WO2013021227A1

WO2013021227A1 - Preparation of femtosecond laser induced oriented crystals in glasses

Info

Publication number: WO2013021227A1
Application number: PCT/IB2011/002076
Authority: WO
Inventors: Bertrand Poumellec; Matthieu Jean-Pierre LANCRY; Chaxing FAN; Huidan ZENG
Original assignee: Universite Paris Sud 11
Priority date: 2011-08-05
Filing date: 2011-08-05
Publication date: 2013-02-14
Also published as: WO2013020912A9; WO2013020912A1

Abstract

The present invention concerns a method for producing monocrystalline region in the bulk of a transparent matrix including Si02 and Li20 and Nb205. This method comprises producing a precipitate of LiNb03 in at least one selected volumic unitary target region, selectively determined within the bulk material of said transparent matrix, and selective irradiation by a femtosecond laser focused onto said unitary target region. According to the invention, successive adjacent unitary target regions may furthermore be produced along a trajectory which is determined so as to be parallel to the c-axis direction of a given crystalline internal orientation. This method enables to produce various components, including optical gratings or arrays, waveguides and/or polarizers, which may include one or several polarizing gratings possibly having different photonic and/or geometry characteristics.

Description

« Preparation of femtosecond laser induced oriented crystals in glasses »

Introduction

The present invention concerns a method for producing monocrystalline region in the bulk of a transparent matrix including Si0₂ and Li₂0 and Nb₂0₅. This method comprises producing a precipitate of LiNb0₃ in at least one selected volumic unitary region, so-called target region, selectively determined within the bulk material of said transparent matrix, for example through known digitally driven electromechanical means, and selective irradiation by a femtosecond laser focused onto said unitary target region. According to the invention, successive adjacent unitary target regions may furthermore be produced along a trajectory which is determined so as to be parallel to the c-axis direction of a given crystalline internal orientation.

This method enables to produce various components, including optical gratings or arrays, waveguides and/or polarizers, which may include one or several polarizing gratings possibly having different photonic and/or geometry characteristics.

Background

Composite materials containing nanoparticules or microcrystals have found an increasing number of applications in different fields of science and technology, such as in the various fields of optics and photonics components and devices.

The preparation techniques of nanostructured glassy materials are various, such as via reduction of metallic ions introduced in glasses during melting, via ion-exchange processes followed by laser processing of the glassy materials or by their thermal processing either in air or in a reducing atmosphere, via microwave-assisted diffusion, or via co-RF-sputtering of glass and metals as well as via introducing conductive carbon nanostructures into glasses during melting .

Controlling properties of nano or microcrystals

Variations of optical properties designed in such nano or microstructured composite materials may produce useful macroscopic effects, in a way that depends on the size, shape, distribution and orientation of the crystalline structure.

Orientation of a crystalline structure here refers to the specific arrangement of its atomic components, as defined in crystallography. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice. The symmetry properties of the crystal are embodied in its point and space group.

The crystal structure and its symmetry play a role in determining many of its properties, such as cleavage directions, electronic band structure, and optical properties. They are thus important for controlling local non-linear properties.

Moreover, crystalline continuity and uniformity is important in obtaining interesting effects at the macroscopic level, or even for obtaining any macroscopic effect at all . For example, polycrystalline non- centrosymmetric materials do not show piezoelectricity, pyroelectricity or non-linear optical properties, i.e. are not macroscopically anisotropic if the crystals are randomly oriented . Only in crystal oriented materials, pyro-, piezo-electricity or NLO properties may be observed . This means that c-axis of the crystal have all the same direction, and the same orientation within this direction, i.e. like an assembly of parallel arrows.

Prior art

Presently, it is known how to obtain a surface layer of such nano- or microstructured composite materials. Such 2D surface layer could be used to fabricate certain optical components such as gratings or polarizers.

Surface layer of oriented crystallized glass-ceramics has been prepared by various methods: electrochemical nucleation, oriented crystallization of glass-ceramics using a thermal gradient.

