Field of Invention

This invention relates in general to the field of waveguides and more specifically to the field of waveguides for surface plasmons. Background of the Invention

Surface plasmon polaritons (SPPs) have been attracting an increasing amount of attention recently for the realization of sub-diffractive optics (see: W. L. Barnes, A. Dereux, and T. W. Ebbesen, "Surface plasmon sub-wavelength optics," Nature 424, 824-830 (2003)) and noninvasive bio-sensors (see: S. LaI, S. Link, and N. J. Halas, "Nano-optics from sensing to waveguiding," Nature Photonics 1, 641-648 (2007)). In particular, single-interface-guided SPPs offer strong confinement in an asymmetrical geometry as well as a noninvasive metal contact surface, and therefore are a favorable choice for use in sensing, especially in the biological area. Conventional excitation methods for single interface SPPs utilize momentum matching through prisms (see: A. Otto, "Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection," Zeit. fur Physik 216, 398-410 (1968)) or gratings (see: H. Raether, Surface plasmons on smooth and rough surfaces and on gratings, (Springer, New York, 1988)). Both of these known excitation methods can be difficult to integrate into a compact sensing platform.

Plasmonic waveguides offer guiding of SPPs (see: Berini P, "Plasmon-Polariton Waves Guided by Thin Lossy Metal Films of Finite Width: Bound Modes of Symmetric Structures", Physical Review B 61, 10484-10503, 2000; Bozhevolnyi S I, Volkov V S, Devaux E, Laluet J-Y and Ebbesen T W, "Channel plasmon subwavelength waveguide components including interferometers and ring resonators," Nature 440, 508-511, 2006; Oulton R F, Sorger V J, Genov D A, Pile D F P, and Zhang X, "A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation", Nature Photonics 2, 496-500, 2008; Berini P, Charbonneau R, Lahoud N, "Long-range surface plasmons on ultrathin membranes", (ACS) Nano Letters, 7, 1376-1380, 2007; Arbel D, Orenstein M, "Plasmonic modes in W-shaped metal-coated silicon grooves", Optics Express 16, 31 14-3119, 2008; and Srivastava T and Kumar A, "Propagation characteristics of channel plasmon polaritons supported by a dielectric filled trench in a real metal," Journal of Applied Physics 106, 043104/1-5, 2009). Due to the chemical stability provided by the metal, SPP waveguides may be compatible with biological materials, and a high sensitivity offered by a field concentration near the surface of the metal may cause them to be particularly suited for sensing. Such SPP waveguides have been adapted for various sensing platforms (see: Ruiyuan Wan, Fang Liu, and Yidong Huang, "Ultrathin layer sensing based on hybrid coupler with short-range surface plasmon polariton and dielectric waveguide," Opt. Lett. 35, 244-246, 2010; Yang Hyun Joo, Seok Ho Song, and Robert Magnusson, "Long-range surface plasmon-polariton waveguide sensors with a Bragg grating in the asymmetric double-electrode structure," Opt. Express 17, 10606-10611, 2009; and Pierre Berini "Bulk and surface sensitivities of surface plasmon waveguides" New J. Phys. 10 105010, 2008).

Examples of waveguides for SPPs include the classic Otto and Kretschmann methods, which use prisms for momentum-matched excitation and therefore are bulky. An integrated platform is preferable for modern applications, where compactness and cost-effectiveness are in demand. Modern studies have shown many coupled-interface plasmon polariton waveguide structures, which can be categorized into metal-insulator-metal (see: S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J. -Y. Laluet, and T. W. Ebbesen, "Channel plasmon subwavelength waveguide components including interferometers and ring resonators," Nature 440, 508-511 (2006)) and insulator-metal-insulator geometries (see: Berini, P., "Plasmon-Polariton Waves Guided by Thin Lossy Metal Films of Finite Width: Bound Modes of Symmetric Structures", Physical Review B 61, 10484-10503, (2000)). However, none of these structures simultaneously meets the demands for sensing of biological cell-surface receptors. These demands include large dimension (> 10 μm cell diameter) and direct cell contact with the field of SPPs that are tightly bound to a single metal-dielectric interface. This unique type of SPP will henceforth be referred to as Interface Plasmon Polaritons ("IPPs").

IPPs are free electron oscillations that have been shown to exist at the surface of bulk metal, away from which the energy decays rapidly (see: H. Raether, Surface plasmons on smooth and rough surfaces and on gratings, (Springer, New York, 1988)). Because the field is localized, IPP oscillation is highly dependent on the material composition in the vicinity of metal surface. Therefore, IPPs are great "agents" for sensor applications. The Channel Plasmon Polaritons ("CPP") concept is another example of waveguides developed for SPPs, as previously published in I. V. Novikov and A. A. Maradudin, "Channel polaritons," Physical Review B 66, 035403/1-13 (2002); and S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J.- Y. Laluet, and T. W. Ebbesen, "Channel plasmon subwavelength waveguide components including interferometers and ring resonators," Nature 440, 508-511 (2006). Other reports have also demonstrated confinement of SPPs in other channel-like structures (see: T. Nishikawa, H. Yamashita, R. Hasui, R. Masuda, S. Fujita, and Y. Okuno, "Development of localized surface plasmon resonance sensor based on nanoimprinting techonology", Plasmonics and Metamaterials (META), Rochester, New York, United States, October 19-24 2008, MWA7; J. Dintinger and O. J. F. Martin, "Channel and wedge plasmon modes of metallic V-grooves with finite metal thickness," Optics Express 17, 2364-2374, (2009); and D. M. Koller, A. Hohenau, H. Ditlbacher, N. Galler, F. Reil, F. R. Aussenegg, A. Leitner, E. J. W. List, and J. R. Krenn, "Organic plasmon-emitting diode," Nature Photonics 2, 684 - 687 (2008)). A common element of known CPPs is that these have been developed to attend to submicron features. In such known CPPs the fundamental mode is strongly localized on the bottom of the channel, and therefore these CPP waveguides have short propagation distances of ~ 100 μm and make the interaction of the plasmon field with the molecules difficult. The submicron CPP waveguides can also create drag force on fluidic flow, and therefore are difficult to implement as sensors. Additionally, these CPP waveguides have been costly to fabricate using current technologies, such as a focused ion beam. Summary of the Invention

