JPH0363632A - Second order nonlinear optical element - Google Patents

Abstract

PURPOSE:To obtain the optical element which has a second order nonlinear optical effect, such as large wavelength conversion effect and a first order electrooptical effect, and has no problems in stability and workability by using a carbonyl compd. having a specific structure. CONSTITUTION:The 4'-nitrobenzylidene-3-ethyl carbonyl amino-4-methoxy aniline which is the monoclinic crystal belonging to a space group CC and point group #9 and has the structure expressed by formula I is used as the quadratic nonlinear optical element to provide at least one substantially optically smooth surface. This surface allows the incidence or propagation of the light having the vector component of the large electrical oscillation in the axial direction of the max. absorption existing within the crystal ac plane. This surface is the (010) face of the crystal and has at least one set of electrodes which impress an electric field having the vector component of the electric field within the crystal ac face.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is extremely useful in the fields of optical information processing and optical communication.
This invention relates to high-performance second-order nonlinear optical elements that apply second-order nonlinear optical effects such as wavelength conversion effects and first-order electro-optic effects (Pockels effect).

[Prior Art] A nonlinear optical effect is a general formula for electric polarization P as a response of a material when an optical electric field E (for example, a laser beam) is applied to the material, P=χ(1)E+χ(21EE+χ''EEE+...・(
Here, χ31ゝ is a linear susceptibility, and χ(n) (an integer of n≧2) is a nonlinear susceptibility), which refers to the effect expressed by a second or higher order term of E.

The effect due to the second term is called a second-order nonlinear optical effect.

Second Harmonic G
Examples include wavelength conversion effects such as energy generation: 5) IG), parametric oscillation, and primary electro-optic effects (Pockels effect).

Applying these effects, the second harmonic generation element (SHG
It is possible to realize industrially important second-order nonlinear optical elements, such as wavelength conversion elements such as optical elements), parametric oscillators, and electro-optical elements such as optical switches and optical modulators.

The second-order nonlinear optical element is made of an optical medium that has a second-order nonlinear optical effect, and has a substantially optically smooth surface through which laser light or the like is incident or propagated.

In the case of an electro-optical element, it further includes an electrode for applying an electric field.

If the element is made with a medium that has higher performance, that is, a larger nonlinear optical effect, it will be possible to: ■reduce the light source output, ■reduce the element size, or ■reduce the applied voltage, and furthermore, ■reduce the price. It is advantageous to aim for it.

Secondary nonlinear optical media have strong anisotropy (
dependence on optical electric field and external electric field orientation). Therefore, there is a need for an element structure that effectively exhibits the effect.

Conventional nonlinear optical media include ■Lithium niobate (LN) and potassium dihydrogen phosphate (
Inorganic ferroelectric crystals such as KDP), ■2-methyl-4-
Organic crystals such as nitroaniline (MNA), ■ Composite systems of organic molecules and polymers with second-order optical nonlinearity (
Bold polymer type (described later), etc.

Nonlinear optical media were the earliest to be considered for use in this field, and their processing methods and other element fabrication techniques are the most well-known. However, its second-order nonlinear optical effect is not large. Therefore, the performance of a secondary nonlinear optical element made of this nonlinear optical medium is generally unsatisfactory, and there are problems in that it is large and expensive.

Among the nonlinear optical media■, L has the highest performance and is widely used.
N-crystal has a further problem in that its performance is degraded by light, which is undesirable for industrial use.

Nonlinear optical media (■) have attracted attention for their large optical nonlinearity and high-speed response of organic molecules caused by intramolecular π-electron fluctuations, as well as their high laser resistance, and in recent years, they have been developed to surpass inorganic ferroelectric nonlinear optical media (■). This is something that is being actively considered.

For example, 2-methyl-4-nitroaniline (MNA) crystal has been reported to have a large nonlinear optical effect that exceeds that of the inorganic ferroelectric nonlinear optical medium LN crystal, which is the best among nonlinear optical media. For example, J.
Appl, Phys, 50 (4), 25
23 (1979), J. Cherrr, Phy.
s, 75 (3), 1509 (1981)
).

However, the magnitude of the nonlinear optical effect of the MNA crystal is not so large compared to that of the LN crystal.

It also has properties that are not desirable in practice, such as water solubility and sublimability at room temperature.

