Patent Application
20110012791
1. Field of the Invention
The invention pertains to a left-handed substance as well as to a wave-guide device and an antenna incorporating this left-handed substance. An object of the invention is also a method for manufacturing this left-handed substance.
Here below in the description, unless otherwise stated, the terms “permittivity ε” and “permeability μ” when used without any other specific information refer to relative permittivity and relative permeability.
Left-handed substances were presented for the first time by Victor Veselago in:
“The Electrodynamics of Substances with Simultaneously Negative Values of ε and μ”, Soviet Physics USPEKHI, vol. 10, n° 4, January-February 1968”.
These materials have the property of simultaneously presenting negative permittivity ε and negative permeability μ within a given range of frequencies. These left-handed substances have many atypical properties, such as:
Because of these atypical properties, these left-handed substances may find numerous applications, especially in the processing of the electromagnetic waves.
It has been proposed especially to use these left-handed substances in wave guides, filters, or antennas. For such applications, it is desirable that the frequency band in which ε and μ are simultaneously negative should in the hyper-frequency domain, i.e. between 1 and 60 GHz.
Various research projects have been conducted to achieve this result. For example, a substance having these properties is described in the following document D1:
D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite Medium with Simultaneously Negative Permeability and Permittivity”, Phys. Rev. Lett., Vol 84, N° 18, p. 4184, 2000.
These known substances are often called “metamaterials”. They comprise a heterogeneous material formed by an array of conductive wires positioned relative to one another in such a way as to present a negative ε relative to the electromagnetic waves which have an electrical field parallel to the biggest dimension of these wires and are propagated at a frequency below the electrical plasma frequency of the substance.
The electrical plasma frequency as well as the sizing of this array of conductive wires to obtain a value of ε below zero has been described especially in the following document D2:
J. B. Pendry, A. J. Holden, W. J. Stewart, and I. Youngs, “Extremely Low Frequency Plasmons in Metallic Mesostructures”, Phys. Rev. Lett., Vol.76, N° 25, 1996.
Broadly speaking, the electrical plasma frequency of the substance is the value of the frequency of the incident electromagnetic wave for which the real part of ε gets cancelled out.
These prior-art substances generally comprise another heterogeneous material formed by another array of conductive patterns that are laid out relatively to one another so as to present a negative value of μ in the desired frequency band. Typically, this other array is a array of conductive split rings (also known as Pendry rings) used to artificially generate a negative μ value through an electromagnetic resonance phenomenon LC in a range of frequencies situated immediately after the magnetic plasma resonance frequency. Broadly speaking, the magnetic plasma resonance frequency is the value of the frequency of the incident electromagnetic wave for which the real part of μ gets cancelled out. Such arrays can be used to obtain a negative μ value after the magnetic plasma resonance frequency. These arrays are for example examined in the following document D3:
J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, “Magnetism from conductors and enhanced nonlinear phenomena “, IEEE Trans. MTT, Vol. 47, N° 11, 1999.
The above two arrays are laid out so as to present both a negative ε value and a negative μ value.
The arrays described here above consist of an elementary pattern called an “elementary cell” repeated at regular intervals in one or more repetition directions. The regular interval is called the “pitch” of the array.
The size of the elementary cell in the direction of repetition is chosen in such a way that the substance behaves like a homogeneous material with respect to the wave illuminating this substance with a frequency included in the range of frequencies for which the values of ε and μ are simultaneously negative. To this end, the size of an elementary cell is chosen to be smaller than and preferably several times smaller than the wavelength of the illuminating wave and typically ten times smaller. At the same time, the pitch of the array is far greater than 1 micrometer so that, at a microscopic scale, the layout of the wires relative to one another can be clearly discerned.
