EP1952492A2

EP1952492A2 - Electrically pumped nd3+ doped solid lasers

Info

Publication number: EP1952492A2
Application number: EP06831149A
Authority: EP
Inventors: Fabrice Gourbilleau; David Breard; Richard Rizk; Jean-Louis Doualan
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2005-11-21
Filing date: 2006-11-21
Publication date: 2008-08-06
Also published as: WO2007057580A8; JP2009516911A; WO2007057580A3; US20100034232A1; WO2007057580A2

Abstract

The invention relates to the field of solid lasers and, more specifically, to laser cavities doped with rare earth ions. The invention also relates to a laser amplification structure, a laser, an associated production method, and a use of said structure. The amplification structure can be electrically excited and comprises an active medium and at least two electrodes arranged either side of said active medium. The active medium comprising a first layer of a silicon oxide is co-doped with silicon nanograins and rare earth ions.

Description

LASER SOLIDS DOPES ND ³⁺ PUMPS ELECTRICALLY

The invention relates to the field of solid lasers and relates more particularly to laser cavities doped with rare earth ions.

The laser market is dominated by laser diodes and lasers based on rare earth or transition metal-based insulating materials (crystals and glasses). Laser diodes mainly consist of a semiconductor diode to produce a light beam. The pumping is done using an electric current that enriches the generator medium in holes on one side and electrons on the other. Light is produced at the junction by the recombination of holes and electrons. This type of laser does not have cavity mirrors. In laser diodes, the population inversion necessary to obtain the laser effect is made possible by means of electrical excitation improving the compactness of such systems. However, these are systems different from those of solid lasers in terms of operation or structures, and therefore the physical problems that arise.

Solid lasers use solid media, such as crystals or glasses as the photon emission medium. The crystal or the glass is only the matrix and is doped at least by an ion which is the laser medium (active medium: medium where the laser effects take place, that is to say the phenomena of excitation and de-excitation of doping elements for light emission). Among the solid lasers, the best known and one of the most common is the YAG laser: Nd ³⁺ . Insulating materials (crystals and glasses) doped with active ions require optical pumping by lamp or diode laser, thus limiting their integration. On the other hand, they offer different and complementary characteristics of laser diodes (short and very short pulses). energy, high brightness, spectral tunability, high light emission power, ...).

The active media are also used in neighboring fields such as doped fibers, waveguide amplifiers,

The path of rare-earth doped fibers has a strong development with the realization of very high power laser. The fiber allows a great flexibility of use, but does not go in the direction of the integration of the lasers.

For integrated systems in the form of guiding thin layers doped with active ions, the effort is mainly on the Er doped telecom amplifiers Er ³⁺ and Tm ³⁺ around 1, 5 microns. Numerous works also exist for the production of laser thin guiding layers, for which high power can be obtained. Such systems are described, by way of example, in patent applications WO-065093 and US-2005/0195472. In this first application, a silicon oxide waveguide codoped with Si silicon nanoclusters (nanograins) and rare earth atoms absorbs visible light but not infrared emissions. Based on this principle, a pumping optical source is disposed above the waveguide. The pumping light injected into the waveguide excites the silicon nanograins (by electron-hole combinations), which in turn excite the Rare Earth elements. Such energy transfer between the silicon nanograins and the rare earth elements has been highlighted in the publication "The Nd-nanocluster coupling strength and its effect in excitation / de-excitation of Nd ³⁺ luminescence in Nd-doped silicon -rich silicon oxide "(Seo et al., Applied Physics Letters, Volume 83, Number 14, October 6, 2003) and increased efficiency (factor of several tens or even a hundred) is obtained. An input optical signal is then amplified in the waveguide using the energy generated by the Rare Earth elements and output as an amplified output optical signal. This system, like all systems incorporating silicon nanograins, requires the combination of a laser diode for optical pumping and a doped active medium, which is a limit to integration. A first object of the invention is to promote the integration of solid lasers in more complex systems.

Another object of the invention is to provide solid lasers as compact as possible.

The presence of laser diodes for optical pumping also implies a loss of energy due to the rate of return thereof.

Another object of the invention is to reduce the energy consumption of solid lasers and thus to increase the efficiency rate of these systems. Laser diodes, although inexpensive, are still an economic expense in the manufacture of the solid laser system.

Another object of the invention is to reduce the manufacturing costs of solid lasers without complicating their manufacture.

In the above-mentioned second application, an optical amplifier consists of an Erbium-doped silicon-based waveguide, which waveguide is supplied with pumping energy of either optical type or electrical type by two electrodes. arranged on the opposite walls of the waveguide, or by Raman effect. In the electrically pumped embodiment, the Erbium ions are excited by the electric field applied between the two electrodes. However, such direct excitation does not work in practice, because the Erbium ions are only weakly sensitive to the electric field, the only possibility for a direct excitation of the active Erbium ions being an absorption of photons by this ion (therefore contrary to the electrical excitation), and the system is then strongly energy consuming to obtain a sufficient amount of excited ions. Another object of the invention is therefore to provide a laser electric pumping solution that allows easier excitation of doping ions, including a less energy-consuming solution and improved rate of return. It should also be noted that the operation of these neighboring systems (waveguide amplifier, laser fiber, etc.) depends to a large extent on the input light signal, which is amplified. In contrast, the solid lasers to which the invention is directed do not require the use of input light signals to emit the output signal.

