JPH11288013A - Wavelength conversion element using stacked structure of polymer thin film generating optical second harmonics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength conversion element in which high technology is not required for its generation and the aging deterioration of conversion efficiency owing to the aged orientation relaxation of a molecule does not occur. SOLUTION: A wavelength conversion element contains an inverted domain stacked body obtained by stacking main chain-type optical non-linear polymer thin films generating optical second harmonics. When a main chain-type optical non-linear polymer material mechanically pulled out in one direction and a band-like film 10 is generated, the polarized axes of molecules constituting the film 10 are highly oriented in a lead-out direction. The film 10 is cut into a plurality of film pieces 101-106 and the film pieces 101-106 are stacked so that the polarization directions of the adjacent film pieces become opposite. Then, the inverted domain stacked body 12 is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic nonlinear optical component using a polymer compound having a large nonlinear optical characteristic and / or a composition thereof. As such an optical component, there is a wavelength conversion element used in a device (for example, an optical computer, an optical disk recording device) for transmitting, processing, and recording information using an optical modulation technique.

[0002]

2. Description of the Related Art
Inorganic materials such as KDP (KH ₂ PO ₄ ) and LNO (LiNbO ₃ ) have been used as nonlinear optical materials, especially as secondary nonlinear materials. However, these inorganic materials are disadvantageous in that they are brittle and have low strength. There is. Therefore, MNA (2-methyl-4-
Single crystals of organic materials represented by nitroaniline) have been developed. However, these organic materials have problems that it is difficult to obtain a large single crystal, and that they have difficulties in workability, heat resistance, and the like. Also, by introducing a low-molecular substance having a large nonlinear optical effect into the polymer matrix and then performing a poling treatment or the like by an external electric field, a material having excellent workability in which the nonlinear optical substance is oriented can be obtained. There are ways. However, since the alignment state formed in this way is relaxed with time, it lacks stability as an optical device material. Furthermore, in a system having a non-linear optical material in a polymer side chain, it is necessary to contain a component having a low glass transition point in order to perform poling treatment sufficiently, so that it is difficult to suppress the orientation relaxation. Derive.

In order to increase the second harmonic generation,
A second-order nonlinear optical material having a large size must be obtained, and a phenomenon called phase matching must be used. This requires obtaining relatively large single crystals. A recently devised method is a method called domain inversion quasi-phase matching.
This is because a structure (inversion domain structure) in which domains having a thickness of about coherent length lc are arranged in a groove having a small width such that the polarization directions are opposite to each other is formed.
As the harmonics are reflected at the domain interface, reflected again, and superimposed on the traveling wave,
This is a method of achieving phase matching. Usually, the coherent length lc is on the order of several μm, so it is technically difficult to form such an inverted domain structure. That is, for example, when an inorganic material is used, formation of an inversion domain structure is attempted by a method in which the inorganic material is cut into thin pieces of equal thickness and arranged, but this requires a great deal of skill. On the other hand, when an organic material is used, an attempt is made to form an inverted domain structure (including a poling process) by applying a polar electric field as described above, but this involves the problem of orientation relaxation as described above. . In any case, the formation of the inverted domain structure requires a great deal of skill and is difficult to realize, so even if it succeeds in a laboratory or on the idea, it has not yet been put to practical use.

[0004]

The present inventor has conducted intensive studies to eliminate the above-mentioned difficulties associated with the formation of the inverted domain structure and to realize a stable light wave manipulation device. As a result, the following wavelength conversion element was obtained.

That is, the present invention provides, as a first invention, a wavelength conversion element including an inverted domain laminated body formed by laminating a main chain type optical non-linear polymer thin film for generating an optical second harmonic. is there.

According to a second aspect of the present invention, there is provided a wavelength conversion element comprising a liquid crystal thin film interposed between adjacent main chain type optical nonlinear polymer thin films in the inversion domain laminate. To provide.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a wavelength conversion element according to the first invention will be described. The wavelength conversion element of the first invention has a rigid main chain skeleton that does not cause orientation relaxation, and is configured using an oriented thin film of a polymer that emits optical second harmonics from the main chain. . Here, the optical second harmonic is light newly generated from the nonlinear optical material due to polarization generated in a substance due to energy of the light when the nonlinear optical material is irradiated with light. Its wavelength is 1/2 of the wavelength of the incident light. Utilization of the above-mentioned alignment body thin film prevents the above-mentioned relaxation of the alignment over time.