However, such methods do not enable actual spatially selection of the crystal location, as the crystals are produced in narrow relation with the surface and starting from it. Moreover, Ba₂TiSi₂0₈ material is not the most interesting material for various applications, inter alia because its non-linear coefficient weaker than several other materials'. Furtherly, as disclosed in Optics Express. 18, 8019-8024 (2010), continuous wave laser has been used to fabricate a, surface layer, through laser induced crystallization of crystalline lines of LiNb03 on surface of a glass by a YAG laser (λ= 1064ηη"ΐ), and through patterning an uniform LiNb03 crystal films on the glass surface by using continuous wave ytterbium YV04 fiber laser (wavelength : 1080nm) irradiations. The planar LiNb03 crystalline patterns had its c-axis orientation along the laser scanning direction.

In 2008, Dai, Y. et al . succeeded in inducing Ba₂TiGe₂0₈ oriented crystals in glass by carefully moving the femtosecond laser focus, as disclosed in "Femtosecond laser-induced oriented precipitation of Ba₂TiGe₂0₈ crystals in glass", Optics Express 2008, 16, 3912-3917. However, this method is limited to the glass surface, which constitutes a limitation to the possible applications.

Furthermore, it would be of great interest for applications in optical, acoustical and actuator devices if crystallized regions could be selectively created inside a bulk glass in a well-controlled manner (i.e. shape, size, orientation).

Very recently, one case of 3D crystallization patterns has been obtained with femtosecond laser in a La₂0-B₂0₃-Ge0₂ glass, as disclosed by A. Stone et al . Optic Express. 17, 23284-23289 (2009). However, such method is not yet very established and necessitates thermal assistance in the process, as the sample was heated at 400°C during the writing process. Such method is thus still technically complex, and the concerned material La₂0-B₂0₃-Ge0₂ is not the most interesting nor the most efficient for numerous applications, e.g . because of its non-linear coefficient.

Oblects of the invention

An object of the invention is to improve and widen the technical possibilities known in the prior art.

Moreover, it is an object of the invention to propose a method for selectively producing spatially oriented crystallization within the bulk of various transparent matrix materials. Thus, the invention also aims to obtain better and more various performances, ranges and types of effects; inter alia for enabling more compatibilities or less complexity in the range of available materials and production processes.

Summary of the invention

The invention proposes a method for preparing composite materials containing nanoparticules or nano or microcrystals with controlled local optical properties, from transparent matrices. According to the invention, said method comprises writing a transparent matrix including Si0₂ and Li₂0 and Nb₂0₅, by producing a precipitate of LiNb0₃ in at least one volumic unitary region, so-called target region, selectively determined within the bulk material of said transparent matrix, through selective irradiation by at least one fast pulsed laser, such as a femtosecond laser, focused onto said unitary target region.

Thus, new materials may be used as transparent matrix, and crystal orientation may be controlled, e.g. according to the detailed effects that are looked for as well as for the precise characteristics of such effects. Such monocrystal may be used for its photoelectric, pyroelectric, piezoelectric, photorefractive, and /or non linear properties, such as in opto-acoustic or photoelectric or non-linear domains.

It has to be noted that LiNb0₃ has numerous advantages, such as having the highest non-linear coefficient, which is seven times higher than for Ba₂TiSi₂0₈.

Furthermore, according to the invention, such method may comprises producing a given continuous region, so-called multipoint region, within the bulk material of the transparent matrix, through implementing a plurality of irradiating steps focused on a plurality of successive adjacent or intersecting unitary target regions within said multipoint region, such as through scanning said continuous region with the focused pulsed laser.

Irradiation of the first region of these successive target regions takes place during a few seconds, so as to produce a precipitated crystal, for example between 1 s and 10 s, or preferably between 2 s and 5 s. Further successive irradiation of further target regions then maintains and propagates a growth front starting from this first precipitated crystal. First irradiation thus acts as a nucleation step while further successive irradiations act as which then act as successive growth steps.

The invention thus enables to obtain a continuous region of precipitated crystals within the bulk of the glass, possibly constituting a non ordered and/or polycrystalline region, such as for producing an isotropic refraction index variation.

Furthermore, the invention propose the successive steps of irradiating steps to be implemented in a spatial and temporal succession determined so as to obtain a monocrystalline region in the given continuous multipoint region.

The invention thus also enables to obtain a continuous region of monocrystals within the bulk of the glass, such as for producing devices implementing anisotropy or second harmonic generation.

