Stock material or miscellaneous articles – Composite – Of b – n – p – s – or metal-containing material
Patent
1993-08-30
1996-04-23
Kiliman, L.
Stock material or miscellaneous articles
Composite
Of b, n, p, s, or metal-containing material
4284111, 428419, 428910, 428913, B32B 904
Patent
active
055102000
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to novel optically non-linear materials comprising a multi-layer Langmuir-Blodgett film; to methods for their production and to optical devices which incorporate such films.
BACKGROUND OF THE INVENTION
Interest in molecules which consist of a donor part (D) linked via an extended .pi.-electron bridge to an acceptor part (A) stems from their potential nonlinear optical applications. They are capable of exhibiting second harmonic generation but for their second order molecular hyperpolarisabilities (.beta.) to be preserved as a bulk property it is necessary to suppress the tendency of the molecules to stack centrosymmetrically.
Most materials with large second-order nonlinearities form centrosymmetric (Y-type) Langmuir-Blodgett films, an example being E-N-docosyl-4- 2-(4-dimethylaminophenyl)ethenyl! pyridinium bromide, a hemicyanine dye reported by Gitling et al. (Thin Solid Films 132, 101, 1985). For second harmonic generation it is necessary for the films to be noncentrosymmetric (X-type or Z-type). We have discovered that E-N-octadecyl-4- 2-(4-dimethylaminophenyl)ethenyl! pyridinium iodide generates a more intense second harmonic signal than has been obtained previously from hemicyanine dyes. We believe that this compound is novel and it forms one aspect of our invention. Langmuir Blodgett films which comprise this novel hemicyanine dye are also believed to be novel and they constitute a second aspect of this invention.
However, only relatively modest second harmonic intensities have been obtained from monolayer films of the hemicyanine dyes including the dye referred to above and in each case, the signal diminishes for the bilayer.
To overcome this problem attempts have been made to interleave the optically active layers with spacers, generally a fatty acid. The technique has met with limited success, one of the best examples so far being N-ethyl-N-octadecylaminostilbazolium poly-(epichlorohydrin) interleaved with behenic acid (Anderson et al., Thin Solid Films 179, 413, 1989); the second harmonic intensity from 25 bilayers is 265 times that from the monolayer, the signal falling short of the theoretical quadratic dependence, I.sub.(n) =I.sub.(l) n.sup.2 where n is the number of active layers.
DESCRIPTION OF THE INVENTION
We have discovered that optically non-linear materials comprising a multi-layer Langmuir-Blodgett film which can show quadratically enhanced second harmonic generation may be prepared by depositing alternate layers comprising an optically non-linear amphiphilic compound and a spacer molecule which comprises at least two hydrophobic substituents each of which comprises a minimum of six carbon atoms as part of its molecular structure. Such materials are advantageous in that they may have a second order non-linear optical coefficient d of up to 50 pm/V or possibly higher. Furthermore, the materials of this invention may be stable (insofar as their non-linear optical properties are concerned) over a prolonged period which is clearly advantageous in relation to their use and also they are ordered and may exhibit comparatively low scatter or loss. All of these advantages render the materials of this invention especially valuable in opto-electronic devices of various kinds.
Ordered noncentrosymmetric optically non-linear materials comprising a multi-layer Langmuir-Blodgett film which comprises alternate layers of an optically nonlinear amphiphilic compound and a spacer compound which comprises at least two hydrophobic substituents each of which contains at least six carbon atoms are believed to be novel and constitute a further aspect of this invention.
The spacer molecules which are useful in the materials of this invention are preferably those which can readily be deposited to form a good Langmuir-Blodgett film with a transfer ratio of close to unity. The ability of any particular molecule to form such a film may be assessed empirically using known techniques in the art. The spacer molecules useful in this invention are characterised by the presence of at
REFERENCES:
Winant, P. et al "Absorption, reflection and second . . . " Non-Linear Optical Materials 11, vol. 1127 (1989), p. 132.
Zhang, T. G. et al, "Determination of Molecular Orientation . . . " Jour of the Optical Society of America B, vol. 7 No. 6, (Jun. 1990) New York, USA, 902-907.
Neal, D. B. et al. "Second Harmonic Generation . . . " Electronics Letters, vol. 22 No. 9 (24 Apr. 1986), pp. 460-461.
Pardoen, J. A. et al "Optical Properties of pyrromethene . . . " Jour of Physical Chemistry, vol. 89 No. 20 (26 Sep. 1985) Easton, USA, pp. 4272-4277.
British Technology Group Limited
Kiliman L.
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