JPH063715A - Optical medium for nonlinear optical devices - Google Patents

Abstract

(57) [Summary] [Object] To provide a non-linear optical medium applicable to a high-performance non-linear optical device which has a wide operating wavelength range, high-speed response, and small operating input light intensity and which can be put to practical use. An optical medium for a non-linear optical device, which contains a trisazo dichroic dye having a tetrahydroquinoline ring at one end of the molecule and at least one naphthalene ring in the molecule. An example of constituents other than dyes is polymethylmethacrylate. [Effect] It is possible to configure a non-linear optical device of practical value that can drive an existing semiconductor laser as a light source.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nonlinear optical medium applicable to optical switches, optical memories, optical signal arithmetic processing devices, etc., which will be used in the future in optical information processing and optical communication systems.

[0002]

2. Description of the Related Art The nonlinear optical effect means that when light is incident on a substance, the electric polarization P of the substance can be expressed by the following equation (Equation 1), while the effect expressed by the second and subsequent terms is expressed. It is something.

[0003]

[Equation 1] P = χ ⁽¹⁾ E + χ ⁽²⁾ E ² + χ ⁽³⁾ E ³ + ...

[Χ ⁽ⁱ⁾ is the i-th order electric susceptibility, E is the electric field intensity of light] Especially, the second high frequency generation (SHG) according to the second term and the third high frequency generation (THG) according to the third term are wavelengths. Although well known as a conversion effect, the third term is also important for giving a change in optical constant according to light intensity, for example, a nonlinear refractive index effect or a nonlinear absorption effect. For example, the nonlinear refractive index effect is such that the refractive index n of a substance changes in proportion to the intensity of incident light, and is expressed by the following equation (Equation 2).

[0005]

(2) n = n ₀ + n ₂ I

(N ₀ is a constant, n ₂ is a non-linear refractive index coefficient) When a medium exhibiting this effect is combined with another optical element such as an optical resonator, a polarizer or a reflecting mirror, an optical bistable element or an optical bistable element is obtained. It is possible to realize important devices such as switches or phase conjugate wave generators that will be used in the future in optical information processing and optical communication systems.

As a conventional example of a nonlinear optical device using a medium exhibiting such a nonlinear refractive index effect, an actual example of an optical bistable element will be shown below with reference to FIG. That is, FIG. 6 is a block diagram showing an example of a conventional nonlinear optical device (optical bistable element).

Reference numeral 11 in FIG. 6 is an optical medium having a nonlinear refractive index (nonlinear refractive index medium), and 42 and 42 'are dielectric vapor deposition mirrors coated on both sides of the nonlinear refractive index medium 11 and having a reflectance of about 90%. is there. In this configuration, if the resonance condition is satisfied by slightly changing the wavelength of the input light, the output light Pt has the characteristics shown in FIGS. 2 and 3 with respect to the input light intensity Pi.

2 and 3 are diagrams showing the operating characteristics of the optical bistable element in terms of the relationship between the input light Pi (horizontal axis) and the output light Pt (vertical axis). FIG. 2 shows the limiter operation. Shown in FIG.
Indicates a bistable operation, and the arrows in the figure show the paths that show the characteristics of Pt when Pi increases and when Pi decreases.

These correspond to a limiter operation and a bistable operation, respectively, and can be applied to waveform shaping of an input optical pulse, an optical switch, an optical signal memory, an optical logical operation operation, etc. in optical communication and an optical information system. Is.

By the way, in this type of non-linear optical device, three values of usable input light wavelength and input light intensity, and a response time capable of following changes in the intensity of an optical signal are important as characteristics. . For example, in the example using a superlattice crystal prepared by alternately and repeatedly growing semiconductor thin films of GaAs and GaAlAs as the nonlinear optical medium 11 in FIG. 6, excitons are excited in the crystal due to light absorption. Since the operating principle is that the refractive index shows light intensity dependence (absorption non-linear effect) by, the coefficient of the third term of the above formula (Equation 1) is large (that is, the efficiency of third-order nonlinearity is high), The required input light intensity is 5 × 10 ⁴ W /
It is excellent in that it can be as small as about cm ^2, but the usable input light wavelength is limited to an extremely narrow range near the exciton absorption spectrum, and the response time is determined by the exciton lifetime. There is a problem that it cannot be used for optical signal processing faster than 10 ^-8 sec.

