WO2004023177A2

WO2004023177A2 - Long range surface plasmon polariton modulator

Info

Publication number: WO2004023177A2
Application number: PCT/DK2003/000579
Authority: WO
Inventors: Sergey Bozhevolnyi; Thomas Nikolajsen; Bonni Kryger; Kristján LEÓSSON; Ildar Salakhutdinov; Thomas Søndergaard; Peter M. W. Skovgaard
Original assignee: Micro Managed Photons A/S
Priority date: 2002-09-06
Filing date: 2003-09-05
Publication date: 2004-03-18
Also published as: AU2003260283A1; AU2003260283A8; WO2004023177A3; EP1546774A2

Abstract

The present invention relates to waveguides for guiding and modulating long range surface plasmon polaritons (LR-SPPs). The guiding devices comprise a core stripe of e.g. metal of finite width embedded in a cladding such as a dielectric cladding. Particularly, in the waveguides of the invention, the complex propagation constant of the guided LR-SPP mode(s) can be dynamically adjusted by heating of the metal core leading to dissipation of heat into the surrounding dielectric material. The devices may be used in the construction of dynamic light guiding integrated components for optical telecommunication.

Description

THERMO ACTIVE LONG RANGE SURFACE PLASMON POLARITON GUIDING DEVICES

FIELD OF THE INVENTION The present invention relates to guiding devices supporting propagation of long range surface plasmon poiaritons where the complex propagation constant of the guided long- range surface plasmon polariton mode(s) can be dynamically adjusted in at least one section of the device. The guiding devices comprise a core stripe of e.g. metal embedded in a cladding such as a dielectric cladding. Particularly the present invention relates to devices where the dynamic adjustment is obtained via dissipation of heat from the core stripe into the surrounding dielectric material. The devices may be used in the construction of dynamic light guiding integrated components for optical telecommunication.

BACKGROUND OF THE INVENTION A surface plasmon polariton is a coupled state between an electromagnetic field and oscillating free electrons at the surface of a conducting medium. Surface plasmon poiaritons can propagate along interfaces between layers of materials with the real part of the dielectric constant being of opposite sign, such as between a metal and a dielectric material. Surface plasmon poiaritons are confined to the interface in the sense that the electromagnetic field associated with a surface plasmon polariton decays exponentially into both media from its maximum at the interface.

Usually the propagation distance of a surface plasmon polariton is limited by large absorption of the part of the field propagating inside the metal. However, so-called long- range surface plasmon poiaritons can be realised in layered structures consisting of two dielectric materials with substantially the same refractive index separated by a very thin layer of metal. For thick metal layers, identical surface plasmon poiaritons can be excited at the interfaces between respectively the top and bottom dielectric layers and the metal layer. If the metal layer is thin enough, the two poiaritons essentially overlap and become coupled, forming two, a symmetric and an asymmetric, supermodes propagating along the interface. Due to a very small overlap between the in-plane component of electromagnetic field of the symmetric polariton supermode and the metal, the loss of such a long-range surface plasmon polariton is drastically reduced compared to a surface plasmon polariton. By comparison, this overlap is much larger and the propagation distance much shorter for the asymmetric (short-range) polariton mode.

Devices relating to the interaction between surface plasmon poiaritons and modes of optical waveguides based on total internal reflection (TIR) have also been realised. Placing the core of a TIR waveguide, such that the electromagnetic field of the light propagating in waveguide overlaps with the electromagnetic field of a surface plasmon polariton, light can be coupled out of the waveguide via evanescent wave coupling. By controlling the amount of power coupled out of the waveguide, an attenuator and/or a variable attenuator can be provided. Furthermore, since the electromagnetic field of a surface plasmon polariton is essentially polarised perpendicular to the interface, only TM (transverse magnetic) modes in the TIR waveguide can be coupled to the surface plasmon polariton. This effect has been used to make fibre optical devices such as polarising filters and polarising beam splitters.

Surface plasmon poiaritons propagating along interfaces of infinite or semi-infinite width, are only confined in the transverse direction orthogonal to the interface whereas the surface plasmon polariton can propagate freely within the plane of the interface, i.e. the surface plasmon polariton is confined only in one dimension. Recently, two-dimensional confinement of long-range surface plasmon poiaritons was demonstrated (see Charbonneau, Berini, Berolo, Lisicka-Shrzek, Opt. Lett. 25, 844 (2000). Charbonneau et. al demonstrated that thin metal stripes of finite width in a dielectric material support propagation of long-range surface plasmon poiaritons along the longitudinal axis of the metal stripe, forming thereby a channel guide for long-range surface plasmon poiaritons.

The complex propagation constant β of modes propagating in a long-range surface plasmon polariton channel guide may be changed so as to induce losses in the long-range surface plasmon poiaritons. Moreover, the phase of the modes may be changed. This may be done by controllably change the complex refractive index of a region of the cladding, e.g. by using an active material in the cladding.

Based on modulations in channel guides, it has been suggested to realise a number of both passive and dynamic devices such as directional couplers, Mach Zehnder interferometers and reflective diffractive gratings, see e.g. WO 01/48521, WO 02/10815 or US 6,442,321. Specifically it has been suggested to realise devices based on thermo active waveguides where the thermo optical effect is achieved by placing a heating electrode adjacent to the channel guide (see WO 01/48521, Figure 47 and corresponding description).

In devices exploiting thermo optical effects in the prior art, e.g. WO 01/48521, the heating is performed by a heating electrode incorporated in the waveguide structure. For the heating electrode to heat up the cladding close to the core, the heating electrode is positioned in the vicinity of the core. It is a disadvantage of these structures that the heating electrode will perturb any propagating long-range surface plasmon polariton mode in the metal core, mostly by causing its scattering and absorption. It is another disadvantage that the heating electrode, being positioned at one side of the metal core, heats the cladding between the heating electrode and the core much more than the cladding on the other side of the core.

SUMMARY OF THE INVENTION. This invention relates to a way of realising the thermo optical effect in long-range surface plasmon polariton (LR-SPP) waveguides by using the waveguide core itself, namely the metal stripe, as the heating element. Hence, according to the present invention, waveguide structures are heated from within outwards, whereas in the devices disclosed by the prior art, the structures are heated from the outside in.

This configuration has a number of advantages. First, it removes any perturbation of an adjacent heating electrode on the propagating LR-SPP mode in the waveguide core.

Also, since the SPP field decreases exponentially from the core-cladding interface, it is the refractive index of the cladding material(s) closest to the core that has the major effect on the propagation properties of the waveguide. The maximum temperature increase induced by heating of the core coincides with the maximum of the electromagnetic field of the guided mode, ensuring thereby the maximum effect of the temperature change on the mode propagation constant. A consequence of this is that devices can be made to perform faster and with lower power consumption than corresponding prior art devices.

Further, it is a major advantage of the present invention that, for sufficiently thick cladding layers, heat will be dissipated isotropically from the metal core into the cladding material(s) adjacent to the core. This allows for substantially symmetric heating of the cladding material(s), which is desirable for maintaining the same level of propagation loss and which is not obtainable with the devices disclosed by the prior art. Also, if the focus is on phase modulation instead of propagation loss, symmetric heating is essential. However, in case asymmetric heating is desired, e.g., to control the propagation loss, this is easily obtainable by introducing cladding materials with different thermal properties and/or different thickness'.

In a first aspect, the present invention provides a thermo-active waveguide section for guiding and/or modulating long range surface plasmon poiaritons (LR-SPPs), the LR-SPPs having at least a first frequency v₀ and a corresponding first wavelength λ₀, the device comprising a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) k_α with a negative real part, Re(kj) < 0, in at least a first frequency range comprising the first frequency, - a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines the waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, wherein the core stripe acts as a heating element in that the thermo-active waveguide section further comprises means for receiving electrical and/or magnetic and/or thermal and/or electromagnetic energy for adjusting a temperature of the core stripe and, via heat transfer from the core stripe, adjacent cladding material(s).