Furthermore, the invention also propose that the successive adjacent unitary target regions are produced along a trajectory which is determined so as to be parallel to the c-axis direction of a given (i.e. wanted) crystalline internal orientation. Thus, for every multipoint region, isolated or included within a pattern, the writing trajectory is controlled so as to follow the direction and orientation that are wanted to be the c-axis direction and orientation.

Thus, it becomes possible to write such a controlled orientation monocrystalline region in a continuous volumic region of almost any shape, size and position. This enables to produce multiple kind of 3D pattern of continuous monocrystalline regions with controlled orientations, which are necessaries for implementing components using anisotropic properties or for generating second harmonics.

Through selectivity possibilities and adjusting the laser parameters, such method enables to control easily the structure, size and orientation within the transparent matrix, also called simply "glass". This is of great interest for researches on laser utilization as well as for research in the science of new materials. Also, precipitated monocrystals may be used for building components or devices for laser frequency multiplexing or multiplication, as well as acoustic surface waves devices, optical modulators, photoelectric cells, optical parametric oscillators, or photoelectric or opto-acoustic devices or components.

According to a particularity of the invention, such a method may be implemented for writing a transparent matrix comprising the following components molar fractions proportions:

Si0₂: less than 34%

Li₂0 and Nb₂0₅: in equal quantities, or with a difference of less than 20%, or possibly 10%.

Preparation of the transparent matrix comprises the following steps:

- melting together the components at a temperature between 1350°C and 1500°C for a duration of 1 to 5 hours;

- casting the matrix in a preheated mold at 400-600°C;

- annealing the matrix for 12 to 36 hours in a cooling down process, from a temperature between 500°C and 600°C toward ambient temperature, preferentially at the decreasing rate of less than 30°C/hr for the first 5 hours, and preferentially between 15°C/hr and 25°C/hr.

According to the invention, writing process is then implemented with one or preferentially all of the following parameters for the femtosecond laser:

- wavelength between 0,4pm to 1,2pm, and preferentially around or at 1,03pm.

- focus with a numerical aperture from 0,1 to 1; preferentially from 0,4 to 0,8.

- pulse duration between 30fs and 2000fs; preferentially between 240fs and 300fs or even lOOOfs.

- pulse energy between 0,2 and 5pJ/pulse; preferentially between 0,5 and 2,5pJ/pulse. According to the invention, the scanning (i.e. movement of the focus point) trajectory while writing a multipoint region may be adjusted according to the following rules.

Particularly, the femtosecond laser focus point is controlled so as to obtain, for the successive unitary target regions within a multipoint region so-called continuous written region, a pulse repetition rate of larger than 200kHz and a scanning speed so-called writing speed smaller than 150 micrometer/s.

As a particularity of the invention, at least one written polarizing array or grating is written with a period lower than one micrometer; and/or is entirely written within an area of less than 10 or even 5pm²; and/or is written in a transparent matrix the dimensions of which range from 5 x 5 mm to 50 x 30 mm (possibly for a plurality of different arrays).

List of figures

The present invention will be better understood and more advantages will become apparent from the following description of examples, with reference to the attached drawings in which :

- FIGURE 1 schematically represents an example of spatially selective irradiation of one target region in the bulk of a transparent matrix, according to the invention;

- FIGURE 2 is a perspective view schematically representing a writing operation according to the invention, comprising irradiation of one multipoint continuous region including several successive target regions, along a writing trajectory comprising two successive non parallel directions;

- FIGURE 3 is a top view schematically representing an example of a array comprising several separated parallel lines, each being written as a multipoint continuous region according to the invention;

- FIGURE 4 is a perspective view schematically representing an example of a 3D array comprising several separated parallel planes, each being written as a succession of three adjacently superposed multipoint continuous linear regions, irradiated according to the invention; - FIGURE 5 is a perspective view schematically representing an example of a 3D component comprising two superposed levels including each two perpendicularly oriented 3D arrays similar to FIGURE 4;

- FIGURE 6a to FIGURE 6b are scanning electron microscope (SEM) images and electron backscatter diffraction (EBSD) images of crystals obtained in the interaction volume from a second example;

- FIGURE 7a to FIGURE 7b are scanning electron microscope (SEM) images and electron backscatter diffraction (EBSD) images of crystals obtained in the interaction volume from a third example;

- FIGURE 8a to FIGURE 8b are graphical results of calculation of the temperature field and of the thermal gradient distribution under scanning according to an exemplary embodiment of the invention, and show respectively:

o for FIGURE 8a : the whole temperature field, and

o for FIGURE 8b : the temperature gradient contour on the temperature background, in the partial area of the square figured in FIGURE 8a;

- FIGURE 9a and FIGURE 9b are SHG images taken with perpendicular polarizations 17px and 17py of a same area within an oriented monocrystal obtained through writing from second example (FIGURE 6).

