Optical: systems and elements – Optical modulator – Having particular chemical composition or structure
Reexamination Certificate
2001-07-23
2004-11-02
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Having particular chemical composition or structure
C359S237000, C359S245000, C359S322000, C343S786000
Reexamination Certificate
active
06813064
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fully tunable photonic band gap (PBG) materials and recipes for synthesis of these materials using periodic composite dielectric materials having differing indexes of refraction in which one or both of the indexes are electro-optically or magneto-optically tunable thereby permitting the photonic band structure to be tuned. More particularly the invention relates to photonic crystals and photonic band gap (PBG) materials having optical stop gaps or complete photonic band gaps in which this stop gap or complete photonic band gap can be opened, closed or readjusted, either locally or globally by means of an external electric, magnetic, or electromagnetic field.
BACKGROUND OF THE INVENTION
Photonics is the science of molding the flow of light. Photonic band gap (PBG) materials, as disclosed in S. John, Phys. Rev. Lett. 58, 2486 (1987), and E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987), are a new class of dielectrics which carry the concept of molding the flow of light to its ultimate level, namely by facilitating the coherent localization of light, see S. John, Phys. Rev. Lett. 53, 2169 (1984), P. W. Anderson, Phil. Mag. B 52, 505 (1985), S. John, Physics Today 44, no. 5, 32 (1991), and D. Wiersma, D. Bartolini, A. Lagendijk and R. Righini, Nature 390, 671 (1997). This provides a mechanism for the control and inhibition of spontaneous emission of light from atoms and molecules forming the active region of the PBG materials, and offers a basis for low threshold micro-lasers and novel nonlinear optical phenomena. Light localization within a PBG facilitates the realization of high quality factor micro-cavity devices and the integration of such devices through a network of microscopic wave-guide channels (see J. D. Joannopoulos, P. R. Villeneuve and S. Fan, Nature 386, 143 (1998)) within a single all-optical microchip. Since light is caged within the dielectric microstructure, it cannot scatter into unwanted modes of free propagation and is forced to flow along engineered defect channels between the desired circuit elements.
The utility of PBG materials arises essentially from their ability to facilitate the localization of light and the controllable inhibition of spontaneous emission of light from atoms and molecules as mentioned above. Although an intensive effort has developed over the past ten years, to microfabricate PBG materials (see “Photonic Band Gap Materials” edited by C. M. Soukoulis, Kluwer Academic Publishing, Dordrecht 1996; Journal of Lightwave Technology IEEE, volume 17, number 11 (1999) Special Issue on Photonic Crystals), it is only recently that a clear route to synthesizing large scale three-dimensional PBG materials with submicron lattice constants has been demonstrated using self-assembly methods, see K. Busch and S. John, Physical Review E58, 3896 (1998); J. E. G. J. Wijnhoven and W. L. Vos, Science 281, 802 (1998); A. A. Zakhidov et. al. Science 282, 897 (1998). The application of this approach to Si, Ge and GaAs based PBG materials may open the door to applications in laser devices and telecommunications as well as to the realization of fundamentally new effects in quantum and nonlinear optics. As pointed out by Sir John Maddox, “If only it were possible to make dielectric materials in which electromagnetic waves cannot propagate at certain frequencies, all kinds of almost magical things would be possible.” John Maddox, Nature 348, 481 (1990).
Recently, it has been shown that PBG materials based on silicon and germanium can be readily fabricated based on inverse opal structures. The value of the photonic band gap depends on the physical dimensions of the air voids so that fabricating the precursor opal with a given sphere size fixes the photonic band gap, U.S. Pat. No. 60/178,773 filed Jan. 28, 2000 and U.S. Pat. No. 60,202,115 filed May 5, 2000, both references being incorporated herein by reference.
For many applications, however, it would be advantageous to obtain some degree of tunability of the photonic band structure through electro-optic or magneto-optic effects. Such devices would find very broad applications in optical networking for telecommunications. The tunable PBG material would act as a tunable mirror or a switch with a latch for routing optical data from one channel to another. If such tunable PBG materials could be made they would form the basic building blocks in photonic circuits, analogous to the role of semiconductors in conventional microelectronics. In the case of semiconductors, the flow of electricity is regulated by means of applied voltages in most applications (but also by magnetic fields in some applications) which modulates the electronic band structure.
It has been proposed that such tunability may be obtained by controlling one or several forms of optical anisotropy of the constituent materials. The science of liquid crystals (see P. G. de Gennes and J. Prost, “The Physics of Liquid Crystals”, Clarendon Press, Oxford 1993; S. Chandrasekhar, “Liquid Crystals”, Cambridge University Press 1992; L. M. Blinov and V. G. Chigrinov, “Electro-Optic Effects in Liquid Crystal Materials”, Springer, N.Y. 1994) has spawned an entire industry related to these electro-optic effects. In earlier works, however, a rather pessimistic conclusion regarding the efficacy of birefringent photonic crystals was drawn (see I. H. Zabel and D. Stroud, Phys. Rev. B48, 5004 (1993); Z. Y. Li, J. Wang, B. Y. Gu, Phys. Rev. B58, 3721 (1998); R. M. Hornreich, S. Shtrikman and C. Sommers, Phys. Rev. E47, 2067 (1993)) since attention was restricted to unrealizable structures that consist of spheres of disconnected anisotropic, high dielectric materials in an air background.
It would be very advantageous to provide an economical and technologically simple recipe for combining the high sensitivity of the photonic band structure to small variations in the refractive index with the known refractive index tunability of liquid crystals or ferro-electrics to produce PBG materials which are tunable.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide photonic band gap (PBG) materials in which the band structure of a photonic crystal can be changed in a controlled manner.
The present invention provides fully tunable photonic band gap (PBG) materials and recipes for synthesis of these materials using periodic composite dielectric materials having differing indexes of refraction whereby changing the refractive index properties of one or more of the dielectric constituents by application of an external field tunes or modulates the photonic band structure in a predictable way. These composite dielectric materials may comprise a high refractive index dielectric material and another optically anisotropic, birefringent, electro-optically tunable, or magneto-optically tunable material with a lower dielectric constant in which the photonic band structure can be globally or locally changed in a controlled manner by application of an external electric, magnetic, or electromagnetic field.
Therefore, in one aspect of the invention there is provided a photonic crystal having a tunable photonic band structure, comprising;
a periodic composite dielectric material having at least two dielectric constituents including a first dielectric constituent having a first refractive index and a second dielectric constituent having a refractive index smaller than the first refractive index so that the periodic composite dielectric material has a photonic band structure; and
at least one of said at least two dielectric constituents having refractive index properties which can be locally or globally changed throughout said photonic crystal in a controlled manner whereby changing the refractive index properties modulates said photonic band structure locally or globally throughout said photonic crystal for providing control of propagation of light through said photonic crystal.
In another aspect of the invention there is provided a photonic crystal having a tunable photonic band structure, comprising;
a periodic composit
Busch Kurt
John Sajeev
Choi William
Epps Georgia
Hill & Schumacher
John Sajeev
Schumacher Lynn
LandOfFree
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