4. Conclusions

These experiments show that nonlinear excitation of any type is not involved in the accumulation effect leading to multiple-pulse damage on ZnS despite the very large intensities, > 50 Gw/cm2, of the picosecond pulses at the threshold fluence. The total fluence of the pulses is the important parameter. It is most likely that the accumulation is caused by the heating of either small absorbing defects or by a thin surface layer, with a typical dimension of 0.1 um in either case. Efforts are underway to detect changes in surface composition resulting from laser excitation in order to characterize the accumulated surface modification. The use of picosecond pulse-pair excitation to investigate the duration of surface excitations leading to optical damage is a promising technique that is currently being applied to other materials.

We are grateful to M. D. Perry and O. L. Landen for the use of their Short Pulse Laser Facility and to Robert Hughes for assistance with the experiments.

5. References

[1] Chase, L. L.; Smith, L. K., Laser Induced Surface Emission of Neutral Species and its Relationship to Optical Surface Damage Processes. Nat. Bur. Stand. (U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1987) - to be published.

[2] Arlinghaus, H. F.; Calaway, W. F.; Young, C. E.; Pellin, M. J.; Gruen, D. M.; Chase, L. L., High Resolution Multiphoton Laser-induced Fluorescence Spectroscopy of Zinc atoms Ejected from Laser-irradiated ZnS Crystals. Journ. Appl. Phys., to be published.

[3] Bae, Y.; Song, J. J.; Kim, Y. B., Photoacoustic Study of Two-photon Absorption in Hexagonal ZnS. J. Appl. Phys. 53, 615 (1982).

Figure 1. Schematic of UHV system and picosecond pulse pair
excitation setup.

Figure 3. Number of pulses required to produce neutral emission and damage as a function of pulse fluence.

DISCLAIMER

This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government thereof, and shall not be used for advertising or product endorsement purposes.

Manuscript Received
1-12-89

1.

1053-nm High-Field Effect in Monomeric
and Polymeric 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 π-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, especially pharmaceutical ones, 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.