JP3545270B2 - Method for manufacturing second-order nonlinear optical material - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a method for manufacturing a second-order nonlinear optical material that can be used for various uses as an optical modulator, an optical switch, and the like, which are widely used in the field of communication devices.
[0002]
[Prior art and problems]
Optical poling has been developed as a polarization treatment that satisfies the phase matching condition of an organic dye-containing organic material having a second-order nonlinear optical effect (J. Si et al., Appl. Phys. Let., Vol. 72, 7). (1998) 762-764). Specifically, a pulse laser of several picoseconds to several nanoseconds is used to irradiate the optical material with the fundamental wave and the second harmonic of the fundamental wave coaxially and in the same polarization direction.
Laser light having a wavelength that is absorbed by the organic dye is used for light poling. The polarization structure that satisfies the phase matching is formed by the optical poling, but the phase matching is satisfied only by the wavelength used for the optical poling. Therefore, when the optical path length of the waveguide structure is increased as an element, the effect of light absorption by the organic dye appears and the second harmonic cannot be efficiently extracted.
[0003]
[Means for Solving the Problems]
The present invention solves such a problem in use in the second-order nonlinear optical element, and causes optical polling by using multiphoton absorption, so that the optical loss is extremely small even if the optical path length is long. An object is to provide an optical element.
In the production method of the present invention, in order to achieve the object, a wavelength band corresponding to multiphoton absorption of three-photon absorption or more of the pulsed laser beam to be irradiated overlaps with the absorption band, and the photoisomerization reaction is caused by light absorption in the wavelength band. Using a pulsed laser, a light flux of two wavelengths without an organic dye absorption band is coaxially applied to a material in which an organic dye molecule or a molecule having a side chain of the organic dye is dispersed or chemically bonded. It is characterized by simultaneous irradiation and light polling.
[0004]
Embodiment
In the present invention, a material containing an organic dye having a second-order nonlinear optical effect is irradiated with light beams of two wavelengths simultaneously and coaxially, thereby optically polling the material. As a material to be irradiated, besides a thin film formed by dispersing or chemically bonding an organic dye molecule having a second-order nonlinear optical effect or a molecule having an organic dye in a side chain, a fiber or a bulk can also be used.
Organic dye molecules having a second-order nonlinear optical effect include, but are not limited to, paranitroaniline, styrylpyridium cyanine, 4-4'tricyanovinyldimethylaminodiazostilbene, and 3-methyl-4-nitropyridine monoxide. , Hemicyanine, 4-4'dimethylamino cyanobiphenyl, dimethylaminophenylurea, 4-amino4'nitrodiphenylsulfide, 4- [NethylN (2hydroxyethyl)] amino4'nitroazobenzene, and the like.
[0005]
When a photoisomerization reaction molecule whose molecular structure changes due to light absorption is used as an organic dye, the electric field alignment is easier even when the laser pulse width is longer than when an organic dye that does not show a photoisomerization reaction is used. Can be caused. Photoisomerization reaction molecules include 4- [NethylN (2hydroxyethyl)] amino 4'nitroazobenzene, hemicyanine, styrylpyridium cyanine and the like. These non-linear organic dyes having dipoles, when an optical electric field is present, generate a rotational moment that aligns the molecules in the direction of the optical electric field. At that time, since a photoisomerization reaction occurs, the orientation is promoted. Even when a laser having a wavelength that does not cause light absorption is used, when a pulse laser beam having a short pulse width and a large peak energy is applied, a photoisomerization reaction occurs due to multiphoton absorption, and orientation polarization by a photoelectric field is promoted.
[0006]
The molecule having an organic dye in the side chain is not particularly limited either. Polymethyl methacrylate-4- [Nethyl N (2hydroxyethyl)] amino 4'nitroazobenzene, polymethyl methacrylate-paranitroaniline, polyimide-4 -[N-ethyl N (2-hydroxyethyl)] amino 4'nitroazobenzene, thermally crosslinked azopolyuracene, and the like.
