FIELD OF THE INVENTION

The present invention relates to new crystal structures useable in nonlinear optics and in electro-optics. It relates more particularly to new crystal structures exhibiting quadratic nonlinear effects and a wide transparency range extending from the visible to the near infrared. Finally, it relates to a process for the preparation of these crystal structures and a device with an electro-optical effect comprising such a crystal structure.

As a result of its noncentrosymmetric structure, which gives it a quadratic nonlinear susceptibility tensor .chi..sup.2, the crystal according to the invention can be used in any configuration of a mixture with three waves of "all-optics" type (second harmonic generation, frequency conversion, parametric effects) and electro-optical type (Pockels effect).

Prior Art

During the past ten years the investigation of crystal structures exhibiting quadratic nonlinear effects which are augumented when compared with materials such as potassium dihydrogenphosophate (Okada et al., Jap. J. Appl. Phys. 16, 55, 1977) as well as a wide transparency range extending from the visible to the near infrared, has centered on organic molecules such as those listed, for example, in the book published by D. S. Chemia and J. Zyss, Academic Press (1987) "Nonlinear Optical Properties of Organic Molecules and Crystals".

These organic crystals may be in the form of a set of molecular dipoles forming a polar stacking in the crystal lattice.

The highest nonlinear coefficient is oriented along these dipoles. Among the crystal structures of this type there may be mentioned:

POM (3-methyl-4-nitropyridine 1-oxide) (J. Zyss, D. S. Chemia and J. F. Nicoud, J. chem. Phys. 74, 4800, (1981)),

NPP (N-(4-nitrophenyl)-L-prolinol) (J. Zyss, J. F. Nicoud and A. Coquillay, J. chem. Phys. 81, 4160 (1984)),

NPAN (N-(4-nitrophenyl)-N-dimethylaminoacetonitrile) (P. Vidakovic, M. Coquillay and A. Salin, J. Opt. Soc. Am. B4, 998, (1987)).

These crystals exhibit an exceptionally high nonlinearity (for example d.sub.21 of the order of 50 pm/V in the case of NPP), but at the cost of a transparency which is limited towards the blue part of the spectrum. Furthermore, the cohesion and the air-stability of such crystals under usual conditions of application may present disadvantages in the case of some of them; this is particularly the case with POM, which must be immersed in a protective liquid and furthermore exhibits a marked tendency to sublimation.

It has recently been proposed by R. Masse and J. Zyss (Molecular Engineering 1, 141, 1991) to encapsulate nonlinear organic molecules in a "framework" array consisting of inorganic anions associated with each other by a single or multiple hydrogen bond, with inorganic anions. The polyanions proposed are of arsenate (H.sub.2 AsO.sub.4.sup.-).sub.n, sulfate (HSO.sub.4.sup.-).sub.n or phosphate (H.sub.2 PO.sub.4.sup.-).sub.n type. A particularly advantageous cystal has been identified in this class: 2-amino-5-nitropyridiniumdihydrogen monophosphate (2A5NPDP).

This crystal structure, of mm2 point symmetry exhibits an inclination of the order of 36.degree. of the transition dipoles of the organic unit in relation to the crystal polar axis, and this favors the coefficient d.sub.31 (of the order of 10 pm/V) and the parametric effects requiring phase tuning by birefringence (second harmonic generation, parametric amplification).

It is nevertheless desirable to find other crystal structures where a coefficient d.sub.ii (with i=1, 2 or 3) would be preferentially optimized by the most perfect possible alignment of the molecules in the lattice along a crystallographic direction i, resembling the crystals of MNA (2-methyl-4-nitroaniline) as described by G. Lipscomb, A. Garito and R. Narang (J. chem. Phys. 75, 1509, (1981)) or crystals of DMACB (4,4'-dimethylaminocyanobiphenyl) as described by J. Zyss, I. Ledoux, M. Bertault, and E. Toupet (Chem. Phys. 150, 125, (1991)).

It is recalled that the coefficients d.sub.21 and d.sub.22 refer to the coefficients of the quadratic nonlinear susceptibility tensor, the axis 2 being the axis of linear symmetry in the crystallographic group P2.sub.1 (contrast rotation).

Reference should also be made, furthermore, to G. R. Meredith (ACS Symposium Series, 233, 27-56 (1983)), who has developed organic salts for quadratic nonlinear optics, in which the cations exhibiting an intramolecular charge transfer character are used in combination with chiral or achiral organic or inorganic anions. The anions employed in this reference are independent units which are not organized into a polyanionic structure, and the mechanical cohesion of such structures is still not sufficient.

SUMMARY OF THE INVENTION

In the light of the prior art set out above, one of the objectives of the invention is to propose new noncentrosymmetric crystal structures exhibiting quadratic nonlinear effects.

A second objective of the invention is to propose crystal structures exhibiting quadratic nonlinear effects which are increased when compared with those of the prior art.

Another objective of the present invention is to propose crystal structure exhibiting quadratic nonlinear effects and an excellent cohesion of the organic molecular units in the crystal.

Further objectives of the invention will appear on reading the description which is to follow.

According to the present invention the noncentrosymmetric crystal structure consists of an, optionally substituted, 2-amino-5-nitropyridinium halide.

The substituents of the 2-amino-5-nitropyridinium cations are those which do not significantly alter the favorable orientation of the polarizable organic unit in the crystal lattice.

The halide is preferably a chloride ion, a bromide ion or a fluoride ion.

According to a preferred alternative form the crystal structure is such that the cation corresponds to the following formula: ##STR1## in which: R denotes a hydrogen atom, a methyl or ethyl radical or a hydroxyl radical.

The cation is also preferably 2-amino-5-nitropyridinium.

