WO2014075994A1 - Method for laser-structuring a nanoparticle network and method for marking a coloured pattern on a support - Google Patents

Abstract

The invention concerns a method for laser-structuring a nanoparticle network, comprising: a] providing (50) a support coated with a layer formed from an essentially inorganic material, b] incorporating (60), into the layer, precursors of nanoparticles giving the material of the layer the property of heating up when said layer is irradiated, for a period of time of less than 10 s, by a linearly polarised laser beam, c] irradiating (64) the layer, with the laser beam, in such a way as to: · transform the precursors of nanoparticles into nanoparticles, and make said nanoparticles grow, and · align the nanoparticles along parallel lines; the support being moved relative to the laser beam, during step c], at a speed of movement of between 50µm.s^-1 and 5000µm.s-1.

Description

 METHOD OF LASER STRUCTURING A NETWORK

 NANOPARTICLES AND METHOD FOR REGISTERING A COLORED PATTERN ON

 The invention relates to a method for structuring a network of nanoparticles. The invention also relates to a method for permanently inscribing a colored pattern on a support.

 [002] It is interesting to form parallel lines of nanoparticles on the surface of a support. Such a structuring makes it possible, for example, to modify the optical properties of the support, to obtain a dichroic effect.

 [003] In this description, an object is said to have a dichroic effect if it presents:

 a first optical spectrum, when this object is observed with a first light radiation having a linear optical polarization;

a second optical spectrum, distinct from the first optical spectrum, when this object is observed, under the same conditions, with a second light radiation, identical to the first light radiation except that it has a linear optical polarization, different from the optical polarization of the first radiation. The term optical spectrum refers to a spectrum selected from the group consisting of electromagnetic transmission, reflection or scattering spectra.

[004] For example, it is known that an object can be imparted with a dichroic effect by disposing metal nanoparticles on its surface in a specific manner. In particular, these nanoparticles must be spatially arranged so as to form lines, in order to promote the appearance of electromagnetic coupling between the nanoparticles, thus modifying the surface plasmon resonance effect due to the presence of these metal nanoparticles. the absence of a specific spatial arrangement. An example of this is described in the document by D. Babonneau et al., "Tunable Plasmonic Dichroism of Au Self-aligned on rippled Al ₂ O ₃ Films", Euro Physics Letters, vol. 93, 2011. This document describes a method of insertion of the nanoparticles in wrinkles previously formed on the surface of an object, to form lines of nanoparticles. This method has several disadvantages. The periodicity of the line spacing depends on the periodicity of the wrinkle spacing, which is difficult to control. In addition, the method requires forming wrinkles of adequate size on the surface of the object, which can complicate the implementation of the method.

[005] The state of the art is also known from the patent FR 2938679 B1. This patent aims to solve a different technical problem of the present invention. Indeed, this patent describes a method of writing a pattern on an object. This pattern is erasable by irradiation with visible light radiation, but has no dichroic effect. In particular, parameters of this irradiation (such as the intensity or the duration of exposure) do not make it possible to obtain an organization of the nanoparticles giving rise to dichroic properties, but only a homogeneous and isotropic distribution of nanoparticles within random patterns defining a texturing on the surface of the material.

[006] From the state of the art is also known from the article of A. Stalmashonak et al., "Metal-glass nanocomposite for optical storage of information," Applied Physics Letters, Vol. 99, p. 201904, 201 1. This article describes a process for imparting a dichroic effect to a soda-lime glass in which silver ions have been introduced by ion exchange. This process involves the formation of nanoparticles on a support, for the permanent marking of colors on this support. This process obtains a dichroic effect by modifying the shape of the nanoparticles and without forming lines of nanoparticles. In addition, this medium does not have dichroic properties in optical reflection, but only in optical transmission.

[007] From the state of the art is also known from the following documents:

 A. Nahal et al, "Photothermal-induced dichroism and micro-cluster formation in Ag + -doped glasses", Applied Physics B, vol. 79, p. 513-518, July 2004;

 - N. Crespo-Monteiro et al, "Role of silver nanoparticles in the laser-induced reversible color-marking and controlled crystallization of mesoporous titania films", Proceedings of SPIE vol. 8096, September 6, 201 1;

 N. Crespo-Monteiro et al, "Reversible and irreversible laser microinscription on silver-containing mesoporous titania films", Advanced Materials, vol. 22, June 2010;

A. Nahal et al. Linear dichroism, produced by thermo-electric alignment of silver nanoparticles on the surface of ion-exchanged glass, Applied Surface Science, vol. 255, June 2009;

 - Kiesow et al, "Generation of wavelength-dependent, periodic line pattern in nanoparticle-containing metal films by femtosecond laser irradiation," Applied Physics Letters, vol. 86, April 2005;

 Kaempfe et al., "Formation of metal particle nanowires induced by ultrashort laser pulses," Applied Physics Letters, vol. 79, September 2001.

 [008] There is therefore a need for a method of laser structuring of nanoparticles on a support, to obtain, in a controlled manner and with a simplified implementation, a spatial distribution of the nanoparticles so as to form periodically spaced lines. on the support and protected from the outside by an essentially inorganic film.