One of the above aims is achieved by the use of silicon nanograins in the active medium, which nanograins are sensitive to an electric field passing through the active medium. The invention thus relies on the electrical excitation of active ions via a semiconductor unlike the known prior art.

For this purpose, the invention firstly relates to a laser amplification structure comprising an active medium and at least two electrodes disposed on either side of said active medium, said active medium comprising a first layer of a silicon oxide co-doped with silicon nanograins and rare earth ions (hereinafter also referred to as co-doped layer). In practice, said electrodes are connected to a power supply so that, in use, an electric current flows in said first layer to excite said silicon nanograins. This laser amplification structure is electrically excitable.

It has been found that silicon nanograins are sensitive to the electric current flowing in the active medium. Thus, the silicon nanograins are excited by the electric current and then transfer their excitation energy to the rare earth ions as explained in the publication Seo et al. supra. The traditional mechanisms of laser stimulated emission then apply for the emission of electromagnetic radiation by de-excitation of the rare earth ions. We note here that silicon nanograins operate as electrical dopants able to transmit their energy to other dopants.

Such a structure thus makes it possible to dispense with a secondary light source (pump laser diode).

An application of these structures is the photonics for which they are generally integrated into semiconductor circuits.

For this purpose, it is provided that said first layer is a thin layer in the sense of the semiconductors, that is to say a layer of thickness less than or substantially equal to one micrometer (1 micron) and generally deposited on a substrate of much greater thickness to benefit from the mechanical properties of the latter. As a result, the structure obtained offers a large compactness that allows easy integration into equipment or integrated circuits. In one embodiment, the active medium consists only of this co-doped thin layer and therefore has a thickness substantially of one micrometer.

The dimensions of the nanograins with respect to the thickness of a silicon oxide layer make it difficult to form a homogeneously codoped layer.

To answer this problem, it is planned to use multilayer structures alternating codoped thin layers and thin films undoped rare earth ions (and possibly nanograins).

For this purpose, it is provided that said active medium further comprises a thin layer of undoped silicon oxide in rare earth ions on which said first layer is deposited. It is also expected that said active medium further comprises a second layer of a silicon oxide co-doped silicon oxide and rare earth ions on which said undoped silicon oxide layer is deposited. In this configuration, it is imperative that the undoped layer is thin because, constituting an insulating layer, it must allow the tunneling effect necessary for the flow of electric current between two codoped layers. By way of example, a thickness of less than or equal to 5 μm for the undoped layer is recommended.

The invention is not limited to an active medium composed of two codoped layers and an undoped layer, but also envisages active media having a large number of codoped layers separated in pairs by an undoped layer. Multilayer structures comprising several tens or even hundreds of layers and in exceptional cases several thousand layers are envisaged. A multilayer active medium is preferably chosen, the two extreme layers (upper and lower) of which are codoped nanograins of Si and rare earth ions. The active medium thus formed includes all of the co-doped layers (thus capable of generating a light wave).

In a multilayer embodiment, the electrodes are positioned parallel to the layers, each electrode covering all or part of the surface of the upper or lower layers (extreme layers) of the active medium. The lower electrode can be either directly in contact with the active medium, that is to say between the latter and the growth substrate, or be positioned on the back of the substrate, the latter allowing electrical conduction.

Active media commonly used are a few millimeters thick. The size of the samples can be reduced to a few hundred microns either to improve the heat dissipation (thin disk) or to increase the compactness (microchip). For microchip devices, the Nd ³⁺ ion with its larger absorption and emission cross sections is very well suited.

Still in the design of a facilitated integration of the structure, the active layer of the structure has a thickness of the order of a few microns, and preferably around the micron according to the current injection characteristics and the indices for the light guidance in the layer. In particular, when a single co-doped thin layer is used in the active medium, a thickness of the order of 1 μm or less is chosen. We recall that the layer active here is the active medium consisting of the multilayer assembly (codoped layers, undoped layers).

In order to optimize the transfer of energy between the silicon nanograins and the rare earth ions, the invention provides that the co-doped layer has a nanocrystalline or amorphous structure, the average distance separating a rare earth ion from a nanograin Si in said active medium being less than or equal to 0.4 nm.

The amplification structure is implemented in lasers as part of the resonant laser cavity. For this purpose, it is provided with reflective surfaces, for example mirrors, preferably said active medium comprises Bragg gratings disposed substantially perpendicular to said electrodes, said gratings consisting of Ge germanium ions photo-inscribed in said layer (s). (s) co-doped (s). To increase the efficiency of the system, all the co-doped layers of the active medium comprise Bragg gratings, preferably aligned from one layer to another. These networks optically close the resonant cavity by acting as mirrors for the electromagnetic emissions of Earth's rare excited dopant ions. It is preferable to place them substantially vertically above the end of the electrodes, and possibly beyond (towards the edges, at the ends of the guiding co-doped layers) when the electrodes cover only part of the surface. active medium. In practice, there are two parallel Bragg gratings that face each other on each side of the electrodes and define the laser resonant cavity.