The organic dye or the molecule having the organic dye in the side chain is dispersed in the matrix. As the matrix material, thermally and chemically stable metal oxides such as silica, titania, zirconia, and alumina, and metal oxides to which a functional group such as a phenyl group is added can be formed by a sol-gel method and are thermally and chemically stable. used. Polymers such as polymethyl methacrylate can also be used for the matrix material.
[0007]
A matrix material in which an organic dye or a molecule having an organic dye in a side chain is dispersed is formed into a thin film on a substrate by spin coating, dip coating, vacuum deposition, or the like. It can also be made into a thin film by a sol-gel method. As the substrate, a glass substrate such as quartz glass or non-alkali glass, a glass substrate coated with a transparent electrode such as ITO, a silicon substrate, or the like is used. The thin film is not particularly limited, but is generally formed on the substrate with a thickness of 100 to 10000 nm (preferably 100 to 1000 nm).
In addition, when the degree of hydrolysis is controlled so that the sol exhibits spinnability during the solution preparation process in the sol-gel method, a gel fiber is produced by a spinning device. By heat-treating this gel fiber, an organic-inorganic composite fiber is produced. In the case of a polymer material, a fiber can be easily formed by controlling the temperature and the amount of a solvent. Further, a bulk material is produced by casting using a mold or the like. The fiber or bulk material obtained in this way can also be used as a material to be irradiated, and is similarly subjected to optical poling.
[0008]
The thin film, fiber or bulk material is optically polled by co-irradiating two wavelengths of light simultaneously. For example, as shown in FIG. 1, an Nd: YVO ₄ laser is used as the excitation light source 1, a Ti: sapphire laser is used as the seed light generation light source 2, and a Nd: YLF laser is used as the light source 4 for exciting the regenerative amplifier 3.
The pulse interval of the laser light emitted from the excitation light source 1 is controlled by the seed light generation light source 2 and the regenerative amplifier 3 from the picosecond order to the femtosecond order. Next, the wavelength of the laser light is converted to 1.1 to 3.0 μm by the optical parametric amplifier 5. The short-wavelength component of the laser light emitted from the optical parametric amplifier 5 is cut off by the short-wavelength cut filter 6, and then passes through the aperture 7 to the polarizing prism 8 to become linearly polarized light.
[0009]
The linearly polarized light is split into two light beams by the beam splitter 9. One light (reflected light) is incident on the second-order nonlinear optical crystal 11 through the shutter 10 to generate a second harmonic. The fundamental component of the second harmonic is cut by the long wavelength cut filter 12.
The other light (transmitted light) is mixed with light from the long wavelength cut filter 12 by the dichroic mirror 14 after adjusting the optical path length by the delay element 13. The mixed laser light is focused by the convex lens 15 and is incident on the sample 16. When simultaneously irradiating a light beam having two wavelengths coaxially, the sample 16 is optically polled.
[0010]
Whether or not the sample 16 has been optically polled is determined by irradiating only the fundamental wave to the sample 16 and transmitting the light transmitted through the sample 16 to the convex lens 17, the short wavelength reflection mirror 18, the shutter 19, the long wavelength cut filter 20, and the interference filter 21. After that, the light is made incident on the detector 22 and detected by the detector 22 to detect the second harmonic.
In optical poling, a periodic DC electric field is generated by a third-order nonlinear optical effect as a result of interference between a fundamental wave and a second harmonic. In conventional light poling, the wavelength of the second harmonic of the two luminous fluxes is the absorption wavelength of the organic dye, and it is assumed that a polarization structure is formed by promoting the orientation of the dye by light absorption. It was thought that light poling would not occur even if a light beam different from the absorption wavelength was irradiated. However, as a result of investigations and studies by the present inventors, it has been found that even at a wavelength where light absorption does not occur, optical polling occurs when the absorption band of the dye is matched by multiphoton absorption.
