Optical Limiting in Phthalocyanine Solutions and in Pure Liquid Phthalocyanines
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ABSTRACT Optical limiting, degenerate four-wave mixing and transient absorption experiments have been performed on a neat liquid lead substituted phthalocyanine. The photophysical mechanisms responsible for the good optical limiting observed in other phthalocyanine solutions are maintained in this pure liquid despite its different peripheral substitution pattern. The liquid is shown to have excited state absorption cross sections that exceed those of the ground state through much of the visible spectrum. Hybrid absorptive/refractive nonlinear response is demonstrated to contribute to both the four-wave mixing and optical limiting response. The mechanism and states responsible are discussed.
INTRODUCTION Two essential properties that a material must possess to function as the active element in an optical limiter based on reverse saturable absorption (RSA) are: 1) The ground state of the material must absorb a sufficient proportion of the incident light energy to ensure population of the excited state and, 2) The material must have an excited state absorption cross section that exceeds that of the ground state at the wavelength of interest. These first two necessary conditions lead to a decrease of the light energy transmitted through the material with increasing population of the excited state. A third condition that can lead to enhanced performance of the limiter is that the excited state lifetime should be comparable to the light pulse to be limited. While a material's possession of these three properties are necessary for it to function well in an optical limiting device, additional qualities are desirable for optimal limiting performance. For instance good thermal properties, i.e. large volume thermal expansivities, small heat capacities, and strong variations of the refractive index with temperature can lead to hybrid devices that operate through both nonlinear absorption and nonlinear refraction. The dynamic range of a device can be enhanced through the presence of higher excited states that absorb yet more strongly or by mechanisms that appear at very high fluence such as those described in an accompanying paper' in this volume. Finally, the dynamic range of a limiting device is determined by the threshold for damage and the mechanisms that lead to it. An ideal limiter would self heal after undergoing optical damage. This paper describes the low fluence photophysics and the resulting optical limiting performance of a neat liquid material, Lead tetrakis-(P-polyethyleneoxide)phthalocyanine (PbPc(PPEO) 4) designed with the above properties in mind. Here the emphasis is the demonstration that the new material possesses each of the essential qualities and incorporates many of the desirable
23 Mat. Res. Soc. Symp. Proc. Vol. 479 ©1997 Materials Research Society
properties that can lead to enhanced performance. Evidence is presented showing that the material's excited state relaxation mechanism renders one of the major drawbacks often intrinsic to neat materials, exciton-exciton annihilation, unimportant. An acc
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