EP0372064A1

EP0372064A1 - Structure generating non-linear electro-optical effects and applications

Info

Publication number: EP0372064A1
Application number: EP89907182A
Authority: EP
Inventors: Dominique Broussoux; Jean-Claude Dubois; Pierre Le Barny; Philippe Robin
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1988-06-10
Filing date: 1989-06-06
Publication date: 1990-06-13
Also published as: WO1989012248A1; US5267076A; FR2632737B1; FR2632737A1

Abstract

L'invention concerne l'obtention d'une structure génératrice d'effets électrooptiques non linéaires, les effets étant dus à des molécules ou à des groupements intramoléculaires possédant une susceptibilité non linéaire et l'orientation des molécules ou des groupements étant induite par un champ électrique appliqué. Selon l'invention, l'orientation des molécules ou groupements intramoléculaires (5) est obtenue au moyen d'un diélectrique à polarisation permanente tel un électret (6). Application aux composants de l'optique non linéaire: doubleur de fréquence, autocorrélateur, mélangeur d'ondes.The invention relates to obtaining a structure generating nonlinear electrooptical effects, the effects being due to molecules or intramolecular groups having nonlinear susceptibility and the orientation of the molecules or groups being induced by a field. electrical applied. According to the invention, the orientation of the molecules or intramolecular groups (5) is obtained by means of a dielectric with permanent polarization such as an electret (6). Application to nonlinear optics components: frequency doubler, autocorrelator, wave mixer.

Description

METHOD FOR OBTAINING A NON-LINEAR ELECTROOPTICAL EFFECT GENERATING STRUCTURE, STRUCTURE OBTAINED AND APPLICATIONS

The present invention relates to a process for obtaining a structure generating non-linear electrooptical effects, that is to say a structure having a susceptibility of order greater than 1. It also relates to the structures obtained by this process and in particular those allowing the generation of the harmonic 2 of an optical wave incident to such a structure. The invention also relates to the possible applications of these structures which can be shaped into planar layers, films or to produce optical guides.

Since the development of the intense light sources provided by lasers, electrooptical effects have been applied in many fields such as optical telecommunications and optical signal processing. Electrooptical effects are induced in dielectric materials (i.e. bodies transparent to an electromagnetic wave) by the application of electric fields. These electric fields can be those associated with incident light beams on the dielectric material.

The refractive index of the transparent dielectric material becomes ε'.jrs as a function of the intensity of the incident beam (s) ¹ which opens up the field of non-linear optics (generation of harmonics, frequency transposition, optical memories, etc. ..)

When an electric field E is applied to a dielectric, the electrically charged particles which form the material of the dielectric (electrons, ionized atoms) are separated from their equilibrium position by a distance which is a function both of the intensity of the E field, of the electrostatic restoring forces to which these particles are subjected, of the frequency of the applied electric field and of the arrangement of dielectric molecules. This results in a polarization of the material which 1 is noted by the vector P and which is equal by definition to the product of the density of the charges displaced by the vector representative of this displacement. Between the electric field E and the polarization vector linked to matter P, there is a matrix relation of the form

^{* p} = l O ii ^

if the intensity of the applied electric field is not too high. If the medium is isotropic, IIII ^is reduced to a scalar. In the general case of anisotropic media, 1 1 1 1 is a tensor.

However, if an intense electromagnetic field of optical frequency is applied to the dielectric, the induced polarization is no longer proportional to the applied optical field, but has non-linear terms proportional to the square and to the cube of the fields applied. All of the effects produced for these nonlinear polarizations have been very studied since the discovery by FRANKEN of the generation of second harmonics by a quartz crystal subjected to the radiation of a ruby laser (PA FRANKEN, AE HILL, CW PETERS, G.

WEIMREICH, Phys. Rev. Lett. , Flight. 7, 1961, p. 118).

We can dec ~ t laugh the response of a medium to the application of an electromagnetic field by the development of the polarization induced in whole series of the applied electric field:

P = ll X ⁽⁽ D ¹⁾ Ml |. T E. * M | | -7- ⁽ (2 ² 2 ⁾ >) I |. | E | .E ₊ The first term of this development represents linear polarization, while the umpteenth term translates the nth order non-linear response of the medium to the applied field. The coefficients 7 _\ are tensors of order (n + 1) which are called tensors of nonlinear susceptibility of the umpteenth order. The different terms contained in P decrease very quickly with the order n and it is only with the appearance of the intense light sources represented by the laser beams that we were able to exploit the non-linear electrooptical effects corresponding to the susceptibility d 'order 2.

