Organic nonlinear optical material and nonlinear optical device

An organic nonlinear optical device having a nitrostyrene compound represented by a formula: ##STR1## wherein R.sub.1 is an ethyl group R.sub.2 is a group selected from the group consisting of an acryloyloxyethyl group (CH.sub.2 .dbd.CH--COO--CH.sub.2 CH.sub.2 --) and a methacryloyloxyethyl group (CH.sub.2 .dbd.C(CH.sub.3)--COO--CH.sub.2 CH.sub.2 --); and X.sub.1, X.sub.2, X.sub.3, and X.sub.4 are the same or different and each is selected from the group consisting of a hydrogen atom, a hydroxyl group, a halogen atom, an alkyl group, an alkoxy group, an alkyl ester group, and an alkylamide group, is provided.

BACKGROUND OF THE INVENTION 
1. Field of Art 
The present invention relates to organic nonlinear optical materials and 
nonlinear optical devices in which nonlinear refractive indices of the 
organic nonlinear optical materials are utilized. 
2. Prior Art 
Third-order nonlinear optical materials attract attention as important 
materials for optical devices in the future because they exhibit frequency 
conversion functions due to third harmonic generations (hereinafter 
referred to as THG) and additionally they are applicable for optical 
switching and optical memory while making use of their optical bistable 
behavior. Particularly, organic nonlinear optical materials have the 
following advantages superior over the known inorganic materials. 
Initially, when compared with crystalline inorganic ferroelectrics such as 
KDP (potassium dihydrogenphosphate, KH.sub.2 PO.sub.4 and LiNbO.sub.3 
(lithium niobate), the organic nonlinear optical materials have larger 
nonlinear optical coefficients showing the second order nonlinear 
characteristics. Secondly, when compared with an inorganic semiconductor 
such as gallium arsenide (Ga--As), the organic nonlinear optical materials 
have ultrafast response times. Thirdly, different from copper(I) chloride 
(CuCl) exhibiting a fast response and being capable of operating at room 
temperature, but being hardly fabricated in a thin film with a thickness 
of micrometer order, the organic nonlinear optical materials can be easily 
fabricated into thin films with the thickness of micrometer order. The 
organic nonlinear optical materials have the possibility of satisfying all 
requirements which have not been satisfied by the known materials, and 
thus eager investigations of such materials are continued. For example, 
the following references disclose organic nonlinear optical materials. J. 
L. Oudar, J. Chem. Phys., Vol 67, No. 2, pp 446 to 457 (1977), "Optical 
Nonlinearities of Conjugated Molecules. Stilbene Derivatives and Highly 
Polar Aromatic Compounds." This reference discloses the results of studies 
on the second- and third-order hyperpolarizabilities .beta. and .gamma. of 
4-dimethylamino-.beta.-nitrostyrene in addition to the derivatives of 
stilbene. Another reference is G. I. Stegeman and C. T. Seaton, 
Proceedings of SPIE--The International Society for Optical Engineering, 
682, pp 179 to 186 (1986), "Third-order Nonlinear Guided-Wave Optics." 
The known third-order nonlinear optical materials include the following two 
groups of materials. The first group includes .pi.-conjugated polymers, 
the typical being polydiacetylene, particularly 
2,4-hexadiyne-1,6-bis(p-toluenesulfonate) and polyacetylene. The second 
group includes low molecular weight compounds each having substituents 
disposed asymmetrically to serve as a donor and an acceptor, the typical 
being aminonitrostilbene, particularly 
4-(N,N-diethylamino)-4'-nitrostilbene. In these compounds, the 
dimethylamino and diethylamino groups serve as the donors and nitro and 
cyano groups serve as the acceptors. 