An example of a polymer thin film which can be suitably used as the oriented thin film is 2-hydroxy-6-naphthoic acid
Copolymerized polyester containing (HNA) and 4-hydroxy-benzoic acid (PHB) as components. In particular, PH
The molar ratio of B to HNA (PHB: HNA) is 30:70
The copolyester in the range of 50 to 50 is preferable because it is rich in liquid crystallinity and easily formed into a film.

Further, in the wavelength conversion element of the first invention, the inversion domain laminated body is formed by laminating the alignment body thin films. The method of forming a polymer oriented body thin film having a uniform thickness and laminating the same is not only not requiring a high degree of skill than the method of slicing and lining up inorganic materials in equal thickness, The accuracy of the inverted domain structure obtained thereby is rather high. It is desirable that the thickness of each domain (that is, a thin film) constituting the laminated body is equal to or slightly smaller than the coherent length, but may be an integral multiple thereof (however, not so large).

Next, a wavelength conversion element according to a second invention will be described. In order to prevent light scattering at the domain interface, it is desirable to apply an equirefractive index liquid to the interface. However, the present inventor has greatly improved the phase matching efficiency by using liquid crystal as the liquid. It has been found that it can be increased. Based on this discovery, the wavelength conversion element of the second invention is an improvement of the wavelength conversion element of the first invention. That is, since the refractive index of the liquid crystal can be changed by an external electric field, the refractive index of the liquid crystal thin film can be adjusted by the electric field. Therefore, by disposing the wavelength conversion element of the second invention in an electric field and appropriately tuning the refractive index of the liquid crystal thin film by appropriately changing the electric field,
Efficient phase matching is realized at a specific refractive index, and the wavelength conversion efficiency is greatly increased.

[0011]

The wavelength conversion element according to the present invention can achieve conversion efficiency sufficient for practical use even with an optical path length of 10 μm, and can be widely used for producing various optical components in various fields. . In particular, it is preferable to form an inverted domain laminate using a main chain type liquid crystalline polymer which is a kind of a main chain type optical non-linear polymer since a wavelength conversion element having high heat resistance can be obtained. In other words, the main chain type liquid crystalline polymer was originally developed with the aim of high strength and high heat resistance, so that a wavelength conversion element constructed using this has naturally high heat resistance. It becomes.

[0012]

EXAMPLES In order to investigate the effects of the present invention, the following experiments were conducted.

1) Preparation of sample substance As a sample substance, 2-hydroxy-6-naphthoic acid (HN
A plurality of copolymerized polyesters having components (A) and 4-hydroxy-benzoic acid (PHB) having different composition ratios (molar ratios) of HNA and PHB were prepared. This sample material is a kind of main chain type liquid crystalline polymer.

2) Mechanical film formation The sample material was heated to its liquid crystal temperature (280 to 310 ° C) to form a liquid crystal state, and formed into a film by a drawing method (a method similar to a normal T-die method). This film is hereinafter referred to as “mechanical film-forming film”.

3) Preparation of Inverted Domain Laminate Using Mechanical Film-Formed Film Next, an inverted domain laminated body was prepared using a mechanical film-formed film. The creation procedure will be described below with reference to FIG. FIG. 1 is a schematic diagram showing a procedure for producing an inverted domain laminate.

According to the method of drawing from the liquid crystal state, a large amount of a uniform film having a relatively smooth surface can be obtained. Therefore, the sample material in a liquid crystal state is drawn out in only one direction by a drawing method, and cut into a long strip in the drawing direction, so as to have a length of about 6 mm and a width of 1 as shown in FIG.
A main chain type liquid crystalline polymer film 10 of about mm (hereinafter, simply referred to as film 10) was formed. Here, the thickness of the film 10 is about 10 to 12 μm, that is, about twice the coherent length (in FIG. 1, the thickness is particularly exaggerated).