Nonlinear optical medium (2) is a material that focuses on the good processing and applicability of polymers, but the magnitude of the second-order nonlinear optical effect is much smaller than that of medium (2), and is currently smaller than that of LN crystal. This is because the presence of polymers reduces the concentration of components with nonlinear optical effects, and
Polling processing (This method is explained in MR8 Conference Material V.
o1.109゜'Non1inear 0ptical
Propertteg of Po17iers'.

Ed, b7^, J, Heeger ef al,
, 1988. This is because there is a limit to the effective orientation of a component having a nonlinear optical effect in methods such as those described in detail in pp. 19). In addition, this type of medium has a problem in terms of stability over time, in which performance deteriorates due to orientation relaxation of a component (dye) having a nonlinear optical effect.

As described above, all three types of nonlinear optical media according to the prior art are unsatisfactory in realizing a high-performance second-order nonlinear optical element.

Below, examples of secondary nonlinear optical elements according to the prior art will be given and the current status will be explained.

Figure 5a shows a Cerenkov type SH having a proton exchange waveguide 51 using an LN crystal disclosed in JP-A-61-18934.
This is a conventional example of the G element 52. Semiconductor laser light 54 that is incident on the proton exchange waveguide 51 and focused by the condensing lens 53 is converted into a second harmonic wave 55 along with the guided wave, and is radiated out of the waveguide. Using a specially made high-power semiconductor laser beam (fundamental wave wavelength 840 nm) of 80 mW as the light source 56, the output of the second harmonic (420 nm) is approximately 1 at maximum.
mW is reported. In practice, an output of several mW or more is required, but an output higher than this cannot be expected using an LN crystal that does not have a large wavelength conversion effect2. Further, FIG. 5 shows a schematic diagram of a Cerenkov type SHG element 52, and the element size is also large, with a length of 6 mm.

In order to construct an SHG element capable of outputting several mW or more using a generally available semiconductor laser with an output of about 40 mW as a light source, a higher performance nonlinear optical medium is required.

FIG. 6 shows Wave He by Auracher et al.
ctron.

4, 129 (1980), this is a conventional example of a branching interference type optical modulator using an LN crystal as a nonlinear optical medium.

Input light 61 guided from one side is split at the Y-shaped branch, and the phases of guided light 62 and guided light 63 are shifted due to the refractive index change caused by the electric field applied between the parallel electrodes, and when they are combined again. This phase shift causes interference and modulation, resulting in output light 64. Modulation is therefore done by the electric field between the electrodes.

In this conventional example, it is reported that when the guided light wavelength is 633 nm, the driving voltage is about 4 .ANG., and the band is 1.3 GHz. The length of the branched portion is as long as 6 mm, and the demand for increasing the degree of integration cannot be met by using an LN crystal that does not have a large primary electro-optic effect.

FIG. 7 is a schematic diagram of an intensity modulation electro-optic element (light intensity modulator) for explaining the operating principle of an electro-optical element that applies a -order electro-optic effect (Pockels effect).

Between the polarizer 72 and the analyzer 73, which are orthogonal to each other, electrodes 74 and 7 are connected.
A birefringent electro-optical element 71 having a diameter of 5 is disposed. For simplicity, the electro-optic element 71 is arranged so that its optical axis (y and 2) is at an angle of 45° with respect to the polarizer and analyzer.

When linearly polarized light enters the electro-optical element 71, the incident light wave is decomposed into ordinary light (y) and extraordinary light (z) components, each of which propagates independently in the crystal, but due to birefringence inherent in the crystal and the electric field applied between the electrodes. Their phases are shifted from each other due to electric field-induced birefringence. In general, only the azimuth component of the polarization axis of the analyzer is emitted from the elliptically polarized light, and therefore the light intensity changes depending on the intensity of the applied electric field.

In the end, the intensity relationship between the input light (I,) and the output light (I) is expressed as: I=Io peak 2 (φ/2) (1). However, φ is the phase difference between the ordinary light and the extraordinary light. Here, . Here, l is the optical path length, λ is the wavelength of light, δ is the phase difference due to birefringence inherent in the crystal, n is the refractive index, r eff is the electro-optic coefficient, and E is the electric field intensity.

In the above equation, 1/2

It can be seen that the intensity can be changed (intensity modulation) within the range of .

The electric field El, that is, the magnitude of the voltage V required to shift φ by π, is an important parameter called the half-wave voltage, and is expressed as follows. Here, d is the distance between the electrodes.