In one aspect, the present disclosure relates to a waveguide for guiding one or more interface plasmon polaritons, characterized in that it comprises: one or more channels to confine the one or more interface plasmon polaritons, have an input and an output; at least one substrate coating applicable to the one or more channels, said one or more channels coated with at least one substrate coating being operable to guide the one or more interface plasmon polaritons in at least one of the following modes: one or more transverse electric polarized modes and one or more transverse magnetic polarized modes; and at least one of the one or more channels being shaped into y-junction, said y-junction being operable to guide one or more of the one or more interface plasmon polaritons polarized by the one or more transverse electric polarized modes or polarized by the one or more transverse magnetic polarized modes. In another aspect, the present disclosure relates to a method of forming a waveguide for guiding one or more interface plasmon polaritons, characterized in that it comprises the following steps: calculating specific dimensions for the waveguide in accordance with one or more characteristics of the one or more interface plasmon polaritons to be guided in the waveguide; etching one or more channels into a substrate, said channels being of a size and a shape corresponding to the specific dimensions calculated for the waveguide; and forming at least one of strips or holes in a surface of the one or more channels.

In yet another aspect, the present disclosure relates to an interface plasmon polaritons mode- matching method of a waveguide characterized in that it comprises the steps of: exciting one or more interface plasmon polaritons in one or more channels of the waveguide by an excitation means; capturing the output of the waveguide by way of an output means, said output means collecting output data; and distinguishing two or more polarized outputs of the waveguide, as captured by the output means, by way of a distinguishing means, said two or more polarized outputs including at least a transverse electric polarized output and a transverse magnetic polarized output, and said distinguishing means collecting distinguishing data.

In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

Brief Description of the Drawings

The invention will be better understood and objects of the invention will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:

FIG. 1 shows simulated and measured dispersion of transverse magnetic (TM) fundamental modes guided in IPP waveguides summarizing the dimension of the two waveguides.

FIG. 2 is a Scanning Electron Microscope (SEM) image of the top view of the IPP waveguide. FIG. 3(a) shows the output intensity of an IPP waveguide the output having polarization.

FIG. 3(b) shows the output intensity of an IPP waveguide of the transverse electric (TE) polarized field.

FIG. 4(a) shows output modes of native uncoated samples observed from the IPP waveguides at 1 mm in length.

FIG. 4(b) shows output modes of samples coated with ~ 6 μm of poly(methyl methacrylate) (PMMA) observed from the IPP waveguides at 1 mm in length.

FIG. 4(c) shows output modes of native uncoated samples observed from the IPP waveguides at 200 μm in length. FIG. 4(d) shows output modes of samples coated with 60 nm of PMMA observed from the IPP waveguides at 200 μm in length.

FIG. 4(e) shows output modes of samples coated with ~ 6 μm of PMMA observed from the IPP waveguides at 200 μm in length.

FIG. 5 shows simulated waveguide dispersion as a function of the PMMA thickness. FIG. 6 shows two sideviews (a and b) of IPP waveguides FIG. 7 shows output of an IPP waveguide. FIG. 8 shows an optical microscope image of the top view of the waveguide results.

FIG. 9, section (a) is a view of an IPP guiding interface, section (b) is a view of an IPP guiding interface having reflective surfaces, and section (c) is a schematic view of a waveguide structure. FIG. 10(a) shows waveguide dispersion of the fundamental, second order and third order modes. FIG. 10(b) shows the magnitude of the Hx field in the cavity. FIG. 11 shows a SEM topview of the Bragg plasmonic waveguide. FIG. 12 shows the reflection spectrum of air- filled Bragg waveguides, having varying periodicities, Λ _Brag g = 765 nm, 770 nm, and 775 nm.

FIG. 13 shows the reflection spectrum of oil-filled Bragg waveguides.

FIG. 14(a) shows varying-density hole-array formed Bragg gratings in a trapezoidal waveguide embedded with gratings.

FIG. 14(b) shows varying-density hole-array formed Bragg gratings in a view of an AFM surface morphology of the density-hole array.

FIG. 15 shows the reflection spectrum of varying-density hole-array Bragg waveguides having varying periodicities. FIG. 16 shows a metal bump dispersive element design.

FIG. 17 shows simulation results of the periodic bump layout reflection spectrum. FIG. 18 shows rectangular waveguide dispersion curves in a vacuum.

In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended as a definition of the limits of the invention.

Detailed Description of the Preferred Embodiment

The present invention is a waveguide structure that facilitates guiding of IPPs, a means of creating the waveguide structure, and a method of utilizing the waveguide structure for IPP mode-matching. The structure of the waveguide of the present invention, and its channels in particular, may be lithographically defined to provide confinement to IPPs. The shape and size of the channel may cause the waveguide to function to guide IPPs in both transverse electric ("TE") and transverse magnetic ("TM") polarized modes. The waveguide may be utilized to facilitate a mode-matching method to excite IPP modes. The waveguide of the present invention may both guide electromagnetic waves and channel analyte material. The IPP waveguide of the present invention may incorporate a channel. This channel may be of a size capable of accommodating IPPs. In the waveguide of the present invention IPPs may be guided at each interface and may be mostly decoupled. As a result the IPPs may have larger propagation distances than known prior art, such as, for example a millimeter. Reactive ion etching may be utilized to control the cross-section shape of the channel. This may extend the interaction region and may also improve the sensitivity of the waveguide in any sensing applications that the waveguide may utilized to achieve.

In another embodiment of the present invention a y-junction may be applied in the waveguide. The y-junction may include a y-junction power splitter that may operate to cause TE polarized IPPs to propagate along the outer walls of the branches. This may cause the TE fields guided on the side walls of the initial input waveguide may to be decoupled.