These prior-art substances have several drawbacks:
Furthermore, the sizing and tunability of the array that make it possible to obtain a negative μ are limited. Indeed, to obtain a negative μ for a given working frequency, it is necessary to build an array having a magnetic plasma resonance frequency neighboring this working frequency. To this end, the dimensions of the split rings must be matched with the wavelength of the working frequency. Now the modification of the size of the split rings cannot be done dynamically, thus preventing the tuning of these metamaterials at a given working frequency after it has been manufactured. Even if the working frequency is known before the manufacturing of the array, the dimensions of the split ring needed to work at this frequency may be impossible to achieve either because they are too small or because on the contrary they are far too great.
It is therefore not easy to use the known substances combining two heterogeneous materials to obtain negative values of ε and μ simultaneously, in physical applications.
Recently, it has been proposed to use only one array of conductive wires arranged in relation to one another so as to present negative permittivity to electromagnetic waves having an electrical field parallel to the greatest dimension of these wires and being propagated at a frequency below the electrical plasma frequency of the substance, each wire being made out of a conductive magnetic material having negative permeability for a range of frequencies of the electromagnetic waves below the electrical plasma frequency of the substance and when there is no external artificial static magnetic field. The wires have a circular cross-section whose diameter is greater than 1 μm.
For example, a substance of this kind is described in the following document D4:
H. García-Miquel, 1,a_ J. Carbonell,2 V. E. Boria,2 and J. Sánchez-Dehesa1, <<Experimental evidence of left-handed transmission through arrays of ferromagnetic microwires>>, APPLIED PHYSICS LETTERS 94, 054103—2009—
In this last embodiment, it is not necessary to plan for another structure in addition to the array of wires, for example an array of split rings, so that this substance will show left-handed properties in a range of frequencies. The structure of this left-handed substance is therefore simpler than that of substances using two heterogeneous materials and especially metamaterials. Indeed, this substance uses the natural ferromagnetic resonance frequency of the material used to form the conductive wires. This ferromagnetic resonance frequency is qualified as being natural because it exists in the absence of any external static magnetic field. The term “static magnetic field” designates a direct magnetic field and not an alternating magnetic field.
Furthermore, the positioning of the ferromagnetic resonance frequency in the neighborhood of the desired working frequency does not call for modifying the pitch or dimensions of the elementary cell of the wireless network. Here, it is sufficient to play on the choice of the conductive ferromagnetic material used to make the wires, i.e. for example, on an external static magnetic field. Given that it is not necessary to adapt the dimensions of the array to bring about a variation in the frequency of the ferromagnetic resonance of this substance, the sizing and tunability of this substance are simplified.
However, in practice, as illustrated by the experimental results shown in the document D4, this material has solely left-handed properties if it placed in an external static magnetic field. This is one particularly major drawback for the use of this type of left-handed substance.
The invention seeks to overcome at least one of these drawbacks by proposing a left-handed substance in which wire comprises at least one strip, made out of a conductive magnetic material, that extends along the greatest dimension of the wire in a plane of the strip and has a thickness at least twice as small as the skin thickness of the conductive magnetic material.
In the above left-handed substance, the material used to make the strips also shows a negative μ value for a range of frequencies below the electrical plasma frequency. Consequently, there is a range of frequencies for which this substance has left-handed properties. Furthermore, because of the small thickness of these strips, it is not necessary for this substance to be placed in an external static magnetic field in order to present left-handed properties. More specifically, the Filing Party is of the view that since the thickness of the strips is at least twice as small as the skin thickness, the electromagnetic field can penetrate the entire cross-section of the strip without any need to resort to an external static magnetic field. Furthermore, the small thickness of the strips naturally boosts the natural magnetization of the magnetic material so as to make it get aligned with the greatest dimension of the wires. Thus, it is no longer necessary to resort to an external static magnetic field to align the magnetization of each strip in parallel with this greatest dimension.
Thus, the left-handed substance has the same advantages as those disclosed in the document D4, without requiring any external static magnetic field.
The embodiments of this left handed material may comprise one or more of the following characteristics:
These embodiments of the left-handed substance furthermore have the following advantages:
An object of the invention is also an electromagnetic wave-guide device comprising:
An object of the invention is also an electromagnetic sender or receiver antenna comprising:
Finally, an object of the invention is also a method for manufacturing the above left-handed substance comprising the etching of a layer made of conductive magnetic material whose thickness is at least twice as small as the skin thickness of this material to form the strip of conductive magnetic material of a plurality of different conductive wires.
