WO1991003683A1

WO1991003683A1 - Acrylic copolymers exhibiting nonlinear optical response

Info

Publication number: WO1991003683A1
Application number: PCT/US1990/005004
Authority: WO
Inventors: Celanese Corporation Hoechst; Bernice I. Feuer
Original assignee: Hoechst Celanese Corp
Priority date: 1989-09-11
Filing date: 1990-09-04
Publication date: 1991-03-21

Abstract

This invention provides novel side chain copolymers which exhibit nonlinear optical response, and which have utility as a transparent optical component in all-optical and electrooptical light switch and light modulator devices. An invention side chain copolymer is illustrated by structure (I).

Description

ACRYLIC COPOLY ERS EXHIBITING NONLINEAR OPTICAL RESPONSE

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS The present patent application has subject matter relative to the disclosures of copending patent application S.N. 915,179, filed October 3, 1986; patent application S.N. 106,301, filed October 9, 1987; patent application S.N. 120,253, filed November 11, 1987; patent application S.N. 148,262, filed January 25, 1988; patent application S.N. 156,051, filed February 16, 1988; and patent application S.N. (CEL-89-67), filed

BACKGROUND OF THE INVENTION

Polymers with a comb structure of pendant side chains are a new class of organic materials which exhibit interesting optical properties.

Comb-like liquid crystalline polymers are described in Eur. Polym. J. , JL8, 651 (1982); Advanced Polymer Science, Liquid Crystal Polymers II/III, Springer-Verlag, New York (1984), pages 215-220; and in United States Patent Numbers 4,293,435 and 4,631,328. The disclosed polymeric structures have been disclosed for their mesogenic optical properties which have prospective utility in opto-electronic display devices.

In the United States Patent Number 4,694,066; 4,755,574; and 4,762,912 liquid crystalline polymers are described which have pendant side chains which exhibit nonlinear optical susceptibility, in addition to meso¬ genic properties. U.S. 4,792,208 discloses nonlinear optically responsive organic compounds and side chain polymers in which the molecular dipoles have an electron donor moiety linked through a conjugated π bonding system to an electron acceptor sulfonyl moiety.

Nonlinear optical properties of organic and polymeric materials was the subject of a symposium sponsored by the ACS division of Polymer Chemistry at the 18th meeting of the American Chemical Society, September 1982. Papers presented at the meeting are published in ACS Symposium Series 233, American Chemical Society, Washington, D.C., 1983.

Thin films of organic or polymeric materials with large second order nonlinearities in combination with silicon-based electronic circuitry have potential as systems for laser modulation and deflection, information control in optical circuitry, and the like.

Other novel processes occurring through third order nonlinearity such as degenerate four-wave mixing, whereby real-time processing of optical fields occurs, have potential utility in such diverse fields as optical communications and integrated circuit fabrication.

Liquid crystalline side chain polymers which exhibit nonlinear optical properties are suitable for application as a nonlinear optical component in optical light switch and light modulator devices. One disadvantage of a liquid crystalline side chain polymer optical medium is a loss of transmission efficiency due to light scattering by deviations from ideal mesogenic order.

There is continuing interest in the theory and practice of optically-responsive polymers which are characterized by an oriented state of comb-like side chain structures. There is also an increasing research effort to develop new nonlinear optical organic systems for prospective novel phenomena and devices adapted for laser frequency conversion, information control in optical circuitry, light valves and optical switches. The potential utility of organic materials with large second order and third order nonlinearities for very high frequency application contrasts with the bandwidth limitations of conventional inorganic electrooptic materials. Accordingly, it is an object of this invention to provide novel optically responsive monomers and polymers.

It is another object of this invention to provide acrylic copolymers having side chains which exhibit nonlinear optical response.

It is a further object of this invention to provide optical light switch and light modulator devices with a transparent polymeric nonlinear optical component comprising a thin film of an acrylic copolymer with nonlinear optically-responsive pendant side chains which can be uniaxially oriented by an external field.

Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.

DESCRIPTION OF THE INVENTION One or more objects of the present invention are accomplished by the provision of an isotropic acrylic copolymer which is characterized by recurring monomeric units corresponding to the formula:

where R is hydrogen or a C^C, alkyl substituent; R¹ is a C^C_g alkyl substituent; m and m¹ are integers which total at least 10; n is an integer between about 1-6; and Z is hydrogen or a nitro substituent. A typical copolymer of the present invention has a weight average molecular weight in the range between about 5000-200,000, and a glass transition temperature in the range between about 40^β-180^*C.