The IPP mode loss and polarization of the present invention may be highly sensitive to near surface material changes. Due to these inherent properties of the present invention, the waveguide may function as an integrated sensor device. The waveguide of the present invention may be of a variety of configurations and may be configured through a variety of means. One embodiment of the waveguide of the present invention may be a simple waveguide etched into material that is not costly, such as, for example silicon. The etching may be achieved using a combination of photolithography and reactive ion etching. The material utilized in the channel may allow for variation of the angle of the sidewall. Such variation may facilitate control of waveguide dispersion of IPPs. The etching may further be created to cause the waveguide to meet certain specific dimension measurements. The specific dimension of the waveguide may have several results, including cause the waveguide to alter the phase and/or group velocity of the electromagnetic fields, and thereby to control the speed of the electromagnetic field momentum. The formation of the waveguide may also include the formation of strips or holes on the waveguide. For example, a coarse waveguide surface may be produced. As described in more detail in the Example section of this patent application, specific bumps and/or gratings may be produced in the waveguide surface. Such bumps or gratings may be formed to a specific configuration though processes, such as, for example heating and/or spinning the waveguide. Another embodiment of the waveguide of the present invention may be created to include a coating. The coating may produce several results. For example, the coating may enhance the excitation optics setup of a simple channel waveguide. Such a waveguide may facilitate a hybrid dielectric-plasmon mode by way of a thin-dielectric-film coating. Yet another embodiment of the waveguide of the present invention may be metalized. Such an embodiment may be formed of silver and/or gold. Such materials may be applied using e-beam evaporation or other means.

An embodiment of the present invention may be utilized for IPP mode-matching. The shape and size of the channel of a waveguide may be critical to the utilization of the waveguide. For example, an embodiment of the waveguide of the present invention may cause the waveguide to function to guide IPPs in both transverse electric ("TE") and transverse magnetic ("TM") polarized modes. Such an embodiment of the waveguide may be utilized to facilitate a mode- matching method to excite IPP modes and may both guide electromagnetic waves and channel analyte material. In one embodiment of the waveguide of the present invention a method of exciting IPPs at each interface may be applied. The excited IPPs may have a large mode size. A variety of electromagnetic energy sources and detectors may be utilized with the present invention. The waveguide may facilitate an efficient mode-matching method to excite IPP modes. In the present invention mode-matching excitation may be applied to setup a more compact sensor platform. Additionally, TE and TM polarized light may both be utilized to excite the waveguide. For example, one embodiment of the present invention may include a channel 10 having the general shape shown in FIG. 6. The channel may have a depth, such as, for example approximately -10 μm, and a bottom width, such as, for example approximately ~5 μm. The sides of the channel 12 may be sloped so that the distance between the sides is shorter at the bottom 14 of the channel than the distance between the sides at the top of the channel. The sides of the channel may be of other configurations as well. The bottom of the channel may be virtually flat, as shown in FIG. 6, or may be of other configurations. A skilled reader will recognize that a channel of other shapes and sizes may also be incorporated into the present invention. The present invention may offer several advantages over the prior art. Known prior art examples have been demonstrated with submicron features, where the fundamental mode is strongly localized around the walls, and therefore they have short propagation distances of ~ 100 μm. For example, CPPs, such as that of Nishikawa et al. (see: T. Nishikawa, H. Yamashita, R. Hasui, R. Masuda, S. Fujita, and Y. Okuno, "Development of localized surface plasmon resonance sensor based on nanoimprinting techonology", Plasmonics and Metamaterials (META), Rochester, New York, United States, October 19-24 2008, MWA7), includes a grating platform that consists of geometric trenches that are structures that are nanometers n size. The Nishikawa CPP uses the momentum matching method to excite localized IPPs. Such prior art waveguides have small propagation distance and require a setup that is bulky. Also these prior art waveguides have been costly to fabricate using a focused ion beam. The present invention offers several benefits over the prior art, such as facilitating extended propagation distances, being less costly to fabricate, and having a compact optics setup.

The present invention also provides a benefit over known CPP prior art examples that confine IPPs in channels that have a depth of ~1 μm and a -20° angled opening in bulk metal substrates. In these geometries, the fundamental CPP mode is strongly confined at the bottom of the channel and into the metal. This creates a short propagation distance of ~100 μm. The present invention facilitates a longer propagation distance, such as for example ~1 mm.

Another benefit of the present invention over the prior art is reflected in the cost of materials. Prior art waveguides are constructed of costly materials, such as semi-infinite silver substrate. To reduce the cost of fabrication of the present invention, as opposed to the cost of prior art waveguides, the channel of the present invention waveguide may be etched into a less costly material, such as, for example {100} silicon substrate using a combination of photolithography and reactive ion etching (RIE) (see: R. Legtenberg, H. V. Jansen, M. J. de Boer, and M. C. Elwenspoek, "Anisotropic reactive ion etching of silicon using SF ₆ ZCVCHF ₃ gas mixtures," Journal of the Electrochemical Society 142, 2020-2028 (1995)). The present invention waveguide may also be metalized, such as, for example with silver using e-beam evaporation. In one embodiment of the present invention the silver may have an approximately 300 nm thickness, well in excess of the penetration depth of approximately 1550 nm excitation wavelength. A skilled reader will recognize that the waveguide may be metalized in other manners as well.

Another embodiment of the present invention may be an IPP waveguide having dielectric surface cladding. The dielectric layers may consist of air and poly(methyl methacrylate) (PMMA) 16, as shown in FIG. 2. A skilled reader will recognize that the dielectric layers may consist of other materials as well. The dielectric layers may further have dielectric permittivity, such as, for example of approximately 1 and approximately 2.25, respectively. The dielectric layer may be modified to form one or more resonant structures. For example, a Bragg resonator, Fabry-Perot resonator, ring resonator, or other resonators may be formed. Such resonators may be utilized to sense the presence of analyte. One or more resonators may also be utilized as elements integrated with the waveguide.

Other embodiments of the present invention may be formed of different materials. Variations of the materials may affect the function of the waveguide and permit the calculations and data derived therefrom. For example, one embodiment of the present invention may be utilized to study the effect of a near surface index change. A waveguide of the present invention created for this purpose may spin-coat PMMA 950A2 (such as, by MicroChem Corp.) above the metal surface of the channel at 6000 rpm. The structure may be baked on a hotplate at approximately 180 ⁰ C for approximately 90 seconds. This process may create a waveguide having a PMMA layer of an approximate thickness of ~ 60 nm. A skilled reader will recognize that other waveguides may be created having a thicker PMMA layer through a variation of this process. For example, the channel of the waveguide may be filled with PMMA and then, baked at approximately 180 ⁰ C for approximately 300 seconds. This may create a PMMA layer with thickness above 6 μm inside the channel. A skilled reader will recognize that a variety of methods and processes may be applied to form the channel of the waveguide of the present invention.