The optical nonlinearies of the .pi.-conjugated polymers are based on the 
polarization of free electrons in the valence electron band, and thus the 
.pi.-conjugated polymers have the disadvantage resembling that of 
inorganic semiconductor materials in that the response time is delayed by 
the resonance effect due to a narrow band gap. Moreover, a .pi.-conjugated 
polymer that is superior over PTS 
(2,4-hexadiyne-1,6-bis(p-toluenesulfonate)) has not yet been found. 
Since the low molecular weight compounds having asymmetrically disposed 
substituents serving as a donor and an acceptor exhibit nonlinear optical 
effects superior over that of 4-(N,N-diethylamino)-4'-nitrostilbene, it 
have been tried to introduce a longer .pi.-conjugated chain in the 
compound and to introduce a more active donor-acceptor pair in the 
compound. However, torsion of the .pi.-conjugated chain is induced as the 
length of the .pi.-conjugated chain in the compound is increased, leading 
to the result that the effective length of the .pi.-conjugated chain (i.e. 
the effective length of delocalized electrons) is decreased. On the other 
hand, when a more active donor-acceptor pair is introduced in a compound, 
spontaneous polarization is enhanced leading to the result that the 
difference in dipole moment between the ground state and the excitation 
state cannot be increased so large as expected. In addition, deterioration 
of the material due to absorption of light and delay in response time is 
accerelated. As the molecular size becomes larger and the spontaneous 
polarization of the nonlinear optical material becomes extremely enhanced, 
the solubility in a solvent or a high polymer matrix is lowered and 
crystallization property is deteriorated to have poor processibility for 
the preparation of an optical element therefrom. 
However, the compounds having substituents disposed asymmetrically and 
serving as a donor and an acceptor are important organic materials for 
eager investigations since they exhibit high speed and highly efficient 
nonlinear optical responses although they have the aforementioned 
complicated problems. 
OBJECTS AND SUMMARY OF THE INVENTION 
The object of this invention is to provide an organic nonlinear optical 
material which exhibits a high speed and highly efficient nonlinear 
optical response and which is improved in solubility not only for organic 
solvents, but also polymeric materials with excellent transparency and 
crystallization property to form a large size crystal. 
A nonlinear optical device utilizing an organic material has been known, in 
which an input power (P.sub.i) is a gating pulse light to obtain an output 
power light responsive to the wave form of the gate pulses. In this 
connection, reference should be made to E. P. Ippen and C. V. Shank, Appl. 
Phys. Lett., 26 (3), 92 (1975). 
FIG. 1 shows diagrammatically a known nonlinear optical device. 
Referring to FIG. 1, polarizers 4a and 4b are disposed with their 
polarization axes perpendicular with each other. A nonlinear refractive 
index kerr medium 1 comprises a 1 mm thick glass cell containing liquid 
carbon disulfide (CS.sub.2). With the construction shown in FIG. 1, the 
linear polarization of the input power transmitting through the polarizer 
4a is converted to the elliptical polarization due to the change in 
refractive index of the nonlinear refractive index kerr medium 1 as far as 
gate pulses P.sub.g are supplied. As the result, a portion of the input 
light can transmit through the perpendicular polarizer 4b, and thus the 
input power is optically switched by the gate light pulses. 
The relative signal intensity T of the input power takes the maximum 
intensity when the angle between the polarized gate light and the input 
power is 45.degree.. The relative signal intensity T is represented by the 
following equations (1) and (2). 
EQU T=sin.sup.2 (.DELTA..phi./2) (1) 
EQU .DELTA..phi.=2.pi.n.sub.2 LI.sub.in /.lambda. (2) 
In the equations set forth above, L is the length of the nonlinear 
refractive index kerr medium, .lambda. is the wavelength of the input 
power, I.sub.in is the intensity of the gate light, n.sub.2 is the 
nonlinear refractive index. When is sufficiently small, the equation (1) 
may be transformed as follows. 