Detailed description

An exemplary implementation of the invention is detailed hereunder in a non limitative way.

Matrix preparation

A glass of formula was produced with the followings molar fractions: Si0₂ : Li₂0 : Nb₂0₅ = 40% : 32.5% : 27.5%.

Approximately 50 g batches of Li₂C0₃, Nb₂0₅ and Si0₂ raw materials were mixed and melted in a platinum crucible in an electronic furnace, at 1430°C for 2 hr. Then the melt was poured onto a steel plate heat-treated at 500°C, and transferred to another electronic furnace pre-set at 540°C cooling down to room temperature. This cool down phase included an annealing phase of the transparent glass to release the stress, with a decreasing rate of 20°C/hr for the first 6 hours.

Monocrvstalline writing

FIGURE 1 schematically represents an example of selective irradiation of one target region 102 in the bulk of a transparent matrix 10 here called glass, according to the invention. A femtosecond laser beam 111 is focused by an objective or a lens 112, into a cone beam 113 toward a focus point 114 located within the bulk of the matrix.

The method of the invention comprises writing this transparent matrix 10 including Si0₂ and Li₂0 and Nb₂0₅, so as to produce a precipitate of LiNb0₃ in at least one selected volumic unitary region 102, so-called target region. This target region is selectively determined within the bulk material of said transparent matrix, for example through known digitally driven electromechanical means, and selective irradiation by a femtosecond laser is focused onto said unitary target region.

Most of the energy of the laser is concentrated in this focus point, 114. Laser irradiation gives a rapid temperature increase (in a few ps) in the laser-irradiated region, consequently resulting in the formation of low viscous melt within an extremely short period (a few 10's ns), in a central region 103. This is true when pulses are separated by more than a few μ≤. When the repetition rate is larger than 200 kHz, the time spent at the nucleation and/or growth temperature is defines by the irradiation time in static mode or respectively the scanning speed in dynamic mode.

Crystallization takes place at the interface between the two phases liquid-solid, from exterior toward interior, in a crystallized region 102.

As shown in FIGURE 2, a writing operation according to the invention, comprises irradiation of at least one multipoint continuous region 220 so- called multipoint region.

Irradiation of the first region of these successive target regions takes place during a few seconds, so as to produce a precipitated crystal, for example between 1 s and 10 s, or preferably between 2 s and 5 s.

According to the invention, this continuous multipoint region 220 is selectively and spatially "written" within the bulk material of the transparent matrix, through implementing - a first initial step of irradiating a first initial target region 102a during 1 s to 10 s, and preferably between 2 s and 5 s; followed by

- a plurality of further irradiating steps focused on a plurality of further successive adjacent or intersecting unitary target regions within said multipoint region, such as through scanning said continuous region with the focalized femtosecond laser 11.

After irradiating the first region 102a for a few seconds until precipitation of at least one nucleation crystal, this continuous multipoint region 220 is scanned with the focused beam 113, 114 of a pulsed laser, such as femtosecond laser 11, along a writing trajectory 221 which can be of any 2D or 3D shape or design. This scanning may be obtained for example by a movement of the laser 11 or its objective 12, or a movement of the matrix, in a relative "scanning movement" shown in FIGURE 1 by the cross of the arrow rear 12. may be In this example, this trajectory comprises two successive non parallel directions 221a and 221b.

Furthermore, according to the invention, the successive steps of irradiating steps are implemented in a spatial and temporal succession determined so as to obtain a monocrystalline region in the given continuous multipoint region, thus adjusting the displacement rate of the growth front along the writing trace.

For low repetition rate (<~200 kHz), the energy deposited by the laser has enough time between two subsequent pulses to diffuse and equilibrate with the glass matrix. This produces a repeated series of rapid heating and quenching and no accumulation of heat, which cannot sustain suitable crystallization temperatures. This is a reason why sufficient repetition rate has to be reached in order to maintain crystallization temperature a time long enough in the written region 220.

