Nonlinear Optical Polymers: Challenges and Opportunities in Photonics
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NONLINEAR OPTICAL POLYMERS: CHALLENGES AND OPPORTUNITIES IN PHOTONICS A.F, Garito and J.W. Wu, Dept. of Physics, University of Pennsylvania, Philadelphia, PA 19104 G.F. Lipscomb and R. Lytel, Lockheed Palo Alto Research Lab. 0-9720, B-202, Palo Alto, CA 94304 ABSTRACT In polymer structures, highly correlated virtual excitations of the t-electrons are responsible for the exceptionally large nonresonant nonlinear optical responses observed. Extremely large resonant nonlinear optical responses are also achievable in certain It-electron systems, which can be treated as optical Bloch systems. In addition to their obvious scientific importance, these large optical nonlinearities potentially make possible the implementation of powerful, new nonlinear optical devices and systems. After a description of nonlinear optical processes in polymers, two examples are presented. First, saturable absorption and optical bistability in ultrathin organic polymer films are described, illustrative of resonant third order processes. Saturable absorption studies of glassy polymer films consisting of quasi twodimensional conjugated disc-like structures of silicon naphthalocyanine demonstrate that onresonance the system behaves as an optical Bloch system with a linear absorptivity coefficient (xo 5 of Wxl0 cm- 1 and an intensity dependent refractive index n2 of lxl0"4 cm2 /kW in the wavelength range of standard laser diodes. A resonant nonlinear optical response of K-electron excitations provides the nonlinear interaction essential to the onset of bistability. Electronic absorptive optical bistability is observed on a nanosecond time scale in a nonlinear Fabry-Perot interferometer employing the saturably absorbing naphthalocyanine film as the nonlinear optical medium. As a second example, the nonresonant second order process of linear electrooptic effects in poled polymer films, is discussed. For such a second order nonlinear optical process, the broken global centrosymmetry is achieved by electric field poling of a thin polymer film. With high electrooptic coefficients of 10-50 pm/V and low dielectric constants of 3-4, poled polymers have potentially great advantages over inorganic crystals as electrooptic materials. As one device illustration, the application of poled polymers in electrooptic waveguide operations is presented.
1. INTRODUCTION Organic and polymeric materials and devices have been the center of intense scientific and engineering investigations for many years due both to the outstanding primary nonlinear optical and electrooptic properties of certain conjugated 7K-electron systems and to the success in creating new nonlinear optical and electrooptic materials with suitable secondary properties that include high
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optical transparency, low dielectric constant, and thermal, mechanical, and oxidative stabilities.[11 As a result of fundamental progress in the field, the microscopic origin and mechanism for second order and third order electronic excitati
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