FR2817049A1 - Photo-crosslinkable material useful for making optical integrated circuits, comprises a chromophore and photosensitizer with different absorption peaks in the visible region in a monomer matrix - Google Patents

Abstract

Photo-crosslinkable material (I) that can be crosslinked with visible light comprises a chromophore (II) and a photosensitizer (III) in a matrix of a monomer having at least three crosslinkable functions, where (II) and (III) have different absorption peaks in the visible region. Independent claims are also included for the following: (1) a crosslinked material produced by crosslinking (I) with visible light; (2) an optical integrated circuit comprising the crosslinked material; and (3) a process for producing an optical integrated circuit, comprising: forming a film of (I); aligning the chromophore molecules by applying an electric field; maintaining the electric field and illuminating the film with visible light through a mask so that only areas corresponding to an optical circuit element are illuminated; switching off the electric field; illuminating the whole film so that the chromophore molecules are aligned only in the areas corresponding to the optical circuit element; and depositing electrodes at desired sites.

Description

<Desc / Clms Page number 1>

PHOTORETICULAR MATERIAL CONTAINING CHROMOPHORE,
CROSSLINKED MATERIAL, OPTICAL INTEGRATED CIRCUIT COMPRISING THIS MATERIAL, AND ITS PREPARATION METHOD
The invention relates to a photocrosslinkable material containing a chromophore, and to a crosslinked material capable of being obtained by photocrosslinking of this photocrosslinkable material.

 The invention further relates to an optical integrated circuit comprising this crosslinked material.

 The invention finally relates to a method for preparing an optical integrated circuit.

 The present invention is in the field of manufacturing optical integrated circuits (CIO).

 Optical integrated circuits dz CIO) are a combination of optical elements, such as waveguides, diffractive optics, BRAGG gratings etc., on the same component or matrix.

 All these elements are constituted by structures having a contrast of refractive index compared to the matrix which supports them. It is also possible, depending on the nature of the material, to modify the value of the refractive index by applying an electric field (this is called the electro-optical effect) . There are thus dynamic components that can be controlled electrically.

<Desc / Clms Page number 2>

 An example of such a device is given in FIG. 1. It is an optical gyroscope using an integrated phase modulator (1), using as matrix (2), a crystal of lithium niobate (LiNbO3 ). The index contrast necessary for the constitution of the waveguides (3) is obtained by ion bombardment of the desired regions. Lithium niobate has important electro-optical properties, the control of the refractive index is therefore obtained by application of an electric field on the waveguide.

 The field is applied by means of the electrodes (4) deposited on the circuit.

 The device, described above in Figure 1, is only an example of the application of a type of material to a given device. There are other materials, such as doped glasses and semiconductors, etc., used for the production of CIO. In addition, the circuit may not be implanted in the volume of the material, but consist of tracks deposited on a substrate. It then appears in relief on the support.

 The materials used for producing the matrices of optical integrated circuits are currently inorganic materials, which have the drawback in particular of the heaviness of their preparation, and their high dielectric constant limiting the switching frequency (capacitive effect).

 It is sought, however, to put at least organic materials capable of replacing the inorganic materials. It is then necessary to

<Desc / Clms Page number 3>

 create in these organic materials a localized and permanent modulation of the refractive index, accompanied by the induction of electro-optical properties.

 There are thus known components prepared essentially by masking and chemical attack on a matrix. Such a procedure, similar to those used in microelectronics, is long and difficult to implement, in particular during the final development stage, and requires the use of numerous solvents, acids and other treatment products.

 We also know that it is precisely the role of chromophores to induce in a material a variation in the index and the electro-optical effect.

 Chromophores are molecules that have several specific properties. They have a dipole moment which allows their orientation in an electric field. They also have an anisotropy of significant linear polarizability; this property can be assimilated to a birefringence on the molecular scale. Chromophores also have quadratic hyperpolarisability which is the basis of the electro-optical effect.

 The chromophores family includes very different molecules. However, these are, most of the time, molecules having an electron accepting group and an electron donor group linked by a conjugation path (n orbitals) allowing the delocalization of the

<Desc / Clms Page number 4>

électrons sur l'ensemble de la molécule. Ces molécules sont appelées molécules push-pull .

electrons on the whole molecule. These molecules are called push-pull molecules.

 One of the difficulties of using these molecules is linked to their tendency to crystallize. In the crystalline phase, the chromophores tend to orient themselves head to tail. The microcrystals, thus formed, have neither dipole moment nor quadratic hyperpolarisability and it is therefore impossible to find these properties on the macroscopic scale.