A present invention copolymer has pendant side chains which exhibit nonlinear optical susceptibility β. A copolymer can be formed into a nonlinear optical medium, such as a transparent film or coating on a substrate. A copolymer can be applied to a supporting substrate by conventional means, such as spin coating, spraying, Langmuir-Blodgett deposition, and the like.

A film or coating fabricated with a present invention copolymer exhibits third order nonlinear optical susceptibility.

A nonlinear optical medium of the present invention can be subjected to an external field to uniaxially orient the copolymer side chains. In one method the polymer medium is heated close to or above the copolymer glass transition temperature T , then an external field (e.g., a DC electric field) is applied to the medium of mobile copolymer molecules to induce uniaxial molecular alignment of polymer side chains parallel to the applied field, and the medium is cooled while maintaining the external field effect.

By this method a present invention nonlinear optical medium has a stable uniaxial alignment of copolymer side chains. The poled optical medium exhibits a second nonlinear optical susceptibility χ^{2). A present invention poled optical medium is capable of exhibiting a χ⁽²⁾ level of 2 x 10'⁸ esu or higher as measured at 1.34 μm excitation wavelength.

In another embodiment this invention provides an optical light switch or light modulator device which contains a polymeric nonlinear optical component comprising a transparent solid medium of an isotropic acrylic copolymer which is characterized by recurring monomeric units corresponding to the formula:

z where R is hydrogen or a C₁-C₄ alkyl substituent; R¹ is a C^C_g alkyl substituent; m and m¹ are integers which total at least 10; n is an integer between about 1-6; and Z is hydrogen or a nitro substituent. In a present invention optical light switch or light modulator device, the polymeric nonlinear optical component exhibits less than about 10 percent scattering of transmitted incident light waves.

The term "transparent" as employed herein refers to a polymeric optical medium which is transparent or light transmitting with respect to incident fundamental and created light frequencies. In a present invention optical device, the copoly eric thin film nonlinear optical medium component is transparent to both the incident and exit light frequencies.

The term "isotropic" as employed herein refers to a transparent copolymeric optical medium in which the optical properties are equivalent in all tensor directions. The term "external field" as employed herein refers to an electric, magnetic or mechanical stress field which is applied to a substrate of mobile copolymer molecules, to induce dipolar alignment of the copolymer molecules parallel to the field. A present invention optical device can be a laser frequency converter, an optical Kerr effect device, an electrooptical Kerr effect device, a degenerate four wave mixing device, an optical interferometric waveguide gate, a wide-band electrooptical guided wave analog-to-digital converter, an all-optical multiplexer, an all-optical demultiplexer, an optical bistable device, an optical parametric device, and the like, as elaborated in U.S. 4,775,215.

The theory of nonlinear harmonic generation by frequency modulation of coherent light is elaborated by A. F. Garito et al in Chapter 1, "Molecular Optics: Nonlinear Optical Properties of Organic And Polymeric Crystals; ACS Symposium Series 233 (1983).

An optical interferometric waveguide gate device is described by A. Lattes et al in IEEE J. Quantum Electronics, 0E-19(11) . 1718 (1983). A wide-band electrooptical guided-wave analog-to- digital converter device is described by R. A. Becker et al in Proceedings Of The IEEE, 22(7), 802 (1984).

Optical multiplexer-demultiplexer devices are described in United States Patent Numbers 3,532,890; 3,755,676; 4,427,895; 4,455,643; and 4,468,776.

Optical bistable devices are described in United States Patents 4,515,429 and 4,583,818; and by P. W. Smith et al in Applied Physics Letters, 1^(6); 280 (1977) and in IEEE Spectrum, June 1981. Optical parametric devices are described in United States Patents 3,371,220; 3,530,301; and 3,537,020.

A present invention optical device can be achieved by constructing one of the optical devices described in the technical literature, except that a present invention polymer medium is utilized as the nonlinear optical component.

