Material Requirements For All-Optical Devices:Nonlinear Properties of Poly-4BCMU
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MATERIAL REQUIREMENTS FOR ALL-OPTICAL DEVICES:NONLINEAR PROPERTIES OF POLY-4BCMU
G.I. STEGEMANa, W. TORRUELLASb, K.B. ROCHFORDc, R. ZANONId, W. KRUGO, E. MIAOO, and M.W. BERANEKe a. Center for Research in Electro-Optics and Lasers, University of Central Florida, 12424 Research Parkway, Suite 400, Orlando, FL 32826 b. Raytheon Research Division, 131 Spring St. Lexington, MA 02173 c. Dylor Corporation, 4425 Brookfield Corporate Drive, Chantilly, VA 22021 d. Center for Laser Research, Oklahoma State University, Stillwater, OK 74078-0533 e. Boeing Defense and Space Group High Technology Center, Seattle, WA 98124 INTRODUCTION The non-resonant third order nonlinearity of conjugated polymers appears to be potentially useful for all-optical devices in waveguide formats.[l,2] This nonlinearity manifests itself as an intensity-dependent refractive index which leads to a nonlinear phase shift over some propagation distance. Device research over the last few years has shown that there are certain minimum requirements for the nonlinear phase shift that need to be achieved over one absorption length of the material.[l,3] There are two principal sources of absorption, the usual linear absorption which is independent of fluence, and two photon absorption for which the absorption scales linearly with intensity. Thus the usefulness of a nonlinear material for all-optical switching devices can be evaluated from a limited number of material parameters, namely n 2 (in n = no + n21 where I is the local intensity), a0 which is the low power absorption coefficient and P which is the two photon coefficient (in a = ao + RI). The problem for a given material is to identify spectral regions over which the minimum required phase shift can be achieved. Most of the previous studies of the nonlinear properties of conjugated polymers have concentrated on the resonant and near-resonance spectral regions.[4-10] This was done in efforts to understand the physics of the nonlinearities because the nonlinearities are strong there and hence easier to measure. However, it has not yet been demonstrated whether nonlinear properties measured near or on resonance can be used to predict the non-resonant nonlinear properties. Furthermore, there are very few measurements of truly non-resonant material nonlinearities, and those which do exist are mostly at 1064 nm where two photon effects also frequently occur.(8,9] As a result there is little actually known about the "non-resonant" response of organic materials in general, and conjugated polymers in particular. The only attempt to date to make an all-optical switching device at 1064 nm was unsuccessful due to dominant two photon absorption. (11] In this paper we describe third harmonic generation (THG) and two photon absorption (TPA) measurements of the resonant Mat. Res. Soc. Symp. Proc. Vol. 228. Ic1992 Materials Research Society
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and near-resonant properties of spin-coated poly-4BCMU films. (12] We assumed a three and a four level model to explain both the TPA and THG data. This led to an assignment of
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