 The solution used to avoid this problem is to disperse the chromophores in a polymer matrix. The matrix must be transparent in the part of the spectrum used and must allow the orientation of the molecules. Again, there are a large number of possible matrices ranging from polymers to sol-gels.

 The collective orientation of the molecules will induce several effects including the appearance on the macroscopic scale of a birefringence linked to the organization of the chromophores, the material passing from an isotropic phase to a non-centrosymmetric phase, there is appearance of 'a quadratic hyperpolarisability on the macroscopic scale and the material is then electro-optical.

 A new problem then appears, that of the sustainability of collective orientation. By orienting the molecules, the material has been placed in an unstable thermodynamic state and there is a strong tendency to disorientate the molecules once the electric field has been removed. Now, if we want to keep

<Desc / Clms Page number 5>

 the desired optical properties, it is important to be able to freeze the molecular orientation in the material.

 There are several solutions for preserving the orientation of the chromophores in the matrix which contains them, but they all have in common an increase in viscosity, after the phase of polarization.

 Two methods are mainly used, the first is to use a rigid matrix at room temperature and to heat it above its glass transition temperature, before applying the electric field. The viscosity having decreased with increasing temperature, the molecules are free to orient themselves. Returning to room temperature, the rigidity of the material prevents rapid relaxation of the orientation of the chromophores, so there is slow relaxation over a few days.

 Another method consists in modifying the rigidity of the matrix by modifying its chemical nature, after polarization. Either polymerization or crosslinking is used, that is to say the formation of a three-dimensional molecular network, or gelation.

 The chemical reaction can be spontaneous or induced. In the latter case, the reaction is mainly caused by a rise in temperature, we are then dealing with thermocrosslinking, or else by irradiation, for example X, or even gamma. Immobilization of chromophores can be enhanced by making them

<Desc / Clms Page number 6>

 intervene in the crosslinking reaction. The chromophore then becomes an integral part of the matrix and its mobility is greatly reduced.

 The essential problem which then arises is the degradation of the chromophore. Indeed, molecules with non-linear optical properties are fragile and do not support high temperatures or harsh radiation.

 The production of optical integrated circuits also requires the possibility of inducing local variations in properties corresponding to the designs of the desired optical circuit. Thermal crosslinking is poorly adapted here. The heating, to be exercised locally, must be induced by significant laser illumination, likely to degrade the chromophore. In addition, the diffusion of heat prohibits fine resolution.

 One could have thought of applying to the crosslinking of organic matrices, containing chromophores, crosslinking by optical excitation or photocrosslinking in the UV range, as it is used in the areas of coatings, varnishes and paints. Indeed, photocrosslinking would allow it to obtain fine spatial resolution.

 However, because the photocrosslinking takes place in the UV range, it would have two drawbacks: a degradation of the chromophores induced by the radiation and a small thickness of UV penetration preventing crosslinking on thicknesses of more than a few microns.

<Desc / Clms Page number 7>

 There is therefore a need for a crosslinkable organic material containing chromophores, suitable in particular for the manufacture of optical integrated circuits, which does not have the drawbacks of the materials of the prior art, described above.

 In other words, there is a need for a crosslinkable organic material, in which the chromophores are neither damaged nor, a fortiori, destroyed during the crosslinking process, in which crosslinking occurs over a considerable depth and in the whole of the material, which allows a fine spatial revolution, and, finally, which can be prepared by a simple and reliable process.

 The object of the present invention is to provide a crosslinkable organic material containing chromophores, which meets, among other things, these needs.

 The object of the present invention is also to provide a crosslinkable organic material, which does not have the drawbacks, defects, limitations and disadvantages of the methods of the prior art and which solves the problems of the prior art.

 This object and others still are achieved, according to the invention, by a material photocrosslinkable by visible light essentially comprising a matrix of a monomer having at least three crosslinkable functions, said matrix containing a chromophore and a photosensitizer having a peak. absorption in the distinct visible region, separate from that of the chromophore.

<Desc / Clms Page number 8>

 The material according to the invention meets, among other things, all of the needs indicated above, and does not have the disadvantages of the materials described above.

 The material according to the invention is a photocrosslinkable matrix in the visible doped by chromophores with non-linear optical properties.

 The material according to the invention being a photocrosslinkable material in the visible range, there will be no destruction or degradation of the chromophores under irradiation. The material according to the invention provides crosslinking over a large thickness, throughout the material.