Synthesis Of Monomers And Copolymers

A. Stilbene Alcohol Intermediate

solvent

polynitrophenyl- acetic acid

B. Monomer and Copolymer

acrylic anhydride tertiary amine 4-dimethylaminopyridine

5 The side chain copolymers of the present invention have a unique combination of physical and optical properties. The stilbene electronic structure in conjugation with an electron-donating amino group and two or three electron- withdrawing nitro groups exhibits exceptional nonlinear

10 optical response. As illustrated in the Table, the presence of multiple nitro electron-withdrawing groups provides an enhanced second order nonlinear optical susceptibility χ^z, as compared to a corresponding acrylic copolymer in which the pendant stilbene side chains have

15 a single nitro electron withdrawing group. TABLE (1)

Material

KHP0₄ 2.4 X 10^" ⁹ esu mononitro-copolymer⁽²⁾ 20 X 10-⁹ esu dinitro-copolymer⁽³⁾ 32 X 10^"δ esu trinitro-copolymer⁽⁴⁾ 36 X 10^"9 esu

(1) As measured at 1.34 micron excitation wavelength.

(2) 4-[N-(2-methacroyloxyethyl)-N-methylamino]-4'- nitrostilbene/ ethyl methacrylate (50/50). (3) 4-[N-(2-methacroyloxyethyl)-N-methylamino]-2' ,4'- dinitrostilbene/methyl ethacrylate (50/50) . (4) 4-[N-(2-methacroyloxyethyl)-N-methylamino]-2' ,4' ,6*- trinitrostilbene/methyl methacrylate (50/50). The polynitrostilbene-containing side chains of a present invention copolymer contribute additional desirable properties as compared to a corresponding copolymer with mononitrostilbene-containing side chains. For example, a polynitrostilbene-containing side chain copolymer exhibits a higher level of thermal stability than does a corresponding mononitrostilbene side chain copolymer.

A present invention polynitrostilbene-containing side chain copolymer also exhibits an enhanced long term structural and optical stability when in the form of a molecularly oriented polymeric film medium which has been poled with an electric field as demonstrated in Example III. A polynitrostilbene-containing side chain copolymer has a higher ground state dipole moment and has a different light absorption spectrum than a corresponding mononitrostilbene-containing side chain polymer.

The following examples are further illustrative of the present invention. The components and specific ingredients are presented as being typical, and various modifications can be derived in view of the foregoing disclosure within the scope of the invention. EXAMPLE I This Example illustrates the preparation of 4-[N-(2- methacroyloxyethyl)-N-methylamino]-2' ,4'-dinitrostilbene- /methyl methacrylate copolymer (50/50).

A. A inobenzaldehyde Starting Material A reactor is charged with 2-(methylamino) ethanol (134 g, 1.8 moles), 4-fluorobenzaldehyde (74.4 g, 0.6 mole), potassium carbonate (82.8 g, 0.6 mole) and dimethylsulfoxide (750 ml), and the mixture is heated at 95^*C for 72 hours. The product mixture is cooled and poured into three liters of ice water. The yellow solid that precipitates is filtered, washed with water, and dried in a vacuum oven, mp 72^*C. The 4-[N-(2-hydroxy- ethyl)-N-methylamino]benzaldehyde product is recrystallized from water as needle-like crystals.

B. Schiff Base

4-[N-(2-hydroxyethyl)-N-methylamino]benzaldehyde

(179 g, 1.0 mole) and toluene (1.2 liters) are charged to a reaction flask, and the reactor is purged with argon.

The reaction is heated to reflux under argon, and water is removed with a Dean-Stark trap.

Methanesulfonic acid (0.2 ml) is added to the refluxing solution, and then aniline (102 g, l.l moles) is added dropwise, and the heating is continued until about 18 ml of water is removed. A yellow precipitate forms on cooling, and is sepa¬ rated by filtration and dried, mp 111.9^*C.

C. Stilbene Alcohol

A reactor is charged with 2,4-dinitrophenylacetic acid (45.23 g, 0.2 mole; Aldrich), toluene (360 ml), and

Schiff base (50.9 g, 0.2 mole) as prepared above. The reaction mixture is stirred at room temperature for one hour, then methacrylic acid (34.4 g, 0.4 mole) is added dropwise, and the reactor contents are heated at 75^βC for three hours and at 110^βC for two hours.

On cooling, the product separates as greenish-black crystals, mp 186^βC-189^βC. Anal. calc. for C₁₇H_l7N₃0₅: C59.47; H,4.99; N,12.24

Founded: C,59.45; H,5.03; N,12.24

D. Acrylate Monomer

A reactor is charged with stilbene alcohol (24 g, 0.07 mole) as prepared above, pyridine (240 ml) and dimethylaminopyridine catalyst (1.71 g, 0.014 mole). The reactor contents are heated to 75^*C, and methacrylic anhydride (29 ml, 0.195 mole) is added, and the reaction is conducted at 75^βC for 20 hours.