 [009] The invention therefore relates to a laser structuring method of a nanoparticle network according to claim 1.

[ooio] The irradiation, using laser radiation, of the layer comprising the nanoparticles or the precursors of nanoparticles, by choosing, as described above, the intensity and duration of irradiation, leads to a heating up the diaper irradiated. In the case where precursors of nanoparticles are present in the layer, the irradiation also leads to the transformation of these precursors into nanoparticles. The applicant has discovered that, under these conditions, the irradiation results in an arrangement of the nanoparticles, along parallel and regularly spaced lines, at the interface between the support and the irradiated layer. In the current state of knowledge of the applicant, the explanations of the physical mechanisms that cause the alignment of the nanoparticles are not yet certain. In addition, the Applicant has found that by changing radiation parameters, such as its wavelength, the spacing between the lines or the average particle size could be adjusted. Thus, the method makes it possible to structure the nanoparticles in a simplified, reliable and controllable manner.

Embodiments of the invention may have one or more of the features of claims 2 to 7.

 These embodiments also have the following advantages:

the metallic nature of the nanoparticles and their position at the interface between the support and the layer, combined with the transparency of at least one or the other of the layer or support, makes it possible to observe the dichroic effect due to the anisotropic organization of the nanoparticles;

 the reduced roughness of the support coated with the transparent layer makes it possible to benefit from specific dichroic properties in specular reflection. These properties are made possible in particular by the fact that the nanoparticles, at the end of step c], are essentially at the interface between the support and the layer;

the use of a porous material facilitates the incorporation of the precursors of nanoparticles in this material, which simplifies the process.

According to another aspect, the invention also relates to a method for permanent inscription, on a support, of a first and a second pattern having different dichroic effects, by means of a laser structuring method. a nanoparticle network according to claim 8.

Embodiments of the invention may have the features of claim 9.

 The invention will be better understood on reading the description which follows, given solely by way of nonlimiting example and with reference to the drawings in which:

 FIG. 1 is a schematic illustration of a plurality of colored patterns inscribed by laser on a support;

 FIG. 2 is a schematic illustration showing, in a very simplified manner and in a view from above, an enlargement of a portion of a colored pattern of FIG. 1;

FIG. 3 is a high-angle annular dark field transmission electron microscopy (Scanning-Tunneling Electron Microscope - High Angle Annular Dark Field (English language) showing in greater detail and in a view from above, an enlargement of a portion of a colored pattern of Figure 1.

FIG. 4 is a diagrammatic illustration, in a cross-sectional view, of the support of FIG. 1;

FIGS. 5A, 5B and 5C schematically illustrate the apparent color differences exhibited by a colored pattern of FIG. 1, along the direction of polarization of the light with which this pattern is observed in diffuse reflection;

FIG. 6 is a flowchart of a method for writing the patterns of FIG. 1; FIGS. 7 and 8 schematically illustrate the support of FIG. 3 during steps of the method of FIG. 5.

 In these figures, the same references are used to designate the same elements.

 In the following description, the features and functions well known to those skilled in the art are not described in detail.

FIG. 1 represents a set 2 of colored patterns permanently inscribed on a surface of an object 4. To simplify FIG. 1, only patterns 5 and 7 carry a reference and only the pattern 5 will be described. in detail thereafter. Unless otherwise specified, all that is said in reference to the reason 5 also applies to the other reasons of the whole 2. By permanent inscription of the reason 5, it is understood that this reason 5 is able to remain inscribed on the object 4 for a period longer than several months or years, and preferably greater than several decades. In particular, this pattern 5 is not erasable by illumination under visible or ultraviolet light. This pattern 5 also has increased resistance to degradations of the surface of the object 4, such as mechanical abrasion or chemical etching, these degradations not destroying the object 4, however. [0019] By colored pattern, it is understood that light transmitted, reflected or scattered by this pattern, in response to electromagnetic illumination, has a specific optical spectrum. For simplicity, in this description, it is considered that the pattern is intended to be observed in visible light. By visible light is meant here a light radiation whose wavelengths are between 350 nm and 780 nm. Thus, in the remainder of the description, and unless otherwise stated, the term color will be used to designate the properties of the wavelength spectrum that the pattern presents. It is known that the same color can be obtained with different wavelength spectra. However, here, for simplicity, it is considered that a given color corresponds to one and only one optical spectrum.

 In this example, the pattern 5 has a square shape, side equal to 2.5mm. This pattern 5 does not occupy the entire surface of the object 4. In this example, the object 4 has a planar shape extending in a horizontal plane.

Such a colored pattern, permanently inscribed on the surface of the object 4, is not obtained from pigments, but thanks to the presence, on this surface, of metallic nanoparticles capable of generating surface plasmon resonance phenomena. Thus, the pattern 5 comprises a plurality of metallic nanoparticles distributed over the surface of this pattern 5.