 Because the crosslinking is carried out by an optical and not a thermal process, the material according to the invention offers the possibility of fine spatial resolution.

 Advantageously, the crosslinkable functions are chosen from acrylate functions and epoxy functions.

 The monomer is advantageously chosen from pentaerythritol tetraacrylate, pentaerythritol triacrylate (PETIA), and tris (2-hydroxyethyl) isocyanurate triacrylate (and others ...).

 The chromophore is advantageously chosen from OCB, MMONS, MOCC, and others, such as chromophores with non-zero quadratic hyperpolarizability.

<Desc / Clms Page number 9>

 The photosensitizer is advantageously chosen from eosin Y and methylene blue and others.

 The material according to the invention may also comprise, in certain cases, a co-initiator, such as methyl diethanolamine.

 Advantageously, in the material according to the invention, all or part of the chromophores is (are) aligned, for example, by application of an electric field.

 The invention further relates to a crosslinked material, in whole or in part, capable of being obtained by photocrosslinking, by visible light, in whole or in part of the photocrosslinkable material, defined above.

 The use, according to the invention, of a photosensitizer suitable for crosslinking and thus blocking the orientation of the chromophores definitively gives the material the desired properties. In addition, this material can present strong dopings in chromophores. These being preferably oriented in whole or in part (in the whole of the material or only in certain selected areas thereof).

 The invention also relates to an optical integrated circuit comprising this crosslinked material; this optical integrated circuit being chosen from amplitude, phase modulators or other devices.

<Desc / Clms Page number 10>

 The invention also relates to a method for preparing an optical integrated circuit, comprising the following successive steps: - putting the photocrosslinkable material, as described above, in which the chromophores are not aligned, in the form of a layer ; - Align the chromophores by applying an electric field; - maintain the electric field and illuminate the layer with visible light, through a mask allowing the illumination on the layer only of the zones corresponding to the pattern of the desired optical circuit; - suppress the electric field; illuminating the entire layer, whereby the chromophores are aligned only in the zones corresponding to the pattern of the optical integrated circuit; - place the electrode in the desired places.

 Thanks to the method of the invention, it is noted that the etching of the circuit can be carried out by leaving the sample in place and that there is no post-processing.

 The process is very simple and requires very little handling. The technique also makes it possible to facilitate the optical connection of the circuit.

 Advantageously, it is possible, during the first step, to embed an optical fiber in the material and to start the circuit starting from the fiber.

<Desc / Clms Page number 11>

 The method according to the invention makes it possible to limit the losses of insertions, an important problem in the context of optical integrated circuits.

0 abscisse la longueur d'onde k (en A) - les figures 3A, 3B, 3C, 3D constituent un schéma de principe qui illustre les différentes étapes de réalisation d'un modulateur d'amplitude selon l'invention. Other details and advantages of the present invention will appear better on reading the description which follows, given by way of illustration and not limitation, and made with reference to the accompanying drawings, in which: - Figure 1 is a block diagram an example of an IOC, which is an optical gyroscope using an integrated double phase modulator; - Figure 2 is a graph which shows the absorption of each of the constituents of the photocrosslinkable material according to the invention; on the ordinate, is absorbed A (arbitrary units) and in

0 abscissa the wavelength k (in A) - FIGS. 3A, 3B, 3C, 3D constitute a block diagram which illustrates the various stages of production of an amplitude modulator according to the invention.

 In detail, the material photocrosslinkable by visible light, according to the invention, essentially comprises a matrix of a monomer having at least three crosslinkable functions.

 By essentially comprises, it is meant that said matrix generally represents more than 50% by weight and preferably from 30 to 90% by weight of the material according to the invention.

<Desc / Clms Page number 12>

 By matrix of a monomer is meant that the monomer is in the form of a homogeneous structure, in which the other elements, namely the chromophore and the photosensitizer are distributed, preferably uniformly, in a homogeneous manner.

 The photocrosslinkable matrix is composed of monomers, also called (precursors)), having at least three crosslinkable functions.

 During the crosslinking reaction which the material according to the invention undergoes, the crosslinkable functions are broken in favor of a bond between two molecules. A cross-linked three-dimensional network is thus formed, the viscosity of which is considerably higher than that of the precursor: for example, this viscosity can go from 500 cps to 1013 ps. The use of precursors having only two crosslinkable functions leads not to a three-dimensional network, in accordance with the invention, but to a polymer, which is clearly less advantageous.