The product mixture is cooled, and poured into 750 ml of water. The resultant black crystalline precipitate is recovered by filtration and dried at 50"C in a vacuum oven, mp 122^β-125^*C. The chemical structure of the pro¬ duct is consistent with a NMR spectral analysis. Recry- stallization of the product from ethyl acetate/ethanol (3.2/1) yields shiny black crystals, mp 125^*-126^βC.

E. Copolymer (50/50) A reactor is charged with 4.11 g (0.01 mole) of acrylate monomer as prepared above and dimethylsulfoxide (41 ml), and dry argon gas is bubbled into the solution. The reactor then is charged with methyl methacrylate (1.0 ml, 0.01 mole) and azodiisobutyronitrile (33 mg) under an argon purge. The reaction mixture is heated at 70^βC for 48 hours to form copolymer product.

The product mixture is poured into a 500 ml volume of methanol to precipitate the copolymer. The copolymer is collected, then dissolved in tetrahydrofuran and reprecipitated in a volume of methanol.

The glass transition temperature (T_g) is 134^*C, and the weight average molecular weight is about 9000, as determined by size exclusion chromatography using Zorbax- PS bimodal columns with tetrahydrofuran as the mobile phase.

The copolymer is soluble in acetone, tetrahydrofuran or N-methylpyrrolidine, and insoluble in ethanol or toluene.

EXAMPLE II This Example illustrates the preparation of 4- [N- ( 2- methacroyloxyethyl ) -N-ethanolamino ] -2 ' , 4 ' , 6 ' -trinitro- stilbene/butyl acrylate copolymer ( 75/25 ) .

A. Aminobenzaldehyde Starting Material Following the procedure of Example I, 2-(ethylamino) ethanol is reacted with 4-fluorobenzaldehyde to provide 4-[N-ethyl-N-(2-hydroxyethyl)amino]benzaldehyde starting material.

B. Schiff Base Following the procedure of Example I, aminoben- zaldehyde starting material as prepared above is reacted with aniline to form a corresponding Schiff base inter- mediate product.

C. Stilbene Alcohol 2,4,6-Trinitrophenylacetic acid (mp 159^β-160"C) is prepared from l-chloro-2,4,6-trinitrobenzene and ethyl malonate in the presence of metallic sodium in accordance with a procedure by M. Kimura in J. Pharm. Soc. Jpn., 73, 1216 (1953).

Following the procedure of Example I, a Schiff base as prepared above is reacted with 2,4,6-trinitrophenyl- acetic acid to provide a corresponding stilbene alcohol product.

D. Acrylate Monomer

Following the procedure of Example I, stilbene alco¬ hol as prepared above is esterified with methacrylic anhydride to provide a corresponding acrylate monomer product.

E. Copolymer (75/25) Following the procedure of Example I, acrylate monomer (0.03 mole) as prepared above is copolymerized with butyl acrylate (0.01 mole) to form a copolymer pro¬ duct, with a T of about 120^*C, and a weight average mole¬ cular weight of 12,000.

EXAMPLE III This Example illustrates the construction and opera¬ tion of an optical frequency converting waveguide module in accordance with the present invention.

A silicon dioxide-coated silicon wafer with a grat¬ ing electrode is constructed by means of the following fabrication procedures.

A commercially available silicon dioxide-coated silicon wafer is placed in a Varian electron beam vacuum deposition system. A 0.1 μm layer of 99.999% purity aluminum is deposited on the wafer. AZ-1518 positive photoresist (Hoechst) is spin- coated on the aluminum-coated wafer with a Solitec model 5100 coater. A 1.5 μm photoresist coating is achieved by spinning at 5000 rpm for 30 seconds. The photoresist coating is dried in a vacuum oven at 90'C for 30 minutes. The photoresist coating is patterned by placing the wafer in contact with a mask of the desired configuration in a Karl Suss model MJB 3 mask aligner, and exposing the masked coating to 405 μm radiation (70 mJ/cm²).

The mask is removed, and a thin piece of silicon (1 cm x 2 cm) is placed on the surface of the patterned photoresist as a protective shield, and the combination is exposed to 70 mJ/cm² of 405 μm radiation. The pat¬ terned photoresist is developed with AZ Developer in water (1:1) over a period of 60 seconds, and the develop- ing cycle is terminated by washing with deionized water.

The photoresist-coating of the wafer is baked in a vacuum oven at 120^*C for 45 minutes. The exposed alumi- num pattern is etched with type A etchant (Transene Corp.) at 50^βC for 20 seconds, and the etched surface is rinsed with deionized water.