Preferably, one of the networks is semi-reflective. In the embodiment comprising only a single co-doped layer, the two networks are arranged in this same layer, perpendicular to the upper and lower surfaces (in the thickness) of said layer. Furthermore, one of said electrodes is gold (Au) allowing a significant electrical conductivity by minimizing energy losses. The other electrode may be chosen from Ni-Cr alloy.

In one embodiment, the electrodes each comprise a conductive layer, for example of ITO (Indium-Tin-Oxide), adjacent to one of the opposite faces of the active medium, respectively.

According to various embodiments, the rare earth ions are chosen from Nd ³⁺ , Yb ³⁺ , Er ³⁺ , Tm ³⁺ and Ho ³⁺ ions.

In particular, the Nd ³⁺ has quickly established itself as the main active ion in laser materials. The success of the YAG: Nd ³⁺ has made the transition at 1, 064 μm a standard in terms of laser emission for applications where the wavelength is not the selection criterion. Among the other rare earth ions I ¹ Yb ³⁺ must be mentioned, whose emission is in the same wavelength range. Optical pumping of the Nd ³⁺ ion has long been achieved by lamps with high pumping efficiency because this ion has many energy levels above the emitting laser level which absorb much of the light emitted by lamps. The development, in the 1980s, of AIGaAs type diodes emitting around 800 nm, leads to the gradual replacement of Nd ³⁺ laser with lamp pumping by lasers with laser diode pumping. The Nd ³⁺ ion mainly used for its emission around 1, 06 μm is of the four-level laser system type in which the population inversion is very easy to obtain because the laser terminal level ( ⁴ ln / ₂ ) has a life time several orders of magnitude smaller than the laser emitter level ( ⁴ F3 / ₂ ), while most other ions (Yb ³⁺ , Er ³⁺ ...) are almost three-level systems. A preferred embodiment of the invention is therefore envisaged with Nd ³⁺ ions. For these "other" ions, the laser terminal level is thermally populated because close to the fundamental level of the ion which requires a greater pumping to achieve the population inversion necessary to reach the laser threshold. The advantage provided by the use of the doping ion Nd ³⁺ is thus that of reducing energy consumption to achieve the desired pumping. In addition, the length of the active medium must be optimized to avoid reabsorption of the laser emission. The stimulated absorption and emission cross sections play a very important role. For the Nd ³⁺ ion, these absorption and emission cross sections are much larger than for the other ions, in particular Yb ³⁺ , for a given matrix.

The invention also relates to an optical laser comprising a resonant laser cavity provided with an amplification structure and an electric current generator connected to said electrodes.

Specifically, the generator is arranged to circulate, in use, an electric current in said active medium to excite said silicon nanograins, in particular in said first layer. According to the mechanisms mentioned above, the rare earth ions are in turn excited by energy transfer from nanograins of

If to them.

The lasers thus produced are as compact as the laser diodes while providing additional performance such as high energy pulses, purity and spectral stability independent of temperature or manufacturing conditions.

The subject of the invention is also a method for manufacturing a laser amplification structure, comprising: a step of deposition of an active medium comprising the deposition of a layer of silicon oxide co-doped with nanograins of silicon and rare earth ions on a substrate, and - a step of depositing electrodes, on either side of the active medium, optionally on at least a portion of said co-doped layer. There are several alternatives for electrode deposition.

In a first alternative, it is envisaged to deposit a metal electrode on the active layer side (here the co-doped layer, but possibly the multilayer assembly mentioned in several places in the present description) opposite to the substrate and another metal electrode on the back of the latter (substrate), if it does not constitute an insurmountable barrier to the electrical conduction in the active medium.

In a second alternative, it is envisaged to deposit on the two opposite faces of the active layer a transparent conductive layer, type ITO (Indium-tin-oxide). In particular, it is possible to deposit an ITO layer on the substrate before depositing the active layers. Metal contacts can then be deposited on these transparent conductive layers to allow connection to a current generator. The assembly consisting of the transparent conductive layer and the contact terminals forms an electrode used for the electrical excitation of the co-doped active medium.

It is thus provided that an electrode is deposited prior to the deposition step of the active medium by the deposition of a conductive layer on said substrate, and in that a second electrode is deposited after the deposition step. active medium on the opposite side of the active medium.

In one embodiment, the deposition step is performed by reactive magnetron co-sputtering of at least one target comprising a first material of a silicon oxide and a second material of

Rare earth. Optionally, said second material is disposed on a portion of said target. Otherwise, multiple targets each provided with only one type of material are expected to be used in a confocal co-spray process.

