Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-08-28
2004-11-02
Mulpuri, Savitri (Department: 2812)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S098000, C385S129000
Reexamination Certificate
active
06812482
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to layered material compositions and related structures, in particular to photonic lattices, and more particularly to a method for fabricating photonic lattices having photonic bandgaps in the optical spectrum.
BACKGROUND OF THE INVENTION
Layered material compositions are materials which exhibit spatial variation of physical properties, composition, or other tangible characteristics, where that spatial variation produces useful bulk properties of the layered material compositions, and the spatial variation can be subdivided a stack of structured layers (the stack can consist of a single layer), which are assembled atop one another with appropriate alignment between the various structured layers. An individual structured layer can exhibit one-, two-, or three-dimensional variation of physical properties, so long as the surfaces of the layers are substantially flat.
Such layered material compositions exhibit a wide range of fascinating, unique, and useful “bulk” physical properties which result from the collective interaction of the spatially varying properties of their constituent materials. One of the most interesting classes of layered material compositions is the photonic lattice, which is a layered material composition which has a spatially varying index of refraction. Photonic lattices will be used as an example throughout this disclosure, and are described in some detail below. It is sufficient at this point to describe two primary optical phenomena which can be exhibited by photonic lattices. A photonic bandgap can appear, being a region of photon energy in which photons cannot propagate. Also, many types of photonic lattices will exhibit rapidly varying “bulk” indices of refraction in certain wavelength regimes. Both of these phenomena are the basis for many useful optical devices.
Many other classes of interesting and useful layered material compositions exist. For example, if a layered material composition has an appropriate spatial variation in, e.g., sound velocity or mass density, it will exhibits a phononic bandgap, i.e., a solid which does not allow propagating sound waves with phonon energies inside the phononic bandgap. It is also possible to produce structures exhibiting unusual and useful electronic properties, such as are associated with superlattices and other layered structures, but where the spatial variation in electronic properties is two- or three dimensional in character. It is also possible to build up two- and three-dimensional active and passive circuitry using the present invention. A further example involves the ability to control mechanical properties, including strength, by introducing a spatial variation in material characteristics on a small size scale. All such compositions, where certain bulk material properties depend intrinsically on the presence of the spatially varying physical properties within and between structured layers, are layered material compositions.
Throughout this disclosure Applicants will focus on the application of the present invention to the fabrication of a particular class of layered material compositions, namely photonic lattices. The term “photonic lattice” is used to describe any structure or material having bulk optical properties associated with a layered spatial variation of refractive index. This includes periodic, quasiperiodic, and aperiodic structures.
The best known property exhibited by some photonic lattices is a photonic bandgap. A material shows a photonic bandgap if there exists a region in energy-momentum space wherein propagating photon modes do not exist. Various structures can exhibit a partial photonic bandgap (a bandgap along some directions), a complete photonic bandgap (a bandgap along all directions, but which do not necessarily overlap in energy), a photonic stopgap (a range of photon energy in which photon propagation is not allowed along any direction), or no photonic bandgap at all. Photonic lattices which do not exhibit a bandgap can still have anisotropic and strongly varying bulk dispersion associated with the spatially varying refractive index. Such bulk optical effects can appear in strictly dielectric layered material compositions, in compositions comprising discrete regions of dielectric and metallic materials, and in various intermediate cases. Any structure exhibiting spatial variation of the local optical properties herein called a photonic lattice. If said structure can be split into a stack of structured layers, it is then also a layered material composition.
Photonic lattices are under investigation for applications in which their unusual interactions with electromagnetic radiation are useful. In their simplest form, such photonic lattices are based on a one-, two-, or three-dimensional periodic refractive index. (Recall that such periodicity is not required.) In such structures the propagation of electromagnetic waves is governed by multiple interference effects leading to wavelength-energy dispersion relationships similar to those describing the motion of electrons in solids. Traditional electron-wave concepts such as reciprocal space, Brillouin zones, dispersion relations, Bloch wave functions, and semiconductor bandgap have electromagnetic counterparts in photonic lattices. Defect states (which allow propagation of very narrow bandwidths in particular directions) can be introduced into the photonic bandgap by adding or subtracting a small amount of material from the ideal structure.
Perhaps the most significant property which can be exhibited by a photonic lattice is the photonic bandgap, a range of photon energies for which no propagating photon modes exist. This effect is analogous to the semiconductor bandgap in solids, which defines a range of energies in which propagating electrons cannot exist. Not all photonic lattices exhibit such a bandgap. Prediction of the properties of a photonic lattice can be carried out using techniques known in the art which are again analogous to those used to calculate electronic band structures in solids. Qualitatively, however, a wide photonic bandgap is encouraged by a number of factors, including:
1. Large ratio between largest and smallest refractive index in the photonic lattice.
2. The existence of continuous sublattices of low and high refractive index throughout the photonic lattice.
3. The volume fraction of the high refractive index sublattice should be less than that of the low refractive index sublattice.
The above list has been simplified by using language which implies the photonic lattice comprises discrete regions having distinct refractive indices. Such discreteness is not required, and any effect which will be discussed in this specification can be found in a photonic lattice having continuously varying refractive index. Also note that whereas the structures which are easiest to analyze are also infinite in physical extent, real photonic lattices have limited spatial dimensions, and as such are technically distinct from theoretical photonic lattices of infinite extent. We shall consider structures with limited physical extent which can be embedded in a photonic lattice of infinite extent also to be a photonic lattice.
The physics which governs photonic lattices and the formation of photonic bandgaps scales with changes in wavelength in a manner which allows (at least in principle) photonic lattices which exhibit bandgaps to exist on any size scale. Indeed, the first demonstration materials were designed for microwave frequencies, and were assembled from bulk epoxy and Styrofoam pieces. Later, silicon micromachining was used to fabricate photonic lattices active in the millimeter wavelength range. Until the present invention was developed, however, only crude demonstrations of two- and three-dimensional photonic lattices had been made which produced a bandgap in what we are calling the optical regime, which comprises optical wavelengths from roughly 20&mgr; down to perhaps 0.1&mgr;. (The long wavelength end represents the ultimate capability of conventional
Fleming James G.
Lin Shawn-Yu
Dudson Brian W.
Hohimer John P.
Sandia Corporation
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