The aluminum grating electrode surface of the wafer then is covered with a 1.5 μm cladding layer of 20% poly- vinyl alcohol (75% hydrolyzed) in water by spin-coating at 5000 rpm for 30 seconds, and the cladding layer is dried in a vacuum oven at 110^βC for two hours.

A nonlinear optically active organic layer of 1.65 μm thickness is spin-coated on the cladding layer at 3000 rpm. The spin-coating medium is a 20% solution of the Example I copolymer (50/50) of side chain monomer/methyl methacrylate in cyclohexanone. The organic layer is dried in an oven at 160^*C for one hour. An upper cladding layer of 1.5 μm thickness is added by spin-coating a medium of polysiloxane (GR-651-L, Owens-Illinois Inc., 25% solids in 1-butanol) at 3500 rpm for 30 seconds. The cladding layer is dried in an oven at 110^*C for 35 minutes. A 0.055 μm coating of aluminum is deposited as an electrode layer on the upper cladding layer.

The fabricated waveguide is placed in a Mettler hot stage, and the unit is raised to 90^*C at l^*C/min. A DC field of 70V/μm is applied across the waveguiding organic layer for ten minutes by means of the electrodes. The electric field is maintained while the waveguide sample is cooled to room temperature at l^*C/min. The χ⁽²⁾ non¬ linear optical response of the waveguiding medium is 32 x 10^*9 esu as measured at 1.34 μm excitation wave- length.

The waveguide structure is cleaved at opposite ends to provide two sharp faces to couple light in and out of the waveguiding organic layer.

Cylindrical lenses are employed to focus and couple 1.34 radiation (0.01 mJ, 10 nsec wide pulse) into the waveguide. The waveguide is situated on a rotation stage, and phase-matched second harmonic generation is observed when the waveguide is rotated until the perio¬ dicity satisfies the value for phase-matching. Under the described operating conditions, a 0.5-1% amount of the fundamental beam is converted into observed second harmonic radiation.

Claims

WHAT IS CLAIMED IS :

1. An isotropic acrylic copolymer is characterized by recurring monomeric units corresponding to the formula:

where R is hydrogen or a C,-C₄ alkyl substituent; R¹ is a C^C_g alkyl substituent; m and m¹ are integers which total at least 10; n is an integer between about 1-6; and Z is hydrogen or a nitro substituent.

2. An acrylic copolymer in accordance with claim 1 which has a weight average molecular weight in the range between about 5000-200,000.

3. An acrylic copolymer in accordance with Claim 1 which has a glass transition temperature in the range between about 40^*-180^*C.

4. A nonlinear optical medium consisting of a transparent film of an isotropic acrylic copolymer which is characterized by recurring monomeric units corres¬ ponding to the formula:

where R is hydrogen or a C₁-C₄ alkyl substituent; R¹ is a C^C_g alkyl substituent; m and m¹ are integers which total at least 10; n is an integer between about 1-6; and Z is hydrogen or a nitro substituent.

5. A nonlinear optical medium in accordance with claim 4 which is characterized by an external field- induced orientation of aligned polymer side chains, and which exhibits second order nonlinear optical suscepti¬ bility χ⁽²⁾.

6. In an optical light switch or light modulator device the improvement which comprises a polymeric non¬ linear optical component comprising a transparent solid medium of an isotropic acrylic copolymer which is charac¬ terized by recurring monomeric units corresponding to the formula :

where R is hydrogen or a 0,-C^ alkyl substituent; R¹ is a C^C_g alkyl substituent; and m¹ are integers which total at least 10; n is an integer between about 1-6; and Z is hydrogen or a nitro substituent.

7. An optical device in accordance with claim 6 wherein the polymer medium exhibits third order nonlinear optical susceptibility χ⁽³⁾.

8. An optical device in accordance with claim 6 wherein the polymer medium has a stable orientation of an external field-induced alignment of polymer side chains, and the medium exhibits second order nonlinear optical susceptibility χ⁽²⁾.

9. An optical device in accordance with claim 6 wherein the polymeric nonlinear optical component exhibits less than about 10 percent scattering of transmitted incident light.

10. An optical device in accordance with claim 6 which is adapted to double the frequency of an incident laser beam.

11. An acrylate monomer corresponding to the formula:

CH = 2

where R is hydrogen or a C₁-C₄ alkyl substituent; R¹ is a C,-C₆ alkyl substituent; n is an integer between about 1- 6; and Z is hydrogen or a nitro substituent.