The invention will be understood more clearly from the following description given purely by way of an example and made with reference to the appended drawings, of which:
In these figures, the same references are used to designate same elements.
Here below in this description, the characteristics and functions well known to those skilled in the art are not described in detail.
The material 2 has an array 4 of conductive wires 6. These wires 6 are for example all identical to one another. The elementary cell of the array 4 contains only one wire 6 herein. This elementary cell is repeated with a regular pitch p1 in a horizontal direction X and with a regular pitch p2 in a vertical direction Z. Here, the pitch values p1 and p2 are for example equal. The number of repetitions of the elementary pattern in the direction X is greater than two and preferably greater than ten. The number of repetitions of the elementary pattern in the direction Z is greater than two and preferably greater than 5.
Each wire 6 extends in parallel to a direction Y perpendicular to the directions X and Z.
The array 4 and especially the pitch values p1 and p 2 are sized in order to present a negative value of c, preferably throughout the hyper-frequency range. The array 4 therefore has an electrical plasma frequency greater than or equal to 20 GHz.
For example, the array 4 is sized through application of the teachings of the document D2.
Here, each wire 6 is obtained by a stacking, in the direction Z, of strips extending in parallel to the direction Y.
Fastener strips 26 and 28 are provided at each end of this stack of ferromagnetic and antiferromagnetic strips. The fastener strip 26 is used especially to fixedly join the stack of strips 20, 22 and 24 to a substrate 30. The substrate 30 is made out of a material that does not modify the magnetic properties of the array 4. To this end, the substrate 30 is typically amagnetic. It is also preferably insulating. For example, the substrate is made out of non-doped silicon, glass, quartz, ceramic or organic material. The substrate 30 may also be made of a preformed substrate.
Here, each strip extends essentially in parallel to the direction Y so that the plane of each strip is parallel to the directions X, Y. Moreover, each strip has a rectangular cross-section. The length of each strip along the direction Y is at least twice as great as the width of the wire n the direction X and advantageously ten times greater than this width. For example, here, the length of each wire 6 is greater than 1 mm.
The thickness of the strips 20 and 24 in the direction Z is at least twice and preferably five or six times smaller than the skin thickness of the conductive magnetic material forming them. For example, the thickness is smaller than 1 μm and preferably smaller than 200 nm. The width of the strips in the direction X is greater than or equal to the thickness. Preferably, the width will be at least ten times greater than the thickness. For example, the width of each strip ranges from 10 to 100 μm.
The natural ferromagnetic resonance frequency of the conductive ferromagnetic material is strictly smaller than the plasma frequency of the substance 2. Preferably, to facilitate use, this ferromagnetic resonance frequency ranges from 1 GHz to 20 GHz. For example, the chosen material has a natural ferromagnetic resonance (called FMR in the graph of
This material also has a magnetic damping coefficient typically smaller than 10−2, corresponding to a mid-height line width Δf (
The material chosen for the strips 20 and 24 here is such that, beyond the ferromagnetic resonance frequency and at least up to 20 Ghz, it has a μ value of less than −10.
Finally, the chosen conductive ferromagnetic material has a conductivity of over 0.5 MS/m. Typically, a conductivity ranging from 0.5 MS/m to 5 MS/m is appropriate.
A material simultaneously having all these properties is for example described in detail in the following document D5:
Y. LAMY and B. VIALA, “Combination of ultimate magnetization and ultra-high uniaxial Anisotropy in CoFe exchange-coupled multilayers”, Journal of Applied Physics 97, 10F910 (2005)”
The graph of
The substance 2 can be manufactured as follows. First of all, at a step 32, the layers 26, 20, 22, 24 and 28 are deposited on the entire surface of the substrate 30 by physical, electrochemical, “chimie douce” (soft chemistry) or other conventional methods. Preferably, at the step 32, the ferromagnetic layers are deposited under magnetic field and/or annealed under magnetic field after depositing, i.e. in an environment in which there is a static magnetic field enabling the natural magnetization of the ferromagnetic material to be oriented in a predefined direction of magnetization.