Thus, the gist of the invention is that the core stripe, by receiving energy heating it, will heat abutting or adjacent cladding materials. As most materials have a temperature dependent refractive index n(T), the heating of the core may lead to changes in refractive indexes of the cladding materials, which will in turn modulate the complex propagation constant(s) of the LR-SPP mode(s) guided by the thermo-active LR-SPP waveguide. This may be used to introduce a loss or a change in phase for the propagating LR-SPP mode.

In the following, a thermo-active waveguide section will also be referred to simply as waveguide section or simply waveguide. Also, the core stripe will be referred to as the core, the stripe, metal core or metal stripe (without implying that the material must be a metal). Also please note that the term section is not meant to imply that a waveguide section must be separated from a neighbouring section, although this will often be the case.

The core stripe may be heated in different ways. In a first preferred embodiment, the cores stripe is heated by a current flowing in the core stripe. Here, the means for receiving energy comprises first and second electrically conducting connectors, each having terminal end parts for being connected to different terminals of a current supply, and - contacting end parts in electrical contact with different parts of the core stripe of the waveguide section so that a current supply connected to the terminal end parts will generate a current through at least part of the core stripe.

Preferably, the thermo-active waveguide section is incorporated in a device also comprising a current supply having at least a first and a second in/outputs in electrical connection with different parts of the core stripe of the thermo-active waveguide section. In other words, the current supply is electrically connected to the core stripe via the first and second connectors, allowing the current supply to generate a current through at least part of the core stripe. In a second preferred embodiment, the current heating the core stripe is induced by a varying magnetic field. In order to induce a current in the core stripe, the means for receiving energy comprises an electrically conducting, closed, induction loop or circuit for receiving magnetic energy. Varying a flux Φ_B of a magnetic field B through the closed loop will induce a current i_B in the loop according to Faraday's law, i_B = k ^■ dΦ_B/dt. By having the core stripe forming part of the loop, a current is generated in the core stripe.

In a third preferred embodiment, the core stripe is heated by absorbing electromagnetic (EM) radiation. Here, the means for receiving energy comprise at least a part of the cladding which is at least substantially transparent to EM radiation having a second frequency, so that the core stripe can be irradiated by said EM radiation through the cladding. Preferably, the thermo-active waveguide section in incorporated in a device also comprising a source of EM radiation for emitting EM radiation of the second frequency.

It may be of interest to introduce controllable losses for the LR-SSP modes guided in the thermo-active waveguide section, e.g. when modulating a signal. Such controllable losses can be introduced by reducing the symmetry in dielectric cladding layers with respect to the core, thereby destroying the symmetric polariton supermode forming the LR-SPP. For example, since the LR-SPP propagation constant depends (among other things) on the refractive index of the cladding materials, realising different refractive indices above and below the core stripe will introduce the desired asymmetry. This may e.g. be obtained by choosing the cladding material(s) so that the asymmetry arises (or disappears) when the structure is heated.

In a fourth preferred embodiment, a first region of the cladding above the core stripe comprises a material having a temperature dependent refractive index nι(T) and a second region of the cladding below the core stripe comprises a material having a temperature dependent refractive index n₂(T), the materials being chosen so that nj(T) and n₂(T) have different thermo-optic coefficients, dn;JdT ≠ dn₂/dT.

In a fifth preferred embodiment, the thermal properties such as heat capacity, thermal conductivity or convection factor of a portion of the waveguide section lying above a first part of the core stripe is different from the thermal properties of a corresponding portion of the waveguide section lying below said first part of the core stripe. The thermal properties may be the thermal properties of the cladding abutting the core stripe or it may be the bulk thermal properties of the waveguide section above/below the core stripe, including layers not in direct contact with the core stripe. In a sixth embodiment, the cladding comprises a first cladding region composed by a material having a refractive index n_α(T) and a second cladding region composed by a material having a refractive index n₂(T), the interface between the first and the second cladding region being patterned to form a grating which can be turned on, n^T) ≠ n₂(T), and off, nι(T) = n₂(T). Preferably, the materials are chosen so that nι(Tj) = n₂(Tj) at a first temperature T|, at which the grating is turned off, and so that n (T_f) ≠ n₂(T_f) at a second temperature T_f/ at which the grating is turned on.

The thermo-active waveguide section according to the first aspect may be used in various components or devices, optical and electrical. Some of these applications will be briefly described in the following sections.

As mentioned in relation to some of the embodiments, the thermo-active waveguide section according to the first aspect may be incorporated in a device for guiding and/or modulating LR-SPPs further comprising a source of electrical and/or magnetic and/or thermal and/or electromagnetic energy for supplying energy to the receiving means of the thermo-active waveguide section. Beside the mentioned sources of electric energy and EM radiation, the source may e.g. be a source of heat in thermal contact with the core stripe.

LR-SPPs may be provided to a thermo-active waveguide section in several ways. Photons may be coupled to SPPs on the core-cladding interfaces by any method described in the prior art, such as grating couplers or simply by end coupling from an optical waveguide such as an optical fibre or a planar waveguide. In a preferred device of the present invention, the LR-SPPs may be provided to the thermo-active waveguide section directly from another LR-SPP waveguide. Such device comprises a thermo-active waveguide section according to the first aspect and another LR-SPP waveguide section positioned in continuation of, but electrically insulated from, the thermo-active waveguide section so that an LR-SPP mode can be coupled from the thermo-active waveguide section to the another LR-SPP waveguide section.

The another LR-SPP waveguide section may or may not be a thermo-active waveguide section. The term - in continuation of - does not mean that the waveguide sections must lie along the same straight line. The another waveguide section should be positioned so that the LR-SPP mode from the thermo active first waveguide section can couple to LR-SPP modes in the another waveguide section and vice versa.

The thermo-active waveguide section according to the first aspect may be used as a phase modulating element in one or more arms of a Mach Zehnder interferometer. Also, the thermo-active waveguide section according to the first aspect may be used in digital optical switch or an in-line modulator.

In a second aspect, the present invention provides a method for adjusting a propagation constant of a LR-SPP mode(s) in a LR-SPP waveguide comprising

- a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) _t with a negative real part, Re(kj) < 0, in at least a first frequency range comprising the first frequency,

- a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide section supporting propagation of LR-SPPs in the cladding, the method comprising the step of adjusting a refractive index, n₂(T), of at least part of the one or more cladding materials by adjusting a temperature of the core stripe.

As described in relation to the first aspect, the temperature of the core stripe may be adjusted using different methods, such as:

- by inducing a current in the core stripe using an external magnetic field, - by conducting an electrical current through the core stripe, by providing a thermal connection between the core stripe and a heat source, and

- by illuminating at least part of the core stripe with electromagnetic radiation.

One or more methods may be applied at the same time.

In an alternative embodiment, the temperature of the core stripe is adjusted by propagating LR-SPPs in the core stripe. Some LR-SPP modes experience significant losses that lead to heating of the core stripe and the surrounding cladding material(s). Preferably, the waveguide section is heated by a (pre- or pump) pulse preceding or co-propagating the pulse which is to experience the modulation. The pre- or pump pulse may have another frequency, mode profile, power, etc. than the pulse to be modulated, enabling a larger absorption of the pre- or pump pulse in the waveguide.