For certain dielectrics and under certain conditions, a polarization induced by an incident light beam and corresponding to a quantity of stored energy gives rise to the radiation of a wave oscillating at a frequency twice that of the incident light beam, which corresponds to the restitution of part of the stored energy.

One of the conditions necessary for a dielectric to be able to generate, by non-linear effect, a wave representing the second harmonic of an incident light beam, is that its susceptibility tensor of order 2 is different from zero. The tensors ^" / \ 'being defined starting from the properties of the crystals, one can strongly restrict the number of independent coefficients of the tensors / \ ^' with 3 components by using the properties of symmetry of the medium.

Therefore 'will only be different from zero in non-centrosymmetric media. This therefore excludes all amorphous media (glasses, lassic polymers) which are commonly used in bedtime optics, thin. Currently, the materials most commonly used to obtain the phenomenon of second harmonic generation in non-linear optics are mineral single crystals such as lithium niobate or potassium dihydrogen phosphate (KDP). However, organic compounds have a number of advantages in this area over mineral salts. Indeed, at frequencies in the field of optics, the nonlinear effects to which they give rise are of purely electronic origin, which leads to an almost instantaneous response of these materials and qualifies them for applications in the field of processing. ultra-fast optical signal. On the other hand, they have a higher optical damage threshold (also referred to as photorefringence). Finally, they offer enormous potential in molecular engineering, making it possible to obtain materials with greater second-order susceptibilities.

Organic materials with an optical function (frequency doubler, Pockels effect, parametric oscillator, optical mixer) can be obtained with different types of materials and by different processing techniques. _G the organic compounds used for the nonlinear optical molecules are formed, or contain molecules having a coefficient called hyperpolarizability second order (or hyperpolarizability).

The best way currently known to obtain molecules having a very high hyperpolarisability is to synthesize compounds formed of a system of 7Î 1 electrons conjugated and having an asymmetry of charges. But for the compound to give rise to the phenomenon of generation of second harmonic, it is also necessary that the molecular arrangement preserve on the macroscopic scale the asymmetry of electronic density which exists on the molecular scale.

Organic materials are known which consist of two-core hyperpolarisability molecules organized in three dimensions in the form of * ^* organic crystals. For these materials to be satisfactory, it is necessary to obtain non-centrosymmetric single crystals. However, the

^'Rate of success in obtaining such crystals is only

20% with non-chiral compounds. Indeed, molecules like 4 N dimethylamino 4 'nitrostilbene (DANS) have a strong permanent dipole moment and go head to tail, which leads to a centrosymmetric system.

We also know the layers formed of second order hyperpolarisability amphiphilic organic molecules, deposited on a substrate by the technique of

Lan muir-Blodgett. These layers have the disadvantage of being formed of polycrystals which causes light scattering phenomena.

Another type of ordered structure of molecules having a very large nonlinear response and capable of generating the second harmonic of an electromagnetic wave consists of polymers. The polymer films are particularly interesting since they can be applied on very large surfaces (relative to the size of the single crystals) and on substrates of various types. Finally, they can be produced much faster than single crystals and at low cost.

The layers formed from mesomorphic copolymers with differentiated and alternating lateral groups are known. The groups are either of a mesogenic nature favoring the establishment of a mesophase, or of high hyperpolarisability of the second order. An electric field applied to these layers orients the mesogenic groups which impose this orientation on groups with high second order hyperpolarisability. The structure obtained is not centrosymmetric (see FR 2,597,109 filed by the applicant).

Also known are the layers formed of molecules with hyperpolarisability of the second order oriented ", under an electric field applied in a matrix of mesomorphic polymer with mesogenic lateral groups promoting the establishment of the mesophase generating the orientation of the active molecules in non-linear optics. This is a first way of obtaining doped layers also called "guest-host" systems. Also known are the layers formed of active molecules in non-linear optics oriented under an electric field in an amorphous polymer matrix with a glass transition temperature above room temperature. As the orientation of the active molecules takes place at a temperature higher than the glass transition temperature, returning to ambient temperature freezes the orientation of the electric dipoles of these molecules. This is a second way of obtaining doped layers. In the case of the layers obtained by the technique of

Langmuir-Blodgett or in the form of single crystals, the orientation of the molecular dipoles is inherent in the Langmuir-Blodgett technique or of crystallogenesis and is stable over time. In the case of polymers active in non-linear optics, a non-centrosymmetric system is obtained by orientation of the corresponding molecular dipoles under the effect of a sufficiently intense electric field, the application time and intensity of which depend on the temperature at which it is applied. As a general rule, amorphous materials with a glass transition greater than the ambient are used so as to freeze the molecular movements at ambient temperature after polarization.