From the above, excluding parts related to the element configuration such as the distance between electrodes, n is determined by the characteristics specific to the electro-optic crystal.
It can be seen that the quantity expressed by 3re11/2 is an important quantity that determines the performance of the element. This quantity is called the figure of merit. The larger this value is, the more it becomes possible to construct an element that can be driven at a lower voltage or is more compact.

Until now, L has been considered exclusively for the production of electro-optical elements.
The figure of merit (n3r33/2) of N crystal is 120 pm/
It is not large, about V. Despite great expectations, truly practical optical modulators and optical switches have not yet appeared because a nonlinear optical medium with a large figure of merit has not been found.

For nonlinear optical media and electro-optical elements other than those described above, see SpringerProceed.
ings in Ph7sics 18. “Ele
clro-optic and Phototefrac
tive Materiala, I! d, b7P
, Gunter, 198? , pp2-17.
pp150-158. pp159-164 or
For details, see Hiroshi Nishihara et al., "Optical Integrated Circuits," Ohmsha, February 25, 1985.

Against the background of the state of the prior art as explained above,
In order to construct a truly practical second-order nonlinear optical element, the current situation is that active research is being conducted mainly on organic crystals to develop a nonlinear optical medium with higher performance.

[Problem to be solved by the invention] The problem of the present invention is to find a high-performance nonlinear optical medium that has a large second-order nonlinear optical effect and has no problems with stability and processability, and to effectively utilize the nonlinear optical effect. It is an object of the present invention to provide a high-performance second-order nonlinear optical element that can be suitably used as a wavelength conversion element, an electro-optic element, etc. by adding a component shape that can be modified.

[Means for Solving the Problems] In order to solve the above problems, the present invention has the following configuration.

"4'-nitrobenzylidene- represented by structural formula (1)
A second-order nonlinear optical element comprising a monoclinic crystal of 3-ethylcarbonylamino-4-methoxyaniline belonging to space group Cc and point group #9, and having at least one substantially optically smooth surface.

(I)'' The present inventors previously discovered in JP-A No. 63-113 that crystals of the benzylidene aniline derivatives shown below are useful as materials with large second-order nonlinear optical effects and excellent stability.
It is disclosed in Publication No. 429.

(D represents a donor substituent and A represents an acceptor substituent. R1 to R9 are hydrogen or any substituent, and at least one is a molecular orientation control group.) However, in this disclosure, The SHG powder method used (J, A
ppl, Ph7s, 39.3798 (1966)
(for details), it is not possible to know the magnitude of the second-order nonlinear susceptibility (third-order tensor), anisotropy, and its relationship with the crystal structure, etc., which are necessary when producing a second-order nonlinear optical element.

Therefore, the present inventors synthesized various benzylidene aniline derivatives, grew high-quality single crystals, performed nonlinear optical and I worked hard to find a medium.

As a result, in the SHG powder method, the SHG relative strength ratio was 3.
It was found that the 4'-nitrobenzylidene-3-ethylcarbonylamino-4-methoxyaniline (hereinafter abbreviated as MNBAEi) crystal, which did not show a very high performance of about 0, had an extremely large second-order nonlinear susceptibility χ (21). This was discovered by measuring the SHG intensity using the Maker fringe method and measuring the first-order electro-optic effect, as described below.

Generally, organic molecules have a strong tendency to exhibit crystal polymorphism, but in MNBAE t, a solution method using acetone or DMF as a solvent, a gas phase method,
Crystals can be grown relatively easily by tactile methods,
No matter which of these methods is used to grow a crystal, a crystal with a specific structure can be obtained, and in the present invention, the nonlinear optical medium is required to have this specific structure.

The specific structure is the space group Cc (Herman-Ma
abbreviation for uguin), point group #9 (interna
tional Tables for Cr7atal
loggraph7. Vat, A″5pace Gt
oupS7mmeter7, Ed, J Theo Ha
hn, The International Union
of Cr7sta11ograpb7(19g3
), that is, it has a lattice constant etc. shown below at 23±1°C.

a = 8.276 (2) Enter β = 115
．． 27 (1) 'b = 28.057 (3) A
z = 4c = 7.651 (1) Figure 1 shows the structure of this crystal. Hereinafter, this structure will be referred to as α type.

However, as is generally accepted in the field of crystals, the values of the lattice constants etc. listed here vary depending on the measurement conditions, and if there is a deviation of about 1%, the space group Cc, point group It can be said that it belongs to #9.