One embodiment of the present invention may include: improved waveguide loss compared to the conventional CPP waveguides; distinguished polarization-supporting geometry; improved integrate-ability of IPP waveguides with micro fluidic channels; and sufficient mode matching for end-fire coupling. A skilled reader will recognize that a variety of waveguide structures may be embodiments of the present invention. An example of waveguide structures formed based on IPP theories is presented below. A skilled reader will recognize that this example does not limit the scope of the possible embodiments of the present invention in any way. Theory-based Waveguide Structures

Another embodiment of the present invention may be formed in adherence with one or more IPP theories. For example, one IPP theory holds that simple IPPs can be excited on bulk silver surface. A waveguide formed to support this theory 18 is shown in FIG. 9. In such an embodiment of the present invention, IPPs may be polarized in the y-axis and have propagation constant _{βipp = k} jε, _n εj(ε _{m + ειl} ) ₌ 4 O7χio ⁶ +389χ lo ² , , where ε _m , _d represent the relative permittivities of metal (e.g., ε _s ,i _ver = -129 +3.16i at 1550 nm - see: P. B. Johnson and R. W. Christy, Optical constants of noble metals," Physical Review B 6, 4370-4379, (1972)) and dielectric (e.g., ε _aa = 1) respectively, and k ₀ may be the momentum of light in vacuum. For the x-axis confinement, the embodiment may include two reflective walls placed on the x-axis 20, as shown in FIG. 9, where all the slopes and surface features of all sides may be modified. As shown, a cavity may be created, and standing waves may be supported in the cavity. The standing waves may be modeled as electromagnetic waves confined in an enclosed metallic resonator, said standing waves having estimated x-axis momentums, A = mπjα , where m is the mode order and α is the width of the cavity. Each mode, propagating in the z-direction, may have dispersion relationship, tan (hα) = (see: A. Yariv and P. Yeh, Photonics: Optical Electronics in Modern Communications (Oxford University Press, 2006), Chapter 3). A similar method of analysis may be applied to analyze the IPPs guided on the wall of the channels. A skilled reader will recognize that the analysis methods of the present invention are not limited to the methods presented.

This embodiment is an IPP waveguide structure having dispersion modeled with an adaptation of the effective index method (EIM). The physical structure design of such an IPP waveguide 22 is shown in FIG. 9, where the IPP may be excited at the bottom surface, and the metal walls may force a near-zero field outside of the cavity. A skilled reader will recognize that the slopes and surface features of the walls of the present invention may vary. It may be possible to measure the IPP flow or movements, or other aspects of the waveguide function, to derive data therefrom. Such data may be utilized to derive further information, such as, for example by the application of one or more algorithms to the data. The further information may relate to phase velocity, group velocity, nonlinear effects, higher order modes, polarization dependence, interference or other information. For example the trust-region-dogleg algorithm may be applied to such data to find the solutions to the slab waveguide transcendental equation.

FIG. 18 shows an example of possible waveguide dispersion properties of the first three lowest order modes. The solid curves (EIM) 72 may represent the real part of the dispersions of the first three lowest order modes of the 1.5 μm wide SPP waveguide, where the fundamental mode has the largest propagation constant. The dotted-line curves (EIM) 74 may represent the imaginary part of the dispersions of the three modes. The fundamental mode index curve in this example is labeled by "1", the second order mode index is labeled by "2", and the third order mode index is marked by "3". In this example, the black line may represent the real part of the mode indices, and the red lines may represent the imaginary mode indices. FIGs 10(a) and 10(b) show the results of the application of the trust-region-dogleg algorithm to data from an EIM waveguide embodiment of the present invention.

A skilled reader will recognize that more than one algorithm may be applied to the data, and the results from the algorithm applications may be compared. For example, to verify an EIM waveguide, it may be possible to utilize MODE (such as is developed by Lumerical Solutions, Inc.) to solve approximately 8 μm deep channels, as shown in FIG. 9. The MODE-solved results are shown as plotted in FIGs 10(a) and 10(b), where they may be compared to the EIM calculations. In FIGs. 10(a) and 10(b), the effective index of a free propagating plasmon at a simple interface, βι _PP lk _o , is represented by black horizontal lines 24 and 26. The comparison in this example shows that EIM solutions are nearly identical to the values calculated using MODE solutions. The ~ 1 % propagation distance discrepancy is a result of the fact that the k-vector in the y-direction is altered by the geometric setup of the channel 22, as shown in FIG. 9. This comparative study of the results calculated using EIM and MODE may validate a theory such as a theory on the proposed guiding mechanisms of the IPP waveguides. Finite element mode (FEM) methods may also be applied to model the properties of the waveguides. Transformation of the current EIM model into another domain may also be employed to analyze similar problems in a variety of geometries.

Other embodiments of the waveguide of the present invention may be formed in accordance with calculations and data, such as those described in this section. For example, FIG. 2 shows a waveguide that may possess structural similarity to the rectangular channel waveguide 22 shown in FIG. 9, which meets requirements discovered through theory-based calculations. Therefore, the guided mode profile shown in FIG. 2 may be obtained by way of experiments involving theory, data and calculations, such as algorithms. Such a channel shape may be controlled lithographically to match the surface curvature of a cell to be utilized with the waveguide, particularly when the waveguide is utilized as a sensor. The result of the control achieved by controlling lithographically the channel shape to match the surface curvature of a cell to be utilized with the waveguide may be maximization of the physical contact of the channel and the cell to facilitate an improved detectability.

The result of such a method may be an embodiment of the present invention, such as a large- dimension IPP waveguide that can be shaped to match the cell surface curvature as well as to modify the dispersion. This waveguide may have reasonably long propagation distance and may not have a stringent demand on fabrication accuracy. The waveguide embodiment may be modeled using an adapted EIM, which shows good agreement with simulated data. A skilled reader will recognize that this is but one embodiment of the present invention and that other embodiments are also possible.