During this deposition step, the substrate acts as anode and the cathode target. By this mechanism, a plasma is created between anode and cathode which allows the detachment of elements of silicon, oxides and rare earth elements coming to condense on the substrate. The reactive magnetron co-sputtering methods are very well suited to the formation of thin layers. They thus make it possible to manufacture quasi-planar structures, thus well adapted to semiconductors and to the manufacture of planar lasers electrically excited. According to two variants, said at least one target is a single silicon oxide target surmounted by a plurality of rare earth oxide wafers (variant 1), and said at least one target comprises a silicon Si target, a target of said first silicon oxide material SiO ₂ and a target of said second rare earth material (variant 2).

The second variant has the advantage of implementing a normal co-sputtering of three parallel cathodes, limiting the interactions between the different elements of the target in the global sense (which are more difficult to control).

The nature of the plasma used during the co-sputtering step is not indifferent to the composition of the co-doped layer formed. Thus, it is expected that said co-sputtering step is carried out in a vacuum chamber comprising an argon plasma and / or hydrogen and / or nitrogen. In the case of a reactive sputtering, the presence of hydrogen makes it possible to reduce the silicon oxide to incorporate an excess of silicon (the silicon nanograins) in the SiO ₂ matrix also deposited. Rare Earth ions are introduced via the plasma into the Si-SiO 2 composite. The level of hydrogen in the plasma therefore constitutes a parameter with regard to the concentrations of rare earth ions and silicon nanoclusters. For this purpose, it is expected that the level of hydrogen in the plasma is between 10 and 90%. This rate favors according to the conditions of deposition (plasma pressure, substrate temperature, interelectrode distance ... etc ..) the formation of Si germs in greater or lesser numbers, and therefore consequently the density of nanoclusters within of the deposited layer. In general, a plasma containing hydrogen is used in the absence of a pure silicon target; which is the case in the hypothesis of a SiO ₂ target surmounted by rare earth oxide platelets or in the hypothesis of two targets SiO ₂ and Nd ₂ θ ₃ . In the latter case, an Ar + H ₂ mixture is used to incorporate silicon Si in the co-doped growth layer (for a single layer), or using an Ar + H ₂ mixture to incorporate silicon Si in the co-doped layer enriched in nanoclusters and a pure argon plasma for the undoped growing silica layer in rare earth ions.

On the contrary, if three Si, SiO2 and Nd2θ3 targets are available, the presence of hydrogen is not essential since the Si target provides the Si nanoclusters without the need for reduction of SiO ₂ silica. Another parameter to take into account is that of the surface of the target occupied by said rare earth material, which is preferably between 3 and 30% of the total surface of the target, a function of the rare earth ion in question. . This parameter makes it possible to vary the proportion of rare earth ions in the final deposit with respect to the excess of silicon (nanograins). For example, for the case of the Erbium ion, the optimal surface is between 23 and 26% whereas in the case of Nd ³⁺ ions, it should not exceed 12%.

In order to obtain Bragg gratings as mentioned above, it is provided that during said co-sputtering step said target is also surmounted by at least one wafer comprising germanium (Ge). Still by reactive magnetron co-sputtering, germanium ions are torn off the wafer and deposited in the layer being deposited. The Germanium ions thus deposited are registered to form said networks. The photo inscription is done using a UV laser. An interference pattern is projected onto the portion of the guide that contains the Ge inducing irradiated areas and non-irradiated areas thus forming the network. Irradiation induces the formation of colored centers that modify the index of the medium. There is thus an alternation of zones with different indices which plays the role of a mirror.

For this purpose, it can be planned to place the germanium wafer only at favorable times for the formation of Bragg gratings, that is to say only during the deposition of the co-doped layers. (since there is no light emission in the undoped layers devoid of rare earth ions).

According to an alternative, it is expected to deposit Ge everywhere in the co-doped layer in growth. Ge ions are inactive from the point of view of the rare earth ion (eg Nd ³⁺ ). Subsequently, the network is entered only in the desired areas (ends of the layers).

Moreover, the molecular defects that can be created during the deposition of the layers must be repaired or attenuated. To do this, the method further comprises a step of annealing said layer thus formed at a temperature of between 800 and 1100 ^° C. for at least ten minutes.

According to one embodiment, the method comprises a step of depositing an N doped polycrystalline silicon layer on said doped layer prior to deposition of said electrodes.

In order to achieve the multilayer active media mentioned above, the method may comprise the succession;

depositing a co-doped layer, and

a subsequent deposition step to form an undoped silicon oxide layer on said co-doped layer.

It is expected that the deposition of a co-doped layer is a step of reactive magnetron co-sputtering of at least one target comprising a first material of a silicon oxide and a second material of

Rare Earth, said second material being disposed on a portion of said target, and said subsequent deposition step is a reactive magnetron sputtering step of a silicon oxide target in a vacuum chamber comprising an argon plasma to form an undoped silicon oxide layer on said co-doped layer.

