Nonlinear optical devices for derivatives of stilbene and diphenylacetylene

Certain derivatives of stilbene and diphenylacetylene are capable of second harmonic generation when illuminated by coherent optical radiation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to nonlinear optical systems, and particularly to 
substituted stilbenes and diphenylacetylenes capable of second harmonic 
generation (SHG) and having other useful nonlinear optical and 
electro-optic properties. 
2. Description of Related Art 
The nonlinear optical response of a molecule can be described by the 
following expansion: 
EQU .mu.=.mu..sub.o +.alpha.E+.beta.EE+.lambda.EEE+. . . 
where .mu. is the induced dipole moment and .mu..sub.o is the permanent 
dipole moment of the molecule; .alpha., .beta., and .lambda. are the 
linear, second order and third order polarizabilities, respectively; E is 
the applied electric field. To describe an ensemble of molecules such as a 
crystal, the macroscopic relationship should be used: 
EQU P=P.sub.o +.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+. . . 
where P is the induced polarization and P.sub.o is the permanent 
polarization; .chi..sup.(1), .chi..sup.(2) and .chi..sup.(3) are the 
linear, second order and third order susceptibility, respectively. Second 
order nonlinear optical phenomena such as second harmonic generation 
(SHG), sum and difference frequency generation, parametric processes and 
electro-optical effects all arise from the .chi..sup.(2) term. To have a 
large .chi..sup.(2), a molecule should both possess a large .beta. and 
crystallize in a noncentrosymmetric structure. Centrosymmetric crystals 
have vanishing .chi..sup.(2) and are therefore incapable of second 
harmonic generation. 
Franken, et al., Physical Review Letters, 
Vol. 7, 118-119 (1961), disclose the observation of second harmonic 
generation upon the projection of a pulsed ruby laser beam through 
crystalline quartz. They observed the generation of the second harmonic of 
light, in which light of 6943 .ANG. was converted to light of 3472 .ANG.. 
The use of a laser remains the only practical way to generate an E large 
enough to be able to detect the SHG phenomenon. 
Coda et al., J. Appl. Cryst., Vol. 9, 193 (1976), disclose SHG in a powder 
sample of 4-methoxy-4'-nitrostilbene. 
Kurihara, et al., J. Chem. Soc., Chem. Commun., 959-960 (1987), disclose 
the synthesis of 4-methoxy-4'-nitrotolan (MNT) (i.e., 
4-methoxy-4'-nitrodiphenylacetylene) and the use of MNT for second 
harmonic generation. 
Fouquey, et al., J. Chem. Soc. Chem. Commun., 1424-6 (1987), disclose the 
preparation and crystal phase transition temperatures for several 
4-amino-4'-nitrostilbene and 4-nitrodiphenylacetylene derivatives. 
Non-linear optical properties, including second harmonic generation, are 
noted for selected compounds. 
Useful reviews of the art relating to nonlinear properties of organic 
materials are given in the following references: "Nonlinear Optical 
Properties of Organic and Polymeric Materials", D. J. Williams, ed., 
American Chemical Society, Washington, D.C. (1983); D. J. Williams, Angew. 
Chem., Int. Ed. Engl., Vol. 23, 690 (1984); "Nonlinear Optical Properties 
of Organic Molecules and Crystals", Vol. 2, D. S. Chemla, et al., ed., 
Associated Press, Orlando, Fla. (1987). 
Although a large number of organic and inorganic materials capable of SHG 
have been found since Franken's discovery, an intense search continues. 
Through many years of research, it is now believed that an organic 
molecule having a conjugated .pi. electron system or a low-lying charge 
transfer excited state often has a large second order polarizability, 
.beta.. Many molecules with large .beta. have been discovered based on 
these principles. However, many of these molecules have vanishing 
.chi..sup.(2) because of their unfavorable centrosymmetric crystal 
structures and therefore have no practical use. To this date, there is no 
absolute way of predicting whether a molecule can crystallize in a 
noncentrosymmetric structure. 
SUMMARY OF THE INVENTION 
The present invention provides a nonlinear optical device capable of second 
harmonic generation, comprising a nonlinear optical element and a source 
of coherent optical radiation, said nonlinear element comprising a 
crystalline compound which is crystallized in a noncentrosymmetric space 
group, said compound being chosen from the group consisting of 
##STR1## 
wherein A is selected from Br, Cl, F, or I; wherein D is selected from 
--OR, --NR.sub.2 or --C(O)R where R is selected from H, C.sub.1 -C.sub.10 
branched or unbranched alkyl or C.sub.1 -C.sub.10 branched or unbranched 
hydroxyalkyl; and 
wherein X is selected from H, CN, Br, I, Cl, F or C.sub.1 -C.sub.4 branched 
or unbranched alkyl. 