Waveguide Structures and Integrated Optics Elements

Another embodiment of the present invention may be a waveguide that may facilitate a y- junction. Such a waveguide may have a channel fabricated on a silicon substrate using a combination of photolithography and reactive ion etching (RIE) to produce a channel, as shown in FIG. 6. The RIE process may allow for variation of the angle of the side walls. The trenches of the channels and/or whole channels may be metalized by depositing a 300 nm thick silver or gold film using e-beam evaporation. The thickness may be chosen to ensure that the IPP remains isolated from the silicon substrate. As shown in FIG. 7, the formation of the channel 28, including its shape and materials, may affect the polarization of the IPP. In another embodiment of the present invention a y-junction power splitter may be applied in the waveguide, as shown in FIG. 1. As shown in FIG. 8, which is an optical microscope image of the top view of the waveguide results 30, a lensed fiber may be used for IPP excitation. FIG. 8 further shows the input waveguide mode and corresponding branched output fields for both TE and TM polarized IPPs 32. For the TE polarized IPPs, the output modes may only propagate along the outer walls of the branches. Consequently, TE fields guided on the side walls of the initial input waveguide may be decoupled.

Other embodiments of the present invention may include a waveguide that supports Mach- Zehnder interferometers and multimode interferometers, and other types of interference-based optics.

Waveguide Excitation and Detection Method

An application of the waveguide of the present invention may involve excitation by an excitation means. It may further involve an input means to couple with light, such as, for example laser light, guided into the waveguide. An output means may also be positioned at the output of the waveguide to capture and/or calculate the output of the waveguide. Such outputs may include two or more polarized outputs that may be observed and distinguished by a distinguishing means.

In accordance with the waveguide of the present invention, TE-excited IPPs may be guided on the two side walls of the channel. The waveguide may also allow TM-excited IPPs to be positioned at the bottom of the channel. The collective metal electron oscillations of IPPs may be perpendicular to the metal surface. Thus the orientation of the metal-dielectric interface may dictate the "polarization" of the IPPs. "Polarized" IPPs may be advantageous compared to the previously known prior art TE-only CPP structures for sensing applications where selective excitations of particles are required. In the present invention, the IPPs may form a localized field that hugs the curvature of the surface, such as, for example a metalized surface, of the waveguide channel. The IPPs may be bound tightly to the metalized surface and may appear to have a curved mode profile, which is a combined effect of IPP polarized in all orientations. In another embodiment of the present invention, one or more waveguides may be excited by end- fire coupling using a lensed single mode fiber that has a focal spot diameter of ~ 3.3 μm. To observe and distinguish the polarized outputs of IPP waveguides, a polarization beam splitter may be placed prior to the detector in the waveguides. The output facet of the waveguides may be magnified by a microscope objective lens and then imaged using a camera.

A waveguide mode is shown in FIG. 7, wherein the plasmon field hugs the curvature of the metalized surface and the field is confined within the channel. The TE-polarized light may be guided on the side walls of the channel. Therefore, by placing the power meter at the appropriate orientation of the polarization beam splitter, it may be possible to characterize the TE and TM waveguide properties separately.

In one embodiment of the present invention the excitation means may be a laser source, such as, for example, a 5 mW tunable laser source, tunable between approximately 1460 nm and approximately 1600 nm. A lensed single mode fiber (SMF) with focal diameter of ~ 3.3 μm may be used at the input to couple the laser light into the IPP mode as an input means. At the output, a 60 x microscope objective lens and a near infrared (NIR) camera may be applied to capture the mode intensity as an output means. It may be possible to observe and distinguish the polarized outputs of the waveguides through a variety of means. For example, a polarization beam splitter (PBS) may be placed prior to the detector as a distinguishing means. A skilled reader will recognize that other excitation means, input means, output means and distinguishing means may be applied in the present invention.

In an embodiment of the present invention, by tuning the laser from 1460 nm to 1600 nm, it may be possible to characterize the waveguide dispersion. Results for two waveguides with different dimensions may be achieved. As shown in FIG. 1, for example, one waveguide may have a = 12 μm, b = 7 μm, and h = 6 μm, and the other waveguide may have dimensions a = 18 μm, b = 13 μm, and h = 6 μm. The two waveguides may have the same tilt on the side walls, θ = 22°. The MODE-solutions (such as, by Lumerical Solutions, Inc.) dispersion relationships may be compared 36 and may exhibit a similar trend.

FIGs. 3(a) and 3(b) show, the mode profile of a polarized IPP isolated using a PBS in a typical 1- mm long waveguide. FIG. 3(a) shows the output intensity of an IPP waveguide having both polarization; whereas FIG. 3(b) shows only the TE polarized field. The scale bars for FIG. 3(a) 38 and 3 (b) 40 are lO μm.

Yet another embodiment of the present invention may involve IPP modes performed using MODE solutions (such as, by Lumerical Solutions, Inc.). Such embodiments may utilize an excitation wavelength, such as, for example of approximately λ = 1400 to 1600 nm. In this wavelength range, silver dielectric permittivity may be modeled using a bulk plasma frequency, f _p =1.39χ l0 ¹⁶ [Hz], an electron relaxation time, r = 3.41χ lθ ^"14 [s], and a high frequency permittivity, ε _∞ = 2 (see: P. B. Johnson and R. W. Christy, "Optical constants of noble metals," Physical Review B 6, 4370-4379, (1972)). FIG. 1 shows the loss dispersion for the TM fundamental modes that may be obtained. Loss dispersion of two waveguides having different geometries may occur, as shown in the inset images (a) and (b) of the table of FIG. 1. The definitions of dimension labeling are marked in the schematic of FIG. 1 to further define the parameters of the simulation.

A comparison of the fields, as shown in FIG. 1 , inset images (a) and (b), may indicate that the fields spreading along the sample surface and at the bottom of the channel can both be eliminated by the PBS. Loss dispersion, as shown in FIG. 1, may be plotted for one or more waveguides.

