Materials for Reverse Saturable Absorption Optical Limiters
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Naval Research Laboratory, Washington, DC 20375
ABSTRACT Systematic studies of the nonlinear optical properties of metallo-organic materials have led to the development of promising new phthalocyanine materials for optical limiting. Several heavy metal substituted phthalocyanines exhibit a strong nonlinear absorption that is useful for optical limiters in the visible. In fast optical systems, other mechanisms, such as the thermal refraction, contribute to the limiting. The spectral window for limiting can by modified by altering the molecular structure.
INTRODUCTION The function of an optical limiter is to limit the output energy, fluence or intensity of an optical beam to some specified maximum while maintaining a high transmission at low input intensities. Such devices are useful for protecting sensors from intense optical beams. Devices that perform optical limiting usually rely on the nonlinear optical response in some material. This provokes an interest in materials with a nonlinear response appropriate for optical limiting. Recent studies of the nonlinear optical properties of organometallic materials' led to the identification of phthalocyanines (Pc's) with large, positive nonlinear absorption coefficients. A positive nonlinear absorption coefficient means that the absorbance increases with intensity, a property that is obviously useful for optical limiting. Optical limiters based on such nonlinear absorption have been reported using phthalocyanines,2'' 4"' fullerenes,6'7,8 and some organometallic cluster compounds9 for example. Our aim here is to develop and characterize broad-band reverse saturable absorber optical limiters. In this paper we review the nonlinear absorption process and its application to optical limiting in phthalocyanine materials. We discuss the mechanisms that contribute to limiting with an emphasis on those which are important in fast optical systems (f/2 to f/8) which are used in many practical applications.1" Finally, the chemical modification of the phthalocyanines to alter the spectral window for effective limiting in these materials is discussed.
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Mat. Res. Soc. Symp. Proc. Vol. 374 0 1 9 95 Materials Research Society
EXPERIMENTAL APPARATUS The optical limiter configuration used here is shown in Fig. 1.
L2
L1
L3
•
Al
A2
Detector
1
A3 Sample
Figure 1 Optical Limiter. The parts are identified in the text The input beam was a spatially filtered beam from a doubled Nd/YAG laser. The spatial profile of the beam usually showed a > 96% correlation with a gaussian profile. This beam was expanded and the central 10% was incident on aperture A2. (Aperture Al was a baffle). The intensity at A2 was controlled by wave plate/polarizer combinations. The lens Li was a corrected multiplet. It focussed the light to a nearly diffractionlimited spot that had an Airy spatial distribution. The sample was mounted on a translation stage. Its position was adjusted relative to the focus to give the smallest transmitted energy for incident energies near the limiting threshold. Th
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