EQU T.alpha.n.sub.2.sup.2 L.sup.2 I.sub.in.sup.2 (3) 
It will be seen from equation (3) that T is in proportion to the square of 
n.sub.2. This known CS.sub.2 optically-gated optical switch was prepared 
and tested. When .lambda. was 0.83 .mu.m, L was 1 mm and I.sub.in was 300 
MW/cm.sup.2, the relative signal intensity T was 1%. Substituting these 
results into the equations (1) and (2), the nonlinear refractive index 
n.sub.2 was calculated as follows. 
EQU n.sub.2 =8.8.times.10.sup.-14 cm.sup.2 /W (4) 
The nonlinear susceptivity .chi..sup.(3) was calculated from the value of 
n.sub.2 to find that .chi..sup.(3) =4.2.times.10.sup.-12 e.s.u. 
The known CS.sub.2 type optically-gated optical switch exhibits a high 
speed response time in the order of 1 picosecond and thus can be used for 
instantaneous photographing or in a high speed spectrographic measuring 
apparatus. However, the nonlinear refractive index of the known CS.sub.2 
optically-gated optical switch is not so high, and thus it requires that 
the gate light has an extremely high intensity. 
Accordingly, another object of this invention is to provide a high speed 
and highly efficient nonlinear optical device which is operative with a 
light having a relatively low light intensity. 
A principal feature of the organic nonlinear optical material, according to 
this invention, resides in that it has a remarkably high nonlinear 
response efficiency as a nonlinear optical material made of a low 
molecular weight compound in which substituents serving as a donor and an 
acceptor are asymmetrically disposed in the molecule. Secondly, the 
material of this invention is improved in processibility in that it can be 
dissolved in a variety of solvents and polymer matrices to form 
concentrated solutions or in that a large single crystal can be formed 
therefrom. 
The organic nonlinear optical materials provided by this invention are 
derivatives of .beta.-nitrostyrene having a substituted amino group at the 
para position as represented by the following general formula (1) or 
derivatives of p-(.beta.-nitroethylenyl)-julolidine another type of 
conjugated system stylene represented by the following general formula 
(2). 
##STR2## 
wherein each of R.sub.1 and R.sub.2 of the substituted amino group at the 
para position stands for an alkyl or hydroxyalkyl group having 2 to 6 
carbon atoms, R.sub.1 and R.sub.2 being same or different groups; and 
X.sub.1, X.sub.2, X.sub.3 and X.sub.4 each stands for at least one 
selected from the group consisting of hydrogen atom, hydroxyl group, 
halogen atoms, alkyl groups, alkoxy groups, alkyl ester groups and 
alkylamide groups, X.sub.1, X.sub.2, X.sub.3 and X.sub.4 being the same or 
different groups. 
##STR3## 
wherein X.sub.1 and X.sub.2 may be the same or different groups and each 
stands for hydrogen atom, hydroxyl group, a halogen atom, an alkyl group, 
an alkoxy group, an alkyl ester group or an alkylamide group. 
The present invention further provides a nonlinear optical device 
comprising an optical medium and an optical element including a polarizer, 
an optical resonator and a reflector, said optical medium comprising an 
organic nonlinear optical material selected from the group consisting of 
derivatives (1) of 4-(N,N-dialkyl)amino-.beta.nitrostyrene and derivatives 
(2) of p-(.beta.-nitroethylenyl)julolidine.

EXAMPLES OF THE INVENTION 
The present invention will now be described in more detail with reference 
to some examples thereof taken in conjunction with the appended drawings. 
Example 1: Synthesis of 4-(N,N-diethyl)amino-.beta.-nitrostyrene (DEANST) 
A solution was prepared by dissolving 118 grams (0.57 mol) of 
p-(N,N-diethyl)aminobenzaldehyde in 500 ml of nitromethane, and added with 
17 grams of ammonium acetate. The mixture was heated at 100.degree. C. for 
5 hours under agitation. The reaction solution was then cooled on a dry 
ice-acetone bath until crystallization had been completed. The separated 
solid (crystal) was filtered off and dried in vacuum. The obtained product 
was recrystallized from ethanol for two times. An amount of 107 grams of 
red crystals was obtained. The yield was 75%. 