The effect of a near-surface index change may be explored using the present invention. For example, using waveguides having lengths of both 1 mm and 0.2 mm. FIGs. 4(a)-(e) show output mode profiles of waveguides coated with 0-nm-thick, ~ 60-nm-thick, and ~ 6-μm-thick layers of PMMA. The native uncoated samples, as shown in FIG. 4(a) and FIG. 4(c), may have the lowest loss. This may be due to the low-refractive-index air that loosely confines the mode. In contrast, waveguides coated with 6 μm of PMMA, as shown in FIG. 4(b) and FIG. 4(e), may have a less intense output, therefore, such waveguides may be function as higher-loss waveguides. Reflection and scattering may occur in the waveguides, as shown in FIGs 4(c) - 4(e), due to the slanted and rough nature of the output facet. The scale bars for FIG. 4(a) 42, 4(b) 44, 4(c) 46, 4(d) 48 and 4(3) 50 are 10 μm.

Atypical waveguiding properties may further occur in a waveguide of the present invention, such as, for example a waveguide coated in 60 nm PMMA. It is possible that the loss may drastically increase and the output power may become negligible after a propagation distance greater than 1 mm. As shown in FIG. 5, the real part of the effective index and the propagation distance are plotted as a function of the PMMA thickness 52. For the TM fundamental mode, a minimum in the propagation distance for a ~ 250 ran thickness may result, and then the high loss may recover as PMMA fills the channel. This dielectric-thickness dependent loss property means that the IPP structure of the present invention may detect the presence of a near-surface index change. This is an important functionality that can be utilized for sensing applications.

A skilled reader will recognize that the present invention may have application as a variety of sensing applications, for example, such as the following sensors, or other sensing applications. a. Biological sensor: The waveguide may be designed to channel material and provide electromagnetic guidance. Analytes may be exposed to the channel simultaneously with excitement of the IPP. Changes to the IPP property may indicate analyte presence. b. Chemical sensor: The waveguide may include channel surfaces that are exposed to a chemical. Such chemical may induce changes in IPP properties. c. Electromagnetic field enhancement: The waveguide may be formed to facilitate slow phase velocity, and consequently for building up one or more electromagnetic fields. Applications of the energy of such an embodiment of the present invention may be one or more lasers or one or more light sources. d. Fluidic channels: The waveguide may have channels coated with metal that facilitate easy surface conjugations. The chemistry and physics of such a waveguide embodiment may be easily modified for the purpose of integration into other technologies.

As shown in FIG. 5 insets (c) and (f), unconventional IPP tight confinement is achieved between the upper dielectric layers and the metal. As a consequence, the mode may be lossy and may not be observed for waveguides longer than 315 μm. An embodiment of the present invention may facilitate polarization rotation or polarization dependence. In an embodiment of the waveguide, as shown in FIG. 4(c), both TE and TM modes may be excited. Stronger TM polarized mode may result due to higher mode overlapping between the TM mode and the fiber mode. The mode detected at the output of the waveguide, which may be coated with 60 nm of PMMA may be TE polarized, as shown in FIG. 4(d). As the waveguide is filled with PMMA the detected mode may become TM polarized, as shown in FIG. 4(e). This effect may be explained by the differences in PMMA thickness. PMMA deposited on the bottom of the channel may not be able to escape during spin-coating, therefore the PMMA may have a thickness that is larger than 60 nm. This PMMA thickness may cause the TM modes to be more lossy. Another possible cause of the polarization dependence may be the coupling of the TE and TM modes. The polarization-dependence may facilitate a revelation of the uniformity of surface index change. Examples

A skilled reader will recognize that a variety of embodiments of the present invention may be possible. Three examples are described below. These merely represent examples of possible embodiments of the present invention. In particular, these examples represent possible modes of creating dispersive elements in accordance with the present invention. In particular, such dispersive elements may be utilized to cause an IPP waveguide function as a sensor. A skilled reader will recognize that the examples provided herein do not limit the scope of the present invention.

As discussed above, embodiments of the present invention may include several types of waveguide shapes and sizes. The examples below discuss embodiments of the present invention that are a trapezoid waveguide. A trapezoid waveguide is merely one embodiment of the present invention and other embodiments are also possible. Additionally, the surfaces incorporated in the waveguide may be smooth or of varying levels of coarseness. Coarse surfaces may further incorporate a variety of configurations. The examples below discuss embodiments of the present invention incorporating coarse surfaces in the form of gratings or bumps. These discussions are provided merely as examples, and other waveguides surfaces may be incorporated into other waveguide embodiments of the present invention as well.

Generally, Example I and Example II relate to trapezoid waveguides. The examples of trapezoid waveguides described in Example I and Example II may be fabricated upon a silicon substrate. The method of fabrication employed in Examples I and Example II may be as follows. Notably, this method of fabrication is presented as an example and a skilled reader will recognize that this method does not limit the scope of the present invention.

A silicon surface may be coated with an 800-nm thick Shipley Sl 811 photoresist. After baking the sample at 115 Celsius for 90 seconds, the waveguide pattern from a shadow mask may be transferred to the photoresist after a 5-second UV exposure (e.g., 35 mW/cm2 at a 365 nm wavelength and 58 mW/cm2 at a 405 nm wavelength). The photoresist may be developed in a Shipley MF-321 developer for ~ 40 seconds, followed by a 1 -minute deionised water rinse.

The sample may be air dried with N2 gas and baked at 100 Celsius for more than 10 minutes. Reactive ion etching (RIE) may be performed on the silicon (see: Legtenberg R, Jansen H V, de Boer M J and Elwenspoek M C 1995, "Anisotropic reactive ion etching of silicon using SF6/O2/CHF3 gas mixtures," Journal of the Electrochemical Society 142, 2020-2028). A Phantom etcher (such as Trion Technology) may be set to supply SF6, O2, and CHF3. The sample may undergo backside cooling with helium.

The RIE process may allow a person fabricating the waveguide of the present invention to control the channel wall angle, θ, as well as depth, h. By varying the gas flow and etching time, the samples produced may have varied wall angles and varied depths. The trenches may be metalized by depositing a chromium adhesion layer, followed by a silver coating using e-beam evaporation. The silver thickness may be chosen to ensure that the SPPs are isolated from the silicon substrate, such that the plasmon propagation may not be allowed to couple to the bottom silicon-silver interface.

