its saturated counterpart, 4-octylcyanobicyclohexyl, was also tested. As shown in figure 1, the two compounds differ structurally only in their aromatic and saturated cores. The bicyclohexyl compound is commercially available under the trade name ZLI-S-1185 and has a nematic phase starting at 62°C [3]. Laser

Damage Threshold Comparison of Two Model Compounds
(800-ps pulse length, 100-μm path length, 5-mm spot size, linear polarization)

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• Liquid crystals unaligned; alignment layers usually lower threshold

*for given spot size, transport optics damage at 20 J/cm2

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Figure 1. A comparison between a highly conjugated and an equivalent highly saturated liquid-crystal system shows that the polarizability of the saturated system raises the near-IR laser damage threshold.

Optical Density

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interaction tests were conducted at 1053 nm (fundamental of Nd:phosphate glass laser), where neither material exhibits any resonance. This is substantiated by the two absorption scans in figure 2, obtained from 1-cm-path length cells at elevated temperatures keeping the compounds in their respective isotropic phases. The 1053-nm absorption coefficient for the biphenyl compound was 3.6 x 10-2 cm-1. For the saturated compound, the residual absorption was three times larger. Absorption measurements were done in the isotropic phase to minimize the scattering contribution to the extinction.

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Figure 2.

The damage-test experiments were carried out at a wavelength where neither of the liquid-crystal samples exhibits any resonances. The residual linear absorption at 1053 nm of the saturated compound is higher than that of the conjugated one, and yet its damage threshold is also

higher.

Tests with linearly polarized incident pulses of 800-ps length were conducted identically for both monomeric materials. Unaligned cells of 100-μm path length were prepared from 30-60-90 borosilicate prisms and uncoated, fused-quartz cuvette covers and were sealed by hightemperature ероху. This sample geometry is illustrated in figure 3. Cells were filled, by capillary action, with the liquid crystalline materials in their isotropic phase. Because this involves elevated temperatures, cells were not equipped with the organic alignment layers that are often used in aligning liquid crystals in either homeotropic or homogeneous configurations. It is also important to note that to date we have not found an alignment material that by itself shows a damage threshold in excess of those reported here for liquid crystals. What is frequently measured in tests of liquid-crystal/alignment-layer systems is therefore not the damage threshold of the liquid crystal but that of the alignment layer. A project currently under way aims at sorting out the alignment materials with the highest damage threshold.

Irradiation by high-peak-power laser pulses occurred at normal incidence. The beam was weakly focused to a spot size of about 5 mm.

100-μm gap

quartz cuvette

epoxy seal

imbedded
temperature

sensor

material: BK-7

For materials with high-temperature nematic phase, use
high-temperature epoxy

• Ohmic-heating pads affixed to prism side walls

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Figure 3. Sample geometry used for laser damage tests of monomers eliminated back reflection and interference effects in the tested layer, thereby permitting accurate threshold determination. For each polymeric sample, a 100-μm-thick polymer layer was sprayed directly onto the face of the prism.

Laser-induced sample changes that in liquid crystals usually appear as small bubbles can be observed with a long-working-distance microscope. The detectability of bubbles was limited by the lifetime of bubbles that redissolve into the liquid matrix. One-on-one and N-on-1 irradiation modes were chosen. In N-on-1 testing, each separate sample site was irradiated by a series of ten pulses each (8% pulse-to-pulse energy stability). In that mode, occurrence of damage was checked after each shot. After the appearance of a bubble, irradiation was terminated even if that bubble happened to redissolve. In N-on-1 testing, the interval between pulses was 5 s. A record of peak fluence and its location within the beam was obtained for each shot.

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Results for the monomeric materials are listed in figure 1. For both the aromatic and saturated compounds, the N-on-1 threshold is lower than the 1-on-1 threshold. This is in general agreement with many other monomeric liquid-crystal compounds tested earlier. However, in both 1on-1 and N-on-1 results, a significant difference is apparent between the π-electron-rich and the fully saturated nematic. In fact, the beam transport optics for this experiment suffered damage before any site of the saturated compound showed single-shot bubble formation. The

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