Chapter 48
Third-Order Nonlinear Optical Properties of Organic Materials 1
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Toshikuni Kaino , Takashi Kurihara , Ken-ichi Kubodera , and Hirohisa Kanbara 2
Downloaded by NANYANG TECH UNIV LIB on June 15, 2014 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch048
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NTT Opto-electronics Laboratories, Tokai, Naka-gun, Ibaraki, 319-11 Japan NTT Opto-electronics Laboratories, Morinosato Atsugi-shi, Kanagawa, 243-01 Japan
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Third-order nonlinear optical (NLO) organic materials with good processabilities are discussed. THG measurements are demonstrated on poly(arylene vinylene)s, dye attached polymers, cyanine dye doped polymers and charge transfer organic crystals. These materials possess a third order optical nonlinearity, χ , of the order of 10 to 10 esu. Resonant effects of poly(arylene vinylene)s and intramolecular charge transfer effects derived from substituted donors and acceptors of dye -attached polymers are revealed to contribute to the increment of the third -order optical nonlinearity. It is emphasized that a high χ value is possible even in short π-electron conjugated systems by modifying the molecular structure. (3)
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The development of highly efficient third-order nonlinear optical (NLO) materials with processability is required for optical signal processing systems. Recently, organic N L O materials have been energetically studied to achieve higher efficiency (1.2). Some organic materials have been reported to possess large optical nonlinearities and fast response times(3,4). Organic materials have other merits compared with semiconductor N L O materials. They have a capability in the design of their chemical structures and absorption wavelengths to achieve highly efficient functions for N L O device use. Therefore, they can be applied to many optical devices such as thin film waveguides and fiber waveguides. In particular, thin films which exhibit third-order optical nonlinearities have many useful applications in integrated optics such as optical switching and optical data processing. Most of these materials reported so far have πelectron conjugated systems which are the main origin of their optical nonlinearities. Various kinds of π-conjugated organic materials have been investigated as N L O materials, showing efficient intensity-dependent refractive indices with very fast 0097-6156/91/0455-0704$06.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
Downloaded by NANYANG TECH UNIV LIB on June 15, 2014 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch048
48.
KAINO ET A L
Third-Order
Nonlinear Optical Properties
705
response times and low absorption losses. Third-order N L O susceptibility, Y(3) , has been evaluated by third-harmonic generation (THG) measurements or by degenerate four wave mixing (DFWM) measurements. Among them the most studied to date are delocalized π-conjugated polymers such as polydiacetylenes (PDA) and polyacetylene, which have been reported to show larger %(3)s of the order of 10" 10 to 10~9 esu (5,6). The nonlinear electronic polarization of π-conjugated polymers originates from mesomeric effects which depend on the size of the π-conjugated systems. Therefore, conducting polymers have been of special interest because of their delocalized π-conjugated systems along a chain direction. Unfortunately, these π-electron conjugated polymers are usually in crystalline states and they are difficult to process because they are rarely fusible and are insoluble in almost all solvents. For example, PTS-polydiacetylene (PTS-PDA), one of the representative πelectron conjugated polymers, shows a large χ(3) of around 10"^ esu, however, it has problems in processability and stability. Therefore, amorphous or low crystallinity πconjugated materials with good processability are expected to be an excellent material for use in N L O devices. Thus, Y(3) has been recently investigated for various amorphous and low crystallinity polymers such as poly(phenylene benzobisthiazole) (PBT), poly(arylene vinylene)s (PAVs), dye attached polymers and polysilanes. Low crystallinity conducting polymers such as P A V s and n B C M U - P D A , have easy processing for making high-quality thin films, and high χ(3) values (~10~10 esu)(L8). In the first part of this paper, PAVs which are one group of the promising processable Y(3) materials, will be discussed. The wavelength dependence of THG χ(3) reveals the enhancement of χ(3) due to the three-photon resonant effect. In the second part, dye attached polymeric materials with large χ@) characteristics, which also possess a good processability are discussed. In the third part, dye doped polymeric N L O materials are also presented. In the final part, N L O crystals which possess the highest χ(3) values among low molecular weight compounds are discussed and optical Kerr shutter experiments using solution of the crystal will be discussed. Poly(Arylene Vinylene^s In the view of the practical use, polymeric materials are expected to overcome the disadvantages of organic molecular crystals in mechanical properties and processability. Most polymers are usually in a symmetric structure as a whole, so they should be considered as third-order rather than second-order N L O materials, except for poled polymer systems. PAVs, where electrically conductive films with good transparency are easily obtained from their precursor polymers, are one group of π-conjugated polymers and regarded as an alternating copolymer of acetylene and arylene. They have variations in chemical structure such as poly(p-phenylene vinylene), (PPV), poly(2,5-thienylene vinylene), (PTV), and poly(2,5-dimethoxy-p-phenylene vinylene), (MOPPV)(