 The crosslinkable functions can be chosen from all known crosslinkable functions. Examples of such crosslinkable functions are the acrylate function, the epoxy function.

 Examples of precursors, monomers, crosslinkable, photopolymerizable are therefore pentaerythritol (II) triacrylate and pentaerythritol (I) tetraacrylate, the formulas of which are given below:

<Desc / Clms Page number 13>

ou encore le tris (2-hydroxyéthyl) isocyanurate triacrylate (et d'autres encore).

or else tris (2-hydroxyethyl) isocyanurate triacrylate (and others still).

According to the invention, the initiation of the reaction by light is not done directly by action on the crosslinkable function with creation of free radicals under UV-with all the harmful effects that this may have on the chromophores, but also on the matrix-but via a photosensitizer compound added to the matrix, advantageously allowing, in accordance with the invention, crosslinking with visible light.

Preferably, according to the invention, it is only illuminated in the range of the absorption peak of the photosensitizer, that is to say only on the peak

<Desc / Clms Page number 14>

 photosensitizer, very precisely, for example with a laser.

 The choice of photosensitizer is conditioned by the fact that, according to the invention, it is desired to induce crosslinking, photopolymerization, in a window of the well defined spectrum corresponding to the wavelength range of visible light.

 The photosensitizer concentration is low, generally from one to a few hundredths by weight, up to 1 to a few thousandths by weight.

 The photosensitizer concentration makes it possible to control the transparency of the precursor-monomer.

 It is thus possible to work - unlike the prior art - on significant thicknesses, which can range up to a few hundred microns, for example 400 μm, for moderate dosages.

 According to the invention, the photosensitizing compound has an absorption peak in the visible light domain, this absorption peak being, moreover, distinct, separate from that of the chromophore.

 FIG. 2 illustrates this situation by showing the contribution to the absorption of each component of the material according to the invention.

 The chromophore (C) absorbs, in general, mainly in the visible, and the photosensitizing chromophore couple (S) is, moreover, adapted so that there is no overlap of the two absorption parts. The absorption peak of

<Desc / Clms Page number 15>

 photosensitizer (S) will therefore also be in the visible, but, for example, at a longer wavelength than that of the chromophore.

 FIG. 2 also shows the absorption peak of the monomer (M) or precursor which is rather located in the UV.

 It is noted that there is an overlap between the absorption peak of the monomer (M) and that of the chromophore (C); this covering would be troublesome if direct photocrosslinking of the monomer was carried out by UV irradiation, since the chromophore, which is extremely sensitive to UV, would then be damaged.

 Because, according to the invention, the photocrosslinking is carried out with visible light in the narrow range corresponding to the absorption peak of the photosensitizer (see arrow), distant, distinct from the chromophore peak, the latter does not undergo any deterioration .

 In addition, chromophore (C) is present in a high concentration, which can generally range from 1% to 50% by weight. Therefore, its absorption would mask that of the photosensitizer, if, according to the invention, the respective absorption peaks of the photosensitizer (S) and of the chromophore (C) were not distinct, separate.

 These conditions being fulfilled, the person skilled in the art will be able to determine which are the appropriate chromophore / photosensitizer associations.

 Examples of chromophores or doping molecules with non-optical properties

<Desc / Clms Page number 16>

are the 5 OCB (III), the MMONS (IV) and the MOCC (V) whose formulas are given below:

Examples of photosensitizers, i.e. dyes, used to induce visible photocrosslinking, are eosin y (VI)

<Desc / Clms Page number 17>

and methylene blue (VII), the formulas of which are given below:

A material, particularly preferred according to the invention, comprises a matrix of PETIA monomer, a chromophore MOCC, in an amount of 10 to 20% by weight and a photosensitizer (eosin), in an amount of 0.1 to 0.001% by weight.

 The material, in this case, also contains a co-initiator, such as methyl diethanol amine, generally at a rate of 1 to 10% by weight.

<Desc / Clms Page number 18>

 The material according to the invention can be prepared by simple mixing of the precursor monomer, the photosensitizer and the chromophore.

 The production of an optical integrated circuit (IOC), using the photocrosslinkable material according to the invention, comprises several steps.

 These steps are illustrated in FIG. 3, which shows the embodiment, by way of example, of an amplitude modulator.

 It is obvious that other optical integrated circuits (CIO) can be prepared in a similar manner, with possible slight adaptations.

 First of all (FIG. 3A), the monomer-precursor, photosensitizer and chromophore mixture is put in the form of a layer (31) of desired thickness, for example from 100 to 200 mm.