Example I

One possible embodiment of the present invention is a structure embedded with corrugated surfacing. Such an embodiment may include a Bragg grating embedded trapezoidal SPP waveguide. Such a structure may be designed, fabricated, and characterized in air and/or under any material system, such as, for example under index matching oil. Testing has shown that the resonance may have a 1100 nm/RIU sensitivity and may be validated by calculation.

An embodiment of the present invention that is a trapezoidal channel SPP waveguide may offer a benefit over the prior art. Unlike conventional SPP-waveguide enabled sensors, the trapezoidal channel SPP waveguide may offer a dual-functionality. The trapezoidal channel SPP waveguide may be operable to both guide SPPs and channel analytes all in one structure. Known prior art waveguides are not capable of performing both guiding and channeling in the same waveguide structure. Recent experiments were performed to demonstrate a Bragg grating embedded trapezoidal SPP waveguide as a functional index sensing structure. The trapezoidal SPP waveguide structure being an embodiment of the present invention may include silver as the active metal surface, and patterned PMMA stripes as Bragg gratings. The waveguides may be formed to have an inverted trapezoid shape that has a bottom width b— 8 μm, top width a = 10 μm, and a height h = 6 μm. The gratings may be formed by a spin coat of a PMMA ebeam-resist (such as, 950 A2 MicroChem Corp.) at 6000 rpm for 90 seconds. The sample may be baked at 180 Celsius to evaporate the solvent. The film thickness may be measured by atomic force microscopy (AFM) and confirmed to be ~ 60 nm. The ebeam may expose the sample at 500 μC/cm ² dose and 2 nA current. The exposed sample may be developed in 1 :3 Methyl Isobutyl Ketone (MIBK) to Isopropyl alcohol (IPA) ratioed solution for 60 seconds. The resulting waveguide may have 100 nm PMMA-stripe Bragg gratings adhering at the bottom of the waveguide. As shown in FIG. 11 that illustrates a SEM image of the top view of the waveguide 54, the gratings may be 100 nm in width and 60 nm in height. Four different waveguides samples may be embedded with four different gratings periodicities, such as, for example Λ _Bragg = 467 nm, 765 nm, 770 nm, and 775 nm. Each Bragg-grating period may be repeated, such as, for example 200 times.

The waveguide material properties may include the silver that has a material dispersion ε _∞ = 2, ω _p = 1.39 ^χ lO ¹⁶ [rad/s], τ = 3.4I xIO ^"14 [s] (see: P. B. Johnson, and R. W. Christy, "Optical constants of noble metals," Phys. Rev. B, 6, 4370-4379, (1972)). The index of PMMA may be measured and approximated to have flat index variation n _PM ivi _A = 1.4807, between 1525 nm and 1565 nm wavelengths; the open channel of the trapezoid SPP waveguide may be filled with air or index matching oil. If air is utilized, the air may be assumed to have index of unity. If index matching oils are utilized, the index matching oils (such as, Series AAA by Cargille Labs) may have dielectric indices n _d = 1.3442, 1.3538, and 1.3634, at a wavelength of 1550 nm. Using the present invention a Bragg resonance shift may be demonstrated, as was shown through experiments conducted using embodiments of the present invention, by changing the grating period, Λ _Bragg , and/or the waveguide dielectric filler index, n^. For the purpose of the recent experiments performed to demonstrate a Bragg grating embedded trapezoidal SPP waveguide as a functional index sensing structure, the Bragg resonance of the samples was characterized by using a tunable laser (such as, JDS Uniphase SWS15101), set to emit 5 mW of power, which sweeps over the 1525 nm to 1565 nm wavelength range. Additionally a 3.3 μm lensed fiber was used to excite the waveguides, and a fiber circulator was used to capture the reflected signal.

In the course of the experiments, a Bragg resonance shift was demonstrated by changing the grating period, Λβ _ragg, and/or the waveguide dielectric filler index, n^. Bragg resonance shifts were shown due to changing grating periodicity 56, as shown in FIG. 12 (the black dashed lines represent the experimentally observed reflected power spectra, the red doted lines represent the calculated spectra). The three samples used had Λβ _ragg = 765 nm, 770 nm, and 775 nm. A distinct

~ 10 nm wavelength shift was the result of each sample for each 5 nm change in the grating period. A Bragg resonance shift was shown due to a changing dielectric filler index 58, as shown in FIG. 13 (the dashed lines represent the experimentally observed reflected power spectra, the red doted lines represent the calculated spectra).

A sample having Λβ _ragg ⁼ 467 nm, and index -matching oils having indices of na = 1.3442, 1.3538, and 1.3634, were utilized for the changing dielectric filler experiment. A distinct ~ 11 nm wavelength shift was observed for each 0.01 change in refractive index, which converts to ~ l lOO nm/RIU.

In order to verify the experimental results the matrix method was utilized to calculate grating resonance (see: A Yariv, Optical Electronics (HRW/Saunders, 1991), Chapter 12). Initially the mode indices of the waveguide structures were calculated, having various material compositions, using the MODE software package (such as, Lumerical Solutions, Inc.). Such mode indices are tabulated in Table 1 below. Table 1. Mode index of plasmonic waveguide having various material composition.

Without PMMA With PMMA

n _d = 1 1.0002 + 9.7743x 10 ⁵ i 1.0214 + 2.7765 x 10 ⁴ i

n _d = 1.3442 1.3503 + 2.6988x 10 ⁴ I 1.3615 + 3.9148x l 0 ⁴ i

n _d = 1.3538 1.3601+ 2.6377x 10" i 1.3707 + 3.8061 x 10 ⁴ i

n _d = 1.3634 1.3700 + 2.6137x 10 ⁴ I 1.3798 + 3.7208x l0 ⁴ i

The mode indices and appropriate lengths for each layer were applied, and the reflectance spectrum was calculated. Using the air filled waveguide having Λβr _agg ⁼ 765 nm as an example, the first air-silver layer was set to n _eff = 1.0002 + 9.7743 ^χ l0 ⁵ i, to be 665 nm in length, and the second air-PMMA-silver layer was set to n _eff = 1.0214 + 2.7765 *10 ⁴ i, to be 100 nm in length. The total length of this unit cell may be 765 nm, and it may be repeated 200 times. As shown in FIG. 12, the calculated reflectance may be plotted (shown as a red dotted line) and overlaid with the experimental results.