In addition, another conventional method is used in which a glass cell filled with carbon disulfide (CS ₂ ) which is a non-linear optical liquid is used as the non-linear optical medium, and an external mirror is used in place of the dielectric vapor deposition mirrors 42 and 42 '. In the example, the operating principle is that the refractive index shows the light intensity dependence due to the rotational arrangement of molecules according to the optical electric field (molecular rotation nonlinear effect).
Although it is excellent in that the usable input light wavelength covers a wide range from the visible region to the near infrared region, the third-order nonlinear effect is not so high, and the input light intensity required for operation is 10 ⁸
It is as large as W / cm ² and the response time is determined by the rotational relaxation time of the molecule, which is 10 ^-11 to 10 ^-12 sec.
There is a problem that it cannot be used for higher speed optical signal processing.

As is clear from the above, the performance of the nonlinear optical device is mostly determined by the characteristics of the nonlinear optical material. Therefore, the development of a nonlinear optical medium that has a wide usable wavelength range, high third-order nonlinear efficiency, low input light intensity required for operation, and short response time is eagerly awaited, and active research is being conducted toward that end. Is the current situation.

Among materials exhibiting the third-order nonlinear effect, π-conjugated polymer materials such as benzene rings and double or triple bonds have recently attracted particular attention. For example, polydiacetylene-bis (paratoluenesulfonate) (abbreviation PTS)
Then, the third-order effect constant χ ⁽³⁾ is 10 ⁻¹⁰ esu [converted into a nonlinear refractive index coefficient, n ₂ = 2 × 10 ⁻¹² (W / cm 2
² ) ^-1 ], which is two orders of magnitude larger than the above CS ₂ liquid. Furthermore, since the mechanism of this non-linear effect is not due to absorption and interaction with molecules or crystal lattices but is purely due to electronic polarization, it is possible to follow the intensity change of the optical signal. Response time is 10
It has an extremely high speed of ^-14 sec, and has the excellent characteristics that the usable input light wavelength covers a wide range from around 0.65 μm to over 2.0 μm.

However, since polydiacetylene such as PTS is generally a crystalline polymer, it is necessary that the entire medium is a single crystal when used as a medium of an optical device. If there is an amorphous part in a part of the medium or if the whole is a polycrystalline state and is an aggregate of minute single crystals, the scattering loss of incident light increases and it becomes useful. I won't. Therefore, in manufacturing an optical device using polydiacetylene,
Techniques such as growth of large crystals or thin film crystallization according to the purpose are indispensable, and even if these crystals are obtained, it is necessary to establish a processing technique for cutting the crystals or polishing the surface. is necessary. These peripheral technologies for polydiacetylene have not yet been resolved. Therefore, none of the non-linear optical devices using polydiacetylene has yet been put to practical use.

[0016]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and utilizes a nonlinear refractive index of a conventional optical bistable element, an optical switch, a phase conjugate wave generator or the like. An object of the present invention is to eliminate the problems in the nonlinear optical device, that is, the drawbacks such as the wavelength limitation, the low speed response, the high operating input light intensity, or the inability to obtain an optical medium having a desired shape as described above. An object of the present invention is to provide a non-linear optical medium applicable to a high-performance non-linear optical device which has a wide operating wavelength range, has a high-speed response, and has a small operating input light intensity and which can be put to practical use. is there.

[0017]

Briefly, the present invention relates to an optical medium for a nonlinear optical device, which has a tetrahydroquinoline ring at one end of a molecule and has at least one tetrahydroquinoline ring in the molecule. It is characterized by containing a trisazo-based dichroic dye having a naphthalene ring.

The present invention relates to a non-linear optical device composed of an optical medium having a non-linear refractive index and an optical element such as an optical resonator, a polarizer or a reflecting mirror, and a specific dichroic dye for liquid crystal display as the optical medium. It is characterized by using.

