WO2012107691A1

WO2012107691A1 - Photonic crystal device.

Info

Publication number: WO2012107691A1
Application number: PCT/FR2012/050272
Authority: WO
Inventors: Sophie Marie-Claude Roseline PERRIN-FASQUEL; Lionel Stéphane CANIONI
Original assignee: Universite Bordeaux 1
Priority date: 2011-02-09
Filing date: 2012-02-08
Publication date: 2012-08-16
Also published as: FR2971344A1

Abstract

The photonic crystal device comprises, on a substrate (3): a stack of a first layer (5) having a first refractive index; and a second layer (9) having a second refractive index that is higher than the first, said second layer comprising a plurality of wells (7) arranged in a periodic two-dimensional structure, each well comprising a solid material having a third refractive index, the second layer with the plurality of wells forming a 2D Bravais lattice of photonic crystals. In addition, at least one well comprises an electronic device for modulating the refractive index of the well by modulating the density of electric charge in at least one part of the well.

Description

PHOTONIC CRYSTAL DEVICE

The present invention relates to a photonic crystal device.

For the record, a photonic crystal is a periodic artificial structure which, when the period is of the order of the wavelength, has a forbidden band for the photons by analogy to the gap of the electrons in the semiconductors. In the field of integrated optics photonic crystals are used to produce guiding structures having many physical properties such as routing, (de) -multiplexing, superprism effect or negative refraction.

In the field of high integration density nanoelectronics, the miniaturization of the components shows that it appears necessary to integrate optical functionalities and electronic functionalities into one and the same system. But the structures and manufacturing processes of these different functionalities are not always compatible. Thus the electronic components are made in the form of a stack of layers of different materials, the whole being shaped by a set of technological steps of the diffusion type, ion implantation, lithography, etching and metallization. The (nano) photonic structures can be made by MESA type technologies, ie a raised form of trays and valleys forming for example a ribbon guide, or more recently by photonic crystals generally obtained by etching holes according to a two or three-dimensional periodic structure belonging to the bravais networks. As a result, integration is usually in the form of hybrid modules. This solution has the disadvantage of increasing manufacturing costs by multiplying the manufacturing steps. In addition, the manufacture of hybrid modules involve module positioning problems so that the waveguides are aligned in front of the photodedectors, for example.

It would therefore be advantageous to obtain devices in which the optical and electronic technologies would be compatible so as to obtain the integration of all the functions on the same support and, if possible, simultaneous manufacturing.

To solve one or more of the aforementioned drawbacks, a photonic crystal device comprises a stack on a substrate of a first layer having a first optical index and a second layer having a second optical index greater than the first, said second layer comprising a plurality of silos arranged in a two-dimensional periodic structure, each silo comprising a solid material having a third optical index, the second layer with the plurality of silos forming a 2D Bravais network of photonic crystals. The device further comprises at least one silo comprising an electronic device for modulating the optical index of the silo by modulating the density of electrical charges in at least a portion of the silo.

Thus, advantageously, integrated photonic crystal devices can be constructed directly from MOSFET components belonging to CMOS technologies on SOI.

Particular characteristics or embodiments that can be used alone or in combination are: the second layer is furthermore at least partially covered by a layer having a fourth optical index smaller than the second and third, and the structure has a stack of first, second and fourth layers, such that the device creates an optical confinement by total reflection internal in the growth axis of these different layers;

it has a band diagram that may include a band gap for radiation having a wavelength of the same order of magnitude as the period of the structure;

the two-dimensional periodic structure has at least one defect in the form of a different, and / or missing and / or supernumerary silo making it possible to perform routing, filtering or negative refraction functions;

it comprises a plurality of defects in the form of different, missing or additional silos arranged in the form of a band along one of the crystallographic axes so that said band forms a waveguide whose mode is confined vertically in the second layer; the silos form a band perpendicular to a crystallographic axis so as to form a flat lens with a negative refractive index; it has properties of metamaterials for a light wave having a predetermined wavelength injected into the plane or normally at the periodicity plane;

the optical index contrast between the silos and the interstices of the photonic crystal is very large so that the period of the structure is substantially ten times smaller than the length predetermined wave and / or permeability and / or permittivity are negative;

• the materials of the layers and silos are organic materials of polymer type and / or semiconductor materials;