[0011]
For example, 4- [NethylN (2hydroxyethyl)] amino 4'nitroazobencene dye having an absorption peak at about 500 nm has no light absorption even when irradiated with light at 1500 nm and 750 nm, so that light polling occurs. Was thought not to be. However, even in this case, when a pulse laser on the order of nanoseconds to femtoseconds is used, the light absorption band of 4- [NethylN (2hydroxyethyl)] amino4'nitroazobencene dye coincides with the three-photon absorption at 1500 nm. . The absorption band of the dye coincides with the absorption band of the dye due to multiphoton absorption, and the orientation polarization of the dye easily occurs due to a photoisomerization reaction, a resonance effect, and the like, so that optical poling occurs.
The intensity of the laser light to be applied is preferably as large as possible to cause light poling, but is determined in consideration of the durability of the thin film material and the substrate material to the laser intensity. For example, when 1500 nm light is used as the fundamental wave, ² MWcm ^{−2 to} 100 GWcm ⁻² (for example, when the pulse width is 150 femtoseconds, 0.3 μJcm ^{−2 to} 15.0 mJcm ⁻² , and when 3.5 nanoseconds) Is 7 mJcm ^{−2 to} 350 Jcm ⁻² ), preferably 2.7 MWcm ^{−2 to} 11 GWcm ⁻² (for example, when the pulse width is 150 femtoseconds, 0.4 μJcm ^{−2 to} 1.7 mJcm ⁻² , 3.5 ns) Is set to a fluence of 9.5 mJcm ^{−2 to} 39 Jcm ⁻² ).
[0012]
Optical poling due to the orientation polarization of molecules is more likely to occur as the peak power for exciting a DC electric field inside the material increases. When compared at the same peak power, the shorter the pulse width of light, the lower the average power. Therefore, even if a large peak power is applied, the material is less likely to be thermally damaged. The temporal and spatial widths that can interfere with each other are reduced. Since the material irradiated with the laser has a difference in group velocity depending on the spectrum, when two light beams are incident, the shorter wavelength of the material proceeds more slowly. For this reason, even when the waveguide structure is produced by irradiating two light beams with the pulse timing completely matched, a width capable of causing interference, that is, a periodic DC electric field is induced, and the length capable of performing optical polling is reduced.
[0013]
For this reason, when an organic dye molecule that does not cause a photoisomerization reaction or a material containing the organic dye molecule is used as a material to be irradiated, the pulse width of the laser beam is set to 100 femtoseconds to 30 picoseconds (preferably, , 150 femtoseconds to 2 picoseconds). On the other hand, in the case of an organic dye molecule that causes a photoisomerization reaction or a material to be irradiated containing the organic dye molecule, the orientation polarization of the dye easily occurs due to the photoisomerization reaction or resonance effect as described above, The width timing is preferably set in the range of 100 femtoseconds to 5 nanoseconds (preferably, 150 femtoseconds to 4 nanoseconds).
The polarization structure formed by the optical poling satisfies the phase matching condition suitable for generating the second harmonic (750 nm in the above-mentioned conditions), and the light having a wavelength other than the wavelength at which the organic dye absorbs the second harmonic. Generate Therefore, when the obtained polarization structure is used as a waveguide structure, it is possible to manufacture an element with extremely small light loss even if the light propagation distance is increased.
[0014]
Embodiment 1
Polymethyl methacrylate is dissolved in tetrahydrofuran, and 4- [Nethyl N (2hydroxyethyl)] amino 4'nitroazobenzene is dispersed in the obtained solution, so that polymethyl methacrylate: tetrahydrofuran: 4- [N [Ethyl N (2hydroxyethyl)] amino 4'nitroazobenzene = 5: 20: 1 coating solution was prepared. The coating solution was spin-coated on a non-alkali glass substrate at 2000 rpm, and the coating film formed on the substrate was immediately dried in an oven at 80 ° C. for 20 minutes.