Examples of such deviations in lattice constants are as follows. Regarding the structure of MNA crystal, it is agreed that it is a monoclinic crystal belonging to space group: Cc and point material: #9, but regarding the lattice constant, B, F. Levine et al. (J, ^pp1. Ph7g , 5G (4), 25
23 (1979)), a=11.17 A, b=11.60 A, C=
7.9OA.

β=137° Also, G, F, 1. ipseomb et al. (J, Ch.
ew, Ph7g, 75 (3), 1509 (1,9
81), a=11.57±0.01 people, b=11.62
±0.01A.

c=8.22±0.01A, β=139.1
Slightly different numbers have been reported by different measurers, such as 8±0.02°.

If the b-axis of an α-type MNBAEt crystal is the y-direction, 1, the axis of maximum absorption in the ac-plane is the
paint group, m) can be expressed by the following equation (4).

(4) Here, when considering the magnitude of each susceptibility from the crystal structure, χ■□1 is the maximum.

Therefore, regarding light, (a) It is preferable to make light that has an electric vibration vector component in the ac plane of the crystal enter or propagate because a large nonlinear optical effect can be obtained. (b) It has a large electric vibration vector component in the X direction. It was predicted that it would be most preferable to allow light to enter or propagate.

Also, if the direction of incidence or propagation of light is
When , regarding the electric field, (1) Applying an electric field with a large electric field vector component in the ac plane of the crystal is preferable because a large first-order electro-optic effect can be obtained. (1) Applying an electric field with a large electric field vector component in the X direction. The most favorable result was expected.

The present inventors have developed the Maker fringe method (
Electron, Left, Vol, 23. N
o, 11. 595 (1987)) by SH
From measurement of G intensity and electric field orientation dependence in optical modulators 2
Understand the relationship between the magnitude of second-order nonlinear susceptibility, anisotropy, and crystal structure, verify the above predictions, and design elements such as the orientation of a substantially optically smooth surface that a second-order nonlinear optical element should have. This has led to the present invention.

In the present invention, in SHG intensity measurement, χ(2)1. , the conventional maximum χ(2)
1. Approximately 3°9X, which is approximately three times that of MNA crystals with
I got 10”” esu. This value means that at a constant incident light density, the SHG element using MNBAEt has a wavelength conversion ability of about 11 times that of the 5) IG element using MNA per unit length. Similarly, when compared with LN, it has an extraordinary wavelength conversion ability of about 560 times.

Therefore, when constructing an SHG element with a certain wavelength conversion efficiency, use of the MNBAEt crystal allows a low-output light source and a compact design.

Furthermore, when an optical modulator was constructed and the figure of merit Ink3rt□-n33r31+/2 of the -order electro-optic effect was determined, it was found that the MNA crystal, which has the conventional maximum figure of merit, is about 2.3
We obtained an extremely large value of about 700 pm/V, which is twice as large. Comparing this to 120pm/V for LN crystal, it is approximately 6
It was a huge value.

Therefore, when using MNBAEt crystal, compactness,
A high-performance electro-optical element with low driving voltage can be constructed.

A substantially optically smooth surface that allows light having an electric vibration vector component to enter or propagate in the directions described in (a) and (b) is provided to an MNBAEt crystal with a specific structure, and a second-order nonlinear optical element is created. If this is the case, it will be an extremely useful device that exhibits a large nonlinear optical effect that has never been seen before.

Such a substantially optically smooth surface may be obtained by using a naturally grown surface by solution growth, but it may also be obtained by a solution method or a melting method between optically smooth substrates such as glass (inside a mold). It may be a growth surface that can be obtained by crystal growth, or it may be a surface that is provided by post-processing of the crystal, such as cutting, creasing, or polishing.

Here, the substantially optically smooth surface is a low-scattering surface, and may be a flat surface, a curved surface, or, in special cases, a grating-like surface having regular irregularities. That is, any surface may be used as long as it allows light to enter, propagate, reflect, or emit light with low loss.

The (010) plane (the widest plane) which is the natural growth plane is (a)
, is preferable as a substantially optically smooth surface that satisfies the conditions described in (a).

Furthermore, as shown in Figure 2, when grown in solution between substrates, a thin film single crystal with the (010) plane as a substantially optically smooth surface is obtained, which guides light in the TE mode. This is particularly useful when constructing devices that function when

(1) A secondary nonlinear optical element configured to include at least one set of electrodes for applying an electric field having an electric field vector component in the direction described in (1) is extremely useful as an electro-optical element.