In all embodiment of the present invention, both the thickness and the width of the core stripe may be varying at least along a part of the axis of the waveguide so as to change the modal properties of the waveguide along the axis of the waveguide. In a third aspect, the invention provides a waveguide section with electro-optic cladding materials to guide and/or modulate LR-SPPs having at least a first frequency modulate. The waveguide section comprising

- a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) k_x with a negative real part, Refkj) < 0, in at least a first frequency range comprising the first frequency,

- a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, wherein the cladding comprises a first cladding layer having a refractive index rii and a second electro-optic cladding layer having a refractive index n₂(E), the interface between the first and the second cladding layer being patterned to form a grating being adapted to be off when n₂(E) = n_t and to be turned on when n₂(E) ≠ n^

In a fourth aspect, the present invention provides a waveguide section having cladding materials with intensity dependent refractive index to modulate or amplify LR-SPPs having at least a first frequency v₀ and a corresponding first wavelength λ₀. The waveguide section comprising a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) kj with a negative real part, Re(k_α) < 0, in at least a first frequency range comprising the first frequency, - a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, wherein at least a first region of the cladding and/or at least a second region of the cladding comprise(s) a material having an intensity dependent refractive index n(I), wherein at least a part of the first region is above the core stripe and/or at least a part of the second region is below the core stripe, and wherein the device further comprises means for providing pumping light of intensity I to pump the material to modulate the refractive index n(I) thereby modulating the complex propagation constant of the LR-SPP modes guided by the LRSSP waveguide. The intensity dependence of the refractive index may be a third order non-linearity and materials having a strong third order non-linearity are often denoted χ(3) - materials. These materials may be used in much the same way as electro-optic materials applying electromagnetic radiation instead of an electrical field. By pumping the χ(3) - material optically, a change in the refractive index is induced so that the guiding properties of the LR-SPP waveguide are modulated. χ(3) - materials may comprise BaTi03, non-linear polymers, etc.

It is an advantage that the pump light may be provided to the material having a refractive index n(I) from a top, bottom or side end of the device and/or the pump light may be guided along a LR-SPP waveguide.

Furthermore, at least a region of the cladding may comprise an optical non-linear material, so that propagation of LR-SPPs having frequencies ω_lr ω₂, etc. generates LR-SPPs having harmonic frequencies 2ω_lr 2ω₂, ω₁ ± ω₂, etc.

It is a further object of the present invention to provide a LR-SPP waveguide section having a core and a cladding wherein the cladding comprises an active material being adapted to provide stimulated emission when pumped either optically or electrically.

Thus, in a fifth aspect, the invention provides a waveguide section for guiding and/or amplifying long range surface plasmon poiaritons having at least a first frequency v₀ and a corresponding first wavelength λ₀, the device comprising

- a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) k_x with a negative real part, Re(k < 0, in at least a first frequency range comprising the first frequency, a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, wherein at least a part of the cladding comprises an active material, the active material being adapted to provide stimulated emission when pumped optically and/or electrically.

The active material may be an active material adapted to provide stimulated emission when pumped either optically or electrically. The active material is characterised as a material where inversion of population of a quantized system can be achieved between an excited state and a lower energy state. Inversion of population may be achieved by optically or electrically pumping. Pumping of the active material optically and/or electrically will induce a change in the complex refractive index of the material and thus also of the propagation constant for an LR-SPP propagating through the part of the guide comprising the active material, so that the LR-SPP is amplified through that region.

The active material may comprise a semiconductor material or a combination of semiconductor materials selected from the group consisting of InP, GaAs, Ge, and Si. Also other semiconductor materials being adapted to provide stimulated emission when pumped either optically or electrically, such as GaAs, GaAlAs, GaAIAsP, etc. may be used. Alternatively, materials, such as glasses, polymers, ferroelectric crystals, etc. being doped with Neodymium, Erbium or other Lanthanides,

A region of active material may, thus, be provided, positioned in the cladding within the extent of the decaying electro magnetic fields and further at least two feedback elements may be provided around the active region. Spontaneous emission captured by the LR-SPP waveguide may be amplified so as to provide laser action.

Preferably, the region comprising the active material is positioned in proximity to the core stripe, or is positioned in the cladding within the extent of the decaying electromagnetic fields of the LR-SPPs guided by the LR-SPP waveguide. It is, furthermore, preferred to provide symmetrically positioned regions of active material so as to preserve the symmetrical mode profile of the LR-SPPs.

LR-SPPs propagating through the part of the guide comprising the active material and thus the population inverted region, induce stimulated emission and are thereby amplified. It is preferred that the frequency of the LR-SPP matches the frequency of a transition between the excited and lower energy state of the quantized system for the stimulated emission to take place.

By providing a region of active material being positioned in the cladding within the extent of the decaying electro magnetic fields and further providing at least two feedback elements in the LR-SPP waveguide, spontaneous emission captured by the LR-SPP waveguide may be amplified so as to provide laser action.

By choosing suitable feedback elements, a narrow output beam of substantially coherent, monochromatic light, thereby providing an LR-SPP waveguide laser. The feedback elements may be any reflectors, such as mirrors, reflective coated end-facets, such as thin film coated end-facets, metal coated end-facet, etc., or gratings in the cladding It is an advantage of the LR-SPP waveguide laser that the electromagnetic field of the long range surface plasmon polariton modes are polarised, whereby the LR-SPP waveguide laser provides a polarised output. It is generally a problem to obtain a mode polarised semiconductor laser so that the LR-SPP waveguide laser will be very useful for many applications, such as devices based on interference of light, such as a Mach-Zehnder interferometer.

It is a further advantage that a waveguide laser may be made without the complex processing involved in manufacturing an active TIR waveguiding core.

According to a sixth aspect, the invention provides a device for guiding long range surface plasmon poiaritons having at least a first frequency v₀ and a corresponding first wavelength λ₀ is provided. The device comprises

- at least one core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) ki with a negative real part, Re(kj) < 0, in at least a first frequency range comprising the first frequency, the core stripe being at least substantially defined in a first plane,

- at least one cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide supporting propagation of the long range surface plasmon polariton (LR- SPP) in the cladding, the device further comprising another type of waveguide being at least substantially defined in a second plane which is displaced and/or rotated in relation to the first plane, the at least two waveguides defining a three dimensional structure, and the at least two waveguides are being adapted to allow for controllable coupling of energy between modes guided by the at least one LR-SPP waveguide and the other type of waveguide along a predetermined length of the LR-SPP waveguide.

Typically, the first waveguide is formed in a layered structure, the layer holding the core stripe of the first waveguide being the first plane, the layer holding a core of the second waveguide being the second plane. In a specific embodiment, the second waveguide the second waveguide may approach the first waveguide from above or below, making the second plane non-parallel to the layers of the structure. The second waveguide may e.g. comprise a metal core stripe which follows a slope formed by etching until it merges with or gets sufficiently close to the first waveguide. In this embodiment, the second plane will not be the plane defined by the width of e.g. a core stripe of the second waveguide. Rather, it will be the plane the core stripe intersects at all times even though it bends up and down.

The devices and components presented above may be integrated on a chip, such as a silicon substrate, comprising at least two optical components is provided, wherein the at least two components may be connected via an LR-SPP waveguide. To optimise the space on the chip, it is preferred to use LR-SPP waveguides that may be bent. Furthermore, at least one of the components on the chip may be any of the above described components.

The structures of the present invention may be fabricated according to the following method for manufacturing long range surface plasmon polariton waveguide.

The method comprises the steps of providing a substrate - applying at least a first cladding layer comprising one or more material(s) having a complex dielectric constant(s) k₂ with a positive real part, Re(k₂) > 0, in at least one frequency range,

- depositing a core stripe comprising one or more material(s) having complex dielectric constant(s) k_x with a negative real part, Re(kι) < 0, in at least one frequency range, the core stripe having a finite width w_m and a thickness t_m on the at least first cladding layer,

- applying at least a second cladding layer comprising one or more material(s) having a complex dielectric constant k₂ with a positive real part, Re(k₂) > 0, in at least another frequency range, so that the cladding layers are substantially embedding the core stripe, wherein the at least first and second frequency ranges comprise the first frequency, and wherein the finite width w_m, and the thickness t_m are selected so that the core stripe defines a waveguide supporting propagation of the long range surface plasmon polariton in the cladding.

The core stripe may be deposited by any metallization process, such as by physical vapour deposition, evaporation, sputtering, chemical vapour deposition, etc. The cladding may be a spin-coated polymer, or the cladding may be a glass, such as a glass deposited by chemical vapour deposition. A top cladding may either be grown or spun directly on the top of the first cladding layer or a top cladding may be bonded to the substrate comprising the first cladding and the core stripe.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1A and B illustrate a thermo active optical waveguide according to the prior art. Fig. 2A and B illustrate a thermo active LR-SPP waveguide according to the prior art.

Fig. 3A and B illustrate a thermo active LR-SPP waveguide according to the present invention.