For example, for an organic layer formed of a poly (methyl methacrylate) or PMMA matrix containing active molecules with high hyperpolarisability, the application of an electric field greater than 1MV / cm at 100 ° C for 10 min with cooling under field provides a second-order susceptibility layer equal to 70 x 10 -12 m / V, or 10 times that of lithium niobate. In fact, the susceptibility value was initially higher but, due to relaxation phenomena, it decreases by about 30% before stabilizing after about 72 hours.

The invention proposes a new way of obtaining a structure which generates non-linear electrooptical effects and which does not have the drawbacks listed above. The molecules active in non-linear optics are oriented by an applied electric field which is induced by a dielectric with permanent polarization which can advantageously be an electret.

The subject of the invention is therefore a process for obtaining a structure generating nonlinear electrooptical effects, said effects being due to molecules or intramolecular groups having a nonlinear susceptibility, the orientation of said molecules or said groups being induced by an applied electric field, characterized in that the method consists in obtaining the orientation of said molecules or of said groups by means of a dielectric with permanent polarization.

The invention further relates to a structure generating non-linear electrooptical effects, the structure comprising molecules or intramolecular groups having a non-linear susceptibility, characterized in that q χ _Ue _has structure also includes a dielectric-polarized, said molecules or said intramolecular groups being subjected to the action of the electric field developed by the dielectric with permanent polarization.

The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation, and thanks to the appended figures among which:

- Figure 1 is illustrative of an electret with excess charges,

FIGS. 2 and 3 represent bilayer structures according to the invention,

FIGS. 4 and 5 represent monolayer structures according to the invention, FIG. 6 represents a bilayer structure with an optical guide according to the invention,

FIG. 7 represents a monolayer structure with an optical guide according to the invention, FIGS. 8 and 9 represent cylindrical structures of the bilayer type according to the invention,

FIG. 10 represents a bilayer structure with a liquid optical layer, according to the invention,

- Figure 11 shows a two-layer structure guided by the optical wave by index variation, according to the invention.

According to the invention, the structure generating non-linear electrooptical effects is a bifunctional structure with one or two layers. It can comprise an optical function layer comprising molecules or intramolecular groups active in non-linear optics, these molecules or these groups being oriented by application of an electric field. In particular, for the generation of second harmonics, these molecules or intramolecular groups will have a high hyperpolarizability). The structure may also comprise a dielectric layer with permanent polarization generating the electric field orienting the molecules or intramolecular groups mentioned above. The structure according to the invention may also comprise only a single layer which performs both of the functions: optical and electrical.

The dielectric with permanent polarization can advantageously be constituted by an electret. If we refer to the article "Electrets" by F. MICHERON published in "Engineering Techniques, E 1893, December 1987": "an electret is a dielectric which carries an almost permanent polarization, ie - say whose decay time is longer than its duration of use. This definition differentiates electrets from dielectrics with strictly permanent polarization, such as pyroelectrics (eg tourmaline), ferroelectrics (eg barium titanate) and polar piezoelectrics (eg lithium niobate). The border between dielectrics with quasi permanent polarization and those with strictly permanent polarization can be explained according to several criteria: for the first, the polarization is always induced artificially, while for the second it is spontaneous. The electrical state of the former is a metastable state, while for the latter it _is strictly stable. Finally, electrets are formed in disordered materials (polymers and amorphous minerals) while spontaneous polarization is always the consequence of a polar crystal structure ".

For reasons relating to the application of the structures according to the invention, it is advantageous to use electrets from polymers. Indeed, these make it possible to easily obtain thin layers or films deposited on a substrate.