Then, at a step 34, the stacking of layers is structured by the same methods as those used in microelectronics, for example lithography and etching or the like. In etching, material is removed to form stacks of strips and therefore wires 6. If the depositing and/or the annealing of the ferromagnetic layers have been done under a magnetic field, then the etching is done so that the ferromagnetic strips extend parallel to the predefined direction of magnetization. The layers may also be deposited directly through a mask or on a substrate having a pre-formed surface.
The Filing Party has noted that the substance 2 has left-handed properties in the ΔT frequency band relative to electromagnetic waves illuminating this substance with an electrical field parallel to the direction Y and a field H parallel to the direction X, i.e. in the plane of the strips. The direction of propagation k of the electromagnetic wave is parallel to the direction Z.
The left-handed properties of the substance 2 are also revealed in the graph of
In this application, the wires 6 of the substance 2 extend along a vertical direction Y and the plane of the strips is parallel to a plane XY, where X is a direction perpendicular to the directions Y, Z. To simplify
In the guide 50, the electromagnetic waves get propagated along the direction Z. Furthermore, the guide 50 is designed so that the guided electromagnetic waves are directed towards the substance 2 with an electrical field parallel to the direction Y and a magnetic field H parallel to the plane of the strips 20, 24. The field H is therefore parallel to the direction X. Thus, for example, the substance 2 makes it possible to open a passband in a bandgap of the guide 50, which can be used to filter the guided electromagnetic waves.
In another example, the substance 2 only partially obstructs the cross-section of the guide 50. This configuration enables a phase-shift in the transmitted wave. A phase-shifter is then obtained.
In both cases, the use of the substance 2 enables the miniaturizing of the devices because the desired effects are obtained for dimensions far smaller than a half wavelength (for example λ/10).
In another example, the invention makes it possible to miniaturize the antenna with not change in gain by using the plate 64 with a size along Z that is smaller than the half wavelength (for example λ/5). These principles of use in substrate form (or superstrate form) can also be applied to a dipolar antenna (which would be positioned here along the axis Y) such as the wires 6 which form the substance 2 extending parallel to the axis of the dipole. The substance 2 is then positioned about this dipole in a direction parallel to the plane XZ.
Many other embodiments are possible. For example, the natural ferromagnetic resonance frequency is not necessarily below 20 GHz.
The magnetic material used to make the magnetic strips is not necessarily homogeneous. For example, the magnetic material may be a material obtained out of a ferromagnetic nano-powder aggregated by means of a binder. In this description, a material is deemed to be homogeneous if it is made out of a single magnetic alloy. Conversely, a material is considered to be heterogeneous if it is made of a magnetic alloy and a dielectric material.
The cross-section of the strips is wider than it is thick, but not necessarily rectangular. For example, the cross-section may ellipsoidal with very low excentricity.
The conductive wires may be made by stacking ferromagnetic and antiferromagnetic layers in the reverse order to the scheme described with reference to
It is also possible to make a wire by stacking several magnetic strips on one another and insulating them electrically from one another by means of strips made of dielectric material. This prevents the appearance of eddy currents and increases the fill rate in magnetic material.
In a simplified embodiment, each wire is formed by a single magnetic strip.
As a variant, the elementary pattern of the array of wires is repeated solely in one direction or in more than two directions.
As a variant, the substrate 30 is made out of a ferromagnetic or piezoelectric material. Thus, the ferromagnetic resonance frequency is adjustable by playing on the voltage applied to this substrate.
Preferably, the wires 6 are surrounded by a dielectric material such as silica or resin, presenting permittivity greater than that of air. However, they can also be surrounded by air.
In another embodiment of the device 48, the plane of the strips is parallel to the plane YZ. In this case the field H of the electromagnetic wave is parallel to the direction Z.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0903549 | Jul 2009 | FR | national |
