JP2651744B2 - Molecular crystal, light wavelength conversion method and module using the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a molecular crystal useful as a nonlinear optical material. The present invention also relates to a light wavelength conversion method and a light wavelength conversion module using a molecular crystal as a nonlinear optical material.

2. Description of the Related Art In recent years, attention has been focused on non-linear optical materials—materials having nonlinearity between polarization and electric field, which appear when a strong optical electric field such as laser light is applied.

Such materials are generally known as non-linear optical materials and are described in detail, for example, in:
“Nonlinear Optical Properties of
"Organic and Polymeric Materials," ACS Symposium Series 233, edited by David J. Williams (American Chemical Society, 1983), "Nonlinear Optical Properties of Organic an
d Polymeric Material "ACS SYMPOSIUM SERIES 233 Davi
d J. Williams (ed., American Chemical Society, 1983), “Organic Nonlinear Optical Materials,” edited by Masao Kato and Hachiro Nakanishi (CMC, Inc., 1985, “Nonlinear Optical Properties of Organic. "Moleculars and Crystals," Volumes 1 and 2, edited by D. S. Schmäller and J. Jis (Academic Press, 1987), "Nonlinear Op
tical Properties of Organic Molecules and Crystal
s "vol 1 and 2 edited by DSChemla and J. Zyss (Academic P
ress company).

One of the applications of the nonlinear optical material is a second harmonic generation (SHG) based on a second-order nonlinear effect and a wavelength conversion device using a sum frequency and a difference frequency. Until now, inorganic perovskites represented by lithium niobate have been practically used. However, recently, it has become known that a π-electron conjugated organic compound having an electron-donating group and an electron-withdrawing group has various properties as a nonlinear optical material, which greatly exceeds the aforementioned inorganic substances.

In order to form a higher-performance nonlinear optical material, it is necessary to arrange compounds having a high nonlinear susceptibility in a molecular state so as not to cause inversion symmetry. It is known that a compound having a long π-electron conjugate chain is useful for expressing a high nonlinear susceptibility, which is one of them, and various descriptions have been given in the above-mentioned literature. As is obvious, the absorption maximum wavelength becomes longer, for example, causing a decrease in the transmittance of blue light, which hinders the generation of blue light as a second harmonic. This also occurs in the p-nitroaniline derivative, and the effect of the transmittance at that wavelength on the efficiency of second harmonic generation is large, as described in Alignment Azema et al., Proceedings of SPII, 400 volumes, New Optical Materials (Alain Azems et al., P
roceedings of SPIE, 400 volumes, New Optical Material
s), (1983), page 186, FIG.

Therefore, the emergence of a nonlinear optical material having a high transmittance for blue light is desired. Conventionally, it has been studied to replace the carbon atom of the benzene nucleus of nitroaniline with a nitrogen atom or the like, but a satisfactory result has not always been obtained.

In addition, the present applicant has disclosed in Japanese Patent Application
-210430 and JP-A-62-210432.

Further, JP-A-62-59934, JP-A-63-23136, JP-A-63-26638, JP-B-63-31768, JP-A-63-163827
No., JP-A-63-146025, JP-A-63-85526, JP-A-63-85526
3-239427, JP-A-1-100521, JP-A-64-56425
Many materials are disclosed in, for example, JP-A-1-102529, JP-A-1-102530, JP-A-1-237625, and JP-A-1-207724.

However, as described above, in order to be useful as a second-order nonlinear optical material, performance in the molecular state alone is not sufficient, and it is essential that the molecular arrangement in the aggregated state has no inversion symmetry. is there. However, at present, it is extremely difficult to predict the molecular arrangement, and the probability of existence in all organic compounds is not high.

When used as an element for wavelength conversion,
Although it is necessary to sufficiently consider the arrangement of molecules in a crystal, many of the above-mentioned ones do not always take this point into consideration. Further, up to the present, a wavelength conversion element using an organic nonlinear optical material has not yet appeared as a commercial product.

 The reason for this can be considered, for example, as follows.

When a fiber-type optical wavelength conversion element is formed using the above-described nonlinear optical material, the crystals are not oriented in a direction that can utilize the maximum nonlinear optical constant of each material, and eventually the wavelength of the optical wavelength conversion element is used. The conversion efficiency is not very high.