Among the various polymers capable of being polarized, there may be mentioned:

- polytetrafluoroethylene (PTFE) or teflon (according to the brand registered by Du Pont de Nemours)

- poly (tetrafluoroethylene - hexafluoropropylene) or Teflon FEP

- poly (tetrafluoroethylene - perfluoromethoxylene) or FPA teflon

0

CF, polypropylene (PP)

"3 These polymers give very interesting electrets because of their exceptional stability which can be maintained for almost a century at room temperature. In addition, these electrets polymers can be manufactured continuously according to industrial processes by implantation of charges. 0 The Corona discharge is an industrial process of implantation of charges. The polarization of the polymer comes from an excess of charges trapped very close under one of the faces of the electret. To implement the Corona discharge, a block or a polymer film to be polarized is brought into contact by one of its faces with an electrode. A point, brought to a high voltage of several kilovolts relative to the electrode, is placed a few millimeters above the face of the polymer opposite the face in contact with the electrode. Ion species move in the electric field thus created. They 0 deposit on the surface and some of these sp charged species transfer their charges to traps located below the surface where they lose all mobility. It is these latter charges which give the polymer an almost permanent polarization, while the surface charges can flow by surface conductivity.

The implantation of charges can also be obtained by other means known to those skilled in the art, for example by the use of an ion beam of sufficient energy.

FIG. 1 schematically illustrates the distribution Q of the charges obtained in an electret with excess of charges. The block or film 1 constituting the electret has two main faces 2 and 3. By the method described above, positive charges which are shown encircled in FIG. 1 have been trapped very close under the face 2.

- represent positive surface charges and signs negative surface charges. All of these charges induce an electric field E in the dielectric 1.

According to the invention, it is possible to obtain a structure generating nonlinear electrooptical effects, these effects being due to molecules or intramolecular groups having a nonlinear susceptibility, by orienting these molecules or these groups by means of an electric field emanating from 'a permanently polarized dielectric such as an electret. Intramolecular molecules or groups with non-linear susceptibility must be able to be influenced by the orientation electric field. The best solutions therefore consist in having either a two-layer structure, each layer having a very specific function, optical or electrical, or a single-layer structure in which the dielectric performs both functions.

The examples which follow, given without limitation, will illustrate some aspects of the invention.

Example 1: bilayer structure. This bilayer structure is shown in the figure

2. A dielectric layer 5 active in non-linear optics is superimposed on a layer 6 constituting an electret of the type represented in FIG. 1. The refractive index of the electret must be appropriate to that of layer 5, it is ie less than the index of layer 5. The electret is obtained, for example, by implantation of positive charges as in the case of FIG. 1. In the case of a PTFE layer of 10 to 30 μ thick, these charges can be implanted about lμ below the surface. This installation can be considered as permanent because the conductivity of this dielectric is almost zero (10 £ L x m).

The face of the electret carrying these charges is that in contact with the active layer 5. The charges very close to the surface can be considered as trapped on the surface and generate an electric field in the active layer 5 in nonlinear optics.

In FIG. 2, to simplify the representation, the trapped positive charges and the surface positive charges are identically represented by the + sign.

The "optical" layer 5 has a thickness 1_. and a dielectric constant £ _ι . The "electric" layer 6 has a thickness 1 „and a dielectric constant ç,„. Let T "be the density of charges placed very close to the surface of the electret. This charge density generates electric fields E_ and E" in the optical layer 5 and in the electric layer 6 respectively.

ε. = ~ f,

-l _λ £? + -L? ^χ £,

The conditions of validity of these formulas are set out in the article by F. MICHERON entitled "Properties and applications of polarized dielectrics: electrets, oriented polymers and ferroelectrics" published in the Technical Review of

THOMSON-CSF, volume 10, n ° 3, p. 457, September 1978 ". If l _j = 2μ, 1 ₂ = lOμ, _± = 5, £ ₂ = 3, <T = lθ ^{" 5} to lθ ^"6 C / m ² , the corresponding electric field E .. is 10 to 10 V / cm. The value of this electric field is sufficient to orient and keep oriented the active molecules of the optical layer.

The optical layer 5 can be, and this is another advantage of the invention, of varied nature. For example, layer 5 can be formed from mesomorphic copolymers - with differentiated and alternating lateral groups, the groups being either of mesogenic nature, or active in non-optical linear. The contacting of layers 5 and 6 then has the effect of orienting the mesogenic groups because of the electric field developed by the electret. The mesogenic groups then in turn impose this orientation on the groups active in non-linear optics.