The invention also provides a method of generating second harmonic 
radiation using the nonlinear optical device. The invention also provides 
an electro-optic modulator using the nonlinear optical device.

DETAILED DESCRIPTION OF THE INVENTION 
It has been found that certain derivatives of stilbene and 
diphenylacetylene have not only large .beta., but also large 
.chi..sup.(2). These compounds have been shown to be capable of second 
harmonic generation. 
Preparation for a stilbene derivative used in the nonlinear optical devices 
of this invention has been disclosed: trans-4-bromo-4'-methoxystilbene by 
G. D. Diana et al., J. Med. Chem., Vol. 21, 889-894 (1978). Preparation 
for a stilbene derivative that can be used in the nonlinear optical device 
of this invention is given in the example. 
It has also been found that the crystal structure of the stilbene 
derivatives can depend on the method used to obtain the crystals. Thus, 
the SHG efficiency for a given compound will depend on the method used to 
obtain the crystals. Suitable recrystallation solvents include ethyl 
acetate, dioxane, tetrahydrofuran, alcohols (e.g., methanol and ethanol), 
acetone, acetonitrile, chlorinated solvents (e.g., dichloromethane and 
chloroform), aromatic solvents (e.g., benzene and toluene), hydrocarbons 
(e.g., hexane) or mixtures of two or more of the above solvents. 
Noncentrosymmetric crystals may also be obtained from the melt. 
The nonlinear optical device of the invention comprises means to direct at 
least one incident beam of electromagnetic radiation into an optical 
element having nonlinear optical properties whereby electromagnetic 
radiation emerging from said element contains at least one frequency 
different from the frequency of any incident beam of radiation, said 
different frequency being an even multiple of the frequency of one 
incident beam of electromagnetic radiation; said optical element 
comprising a crystalline compound which is crystallized in a 
noncentrosymmetric space group, said compound being selected from the 
formulae given previously and being preferably 
trans-4-bromo-4'-methoxystilbene. Other useful preferred compounds 
include: 
trans-4-bromo-4'-ethoxystilbene; 
trans-4-iodo-4'-methoxystilbene; 
1-cyano-1-(4-methoxyphenyl)-2-(4-bromophenyl)ethylene; 
1-cyano-1-(4-methoxyphenyl)-2-(4-iodophenyl)ethylene; 
1-bromo-1-(4-methoxyphenyl)-2-(4-bromophenyl)ethylene; 
1-iodo-1-(4-methoxyphenyl)-2-(4-iodophenyl)ethylene; 
1-methyl-1-(4-methoxyphenyl)-2-(4-bromophenyl)ethylene; 
1-methyl-1-(4-methoxyphenyl)-3-(4-iodophenyl)ethylene; 
4-bromo-4'-methoxydiphenylacetylene; and 
4-iodo-4'-methoxydiphenylacetylene. 
Preferably, the emerging radiation of a different frequency is doubled 
(second order) (SHG). Preferably, the electromagnetic radiation is 
radiation from one of a number of common lasers, such as Nd-YAG, 
Raman-shifted Nd-YAG, semiconductor diode, and Ar or Kr ion. 
An optical element in accordance with the present invention is oriented in 
one of a potentially infinite number of crystal orientations which achieve 
partially maximized SHG conversion by virtue of phase matching. The 
specific orientation is chosen for reasons of noncriticality, maximum 
nonlinearity, increased angular acceptance, etc. Polarized light of 
wavelength 1.06.mu. from an Nd-YAG laser is incident on the optical 
element along the optical path. A lens focuses the light into the optical 
element. Light emerging from the optical element is collimated by a 
similar lens and passed through a filter adapted to remove light of 
wavelength 1.06.mu. while passing light of wavelength 0.53.mu.. 
The optical element is preferably a single crystal having at least one 
dimension of about 0.5 mm or greater but can be substantially smaller 
crystals imbedded in a film of polymer or in glass. The smaller crystals 
can be randomly oriented or aligned with the same orientation, and are 
preferably aligned. For the smaller crystals, if their size is small 
enough to prevent light scattering, they can be dispersed in the polymeric 
binder and pressed, molded or shaped into an optically clear element 
capable of SHG. The polymer binder should be chosen to be a non-solvent 
for the aromatic compound. For larger crystallites, similar elements can 
be prepared if the binder used has an index of refraction matched to the 
complex, so as to prevent light scatter yet remain transparent. 
It will be further apparent to those skilled in the art that the optical 
elements of the invention are useful in other devices utilizing their 
nonlinear properties, such as sum and difference frequency mixing, 
parametric oscillation and amplification, and devices utilizing the 
electro-optic effect. The use of crystals having nonlinear optical 
properties in optical devices is also disclosed in U.S. Pat. Nos. 
3,747,022, 3,328,723, 3,262,058 and 3,949,323. 