Next, the nonlinear optical constant of the film 10 was measured. Usually, the nonlinear optical constant is defined as a physical property value of a crystal.
Since a very high degree of orientation is recognized in the solid structure of No. 0 as described later, in this experiment, a nonlinear optical constant for the film 10 is defined for the sake of convenience as in the case of the crystal. In this measurement, the direction of the molecular chain axis of the film 10 (the direction of the arrow in the film 10) was taken as the z-axis, the plane of the film 10 was taken as the xz plane, and the thickness direction of the film 10 was taken as the y-axis. In this case, it is d ₃₃ nonlinear optical constant obtained when a light wave having a vibration plane parallel to the z-axis is parallel to incident on the y-axis.

The nonlinear optical constants d ₁₁ and d of the film 10
_{When 33} was measured, the former was almost zero, while the latter was about 10 times the nonlinear optical constant d ₁₁ of Y-cut Quartz. In addition, as a result of observation of the film 10 by a polarizing microscope and X-ray diffraction measurement, it was found that the long axes of the molecules constituting the film 10 were highly oriented in the drawing direction (machine direction). From the results of the above observations and measurements, although it is not possible to immediately determine which of the positive and negative poles of the polarization direction in the film 10 is oriented in the drawing direction,
Regardless of which part of the film 10 is taken, it is considered sufficiently reasonable to assume that one pole is oriented in the pull-out direction and the other pole is oriented in the opposite direction. In the following description, it is assumed that the positive electrode faces upward and the negative electrode faces downward as shown in FIG.

After forming the film 10 by the drawing method, the film 10 was cut into a plurality. In this experiment, as shown in FIGS. 1A and 1B, the film 1
Although 0 was divided into six parts 101 to 106 (hereinafter, referred to as “film pieces”), the number of divisions need not be six, and may be an arbitrary number. The six film pieces 101 to 106 thus obtained were
As shown in (b), two adjacent film pieces were stacked so that the polarization directions were opposite to each other, and a structure 12 having six layers was prepared. Hereinafter, this structure 12 is referred to as an inverted domain laminate 12. Although the thickness of the inverted domain laminated body 12 is exaggerated in the drawing, it is actually a film having a thickness of about 60 to 72 μm.

4) Preparation of Wavelength Conversion Cell Next, a wavelength conversion cell 20 was prepared using the above-mentioned inverted domain laminated body 12. FIG. 2 is a sectional view showing the wavelength conversion cell 20. The wavelength conversion cell 20 includes a pair of glass plates 2.
A structure in which the inverted domain laminated body 12 is sandwiched between 1 and 22 in a sandwich manner is held by a cell frame 24. In addition, between adjacent film pieces in the inverted domain laminated body 12 or between the inverted domain laminated body 12 and the glass plate 21 or 22, an equal refractive index liquid 25 is applied to prevent scattering of light at an interface. It is sealed in a thin layer.

5) Wavelength Conversion by Wavelength Conversion Cell When the intensity of the second harmonic light (SH light) generated from the cell when the light is incident on the wavelength conversion cell 20 is measured, the intensity is one mechanical sheet. When the light was applied to the film-forming film, the intensity was as large as 15 to 20 times the intensity of SH light generated from the film. As described above, according to the present invention, a wavelength conversion cell exhibiting an excellent nonlinear optical effect can be obtained only by a simple process of cutting and laminating a mechanical film-formed film into a plurality of film pieces. .

Next, a wavelength conversion cell according to an embodiment of the second invention will be described with reference to FIGS. FIG. 3 is a perspective view showing the inverted domain stacked body 28 of the present embodiment, and FIG. 4 is a cross-sectional view showing a wavelength conversion cell 32 formed using the inverted domain stacked body 28. Although the inverted domain laminate 28 is actually in the form of a film, its thickness is exaggerated in the figure.

As shown in FIG. 3, the inverted domain laminated body 28 is composed of six film pieces 108 obtained by the same method as the film pieces 101 to 106 of FIG.
0 are alternately stacked. As shown in FIG. 4, the inverted domain laminate 28 is sandwiched between a pair of glass plates 35 and 36, and furthermore,
The wavelength conversion cell 32 is obtained by disposing a pair of electrodes 33 and 34 for tuning the refractive index of the liquid crystal thin film 30 at predetermined positions. The periphery of the space in which the liquid crystal thin film 30 is inserted is sealed with a sealing material 31 to prevent leakage of the liquid crystal.