[0015]
Using the optical polling apparatus of FIG. 1, the thin film on the substrate was optically polled as follows. A pulse laser beam having a wavelength of 800 nm, a pulse width of 150 femtoseconds and a repetition frequency of 1 kHz is emitted from the seed light generation light source 2 and the regenerative amplifier 3, the wavelength is converted to 1500 nm by the optical parametric amplifier 5, and the short wavelength cut filter 6 shortens the wavelength. After the wavelength component was cut out to extract light of 1500 ± 50 nm, the light was linearly polarized by the polarizing prism 8. The linearly polarized light was split into two light beams by the beam splitter 9. One light (reflected light) was made incident on a potassium / tantalum / phosphoric acid crystal (second-order nonlinear optical crystal 11) to generate light of 750 nm. The 1500 nm light component of the linearly polarized light was cut by the long wavelength cut filter 12. The other light (transmitted light, 1500 nm) was mixed with the 750 nm light transmitted through the secondary nonlinear optical crystal 11 by the dichroic mirror 14 after adjusting the optical path length by the delay element 13. The mixed light was converged by the convex lens 15, coaxially synchronized with the pulse in time, incident on the sample 16 from the direction perpendicular to the film, and the sample 16 was subjected to optical polling. Note that the fluence of 1500nm and 750nm pulsed laser light was respectively adjusted to 9MyuJcm ^-2 and 0.5μJcm ^-2.
[0016]
In order to check whether or not the sample 16 was optically polled, the sample 16 was irradiated with light of 1500 nm and the generation of the second harmonic at that time was monitored by the detector 22, and the second harmonic was detected. From the detection of the second harmonic, it was confirmed that the dye was oriented and polarized by light poling, and second-order nonlinearity was induced in the sample 16. Since the generated second harmonic reaches a peak and saturates in only 30 seconds, it was found that the polarization structure was induced very quickly. The induced second-order nonlinearity susceptibility was 0.3 pmV- ¹ .
[0017]
Embodiment 2
Ethanol: tetraethoxysilane: dimethylformamide: 4- [Nethyl N (2hydroxyethyl)] amino 4'nitroazobenzene: hydrochloric acid: water = 4: 1: 2: 0.05: 1.18: 6 by molar ratio Was prepared as follows. First, tetraethoxysilane and dimethylformamide were added to half of the ethanol in a molar ratio and stirred, and 4- [NethylN (2hydroxyethyl)] amino 4'nitroazobenzene was further added and stirred to uniformly disperse. The prepared solution A was prepared. Separately from solution A, the remaining ethanol was mixed with hydrochloric acid and water to prepare solution B. The sol solution was prepared by drop-mixing the solution B with the solution A.
After stirring the sol solution for 1 hour, it was spin-coated on a non-alkali glass substrate at 2000 rpm. The coating film formed on the substrate was immediately heat-treated in an oven at 150 ° C. for 20 minutes to produce a pigment-dispersed silica composite thin film.
[0018]
The obtained dye-dispersed composite thin film was subjected to light poling in the same manner as in Example 1. In Example 2, it was adjusted 1500nm and 750nm of the laser beam respectively 9MyuJcm ^-2, the fluence of 0.5nJcm ^-2.
When the light-poled thin film sample was irradiated with light of 1500 nm, and the generation of the second harmonic at that time was monitored by the detector 22, the second harmonic was detected. Therefore, it was confirmed that the dye was oriented and polarized by light poling, and second-order nonlinearity was induced in the thin film sample. Since the generated second harmonic reaches a peak during 120 seconds and saturates, it was found that a polarization structure was induced very quickly. The induced second-order nonlinearity susceptibility was 0.17 pmV- ¹ .
[0019]
Embodiment 3
Ethanol: tetraethoxysilane: tetraethylphenylsilane: dimethylformamide: 4- [NethylN (2hydroxyethyl)] amino 4'nitroazobenzene: hydrochloric acid: water = 4: 0.95: 0.05: 2 by molar ratio : 0.05: 1.18: 6 was prepared as follows. First, tetraethoxysilane, tetraethylphenylsilane, and dimethylformamide are added to half the ethanol in a molar ratio and stirred, and 4- [NethylN (2hydroxyethyl)] amino 4'nitroazobenzene is further added and stirred. Then, a solution C uniformly dispersed was prepared. Separately from solution C, the remaining ethanol was mixed with hydrochloric acid and water to prepare solution D. The sol solution was prepared by drop-mixing the solution C with the solution C.