Crystalline orientation

According to the invention, successive adjacent unitary target regions may furthermore be produced along a trajectory 221 which is determined so as to be parallel to the c-axis direction of a given (i.e. wanted) crystalline internal orientation, shown in the figures by the short arrows 104a to 104j within the circle showing the central circular section of the unitary target regions 102a to 102j. In principle, the crystallization of glasses is dominated by two factors based on the theory of crystallization : nucleation frequency and growth rate. The speed to transform the glass state into crystal is relevant from the kinetics and it is determined by two steps: the nucleation and the growth of the nuclei. In order to induce a phase transformation, it needs a large free enthalpy difference between the two phases, some seeds (fluctuations of molecular arrangement) and an activation energy barrier (interface energy) small enough to be overcome by vibration energy. The stability of the seeds is determined by two terms. The first one is the volumic enthalpy difference between glass and crystal organization. It is defined by the following expression in which L is the latent heat from glass to crystal :

. This term, which is negative, has to be multiplied by the seed volume (assumed spherical here). The second one is the interface energy γ₅ι, which is positive. This one has to be multiplied by the surface of the seeds, and thus the total seed enthalpy depends on its radius and experience a maximum for a critical radius.

The following equations express forces relevant for orienting the growth : Ο_ν {T_meh - T) 3(AG_r y {T_mell - Tf

Possibilities to orient the crystallization arise from anisotropy in the free enthalpy expression on one hand, and from gradients in kinetics on the other hand [10] . The minimum of enthalpy and the maximum in kinetics according to the orientation will favour crystallization in this orientation.

The enthalpy in each unitary target region depends on space and time coordinates. When considering the first target region 102a as a starting point, the orientation for which the enthalpy decreases the fastest (i.e. for which the kinetics is fastest) will definite the crystallization orientation.

It has been shown that nucleation and growth involve atom migrations. These diffusions are driven, on one hand by the gradient of free enthalpy (i.e. gradients of chemical potentials, gradient of concentrations), of electric potential, or of stress but also by gradient of mobility that can be produced by temperature or light intensity gradient. Therefore, by controlling the field orientation, especially the temperature gradient orientation, it is possible to control the precipitation and eventually the crystal growth orientation. This is what we have to perform when using laser beam.

As explained above, the particle formation is achieved in a space configuration 102 centered on the focus 104 of the beam. With heat diffusing radially, the temperature gradient is oriented towards the focus center. Without other forces, such as in the first target region 102a, the orientation for the maximum crystallization rate is thus towards the center.

When the laser spot is moved (in fact at a speed large enough after the nucleation stage), the temperature field departs from circularity and left a larger heated region and a weaker gradient field behind the spot than in front of it, as illustrated in region 1 of FIGURE 8. All the points situated in front of the spot will be heated and thus even if they have crossed the nucleation area, they will forget this period of time because they will be heated at higher temperature.

By moving the beam in a scanning movement 12, thermal gradient is oriented differently inside the line than outside and this is detrimental for uniform orientation. However, by moderating the energy deposition, it is possible to write thinner lines uniformly oriented . This may explain some of the difficulties of adjusting the writing parameters.

The beam scanning movement modifies the previous radial geometry into a plane one. Thus, crystallization is achieved in a very large temperature gradient that has a plane geometry with a gradient perpendicular to it and to the direction of scanning 221.

By producing ferro-electric crystal with a femtosecond laser, the inventors showed that when they changed the orientation of scanning, the crystal orientation changed at the same time. In addition, they also showed that instead of producing an abrupt grain boundary to accommodate the change, the lattice orientation changed gradually, probably thanks to lattice strain, preserving a continuity between crystals differently oriented.

Thus, crystallization is sensitive to temperature gradient and it will be defined by the orientation of the laser scanning movement in the glass. The orientation of the growth takes place during the step of writing the further target regions 102b to 102j, after nucleation in static mode in the first target region 102a. Therefore, as shown above, the particle nucleation is achieved in a space configuration centred on the focus for a static beam.