In such a wavelength conversion cell 32, the arrangement of the pair of electrodes 33 and 34 is determined in consideration of the type of liquid crystal constituting the liquid crystal thin film 30. For example, the liquid crystal thin film 3
When the liquid crystal constituting 0 is a p-type nematic liquid crystal, the electrodes 33 and 34 are arranged so that an electric field parallel to the polarization axis of the inversion domain stack 28 is applied. The electrode arrangement shown in FIG. 4 corresponds to this case. On the other hand, when the liquid crystal forming the liquid crystal thin film 30 is an n-type nematic liquid crystal, the electrodes are arranged so that an electric field perpendicular to the polarization axis in the inversion domain stack 28 is applied.

According to the above configuration, by appropriately adjusting the voltage applied between the electrodes 33 and 34, the refractive index of the liquid crystal thin film 30 and that of the film piece 108 are completely matched with each other. It is possible to prevent scattering of light at the interface between the film pieces 108 and the interface between the film piece 108 and the glass plate 35 or 36.

While various embodiments of the wavelength conversion device according to the present invention have been described with reference to the drawings, the embodiments are not limited to these embodiments and can be variously modified within the spirit and scope of the present invention. It is. For example, in the above embodiment, a copolymer polyester which is a main chain type liquid crystalline polymer was used, but the present invention can be practiced using any other main chain type optical non-linear polymer.

In the above embodiment, the width of the belt-like film is set to 1 mm, but this width can be changed as appropriate. However, when a liquid crystal thin film is interposed at the interface in the inversion domain laminate as in the second invention, it is necessary to consider the following constraints. That is, in order to effectively change the refractive index of the liquid crystal, it is necessary to apply an electric field having an intensity corresponding to the driving voltage of the liquid crystal. For example, when the liquid crystal thin film 30 is composed of cyanobiphenyl which is a kind of p-type nematic liquid crystal, the required electric field strength is 1 V per 10 μm (that is, 1 × 10 ⁵ V / m).
It is about. It is necessary to increase the voltage applied between the electrodes as the width of the film increases, so it is not preferable to unnecessarily increase the width of the film. It is preferable that the width be an appropriate width considering the above.

In the embodiment of the second invention, the liquid crystal thin film 3
Although the p-type nematic liquid crystal and the n-type nematic liquid crystal have been mentioned as examples of the liquid crystal that can be used for 0, other than this, for example, a mixed liquid crystal including a plurality of types of different p-type liquid crystals, a plurality of types of different n-type liquid crystals, It is also possible to use a mixed liquid crystal containing. In such a mixed liquid crystal, the response and the refractive index change variously according to the components and the composition ratio. If such a mixed liquid crystal is used, the components and the composition ratio of the mixed liquid crystal are appropriately determined according to the type of the main-chain type optical non-linear polymer forming the inversion domain laminate, the intended use of the wavelength conversion element, and the like. A wavelength conversion element having desired performance can be obtained.

Further, in the description of the second invention, it has been described that the refractive index of the liquid crystal thin film can be adjusted by appropriately changing the electric field. As a method of changing the electric field, the voltage applied to the electrode is changed. In addition to the method of changing the position of the electrodes, a method of changing the arrangement of the electrodes can be used. For example, when an electric field is applied using a pair of electrodes as in the above embodiment, the pair of electrodes can be rotated about the optical axis of the cell. By doing so, the refractive index of the liquid crystal thin film can be tuned with higher accuracy by appropriately adjusting the voltage and changing the arrangement of the electrodes.

[Brief description of the drawings]

FIGS. 1A to 1C are schematic views showing a procedure for producing an inverted domain laminate.

FIG. 2 is a cross-sectional view showing a wavelength conversion cell created by using the inversion domain laminate of FIG.

FIG. 3 is a perspective view showing an inverted domain laminate used for a wavelength conversion cell according to an embodiment of the second invention.

FIG. 4 is a cross-sectional view showing a wavelength conversion cell formed by using the inversion domain laminate of FIG. 3;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Main-chain type liquid crystalline polymer film 101-106, 108 ... Film piece 12, 28 ... Inversion domain laminated body 20, 32 ... Wavelength conversion cell 21, 22, 35, 36 ... Glass plate 24 ... Cell frame 30 ... Liquid crystal Thin film 31: sealing material 33, 34: electrode

Claims (1)

[Claims] 1. A wavelength conversion element including an inversion domain laminate formed by laminating a main chain type optical non-linear polymer thin film for generating an optical second harmonic.