Fig. 4A and B illustrate a thermo active LR-SPP waveguide according to the present invention where a current is used to heat the core stripe.

Fig. 5A and B show a device with a thermo active LR-SPP waveguide according to the present invention.

Fig. 6A and B illustrates different thermo active LR-SPP waveguide sections according to the present invention.

Fig. 7A-C show different configurations for crossings of LR-SPP waveguide sections.

Fig. 8 illustrates a method for making electrical connections to core stripes in a planar structure.

Fig. 9 illustrates a method for making several electrical connections to core stripes in a planar structure.

Fig. 10A and B show an inline modulator based on asymmetric thermal properties for thermo active LR-SPP waveguide sections according to the present invention.

Fig. 11A and B show different configurations for MZIs based on thermo active LR-SPP waveguide sections according to the present invention.

Fig. 12 shows an LR-SPP directional coupler based on thermo active LR-SPP waveguide sections according to the present invention.

Fig. 13A and B show different configurations for digital optical switches based on thermo active LR-SPP waveguide sections according to the present invention.

Fig. 14 illustrates a thermo active waveguide device where the core guide is modulated in a periodic way to create a reflective diffractive grating.

Fig. 15 shows an integrated device comprising components of the previous figures. Fig. 16 illustrates the use of a preceding SPP pump pulse to heat op the core and adjacent cladding layers.

Fig. 17A and B show a waveguide section adapted to have a current induced in it by an external varying magnetic field.

Fig. 18 shows a thermo active LR-SPP waveguide section according to the present invention comprising a thermo-optic grating.

Fig. 19A and B show a MZI based on thermo active LR-SPP waveguide sections according to the present invention, used in experimental verification of the invention.

Fig. 20 is a graph showing the extinction ratio of the MZIs of Figure 19.

Fig. 21 is a graph showing the transient behaviour of the performance of the MZIs of Figure 19.

Fig. 22 is a graph showing the exponential rise/decay for determining the time constant of the transient behaviour illustrated in the graph of Figure 21.

Fig. 23 shows a digital optical switch based on thermo active LR-SPP waveguide sections according to the present invention used in experimental verification of the invention.

Fig. 24 is a graph showing the development of the intensity of arm I and arm II of the digital optical switch with a 6 mm long heating element of Figure 23.

Figure 25 shows the results of transmission through an in-line modulator having a 4 mm long heating element.

Fig. 26 shows an LR-SPP device comprising an electro-optic grating.

Fig. 27A and B are schematic drawings of LR-SPP devices based on χ(3) materials.

Fig. 28 shows an LR-SPP device comprising a Mach-Zehnder modulator, modulation being introduced via by optical pumping.

Fig. 29A-C are schematic drawings of LR-SPP devices comprising active materials to achieve amplification and lasing properties. Fig. 30 is a schematic drawing of an LR-SPP device comprising non-linear frequency conversion.

Fig. 31A-C are schematic drawings of multi layered waveguide structures incorporating LR- SPP guides.

Fig.ι 32A and B show an example of a switching device incorporating optical fibres with LR- SPP waveguides.

Figs. 33A and B show dispersion relations for the propagation constant of a LR-SPP waveguide and for a standard single mode fibre and a dispersion compensated fibre, respectively.

DETAILED DESCRIPTION OF THE FIGURES

Figure 1A and B show an example of a prior art thermo active optical waveguide section. On Figure 1A, a cross section of the device perpendicular to the direction of propagation of the light is shown while on Figure IB the cross section is along the direction of propagation. The device consists of a core of dielectric material 1 embedded into a cladding 2 consisting of another dielectric material with a lower refractive index.

Above the cladding layer is a heating element 3, e.g. a metal electrode connected to a current supply. The propagation constant of the modes sustained by the guide mainly depends on the refractive index of the core. When the heating element is heated, the temperature increase changes the effective refractive index of the guide and thus the propagation constant.

Figure 2A and B shows an example of a thermo optic LR-SPP waveguide section. The waveguide consist of a core metal stripe 4 surrounded by a top 5 and a bottom 6 cladding layer of substantially equal refractive index. Such LR-SPP waveguides are described by the inventors in e.g. Applied Physics Letters 82, 668 (2003).

The propagation of LR-SPP modes in a LR-SPP waveguide can be characterised using a complex propagation constant β. The electromagnetic field describing a LR-SPP mode in a given system can be expressed as:

E = E(x, y) exp i(βz - ωt) where the mode at frequency ω propagates along the z-axis. E(x,y) describes the amplitude of the field distribution perpendicular to the direction of propagation, while the amplitude evolution along the direction of propagation is described by β. As β \s in general a complex number, β = β_r + iβ„ the field can be written as

E = E(x, y) exp(-/7, z) ex i(β_rz - ωt) .

From this expression, it is evident that the real part of the complex propagation constant describes the dependence of the LR-SPP phase on distance, while the imaginary part describes the LR-SPP attenuation with propagation.

The complex propagation constants β of modes sustained by such a guide are dependent on the refractive indices of the core and the cladding, approaching, e.g., for the symmetric configuration and an exceedingly thin core the light wave number in the cladding. The refractive index of a material is in general temperature dependent. Hence the propagation constants of the guide can be manipulated by providing thermal energy Q to the structure. In typical waveguide structures this is achieved by placing a heating element 3 on the outer boundary of the cladding where the element itself is not interacting with the electromagnetic field of the optical mode.

In thermo optic LR-SPP devices according to the prior art, a heating element 3, e.g. a metal electrode connected to a current supply, is positioned above the cladding layer. The propagation constant of the modes sustained by the guide mainly depends on the refractive index of the surrounding cladding layer. When the heating element is heated, the temperature difference between the element 3 and the stripe 4 creates a thermal gradient in the intervening top cladding 5. The heating induces an asymmetry in the structure as the thereby manipulating the propagation constant of the bound LR-SPP modes.

The main idea of the present invention is to use the guiding metal core as a heating element, which dissipates heat into the surrounding dielectric material, this is illustrated in Figure 3A and B.

Figure 4A and B show a thermo active waveguide in which heating via the core metal stripe is obtained by sending an electrical current through the stripe. Consider a gold core stripe of 10 μm width and 15 nm thickness. Assuming a resistivity of gold of p = 2,36* 10^"8 Ωm the resistance R per unit length of the gold stripe can be estimated to be

R = £- = \ 51Ωlmm ,

A where A is the cross sectional area of the stripe. This level of resistance is sufficiently large as compared with the resistance of contacting wires, suggesting that that metal stripes with dimensions suitable for propagation of long range surface plasmon poiaritons, are also very suitable for conversion of electrical energy into heat.

Figure 5A and B show a sketch of a thermo active waveguide, according to Figure 4, seen from the top. Figure 5A shows the whole structure while Figure 5B shows a magnification of a section where contact is made. Electrical contact to the guiding core stripe device is established via small contact stripes 7. The dimensions of these stripes are chosen in order to minimise the perturbation of the contact stripes 7 on the propagating LR-SPP mode in the core stripe 4.

Typically the thickness of the contact stripes 7 is chosen to be the same as that of the core stripe 4 in order to limit the number of steps during processing. The thickness t_m of the core stripe is typically selected between 1-50 nm. The contact stripes 7 are connected to wires 8, which are defined as any conductor with low resistance. In the devices manufactured, the wires 8 consist of thick (100 nm -300 nm) and wide (20 μm - 1 mm) metal stripes deposited in the same layer as the guiding core 4. The current supply 20 for operating the device is connected to the wires 8.

The minimum separation, d, between the wire 8 and the guiding core 4, that is the length of contact stripe 7, should on one hand be chosen long enough (> 1 μm) to minimise the perturbation of the field in the wire 8 on the LR-SPP modes, and on the other hand short enough (< lOOμm) to minimise heating and thereby heat dissipation by the contact stripes 7. The width and thickness of contact stripe 7 should be chosen large enough to not create substantial ohmic heating of contact stripe 7, while small enough to not perturb (by scattering and absorption) the LR-SPP modes.