In this case, the (010) plane may be used as the plane on which the electrode is provided. Example 3 is an example of this, in which the (010) plane or (0-10) thin film single crystal obtained by solution growth between substrates is
) is a preferred embodiment of the device having at least one set of electrodes on the surface. In this case, since the effect is greatly expressed in the TE waveguide mode, it is desirable to further process the material into a three-dimensional waveguide type (channel type) electro-optical element.

In the configuration example of the SHG element of Example 4, YAG laser light (1064 nm) is guided in the TE mode in a tantalum pentoxide (tatos) passive waveguide provided on the (010) plane, that is, crystalline When the electric vibration vector component is incident in a plane perpendicular to the axis, S
HG light (532 nm) was emitted.

In this example, the substantially optically smooth surface is the (010) plane in contact with the passive waveguide, and an evanescent wave with an electrical oscillation vector in a plane perpendicular to the crystal axis bleeds from the waveguide into the crystal. This element functions as follows. Therefore,
An example of an element having only one substantially optically smooth surface.

As explained above, MN having the specific structure of the present invention
When a BAEt crystal is provided with an element structure capable of producing a large nonlinear optical effect, a second-order nonlinear optical element with higher performance than ever before can be constructed, which is extremely useful industrially.

Note that the method for growing the MNBAEt crystal with a specific structure is not limited to the method described above. Further, the second-order nonlinear optical element made of MNBAEt crystal with a specific structure and having at least one substantially optically smooth surface is not limited to the above-mentioned element. Various elements can be configured. Furthermore, the element referred to here may be composed of a single crystal, or may have electrodes, a protective film, a waveguide layer, an anti-reflection coating layer, etc., or may be constructed on a substrate other than glass. , or may be a part of the whole. In short, if it is made of MNBAEt crystal with a specific structure and has at least one substantially optically smooth surface,
It is capable of functioning as a nonlinear optical element and is within the scope of the present invention.

Hereinafter, the present invention will be explained in more detail with reference to Examples, but the effectiveness of the present invention is not limited in any way by these Examples.

[Examples] Example 1 (i) Synthesis, purification, and evaluation of physical properties and stability of 4-nitrobenzylidene-3-ethylcarbonylamino-4-methoxyaniline (MNBAEt).

MNBAEt was synthesized by the following route (Synthesis 1 to Synthesis 3).

(Synthesis 1) 3 equipped with a reflux condenser, nitrogen inlet pipe and dropping funnel
Into a 00 ml Mitsuro flask, add about 100 ml of 1,
2-dichloroethane, 8.40 g (5 Q mmol) of 2-amino-4-nitroanisole, and 1.2 ml of pyridine were added, and the mixture was stirred with a magnetic stirrer at room temperature for about 10 minutes under a nitrogen atmosphere.

The above solution was cooled with ice water, and about 100 ml of 1
, 4.63 g (50 mmol
) of chloroacetyl chloride was added using a dropping funnel over about 15 minutes.

The reaction solution, which was initially orange, immediately turned pale yellow.

The temperature of the reaction solution was raised to room temperature and further to about 50° C., and stirring was continued for about 5 hours.

After confirming the completion of the reaction by thin layer chromatography, the reaction solution was transferred to a separating funnel, and about 150 mm1 of chloroform was added thereto to completely dissolve the precipitated crystals, washed thoroughly with about 1 N diluted hydrochloric acid, and then diluted with anhydrous sulfuric acid. Dry with sodium,
Thereafter, the solvent was distilled off using a rotary evaporator to obtain pale yellow crude crystals, which were dried under vacuum heat. 10.26g (yield 91.5%) of the target product could be obtained.

(Synthesis 2) After dissolving 20.2 g (90 mmol) of stannous chloride dihydrate in about 30 cc of concentrated hydrochloric acid, 6.73 g (30 mmol) of 4-nitro-
2-Ethylcarbonylaminoanisole was added and stirred at room temperature.

The yellow reaction solution turned pale pink as it generated heat.

After about 2 hours, the pale pink reaction solution was filtered and washed with cold water to obtain white crude crystals, which were vacuum dried. 2.48g (yield 35.8%) of the target product was obtained.