 Advantageously, this pattern 5 also has a dichroic effect in optical transmission, optical specular reflection and optical diffuse reflection. Thus, the apparent color of this pattern depends on the optical polarization with which this pattern is illuminated or observed.

 Figures 2 and 3 describe in more detail the arrangement of the nanoparticles of the pattern 5. For clarity, the colors of Figure 3 have been reversed. In the portion of the object 4 on which the pattern 5 is formed, these nanoparticles are arranged, in the horizontal plane, so as to form a plurality of lines substantially parallel to each other and spaced periodically. For example, the number of these lines is greater than several hundred. For simplicity, only two lines 6, 8, immediately adjacent to each other, are shown in Figure 2. These lines 6 and 8 are here statistically identical, only the line 6 will be described in detail. Line 6 extends in the horizontal plane. Two lines immediately adjacent to each other are separated from each other by an equal distance, within 2% or within 5%, of a spacing value L. This distance is defined as being the smallest distance, measured in the horizontal plane, separating two lines immediately adjacent to each other. This value L here is strictly less than the wavelength of the laser radiation used to form the lines. This value L is advantageously between 150 nm and 800 nm. Here, the value L is between 260 nm and 440 nm.

To simplify Figure 2, the nanoparticles are drawn in the same way. Only nanoparticles 10 and 12 of line 6, immediately adjacent to each other, bear a numerical reference. Subsequently, in the rest of the description, the reference 10 is also used to reference the nanoparticles of the lines of the pattern 5. In this example, the nanoparticles have a substantially spherical shape. The diameters of the nanoparticles of a given line have a statistical distribution that is identical or very similar to that of the other lines. "Very close" means that the average diameter varies from one line to another by less than 10% or 5%. D denotes the average diameter of the nanoparticles of this line 6. This diameter D is here greater than or equal to 10nm or 25nm and less than or equal to 200nm or 400nm. The diameter of a nanoparticle is here defined as being the maximum diameter of this nanoparticle among all the diameters measured in nanoparticle cutting planes parallel to the horizontal plane. Here, the nanoparticles forming the line 6 are substantially aligned along and on either side of a straight line 9 of the horizontal plane. More exactly, the nanoparticles forming the line 6 are typically separated from this line 9 by at most twice the average diameter D. More than 80% or 90% or 95% of the nanoparticles of the line 6 are here entirely within an area extending along this line 9, symmetrically on either side of the line 9 with a width equal to 2 * D. The nanoparticles 10 and 12 are, on average, separated from each other by a distance W which is of the order of magnitude of the average diameter D. This distance W is here greater than 10 nm and less than 250 nm. These nanoparticles are here made of a noble metal, such as silver or gold.

The pattern 5 is thus defined on the surface of the object 4 by arranging the nanoparticles 10 so as to form the lines 6 and 8. This organization of the nanoparticles 10 allows the pattern 5 to have a dichroic effect. Thus, when it is illuminated by visible light, this pattern exhibits, in specular reflection, in diffuse reflection and in transmission, in the visible range, an apparent color which depends on the polarization of the light with which it is enlightened. This apparent color also depends on the values of D and W.

FIG. 4 shows in more detail the pattern 5. A support 14 is covered with a layer 16 comprising the nanoparticles 10. Here, the surface of the object 4 is formed by the support 14.

 The support 14 is able to be covered by the layer 16. Here, the support 14 is a rigid support, extending in the horizontal plane. Advantageously, the mechanical and structural properties of the support 14 are not altered when this support 14 is brought to a temperature between 340 ° C and 400 ° C. For example, the support 14 is a layer of soda-lime glass. This support here has a thickness greater than or equal to 0.5 mm or 1 mm.

 The layer 16 is formed of an essentially inorganic material. In this example, the layer 16 is transparent for radiation whose wavelength corresponds to visible light, so as to allow observation of the dichroic effect in optical reflection. The layer 16 is here a layer of nanocrystallized titanium dioxide. This layer 16 here has a thickness greater than or equal to 50 nm or 100 nm. This thickness is, for example, less than or equal to Ι ΟΟμιτι or 10μηη or 1 μηη and, preferably, less than or equal to 500nm. This is a necessary condition to obtain white light interference in specular reflection and thus a colored luster effect. This effect of colored luster will be described in more detail in the following.

 The nanoparticles 10 are located at the interface between the support 14 and the layer 16. For example, more than 90% of the nanoparticles 10 are located in the 50% thickness of the layer 16 which are at closer to the interface between the support 14 and the layer 16. The nanoparticles 10 may be partially included in the support 14.

Advantageously, so that the pattern 5 has the dichroic effect visible in optical transmission, the support 14 is transparent, to allow the transmission of light through the object 4. Advantageously, the support 14 has a surface roughness ("roughness surface" in English) less than the wavelength of the light with which it is desired to observe a dichroic effect. Here, this surface roughness is less than or equal to 200 nm and, preferably, less than or equal to 50 nm or 20 nm. In this description, the roughness of a colored surface is defined as the roughness RMS ("root mean square" in English) of this surface. This roughness gives the support 14 the ability to reflect a luminous flux at least ten times greater in the specular direction than in the non-specular direction. In this case, the combination of the optical properties of the layer 16 and the particles 10, gives the pattern 5 an effect called "colored luster" (also called "colored luster"), that is to say that the reflected light by the pattern 5, in a specularly reflective direction, has a different color from the light reflected by this pattern 5 in a non-specular direction.