Also, the wavelength conversion efficiency of an optical wavelength conversion element increases as the element lengthens, but it is difficult to obtain a uniform single crystal from the above materials, and thus it is not suitable for producing a long optical wavelength conversion element. There are also problems.

(Problems to be Solved by the Invention) Accordingly, a first object of the present invention is to provide a molecular crystal having a molecular arrangement suitable for a wavelength conversion element having excellent blue light transmittance and no inversion symmetry. . The second purpose is
It is an object of the present invention to provide a method that utilizes a response related to conversion of an optical wavelength among nonlinear responses. A third object is to provide an optical wavelength conversion module that has high wavelength conversion efficiency and can easily obtain the second harmonic in the blue region.

(Means for Solving the Problems) As a result of intensive studies, the present inventors have obtained a molecular crystal characterized by being constituted by a molecule represented by the following formula (I). Was found achievable.

Formula (I) The compound of the formula (I) can be generally synthesized by the following method. That is, it can be obtained by a reaction between 3,5-dimethyl-1H-1,2,4-triazole and 2-chloro-5-nitropyridine. Examples of the solvent include hydrocarbons such as n-hexane, ethers such as tetrahydrofuran, 1,2-dimethoxyethane, amides such as N, N-dimethylformamide, N-methylpyrrolidone, dimethylsulfoxide, and sulfolane. Sulfur compounds, nitriles such as acetonitrile, esters such as ethyl acetate and the like are used. Among them, amides, sulfur-containing compounds,
Nitriles are preferred. In the reaction, it is preferable to use a base, and as the base to be used, pyridine, triethylamine, an organic base such as 1,8-diazabicyclo [5,4,0] -7-undecene, potassium carbonate, sodium hydrogen carbonate,
Any inorganic base such as sodium hydroxide may be used.
The reaction temperature is preferably -10 ° C to 150 ° C, more preferably 20 ° C to 100 ° C.

 Next, an example will be described.

Synthesis example Synthesis of compound of formula (I) 4,5-dimethyl-1H-1,2,4-triazole 48.5 g (0.5
Mol), 79.3 g (0.5 mol) of 2-chloro-5-nitropyridine and 69.0 g (0.5 mol) of potassium carbonate were weighed into a 500 ml three-necked flask equipped with a stirrer and a thermometer, and further 250 ml.
Of dimethyl sulfoxide (DMSO) was added. After stirring at 50 ° C. for 5 hours, the mixture was allowed to cool to room temperature and poured into 500 ml of ice water. The precipitated crystals were collected by filtration and washed with water. The crystals were recrystallized with acetone while decolorizing with activated carbon.
Work-up gave the compound of formula (I). Yield 48 g (yield 43.
8%) Melting point: 147 ° C 'H-nmr (δppm): 2,433 (3Hs), 2,914 (3Hs),
8,069 (1Hd), 8,617 (1Hdd), 9,316 (1Hd) Next, the powder obtained here is single-crystallized. The method of single crystallization is a solvent evaporation method and a temperature drop method. , A solution method such as a vapor diffusion method, a melt method such as a Bridgman method, and a method by sublimation.

For single crystallization, see “Crystal Engineering Handbook (Kyoritsu Shuppan, 1971)” edited by the Editing Committee for Crystal Engineering Handbook.
This can be done by referring to the description in Chapter VII, Chapter 8.

For the wavelength conversion method, use a single crystal of appropriate size,
There are methods based on angular phase matching and temperature phase matching, and methods based on Cherenkov radiation using a waveguide.

As an example of the latter, there is a fiber optical wavelength conversion element and a light source device. In the case of the present invention, the core of the optical wavelength conversion element is a nonlinear optical material represented by the formula (I). Is used in a single crystal state, and the crystal orientation direction of (I) constituting the core is set so that the a-axis thereof extends substantially in the major axis direction of the core.
It is characterized in that a fundamental wave linearly polarized in the direction of the b-axis or c-axis of the crystal orthogonal to the b-axis is incident on the light wavelength conversion element.

As a laser light source used as a fundamental wave, for example, those shown in Table 1 can be mentioned. The wavelength of the fundamental wave is not limited at all except for the influence of the absorption of the material described above. This is because laser and optronics (La
Ser & Optronics), p. 59 (November 1987).