(Synthesis 3) 200m equipped with reflux condenser and magnetic stirrer
1. 2. ．． Add 31 g (10 mmol) of 3-ethylcarbonylamino-4-methoxyaniline hydrochloride and 1.56 g (10 mmol, 1) of p-nitrobenzaldehyde, and add about 60 ml of ethanol/
Water (2:1) was used as a reaction solvent, and the mixture was stirred at room temperature for about 10 minutes.

Next, gradually add sodium hydroxide aqueous solution to pH 6~
7, the reaction solution turned yellowish brown. Stirring was then continued at room temperature for about 3 hours.

After confirming completion of the reaction by thin layer chromatography using chloroform as a developing solvent, stirring was stopped.

The precipitated crude product was collected by filtration and washed with cold ethanol.

The yellow-brown crude product obtained here was dissolved in acetone, insoluble materials were removed, and the solvent was removed using a rotary evaporator to obtain yellow-orange crude crystals.

When the crude crystals were recrystallized with a mixed solvent of chloroform/benzene (1/3), yellow crystals were obtained, which were collected by filtration and dried under vacuum. The target product was 2°0.8g (yield 63.
5%) was obtained.

Crystals of MNBAEt are grown by a solution method using various solvents, a melting method, or a gas phase method, and the crystals are pulverized to a size of 100μ.
Particles with a particle size of about m are processed using the SHG powder method (J, Appl, P
b7s,, 39. 3798 (1966)) sample.

The light source used for the measurement was a Nd:YAG laser (wavelength 1.
06 μm), and the irradiation conditions were a pulse width of 200 ns, a repetition rate of 10 Hz, and a peak power density of about 30 MW/al. 5) (G relative strength (urea ratio) is shown in Table 1.

As is clear from the results, MNBAEt has crystal polymorphism (
There is no polymorphism. That is, crystals with the same structure can be stably grown regardless of the crystal growth method.

Next, the physical properties and stability of the MNBAEt crystal were investigated.

The melting point was approximately 169°C.

Yellow angular crystals (5 x 4 x 3 mm) that have grown into relatively large sizes.
3) was placed in a glass sample tube at room temperature, one end was tightly capped, and the other end was left open.

Even after 30 days had passed, there was no change in weight in either case, and the glass wall of the sealed one did not become discolored.

Therefore, it was found that this crystal has no sublimation property at room temperature. The crystals after standing were crushed and the SHG strength was examined using a powder method, but the measured values were the same as before standing. Therefore, this crystal was found to be stable at room temperature in terms of optical nonlinearity.

The above-mentioned sublimation and optical nonlinearity stability checks performed at room temperature were performed at 80°C. There was no change in weight or powder method SHG strength, and it was found that this crystal was stable even at 80°C.

The weighed crystals were placed in a vacuum ampoule tube, connected to a vacuum line, continued to be evacuated, and the sublimation property was examined at room temperature and under a pressure of about 10-' Torr. After 24 hours, there was no change in the weight of the crystal, and it was found that there was no sublimation property even at room temperature and under high vacuum.

The crystals and water were placed in a glass sample tube, sealed tightly, and left at room temperature. The water did not become colored even after 30 days, indicating that the crystals had no solubility in water.

(ii) X-ray structural analysis and optical properties of MNBAE crystals.

We analyzed the structure of MNBAEt crystal, which is useful as a secondary nonlinear optical medium, and identified its structure.

Among the crystals obtained in (1) above, the single crystal sample was obtained as a yellow angular crystal by slow evaporation from an acetone solution (0,450xO,450x
o, 370 mm') was used.

The measurement was carried out using a Rigaku AFC6R diffractometer using the MoKa line (
λ = 1.71069 people).

The results are shown in Table 2 and Figures 1a and 1s.

In FIG. 1, OA, OB, and QC represent unit cells and correspond to crystal axes a, b, and c, respectively.

A monoclinic crystal belongs to what is called an optically biaxial crystal in terms of crystal optics, and it is generally known that all three principal refractive index values are different, and this was also confirmed in this example.

As a result of observing an interference image using a conoscope from a direction perpendicular to the flat plate surface of the prism-shaped article, it was found that one of the principal refractive index directions (optical elastic axis) was in the b-axis direction. Therefore, the other two optoelastic axes are in the ac plane.

MNBAEt crystal useful as a secondary nonlinear optical medium,
It was identified as having the structure and optical properties shown above.

(i) Second-order nonlinear susceptibility (χ′
measurement).