Different design parameters are possible for the connections (contact stripes 7 and wires 8) in order to make allowance for the above considerations. The length, d, of contact stripe 7 is preferably chosen to be longer than 1 μm, such as longer than 3 μm, 5 μm or 7 μm while shorter than 100 μm such as shorter than 50 μm or 20 μm. The cross sectional dimensions, width and thickness, of contact stripe 7 is preferably the same as for the core stripe. Typically, the width of the contact stripe 7 is selected between 1 μm and 100 μm, such as between 1 μm and 20 μm, such as between 1 μm and 12 μm, such as between 3 μm and 8 μm, narrower than 20 nm such as narrower than 10 nm, while wider than 3 nm such as wider than 5 nm. Typically, the thickness of the contact stripe 7 is selected between 1 nm and 100 nm, such as between 1 nm and 50 nm, preferably between 1 and 15 nm, such as 10-50 nm. Alternatively, the contact stripe 7 can be tapered, narrowing from the wire 8 towards the core stripe 4. Figure 6A shows how to electrically insulate sections of the guiding core from each other to create thermo active waveguide sections 12. By introducing a small gap 9 the currents within each section can be controlled individually. Figure 6B shows a magnification of the gap 9. The size (length) of the gap should be chosen taken the following considerations into account

Fabrication. The gap should electrically insulate two core stripe sections, and the precision of the fabrication technique sets a lower limit to how short the gap can be made so as to guarantee electrical insulation every time, also in a large scale production. With the lithographical methods used in the fabrication of the devices in the present work, the lower limit is roughly 0,1 μm while a length of about 1 μm would give a safe insulation every time. - Short circuit. If the gap is too narrow as compared to the potential over it, the electric field strength in the intermediate dielectrica may become so large as to generate a disruptive discharge. - Capacitance. If an alternating current is used, the gap may still become conductive in the sense of a capacitor. Hence, the effect capacitance of the gap should be estimated.

Optical losses. LR-SPP modes should couple between insulated sections with acceptable losses, such as below 1 dB or 10 dB.

Dividing the core stripes into sections by introducing small gaps 9 between them allows one to realise crossings 14, 15 and 22 of the core stripes 4, as shown in Figure 7A-C. This ensures both very low optical cross talk (between modes propagating along crossed stripes) and electrical insulation. In such crossings 14, 15 and 22, an LR-SPP mode propagation along one of the directions will not be perturbed by the crossing guide if the crossing gap is sufficiently large (>5μm) so as to eliminate overlap between the LR-SPP mode and the crossing core stripe. This way crossing with very low or even zero cross talk between waveguides can be realised. The shaping of the core stripes 4 at the crossing can be altered in order to minimise optical cross talk as well as SPP coupling losses. The configurations 14, 15 and 22 of Figures 7A-C are three possible embodiments, but many more can be envisaged.

Figure 8 illustrates how the possibility of sectioning the waveguides can be used to cross contact stripes with core guides. This possibility is convenient for the design of complex structures where several waveguides and wires are to be combined in a single plane.

Figure 9 illustrates a device for controlling two individual core stripes using only one electrical source. Especially a device like this can be used to design a device where exactly the same change in propagation constant of the two stripes is achieved with the same source. This will be the case if the two paths between the poles of the electrical source are identical.

Figure 10A and B shows the principle of an inline modulator 13 by which the loss of a LR- 5 SPP mode propagating through the stripe can be controlled by heating. The principle of operation is, by heating of the core stripe 4, to break the symmetry of the refractive index distribution above and below the guiding stripe thereby inducing additional ohmic loss in the device 13. This can be done placing the top and bottom surface of the cladding layers 5 and 6 in contact with materials 101 and 102, e.g. air and silicon, with different thermal 10 properties such as heat capacitance, thermal conductivity or convection factor.

Figure 11A and B shows different configurations of Mach Zehnder Interferometers (MZI) 15 based on thermo active waveguide sections 13. An incoming electro magnetic wave is split into two branches by a Y-splitter 11. The propagation constant of the LR-SPP mode

15 propagating in one of the arms is controlled by the application of an electric current in the arm through wires 8 and contact stripes 7. Thereby, the core stripe 4 is heated and the refractive index of a thermo-optic cladding abutting the core stripe 4 is changed. A second Y-splitter 11 combines the modes from the two branches. The main thing to notice in the design of the MZIs 15 in Figure 11 is that the possibility of electrically insulating sections

20 13 of the core stripe is essential for the realisation of this devices as power otherwise will be dissipated from both arms. The insulating gap 9 is introduced in both arms in order to eliminate any effects due to a phase shift introduced by one of the gaps. Experimental results from the device similar to the one in Figure 11 are presented in the experimental verification.

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Figure 12 shows an example of how a thermo active waveguide section 13 can be used to realise a directional coupler switch 16 by changing the propagation constant within the region of interaction between the two guides.

30 Figures 13A and B show two examples of digital optical switches 17 and 18 realised using the thermo active waveguide sections 13. Such devices are designed in such a way that light will follow the path of the highest refractive index. Different paths can therefore be configured using the thermo active waveguide sections.

35 Figure 14 illustrates a thermo active waveguide device 19 where the physical dimension of the core guide is modulated in a periodic way to create a reflective diffractive grating 10. The variation of the dimension of the core guide can be either in the width, the thickness or both. The grating is designed to reflect LR-SPP modes with specific propagation constants and allow other LR-SPP modes to pass. The propagation constant of the propagating LR-SPP mode can be tuned in and out of resonance by adjusting the temperature by sending a current through the core guide. The device 11119 can thereby work as an adjustable filter. For a suitable choice of period this can used to switch between conditions where the mode is completely or partially reflected or transmitted.

In Figure 15 illustrates how most of the principles and component of the previous figures can be combined into an integrated device, which acts as a thermo active tuneable add/drop switch. Entering the device via the "in" waveguide the first TO section decides if light is to proceed straight through the device or couple to the Mach Zehnder section of the device. The arms of the Mach Zehnder device are configured with identical tunable gratings according to the sketch in Figure 14. For a suitable design of the first directional coupler of the MZI any light reflected by the grating will be dropped via the drop waveguide. Light which is not being reflected will continue and couple to output waveguides via the second directional coupler together with light added to the device via the add waveguide and reflected by the grating.

Figure 16 illustrates a thermo active guide section where heat dissipation by the core stripe is due to transformation of energy from a preceding LR-SPP mode into heat. By propagating a SPP, ω₂, immediately before or together with the LR-SPP of interest, ω_, will experience a structure heated by the absorption of ω₂. ω₂ can be chosen so as to have a much higher absorption than ω_r. The pump pulse ω₂ can thereby be used to switch the subsequent LR-SPP on or off depending on the initial conditions.

Figure 17A and B illustrates a thermo active guide section 23 where the heat dissipation by the core stripe is due to a current i_B induced in the core stripe by an external varying magnetic field B. The section 23 includes a closed conducting loop allowing generation of the current i_B. The induced current heats the core stripe 4 and the surrounding cladding materials 5 and 6 so as to generate a detectable change in the propagation constant of a bound LR-SPP mode, i.e. to increase or decrease the guided power. The magnetic field B can be varying in space and/or time, whereby such a device can potentially be used as an all-optical magnetic flux sensor.

In a device illustrated in Figure 18, the cladding comprises a first cladding layer 32 having a refractive index n^T) and a second thermo-optic cladding layer 30 having a refractive index n₂(T), the interface between the first and the second cladding layers being patterned to form a grating. The material compositions may be chosen so that the grating will be off at working temperatures, n^To) = n₂(T₀), and be turned on when heated, nι(T_f) ≠ n₂(T_f). In a preferred embodiment the grating may be adapted to controllably couple at least a part of the LR-SPP guided by the device out of the waveguide.

In order to support the invention described above the following experimental verification study has been conducted.