The dichroic dye for liquid crystal display is a special dye used for guest-host (GH) type liquid crystal display and is linear and rigid so that the dye molecules are fixed in the alignment direction of the liquid crystal. It has a characteristic molecular structure,
In many cases, the π-conjugated system is extended by an azo bond. The basis for the development of dichroism is that the molecule has a π-conjugated system that exhibits a strong light absorption transition moment in the visible region, and that the transition moment direction is aligned with the long axis of the molecule. Is. In addition, a method of introducing a naphthalene ring into a π-conjugated chain has been adopted in order to impart a practically useful bathochromic property. Further, in order to improve the solubility in the host liquid crystal, a tetrahydroquinoline ring is used in place of the commonly used aniline derivative, or a flexible alkyl group is introduced at the molecular end. In particular, the electron-donating tetrahydroquinoline ring not only contributes to improving the solubility,
It is useful because it has the function of retaining the conjugation of the benzene ring and the nitrogen atom loan pair, which is essential for improving dichroism and bathochromism.

Here, the present inventors have characterized the molecular structure of the dichroic dye for liquid crystal display as described above, that is, have a rigid π-conjugated system and have strong absorption in the visible region. , It was found that the transition moment of light absorption is in the same direction as the π-conjugated chain is completely in agreement with the factor that increases the third-order nonlinear optical effect. Furthermore, the molecular design for improving the bathochromicity / solubility of the dichroic dye for liquid crystal display, that is, introducing a naphthalene ring into the π-conjugated system or substituting a tetrahydroquinoline ring or an alkyl group at the molecular end is not possible. , Is a means to counter the π-conjugated chain expansion method with reduced solubility. This method is effective in obtaining a large third-order effect constant χ ⁽³⁾ without using long rigid linear chains such as π-conjugated polymers. It was found to be an extremely effective molecular design.

In addition, the dichroic dye having a naphthalene ring and a tetrahydroquinoline ring has a molecular structure excellent in mechanical, thermal and electrical durability, and is laser resistant, because of its practical requirements. It is also convenient when used as a non-linear optical material that requires process resistance in device processing.

The first feature of the present invention is that the dichroic dye having a naphthalene ring and a tetrahydroquinoline ring has a third-order nonlinear susceptibility [χ ⁽³⁾ ] of 10 ⁻¹⁰ equivalent to that of a π-conjugated polymer.
This is the first clarification that it is an extremely large value of esu or more. In addition, it was confirmed that the nonlinear refractive index coefficient (n ₂ ) has a large value due to the large third-order nonlinear optical effect, and these dichroic dyes are used as nonlinear optical media, and optical bistable elements, optical switches, phase conjugate waves are used. The second characteristic is to construct a nonlinear optical device such as a generator and to clarify that they can be put to practical use.

Further, the optical medium of the present invention may be composed of the above specific trisazo dichroic dye and polymethylmethacrylate.

[0024]

EXAMPLES The third-order nonlinear optical characteristics of the dichroic dye according to the present invention and the features when the dye is used as a nonlinear optical medium for a nonlinear optical device will be described in detail below with reference to Examples. The invention is not limited to these examples.

Example 1 A trisazo dichroic dye having a molecular structure represented by the following formula (Formula 1) was dissolved in xylene and applied on a glass substrate by a spin coating method to form a blue film having a thickness of 210Å. T
Χ ⁽³⁾ of this film was evaluated by the HG maker fringe method. Χ ⁽³⁾ value at communication wavelength 1.55 μm is 4 × 10
It was ^-10 esu. When this value is converted into a nonlinear refractive index coefficient n ₂ , it becomes 10 ⁻¹² (W / cm ² ) ⁻¹ or more, and CS ₂
It is comparable to PTS, which is more than two orders of magnitude larger than liquid. Considering that the mechanism of the nonlinear optical effect is due to pure electronic polarization as in PTS, the response time of this material is estimated to be about 10 ⁻¹⁴ sec as in PTS, and it also has sufficient high speed.

