Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2002-06-21
2004-05-25
Healy, Brian (Department: 2874)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S129000, C385S002000, C385S014000, C385S031000
Reexamination Certificate
active
06741782
ABSTRACT:
TECHNICAL FIELD
The invention relates to optical devices and is especially applicable to waveguide structures and integrated optics.
BACKGROUND ART
This specification refers to several published articles. For convenience, the articles are cited in fill in a numbered list at the end of the description and cited by number in the specification itself. Their contents are incorporated herein by reference.
In the context of this patent specification, the term “optical radiation” embraces electromagnetic waves having wavelengths in the infrared, visible and ultraviolet ranges.
The terms “finite” and “infinite” as used herein are used by persons skilled in this art to distinguish between waveguides having “finite” widths in which the actual width is significant to the performance of the waveguide and the physics governing its operation and so-called “infinite” waveguides where the width is so great that it has no significant effect upon the performance and physics of operation.
As explained in copending international patent application No. PCT/CA00/01525, the contents of which are incorporated herein by reference, at optical wavelengths, the electromagnetic properties of some metals closely resemble those of an electron gas, or equivalently of a cold plasma. Metals that resemble an almost ideal plasma are commonly termed “noble metals” and include, among others, gold, silver and copper. Numerous experiments as well as classical electron theory both yield an equivalent negative dielectric constant for many metals when excited by an electromagnetic wave at or near optical wavelengths [
1
,
2
]. In a recent experimental study, the dielectric function of silver has been accurately measured over the visible optical spectrum and a very close correlation between the measured dielectric function and that obtained via the electron gas model has been demonstrated [
3
].
It is known that the interface between semi-infinite materials having positive and negative dielectric constants can guide TM (Transverse Magnetic) surface waves. In the case of a metal-dielectric interface at optical wavelengths, these waves are termed plasmon-polariton modes and propagate as electromagnetic fields coupled to surface plasmons (surface plasma oscillations) comprised of conduction electrons in the metal [
4
].
It is known to use a metal film of a certain thickness bounded by dielectrics above and below as an optical slab (planar, infinitely wide) waveguiding structure, with the core of the waveguide being the metal film. When the film is thin enough, the plasmon-polariton modes guided by the interfaces become coupled due to field tunnelling through the metal, thus creating supermodes that exhibit dispersion with metal thickness. The modes supported by infinitely wide symmetric and asymmetric metal film structures are well-known, as these structures have been studied by numerous researchers; some notable published works include references [
4
] to [
10
].
In general, only two purely bound TM modes, each having three field components, are guided by an infinitely wide metal film waveguide. In the plane perpendicular to the direction of wave propagation, the electric field of the modes is comprised of a single component, normal to the interfaces and having either a symmetric or asymmetric spatial distribution across the waveguide. Consequently, these modes are denoted s
b
and a
b
modes, respectively. The s
b
mode can have a small attenuation constant and is often termed a long-range surface plasmon-polariton. The fields related to the a
b
mode penetrate further into the metal than in the case of the s
b
mode and can be much lossier by comparison. Interest in the modes supported by thin metal films has recently intensified due to their useful application in optical communications devices and components. Metal films are commonly employed in optical polarizing devices [
11
] while long-range surface plasmon-polaritons can be used for signal transmission [
7
]. In addition to purely bound modes, leaky modes are also known to be supported by these structures.
Infinitely wide metal film structures, however, are of limited practical interest since they offer one-dimensional (1-D) field confinement only, with confinement occurring along the vertical axis perpendicular to the direction of wave propagation, implying that modes will spread out laterally as they propagate from a point source used as the excitation. Metal films of finite width have recently been proposed in connection with polarizing devices [
12
], but merely as a cladding.
In addition to the lack of lateral confinement, plasmon-polariton waves guided by a metal-dielectric interface are in general quite lossy. Even long-range surface devices exploit this high loss associated with surface plasmons for the construction of plasmon-polariton based modulators and switches. Generally, known plasmon-polariton based modulator and switch devices can be classified along two distinct architectures. The first architecture is based on the phenomenon of attenuated total reflection (ATR) and the second architecture is based on mode coupling between a dielectric waveguide and a nearby metal. Both architectures depend on the dissipation of optical power within an interacting metal structure.
ATR based devices depend on the coupling of an optical beam, which is incident upon a dielectric-metal structure placed in optical proximity, to a surface plasmon-polariton mode supported by the metal structure. At a specific angle of incidence, which depends on the materials used and the particular geometry of the device, coupling to a plasmon mode is maximised and a drop in the power reflected from the metal surface is observed ATR based modulators make use of this attenuated reflection phenomenon along with means for varying electrically or otherwise at least one of the optical parameters of one of the dielectrics bounding the metal structure in order to shift the angle of incidence where maximum coupling to plasmons occurs. Electrically shifting the angle of maximum coupling results in a modulation of the intensity of the reflected light. Examples of devices that are based on this architecture are disclosed in references [
23
] to [
36
]. Reference [
42
] discusses an application of the ATR phenomenon for realising an optical switch or bistable device.
Mode coupling devices are based on the optical coupling of light propagating in a dielectric waveguide to a nearby metal film placed a certain distance away and in parallel with the dielectric waveguide. The coupling coefficient between the optical mode propagating in the waveguide and the plasmon-polariton mode supported by the nearby metal film is adjusted via the materials selected and the geometrical parameters of the device. Means are provided for varying, electrically or otherwise, at least one of the optical parameters of one of the dielectrics bounding the metal. Varying an optical parameter (the index of refraction, say) varies the coupling coefficient between the optical wave propagating in the dielectric waveguide and the lossy plasmon-polariton wave supported by the metal. This results in a modulation in the intensity of the light exiting the dielectric waveguide. References [
37
] to [
40
] disclose various device implementations based upon this phenomenon. Reference [
41
] farther discusses the physical phenomenon underlying the operation of these devices.
These known modulation and switching devices disadvantageously require relative high control voltages and have limited electrical/optical bandwidth.
The aforementioned co-pending international patent application No. PCT/CA00/01525 disclosed a waveguide structure comprising a thin strip having finite width and thickness with dimensions such that optical radiation having a wavelength in a predetermined range couples to the strip and propagates along the length of the strip as a plasmon-polariton wave. The strip may comprise a materi
Adams Thomas
Healy Brian
Spectalis Corp.
Wood Kevin S
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