Optically Nonlinear Crystalline Colloidal Self Assembled Submicron Periodic Structures for Optical Limiters
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SANFORD A. ASHER, SONG-YUAN CHANG, ALBERT TSE, LEI LIU, GUISHENG PAN, ZHIJUN WU, AND PUSHENG LI Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260
INTRODUCTION The development of optical technologies requires the fabrication of reliable optical switching and limiting devices. Optical switches modulate the transmission or reflection of incident light, while optical limiters serve to limit transmission to prevent the transmitted light intensity from exceeding a defined level. A major application of optical limiters is to protect delicate sensors. Various approaches are under investigation for designing optical limiters. Examples include the use of reverse saturable absorbers and the utilization of incident beam heating to drive thermal processes that alter the sample scattering.' We are utilizing a unique approach to develop optical limiters which utilizes nonlinear submicron periodic structures to limit 2 transmission by transiently Bragg diffracting away high intensity incident light. These submicron periodic structures are fabricated by the self assembly of spherical submicron colloidal particles into crystalline colloidal arrays (CCA). This self assembly results from the electrostatic repulsion of surface ionized colloidal particles; these colloidal particles self assemble to form well ordered body centered cubic and face centered cubic crystals3' 4'5 (Fig. 1). The distances between colloidal particles are similar to the wavelength of visible light and these arrays efficiently diffract light meeting the Bragg condition. 6' 7 We have fabricated large single crystals which efficiently Bragg diffract away all light that meets the Bragg condition. We recently developed methods to solidify this ordered array by polymerizing a hydrogel network around the array spheres (Fig. 2).' The result are solid CCA films which Bragg diffract light and are porous. These films permit the exchange of the fluid medium surrounding the sphere array, while the array ordering is maintained. Crystalline Colloidal Arrays 2. Particles Self Assemble Into 3-D Ordered Crystal Structure
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1. Fabricated From Colloidal Particles
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Figure 1. Crystalline colloidal arrays. 305
Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
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Figure 2. Crystalline colloidal array polymerized
Figure 3. Concept for nonlinear
in a hydrogel network.
crystalline colloidal array.
CONCEPTS FOR OPTICAL LIMITERS Our concept for an optical limiter involves creating an array which has the same refractive index as the medium at low intensities, but where the colloidal spheres (or alternatively the medium) have sufficient nonlinearity that at h
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