The product, p-(N,N-diethyl)amino-.beta.-nitrostyrene, had a melting point 
of 95.degree. C. The .sup.1 H-NMR spectrum of the product is shown in FIG. 
2, and details of the NMR analysis are set forth below. 
Solvent: CDCl.sub.3, .delta..sub.C-H (aromatic and vinyl): 6.8 to 8.0 ppm, 
.delta..sub.C-H (methyl) 1.2 ppm, 
.delta..sub.C-H (methylene): 3.4 ppm, 
Elemental Analysis: C.sub.12 H.sub.16 N.sub.2 O.sub.2 (MW=220.27) 
Cald.: C 65.44, H 7.32, N 12.72; 
Found: C 65.57, H 7.20, N 12.92 
Example 2: Synthesis of p-(.beta.-nitroethylenyl)-julolidine (JANST) 
A solution was prepared by dissolving 60 grams (0.3 mol) of 
p-formyljulolidine in 250 ml of nitromethane, and added with 8 grams of 
ammonium acetate. The mixture was heated at 100.degree. C. for 5 hours 
under agitation. The reaction solution was then cooled on a dry 
ice-acetone bath until crystallization had been completed. The separated 
solid (crystal) was filtered off and dried in vacuum. The obtained product 
was recrystallized from methanol for two times. An amount of 50 grams of 
red crystals was obtained. The yield was 69%. 
Example 3: Synthesis of 4-(N-ethyl-N-hydroxyethyl)amino-.beta.-nitrostyrene 
(EOEANST) 
A solution was prepared by dissolving 60 grams (0.31 mol) of 
4-(N-ethyl-N-hydroxyethyl)aminobenzaldehyde in 250 ml of nitromethane, and 
added with 8 grams of ammonium acetate. The mixture was heated at 
100.degree. C. for 5 hours under agitation. The reaction solution was then 
cooled on a dry ice-acetone bath until crystallization had been completed. 
The separated solid (crystal) was filtered off and dried in vacuum. The 
obtained product was recrystallized from acetonitrile for two times. An 
amount of 42 grams of red crystals was obtained. The yield was 66%. 
Example 4: Synthesis of 4-(N,N-diethyl)amino-.beta.-nitro-2-hydroxystyrene 
(Oh-DEANST) 
A solution was prepared by dissolving 60 grams (0.31 mol) of 
2hydroxy-4-(N,N-diethyl)aminobenzaldehyde in 250 ml of nitromethane, and 
added with 8 grams of ammonium acetate. The mixture was heated at 
100.degree. C. for 5 hours under agitation. The reaction solution was then 
cooled on a dry ice-acetone bath until crystallization had been completed. 
The separated solid (crystal) was filtered off and dried in vacuum. The 
obtained product was recrystallized from acetonitrile for two times. An 
amount of 34 grams of red crystals was obtained. The yield was 53%. 
Example 5: Synthesis of 4-(N,N-diethyl)amino-.beta.-nitro-3-chlorostyrene 
A solution was prepared by dissolving 34 grams (0.16 mol) of 
3-chloro-4-(N,N-diethyl)aminobenzaldehyde in 150 ml of nitromethane, and 
added with 5 grams of ammonium acetate. The mixture was heated at 
100.degree. C. for 5 hours under agitation. The reaction solution was then 
cooled on a dry ice-acetone bath until crystallization had been completed. 
The separated solid (crystal) was filtered off and dried in vacuum. The 
obtained product was recrystallized from acetonitrile for two times. An 
amount of 16 grams of red crystal was obtained. The yield was 39%. 
