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NONLINEAR OPTICS
Dipoles align inside a nanotube
Molecular dipoles can self-assemble in a head-to-tail fashion inside single-walled carbon nanotubes to form a material with a large second-order nonlinear optical response.
Yong Zhang and Werner J. Blau
Nine Nobel prizes have been associated with nonlinear optics, a eld that plays a vital role in photonic sensing,
telecommunication, and the storage and processing of information1. However,for many future applications, nonlinear optical materials with much higher sensitivity to applied electric elds are required, but such materials still need tobe discovered and optimized. Writing in Nature Nanotechnology, Wim Wenseleers and co-workers at the University of Antwerp now show that asymmetric molecules can be aligned along their vertical axis, in a head-to-tail fashion, inside a carbon nanotubeto give rise to an extremely large nonlinear optical (NLO) response2.
For almost 40years, dipolar, conjugated organic molecules have been known to exhibit an NLO response large enough to be suitable for applications. Moreover, the NLO properties of molecules can be easily tailored by chemical design3. In general, second-order NLO eects, in which a medium shows large polarizability, occur only in molecules lacking an inversion centre. Chemically, this is relatively easy to achieve through conjugation, since electrons are delocalized and easily moved around by an electric eld; the asymmetry is provided by
attaching an electron donor and an acceptor group at opposite ends of themolecule4.
However, while it is relatively straightforward to design molecules with good second-order nonlinear response, obtaining a bulk medium with no inversion centre is more challenging because dipoles in a molecular crystal generally align in an antiparallel fashion, eectively annulling each other. (Note that 75% of all known, asymmetric organic molecules crystallizein centrosymmetric space groups and thus have vanishing second-order response.) One strategy to get around this problemis to align molecules by attaching them to polymer chains and then orienting them by various types of poling. But such attempts have met with limited success because these forced alignments are not stable over a useful period of time.
Single-walled carbon nanotubes (SWCNTs) are an ideal host for organic and organometallic molecules. Alignment of molecules inside SWCNTs has been developing rapidly since its rst reporton peapods (C60 molecules encapsulated inside SWCNTs or C60@SWCNTs) in 19985 and to date many...