 FIGS. 5A, 5B and 5C illustrate an example of the dichroic effect of the pattern 5. In this example, only the dichroic effect in diffuse reflection is considered. This dichroic effect is manifested by an apparent color difference of the pattern 5 when this pattern 5 is illuminated by, successively, lights having different linear optical polarizations and observed in the same direction. Here, the light illuminating the pattern 5 is polychromatic and its spectral components are in the range of wavelengths corresponding to visible light. This light is here white light. In a first case (Figure 5A), the pattern 5 is illuminated with unpolarized white light. The pattern 5 then has, in diffuse reflection, a first color 20. In a second case (FIG. 5B), the pattern 5 is illuminated by white light having a linear polarization oriented in a first direction, for example parallel to the lines of nanoparticles, symbolized by the arrow 22. The pattern 5 then has, in diffuse reflection, a color 24, different from the color 20. In a third case (Figure 5C), the pattern 5 is illuminated by white light having a polarization linear direction oriented in a second direction, for example perpendicular to the first direction, symbolized by the arrow 26, orthogonal to the direction 24. The pattern 5 then has, in diffuse reflection, a color 28 different from the colors 20 and 24.

An example of a method for inscribing the pattern 5 on the object 4 will now be described, with reference to the flow chart of FIG. 6 and with the aid of FIGS. 1 to 4, 7 and 8.

This method involves structuring, by laser, nanoparticles 10 to form lines 6 and 8.

For this purpose, during a step 50, the support 14 is provided, coated with a layer 54 (Figure 7) of an essentially inorganic material capable of forming the layer 16 after irradiation with laser radiation. The support 14 here forms the object 4. The layer 54 is particularly capable of crystallizing locally, in response to induced heating by means of irradiation with laser radiation. This layer 54 is particularly capable of forming the layer 16 after crystallization. In this example, the layer 54 is transparent for radiation whose wavelength corresponds to the wavelength of the laser radiation used for the irradiation to be described in the following. Here, the layer 54 is transparent for visible light. This layer here has an amorphous structure and comprises a plurality of pores whose diameter is between 1 nm and 100 nm. For example, the layer 54 is a mesoporous layer of titanium dioxide (TiO ₂ ) in the amorphous state. This layer 54 has a thickness greater than or equal to 100 nm. This layer 54 extends here in the horizontal plane.

 Step 50 comprises an operation 56 for supplying the support 14, and then an operation 58 for depositing the layer 54 on the support 14.

In this example, the layer 54 is produced by a sol-gel process. For this purpose, operation 58 comprises the preparation of a solution as follows: the supply of a molar amount N ₀ (expressed in moles) of a titanium precursor, for example titanium tetraisopropoxide; (TTIP);

-l'ajout, the titanium precursor, 0.5 * N ₀ mol of acetylacetone (AcAc) and then 0.015 * N ₀ mole of Hydrochloric acid (HCI);

-l'ajout an amount of 0.025 * N ₀ mole of a triblock copolymer, for example the triblock copolymer marketed by BASF under the reference "Pluronic P 123", previously dissolved in 28.5 * N ₀ mole ethanol;

-l'ajout 29.97 * N ₀ mol of ultrapure water, the mixture thereof forming the solution.

The quantity N ₀ is chosen as a function of the volume of the layer 54 to coat the support 14. For example, N ₀ is greater than or equal to 1 mol.

The solution thus formed is then deposited on the support 14, by means of a so-called dip coating process in order to form films called "organic-inorganic hybrids". By "organic-inorganic hybrid" is meant a composite material which has two distinct components which are of nature, respectively, organic and inorganic. The support 14 is thus immersed in the solution and then removed with a pulling speed of between 10 mm. min ^-1 and 100 mm ^-1 , in the presence of an ambient humidity of between 20% and 80% and preferably of between 40% and 50%. These hybrid films are then heated to calcine the copolymer present in these films. For example, these films are heated to a temperature of 340 ° C. for a duration of 4 hours. At the end of the operation 58, the layer 54 is formed on the support 14.

Then, during a step 60 (FIG. 6), precursors of nanoparticles 62 (FIG. 8) are incorporated in the layer 54. These precursors 62 are capable of forming the nanoparticles 10 when the layer 54 is irradiated by a laser radiation linearly polarized. These precursors 62 (and, where appropriate, these nanoparticles 10) in particular give the layer 54 the property of being heated when it is irradiated, for a period of less than 10 s, by a linearly polarized laser radiation having an intensity greater than or equal to I kW.cnrr ² . These nanoparticles or precursors 62 are here distributed homogeneously and randomly in the layer 54.