If layer 5 constitutes a "guest-host" system in which molecules active in non-linear optics are scattered in a matrix of mesomorphic polymer with mesogenic lateral groups, here again, the placing in contact of layers 5 and 6 will cause the orientation active groups in nonlinear optics.

If layer 5 constitutes a guest-host system in which the molecules active in non-linear optics are disseminated in an amorphous polymer matrix, the contacting of layers 5 and 6 will take place at a temperature higher than the transition temperature glass of the amorphous polymer to allow the orientation under field of the molecules active in nonlinear optics.

The electric field permanently prevailing in the optical layer because of the proximity of the electret makes it possible to obtain a permanent alignment of the active molecules in nonlinear optics, parallel to the field lines and thus to avoid the decrease in susceptibility as a function time. Example 2 This example relates to a two-layer structure placed on a glass substrate covered with an electrode made of mixed tin and indium oxide (ITO electrode). Is deposited, as shown in Figure 3, on the substrate 10, on the side of the electrode II, a solution containing polypropylene so as to obtain ^" after drying and evaporation of the solvent a layer 12 having a thickness of about one ten μ. The deposit can be done by the so-called "spinning" method. The solvent used can be chlorobenzene or trimethylbenzene. The layer 12 thus obtained then receives an implantation of charges by the Corona effect which makes it possible to trap a certain number of surface charges. This operation is done at high temperature (above 80 ° C) in order to populate the deep traps. The electrode 11 then serves as a contact electrode with the dielectric to be polarized. The voltage necessary for the implementation of the Corona effect is applied between the electrode 11 and a tip which is moved above the surface to be trapped. In this example, positive ions have been injected, as indicated by the + signs represented in FIG. 3. Once the electret, which constitutes the layer 12, has been made, another solution is deposited on this layer to obtain the layer optical. This solution can also be deposited by the so-called spinning method. It contains, for example, a polymer of polymethylmethacrylate type with hyperpolarisability molecules, of azo type comprising donor and acceptor groups at the ends. Such molecules are known to those skilled in the art. The solvent for the solution can be methylisobutylketone or methyl ethyl ketone. In order to evaporate and dry the film, the structure is brought to high temperature (above 60 ° C.) for one hour under vacuum.

If the solution used to deposit the second layer is sufficiently resistive, the electric field radiated by the electret is sufficient to orient the active molecules in non-linear optics during the evaporation of the solvent.

If the solution is not sufficiently resistive, charges of opposite sign to the trapped charges will come to screen the radiated electric field and, therefore, no orientation will be '. impossible. In this case, it is necessary to deposit on the optical layer 13, after drying and evaporation of the solvent, an electrode (for example made of aluminum or gold) and to polarize the composite structure by application of an electric field, taking account of the direction of the electric field radiated by the electret. Then simply eliminate this electrode to obtain the final structure. The elimination of this electrode can be done by methods well known to those skilled in the art.

The electric field radiated by the electret will keep the dipoles of the optical layer permanently aligned over a temperature range defined by the stability of the electret and which, as a general rule, depends on the polarization temperature (greater than 80 ° VS) . In FIG. 3, some of these dipoles are shown in summary form, the general direction of which is well perpendicular to the deposited layers.

The thickness of the optical layer thus obtained can be of the order of a few μ.

Example 3: monolayer structure. In this case, the dielectric layer provides both optical and electrical functions.

In the application to the generation of harmonic 2, the layer contains molecules or intromolecular groups with hyperpolarisability (oriented in parallel and is electrically charged by implantation of charges.

FIG. 4 schematically represents a bifunctional layer 15 by adopting for the electrical charges the representation taken for FIG. 1. We have also shown briefly some of the dipoles of molecules with high β hyperpolarizability.

Can be used as optically active molecules those described in Example ' ^~ . These molecules can be contained in a tal material such as poly (methyl methacrylate) PMMA or poly (methyl acvylate) PMA.