 In this layer, the chromophores (32) doping the precursor are randomly oriented, they are not aligned.

 In a second step, the layer is polarized, that is to say that an electric field (E) is applied (polarization coronna) so as to achieve the alignment of the chromophores, that is to say orient them all in the same direction (Figure 3B).

The electric field is generally perpendicular to the main plane of the layer (31). The electric field being maintained, a first photocrosslinking is carried out, by lighting with visible light (33) (suitable wavelength) the layer (31), through a mask (34) provided with a pattern ( 35)

<Desc / Clms Page number 19>

 corresponding to the pattern, to the layout of the optical circuit which it is desired to obtain on the layer, that is to say that the mask (34) allows the illumination on the layer only of the areas corresponding to the pattern, to the optical layout that we want to get.

 This second step of the method, which is in fact a first crosslinking operation along the path of the desired optical circuit, has in particular three consequences: there is a densification of the matrix, accompanied by an increase in the refractive index, and there is immobilization of the chromophore, and therefore, as a result, an induction of birefringence and permanent electro-optical properties.

 In the third step of the method, the polarization field is cut, in other words, the electric field is suppressed. The orientation of the chromophores in the non-crosslinked, non-illuminated regions (protected by the mask) during the previous step, that is to say in the regions (36) outside the pattern, of the layout of the optical circuit , which one wishes to obtain, then relaxes (37).

 The entire layer is uniformly illuminated with visible light (33) (FIG. 3C) and the orientations are thus frozen. The chromophores are aligned, have a preferred orientation only in the region (38) corresponding to the drawing of the mask motif in FIG. 3B, through which the light could pass. A contrast of optical properties is thus obtained between the regions where the orientation has been blocked and those where it has been relaxed. This contrast

<Desc / Clms Page number 20>

 constitutes the optical circuit. In other words, this induces an index jump ensuring the guidance conditions.

 The fixed orientation, the alignment of the chromophores in a direction which is perpendicular to the plane, the layer gives the material electro-optical properties. To use the electro-optical effect, the electrodes (39,40) necessary for the application of an electric field must still be deposited at the desired locations (Figure 3D). The deposition of electrodes makes it possible, in fact, to apply an electric field which modulates the refractive index of the guide.

In FIG. 3D, an amplitude modulator is thus obtained.

Claims (13)

 1. A material photocrosslinkable by visible light essentially comprising a matrix of a monomer having at least three crosslinkable functions, said matrix containing a chromophore and a photosensitizer having an absorption peak in the visible domain distinct from that of the chromophore.  2. Material according to claim 1, in which said crosslinkable functions are chosen from acrylate functions and epoxy functions.  3. Material according to claim 2, wherein said monomer is chosen from pentaerythritol tetraacrylate, pentaerythritol triacrylate (PETIA) and tris (2-hydroxyethyl) isocyanurate triacrylate.  4. Material according to claim 1, in which said chromophore is chosen from OCB, MMONS, MOCC and chromophores with non-zero quadratic hyperpolarisibility.  5. Material according to claim 1, wherein said photosensitizer is chosen from eosin Y and methylene blue.  6. Material according to claim 1, further comprising a co-initiator.  7. Material according to claim 1, in which all the chromophores or a part of them (is) are aligned, for example, by application of an electric field. <Desc / Clms Page number 22>  8. Crosslinked material, in whole or in part, capable of being obtained by photocrosslinking, in whole or in part, by visible light of the photocrosslinkable material according to any one of claims 1 to 7.  9. Material according to claim 8, in which said photocrosslinking is carried out by illuminating only on the absorption peak of the photosensitizer.  10. An optical integrated circuit comprising the crosslinked material according to claim 8 or claim 9.  11. A method of preparing an optical integrated circuit, comprising the following successive steps: - putting the photocrosslinkable material according to any one of claims 1 to 6, in which the chromophores are not aligned, in the form of a layer ; - Align the chromophores by applying an electric field; - maintain the electric field and illuminate the layer with visible light, through a mask allowing the illumination on the layer only of the zones corresponding to the pattern of the desired optical circuit; - suppress the electric field; - Illuminate the entire layer, whereby the chromophores are aligned only in the zones corresponding to the pattern of the optical integrated circuit; <Desc / Clms Page number 23>  - place the electrode in the desired places.  12. The method of claim 11, wherein during the first step, an optical fiber is embedded in the material.  13. The method of claim 11, wherein the optical integrated circuit is an amplitude modulator, or a phase modulator.