 In this example, step 60 is carried out by incorporating the precursors 62 in the layer 54 by impregnating the layer 54 with an aqueous solution comprising the precursors 62, so that these precursors 62 fill the pores of the layer 54.

Here, the precursors 62 comprise metal cations of the same chemical species. The nanoparticles 10 formed from these precursors 62 are therefore metal nanoparticles. Preferably, these metal cations are selected from the group consisting of noble metal cations. In this example, the precursors 62 comprise an aqueous solution of silver nitrate. The metal cations are silver ions Ag ⁺ or silver complexes such as the chemical compound of formula [Ag (NH ₃ ) 2] ⁺ NO ₃ ^- , the nanoparticles 10 are therefore formed of metallic silver.

The impregnation of the layer 54 is thus carried out as follows: first, by preparing an aqueous solution, by mixing silver nitrate salts in ethanol diluted in water with a concentration greater than or equal to 0.5 mol.l ^"1 or 1, 5 mol.l ^{" 1} or 3 mol.l ^"1 , then by adding ammonia (NH ₄ ⁺ _aq + HO ^" _aq ) in this solution, so that the molar concentration of silver ions in the solution is greater than 0.5 mol.l ^-1 or 1 mol.l ^-1 and preferably 1.5 mol.l ^-1 ; then, immersing the support 14 coated with the layer 54 in this solution for a period greater than or equal to 30 minutes.

 In this example, the atomic concentration in the layer 54 of the precursors 62 is sufficient so that, at the end of the irradiation, the distance W is of the order of magnitude of the diameter D of these nanoparticles. a plane parallel to the lines. Here, the atomic concentration in the layer 54 of the silver element is greater than 0.01 times or 0.1 times the atomic concentration of the titanium element in the layer 54. The ratio of these atomic concentrations can be determined experimentally, for example by means of elementary analysis techniques, such as the so-called "microprobe probe" ("electron probe microanalysis" technique in English).

 Then, during a step 64, the layer 54 is irradiated with a linearly polarized laser radiation, to obtain the nanoparticles 10 arranged along the lines 6, 8. Here, only a portion of the layer 54, corresponding to the location of the pattern 5, is irradiated.

For this purpose, this step 64 comprises here: an operation 66 for calibrating the physical parameters of the laser radiation and the irradiation so that the pattern 5 has a specific color, and then

an operation 68 for applying the laser radiation to the layer 54.

The physical parameters of the radiation calibrated during the operation 66 are chosen from the group consisting of the intensity, the wavelength, the angle of incidence of the radiation relative to the normal to the support, and the direction of the optical polarization of the laser radiation. The parameters of the irradiation include in particular the irradiation time of the layer 54 by the laser radiation. A modification of one or more of these parameters results in a change in the L value of the spacing of lines 6, 8, D and W.

 In this example, the laser radiation is generated continuously by an argon-krypton laser source. This laser radiation is adjusted to present:

 a wavelength here comprised in the field of visible light and preferably equal to 488 nm,

an intensity greater than or equal to I kW.cnrr ² or SkW.cnrr ² and, preferably, greater than or equal to 10kW.cnrr ² ;

 a zero angle of incidence relative to a direction perpendicular to the surface of the irradiated layer 54.

The irradiation time is less than 10s or 1 s or 10 ^"1 s and, preferably, less than 5 * 10 ^{" 2} s. The irradiation time is here between 4.5 * 10 ^-3 s and 2 * 10 ^-1 sec, and preferably between 5 * 10 ^-3 s and 5 * 10 ^-2 sec.

 In the operation 68, the layer 54 is irradiated by the laser radiation. In this example, this irradiation allows:

to convert the precursors 62 into nanoparticles 10, and

to cause the formation of lines 6, 8.

The formation of the nanoparticles 10 by transformation of the precursors 62 into nanoparticles is here obtained by a reduction of the precursors 62, triggered by the irradiation. Here, the Ag ⁺ cations are reduced to Ag silver atoms, forming the nanoparticles 10.

 On the other hand, the irradiation leads to the absorption, by precursors 62, of a portion of the laser radiation. This absorption leads to a heating of the layer 54. The depositor estimates that it is this heating of the layer 54 which, once a certain threshold is exceeded, triggers:

crystallizing part of the layer 54 to form the layer 16, and

 the growth of the nanoparticles 10 and a spontaneous alignment of these nanoparticles 10.

In this example, the precursors 62 comprise, in addition to metal cations, a plurality of small metallic nanoparticles, formed spontaneously from a portion of these metal cations. These small nanoparticles are able to absorb some of the laser radiation. These small nanoparticles have an average diameter of less than or equal to 5 nm or 3 nm. These small nanoparticles are formed in the same material as the nanoparticles 10, but are distinct from these nanoparticles 10, in particular, because of their smaller average diameter than that of the nanoparticles 10, but also of their homogeneous and disordered spatial arrangement in the layer 54. There is, in particular, no dichroic effect associated with the presence of these small nanoparticles.