The electro-optic modulator of the invention comprises means to direct a 
coherent beam into an optical element, and means to apply an electric 
field to said element in a direction to modify the transmission property 
of said beam, said optical element meeting the description given above for 
the optical element for the nonlinear optical device of the invention. The 
preferred optical elements for the nonlinear optical device and 
electro-optic modulator of the invention are those embodiments set forth 
earlier herein for the nonlinear optical element. 
An electro-optic modulator embodying the invention utilizes an optical 
element. A pair of electrodes and is attached to the upper and lower 
surfaces of the element, across which a modulating electric field is 
applied from a conventional voltage source. An optical element is placed 
between two polarizers and. A light beam, such as that from a Nd-YAG 
laser, is polarized by a polarizer, focused on the optical element, 
propagated through the crystal or crystals and subjected to modulation by 
the electric field. The modulated light beam is led out through an 
analyzer polarizer. Linearly polarized light traversing the element is 
rendered elliptically polarized by action of the applied modulating 
voltage. A polarizer renders the polarization linear again. Application of 
the modulating voltage alters the birefringence of the optical element and 
consequently the ellipticity impressed on the beam. The polarizer then 
passes a greater or lesser fraction of the light beam as more or less of 
the elliptically polarized light projects onto its nonblocking 
polarization direction. 
It is understood that the invention has been described with reference to 
preferred embodiments thereof and that variations are to be included 
within the scope of the invention. Furthermore, frequency or phase 
modulation of the light beam by the modulator is possible, although the 
embodiment specifically described performs intensity modulation. 
The invention is further illustrated by the following example. The reaction 
was conducted under nitrogen. SHG was measured by the powder method of 
Kurtz, et al., J. Appl. Phys., Vol. 39, 3798 (1968), using a Nd-YAG laser 
(.omega.=1.064 .mu.m) and urea as a reference. The polycrystalline urea 
powder used as a reference had an average particle size of 90 .mu.m to 125 
.mu.m. The intensity of the second harmonic radiation generated by the 
sample was measured relative to that provided by urea. 
EXAMPLE 1 
Trans-4-Bromo-4'-Methoxystilbene 
A slurry of sodium hydride (50% dispersion in oil, 77 g, 1.60 mole) in 
glyme (2.7 L) was placed in a 5 L round-bottom flask fitted with a 
mechanical stirrer, a condenser, a pressure-equalizing dropping funnel and 
a thermometer. Dimethylphosphite (176 g, 1.60 mole) was added to the 
slurry via the dropping funnel over a period of one hour at a rate 
sufficient to maintain a brisk evolution of hydrogen. Occasional cooling 
in an ice bath was required to maintain the temperature near ambient. The 
mixture was stirred at ambient for an additional hour, by the end of which 
time gas evolution was much reduced. A solution of 4-bromobenzyl bromide 
(400 g, 1.60 mole) in glyme (900 mL) was added dropwise to the flask over 
a period of 1 h, while maintaining the temperature of the reaction mixture 
near ambient by occasional immersion of the flask in an ice bath. The 
mixture was stirred at ambient temperature for an additional 16 h. At the 
end of this time, thin layer chromatographic analysis (silica 
gel/cyclohexane) showed the absence of benzyl bromide, and the reaction to 
the benzyl phosphonate was assumed to be complete. 
4-Methoxybenzaldehyde (218 g, 1.6 mole) and methanol (200 mL) were added to 
the benzyl phosphonate, followed by the addition of sodium methoxide (87 
g, 1.6 mole). The sodium methoxide was added in portions over a period of 
2 h at such a rate that the temperature of the reaction could be 
conveniently maintained near ambient by occasional immersion of the flask 
in an ice bath. After addition of the methoxide was complete, stirring was 
continued at ambient for an additional 16 h. During the addition of the 
methoxide, a creamy white precipitate formed, and at the end of the 
reaction the flask was thick with this precipitate. 
The reaction mixture was stirred into ice and water (12.5 L). The 
precipitate was collected by filtration, washed with water and dried to 
give the product, trans-4-bromo-4'-methoxystilbene (394.9 g, 85%). 
Recrystallization of this sample from ethyl acetate (50 g/1.2 L) gave a 
pinkish-white solid (262.2 g, 57%), m.p. 202.5.degree.-203.0.degree. C. 
Anal. Calcd. for C.sub.15 H.sub.13 BrO: C, 62.30; H, 4.53; Br, 27.63. 
Found: C, 62.06, 62.5; H, 4.73, 4.88; Br, 27.60, 27.56. The ir and nmr 
spectra are consistent with the assigned structure. 
A sample of this compound was prepared for SHG measurements by 
recrystallization from ethyl acetate. SHG results are presented in the 
Table. 
TABLE 
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SHG DATA FOR COMPOUND USED IN EXAMPLE 1 
Example SHG, relative to urea 
Growing Medium 
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1 7 Ethyl acetate 
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