The electronic device is a MOSFET transistor and the optical index is modulated by an electric voltage applied to the gate of the MOSFET transistor and / or between the drain and the source of the MOSFET transistor;

The first layer deposited on a silicon substrate is a silicon oxide layer, the second silicon layer containing silicon oxide silos containing smaller silicon silos so that the combination of each large and small silo forms a SOI MOSFET transistor with silicon oxide isolation housings; the assembly may include defects of one or more transistors with or without isolation oxide; and the silos and the second silicon layer may be at least partially covered by a silicon oxide layer; and or

It further comprises a plurality of stack of second layers or first and second layers so as to form a three-dimensional photonic crystal or a metamaterial.

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

FIGS. 1A, 1B, 1C and 1D are perspective diagrams of an embodiment of photonic crystals according to the invention; FIG. 2 is a sectional view in the layer containing the silos 7 of the embodiment of FIG. 1;

FIG. 3A is a first variant of the embodiment of FIG. 1 and 3B the associated sectional view;

FIG. 4 is a second variant of the embodiment of FIG. 1;

- Figures 5 are sectional views of a variant of the embodiment of Figure 4 having defects which are transistors without isolation of silicon oxide. In A, a straight guide structure formed by a row of defects and the associated band diagram in the direction of the guide. In B, a directional filtering structure composed of two parallel guides separated by a resonant cavity. In C, a flat lens with negative refractive index, the associated diagram shows the focusing of a point source placed to the left of the lens; and

- Figure 6 is a graph showing optical measurements. It shows the evolution of the silicon index under the gate modulated by a drain-source voltage in a MOSFET transistor.

With reference to FIG. 1A, a device 1 for photonic crystals comprises a stack on a support 3 of a first layer 5 having a first optical refractive index, abbreviated hereinafter as an optical index.

The support 3 is typically a monocrystalline silicon substrate, the layer 5 is silicon oxide. On this slice, a selective etching process makes it possible to obtain, FIG. 1B, a plurality of silicon oxide silos 7 arranged in a two-dimensional periodic structure, FIG. 1C. The interstice 9 between the silos is filled, for example by a deposition process, with a material having a second optical index. The interstice 9 between the silos typically corresponds to silicon. As is known, the optical index of silicon of layers 3 and 9 is equal to 3.45 and is greater than the optical index of the silicon oxide of layer 5 and silos 7 equal to about 1.45 . The entire structure could still be covered by a layer of silicon oxide, particularly in a particular case of a network of silos and interstices having an index greater than silicon oxide.

Figure 1D is a three-dimensional view of the photonic crystal obtained with a cubic mesh.

Although Figures 1B, C and D represent parallelepipedal silos, these can have various shapes. In particular, the physical phenomena of etching and doping round the angles to a greater or lesser extent.

The plurality of silos 7 inserted in a silicon layer 9 forms a photonic crystal which belongs to 2D Bravais networks. For example, the symmetry chosen in FIG. 2 corresponds to a hexagonal (or triangular) mesh symbolized by the KM Brillouin zone. The distance between the center of two silos along one of these axes defines the periodicity noted a of the photonic crystal on this axis.

By way of example, for a photonic structure operating at 1.55 m.sub.2, the support 3 has a thickness of 375.sub.m, the layer 5 has a thickness of 375 nm and the second layer 9 with its silos 7 has a thickness of 150 to 200 nm. These thicknesses are based on a technology of CMOS-130 nm SOI transistor with silicon oxide isolation boxes. This configuration makes it possible to build a photonic crystal operating around 1.55 μm directly from the basic technological bricks of an integrated circuit.

It should be noted that this type of integrated photonic crystal device offers the possibility of working with technologies of very different sizes. Indeed, to work at a given wavelength, relying on a well-chosen technology, it suffices to adapt the opto-geometric parameters of the photonic crystal as the period.

By way of example, the following description details this embodiment of photonic crystal for integration on a circuit in SOI CMOS technology.