Separate cathodes are chosen for each of the co-spray and spray steps because the reaction parameters are distinct. More specifically, the method comprises a plurality of alternating co-spray and spray step steps to form a multilayer structure, the first and last steps performed being preferably co-spray steps for the deposition. co-doped layers, the electrode being deposited on the top of the layer opposite to the substrate in the structure (the other being on the back of the substrate).

According to two alternatives, during said spraying step, the argon plasma is a pure argon plasma, making it possible to obtain an undoped layer having no Si nanograins, said plasma being enriched in hydrogen, so as to incorporate an excess of Si in the undoped insulating layer. This makes it possible to facilitate electrical conduction in the insulating thin layer and to satisfy the tunneling effect.

In addition, the rare earth ions are of at least one kind chosen from Nd ³⁺ , Yb ³⁺ , Er ³⁺ , Tm ³⁺ and Ho ³⁺ ions.

Furthermore, it is also envisaged to proceed to the manufacture of a ribbon guide to allow lateral guidance of the light. The ribbons can be made by etching once the layers have been deposited. We start from the multilayer device on which we engrave a ribbon guide from a few nm to a few tens of nm using a mask. A suitable technique may be plasma assisted etching (RIE).

The invention also relates to a method of operating a laser comprising a laser cavity provided with a laser amplification structure and an electric current generator, said laser amplification structure comprising an active medium and at least two electrodes disposed on either side of said active medium, said active medium comprising a first layer of a silicon oxide is co-doped in silicon nanograins and in rare earth ions, and said electric current generator connected to said electrodes, said method comprising a step of circulating an electric current in said active medium by the application of a power supply to the terminals said electrodes.

The invention also relates to a use of a laser amplification structure comprising a layer of a silicon oxide co-doped silicon oxide and rare earth ions, electrically pumped as a laser amplifier. This use can also be applied to a laser or a laser fiber.

The invention will be better understood with the aid of the detailed description below and the appended figures in which: FIG. 1 schematically represents a target cathode and the substrate-anode within the deposition frame in the configuration of co-doped thin film deposits;

FIG. 2 is a graph illustrating the absorption spectrum of the Nd ³⁺ ion in a SiO ₂ -Nd film deposited by magnetron sputtering; FIG. 3 is a graph illustrating the influence of the hydrogen level in the plasma on the emission intensity of the Nd ³⁺ ion in films annealed at 900 ^° C.

FIG. 4 is a graph illustrating the PL intensity of a film made with 80% H2 in plasma and annealed under different conditions;

FIG. 5 is a schematic representation of the two target cathodes and the substrate-anode within the deposition frame for a multilayer deposition according to the invention; and

FIG. 6 and 7 illustrate multilayer embodiments implemented by the present invention.

The example described below makes it possible to highlight the mechanism and the optimization of efficient energy transfer between the Si nanograins and the Nd3 + ion, for thin layers manufactured by reactive magnetron sputtering, thereby enabling the manufacture of an electrically excited planar laser.

The confinement of the carriers (excitons) in silicon nanograins inserted in a silica matrix makes it possible to obtain visible luminescence by electrical or optical excitation of such a composite system. Many studies concern the study of silicon nanograins associated with the Er ³⁺ ion to produce amplifiers in the wavelength range favorable to telecommunications (1. 54 .mu.m). Effective energy transfer exists between the nanograins and the rare earth, which can emit with an intensity 100 times greater than that which prevails without the presence of nanograins.

A silica matrix containing silicon nanograins associated with the Nd ³⁺ ion is used for the realization of integrated laser. Indeed, it has recently been demonstrated that there is a transfer between Si nanoclusters and the Nd ³⁺ ion in layers made by ECR-assisted electrochemical cyclotron resonance (PECVD). ) [Seo et al., Appl. Phys. Lett. , 83, 2778 (2003); "Nd-nanocluster coupling strength and effect in excitation / de-excitation of Nd ³⁺ luminescence in Nd-doped silicon-rich silicon oxide."]. The invention makes it possible to electrically excite the silicon nanograin, which transfers its energy to the Nd ³⁺ ion, thereby creating a population inversion between the two laser levels of this ion. A laser emission around 1.1 μm, depending on the nature of the matrix (material and dopants), is thus obtained.

The aim is to produce reactive magnetron co-sputtering thin layers of neodymium co-doped silica glass and Si nanograins. These guiding layers will lead after etching and deposition of the electrodes to an electrically excited laser system, which can emit around 1, 1 μm. This approach is compatible with silicon technology and is perfectly consistent with the current trend of manufacturing compact systems easily integrable. The reactive co-spray technique used, inter alia, for deposition of thin films is shown schematically in FIG. 1.

