Periodic dielectric structure of the three-dimensional...

Details Periodic dielectric structure of the three-dimensional... Periodic dielectric structure of the three-dimensional...

2000-05-10

2002-10-22

Wong, Don (Department: 2821)

Communications: radio wave antennas

Antennas

Antenna components

C343S91100R

Reexamination Certificate

active

06469682

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a crystalline structure with three-dimensional photonic band gap. It also relates to a manufacturing method for said structure.
Recent discoveries have allowed emphasizing a particular physical phenomenon, which develops within periodic dielectric structures, such as some crystals., wherein differences of electrical properties are periodically repeated through their thickness. This phenomenon has been called “photonic band gap”, or “PBG” (by analogy with the electronic band gap which can be created in semiconductor crystals). Structures with a photonic band of this type will hereinafter be referred to as “photonic crystals”.
Exemplary, non-exhaustive structures of this type were described in the following article or book;
the book “Photonic Crystals, Molding the Flow of Light”, J. D. JOANNOPOULOS and al. “Princeton University Press”, 1995; and
the article: “Photonic Band gap materials”, in “Proceedings of the NATO Advanced Institute-on Photonic Band gap Materials”, ELOUNDA, Greece, June 1995.
Although one-dimension photonic crystals have been used for long, the idea of creating two- or three-dimension photonic band gap structures only appeared some ten years ago. Ever since, and due to the considerable possibilities, they offer, the interest for photonic crystals has been growing. Indeed, photonic crystals are appropriate for a large number of applications and they are actually in use in devices such as semiconductor lasers, solar cells, high quality resonant cavities, and filters.
Due to their remarkable features, the photonic crystals have found an application more recently in hyper frequency circuits or in components operating in the millimeter or sub-millimeter wave ranges. More particularly, such crystals have found an advantageous application in the so-called photonic band gap antennas, where they are used as substrates for antenna systems.
In an antenna of this type, the underlying photonic crystal prevents that electromagnetic waves of the considered frequency that reach it to propagate therein, and thus it reflects towards the surrounding air the entire electromagnetic radiation emanating from the system it supports, inasmuch as the antenna working frequency matches the band gap frequency.
It is thus effectively possible to consider the photonic crystals as being the electromagnetic homologues of the semiconductor crystals, so far as their behavior towards the electrons is concerned. The photonic crystals have a frequency band for which the propagation of electromagnetic wave through them is inhibited. The operating parameters of the photonic band gap structures are to be found in the periodicity of the variation of their dielectric properties, in the dielectric contrast and in the achieved structural arrangement.
The photonic crystals offer very substantial opportunities for the future, since they allow reducing the global mass of the system where they are used and improve the radiation efficiency performances that are essential for systems operating in a radio frequency or optical frequency range of a predetermined extent.
Those photonic band gap crystals the structure of which is only two-dimensional offer the substantial advantage of being easy to produce. But, although they can be useful sometimes, it appears that the three-dimensional photonic band gap crystal structures prove more appropriate for uses such as antenna substrates. Basically, an aitenna radiates in a three-dimensional space. It follows therefrom that it is preferable to provide a structure ensuring a completely three-dimensional photonic band gap, that will be able to prevent any propagation of the electromagnetic radiation through it by covering all spatial directions, and that will consequently be effective as an omni-directional reflector,
A few three-dimensional configurations have been proposed in the prior art. Typical exemplary structures will be indicated below, and their main features will briefly be reminded, in connection with the manufacturing method.
U.S. Pat. 5,172,267 “Optical reflector structure, method of fabrication, and communication method”, (filed by Eli YABLONOVITCH) describes a first type of photonic crystals with a three-dimensional band gap.
The appended
FIG. 1A
illustrates both the main features of the crystal structure la and the manufacturing process thereof. A mask
2
, with a lattice of holes
2
a
regularly distributed in a triangular mesh configuration, is arranged on a crystal block
10
a
, simply. shaped as a rectangular parallelepipedal bar. Three successive micromachining operations are conducted, each of which provides straight holes or channels
100
a
in the crystal along a different determined orientation.
These micro-machining operations, for instance, can be performed by resorting to mechanical drilling, micro-lithography or ion beam technics, as suggested by the presence of three beams f
1
-f
3
in
FIG. 1
a
. More precisely, the three micro-machining operations can be performed along three different respective directions &Dgr;
1
to &Dgr;
3
, each tilted by an angle &agr; of predetermined identical amplitude with respect to a vertical axis &Dgr;V, orthogonal to the upper face of mask
2
, and also consequently to both main faces of bar
10
a
. The angle &agr; precisely equals 35.26 degrees. All three directions &Dgr;
1
to &Dgr;
3
are equally distributed in space; they consequently form angles of 120 degrees between one another. Those micro-machining operations result in a three-dimensional set of holes
100
a
providing empty channels in the material and crossing each another within the bar
10
a.
By appropriately selecting the physical characteristics (dielectric constant of the material forming the original crystal) as well as the geometric features (diameter of the holes in the mask, pitch distance between the adjacent holes), the structure with the desired three-dimensional photoninc band gap is obtained. For a more detailed description concerning the features of this structure according to the prior art, reference can be made to the above mentioned U.S. Pat. No. 5,172,267.
By appropriately selecting the physical characteristics (dielectric constant of the material forming the original crystal) as welle as the geometric features (diameter of the holes in the mask, pitch distance between adjacent holes), the structure with the desired three-dimensional photonic band gap is obtained. For a more detailed description concerning the features of this structure according to the prior art, reference can be made to the above mentioned patent U.S. Pat. No. 5,172,267.
However, it will be easily ascertained that the manufacturing method taught by the above prior art is quite complex, notably because three micro-machining, must be performed along three directions with different slopes with respect to the vertical axis &Dgr;V.
A second type of photonic crystal structures of the three-dimensional photonic band gap type is described in the U.S. Pat. No. 5,406,573 (Ekmel OZBAY and al.). The general configuration of the proposed structure, as described according to several alternatives, resembles a wood stack in which layers of logs, alternately oriented in two perpendicular directions, are piled successively on top of one another.
The appended
FIG. 1B
, which reproduces the main features of
FIG. 2
in the above-mentioned prior art patent, illustrates one of the structure alternatives described in this document, said structure being designated as
1
b
in FIG.
1
B. The
1
b
structure consists of a pile of rods
1
b
, arranged in layers on several levels, numbered N
1
to N
4
in
FIG. 1B
, and more generally N
1
to Nn.
All rods are arranged parallel to an arbitrary reference plane with orthononnative axes X and Y, and rods located within a particular layer at a same level parallel to this plane are parallel to one another. Rods arranged in two if successive levels, for instance N
1
and N
2
, are orthogonal to one another, i.e. they are alternately parallel to one then the other of

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