During irradiation, the nanoparticles 10 are generated towards the interface between the support 14 and the layer 54 so as to form these lines 6 and 8. Here, clusters of precursors 62 can also, according to their conditions of use. exposure to laser radiation, grow or decrease in volume in response to irradiation. However, in the current state of knowledge of the applicant, the explanation of the physical mechanisms that cause the alignment of the nanoparticles is not yet certain.

 This results in a spontaneous organization of the nanoparticles 10 which form the lines 6 and 8, at the interface between the support 14 and the layer 16. The direction in which lines 6 and 8 are formed corresponds to the direction orientation of the polarization of the laser radiation.

Advantageously, the laser radiation is focused on the layer 54 so as to form a beam having a diameter greater than or equal to 1 μιτι or 5μηη or 10μηη. Here, the laser radiation is focused using a lens with x10 magnification.

In this example, the pattern 5 has an area greater than the area directly illuminated by the beam. Also, the object 4 must be moved relative to the beam to successively illuminate different portions of the layer 54, so as to form the entire pattern 5. For example, the object 4 is moved relative to the beam in one direction displacement that is perpendicular to the direction of propagation of the beam. This displacement is here realized horizontally. In this example, to reduce the duration of the inscription process, only a portion of the layer 54 is irradiated to structure the nanoparticles 10. More exactly, the object 4 is moved, so as to form a plurality of separate bands, spaced from each other and essentially parallel to each other. In each of these bands, the nanoparticles 10 form, after irradiation, the lines 6 and 8. These bands are here produced under the same experimental conditions and, in particular, with identical physical radiation parameters. Each of these strips thus constitutes a colored pattern having the same properties as the pattern 5. The joining of these bands thus forms the pattern 5. The spacing distance between these strips is chosen here so that the strips are contiguous. The width of the bands can be adjusted by changing the focus of the laser. This width is chosen according to the optical resolution power of the optical receiver to which the visualization of the pattern 5 is intended, so that the bands are not perceptible by this optical receiver. For example, the distance can be chosen so that the pattern 5 is visible to an emmetropic human without an optical instrument. Here, the distance between two consecutive bands is chosen less than or equal to 50μηη or 25μηη and, preferably, less than or equal to 20μηη. Here, the strips are spaced two by two by the same distance.

The speed of displacement of the object 4 is determined by the irradiation time chosen during the operation 66. Here, this speed of displacement is between δθμηη.ε ^"1 and δΟΟΟμηη.ε ^{" 1} .

For example, the following parameters are chosen to form the pattern 5: displacement speed: Ι ΟΟΟμηη.ε ^"1 ,

Intensity: 196kW.cm ^"2 ,

-width of the band: 18μηη,

 -spacing between two consecutive bands: 18μηη.

 Advantageously, during a step 70, the pattern 7 of the set 2 (Figure 1) is written on the object 4. This step 70 includes operations 72 and 74, respectively, identical to operations 66 and 68 , except that :

 during operation 72, at least one of the parameters of the laser radiation or of the irradiation is modified, so that the radiation and irradiation parameters used during the operation 74 are different from those used during operation 68 to form the pattern 5;

 during operation 74, the laser radiation irradiates a location of the layer 54 distinct from the irradiated location during the operation 68 to form the pattern 5.

 The speed of movement of the object 4 with respect to the beam during the step

70 can also be different from the movement speed during step 60, while remaining between δθμηη.ε ^"1 and δΟΟΟμηη.ε ^{" 1} . For example, the following parameters are chosen to form pattern 7:

displacement speed: 700μηη.5 ^"1 ,

Intensity: 196kW.cm ^"2 ,

 -width of the band: 18μηη,

-spacing between two consecutive bands: 18μηη.

 Thus, the pattern 7 has a distinct color of the pattern 5.

 [0065] Many other embodiments are possible.

Alternatively, the pattern 5 has no effect of colored luster. In this case, the roughness of the support 14 has a value greater than the wavelength at which it is desired to observe a dichroic effect. The object 4 does not necessarily have a planar shape. For example, the object 4 has a curved shape.

 The pattern 5 may not have a dichroic effect in transmission. In this case, at least one or the other of the support 14 or the layer 16 is not transparent. The line 9 is not necessarily rectilinear. Alternatively, the straight line 9 has a non-zero curvature and has no point of inflection. In this case, the line 6 has a curvature.

 For simplicity, the nanoparticles 10 have been described as having a spherical shape. In reality, these nanoparticles 10 may differ from each other in their shape and in particular may have a shape other than spherical, such as an ellipsoid shape. When these nanoparticles 10 are not spherical, the diameter of a nanoparticle is replaced, for example, by the ferret diameter or the hydraulic diameter of this nanoparticle.

 The nanoparticles 10 may be formed of an alloy of different metals in the same zone, for example gold and silver.

 The nanoparticles 10 are not necessarily made of a noble metal. For example, the nanoparticles 10 are formed based on cobalt.