The magnitude of the second-order nonlinear optical effect and its anisotropy of the MNBAEt crystal with this specific structure were determined using a YAG laser (1064n
Maker fringe method (1) using a tabular single crystal according to m)
! 1ectton, Le order, 23(11), 5
95 (1987)) by measuring the orientation dependence of SHG intensity.

About 100 μm thick (0
10) Set the MNBAEt flat single crystal with the widest plane on a goniometer, and measure the rotation angle θ around the axis of linearly polarized laser light (YAG laser, 101064n) incident on the (010) plane and the laser beam incidence angle on the (010) plane. The relationship between observed SHG intensity and φ was determined.

The θ dependence was approximately a cos2θ function with a period of π, and the φ dependence was a so-called fringe pattern.

When the electric vibration vector (polarization plane) of the incident light is made to match the X-axis direction in the (010) plane, the maximum nonlinear susceptibility (χ(
2) The values of , , ) were calculated.

This χ(2)1. The value of
The value of A crystal is 1.2X10"" esu, MNBA
Using 1.7 and 2.6 as the principal refractive index values for the fundamental wave and second harmonic in the X-axis direction of the Et crystal, approximately 3
．． The result was an extremely large value of 9X10-6esu.

Table 1. SHG relative intensity of MNBkEt crystal Table 2. We have developed an MNBAEt crystal with a specific structure.

Example 2 Preparation of MNBAEt thin film single crystal having an optically smooth (010) plane.

N,N-dimethylformamide solution of MNBAEt (
(approximately 9 wt%) was prepared, sandwiched between two optically polished glass substrates, and left in a constant temperature bath at 53°C. Approximately 1
On day 0, due to slow evaporation, up to 20
A thin film single crystal measuring mm×4 mm×5 μm was grown.

According to analysis of the X-ray diffraction pattern, the smooth plane grown parallel to the glass substrate was a (010) plane. The morphology of the crystal is shown in Figure 2.

Note that by observing the interference image using a conoscope, it was confirmed that one of the optical elastic axes was perpendicular to a plane parallel to the glass substrate.

Next, one side of the glass substrate was peeled off to expose the smooth (010) surface. MNBAET with glass substrate on one side
The thin film single crystal can be used as it is as a two-dimensional waveguide type nonlinear optical element.

Linearly polarized He-Ne laser light was guided in TE mode. In this case, the substantially optically smooth surfaces for waveguiding are the (010) and (0-10) surfaces. It was confirmed that the maximum nonlinear optical coefficient of MNBAEt could be effectively utilized due to the good waveguiding state.

Example 3 Fabrication of optical modulator using electro-optic element and measurement of first-order electro-optic effect MNBAEt thin film single crystal (10 mm x 3
(010) side (or its back side (0
An optical modulator is constructed with an electro-optic element using a substantially optically smooth surface (-10) plane (these two planes are equivalent), and the magnitude of the -order electro-optic effect and the electric field orientation dependence are Examined.

The configuration of the optical modulator is shown in FIG. 3a.

An MNBAEt thin film single crystal 31 was bonded to a glass substrate 32 with a smooth (O-10) surface, and aluminum parallel electrodes 34 (thickness approximately 2000 mm, distance between electrodes 5 μm 1 Electrode width 1
mm, length 40 mm) was pressed onto the (010) plane to obtain an electro-optical element 3.

Linearly polarized He-Ne laser light (633 nm) that has passed through a polarizer is incident on the element from the (010) plane direction so that the electric vibration vector matches the aC plane, and is passed through the Soleil Babinet compensator 35 to generate polarized light due to inherent birefringence. eliminate the effect,
The intensity of the output light coming out through the analyzer 39 orthogonal to the polarizer 38 was detected by the photodetector 36. A sine wave voltage of 1QkHz was applied to the parallel electrodes 34 by a function generator 37. Figure 3 shows an example of the relationship between a signal drawn on an oscilloscope and a sine wave voltage.

The glass substrate 33 provided with the parallel electrodes 34 is (010)
On the plane, the electric field was rotated by 15 degrees from the direction corresponding to the X-axis direction, and the dependence of the magnitude of the -order electro-optic effect on the electric field direction was measured. The maximum first-order electro-optic effect was obtained when the electric field orientation coincided with the X-axis orientation.

Using the relationships of equations (1), ■, and (3) shown in the prior art section, the figure of merit 1 n+ 'rxt-ns 3r3□l
/2 was calculated. Approximately 6 times that of LN crystal, approximately 2.3 times that of MNA crystal (n l3r11/2), and approximately 700 pm/V when the figure of merit of LN crystal is 120 pm/V.
The result was an extremely large value of V.