The optically active intramolecular molecules or groups spontaneously orient themselves under the influence of the implanted charges. This orientation is all the better if one proceeds at a temperature higher than the ambient temperature, for example at 60 ° C. Example 4

This example relates to a monolayer structure arranged on a substrate. Figure 5 shows such an arrangement. The substrate 20 has an optical refractive index lower than that of the layer 21 which is deposited thereon and which must ensure the double optical and electrical function. The layer 21 is deposited for example by the so-called spinning method, followed by evaporation and drying. It contains molecules or intramolecular groups with high fixed hyprpolarizability. Charges, for example positive, are injected into the face of the layer 21 which is opposite to the substrate 20. If the substrate 20 is not conductive, it is necessary, as in Example 2, to provide an electrode between the substrate and the layer 21. If the density of charges trapped on the surface is sufficient, the electric field induced by these charges will orient the molecules or intramolecular groups with high p-hyperpolarisability, and will keep them permanently aligned. The materials that can be used can be those of Example 3. Example 5

This example relates to a bilayer structure with an optical guide. The structure described in this example is the same as that of Example 2. The essential difference between the two examples is constituted by the use of a negative photoresist polymer.

FIG. 6 represents such a bilayer structure with an optical guide. A glass substrate 25 supports an ITO electrode 26 itself covered by the electret 27 which supports the optically active layer 28. The arrangement of this structure can be carried out in the same way as in the case of Example 2 Instead of being a uniform layer, layer 28 forms an optical waveguide. To obtain this optical waveguide, a uniform optical layer is first deposited on the electret 27 as in the case of FIG. 3. This optical layer is made of a negative photoresist type polymer (crosslinkable under radiation) in order to obtain only the part irradiated during masking under the effect of ultraviolet radiation, an electron beam or X-rays and after development by a solvent. The orientation of optically active intramolecular molecules or groups must be done before irradiation. In order for the optical waveguide to fulfill its function, the layer 27 playing the role of electret must have an optical refractive index lower than the refractive index of the waveguide 28. Example 6

This example relates to a monolayer structure with an optical guide, the term monolayer always being reserved for the fact that the same layer performs both the optical and electrical functions. To develop this structure, it is possible to start, as shown in FIG. 7, from a glass substrate 30 on which a thin insulating layer is first deposited, then a bifunctional layer made of a negative photoresist type polymer and intended to ensure the both optical and electrical functions. After irradiation and development of the photoresist polymer to obtain the waveguide shape 32, the electret function of the guide is carried out, for example by implantation by means of an ion beam. An insulating protective layer 33 is then deposited until the waveguide 32 is covered, which is thus buried. The layer 33 has a refractive index lower than that of the guide 32. Under the reference 33, two layers have in fact been designated which are deposited in two different stages, but which can be of the same material, for example polyfluoride de vinylidene. By providing means for applying an external electric field (one electrode on the substrate 30 and another on the layer 33), an electro-optical modulator is obtained.

Example 7 This example relates to a structure relating to the bilayer type, but the components of which are cylindrical as shown in FIG. 8. A sheath 35 is obtained by axial drilling of a cylinder of revolution made of dielectric material. From this sheath, an electret can be produced by implantation of charges on the external tubular surface 36. The implantation of charges can be obtained by placing the sheath 35 under an ion beam and by rotating it around its axis. These charges are positive in the 0 case shown in Figure 8, but they can of course be negative. The sheath 35 then radiates a radial electric field, of cylindrical symmetry.

An optical fiber 37, made of a material containing intramolecular molecules or groups active in t - nonlinear optics, is introduced inside the sheath 35. Under the effect of the electric field radiated by the sheath and the temperature (upper at 60 ° C) imposed, the optically active molecules or groups of the fiber orient radially and will remain fixed in this orientation so

20 permanent. The optical refractive index of the sheath 35 is lower than that of the fiber 37.

Example 8

This example relates to a structure similar to that of Example 7, but which differs therefrom by the nature and the distribution of the implanted loads. Figure 9 illustrates such a structure. This figure shows the sheath 40 and the fiber 42 as before. The difference is that the charges implanted on the outer tubular surface do. ut negative at one end of the sheath and positive at the other _Q end. The resulting polarization is then parallel to the axis of the fiber.

Example 9

This example concerns the bilayer structure represented in FIG. 10. By one of the methods already

- described, a polarized electret film 50 is deposited on the face a substrate 51 which is covered by an electrode 52. The substrate 51 can be made of glass and the electrode 52 of a mixed alloy of tin and indium oxides (ITO). A liquid layer 53 containing non-oriented hyperpolarizable molecules is deposited between the electret film 50 and a glass slide 54.

The seal is ensured by seals 55, for example made of epoxy.