The support 14 is not necessarily rigid and may be formed of a flexible material. Thus, the support 14 may be made of a material other than glass. For example, the support 14 is a ceramic or a metal or also comprises an organic material, such as plastic. Preferably, the material forming the support 14 is chosen so that its structural properties are not degraded during heating for calcining the copolymer. Indeed, some materials, such as organic materials, may be degraded if they are brought to a temperature of several hundred degrees Celsius. Alternatively, the heating is replaced by a rapid annealing carried out by means of a rapid annealing oven ("rapid thermal annealing" in English) by exposure to infra-red radiation for a period of less than 10 seconds or 1 minute or at 5 minutes. In this case, the organic material present in the layer is formed of a material absorbing infra-red radiation. Thus, the layer 54 can be formed without damaging the organic materials of the support 14.

The support 14 may be formed of several layers.

 Alternatively, the layer 54 is covered with a protective layer, preferably transparent. The layer 16 may also be covered with such a protective layer.

The process of forming the layer 54 of TiO ₂ may be different. For example, the layer 54 is deposited by inkjet process or by centrifugation. Alternatively, the layer 54 of TiO ₂ is not porous. The method of producing this layer 54 is then modified to allow the incorporation of the precursors 62 during the preparation of this layer 54. For example, the support 14 is coated with a hybrid material comprising a titanium precursor and an organic material capable of self-organization. The precursors 62 are thus incorporated directly into this hybrid material. In this case, operation 58 and step 60 are performed simultaneously. For example, this hybrid material is produced according to a method identical to the method described with reference to operation 58, except that:

a molar amount of 0.2 N ₀ of silver nitrate AgNO ₃ is incorporated into the solution after 4 hours of stirring of the solution,

 the calcination of the deposited layer is omitted.

The layer 54 may be made of a material different from TiO ₂ . For example, the layer 54 is made of zinc oxide ZnO.

The layer 54 is not necessarily amorphous, but may be made of a material already having, at least in part of its volume, crystalline properties, for example a layer containing nanoparticles of crystallized TiO ₂ in the phase. anatase.

The silver nitrate solution used for filling the pores of the layer 54 may have a different composition. For example, the silver nitrate can be dissolved in water (and not in a water-ethanol mixture) with or without the addition of ammonia. The concentration of silver in the solution can vary.

In a variant, metallic nanoparticles, called intermediate nanoparticles, are formed in the layer 54, from the metal cations, before generation of the nanoparticles 10. These intermediate nanoparticles are distinct from the nanoparticles 10, for the same reasons as that which has previously described with reference to small nanoparticles. These intermediate nanoparticles have a mean diameter of less than 20 nm or 15 nm and are spatially distributed in a disordered and homogeneous manner in the layer 14. The formation of these intermediate nanoparticles can be triggered by means of illumination by UV radiation of lower intensity to 1 kW.cnrr ² , or by means of a heat treatment whose temperature is for example between 100 ° C and 300 ° C, or of a chemical reduction, with for example a reducing agent such as tetrahydruroborate of sodium NaBH ₄ . In step 64, the nanoparticles 10 are then formed from these intermediate nanoparticles.

In steps 64 and / or 70, the physical parameters of the laser radiation can be more finely tuned, in order to more precisely control the crystalline phase obtained for the layer 16. In fact, the TiO ₂ can crystallize in several distinct phases. , say rutile, brookite and anatase. Each of these phases has distinct optical properties (such as refractive index or optical distance) that can change the color of the pattern 5.

The process for structuring the nanoparticles 10 may be implemented independently of the method of inscription of the pattern 5. For example, this method may be used to have the nanoparticles arranged spatially in the form of lines 6, 8 but without any attempt to obtain a dichroic effect or even the inscription of a colored motif. In this case, the nanoparticles 10 are then not necessarily metallic. The wavelength of the laser radiation irradiating the layer 54 is then chosen as a function of the optical absorption spectrum of the material forming the precursors 62 of these nanoparticles 10, so that these precursors 62 are capable of generating heating of the layer 54 during irradiation with this laser radiation. Still in this case, operation 66 can be omitted. Finally, the layer 16 may not be transparent.

 The wavelength of the laser radiation may be chosen outside the range of visible light. For example, the wavelength is chosen in the field of ultraviolet. In this case, the optical properties of the layer 54 (in particular the range of wavelengths for which it has transparency) may be different.

 Alternatively, the nanoparticles 10 can be aligned over the entire surface of the layer 54. The pattern 5 can occupy the entire surface of the object 4.

The pattern 5 is not necessarily visible to the naked eye and may therefore have a smaller area than that described. For example, the pattern 5 has an area less than Ι ΟΟμιτι ² .

Alternatively, the laser radiation is not applied continuously but has a succession of pulses. The laser is for example a femtosecond laser.

 The diameter of the focused laser beam may be different.

Step 70 may be omitted. In this case, only a single pattern 5 is written on the object 4.