Example 4 An example of the configuration of an SHG element using a flat MNBAEt single crystal having a specific structure of the present invention will be shown.

The structure of the 5t (G element is shown in FIG. 4. Tantalum pentoxide (Ta
2 (1 g) was deposited to a thickness of approximately 8000 to 9000 Å to form a passive planar waveguide 42. In this configuration, the refractive index of tantalum pentoxide is high at the YAG laser beam (1064 nm) wavelength, and the refractive index of the MNBAEt tabular single crystal is high at the second harmonic wavelength.

Linearly polarized light from a YAG laser was incident on this passive planar waveguide 42 such that its plane of polarization was parallel to the (010) plane. That is, the wave was guided in TE mode. The incident YAG laser light density was approximately IMW/d.

It was confirmed that green SHG light was emitted from the MNBAEt crystal side. The SHG light with the maximum intensity was observed when the electric vibration vector of the YAG laser light coincided with the X axis.

[Effects of the Invention] According to the present invention, the crystal structure 4 has an unprecedented large wavelength conversion effect, a second-order nonlinear optical effect such as a first-order electro-optic effect, and has a specific crystal structure without problems in stability and workability. ′−
Nitrobenzylidene-3-ethylcarbonylamino-4
- By applying methoxyaniline crystals, it is possible to provide extremely high-performance secondary nonlinear optical elements that are suitably used as wavelength conversion elements such as SHG elements, electro-optical elements such as optical switches and optical modulators, This can be greatly utilized in the fields of optical information processing and optical communications.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the structure of MNBAEt crystal. FIG. 2 is a drawing showing the morphology of an MNBAEt thin film single crystal with a smooth (010) plane. FIG. 3a is a drawing showing the structure of an optical modulator using an electro-optical element made of MNBAEt thin film single crystal. FIG. 3 is a diagram showing the relationship between the applied voltage drawn on the oscilloscope and the detected output voltage of the photodetector corresponding to the output light intensity. FIG. 4 is a drawing showing the configuration of the SHG element. FIG. 5 is a drawing showing the configuration of a conventional Cerenkov type SHG element using an LN crystal. FIG. 6 is a drawing showing the configuration of a conventional branching interference type optical modulator using an LN crystal. FIG. 7 is a diagram illustrating the operating principle of an electro-optical element that applies the -order electro-optic effect. 3: 31: 32゜34: 35: 36 = Electro-optical element, MNBAET thin film single crystal, 33 glass substrate, aluminum parallel electrode, Soleil Babinet compensation plate, photodetector 7 41 2 1 2 5 1 4 1 3 Function Generator MNBAET flat single crystal, passive planar waveguide, proton exchange waveguide Cerenkov type SHG element second harmonic input light, 62, 63: guided light, output light, electro-optical element, 72: polarizer, analyzer, ? 4.75: Electrode,

Claims (1)

[Scope of Claims] (1) Monoclinic system belonging to space group Cc, point group #9 of 4'-nitrobenzylidene-3-ethylcarbonylamino-4-methoxyaniline represented by structural formula (1) A second-order nonlinear optical element consisting of a crystal and having at least one substantially optically smooth surface. ▲Mathematical formulas, chemical formulas, tables, etc.▼(1) (2) At least one substantially optically smooth surface is
2. The second-order nonlinear optical element according to claim 1, wherein the surface is a surface on which light having an electric vibration vector component enters or propagates within the crystal ac plane. (3) The at least one substantially optically smooth surface is a surface through which light having a large electric vibration vector component in the axial direction of maximum absorption in the ac plane of the crystal is incident or propagated. A second-order nonlinear optical element. (4) at least one substantially optically smooth surface,
The second-order nonlinear optical element according to claim 1, which is a (010) plane of a crystal. (5) The second-order nonlinear optical element according to claim (1), further comprising at least one set of electrodes for applying an electric field having an electric field vector component in the ac plane of the crystal. (6) The second-order nonlinear optical element according to claim (5), further comprising at least one set of electrodes for applying an electric field having a large electric field vector component in the axis direction of maximum absorption in the crystal ac plane. (7) 2 according to claim (4), wherein at least one set of electrodes is provided on the substantially optically smooth (010) plane of the crystal.
order nonlinear optical element.