FIGURE 8a and FIGURE 8b shows the results of calculation of the temperature field and of the thermal gradient distribution under scanning for the glass 0.6(Li₂O.Nb₂O5)-0.4SiO2, for pulse energy of 1.5 μ-1/pulse, scanning speed of 100 μηι/s toward the right hand side.

FIGURE 8a shows the whole T field, with the elliptical shape of the isotherms. The temperature range around 750 K should be considered adapted for producing growth (light blue). The black square marks the location of the results shown in FIGURE 8b.

FIGURE 8b displays the temperature gradient contour, in lines above the temperature background . The large gradients are concentrated on the front of the beam. Note that the gradient in the growth zone is around 20 Κ/μΓΠ in region 1 to 40 Κ/μηη in region 2, i.e. much smaller than that in static mode.

Writing parameters

In unitary target regions such as shown in FIGURE 1, chemical diffusion is driven by thermal gradient, which determines the crystal growth and orientation. Thermal diffusion and heat accumulation effects play an important role in forming crystals in glasses; and adjusting such parameters is relevant for the fabrication oriented crystals within such glasses.

In writing such multipoint region 220, a nucleation at the crystal growth front should be avoided for obtaining homogeneous crystal lines or regions 220, and moreover for obtaining regions 220 with oriented 104 crystals.

Thus, it can be seen that the invention makes it possible to use femtosecond laser to orient the crystals along a preferred direction in glasses by choosing suitable glass-matrices and accurately adjusting its parameters.

Various possibilities and advantages

A most innovative aspect of the invention is that it will be possible to grow the microcrystals in a selected direction or orientation in the bulk and not at the surface only.

Also, it becomes possible to shape the microcrystals directly by controlling the scanning movement of the laser (and of its focus).

Such flexibility of the pattern design may also enable to shape nonlinear optical properties in more various forms, e.g. for obtaining second and third order non linear susceptibility.

Furthermore, such method makes it possible to produce components in a contactless, giving more flexibility in many part of the production and design field .

Also, writing may be obtain with little or no surface damage on the matrix.

More generally, spatial selectivity through laser scanning control and various possibilities of parameters such as wavelength, numerical aperture, pulse energy, polarization direction, but also spatio-temporal pulse shaping and writing direction, makes this process a very flexible tool for a versatile and easy to set up production.

Short pulse duration (femtosecond) allows applying a very large light intensity (more than TW/cm²) in delivering a rather small quantity of energy (smaller than a few 10s μ-l/pulse). Specifically, femtosecond laser direct writing technique allows thus 3D engineering of the optical properties of the material via gaining control over the 3D spatial distribution of nanocrystals in the glass down to sub-micrometer scale. This presents interesting prospects for shaping novel 3D photonic structures for optical telecommunication applications, high power laser and so more.

Contribution to unify the writing methods for various components: Today's advanced ultra-short laser systems offer a great number of glass interactions, from surface machining, annealing, voids formation, nano or micro crystals precipitation, dissolution and shaping or refractive index changes (isotropic or anisotropic) writing . It is thus a single processing tool that can enable the manufacturer fabricating disparate components on a common substrate, and enabling their integration into functional and compact systems.

Examples of custom-made components

Spatially selective writing of monocrystalline lines with controlled orientation makes it possible to produce many kinds of custom made or even on-demand optical components, such as the components using the above-cited effects.

By enabling fabrication of custom-made 3D nano-structured glasses., the invention enables producing new structures in 3D and new original devices based on nanocrystals embedded within glass matrix, in either polycrystalline or oriented monocrystalline traces.

Polycrystalline traces may be used for example for producing various 3D volumic devices, for example of a kind that uses isotropic refraction index variation, such as waveguides or optical diffractive devices.

Monocrystalline traces, possibly oriented, may be used for producing various 3D volumic devices, for example of the kind using anisotropy or second harmonic generation, such as for converting Infra Red light into visible light.

For example, devices such as polarizer, diffractive optics, waveguides, sensors, frequency doubling devices, photonic crystals and even metamaterials (i.e. ordered composites) can be developed. Such components could enable to be implemented in various fields and under various forms, such as integrated-optics, nano-optics, optical components, optoelectronics or bio-photonics.