Step 70 may be applied repeatedly to form a plurality of patterns. For example, to form the different patterns of the set 2 other than the patterns 5 and 7, the step 70 is applied repeatedly, changing, each time, a physical parameter of the laser radiation or irradiation. The speed of movement of the object 4 with respect to the beam can also be modified. For example, the combinations of parameters described in the table below can be used to form these patterns: speed intensity width band spacing

1000 95 20 20

 600 95 20 20

 100 191 20 20

 200 95 20 20

 900 157 18 18

 1000 157 18 18

 2000 157 18 18

 3000 118 18 18

 3000 157 18 18

 4000 157 18 18

 Where:

"Speed" refers to the speed of movement of the object 4 relative to the beam, expressed in μm.s ^-1 ;

"Intensity" refers to the intensity of the laser radiation on the object 4, expressed in kW.crrr ² ;

 - "bandwidth" means the width of a band drawn with the aid of the laser beam, expressed in μιτι;

 - "spacing" means the distance separating two bands immediately adjacent to each other, expressed in μιτι.

Claims

1. Method of laser structuring of a network of nanoparticles, characterized in that it comprises: a] the supply (50) of a support coated with a layer formed of an essentially inorganic material, b] the incorporation (60), in the layer, of nanoparticle precursors, these nanoparticle precursors being distributed homogeneously and disorganized in the layer and conferring on the layer material the property of being heated when this layer is irradiated; for a period of less than 10 s by linearly polarized laser radiation having an intensity of 1 kW.crn ^{"2 or more} , c) the irradiation (64) of the layer, by the laser radiation defined in step b], so as to:  · Transform the precursors of nanoparticles into nanoparticles, and increase the size of these nanoparticles, and  Align the nanoparticles along lines parallel to each other, the periodicity of the spacing is in particular a function of the wavelength of the laser radiation and whose orientation is a function of the orientation of the polarization of the laser radiation; the support being displaced relative to the laser radiation, during step c], with a displacement speed of between 50 pm.s ^-1 and 5000 pm.s ¹ . The method of claim 1, wherein: step b] comprises the incorporation (60), in the layer, of precursors (62) of metal nanoparticles, so as to obtain, after step c], a nanoparticle network having an effect dichroic;  step a] comprises providing the support or the layer formed in a transparent material in the wavelength range for which it is desired to observe the dichroic effect. 3. The method of claim 2, wherein step b] comprises the incorporation (60), in the layer, of precursors (62) of metal nanoparticles, comprising metal cations chosen from the group consisting of noble metal cations. . 4. The method of claim 2 or 3, wherein: step a] comprises the provision of a support whose roughness of the face coated by the layer is less than or equal to X nm RMS, where X is equal to the wavelength for which it is desired to observe a dichroic effect , so as to obtain a dichroic effect in specular reflection; at the end of step c], the nanoparticles are aligned at the interface between the support and the irradiated layer. 5. Method according to any one of the preceding claims, wherein: step a] comprises the provision of a support coated with a layer of substantially inorganic material, this layer being furthermore porous;  step b] comprises filling the pores of the layer of the essentially inorganic material with the precursors of nanoparticles. 6. Method according to any one of the preceding claims, wherein the layer is amorphous and is able to crystallize locally in response to the heating of the layer, in step c]. The method of any one of the preceding claims, wherein step a] comprises providing a support coated with a layer (54) of mesoporous titanium dioxide. 8. A method of permanent inscription, on a support, of a first and a second pattern having different dichroic effects, by means of a laser structuring method of a network of nanoparticles, characterized in that method comprises: a] providing (50) a layer of substantially inorganic material on the support, b] the incorporation (60) into the layer of precursors of metal nanoparticles, these precursors of metallic nanoparticles being homogeneously distributed and disorganized in the layer and conferring on the layer material the property of heating up when this layer is irradiated, for a period of less than 10 s, by a first and, alternately, a second linearly polarized laser radiation having an intensity greater than or equal to 1 kW.cnrr ² , c] irradiating (64) the layer, at the location of the first pattern, with the first laser radiation defined in step b], so as to: Transforming, at the location of the first pattern, these precursors of metallic nanoparticles into metal nanoparticles and increasing the size of these metal nanoparticles, and • Align, only at the location of the first pattern, the metallic nanoparticles along lines parallel to each other; d] irradiating (70) the layer, at the location of the second pattern, with the second laser radiation defined in step b], so as to:  Transforming, at the location of the second motif, these precursors of metallic nanoparticles into metal nanoparticles and increasing the size of these metal nanoparticles, and  Aligning, only at the location of the second pattern, the metal nanoparticles along lines parallel to each other; the support being displaced with respect to the laser radiation, during steps c 1 and d 1, with a displacement speed of between 50 μm ^-1 and 5000 ms ^-1 ; the steps c] and d] differing from each other also by the duration of irradiation or by a physical parameter of, respectively, first or second laser radiation, this physical parameter being chosen from the group consisting of intensity, the wavelength, the angle of incidence of the radiation relative to the normal to the support, and the direction of the optical polarization, so that at the end of the steps, respectively, c] and d], the first and second patterns have distinct reflection, transmission or optical scattering spectra. 9. The method of claim 8, wherein steps c] and d] further differ from each other by the speed of movement of the carrier relative to the laser radiation.