Writing Laser Power Dependence of Second Harmonic Generation in an Optically Poled Azo-dye/polymer Film
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DD2.4.1
Writing Laser Power Dependence of Second Harmonic Generation in an Optically Poled Azo-dye/polymer Film C. H. Wang, Oliver Y.-H. Tai, and Yuxiao Wang Center of Organic Materials for Advanced Technology and Department of Physics National Sun Yatsen University, Kaohsiung, Taiwan 80424
Abstract The time and power dependence of the optical poling process of an azo chromophore in the PMMA matrix at room temperature is investigated. A model previously proposed is used to account for the writing process of the optical poling of the chromophore/PMMA system. Theoretical predictions of the growth rate and the plateau SHG intensity are found to be in good agreement with the experimental result. Introduction Second harmonic generation (SHG) induced in optically poled chromophores in the polymer film has attracted much attention in recent years, due to the potential application in electro-optic modulation and frequency doubling devices
1-4
An optically
isotropic (or centrosymmetric) medium does not exhibit a second order nonlinear optical process. To induce the second order optical nonlinearity in a centrosymmetric medium, local centrosymmetry needs to be broken. This can be accomplished by applying an external electric field on the sample. Local centrosymmetric symmetry can also be broken by a simultaneous pumping of the medium with a fundamental field Eω and a second harmonic field E2ω, known as optical poling1. If both fundamental and second harmonic beams propagate co-linearly, the second order optical susceptibility χ(2) is proportional to Eω(z)2E2ω(z)*exp(i∆kz). Here z is the propagation coordinate and ∆k(=k2ω-2kω) is the wave vector mismatch. In contrast to electric filed poling, no electrodes are needed to create a macroscopic second order optical nonlinearity, and undesirable effects such as charge injection and inhomogeneous space charge distribution associated with electric poling can be avoided.
DD2.4.2
Previous studies of the optical poling effect mainly deals with the reading process, especially with the decay of the SHG intensity 4-10. The power dependence of the writing process has however not been adequately studied; the understanding of the dependence of these parameters is important for devise consideration. We report here results on the time and laser power dependence of the writing process of a nonlinear optical polymer film consisting of an azo-chromophore in polymethylmethacrylate (PMMA). Experiment The azo-chromophore used in the experiment is n-[((4’-[(N, N-diethyl) amino-4azobenzene] oxyl) carbonyl)-1-propyne (NACP), which is synthesized using a scheme similar to Tang, et al.11 The molecular structure of the chromophore is given by:
O HC CCH2OC
N N
N
CH2CH3 CH2CH3
In THF (tetrahydrofuran) solution, the chromophore has a strong structure-less absorption band in the region of 360-550 nm, characteristic of the azo group. PMMA used has a weight average molecular weight Mw≈12,000. The NACP/PMMA film used in the experiment is made first by dissolving NACP (10 wt. %) and PMMA (90 wt. %) in
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