The desired functions to be performed are:

- Variable optical attenuation (VOA)/ modulation

- Optical switching

In order to perform these functions it was chosen to investigate the following structures:

- Thermo active Mach-Zehnder interferometers (MZI) Digital optical switches (DOS)

- In-line modulators (IL)

The MZ interferometer is a basic component of integrated optics and in different configurations can be used for modulation, power control and even switching. A MZI based on the splitting of the intensity of a LR-SPP guide into two separate arms has been fabricated to verify the performance of the present invention. As the two arms are again recombined into one, either using a Y-splitter or a directional coupler, the intensity coupled to the guide depends on the relative phase between the LR-SPP modes coming from the two arms. This phase sensitivity makes the device very sensitive to thermo optically induced refractive index changes introduced in one of the arms and thus very suitable for applications, which require low electrical power and fast response.

Digital optical switches are included in the verification study to provide a way for switching, which is less sensitive to the precision of the control power and which can act as a broadband device. Digital optical switches in the simplest form consist of a Y-splitter with a shallow angle of separation between the two branches. If a refractive index difference is introduced between the two arms light will propagate along the path of the highest index.

Finally, in-line modulators are included in the verification study as these structures potentially provide a very simple way of providing a VOA, which is unique for the MMP wave guiding technology. The operational principles of the in-line modulator are based on designing the waveguide in such a way that heating of the core creates a refractive index difference between the top and bottom cladding layers. The consideration prior to the design of the components was done with the aim of showing proof of principle of the device functionality. The designs where based on the following considerations.

The thermo optical coefficient of the polymer used as cladding material (BenzoCycloButene from Dow Chemicals, BCB) has been reported to be

« — 3 - 10^" C~ APL68(14),1910 - 1912,1996) . dT ^{L J}

This value is rather low for a polymer material, which typically has coefficients which are significantly higher, e.g. PMMA with a coefficient of -1,2- 10^"4 "C^"1 [JLT, 7(3),449, 1989]. The main physical mechanism leading to the thermally induced refractive index change is thermal expansion of the material.

Consider now a MZI where the length of the arm, which is heated, is L = 5,7 mm (similar to our devices). In order to switch the device from e.g. an on to an off state, a phase shift, θ, of π is needed. The refractive index change Δn needed to generated this phase shift is

Δ« = -^ « 10-⁴

2πL

Given the thermo optical coefficient of BCB an increase in temperature of the order of 3-4 degrees is therefore needed.

Now, consider the device to consist of a heating source, which is embedded into BCB with

Q = ^^ * 100mW/mm². d d = 15 μm on either side of the element, and consider the top and bottom surface of the BCB to be kept at room temperature. The amount of power to be dissipated by the heating element in order to set up a temperature gradient between the centre and the faces of the BCB is

Considering a stripe of 10 μm width, this corresponds to a heat dissipation of 1 mW per mm of stripe.

The estimate above is based on assuming a thermal conductivity of k = 0.2 W/mK. This value is used as it is referred to as a general value for the conductivity of polymers [[JLT, 7(3), 449, 1989], no cited value specifically for BCB has been found.

Due to this, and due to the crude assumptions used to estimate the influence of the temperature change and the influence on the propagation constant of the guide, the above estimates only provide the order of magnitude of the electrical powers needed to drive the devices.

As mentioned, the aim of the verification study is to investigate the possibility of using metallic core stripes as heating elements. Consider a stripe waveguide of 10 μm width and 15 nm thickness. Assuming a resistivity of gold of p = 2,36* 10^"8 Ωm the resistance per unit length of the gold stripe is estimated to be

R = £- = \51Ω.lmm . A

Based on this value it follows that the current needed in order to obtain the % phase shift in the MZI is of the order of 2-3 mA and the voltage for a 5,7 mm long heating element 1,4V.

The initial considerations presented above are related to the MZI. For the DOS and IL devices it is expected that thermally induced refractive index change and, hence, the power requirements should be an order of magnitude higher. Even with this in mind, the above considerations suggest that the principle of using the metallic core stripes as heating element seem like a very attractive idea. The estimated current/voltage/power levels are very suitable for practical applications as they are easily realised.

For the manufacturing of the devices a set of photolithography masks were prepared. - Waveguides and heating elements for the intermediate plane (positive). - Contact pads and wires for the intermediate plane (negative).

The wires referred to are metal stripes with dimensions of a minimum of 50 nm thickness and 100 μm width. These stripes have a very low resistance of a few ohms per mm and are used to connect the contact pads (l x l mm² and at least 50 nm thick) with the heating elements.

The procedure for manufacturing the devices is basically the same as for the manufacturing of stripe waveguides. For the specific devices investigated in this study the following recipe was used.

Spin-on adhesion promoter, 14 μm thick layer of BCB and edge-bead remover. Polymer soft-cured (70-80% crosslinked) at 210°C.

Resist coating AZ5214E, 1.5 μm thickness/ UV exposure with mask 2/ development. - Metal deposition, 15 nm Au, deposition rate 2 A/s, followed by lift-off in an acetone bath with ultrasound.

Resist coating AZ5214E, 2.2 μm thickness/exposure with mask 3/image reversal/ development. - Metal deposition, 3 nm Ti + 200 nm Au, deposition rate 10 A/s, followed by lift-off in an acetone bath (no ultrasound).

- Spin-on of top 14 μm thick layer of BCB. Final structure hard-cured at 250°C.

Following the manufacturing the contact pads in the intermediate plane was exposed by removing the top BCB layer. This was done primarily using a C0₂-laser burning system.

The configuration of the Mach-Zehnder interferometer investigated in this study is shown in Figure 19A. The length of the thermo active waveguide section 13 in one of the arms is 5,7 mm between the contact stripes 7. Two different configurations of this device was investigated the difference between the two being the size of the gap inserted for electrical insulation of the arm from the rest of the structure, shown in Figure 19B (note that the same gap was introduced in both arms). The stripes 4 had thickness t = 15 nm and width w = 10 μm. The distance between the arms was 250 μm. The resistance of the arm was measured to be 1,6 kΩ for both devices.

Figure 20 shows a plot of ,a measurement aimed at determining the extinction ratio of the devices of Figure 19. The output from the MZIs was recorded while changing the electrical current through the active arm and, hence, the power dissipated into the thermo active section. This way an extinction ratio better than 35 dB was achieved for the best device while the other device displayed a better than 25 dB extinction ratio.

The result of the transient behaviour of the MZI devices of Figure 19A is shown on Figure 21. The current through the active arm was generated using a 15 MHz function generator which were adjusted in order to maximise the amplitude and minimise the zero level of the output from the MZI. Optimum operation was found for V_p-P = 3,8 V and an offset of 2 V. Once adjusted to optimum performance the devices under continuous modulation was observed to be stable during several hours of operation. From the plots of Figure 22, the exponential time constant describing both the rise and fall of the signal was fitted to be around 0.7 ms. The time constant is a measure of the time needed to reach a steady-state temperature distribution for a given heat dissipation from the core stripe into the abutting cladding.

In conclusion, MZIs with nice performance in terms of high extinction ratio, low power consumption, fast response and low insertion loss is observed. The performance has to be considered taking into account that the thermo-optical coefficient of BCB significantly below what is typical for polymers used in thermo-optical devices. The configurations of the DOS investigated in this report is shown in Figure 23. The core stripe 4 had thickness t = 15 nm and width w = 7,5 μm. The resistance of the arms where measured to be 2,15 kΩ.

On Figure 24 the development of the intensity of arm I and arm II are shown as the power dissipated by the thermo active waveguide section 13 in arm II is increased. The device under investigation has a 6mm long heating element.

As the power is turned on, it can be seen how the intensity shifts away from the heated arm II toward arm I. A full 3 dB increase of the power in arm I is not achieved. However, the dependence of the intensity of the two arms serves to provide a proof of principle of digital optical switching.

The in-line modulators are added to this study as they potentially provide a conceptually simple way of controlling the loss of light propagating through the guide. The principle of operation is, by heating, to break the symmetry of the refractive index distribution above and below the guiding stripe thereby inducing additional ohmic loss in the device.

Figure 25 shows the results of transmission through a waveguide with a thermo active waveguide section of 4mm length. The bottom BCB surface is in contact with the Si wafer while the top surface is in contact with air, which makes the overall structure asymmetric.