(Examples) Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

Example 1 The compound of the formula (I) obtained by the above method was dissolved in tetrahydrofuran, and a substantially colorless and transparent columnar crystal was obtained by a solvent evaporation method. The largest one is 3mm x 1mm x
It was 0.5 mm.

At the same time, the obtained smaller crystals were shaped (FIG. 1) and subjected to X-ray crystal structure analysis. The results are shown below.

Crystallographic data orthorhombic, space group Pna2 ₁ lattice constants a = 8.054 (1) Å b = 18.790 (1) Å c = 6.5573 (1) Å v = 992.3 (1) Å 3 number of molecules per unit cell Z = 4 The crystal structure diagram is shown in FIGS. 2a), b) and c).

It can be seen from the crystallographic data space group that the present crystal has no inversion symmetry.

Similar crystals could be obtained by using acetone instead of tetrahydrofuran as a solvent. Further, when crystals were formed by a temperature drop method using N, N-dimethylformamide, crystals of 10 mm × 5 mm × 3 mm (FIG. 3) could be obtained.

The crystal obtained here was confirmed to be a single crystal by confirming the extinction position with a polarizing microscope.

The size of the single crystal preferably has a maximum side length of at least 1 mm.

Example 2 Second harmonic generation was performed using the single crystal shown in FIG.
Actually, the apparatus shown in FIG. 4 was used.

Green light of 532 nm was observed in the form of a beam.
This indicates the possibility of phase matching, and thus indicates that the crystal of the present invention is useful as a nonlinear optical material for light wavelength conversion.

Reference Example 1 An absorption spectrum was measured to confirm blue light transmittance. The results are shown in Table 2.

In the table, Has a transmittance of 95 in a 4 × 10 ^-4 mol / l ethanol solution.
%.

Therefore, it is clear that the compound of the present invention is extremely excellent in blue light transmittance.

Reference Example 2 In actually forming a fiber-type optical wavelength conversion element, how to set the crystal orientation and what direction the polarization direction of the fundamental wave to be incident on it should be set to obtain high wavelength conversion efficiency Was unknown.

Hereinafter, a method for setting the crystal orientation of the nonlinear optical material and the linear polarization direction of the fundamental wave suitable for obtaining high wavelength conversion efficiency will be described.

The crystal of (I) has an orthorhombic system, and the point group is mm2.
Therefore, the tensor of the nonlinear optical constant is Becomes Here, d ₃₁ is the crystal axis a, as shown in FIG.
When considering the optical axes X, Y, and Z determined for b and c,
Light linearly polarized in the X direction (hereinafter referred to as X polarized light, Y, Z)
The same applies to ) Is a non-linear optical constant when the second harmonic of the Z-polarized light is taken out as a fundamental wave. Similarly, d ₃₂ is a nonlinear optical constant when the second harmonic of the Z-polarized light is taken out by entering the fundamental wave of the Y-polarized light. nonlinear optical constant, d ₃₂ is Z is incident fundamental wave of the polarization nonlinear optical constant in the case of taking out the second harmonic of the Z polarized light, d ₂₄ is Y and Z the fundamental wave of the polarized light is incident second Y-polarized light nonlinear optical constant in the case of taking out the higher harmonic wave, d ₁₅ is a nonlinear optical constant in the case of taking out the second harmonic of the X polarized light is incident fundamental wave of X and Z polarized light. Hereinafter, the magnitude of each nonlinear optical constant will be described.

Since the refractive index of the (I) is not yet clear, the following equation _{d IJK = N · f I (} 2ω) f J (ω) f K (ω) in b _IJK of b _IJK derivable nonlinear optical constant d _IJK Indicates a value. N is the number of molecules per unit volume, f (ω), f (2ω)
Are local electric field correction factors for the fundamental wave and the second harmonic, respectively.

These b _IJK values were determined by X-ray crystal structure analysis and PP
The value is based on β calculated using the P-CIMO method and Ward's formula, and the unit is [× 10 ⁻³⁰ esu].