Example 6: Synthesis of 4-(N,N-diethyl)amino-.beta.-nitro-2-methoxystyrene 
(MeO-DEANST) 
A solution was prepared by dissolving 30 grams (0.14 mol) of 
2-methoxy-4-(N,N-diethylamino)benzaldehyde in 150 ml of nitromethane, and 
added with 4 grams of ammonium acetate. The mixture wa heated at 
100.degree. C. for 5 hours under agitation. The reaction solution was then 
cooled on a dry ice-acetone bath until crystallization had been completed. 
The separated solid (crystal) was filtered off and dried in vacuum. The 
obtained product was recrystallized from acetonitrile for two times. An 
amount of 16 grams of red crystals was obtained. The yield was 44%. 
Example 7: Synthesis of 
4-(N,N-diethyl)amino-.beta.-nitro-2-acetyloxystyrene (AcO-DEANST) 
A solution was prepared by dissolving 23.5 grams (0.10 mol) of 
2-acetyloxy-4-(N,N-diethylamino)benzaldehyde in 100 ml of nitromethane, 
and added with 3 grams of ammonium acetate. The mixture was heated at 
100.degree. C. for 5 hours under agitation. The reaction solution was then 
cooled on a dry ice-acetone bath until crystallization had been completed. 
The separated solid (crystal) was filtered off and dried in vacuum. The 
obtained product was recrystallized from acetonitrile for two times. An 
amount of 10 grams of red crystals was obtained. The yield was 36%. 
Example 8: Synthesis of 4-(N,N-diethyl)amino-.beta.-nitro-3-methylstyrene 
(Me-DEANST) 
A solution was prepared by dissolving 30 grams (0.16 mol) of 
3-methyl-4-(N,N-diethylamino)benzaldehyde in 150 ml of nitromethane, and 
added with 5 grams of ammonium acetate. The mixture was heated at 
100.degree. C. for 5 hours under agitation. The reaction solution was then 
cooled on a dry ice-acetone bath until crystallization had been completed. 
The separated solid (crystal) was filtered off and dried in vacuum. The 
obtained product was recrystallized from acetonitrile for two times. An 
amount of 17 grams of red crystals was obtained. The yield was 45%. 
Example 9: Synthesis of 
4-(N,N-diethyl)amino-.beta.-nitro-3-acetylaminostyrene (AcNH-DEANST) 
A solution was prepared by dissolving 28 grams (0.12 mol) of 
3-acetylamino-4-(N,N-diethylamino)benzaldehyde in 120 ml of nitromethane, 
and added with 4 grams of ammonium acetate. The mixture was heated at 
100.degree. C. for 5 hours under agitation. The reaction solution was then 
cooled on a dry ice-acetone bath until crystallization had been completed. 
The separated solid (crystal) was filtered off and dried in vacuum. The 
obtained product was recrystallized from acetonitrile for two times. An 
amount of 10 grams of red crystals was obtained. The yield was 36%. 
EXAMPLE 10: Preparation of 
4-(N,N-diethyl)amino-.beta.-nitrostyrene(DEANST)/Polymethylmethacrylate(PM 
MA) (Doping) 
A 9.8 wt % solution of p-(N,N-diethyl)amino-.beta.-nitrostyrene in 
chloroform and a 18.2 wt % solution of poly(methyl methacrylate) in 
chloroform were mixed in a mixing ratio of 1:1. The mixed solution was 
coated on a glass plate by spin coating to form a 1 micron thick film of 
35 wt % p-(N,N-diethyl)amino-.beta.-nitrostyrene-polymethyl methacrylate. 
The visible light absorption spectrum of the thus produced film is shown 
in FIG. 3. 
Example 11: Preparation of 
4-(N,N-diethyl)amino-.beta.-nitrostyrene(DEANST)/(2,2,3,3-tetrafluoropropy 
lmethacrylate(Fluoro Resin)-MMA Copolymer 
A 12 wt % solution of 4-(N,N-diethyl)amino-.beta.-nitrostyrene in acetone 
and a 18 wt % solution of a 2:1 copolymer of 
2,2,3,3-tetrafluoropropylmethacrylate/ methyl methacrylate in acetone in a 
mixing ratio of 1:1. The mixed solution was coated on a glass plate by 
spin coating to form a 1.5 micron thick film of 40 wt% 
4-(N,N-diethyl)amino-.beta.-nitrostyrene(DEANST)/2,2,3,3-tetrafluoropropyl 
methacrylate-MMA copolymer. 