After stirring the sol solution for 1 hour, it was spin-coated on a non-alkali glass substrate at 2000 rpm. The coating film formed on the substrate was immediately heat-treated in an oven at 150 ° C. for 20 minutes to produce a dye-dispersed phenyl-based silica composite thin film.
[0020]
The substrate S dye dispersed phenyl silica composite thin film is formed, after the gel membrane G was applied (a) as shown in FIG. 2, leaving the waveguide portion by reactive ion etching using the polyimide film F ₁ The gel film G was removed (b). The waveguide portion are formed surface, further forming a polyimide film F ₂ by spin coating (c), was thermally cured at 0.99 ° C.. By polishing both end surfaces of the waveguide portion, a waveguide W having a length of 1 mm was formed.
Next, the poling light beam L was made incident from the polished end face P of the waveguide W, and the gel film G was subjected to light poling. In Example 3, a laser beam having a pulse width of 3 picoseconds was used, and lights of 1500 nm and 750 nm were simultaneously incident coaxially (d). At this time, the light of 1500nm to fluence of 9MyuJcm ^-2, light 750nm were prepared fluence 0.5nJcm ^-2. Otherwise, the same conditions as in Example 1 were employed.
When a fundamental wave B of 1500 nm was made incident on the waveguide W in order to check whether or not optical polling was performed, the second harmonic H was detected (e). The induced second-order nonlinearity susceptibility was 0.25 pmV- ¹ .
[0021]
Embodiment 4
The sol solution containing ethanol: tetraethoxysilane: diphenyldimethoxysilane: dimethylformamide: hemicyanine: hydrochloric acid: water = 4: 0.8: 0.2: 2: 0.003: 1.18: 6 in molar ratio is as follows. It was prepared as follows. First, a solution E in which tetraethoxysilane, dimethylphenyldimethoxysilane, and dimethylformamide were added to and stirred with half of ethanol in a molar ratio of half of the total amount, and hemicyanine was further added and stirred to be uniformly dispersed was prepared. Separately from the solution E, the remaining ethanol was mixed with hydrochloric acid and water to prepare a solution F. The sol solution was prepared by adding the solution F dropwise to the solution E.
After stirring the sol solution for 1 hour, it was spin-coated on a non-alkali glass substrate at 2000 rpm. The coating film formed on the substrate was immediately heat-treated in an oven at 150 ° C. for 20 minutes to produce a dye-dispersed phenyl-based silica composite thin film.
A 1-mm-long waveguide was formed from the dye-dispersed phenyl-based silica composite thin film in the same manner as in Example 3, and subjected to optical poling. Next, to check whether or not optical polling was performed, a fundamental wave of 1500 nm was made to enter the waveguide, and the second harmonic was detected (e). The induced second-order nonlinearity susceptibility was 0.26 pmV- ¹ .
[0022]
Embodiment 5
By dispersing paranitroaniline instead of 4- [NethylN (2hydroxyethyl)] amino4'nitroazobenzene in the same manner as in Example 1, polymethylmethacrylate: tetrahydrofuran: paranitroaniline = A solution containing 5: 20: 1 was prepared, and a paranitroaniline-dispersed polymethyl methacrylate thin film was prepared from the solution.
Optical polling was performed in the same manner as in Example 1 except that the pulse width of the laser beam used for optical polling was changed to 20 picoseconds and 30 picoseconds, and the thin film was irradiated for 20 minutes from the vertical direction. When a laser beam of 20 picoseconds was used, a very slight second harmonic signal was detected, but at 30 picoseconds, the second harmonic was not detected at all. For this reason, with a pulse laser beam of the same output, when a laser beam with a long pulse width is used, the DC electric field induced with a decrease in peak power is reduced, and photoisomerization reaction due to multiphoton absorption is less likely to occur. As a result, it can be seen that optical polling is no longer possible.