As seen the transmission through the stripe starts to decrease significantly for a power dissipation of the order of 12mW/mm. This power level is rather high for a practical device.

However, taking into account the TO coefficient of BCB this number can be reduced by an order of magnitude by suitable choice of material. In that case the in-line modulator constitutes an attractive way of realising variable optical attenuators.

In the following, a number of different ways to dynamically adjust the complex propagation constant in LR-SPP waveguides are described.

The device shown in Figure 26 is an electro-optic device, wherein the top cladding comprises a cladding layer 32 having a refractive index n_α which is not dependent upon the electric field E, and a part 30 comprising an electro-optic material having a field-dependent refractive index n₂(E). The interface between the two cladding materials is shaped to form a grating 31. For E=0, the refractive index n₂(E) = rii so that the grating 31 is turned off. For E ≠ 0, n₂(E) ≠ nj so that the grating 31 is turned on. When the grating 31 is turned on, it is possible to couple light out of the LR-SPP waveguide 4. Hereby, an electrically controlled coupling is provided. In Figures 27A and B, examples of devices providing optical control of the properties of the LR-SPP waveguides 4 are shown. The cladding material 33 comprises at least a region of materials having intensity dependent refractive indices n(I). These materials having a strong third order non-linearity are often denoted χ(3) - materials. By pumping the χ(3) - 5 material optically, a change in the refractive index is induced so that the guiding properties of the LR-SPP waveguides 4 are modulated. In Figure 27A, an electromagnetic wave source 34, such as a light emitting diode is provided, whereby the intensity dependent refractive index in the part of the cladding 35 being illuminated by the light source 34 is changed so that propagation properties of the LR-SPP waveguide 4 are changed. In Figure 10 27B, the pumping light 36 is guided along with a signal 37 in the LR-SPP waveguide, so that the, part of, the cladding layer having a refractive index dependent on the intensity will be changed.

In Figure 28, a Mach-Zehnder modulator 38 based on a construction comprising χ(3) -

15 materials is illustrated. The input arm 4 receives a LR-SPP signal 36 which is split into two arms 39 and 42 in splitter 41. Instead of providing an electrode for application of an electric field as in standard electro-optic MZ modulators, a pump beam 37 is provided by a waveguide 40 to the branch 39 in the area where the cladding comprises the χ(3) - material. The phase of the signal 36 in arm 39 are thus modulated by the signal 37 in

20 waveguide 40. The waveguide 40 is here an LR-SPP waveguide, but it may be any other type of waveguide capable of coupling light into the Mach-Zehnder component by evanescent wave coupling. The device can be constructed in such a way that the pumping light is totally coupled out of the Mach-Zehnder interferometer at the end of the region of evanescent wave coupling.

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Also, an active material may be provided in the cladding so as to obtain amplification and lasing by LR-SPP waveguides. In Figure 29A, the cladding 51 comprises active materials for providing stimulated emission upon either electrical or optical pumping. In Figure 10B, the stimulated emission in the active region of the cladding layers 51 amplifies an

30 incoming LR-SPP mode having a power P_in, so as to obtain an output mode Pout being amplified by the obtained gain so that P_out = Gain * P_rn. Figure 10C shows an example of an LR-SPP waveguide laser, wherein two feedback elements 53 and 54 are provided as coated end-facets of a waveguide sample. Hereby, spontaneous emission captured by the LR-SPP waveguide is amplified so as to provide laser action. By choosing a feedback

35 element 53 being a reflector with a reflectivity being higher than 98 % (high reflectivity) and a feedback element 54 being an output coupler having a reflectivity of less than 98 %, a laser beam having an output power P_out may be generated. In Figure 30, an example of a component comprising a 2.nd order non-linear material in the cladding 55 is shown. By providing electrical or optical pumping, as described above, such components may be used for frequency conversion, such as second harmonic generation or sum frequency generation.

Figure 31A, B and C show an example of stacked or layered structures comprising LR-SPP waveguides and/or other types of waveguides. Provided that the cores of the waveguides are in close proximity and provided that the propagation constants of two coupled modes in either waveguide are the same, the modes may couple from one plane to another. In Figure 31A, a standard TIR waveguide having a core 60 is embedded in a cladding layer 61. On top of the cladding 61, a core stripe 4 is deposited and a top cladding 62 is provided. Optionally, an electrode 63 is provided for electrical control of the coupling by tuning the coupling optimum frequency. In Figure 31B, two core stripes 4 and 64 are embedded in cladding 62. The distance between the stacked waveguides may be selected so as to allow for coupling between modes propagating in the two LR-SPP waveguides 4 and 64. In Figure 31C, a LR-SPP waveguide 4 is deposited on top of a fibre 65. A part of the fibre cladding is polished off and a core stripe 4 is deposited on the remaining polished cladding followed by deposition of a LR-SPP cladding 62. The cladding is polished off so that the distance between the fibre core 67 and the core stripe 4 allows for coupling between the modes propagating in the fibre 65 and the LR-SPP waveguide 4. An electrode 63 is provided to electrically control the coupling.

It is envisaged that such stacked structures may comprise more than two layers, such as 3, 4 and 5 layers and even up to 10 or 15 layers. It is an advantage of such stacked structures that they are easy to manufacture, especially compared to stacked standard TIR waveguides, being a very cumbersome process.

In Figure 32A and B, an add/drop switch 71 based on a structure comprising stacked wave guides is shown. The add/drop switch 71 is an example of a switching device incorporating optical fibres, such as single mode fibres or dispersion compensated fibres with LR-SPP waveguides. Light is coupled from one fibre to another via the LR-SPP guides. Light couples from the fibre to the LR-SPP guide only when the propagation constants are matched. A number of fibres 71 are provided being embedded in a substrate 72 having a refractive index substantially equal to the refractive index of the fibre cladding. The substrate is prepared so as to remove part of the upper cladding of the fibres 71. Along a predetermined length of the fibres, a LR-SPP waveguide core 4 is deposited as shown in Figure 31C. The top cladding layer 73 comprises an electro-optic material so that the application of an electric field from electrode 63 can modulate the propagation properties (the propagation constant) of the LR-SPP guide. The design shown comprises a number of LR-SPP to fibre couplers as shown in Figure 31C so as to be able to actively control the exchange of signal between the fibres.

To obtain a good spectral selectivity, so that only a specific selected mode is coupled from one waveguide to another, it is important that the propagation constant of the LR-SPP mode match the propagation constant in the other type of waveguide. The spectral width of the modes coupled from one waveguide to another is related to the difference in slopes of the dispersion curves for each of the waveguide. In Figure 33A, the dispersion curve 75 for a standard TIR waveguide and the dispersion curve 76 for a LR-SPP waveguide is shown. It is seen that the slopes of the curves 75 and 76 are very similar around the point of intersection 77 where phase matching occurs. In Figure 33B, the dispersion curve 76 for a LR-SPP is shown together with the dispersion curve 78 for a dispersion compensated fibre. It is seen that the slopes of the curves 76 and 78 are not similar around the point of intersection 79 where phase matching occurs. The coupling between a dispersion compensated fibre and the LR-SPP waveguide may, thus, provide an increased spectral selectivity.

Claims

1. A thermo-active waveguide section for guiding and/or modulating long range surface plasmon poiaritons (LR-SPPs), the LR-SPPs having at least a first frequency v₀ and a corresponding first wavelength λ₀, the device comprising

- a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) kj with a negative real part, Re(kj) < 0, in at least a first frequency range comprising the first frequency,

- a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines the waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, wherein the core stripe acts as a heating element in that the thermo-active waveguide section further comprises means for receiving electrical and/or magnetic and/or thermal and/or electromagnetic energy for adjusting a temperature of the core stripe and, via heat transfer from the core stripe, adjacent cladding material(s).

2. A thermo-active waveguide section according to claim 1, wherein the means for receiving energy comprises first and second electrically conducting connectors, each having

- terminal end parts for being connected to different terminals of a current supply, and contacting end parts in electrical contact with different parts of the core stripe of the waveguide section so that a current supply connected to the terminal end parts will generate a current through at least part of the core stripe.