In the instance of the experiments performed upon the embodiment of the present invention as described herein, the theoretical calculations were found to validate the experimental observations. Moreover, the results suggest that having corrugated surfaces may be used as fluidic chemical sensors and may be adapted by biological lab-on-a-chip devices. Example II

Another possible embodiment of the present invention incorporates varying-density hole-arrays in the structure, such as, for example hole-arrays in trapezoidal plasmonic waveguide. To form such an embodiment of the present invention the following method may be used to make gratings in PMMA. A PMMA ebeam-resist (such as, 950 A2 MicroChem Corp.) may be spin- coated at 6000 rpm for 90 seconds. The sample may be baked at 180 Celsius to evaporate the solvent. The ebeam may expose the sample at 500 μC/cm2 dose and 2 nA current. The exposed sample may be developed in 1 :3 MIBK to IPA ratioed solution for 60 seconds. The resulting waveguide may have hole-arrays adhering at the bottom of the waveguide. A skilled reader will recognize that a variety of varying-density hole-array Bragg grating embedded in trapezoidal SPP waveguides may be designed, fabricated, and characterized. For example, the holes may be patterned in thin PMMA layers. An advantage of using the density hole-array over known waveguides may be that it can narrow down the resonant spectral width. The varying-density hole-array gratings may have grating periods of A _Bragg ⁼ 760 nm, 765 nm, 770 nm, and 775 nm (indicated as "~ 750 nm", as shown in Figure 15b). An example of a reflection spectrum of varying-density hole-array Bragg waveguide having varying periodicities 60 is shown in FIG. 15. As shown in FIG. 14(a), varying-density hole-array formed Bragg gratings may be included in a trapezoidal waveguide embedded with gratings 62. Each Bragg- grating period may be repeated 200 times. The hole-to-hole separation distance may be ~ 100 nm (indicated as "~ 100 nm" 64, as shown in Figure 14(b)).

Example III

Still another possible embodiment of the present invention may include laser irradiated bumps (instead of gratings or hole-arrays formed in PMMA, as described in Example I or Example II). The method to form a waveguide having laser irradiated bumps may be used to make periodic structure in metal. The method may involve metal grating. The bumps may be laid into the trapezoidal channel of the waveguide of the present invention and function as a dispersive element.

One technique of metal bump making is published in Nakata Y, Miyanaga N, Okada T 2007, "Effect of pulse width and fluence of femtosecond laser on the size of nanobump array," Applied

Surface Science 253 6555-6557. The present invention may utilize such a technique or may involve a laser-material interaction mechanism to form bumps on metal surfaces. The bumps integrated into the waveguide may be formed to be 1 -dimensional, 2-dimensional, or 3- diemensional periodic arrays. Such arrays may be used in the trapezoidal waveguide as dispersive elements (and may further replace PMMA gratings described as integrated in the embodiments of the present invention described in Example I and Example II).

The bumps may be formed on the metal surface to be separated at a desired periodicity. For example, FIG. 16 shows a 5-by-5 array wherein the number of bumps 68 is arbitrary in the layout. The bumps may achieve local plasmon resonance as well as resonance caused by the periodic layout of the bumps.

FIG. 17 shows a simulation result of reflection measurement 70. The peak is shown to be a localized plasmonic resonance at the near field. The dip shown may be due to resonance of the 2- dimensional bump array periodic structure layout, which is 500 nm in the example shown in FIG. 17. A skilled reader will recognize that the periodicity of the bump-array may be altered to tune the dip location on the spectrum.

It will be appreciated by those skilled in the art that other variations of the embodiments described herein may also be practiced without departing from the scope of the invention. Other modifications are therefore possible. For example, an IPP waveguide embedded with, or without, the dispersive elements, may be utilized as a biological sensor, a chemical sensor, an environmental sensor, a device to alter the propagation of electromagnetic fields, or a device to confine fluid. Examples of embodiments of the present invention that may provide for such utilization are provided below. A skilled reader will recognize that such embodiments of the present invention may be of various types and include a variety of elements described in this patent application. The embodiments described below for, each of these utilizations, are merely possible examples of waveguides of the present invention to be used for these purposes.

An embodiment of the present invention may be utilized as a biological sensor. For example, in such an embodiment, the surface of the IPP waveguide may be functionalized or paved with a receptor molecule. Such a receptor molecule may be complimentary of a molecule under detection. During a sensing experiment or procedure, the analyte solution, which may be a mixture of many types of molecules, may be placed on the waveguide. When the molecule under detection is present in the analyte solution it may bind to the receptor molecule and the plasmon characteristic may change at that point. Such a change may cause the dispersive element to shift, for example, to peak or dip, in a manner similar to the manners discussed in the context of Examples I, II or III.

Another embodiment of the present invention may be utilized as a chemical or environmental sensor. Such an embodiment may sense occurrences when chemical or environmental factors changes around a dispersive element in the waveguide, as upon such an occurrence the dispersive element may shift, for example, to peak or dip in a manner similar to the manners discussed in the context of Examples I, II or III. Such chemical or environmental factors may include the refractive index, temperature, humidity, vibration and structure.

Yet another embodiment of the present invention may be utilized as a device to alter the propagation of electromagnetic fields. For example, such an alteration may be to slow down or to speed up the propagation of electromagnetic fields. A waveguide of this embodiment may be formed to have a specific dimension. The specific dimension may cause the phase and/or group velocity of the electromagnetic fields to change. Therefore, the IPP waveguide dimension may be utilized to control the speed of electromagnetic field momentum. A skilled reader may recognize that the required specific waveguide dimension may be derived in accordance with the discussion of the Theory-based Waveguide Structures section of this patent application.

Still another embodiment of the present invention may be utilized to confine fluid. A skilled reader may recognize that an IPP waveguide structure may have a channel shape. The channel may be utilized to confine matter. One or more channels shaped in the waveguide may be coated, such as, for example with a metal coating. Such coating may facilitate easy surface conjugations. A skilled reader will recognize that the chemistry and physics of such a waveguide embodiment may be easily modified for the purpose of integration into and/or with other technologies.