[0023]
Embodiment 6
In the same manner as in Example 1, a 4- [N-ethyl N (2-hydroxyethyl)] amino 4′-nitroazobenzene-dispersed polymethyl methacrylate thin film was produced. As a laser beam used for optical poling, a third harmonic having a pulse width of 3.5 nanoseconds and a wavelength of 355 nm, oscillated from an Nd: YAG laser, was converted into a wavelength of 1500 nm (fundamental wave) by an optical parametric amplifier. Light of 750 nm was generated from the fundamental wave by the second-order nonlinear optical crystal in the same manner as in Example 1, and the 4- [N-ethyl N (2-hydroxyethyl)] amino 4'nitroazobenzene-dispersed polymethyl methacrylate thin film was subjected to photo-polling. At this time, fluence, 1500 nm, respectively, for 750nm ^35Jcm -2, was 10mJcm ^-2. Since 4- [NethylN (2hydroxyethyl)] amino4'nitroazobenzene, which causes photoisomerization reaction, is used as a non-linear organic dye, even a laser having a pulse width as long as 3.5 ns can be used. It was found that light poling was performed after irradiation for a minute.
[0024]
Embodiment 7
Polymethyl methacrylate: tetrahydrofuran: 4- [Nethyl N (2hydroxyethyl)] amino 4'nitroazobenzene = 1: 4: 0.2 is mixed and dissolved in a weight ratio, and after the solution becomes homogeneous, the solution is dissolved. Was heated to 80 ° C. and the solution was concentrated until it exhibited spinnability. A fiber was spun from the concentrated solution according to a conventional method, and dried at 80 ° C. for 2 hours to prepare 4- [N-ethyl N (2-hydroxyethyl)] amino 4′-nitroazobenzene-dispersed PMMA fiber. A fiber was sandwiched between two glass plates using an ultraviolet curable resin as a cladding material, sealed by irradiation with a high-pressure mercury lamp, and the end face of the fiber was polished to produce a fiber waveguide.
The obtained fiber waveguide was subjected to optical polling in the same manner as in Example 1. Fluence, 1500 nm, respectively, for 750nm laser light 1.7mJcm ^-2, and adjusted to 0.5mJcm ^-2. When the fiber waveguide irradiated with the laser was evaluated in the same manner as in Example 2, the generated second harmonic reached a peak in about 30 seconds and was saturated. The induced second-order nonlinear susceptibility was 0.5 pmV ^-1 .
[0025]
【The invention's effect】
As described above, in the present invention, a material such as a thin film, a fiber, or a bulk containing an organic dye having a second-order nonlinear optical effect or a polymer having an organic dye in a side chain is irradiated with a pulse laser to perform multiphoton absorption. The light is polled. Since multi-photon absorption is used, optical poling is possible even when laser light of a wavelength that does not absorb light of the organic dye is used. A second-order nonlinear optical element is manufactured.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an optical polling device used in the present invention. FIG. 2 is a diagram illustrating the optical polling of an optical waveguide formed on a substrate in the order of processes.
1: Excitation light source 2: Seed light generation light source 9: Beam splitter 11: Second-order nonlinear optical crystal 13: Delay element 14: Dichroic mirror 16: Thin film sample S: Substrate G: Gel film F1, F2: Polyimide film L: Two Polling beam having two wavelengths W: waveguide H: second harmonic

Claims (1)

The wavelength band corresponding to the multiphoton absorption of three or more photons absorbed by the pulsed laser light to be irradiated overlaps with the absorption band, and has an organic dye molecule or an organic dye in the side chain that causes a photoisomerization reaction due to light absorption in the wavelength band. Second-order nonlinearity characterized in that a pulsed laser is used to simultaneously irradiate a light beam of two wavelengths that do not have an absorption band of an organic dye onto a material in which molecules are dispersed or chemically bonded to a material in which there is no absorption band of an organic dye, and light polling is performed. Manufacturing method of optical material.

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