A target of pure silica 10 is surmounted by a variable number of platelets of Nd2U3 (rare earth oxide) 1 1 so as to modulate the concentration of Nd ³⁺ ions incorporated in the deposited layer, as well as pieces of Ge or Geθ ₂ 12 to increase the photosensitivity of silica by reducing its gap [see Nishii et al. , Optics Lett., Vol. 20, issue 10, 1,184 (1995); "Ultraviolet-radiation induced chemical reactions through one-and-two-photon absorption processes in Geθ2-SiO ₂ glasses"]. Thus, Bragg gratings, playing the role of future mirrors of the laser cavity, are photoinscribed at both ends of the Nd ³⁺ doped silica guide and containing Si nanograins. The silicon excess incorporated in the layer is obtained through the reactive nature of the plasma which consists of a mixture of argon and hydrogen ionized sputtering and interact with the Siθ2-Nd ₂ θ3-Ge. Given the reducing nature of hydrogen with respect to oxygen, such a process leads to a deficit of oxygen in the layer and thus makes it possible to control the amount of excess silicon by varying the partial pressure of hydrogen. In addition to this parameter, the use of a reactive gas such as hydrogen leads to a phenomenon of selective etching of the growing layer thus favoring the multiplicity of nucleation sites of Si nanograins. The highest density of nanograins which are then ensured a higher coupling rate with Nd ³⁺ ions and thus a high proportion of optically active ions. Deposits take place at room temperature under a power ranging between 5OW and 120W and a total gas pressure not exceeding 6x10 ^'2 Torr. In this configuration, the ionization of the gases makes it possible to tear off elements of the target and deposit them on the substrate (anode) 13.

The post-deposit heat treatments are carried out between 800 and 1100 ^° C., as well as with time, under a stream of pure Ar or N ₂ . An absorption spectrum of the Nd ³⁺ ion from a SiO ₂ -Nd film (without the presence of excess silicon) made by spraying magnetron, is shown in FIG. 2. This spectrum emphasizes the presence of the strong absorption (20) of the Nd ³⁺ ion at 800 nm, which makes it possible to envisage the energy transfer between excited nanograins of Si and the Nd ³⁺ ion. In addition, it can be seen that the 488 nm line provided by an Argon laser is a very weakly resonant line for the Nd ³⁺ ion, which allows us to use this excitation wavelength to highlight this energy transfer with the rare earth ion.

FIG. 3 shows the evolution of the photoluminescence intensity of the Nd ³⁺ ion as a function of the hydrogen content ΓH = PH2 / (PH ₂ + PAΓ) in the plasma for thin films produced at a RF power of 6OW.

It can be seen that the film deposited with the highest hydrogen content ΓH = PH ₂ / (PH ₂ + P _AΓ ) has the highest intensity (30) with a split emission peak. This is due to the maximum density of Si nanograins playing the role of excitation relay Nd ³⁺ ions, as already mentioned. The sensitizing role played by the nanograin of

If is clearly demonstrated with a sharp increase in the intensity of films containing silicon in comparison with that which was manufactured with a pure plasma (or almost pure) argon (31) and therefore that does not contain (or little) Si in excess.

The effect of the heat treatment applied on the emission of the Nd ³⁺ ion is shown in FIG. 4. The increase in temperature strongly favors one of the emission peaks at the expense of the other while increasing the intensity of an order of magnitude.

The simple linear guide with or without coating (cladding) is obtained after etching and deposition of electrodes to allow the injection of current. These electrodes (Al, Au, Al-Si alloy ...) are deposited all along the guide, as well as on the underside of the substrate. Electrical excitation leads to the formation of an exciton within the Si nanograin which, by recombining, either emits at 800 nm, or transfers its energy to the Nd ³⁺ ion in the vicinity and therefore allows the emission of this rare earth at the desired wavelength. Electrical excitation in such systems can be promoted through multilayer structures. Indeed, to allow an optimal electrical excitation of Si nanograins which are embedded in an insulating matrix (SiO 2), they must have a sufficient density to be separated from each other by a distance less than the "tunnel distance" ( about 2 to 4 nm) for the injection of electrons. In addition, these nanograins must keep a size of less than about 5-6 nm and be surrounded by oxide to maintain the property of quantum confinement carriers that will enable them to play their role of effective sensitizer vis-à-vis the rare earth ion in their close environment, the optimal interaction distance being less than 1 nm.

The reactive magnetron sputtering technique used to deposit these multilayers is shown schematically in FIG. 5. It allows the control of the thicknesses of the different types of layers deposited as well as the location of the rare earth ion to optimize its emission. It is characterized by two aspects, the technical aspect related to the use of two cathodes (50a and 50b) and alternate deposits by sequential rotation of the substrate carrier (anode 51) and the reactive aspect related to the presence of hydrogen in the plasma and thus interacting with the silica target. The two targets are silica, one of which is surmounted by a variable number of rare earth oxide platelets (52) so as to modulate the concentration of rare earth ions incorporated.

The deposit sequence is as follows:

- when the substrate (or the anode) is facing the target surmounted by pieces of rare earth oxide (POSITION B), the deposition takes place under argon plasma mixed with hydrogen (~ 1: 1 ) (53). Given the reducing power of hydrogen towards oxygen from the silica target, the deposited layer then contains an excess of silicon next to the rare earth ions incorporated by the co-sputtering of the platelets. oxide. the deposition of the SiO ₂ layer is done by placing the substrate facing the pure silica target (POSITION A) under a pure argon plasma (54).