Example 12: Synthesis of 
4-(N-ethyl-N-acryloyloxyethyl)-amino-.beta.-nitrostyrene(DEANST-Ap)/Methyl 
Methacrylate(MMA) Copolymer 
A solution was prepared by dissolving 4.5 grams of methyl methacrylate, 
24.2 grams of 4-(N-ethyl-N-(2-acryloyloxyethyl)amino)-.beta.-nitrostyrene 
and 0.14 gram of 2,2'-azobis(2-methylpropanenitrile) in 70 ml of 
dehydrated dioxane. The solution was charged in a glass polymerization 
ample, followed by evacuation and sealing, and then the solution was 
allowed to react at 60.degree. C. for 24 hours. The reaction solution was 
poured into hexane, whereby a precipitate was separated. The precipitate 
was filtered and then rinsed with methanol. The rinsed precipitate was 
dried to obtain a product polymer. The molar fraction of the copolymer was 
0.3 for MMA and 0.7 for DEANST-Ap. The following is the structural formula 
of the copolymer. 
##STR8## 
Example 13 
A neodymium-yttrium-aluminium garnet (Nd-YAG) laser was used as the light 
source for the measurement of the third harmonic generation (THG). The 
laser had a wavelength of 1.06/.mu.m and an intensity of 50 MW/cm.sup.2. 
After filtering visible light, the laser beam was focused by a lens and 
irradiated on a sample and the light rays emitted from the sample was 
passed through a filter so that only the intensity of THG was detected 
while using an photomultiplier tube. The sample subjected to measurement 
was prepared by pulverizing a crystal of an organic nonlinear material to 
adjust the particle size to 105 to 120 microns. In order to ascertain that 
the origin of the THG was not the second-order cascading in third-order 
nonlinear optical processes, as observed in urea or 
2-methyl-4-nitroaniline (MNA), but the pure THG effect, the second 
harmonic generation (hereinafter referred to as SHG) of the same sample 
was measured. Table 1 shows the results of measurements of harmonic 
generation intensities for the organic nonlinear optical materials of this 
invention and for comparative samples. 
The relative SHG intensities compared to urea of individual samples are 
shown in the third column of Table 1, which reveal that the results are 
approximately zero (.apprxeq.0). This means that the measured results are 
not the second-order cascading in third-order nonlinear processes (w+2w 
.fwdarw.3w), but show pure third-order effects. In the fourth column of 
the Table, the relative THG intensities compared to p-nitroaniline are 
shown. As shown, the organic nonlinear optical materials of this invention 
have the relative THG intensities ranging from 300 to 720, whereas the 
results of the comparative samples or comparative examples range from 50 
to 80. The compounds used in the Comparative Examples will be set forth 
below. 