From this table, it can be seen that d ₃₂ , d ₃₂ , d ₂₄ , and d ₁₅ can take large values. Therefore, as shown in FIG. 5, in forming the fiber type optical wavelength conversion element 10 in which the core 11 made of (I) is filled in the cladding 12, the crystal of (I) is moved along its a-axis (in the optical axis, (X axis) is oriented so as to extend in the core axis direction (this can be realized by the method described below), and the c-axis (Z-axis in the optical axis) or b If a fundamental wave linearly polarized in the direction of the axis (Y axis in the optical axis) is made incident, the above large nonlinear optical constants d ₂₄ , d ₃₂ and d ₃₃ can be used.

Embodiment 3 FIG. 6 shows an optical wavelength conversion module according to a third embodiment of the present invention. This optical wavelength conversion module includes a fiber type optical wavelength conversion element 10 and this optical wavelength conversion element 10.
And a light source device 20 for inputting a fundamental wave to the light source device.

Here, a method for producing the optical wavelength conversion element 10 will be described.

First, an air glass fiber serving as a clad is prepared. This glass fiber is made of SFS3 glass fiber as an example, and has an outer diameter of about 100 μm and a hollow diameter of 6 μm.
μm. On the other hand, (I) is a solvent 1 of acetone.
To prepare a saturated solution of (I) (at a temperature of 35 ° C.). The saturated solution of (I) is kept at a constant temperature of 35 ° C. in a thermostat, and one end of a glass fiber is introduced into the solution as shown in FIG. Then, the solution of (I) enters the glass fiber by capillary action. When stored in this state, acetone as a solvent evaporates and becomes supersaturated. And crystal nuclei are generated inside the glass hollow tube,
A single crystal grows. As a result, a single crystal state having a uniform crystal orientation over a long range of 20 mm or more can be obtained.

When (I) is filled in the glass fiber 12 in a single crystal state as described above, the crystal orientation state is such that the a-axis (the optical axis is the X-axis) extends in the core axis direction as shown in FIG. State.

After the core 11 was filled as described above, both ends of the glass fiber 12 were cut with a fiber cutter to form a light wavelength conversion element 10 having a length of 10 mm. As shown in FIG. 6, this optical wavelength conversion element 10 is combined with a light source device 20 to constitute an optical wavelength conversion module. In this embodiment, a semiconductor laser 21 is used as a light source for generating a fundamental wave, and a wavelength of 820 nm emitted therefrom.
Is converted into a parallel beam by a collimating lens 22, then passed through an anamorphic prism pair 23 and a λ / 2 plate 25, focused by a condenser lens 26 to a small beam spot, and The light is irradiated on the incident end face 10a of the wavelength conversion element 10. As a result, this fundamental wave 15
Enters the light wavelength conversion element 10. As mentioned earlier, the core
(I) constituting 11 has a crystal orientation state in which the X-axis extends in the core axis direction.
By rotating the plate 25, the fundamental wave 15 in the Z-polarized state is obtained.
Is input to the optical wavelength conversion element 10.

The fundamental wave 15 incident on the optical wavelength conversion element 10 is converted into a second harmonic 15 'having a wavelength of 1/2 (= 410 nm) by (I) constituting the core 11. This second harmonic 15 'is clad
The device travels through the element 10 by repeating total reflection between the outer surfaces of the 12 and the waveguide mode of the core of the fundamental wave 15 and the second harmonic 1
Phase matching is performed with the radiation mode to the 5 ′ cladding (so-called Cherenkov radiation).

From the emission end face 10b of the optical wavelength conversion element 10, a beam 15 ″ in which the second harmonic 15 ′ and the fundamental wave 15 are mixed is emitted. The emitted beam 15 ″ is passed through the condenser lens 27 and collected. After being illuminated, the second harmonic 15 'at 410 nm is well transmitted, while the fundamental 15 at 820 nm is passed through an absorbing bandpass filter 28 to extract only the second harmonic 15'. Using a polarizing plate or the like, it was confirmed that the second harmonic 15 'was Z-polarized light. In other words, in this example, the nonlinear optical constant d ₃₃ of the aforementioned (I) is utilized. The light intensity of the second harmonic 15 'is measured by the optical power meter 29, and
When the wavelength conversion efficiency was determined, it was about 1% in terms of 1 W.