[0026]

[Chemical 1]

Example 2 FIG. 1 is a diagram for explaining an example of a non-linear optical device when the dichroic dye shown in Example 1 of the present invention is used as an optical medium, and the non-linear refraction used in the present invention is shown. The index medium 11 is a non-linear optical film produced by the method described below, and in this device, dielectric multilayer vapor-deposition film mirrors 12 and 12 'which reflect about 90% of input light and transmit the rest are arranged to face each other. It is an external optical resonator. The method for producing the dichroic dye film will be described. A dimethylacetamide (DMAc) solution of the dichroic dye and a chlorobenzene solution of polymethylmethacrylate (PMMA) shown in Example 1 were prepared, the dye and PMMA were mixed, and this mixed solution was poured into a flat Petri dish, and a nitrogen stream was introduced. After the solvent was slowly evaporated under the following conditions, the dye content was reduced to 2 by drying under reduced pressure.
A ˜25 wt% PMMA film (film thickness: 1 mm) was obtained.

Unless otherwise specified, the examples of the present invention described below were carried out by using a 1 mm thick PMMA film containing 17 wt% of the dichroic dye shown in Example 1.

In this way, 10 × 1 for a nonlinear optical device
A film-like optical medium having a size of about 0 × 1 mm ³ was obtained. In order to operate the device shown in FIG. 1, the resonance condition may be adjusted by slightly changing the input light wavelength or slightly changing the resonator length (mirror interval) as in the case of FIG. 6 of the conventional example. In the case of the present embodiment, since 1.064 μm light from the Nd ³⁺ -YAG laser was used, the adjustment was performed by changing the resonator length. The limiter operation and the bistable operation as shown in FIGS. 2 and 3 were obtained between the input light intensity Pi and the output light intensity Pt.

The minimum input light intensity necessary for the operation (Pi ^min)
Is analytically given by the following equation (3).

[0031]

## EQU3 ## Pi ^min = (Kλ) / (n ₂ l)

[Where λ is the wavelength of light, l is the optical medium length,
K (-0.001) is a coefficient determined by the reflectance of the mirror and the resonator length adjustment] In this embodiment, λ = 1.064 µm and l = 1 mm, so Pi ^min is about 10 ⁷ W / cm ^2. It was When a semiconductor laser with an effective output of 300 mW (pulse oscillation) and an oscillation wavelength of 0.83 μm is used as the light source, if the beam diameter is narrowed to 1 μm, the light intensity is calculated to be 3.8 × 10 ⁷ W / cm ^2, and at this wavelength It is sufficiently larger than the value of Pi ^min of the above nonlinear optical device. In fact, this nonlinear optical device could operate even when a semiconductor laser was used as the light source.

It has already been described that the response time t of this material is estimated to be about 10 ^-14 seu. However, the reaction time of the device is determined by the larger value of the medium response time t and the intracavity photon lifetime tp. Intracavity photon lifetime t
p is calculated from the following equation (Equation 4).

[0034]

(4) tp = -lop / (C ln R)

(Where lop is the optical length of the resonator, C is the speed of light, and R is the reflectivity of the mirror) Under the experimental conditions of this embodiment, tp was calculated to be 6 × 10 ^-11 sec from the equation (Equation 4). It was t
Since p> t, this value becomes the response time of the device. In this example, it was confirmed that the response time was shorter than 10 ^-10 sec.

Example 3 FIG. 4 is a diagram for explaining an example of an optically controlled optical switch when the dichroic dye for liquid crystal display shown in Example 1 is used as an optical medium. Reference numerals 21 and 21 'in the figure are orthogonal polarizer systems each including two polarizers arranged so that their polarization axes are orthogonal to each other, and 11 is the medium obtained in the second embodiment. In this configuration, linearly polarized light that has passed through the orthogonal polarizer 21 only while the gate pulse light Pg is incident is
The polarization angle is rotated by the change in the refractive index of the non-linear refractive index medium 11, and passes through the orthogonal polarizer 21 '. That is, the incident light is optically switched by the gate light. Also in this device, the usable wavelength range, the response time, and the value of the operation input light intensity (gate pulse light) are important, but it is clear from Example 2 that the usable wavelength range and the response time are extremely excellent. It goes without saying.

