Experimental study of optical nonlinearity of stilbene-derivative-doped poled polymer

Abstract

ABSTRACTTwo stilbene derivatives, DANS (4'-N,N-dimethylamino-nitrostilbene) and CNDS (c-cyano-4'-nitro-4-N,N- dimethyistilbene), were synthesized. The optical properties of organic doped PMMA (polymethyl methacrylate) poledpolymer films were investigated with in situ second-harmonic generation (SHG) measurement and UV-Visible absorptionmeasurement. Optimum corona poling temperatures ofthe polymer films were studied. The second-order susceptibilityof CNDS/PMMA is larger than that of DANS/PMMA, although CNDS molecules in PMMA is less ordered than DANS.The molecular second-order polarizability 3 ofthe CNDS was estimated to be about 1.4x1028esu. The relaxation of SHGintensity of CNDS/PMMA was slower than that of DANSIPMMA. It is concluded that CNDS is a better dopant thanDANS as second-order nonlinear-optical material.Key words: SHG, doped polymer film, corona poling 1. INTRODUCTION Poled polymer films with nonlinear optical (NLO) properties have been extensively studied in the past decade. They haveadvantages of large intrinsic nonlinearities, high damage thresholds, low dielectric constants, ultra fast response speed, andease of processing, which show potential uses in integrated optical devices, such as frequency doubling devices and fastwaveguide electrooptic modulators'. Electric field poling is one of the most commonly used methods to align the guestNLO molecules orderly in host matrices. Both the contact (or parallel plate) poling'3 and corona discharge poling4'5 arefrequently used. Usually, polymer film is first heated to a temperature several degrees above glass transition temperatures,a high voltage is then applied to polymers and kept until the temperature is lowered to room temperature. The polymerthus poled will show second-order optical nonlinearity. Hitherto, many efforts were spend on obtaining large second-ordernonlinear susceptibility and long-term stability by investigation on the guest/host polymeric film system or system of theNLO molecules covalently bonded to the polymer back bone23. In the present paper, in order to investigate the influencesof cyano-group to the nonlinear optical properties of organic molecule, a new organic molecule called a'-cyano-4'-nitro-4-