(Effect of the Invention) As described above, the molecular crystal composed of the molecule represented by the formula (I) of the present invention has high blue light transmittance and no inversion symmetry in the molecular arrangement, so that the second-order nonlinearity is obtained. Has an optical effect. Therefore, it is a material useful for wavelength conversion using the second-order nonlinear optical effect. It is particularly useful for generating a converted wave in the blue region. Further, as described in detail, according to the optical wavelength conversion module of the present invention, the high nonlinear optical constant of (I) can be actually used in a fiber type nonlinear optical material, and the optical wavelength conversion element is sufficiently long. Since it can be formed, extremely high wavelength conversion efficiency can be realized. Also, since (I) has an absorption edge near 400 nm,
According to this light wavelength conversion module, it is also possible to efficiently extract the second harmonic in the blue region using a laser beam of about 800 nm as a fundamental wave.

In the above, the method using the Cherenkov radiation method has been described. However, the present invention is not limited to these, and waveguide-to-waveguide phase matching is also possible. The converted wavelength wave is not only limited to the second harmonic, but is also used for third harmonic, sum and difference frequency generation.

In addition, the compound can be single-crystallized, a bulk single crystal can be cut out therefrom, and a second optical harmonic can be generated by inputting a YAG laser beam. At this time, an angle phase matching is used as a phase matching method. The wavelength conversion efficiency can be increased by using these bulk single crystals not only outside the laser cavity but also inside a solid laser cavity such as an LD pumped solid laser. Furthermore, the wavelength conversion efficiency can be increased by arranging it in the resonator of the external resonator type LD.

For the above-mentioned single crystallization, a Bridgman method, a solvent evaporation method, or the like is used.

The wavelength converted wave is used not only for the second harmonic, but also for the generation of the third harmonic and the sum difference frequency.

[Brief description of the drawings]

FIG. 1 shows the shape and plane index of a single crystal prepared by a solvent evaporation method of a tetrahydrofuran solution. The size of the crystal is almost colorless and transparent, and has the property of extending in the c-axis direction. FIG. 2 shows a crystal structure diagram. a) is an a-axis stereo projection view, b) is a b-axis stereo photography view, and c) is a c-axis stereo projection view. FIG. 3 shows the structure of the crystals obtained by the temperature drop method of the N, N-dimethylformamide solution. FIG. 4 is a schematic diagram of a second harmonic generation experiment apparatus using the crystal of FIG. The numbers in the figure indicate the following. 1: Nd: YAG laser 2: Fundamental wave (λ = 1.064 µm) 2 ': Second harmonic (λ = 0.532 µm) 3: Single crystal of compound of formula (I) 4: Fundamental wave cut filter FIG. 3 is a schematic view showing a crystal orientation of a core in the light wavelength conversion element according to the present invention. FIG. 6 is an explanatory view for explaining a method for producing an optical wavelength conversion element according to the present invention. 10: Optical wavelength conversion element, 11: Core 12: Cladding, 15: Fundamental wave 15 ': Second harmonic, 20: Light source device 21: Semiconductor laser, 22: Collimating lens 23: Anamorphic prism pair 25: λ / 2 Plates 22, 27: Condensing Lens FIG. 7 shows an apparatus for growing a single crystal by a solvent evaporation method of a fiber having a single crystal of the compound of the formula (I) as a core. Saturated solution of compound of formula (I) Clad (glass fiber)

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Akinori Harada 798 Miyadai, Kaisei-cho, Ashigara-gun, Kanagawa Prefecture Fuji Photo Film Co., Ltd. Examiner Motofumi Tabe (56) References JP-A-4-77483 (JP, A)

Claims (3)

(57) [Claims] 1. A orthorhombic, characterized in that it is constituted by a molecule represented by the following formula (I), molecular crystals having Pna2 ₁ space group. Formula (I) 2. A method for converting a light wavelength using a single crystal made of a molecular crystal according to claim 1, wherein when converting the light wavelength using a laser beam and a nonlinear optical material. 3. A single-crystal non-linear optical material characterized by being constituted by a molecule represented by the formula (I) in a cladding and filled as a core, and the crystal of the optical material has an a-axis substantially a core axis. A light wavelength conversion element that is oriented so as to extend in a direction, and a light source device that causes the light wavelength conversion element to enter a fundamental wave linearly polarized in the direction of the b-axis or c-axis of the crystal orthogonal to the a-axis. Optical wavelength conversion module. (However, how to determine the crystal axis is the two-fold axis as the C axis, and the other crystal axes follow the right-handed system.)