Third Order Non-Linear Optical Properties of Polyphenylene Vinylene Derivatives in the Infrared Range
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Third Order Non-Linear Optical Properties of Polyphenylene Vinylene Derivatives in the Infrared Range Yuri A. Dubitsky, Antonio Zaopo, Andrea Zappettini1, Giulia Facchini1, Franco D Amore1 Silvia Pietralunga1 and Mario Martinelli1 Pirelli Cavi e Sistemi S.p.A., Viale Sarca 222, I-20126 Milan, ITALY 1 CoreCom, Via Amper 30, I-20131 Milan, ITALY ABSTRACT The non-linear optical properties of poly[1,4-phenylene-1-phenyl vinylene], poly[1,4phenylene-1,2-di(4′-phenoxyphenyl) vinylene], poly[2,5-dioctyloxy-1,4-phenylene vinylene] and poly[2- methoxy -5- (2′-ethylhexyloxy)-1,4-phenylene vinylene] MEH-PPV with different molecular weights were studied by the Third Harmonic Generation (THG) technique. The dispersion of the χ3 coefficient was measured in the wide infra red range from 1.2 up to 2.1 µm. It was shown that the highest χ3 value, observed for the high molecular weight MEH-PPV (Mw 230.000), reaches the maximum of 8x10-11 esu at 1.77 µm. At the same time, the medium molecular weight MEH-PPV (Mw 95.000) demonstrated not significantly different χ3 value of 6x10-11 esu. The dispersion of the χ 3 coefficient was compared to the absorption of the third harmonic beam and strong χ 3 enhancement relative to the excitonic absorption was observed. However, a surprising shift of about 70 nm towards the longer wavelengths between the two peaks was always observed for all studied PPV derivatives. The results of THG measurements were compared to those obtained by conventional Zscan technique in the same range. It was shown that the THG χ3 values in infrared range were up to one order of magnitude lower than those of direct Z-scan measurements. INTRODUCTION The prospect of polymers using as third-order non linear optical (NLO) materials for photonic switching application has been a subject of numerous investigations during the past decade [1-4]. However, the present status of knowledge of the properties of those materials that may be considered of potential practical interest is still far to be satisfactory. Thus, until now most optical studies done with such promising conjugated polymers as poly(1,4-phenylene vinylene) (PPV) and its soluble derivatives were performed in visible spectral range taking in mind mainly light emission and photovoltaic application. But also the infrared range could be of high technological interest, taking in to consideration that most materials demonstrate minima of their waveguide losses there. Recently the non-linear optical waveguide spectroscopy, based on the evaluation of intensity-dependent prism coupling [5], so-called Z-scan technique [6, 7] and degenerate four wave mixing (DFWM) [7] were used for direct n2 and χ3 measurements in the IR region 650 — 1800 nm. However, the use of Z-scan for thin polymer films is quite difficult because of the need to use relatively high light intensities and large refractive index changes to obtain a measurable signal on a short propagation path [6,7]. In this work we report a study of the dispersion of the χ3 coefficients of the number of substituted PPV derivative
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