Due to the spatial selectivity, the design of the nano or micro structured material is optimized in terms of spatial distribution, concentration, size, shape and orientation of the nanocrystals. The variability of the final product may be adjusted based on this optimization. Polarizing components

While gratings and polarizers were already obtained in 2D structures, the method of the invention enables to directly write 3D components in various materials with more possibilities and flexibility. It also makes it possible to produces new individuals or integrated components in existing fields as well as new ones.

As shown in FIGURE 3 to FIGURE 5, the invention thus also proposes to produce a polarizing component through writing a method according to method here above within at least one transparent matrix.

As shown in FIGURE 3, such component may be or include a polarizing grating 3 produced by writing in an essentially 2D (plane) matrix 30 a plurality of multipoint regions 321 to 327 through a method as exposed here above, in a pattern designed to produce at least one polarizing array 32.

As shown in FIGURE 4, such a component may be a 3D polarizer 4 produced by writing within a thick volumic matrix 40 a plurality of parallel "planes" 421 to 427 forming a 3D array 42, each plane being obtained by writing a plurality of superposed adjacent or intersecting multipoint regions 421a to 421c.

Method according to the invention may further comprises writing within the at least one transparent matrix (and possibly a single matrix) a plurality of polarizing arrays having different photonic and/or geometry characteristics.

In the example shown in FIGURE 5, a single monolithic volumic matrix 50 is written with four 3D arrays 51 to 54 spread on different levels 50a and 50b, and having two perpendicular orientations on each level .

Example 2

Another exemplary implementation of the invention is detailed hereunder in a non limitative way, related to FIGURE 6a and FIGURE 6b.

A glass was produced with the molar fractions formula : 34Si0₂-33Li₂0-33Nb₂0₅ (mol%).

Main raw materials are 7.14g of quartz sand, 13.44g of lithium carbonate and 30.73g of niobium oxide. These components were mixed and grinded to an homogeneous composition. This mix was put in a quartz or platinum crucible which was then kept in an oven for 2h at 1400°C, so as for the components to be melted and dispersed in an homogeneous way. This crucible was taken out, and melted mix was cast in a preheated mold to produce glass. This glass was then slowly cooled in another oven, through an annealing process of 12h from 550°C to room temperature. Glass was then cut and polished to produce transparent (pale yellow) samples.

For the writing, the beam of the femtosecond laser was focused in a point spatially selected within the bulk of the glass, and further scanning movements along vector k may be obtained through software control of the motorized laser tray.

Following laser settings were used for obtaining precipitation and growth of crystals. Laser frequency: v=300kHz, laser wavelength : λ = 1030nm, numerical aperture of the focusing lens: NA = 0.6, pulse duration : T = 300fs, focalization depth : h = 400μη"ΐ, scanning speed : V = 5Mm/s, pulse energy: E = 2.1 J.

FIGURE 6a and FIGURE 6b show written crystals from example 1, through scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) images. FIGURE 6a image shows a lenticular trace of the laser, with maximal width and length of respectively lOprn and 30pm. FIGURE 6b shows the distribution of crystal orientation in the laser irradiated area 620. In the middle region 604 of this area, we can see that the crystal is sensibly parallel to the c(0001) axis, which is often the most important for many properties. Coordinates of orientations are shown in the nearby triangle, where different orientations are figured by different shades (originally in colors).

Example 3

Here is still another non limitative example of the method according to the invention, implemented with the same matrix as in example 1.

Following laser settings were used for obtaining precipitation and growth of monocrystals. Laser frequency: v=300kHz, laser wavelength : λ = 1030nm, numerical aperture of the focalizing lens: NA = 0.6, pulse duration : τ = 300fs, focalization depth : h = 400pm, scanning speed : V = 5pm/s, pulse energy: E = 1.5pJ instead of 2.1pJ for the previous example.

FIGURE 7a and FIGURE 7b show crystals from example 2, through scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) images. FIGURE 7a image shows a lenticular trace of the laser under scanning, with maximal width and length of respectively 9pm and 18pm. FIGURE 7b shows the distribution of crystal orientation in the laser irradiated area 720. An homogeneous region 704 can be seen in the middle of this area, in which the crystal orientation is parallel to a direction D704 situated between the c (0001) axis and the 1100 direction.

These figures show that most of the interaction volume is made of oriented crystal of size of a few μηη, which was grown from a precipitated crystalline nucleus.