1. 4-(N,N-dimethylamino)-.beta.-nitrostyrene, DMA-NS 
2. 4-(N,N-diethylamino)-4'-nitrostilbene, DEANS 
3. 4-(.beta.-nitrophenonyl)ethylennyl-julolidine, JANS 
4. 4-(N,N-dimethyl)amino-.beta.-nitro-2-hydroxystyrene, Oh-DMANST 
TABLE 1 
______________________________________ 
Organic Relative Relative Inten- 
Nonlinear Intensity sity of THG to 
Example 
Optical of SHG to p-Nitroaniline 
No. Material Urea Ratio 
Ratio 
______________________________________ 
Working Examples of the Invention 
1 DEANST .apprxeq.0 
720 
2 JANST .apprxeq.0 
500 
3 EOEANST .apprxeq.0 
680 
4 Oh-DEANST .apprxeq.0 
630 
5 Cl-DEANST .apprxeq.0 
480 
6 MeO-DEANST .apprxeq.0 
460 
7 AcO-DEANST .apprxeq.0 
560 
8 Me-DEANST .apprxeq.0 
510 
9 Ac-NH-DEANST .apprxeq.0 
450 
10 DEANST-PMMA .apprxeq.0 
300 
(Doping) 
11 DEANST-Fluoro .apprxeq.0 
320 
Resin/MMA 
Copolymer 
12 DEANST-Ap/MMA .apprxeq.0 
400 
Copolymer 
Comparative Examples (Comparative Samples) 
1 DMA-NS .apprxeq.0 
50 
2 DEANS .apprxeq.0 
60 
3 JANS .apprxeq.0 
80 
4 Oh-DMANST .apprxeq.0 
60 
______________________________________ 
Example 14 
An optically-gated optical switch device, in which 
p-(N,N-diethyl)amino-.beta.-nitrostyrene prepared by Example 1 is used as 
an optical medium, will now be described. 
A solution prepared by dissolving p-(N,N-diethyl)-amino-.beta.-nitrostyrene 
in dimethylformamide (DMF, .beta.=37.8) was contained in a sealed glass 
vessel to be used as a nonlinear refractive index kerr medium. The other 
parts of the construction of the device were the same as shown in FIG. 2. 
FIG. 4 shows the change in relative signal intensity of the optical switch 
in terms of the change in concentration of 
4-(N,N-diethyl)amino-.beta.-nitrostyrene. As seen from FIG. 4, the 
instantaneous transmittance is increased in proportion to the second power 
of the intensity. As the concentration is increased above 23 wt%, the 
instantaneous transmittance exceeds the instantaneous transmittance of 
liquid carbon disulfide (CS.sub.2) which has been well known and used 
widely to date. The nonlinear refractive index n.sub.2 of the medium when 
the concentration of 4-(N,N-diethyl)amino-.beta.-nitrostyrene is 40 wt% 
was calculated from Equation (3) set forth hereinbefore to find that 
n.sub.2 took the following value. 
EQU n.sub.2 =1.5.times.10.sup.-13 cm.sup.2 /W(.chi..sup.(3) 
=7.3.times.10.sup.-12 e.s.u.) 
The abscissa of the graph of FIG. 5 indicates the dielectric constant of 
various solvents used for dissolving the organic nonlinear optical 
material of the invention, individual solvents being used at the same 
molar concentration. The nonlinear refractive indices n.sub.2 of the 
organic nonlinear optical materials when dissolved in individual solvents 
are plotted in the graph in terms of the dielectric constants of 
respective solvent group. The used solvents are aromatic solvents 
including nitrobenzene (A), a mixture of nitrobenzene and chlorobenzene 
(B), chlorobenzene (C) and benzene (D), and non-aromatic system solvents 
including dimethylformamide (E), acetone (F) and chloroform (G). 
As seen from FIG. 5, the value of n.sub.2 becomes larger as the dielectric 
constant of the used solvent is higher, and aromatic solvent groups are 
generally more preferable than non-aromatic system solvents. The highest 
efficiency was obtained when 4-(N,N-diethyl)amino-.beta.-nitrostyrene was 
dissolved in nitrobenzene in an amount corresponding to the highest 
dissoluble concentration. At that time, n.sub.2 took the value of 
2.2.times.10.sup.-13 cm.sup.2 /W. The value was about 2.5 times as high as 
that obtainable by using the known CS.sub.2. The found effect indicated by 
the transmittance was about six times (2.5.sup.2 =6.25) as high as that 
obtainable by the use of CS.sub.2. Search and identification of optimal 
solvents disclosed herein are the pioneer work done by us. 