In a first variant, the device 1 is completed, FIG. 3A and 3B, by the production of a silicon silo 11 inserted in the silicon oxide silo 7. This structure makes it possible to produce a photonic crystal from a network of SOI MOSFET transistors with isolation boxes using developed and widely developed technology. In this first variant, the photonic crystal is obtained by a network of transistors without the realization of the gate, drain and source contacts. The photonic crystal is constructed solely from the different layers of silicon and S1O ₂ constituting a SOI MOSFET transistor. This type of photonic crystal structure directly integrable on SOI MOSFET technology has the advantage of controlling light in all directions of space. In the stacking direction of the different layers, the light is confined by total internal reflection, in the plane of the photonic crystal the light is controlled by band gap effect for example. In a second variant, FIG. 4, a silicon oxide layer 13 at least partially covers the silos 11. These silos 11 comprise complete MOSFET transistors comprising a gate 15, a drain 17 and a source 19 with the necessary dopings and metal contacts. These are, for example, partial or total depletion transistors. The silicon oxide of the second layer 7 corresponds to an isolation box for these transistors. As is explained below, the addition of the polarization elements of the transistor introduces a very original concept based on the modulation of the photonic crystal by a chosen number of repeating patterns, allowing many optical modulation architectures.

Thus, the manufacture of the photonic crystal uses conventional techniques of microelectronics (deposition, etching, lithography, etc.) and is therefore fully compatible with the manufacture of SOI MOS circuit for performing the conventional electronic functions. The realization of optical functions can be obtained on the one hand by using the crystal without defect in forbidden band regime for example. On the other hand by the insertion of defects in the structure of the photonic crystal can trap the photons so as to achieve confinement structures. By default, one understands the addition or the suppression, without necessarily respecting the periodicity, of silos of any type in the structure by analogy with a defect in a crystal linked to the lack of an atom or, on the contrary, to the presence of 'a supernumerary atom.

In a first example, FIG. 5A a band of transistors 31 without the SiO 2 insulations, makes it possible to produce a linear defect constituting an optical guide along one of the crystallographic axes, ΓΚ. By positioning a light source of wavelength λ to the entry of this band and by choosing well the periodicity of the silos, it is possible to realize a monomode waveguide. In fact, in FIG. 5A, the band diagram of the guide in the direction of the guide shows monomodal spectral ranges at 33 in particular around the point a / λ = 0.25 where a is the interval step of the silos in a hexagonal mesh. The operating point 35 at the edge of the forbidden band indicates the point from which the mode is no longer guided.

In a second example, FIG. 5B, two parallel guides are coupled by a resonant cavity so as to produce a directional insertion-extraction filter called "Add-drop". In a third example, FIG. 5C, the device forms a band of faultless silos arranged perpendicularly to a light source to form a flat lens with a negative refractive index.

Furthermore, the use of MOSFET in the silos makes it possible to create a modulation of the optical index of the silicon under the gate which makes it possible to modulate the guided optical field in the photonic structure. Indeed, by using the V _DS (drain-source) and / or V _GS polarizations it is possible to inject carriers into the drain-source channel, under the gate. This injection or desertion of carriers makes it possible to modulate the optical index under the gate and thus to modulate the optical field guided in this layer. By way of example, the architectures of FIGS. 5A and 5B show that the optical modulation effect by the transistors is in this case particularly advantageous where the light is confined, that is to say in the defect. In this configuration, it is necessary to remove the silicon oxide insulations that isolate the portion modulated by the transistor of the optical field. For example, Figure 5A shows that by modulating the index we can pass type points 33 to 35 so as to provide an optical modulator. In other words, the state of the guide, passing or blocking, is controlled by modulating the polarization of the transistors.

An optical modulator is thus produced by the effect of injection or desertion of charges (according to the N or P type) in a field effect transistor. Experimental measurements carried out in the inventor's laboratory show the evolution of the silicon index variation under the gate as a function of the polarization, FIG. 6, from a grid-width AMS Bi-CMOS MOSFET transistor. 0.35 μm. These measurements were performed by laser illumination of the non-metallized grid; the variation of the index being obtained by measurements in reflection. A maximum variation of the index of the order of 3.times.10.sup.- ³ was measured with a transistor saturated at about 500 .mu.M for a gate-source voltage of 3.3V.

The invention has been illustrated and described in detail in the drawings and the foregoing description. This must be considered as illustrative and given by way of example and not as limiting the invention to this description alone. Many other embodiments are possible.