These sequences are repeated as many times as necessary to obtain the structural characteristics and the adequate optical properties of the manufactured film.

FIG. 6 illustrates a multilayer structure in which three co-doped Si nanograins 60 layers and Rare Earth ions are separated by two insulating layers 61 of silicic oxide. Bragg gratings 62 are inscribed in each of the two end layers to form the resonant cavity of a laser. The electrodes 63 are placed on either side of a multilayer active medium and fed by a current generator G which passes a current through the stack of layers. The networks 62a on one side of the structure (left side in the figure) are semi-reflective, those 62b on the other side of the structure are almost perfect mirrors. As a result, the light emission 64 from the laser structure excited by the circulating electric current is performed on the same side (left side in the figure).

FIG. 7 illustrates another structure according to the invention, the specificities related to the electrodes can be combined in whole or in part with the structure of Figure 6. The deposition of the various layers of this structure is achieved by reactive magnetron sputtering methods as described previously.

The structure is deposited in layers on a silicon substrate

(70). It comprises, in order: - a first intermediate layer (buffer) 71 of SiO ₂ silica of about

5 microns thick and medium index substantially equal to 1.45;

a transparent conductive layer of ITO (Indium tin oxide - tin oxide and indium) 72 with a thickness of less than 100 nm and an index approximately equal to 1.9; the active medium 73 comprising a co-doped layer of silicon nanograins and rare earth ions, with a thickness of between 500 nm and 1 μm and an index of between 1.5 and 2. A multilayer structure as described with reference to FIG. FIG. 6 can also be considered;

one (or more) metal terminal 74 on a part of the layer 72 not covered by the active medium 73, preferably on one side of the active medium other than that which serves as a laser output (typically the side provided with a partially reflective Bragg mirror). This terminal 74 may be deposited either by removal of a portion of the active medium 73 from the ITO layer, or when initially the deposition of the active medium 73 has been provided on only a portion of the ITO layer 72, then by the localized deposit of metal;

a second transparent conductive layer of ITO 75 on the active medium 73, having characteristics similar to the first ITO layer 72;

a second SiO 2 silica intermediate layer 76 approximately 200 nm thick and with a medium index substantially equal to 1.45. This layer is deposited on only a portion of the ITO layer 75; one or more metal terminals 76 in direct contact with the second layer of ITO 75.

The metal terminals 74 in contact with the first ITO layer 72 are connected to a terminal of a current generator 78 and the metal terminals 77 in contact with the second layer of ITO 75 are connected to the other terminal. of the current generator 78 so as to pass through a current in the active medium 73 thanks to the conductivity of the ITO layers 72 and 75. In this embodiment, each ITO layer and metal terminal assembly is seen as an electrode of the laser. electrical excitation.

This results in a distribution of photons (resulting from the deexcitation of the rare earth ions at the origin of the laser phenomenon) inside the guide formed by the active medium. This structure has the advantage of being free of the potential barrier induced by the presence of the intermediate layer of silica between the substrate and the active layer.

To ensure the guiding of the light in the active medium (amplification medium), a lower index is used on either side of the guide (layers 71 and 76 of index 1, 45 against 1, minimum 5 for the active layer 73). To reduce the impact of the ITO transparent conductive layers 72 and 75 in guiding and amplifying the light signal and in particular to avoid evanescent wave losses, a thin transparent layer thickness is chosen, of the order of a few 10 nm. . In addition, these layers potentially represent a potential barrier for load carriers to overcome. Thus, the layer is optionally extremely thin, of the order of a few nm to 10 nm.

Among the rare earths likely to benefit from the role of effective sensitizer played by Si nanograins, there are also:

- The Yb ³⁺ ion for an emission around 1 μm - The Er ³⁺ ion for a 1. 54 μm emission that would allow the optical transport and transfer of information, especially in computers.

- Tm ³⁺ ion for emission to 2 μm for telemetry applications, eye safety, in the medical field. - The ion Ho ³⁺ for emission also around 2 microns and therefore for the same fields of application as the ion Tm ³⁺ .

Moreover, different types of matrix can be used:

silicon di-oxide SiO ₂ ; silicon oxides SiO _x ;

silicon oxynitrides SiO _x Ny;

For silicon oxynitride matrices, it is expected that said plasma is enriched in nitrogen, thus making it possible to form a matrix oxynitrided which facilitates the electrical conduction within it in the presence of Si nanograins. The presence of nitrogen may be complementary to that of hydrogen.

Claims

A laser amplification structure comprising an active medium and at least two electrodes disposed on either side of said active medium, said active medium comprising a first layer of a silicon oxide doped with rare earth ions, characterized by the said first layer of a silicon oxide is co-doped with silicon nanograins and rare earth ions.

2. Structure according to claim 1, characterized in that said active medium further comprises a thin layer of undoped silicon oxide rare earth ions on which said first layer is deposited.

