Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2001-07-20
2004-08-24
Stafira, Michael P. (Department: 2877)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S130000, C385S129000, C356S445000
Reexamination Certificate
active
06782179
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides a method and devices for controlling the propagation of Surface Plasmon Polaritons (SPPs) using Surface Plasmon Polariton Band Gap (SPPBG) regions. The SPPBG regions are regions of one or more interfaces supporting the propagation of SPPs on which SPPs experience a periodic modulation of the dielectric properties of the media into which its electromagnetic field extend. The frequency range of the band gap is determined by the period of the modulation. SPPBG regions prohibit propagation of SPPs having a frequency within its band gap.
By forming transmitting regions in the SPPBG regions the present invention provides ultra-compact SPP waveguides. The present invention can be utilised to form compact integrated SPP/optical circuits. Also, the present invention provides cavities supporting standing SPP-waves for field localisation. Such field localisation can provide very high field intensities and can be used in various sensor applications.
The devices of the present invention provide a number of advantages over photonic components since SPPs propagates on 2-dimensional interfaces, and only confinement in the plane of propagation is needed. This allows for a very simple production of the devices according to the present invention.
BACKGROUND OF THE INVENTION
Surface plasmon polaritons are quasi-two-dimensional electromagnetic (EM) modes propagating along an interface between a conducting and a dielectric material.
FIG. 1
shows an SPP propagating along the interface
4
between metal
2
and air
5
. The EM field amplitudes
6
decay exponentially in both neighbouring media in the directions perpendicular to the interface
4
, as illustrated in FIG.
1
.
Typically, SPPs are excited by matching the propagation of electromagnetic radiation from a laser beam to the propagation constant &bgr; of the SPP whereby the EM field can be coupled to SPPs.
FIG. 1
shows a schematic representation of the SPP excitation in the Kretschmann configuration at a glass-metal interface
1
or an air-metal interface
4
of a metal film
2
deposited on a glass substrate
3
. The angle of incidence &thgr; of the light through a glass prism
7
on the backside of the glass substrate
3
should be adjusted to satisfy the phase matching condition: &bgr;=(2&pgr;/&lgr;) n sin &thgr;, where n is the glass refractive index. The exact phase matching conditions determine which interface the plasmon is coupled to, but as the metal film
3
is typically much thinner than the transverse extension of the field amplitude
6
, the SPP can be considered as primarily propagating in the dielectric layers
3
and/or
5
and following the metal-dielectric interface
2
and/or
4
.
Several methods and devices for performing this coupling are known; e.g. prism couplers as illustrated in
FIG. 1
or described in U.S. Pat. No. 4,565,422 and grating couplers such as described in U.S. Pat. No. 4,567,147 and U.S. Pat. No. 4,765,705. The propagating SPPs can be converted back to photons again by making use of a similar device.
Some simple optical elements able to govern SPP propagation have been suggested by Smolyaninov et al. (Phys. Rev. B 56, 1997, 1601). These elements utilise diffraction and refraction of SPPs on surface defects according to the Huygens-Fresnel principle.
The existence of surface plasmon polariton band structure have been mentioned in a number of articles such as Scherer et al. (Journal of Lightwave Technology 17, 1999, 1928); Smolyaninov et al. (Phys. Rev. B 59, 1999, 2454); and Kitson et al. (1996) (Phys. Rev. Lett. 77, 1996, 2670). Such band structures arise from periodic structures fabricated at a metal-dielectric interface. When the excited SPP propagate along the periodic structure, the SPP propagation constant is periodically modulated resulting eventually in a “plasmonic band gap” effect.
Plasmonic band gaps structures in 2-dimensional crystals have been reported by Kitson et al (1996). The article describes the coupling of photons to SPPs on a textured interface using a prism as described in relation to
FIG. 1
of the present description, the texture describing a periodic hexagonal pattern. The reflection of the incident laser beam is a measure of the coupling of photons to SPPs on the interface and
FIG. 3
of the article illustrates the resulting reflectivity of the coupling region. Thus,
FIG. 3
shows that for photons having energy in the interval 1.91-2.00 eV, there is a poor coupling of photons to SPPs illustrating a plasmonic band gap for the corresponding SPPs.
A further article by Kitson et al. (J. Appl. Phys., 84, 1998, 2399) relates to reducing losses in microcavities using metallic mirrors (e.g. organic LEDs). The article proposes a method to avoid losses due to non-radiative coupling from microcavity modes to SPP modes in the metal mirrors. Using textured mirror surfaces, a band gap may be introduced, which prohibit coupling to SPPs having energies within the band gap (the prohibition of this coupling is described in detail in the article by the same authors in the previous section, Kitson et al. (1996)). Tuning the band gap to the microcavity mode will thereby reduce the coupling losses of the microcavity mode. The article describes a microcavity with a one-dimensional texturing of one of the metal mirrors.
Photonic band-gap (PBG) materials have been used for providing wave guiding, light localisation, low losses for bending and strong wavelength dependent light transmission. The photonic band gap effect relies on periodic scattering of light by a wavelength scale periodic structure of scatters similar to the effect experienced by electrons in atomic lattices, namely that the photon/electron energies are arranged into energy bands separated by gaps in which propagation states are prohibited. Existing PBG-based structures utilises 3-dimensional periodic structures which are typically difficult to fabricate and have so far very little design flexibility. Also, existing planar PBG based waveguides have high optical losses in the out of plane dimension.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and a device for guiding and localisation of electromagnetic radiation.
It is another object of the invention to provide a substantially 2-dimensional structure for guiding of electromagnetic radiation.
It is a further object of the invention to provide compact and low loss integrated optical circuits comprising passive and active components such as waveguides, bends, splitters, couplers, filters, multiplexers, de-multiplexers, interferometers, resonators, sensors, tuneable filters, amplifiers, switches, sensors, etc.
It is a still further object of the invention to provide compact and low loss integrated optical circuits, which can process signals faster than known optical circuits due to their smaller size.
It is a still further object of the invention to provide compact and low loss integrated optical circuits, which are easy and cheap to fabricate.
It is a still further object of the invention to provide localised high intensity electromagnetic fields for use in sensor applications.
The present invention fulfil these objects by providing a method and a device providing a controlled propagation of Surface Plasmon Polaritons (SPPs) in Surface Plasmon Polariton Band Gap (SPPBG) structures. By leaving channels in SPPBG structures free from periodic modulation, the present invention provides ultra-compact waveguides in SPPBG structures, that is, an energy/frequency dependent guiding of the SPPs which can be utilised to form compact integrated SPP/optical circuits.
Thus, the present invention is based on processing light signals in a 2-dimensional system by guiding and/or localising corresponding SPP fields. The coupling of light signals to interface propagating EM fields (SPPs) can be done with a close to 100% conversion efficiency in a thin conducting film using gratings or prism couplers. The SPPs have a wavelength very similar to that of photons and the same signal processing opportunities using band
Bozhevolnyi Sergey
Østergaard John Erland
Harness Dickey
Micro Managed Photons A/S
Stafira Michael P.
Valentin II Juan D
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