FIGURE 9a and FIGURE 9b are two second harmonic generation (SHG) images, which demonstrate the monocrystalline properties and the unicity of orientation of the polar axis direction of the written trace. These two images are taken under different polarizations 17px and 17py, enlightened at 1.030 nm and read at 515 nm.

Image FIGURE 9b show a far more visible pattern than FIGURE 9a, thus showing that polar axis orientation of the crystal is actually parallel to the polarization 17py, and thus parallel to the writing direction k .

Of course, the invention is not restricted to the above described examples and embodiments. Numerous variations may be applied inside the scope of the invention.

Claims

1. Method for preparing composite materials (3, 4, 5) containing nano or microcrystals with controlled local optical properties, from transparent matrices,

wherein said method comprises writing a transparent matrix (10, 30, 40, 50) including Si0₂ and Li₂0 and Nb₂0₅, by producing a precipitate of LiNb0₃ in at least one volumic unitary region (102), so-called target region, selectively determined within the bulk material of said transparent matrix, through selective irradiation by at least one fast pulsed laser, such as a femtosecond laser (11), focused (112) onto said unitary target region.

2. Method according to claim 1, further comprising producing at least one given continuous region, so-called multipoint region (220, 321 to 327, 421a to 427), within the bulk material of the transparent matrix (10, 30, 40, 50), through implementing

- a first initial step of irradiating a first initial target region (102a) during 1 s to 10 s; followed by

- a plurality of further irradiating steps focused on a plurality of further successive adjacent or intersecting unitary target regions (102b to 102j) within said multipoint region, such as through scanning said continuous region with the focalized pulsed laser (11).

3. Method according to 2, wherein the successive steps of irradiating steps are implemented in a spatial and temporal succession determined so as to obtain a monocrystalline region in the given continuous multipoint region (220, 321 to 327, 421a to 427).

4. Method according to 3, wherein the successive adjacent unitary target regions are produced along a trajectory (221) which is determined so as to be parallel to a given crystalline internal orientation, thus obtaining the c-axis (104a to 104j) to be oriented along to said trajectory (221).

5. Method according to anyone of the preceding claims, wherein the transparent matrix (10, 30, 40, 50) comprises the following components molar fractions proportions:

Si0₂: less than 34%

6. Method according to the preceding claim, wherein the transparent matrix preparation comprises the following steps:

Preparation of the transparent matrix comprises the following steps:

- casting the matrix in a preheated mold at 400-600°C;

7. Method according to anyone of the preceding claims, wherein the femtosecond laser (11) operates at a wavelength between 0,4pm to 1,2pm, and preferentially around or at 1,03pm.

8. Method according to anyone of the preceding claims, wherein the femtosecond laser (11) is focused with a numerical aperture from 0,1 to 1; preferentially from 0,4 to 0,8.

9. Method according to anyone of the preceding claims, wherein the femtosecond laser (11) is operated with a pulse duration between 30fs and 2000fs; preferentially between 240fs and 300fs or even lOOOfs.

10. Method according to anyone of the preceding claims, wherein the femtosecond laser (11) is operated with a pulse energy between 0,2 and 5pJ/pulse; preferentially between 0,5 and 2,5pJ/pulse.

11. Method according to anyone of claims 2 to 10, wherein the femtosecond laser (11) focus point (114) is controlled so as to obtain, for the successive unitary target regions (102a to 102j) within a multipoint region (220) so-called continuous written region, a pulse repetition rate of 200 kHz to 80 MHz and a scanning speed so-called writing speed of 0,21 mm/s to 1,0 mm/s.

12. Method for producing a photonic component, comprising at least one step of writing a plurality of multipoint regions through a method according to anyone of claims 1 to 11 within at least one transparent matrix (30, 40, 50), in a pattern (32, 42, 51 to 54) designed to produce at least one polarizing grating or array.

13. Method according to the preceding claim, further comprising writing within the at least one transparent matrix (50) at least two polarizing gratings or arrays (51 to 54) having different photonic and/or geometry characteristics.

14. Method according to anyone of claims 12 to 13, wherein at least one written polarizing grating or array (32, 42, 51 to 54) is written with a period lower than one micrometer; and/or is entirely written within an area of less than 10 or even 5pm²; and/or is written in a transparent matrix (30, 40, 50) the dimensions of which range from 5 x 5 mm to 50 x 30 mm.