For example, the first and second layers can be machined and successively stacked to form a three dimensional network of pads S1O ₂ in silicon for example. This would result in a three-dimensional photonic crystal.

The optical index variation can be done using a device other than a MOSFET transistor such as, for example, a PN junction or capacitance.

It is also possible to obtain optical behaviors of the metamaterial type. As a reminder, metamaterials are materials whose structuring is at least ten times smaller than the wavelength. For example, in a configuration of very high index silos in a layer of SiO ₂ or silicon, it is possible to approach a meta-materials behavior in which a permeability and a negative permittivity can be obtained. The modulation effect of the current in the transistor channel makes it possible to glimpse a new class of integrated meta-materials whose permeability, in this example, could be modulated by a transistor effect.

Finally, it is noted that although this description is based on technology and semi ^¬ conductive materials, it is also possible to use organic materials such as polymeric or metallic materials and that, in accordance with the properties of optical index as described above.

Other applications in the field of biochemical sensors based on the modulation or not of an integrated photonic crystal structure, derive directly from the present invention. For example, it is conceivable to detect the grafting of biochemical molecules on the silos or another part of the photonic crystal. By modifying the silo index induced by this grafting, a frequency shift of the guided optical modes appears.

In the claims, the word "comprising" does not exclude other elements and the indefinite article "one" does not exclude a plurality.

Claims

Photonic crystal device comprising a stack on a substrate (3) of a first layer (5) having a first optical index and a second layer (9) having a second optical index greater than the first, said second layer comprising a plurality silos (7) arranged in a two-dimensional periodic structure, each silo comprising a solid material having a third optical index, the second layer with the plurality of silos forming a 2D Bravais network of photonic crystals characterized in that at least one silo comprises an electronic device for modulating the optical index of the silo by modulating the density of electrical charges in at least part of the silo.

Device according to Claim 1, characterized in that the second layer is furthermore at least partly covered by a layer (13) having a fourth optical index which is smaller than the second and third, and the structure has a stack of first, second and fourth layers, such that the device creates an optical confinement by total internal reflection in the growth axis of these different layers.

Device according to claims 1 or 2, characterized in that it has a band diagram which may comprise a forbidden band for a radiation having a wavelength of the same order of magnitude as the period of the structure.

Device according to any one of claims 1 to 3, characterized in that the two-dimensional periodic structure has at least one defect in form a different silo, and / or missing and / or supernumerary for carrying out routing functions, filtering or negative refraction.

Device according to Claim 4, characterized in that it comprises a plurality of defects in the form of different, missing or additional silos arranged in the form of a strip along one of the crystallographic axes so that said strip forms a waveguide of which the mode is confined vertically in the second layer.

Device according to claim 4, characterized in that the silos form a strip perpendicular to a crystallographic axis so as to form a flat lens with a negative refractive index.

Device according to claim 1 or 2, characterized in that it has properties of metamaterials for a light wave having a predetermined wavelength injected in the plane or normally at the periodicity plane.

Device according to claim 7, characterized in that the optical index contrast between the silos and the interstices of the photonic crystal is very large so that the period of the structure is substantially ten times lower than the predetermined wavelength and / or permeability and / or permittivity are negative.

Device according to any one of the preceding claims, characterized in that the materials of the layers and silos are organic materials of the polymer type and / or semiconductor materials.

10. Device according to claim 1, characterized in that the electronic device is a MOSFET transistor and that the optical index is modulated by an electric voltage applied to the gate of the MOSFET transistor and / or between the drain and the source of the MOSFET transistor. .

11. Device according to claim 10, characterized in that the first layer deposited on a silicon substrate is a silicon oxide layer, the second silicon layer containing silicon oxide silos containing smaller silicon silos of silicon. so that the association of each large and small silo form a SOI MOSFET transistor with silicon oxide isolation boxes.

12. Device according to claim 10 or 11, characterized in that the silos are at least partially covered by a layer of silicon oxide.

13. Device according to claim 12, characterized in that the second silicon layer is at least partially covered by a silicon oxide layer.

14. Device according to claim 11, comprising defects according to claim 4 of one or more transistors with or without insulating oxide.

15. Device according to any one of the preceding claims, characterized in that it further comprises a plurality of stack of second layers or first and second layers so as to form a three-dimensional photonic crystal or a metamaterial.