Example 15 
Another embodiment of the nonlinear optical device, according to this 
invention, is shown in FIG. 6(a). A nonlinear optical kerr medium 1 
comprises a solution of 4-(N,N-diethyl)amino-.beta.-nitrostyrene dissolved 
in a solvent, and the medium is disposed between to mirrors 3a and 3b each 
of which is made of a multi-layered film of ferroelectic material 
reflecting 90% of input light and transmitting the remaining 10% of input 
light. The medium 1 and the opposing mirrors 3a and 3b constitute an 
optical resonator. 
The aforementioned device may be operated by varying the wavelength of 
input light a little or by changing the resonator length, i.e., the 
spacing between the mirrors 3a and 3b, so that the resonator is adjusted 
to resonate. 
In this device, a light ray having a wavelength of 1.064 microns from an 
Nd-YAG laser was used, and the device was actuated by changing the 
resonator length. The interrelations between the intensity of input power 
P.sub.i and the intensity of output power P.sub.t are shown in FIGS. 6(b) 
and 6(c) to find that the limiting operation and the bistable operation 
took place. In each of FIGS. 6(b) and 6(c), the abscissa indicates the 
input power P.sub.i and the ordinate indicates the output power (signal) 
P.sub.t. 
The minimum input power (P.sub.i.sup.min) can be analytically obtained from 
the following equation of: 
EQU P.sub.i.sup.min =(K.lambda.)/(n.sub.2 1); 
wherein .lambda. is the wavelength of used light, 1 is the length of the 
optical medium, k is a coefficient determined by the reflectivity of a 
mirror and the adjustment of resonator length and generally taking a value 
of about 0.001. When the effective output of the pulse oscillation 
is set to 50 mW and a semiconductor laser having an oscillation wavelength 
of 0.83 micrometer is used, the intensity of output power is calculated to 
be 6.times.10.sup.6 W/cm.sup.2 by focusing the beam diameter to 1 
micrometer. The intensity of the output power is sufficiently high as 
compared to the minimum input power P.sub.i.sup.min of the aforementioned 
nonlinear optical device at that wavelength. This nonlinear optical 
device, according to this invention, could be operated while using a 
semiconductor laser as the light source. 
The response time of 4-(N,N-diethyl)amino-.beta.-nitrostyrene of this 
invention is estimated to be approximately 10.sup.-12 second. However, the 
response time of the device is determined by the longer one of the 
response time of the used medium and the lifetime t.sub.p of a photon in 
the oscillator. The lifetime of a photon t.sub.p is calculated from the 
following equation of: 
EQU t.sub.p =-1.sub.op /(c.times.1nR); 
wherein 1.sub.op is the wavelength of the oscillator, c is the velocity of 
light, and R is the reflectivity of the mirror. The lifetime of a photon 
is calculated to be 6.times.10.sup.-11 seconds and thus t.sub.p &gt;t so that 
the lifetime of a photon determines the response time of the device. It is 
thus ascertained that the response time of this embodiment of the 
invention is shorter than 10.sup.-10 second. 
Example 16 
A phase conjugated wave generator according to this invention will now be 
described with reference to FIG. 7. The device comprises half-mirrors 5a, 
5ba, reflector 6 and a liquid optical medium 1 composed of 
4-(N,N-diethyl)amino-.beta.-nitrostyrene descibed in Example 14. The 
device is an optical alignment referred to as a degenerated four wave 
mixing. In detail, when three input light waves, i.e., a light wave 
A.sub.1, a light wave A.sub.2 incident from the direction reverse to the 
incident direction of the light wave A.sub.1 and a light wave A.sub.p 
incident obliquely, are incident upon an optical medium having a nonlinear 
optical refractive index, the fourth light wave A.sub.c where conjugated 
in regard to the light wave A.sub.p only the special phase term is 
generated. The phase-conjugated-wave attracts attention in the image 
processing technology as it may be effectively used for correction of an 
image or an effective means for real time holography. 
It was ascertained that the device of this embodiment exhibited a high 
speed response could be operated with an input power of relatively low 
intensity.