3. Structure according to the preceding claim, characterized in that said active medium comprises a plurality of layers of silicon oxide co-doped in silicon nanograins and rare earth ions, said co-doped layers being separated by layers. undoped silicon oxide, the upper and lower layers of said active medium being co-doped layers.

4. Structure according to any one of the preceding claims, characterized in that said co-doped layer has a structure in which the average distance between a rare earth ion of a silicon nanograin in said active medium is less than or equal to 0.4 nm.

5. Structure according to any one of the preceding claims, characterized in that the rare earth ions are of at least one selected from Nd ³⁺ , Yb ³⁺ , Er ³⁺ , Tm ³⁺ and Ho ^{3 +} .

6. Structure according to any one of the preceding claims, characterized in that said active medium comprises Bragg gratings disposed substantially perpendicular to said electrodes, said arrays consisting of Ge germanium ions photo-inscribed in said co-doped layer (s).

7. Structure according to any one of the preceding claims, characterized in that said electrodes each comprise a conductive layer adjacent to one of the opposite faces of the active medium respectively.

8. Structure according to any one of the preceding claims, characterized in that the (said) layer (s) codoped (s) is (are) thin layer (s).

An optical laser comprising a laser cavity provided with an amplification structure according to any one of the preceding claims and an electric current generator connected to said electrodes.

Laser according to claim 1, characterized in that said current generator is arranged to circulate, in use, an electric current in said active medium to excite said silicon nanograins.

A method for manufacturing a laser amplification structure, comprising: a step of deposition of an active medium comprising the deposition of a layer of silicon oxide co-doped with silicon nanograins and terrestrial ions Rare on a substrate, and a step of depositing electrodes, on both sides of the active medium.

12. Method according to the preceding claim, characterized in that an electrode is deposited prior to the deposition step of the active medium by the deposition of a conductive layer on said substrate, and in that a second electrode is deposited after the deposition step of the active medium on the opposite face of the active medium.

13. The method of claim 1 1 or 12, characterized in that said first step is performed by reactive magnetron co-sputtering of at least one target comprising a first material of a silicon oxide and a second material of Rare earth, said second material being disposed on a portion of said target.

14. Method according to the preceding claim, characterized in that said at least one target is a single silicon oxide target surmounted by a plurality of rare earth oxide wafers.

15. Method according to the preceding claim, characterized in that the surface of the target occupied by said material in rare earths is between 3 and 30% of the total surface of said single target.

16. The method of claim 13, characterized in that said at least one target comprises a silicon Si target, a target of said first SiO 2 silicon oxide material and a target of said second rare earth material.

17. Method according to one of claims 13 to 16, characterized in that said co-sputtering step is carried out in a vacuum chamber comprising an ionized argon and hydrogen plasma.

18. Process according to the preceding claim, characterized in that the level of hydrogen in the plasma is between 40 and 90%.

19. Process according to any one of claims 13 to

18, characterized in that during said co-sputtering step, said target is also surmounted by at least one wafer comprising germanium (Ge).

20. Process according to any one of claims 1 1 to

19, characterized in that it further comprises a step of annealing said layer thus formed at a temperature between 800 and 1100 ⁰ C for at least ten minutes.

21. Method according to any one of claims 11 to 20, characterized in that the step of depositing the active medium comprises the succession;

depositing a co-doped layer, and a subsequent deposition step to form an undoped silicon oxide layer on said co-doped layer.

22. Method according to the preceding claim, characterized in that the deposition of a co-doped layer is a reactive magnetron co-sputtering step of at least one target comprising a first material of a silicon oxide and a a second rare earth material, said second material being disposed on a portion of said target, and said subsequent deposition step is a reactive magnetron sputtering step of a silicon oxide target in a vacuum enclosure comprising a plasma of argon to form an undoped silicon oxide layer on said co-doped layer.

23. Method according to the preceding claim, characterized in that it comprises a plurality of alternations of co-doped layer deposition and subsequent deposition steps to form a multilayer structure, the first and last steps of said plurality of co-doped layers and alternations being co-sputtering steps for the deposition of co-doped layers.

24. Method according to one of claims 22 and 23, characterized in that, during said spraying step, the argon plasma is a pure argon plasma.

25. Method according to one of claims 22 and 23, characterized in that, in said spraying step, said plasma is enriched in hydrogen.

26. Method according to any one of claims 1 1 to 25, characterized in that the rare earth ions are of at least one selected from Nd ³⁺ , Yb ³⁺ , Er ³⁺ , Tm ³⁺ and Ho ³⁺ .

27. A method of operating a laser comprising a laser cavity provided with a laser amplification structure and an electric current generator, said laser amplification structure comprising an active medium and at least two electrodes arranged on each side and other said active medium, said active medium comprising a first layer of a silicon oxide is co-doped silicon nanograins and rare earth ions, and said electric current generator connected to said electrodes, said method comprising a step of circulating an electric current in said active medium by the application of a power supply across said electrodes.

28. Use of a laser amplification structure comprising a layer of a silicon oxide co-doped silicon oxide and rare earth ion, electrically pumped as a laser amplifier.