Without being bound in any way whatsoever to a scientific theory, the Applicant nevertheless believes that these structures have a specific cohesion due to hydrogen bonds which are short and therefore highly energetic, permitting the aggregation of the cations around the halide anions. Bonding of this type gives the crystal structure according to the invention stability, transparency and ease of growth.

Also preferably, the crystal structure according to the invention is a monocrystal.

The invention also relates to a process for the preparation of the crystal structures as just described in the above description, in which one mole of an optionally substituted 2-amino-5-nitropyridine is mixed with at least one mole of a hydrogen halide in a suitable solvent, the mixture is caused to crystallize and the crystal structures are recovered.

The suitable solvent is especially water or an acetone/water mixture.

One mole of an optionally substituted 2-amino-5-nitropyridine will be preferably mixed with between 0.3 and 2 moles of a hydrogen halide.

The crystallization is performed by slow evaporation of the solvent. A method which is well known to a person skilled in the art is therefore involved.

Advantageously, in a solution which is saturated in rotation with an anion-cation mixture, a crystal seed of a crystal structure such as described above will be present. Slow evaporation results in the crystallization which is sought, at constant temperature (especially ambient temperature). This temperature can also be gradually lowered.

The monocrystals have the required properties, that is to say a polar crystal structure compatible both with the requirements of nonlinear optics (quadratic effects) and of ferroelectricity, a nonlinear response and a satisfactory transparency-efficiency compromise.

Finally, the invention relates to a device with an electro-optical effect comprising a crystal structure as described above. These devices generate a second harmonic or more generally a sum or difference of frequencies. The applications which stem from these advantageous properties are numerous, both as bulk crystal and as a guiding structure, such as,

phase modulation of an optical carrier

amplitude modulation

directive coupler,

electro-optical crossover and more generally electro-optical switching point,

electro-optical sampling,

electronic logic'

electro-optical triggering of lasers ("Q-Switch") without prejudice to applications requiring a phase tuning using birefringence, such as:

second harmonic generation,

frequency conversions

parametric amplification, emission and oscillation.

By way of example, a laser cavity with electrooptical triggering, provided with a crystal structure according to the invention is described below.

In such a device a crystal structure according to the invention is introduced into the cavity of a laser (for example a YAG laser emitting at 1.06 or 1.34 m and more generally any laser emitting in the crystal transparency range of 0.45 to 1.7 m) in a configuration of the Pockels cell type. The interplay of polarizations and reflections results in a blocking of the emission when the crystal is under tension. In the absence of tension the crystal becomes transmitting again and the cavity can emit. A general scheme of the principle is shown, for example in R. W. Hellwarth "Q Modulation of lasers" in Lasers, vol. 1, A. K. Levine ed. (New York, Marcel Dekker, 1996) p. 253.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now illustrated by the embodiments given by way of guidance below and by FIGS. 1 and 2, appended.

FIG. 1 shows the crystal lattice of the 2-amino-5-nitropyridinium chloride crystal projected in the a, c! crystallographic plane.

FIG. 2 shows the same crystal projected in the a, b! plane.

The noncentrosymmetric structure of the crystals according to the invention can be seen on reading these figures, where a layer of anions separating the organic dipoles can be noted, which are oriented in patterns of herringbone type in a single polar direction (P2.sub.1 structure).

EXAMPLE 1

One mole of 2-amino-5-nitropyridine (2A5NP) is dissolved completely in the minimum volume of acetone at ambient temperature. 30 cc of a molar solution of hydrochloric or hydrobromic acid are added. The solutions obtained are slowly evaporated at ambient temperature and easily yield transparent crystals 4 mm.times.4 mm.times.4 mm in size. The reaction can also be performed in water at 40.degree. C.: 0.8 mole of 2A5NP will then be dissolved per one mole of hydrochloric or hydrobromic acid. Raising the temperature makes it easier to dissolve the 2-amino-5-nitropyridine nitropyridine in the acetone-water mixture.

The chemical reactions are written:

C.sub.5 H.sub.5 N.sub.3 O.sub.2 +HCl.fwdarw.C.sub.5 H.sub.6 N.sub.3 O.sub.2.sup.+ +Cl.sup.- .fwdarw.C.sub.5 H.sub.6 N.sub.3 O.sub.2.sup.+.Cl.sup.- (2A5NPCl)

C.sub.5 H.sub.5 N.sub.3 O.sub.2 +HBr.fwdarw.C.sub.5 H.sub.6 N.sub.3 O.sub.2.sup.+ +Br.sup.- .fwdarw.C.sub.5 H.sub.6 N.sub.3 O.sub.2.sup.+.Br.sup.- (2A5NPBr)

Investigation of the crystal structures has made it possible to determine unambiguously the chemical formulae and the isomorphology of the two compounds. The active phases are perfectly characterized:

2A5NPCl a=9.956(7), b=7.737(1), c=4.813(1) A beta=95.86(9).degree.P2.sub.1

2A5NPBr a=10.074(7), b=7.805(1), c=4.949(1) A beta=95.72(9).degree.P2.sub.1

EXAMPLE 2: crystal growth

Evaporation of solutions containing one mole of 2A5NP per one mole of HCl (or HBr) in air at ambiant temperature produces beautiful crystals of 2-amino-5-nitropyridinium chloride or bromide, both of which have the same morphology (P2.sub.1 symmetry). Sturdy crystals of 7 mm.times.7 mm.times.5 mm, or even of 10 mm.times.6 mm.times.5 mm in the case of the chlorides, have been easily obtained.

These crystals have thermal and mechanical properties equivalent to those of 2A5NPDP.

Furthermore, according to the tests on powder for second harmonic generation (illumination with an Nd:YAG laser, .lambda.=1.06 .mu.m) their efficiency is closely related to that of POM and of NPP.