The required gate pulse light intensity (Pπ _{/ 2} ) is analytically given by the following equation (Equation 5).

[0038]

[Equation 5] Pπ _{/ 2} = λ / (2 × n ₂ l)

In this embodiment, λ = 1.064 μm and l = 1
Since it is mm, Pπ _{/ 2} was about 10 ⁹ W / cm ² . In fact, even at lower gate pulse intensity,
The transmittance T follows the following formula (Equation 6).

[0040]

[Equation 6] T = sin ² {(Pi / Pπ _{/ 2} ) × π / 2}

Therefore, it was found that the semiconductor laser light whose beam diameter was narrowed down could also be operated.

Example 4 FIG. 5 is a diagram for explaining an example of a phase conjugate wave generator when the dichroic dye shown in Example 1 is used as an optical medium. In the figure, reference numerals 31 and 31 'are semitransparent mirrors, 32 is a total reflection mirror, and 11 is an optical medium obtained in the second embodiment. This configuration is an optical arrangement called degenerate four-wave mixing, and when three light waves A ₁ , A ₂ (opposite direction to A ₁ ) and Ap (tilt incidence) enter a medium having a nonlinear refractive index, Ap
To the fourth light wave (A
c) occurs. This phase conjugate wave is drawing attention as an effective means for image correction and real-time holography in image information processing technology [O plus (O plus
E) March issue, page 73 (1982)].

[0043]

As described above in detail, the dichroic dye having a naphthalene ring and a tetrahydroquinoline ring according to the present invention has an extremely fast response speed and a large nonlinear refractive index, and further has a nonlinear refractive index. It has excellent light transmission and processability as a medium. Therefore, a film of a dichroic dye having a naphthalene ring and a tetrahydroquinoline ring can be used for an optical bistable device, an optical switch, a phase conjugate wave generator, or another optical device for nonlinear optical devices important in the future optical information processing or optical communication fields. It can be applied as a medium. As a result, the optical medium containing the dichroic dye having the naphthalene ring and the tetrahydroquinoline ring of the present invention has a wavelength dependency, a response speed, a response speed, as compared with a device using a conventional nonlinear optical liquid or a semiconductor superlattice crystal. It is possible to realize a device which is remarkably excellent in terms of the intensity of input light necessary for operation, and it is possible to configure a non-linear optical device of practical value that can drive an existing semiconductor laser as a light source.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical bistable element showing one example of a nonlinear optical device using an optical medium (nonlinear refractive index medium) of the present invention.

FIG. 2 is a diagram showing a limiter operation characteristic of an optical bistable element.

FIG. 3 is a diagram showing bistable operation characteristics of an optical bistable element.

FIG. 4 shows a second embodiment of the nonlinear optical device using the optical medium (nonlinear refractive index medium) of the present invention, and is a configuration diagram of an optical control optical switch.

FIG. 5 shows a third embodiment of a nonlinear optical device using an optical medium (nonlinear refractive index medium) of the present invention, and is a configuration diagram of a phase conjugate wave generator.

FIG. 6 is a configuration diagram showing an example of a conventional nonlinear optical device (optical bistable element).

[Explanation of symbols]

11 ... Non-linear refractive index medium, 12 and 12 '... (External) dielectric multilayer vapor deposition film mirror, Pi ... Input light, Pt ... Output light,
21 and 21 '... Polarizer (constituting orthogonal polarizer system), Pg ... Gate pulse light, 31 and 31' ... Semi-transmissive mirror, 32 ... Total reflection mirror, 42 and 42 '... Dielectric vapor deposition mirror ( (Reflectance about 90%)

Claims (2)

[Claims] 1. An optical medium for a nonlinear optical device comprising a trisazo dichroic dye having a tetrahydroquinoline ring at one end of the molecule and at least one naphthalene ring in the molecule. 2. The optical medium for a nonlinear optical device according to claim 1, wherein the medium is composed of a trisazo dichroic dye having a tetrahydroquinoline ring and at least one naphthalene ring, and polymethyl methacrylate. An optical medium for a non-linear optical device characterized by the above.