If the liquid layer 53 is sufficiently resistive, the optically active intramolecular molecules or groups orient themselves under the effect of the electric field induced by the electret 50.

If the liquid layer 53 is not sufficiently resistive, an electrical control element must be associated with the structure. In this case, the internal face of the blade 54 is covered with an ITO electrode 56. To cause the orientation of the optically active molecules or groups of the liquid layer 53, it is necessary to apply, between the electrodes 52 and 56, a pulse DC voltage developing an electric field in the opposite direction to that developed by the electret. Once the optically active intramolecular molecules or groups are correctly oriented, the electric field developed by the electret will maintain this arrangement for a certain time which will depend on the greater or lesser resistivity of the liquid layer 53. It will be necessary to periodically reorient the optically active molecules by means of a voltage pulse.

Example 10

The structure shown in Figure 11 is of the bilayer type. On an substrate 60 (glass, silicon, gallium arsenide, etc.), an optical layer 61 has been deposited comprising molecules or intramolecular groups active in non-linear optics. The deposition of the optical layer 61 can be obtained from a solution, as described above. A layer 62 of dielectric material is then deposited on the optical layer 61. In the dielectric layer 62, an ion beam is created by implanting ions. two zones 63 and 64 playing the role of electrons. Zones 63 and 64 are implanted by loads of opposite signs. It is thus possible to obtain a structure guided in the layer 61 only by variation of the optical index. The radiating field lines in the layer 61, and joining the zones 63 and 64, orient the optically active molecules. As already mentioned, a high temperature (around 60 ° C) facilitates the orientation of the molecules during the ion implantation operation. This induces a permanent variation of index Δ n since the applied electric field is permanent. With this type of structure, the injection of an optical wave can be done by the edge.

An improvement to the structure shown in FIG. 11 consists in modulating the electric field induced by the electrets by superimposing on it another electric field. Indeed, it is possible, during the development of the structure, to deposit electrodes connected to a voltage generator (for example one electrode on the substrate 60 and the other on the optical layer 61, in the region between the two electrets) so as to frame the optical guide. The modulating electric field can be continuous or alternating. Another solution would be to deposit electrodes on zones 63 and 64 with the interposition of a protective layer.

Claims

1. Method for obtaining a structure generating non-linear electrooptical effects, said effects being due to molecules or intramolecular groups having a non-linear susceptibility, the orientation of said molecules or said groups being induced by an electric field applied, characterized in that the method consists in obtaining the orientation of said molecules or of said groups by means of a dielectric with permanent polarization.

2. Structure generating non-linear electrooptical effects, the structure comprising molecules or intramolecular groups (5) having a non-linear susceptibility, characterized in that the structure also comprises a dielectric with permanent polarization (6), said molecules or said intramolecular groups being subjected to the action of the electric field developed by the dielectric with permanent polarization.

3. Structure according to claim 2, characterized in that the dielectric with permanent polarization is an electret.

4. Structure according to one of claims 2 or 3, characterized in that said molecules or said intramolecular groups are included in a layer called optical layer (13) which is superimposed on the dielectric with permanent polarization (12).

5. Structure according to claim 4, characterized in that said optical layer is produced in the form of a waveguide (28).

6. Structure according to claim 5, characterized in that said optical layer has the shape of a fiber (37), the dielectric with permanent polarization having the shape of a sheath (35) enveloping the fiber.

7. Structure according to claim 6, characterized in that the polarization of the dielectric with permanent polarization (35) is such that the electric field induced in the fiber (37) is radial.

8. Structure according to claim 6, characterized in that the polarization of the dielectric with permanent polarization (41) is such that the electric field induced in the fiber (42) is axial.

9. Structure according to claim 4, characterized in that said optical layer (53) is liquid.

10. Structure according to claim 4, characterized in that the dielectric with permanent polarization is formed of two zones (63, 64) whose electric charges are of opposite signs, these zones inducing in the optical layer (61) a localized electric field .

11. Structure according to one of claims 2 or 3, characterized in that said molecules or said intramolecular groups are included in the dielectric with permanent polarization (15).

12. Structure according to claim 11, characterized in that the dielectric with permanent polarization including said molecules or said intramolecular groups has the form of a waveguide (32).

13. Structure according to any one of claims 2 to 11, characterized in that it further comprises means (52, 56) for applying an external electric field.