3. A thermo-active waveguide section according to any of the preceding claims, wherein the means for receiving energy comprises an electrically conducting, closed, induction loop for receiving magnetic energy, the core stripe forming part of said electrically conducting, closed, induction loop.

4. A thermo-active waveguide section according to any of the preceding claims, wherein the means for receiving energy comprises at least a part of the cladding which is at least substantially transparent to EM radiation having a second frequency.

5. A thermo-active waveguide section according to any of the preceding claims, wherein a first region of the cladding above the core stripe comprises a material having a temperature dependent refractive index n_α(T) and a second region of the cladding below the core stripe comprises a material having a temperature dependent refractive index n₂(T), wherein n^T) and n₂(T) have different thermo-optic coefficients, dnJdT Φ dn₂/dT.

6. A thermo-active waveguide section according to any of the preceding claims, wherein the thermal properties such as heat capacity, thermal conductivity or convection factor of a portion of the waveguide section lying above a first part of the core stripe is different from the thermal properties of a corresponding portion of the waveguide section lying below said first part of the core stripe.

7. A thermo-active waveguide section according to any of the preceding claims, wherein the cladding comprises a first cladding region composed by a material having a refractive index nj(T) and a second cladding region composed by a material having a refractive index ⁿ ₂θ^~), the interface between the first and the second cladding region being patterned to form a grating and the materials being chosen so that nι(T,) = n₂(T,) at a first temperature T„ at which the grating is turned off, and so that n_α(T_f) ≠ n₂(T_f) at a second temperature T_f, at which the grating is turned on.

8. A device for guiding and/or modulating long range surface plasmon poiaritons (LR-SPPs) comprising a thermo-active waveguide section according to any of the preceding claims and a source of electrical and/or magnetic and/or thermal and/or electromagnetic energy for supplying energy to the receiving means of the thermo-active waveguide section.

9. A device according to claim 8, wherein the source comprises a current supply having at least a first and a second terminal in electrical connection with different parts of the core stripe of the thermo-active waveguide section.

10. A device according to claim 8, wherein the source comprises a current supply electrically connected to the core stripe of the thermo-active waveguide section allowing the current supply to generate a current through at least part of the core stripe.

11. A device according to claim 8, wherein the source comprises a source of heat in thermal contact with the core stripe.

12. A device according to claim 8, wherein the source comprises a source of electromagnetic (EM) radiation for emitting EM radiation of a second frequency, and wherein at least part of the cladding is at least substantially transparent to EM radiation with said second frequency.

13. A device for guiding and/or modulating long range surface plasmon poiaritons (LR- SPPs) comprising a thermo-active waveguide section according to any of claims 1 to 8 and another LR-SPP waveguide section positioned in continuation of, but electrically insulated from, the thermo-active waveguide section so that an LR-SPP mode can be coupled from

5 the thermo-active waveguide section to the another LR-SPP waveguide section.

14. A Mach Zehnder interferometer comprising at least one thermo-active waveguide section according to any of claims 1 to 8 in at least one arm.

10 15. A digital optical switch comprising at least one thermo-active waveguide section according to any of claims 1 to 8 in at least one arm.

16. An in-line modulator comprising at least one thermo-active waveguide section according to any of claims 1 to 8 in at least one arm.

15

17. A method for adjusting a propagation constant of a long-range surface plasmon polariton (LR-SPP) mode(s) in a LR-SPP waveguide comprising

- a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) i with a negative real

20 part, Re(kχ) < 0, in at least a first frequency range comprising the first frequency,

- a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a 25 first waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, the method comprising the step of adjusting a refractive index, n₂(T), of at least part of the one or more cladding materials by adjusting a temperature of the core stripe.

30 18. A method according to claim 17, wherein the temperature of the core stripe is adjusted by inducing a current in the core stripe using an external magnetic field.

19. A method according to claims 17 and 18, wherein the temperature of the core stripe is adjusted by propagating SPPs and/or LR-SPPs in the core stripe.

35

20. A method according to claims 17 to 19, wherein the temperature of the core stripe is adjusted by conducting an electrical current through the core stripe.

21. A method according to claims 17 to 20, wherein the temperature of the core stripe is adjusted by providing a thermal connection between the core stripe and a heat source.

22. A method according to claims 17 to 21, wherein the temperature of the core stripe is adjusted by illumination at least part of the core stripe with electromagnetic radiation.

23. A waveguide section for guiding and/or modulating long range surface plasmon poiaritons having at least a first frequency v₀ and a corresponding first wavelength λ₀, the device comprising - a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) k_α with a negative real part, Re(k_x) < 0, in at least a first frequency range comprising the first frequency, - a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, wherein the cladding comprises a first cladding layer having a refractive index n_t and a second electro-optic cladding layer having a refractive index n₂(E), the interface between the first and the second cladding layer being patterned to form a grating being adapted to be off when n₂(E) = ni and to be turned on when n₂(E) Φ n .

24. A waveguide section according to claim 23, further comprising means for applying an electric field E over the first and second cladding layers.

25. A waveguide section for guiding and/or modulating long range surface plasmon poiaritons having at least a first frequency v₀ and a corresponding first wavelength λ₀, the device comprising - a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) kj with a negative real part, Re(kj) < 0, in at least a first frequency range comprising the first frequency, a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, wherein at least a first region of the cladding and/or at least a second region of the cladding comprise(s) a material having an intensity dependent refractive index n(I), wherein at least a part of the first region is above the core stripe and/or at least a part of the second region is below the core stripe, and wherein the device further comprises means for providing pumping light of intensity I to pump the material to modulate the refractive index n(I) thereby modulating the complex propagation constant of the LR-SPP modes guided by the LRSSP waveguide.

26. A waveguide section according to claim 25, further comprising a pump light source for illuminating the cladding regions having a refractive index n(I) from a top, bottom or side end of the device and/or along the LR-SPP waveguide.

27. A waveguide section according to claim 25, wherein the first region of the cladding and/or the second region of the cladding comprises an optical non-linear material, so that propagation of LR-SPPs having frequencies α>ι, co₂, etc. generates LR-SPPs having harmonic frequencies 2α>ι, 2ω₂, ωi ± ω₂.

28. A waveguide section for guiding and/or amplifying long range surface plasmon poiaritons having at least a first frequency v₀ and a corresponding first wavelength λ₀, the device comprising a core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) k₂ with a negative real part, Re(k_α) < 0, in at least a first frequency range comprising the first frequency, - a cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide section supporting propagation of the long range surface plasmon polariton (LR-SPP) in the cladding, wherein at least a part of the cladding comprises an active material, the active material being adapted to provide stimulated emission when pumped optically and/or electrically.

29. A waveguide section according to claim 28, wherein the active material comprises a semiconductor material selected from the group consisting of InP, GaAs, Ge, and Si.

30. A waveguide section according to claim 28 or 29, wherein the active material comprises a material being doped with rare earths, such as Erbium.

31. A waveguide section according to any of claims 28-30, wherein the device is adapted to amplify an incoming wave.

32. A waveguide section according to any of claims 28-31, wherein the device further comprises at least two feedback elements positioned so as to form a cavity embedding the active material, whereby an incoming wave is amplified by stimulated emission so as to provide a narrow output beam of substantially coherent, monochromatic light.

33. A device for guiding long range surface plasmon poiaritons having at least a first frequency v₀ and a corresponding first wavelength λ₀, the device comprising

- at least one core stripe having a finite width w_m and a thickness t_m, the core stripe comprising one or more material(s) having complex dielectric constant(s) k_α with a negative real part, Re(k < 0, in at least a first frequency range comprising the first frequency, the core stripe being at least substantially defined in a first plane, - at least one cladding at least substantially embedding the core stripe, the cladding comprising one or more material(s) having complex dielectric constants k₂ with a positive real part, Re(k₂) > 0, in at least a second frequency range comprising the first frequency, the finite width w_m, and the thickness t_m being selected so that the core stripe defines a first waveguide supporting propagation of the long range surface plasmon polariton (LR- SPP) in the cladding, the device further comprising another type of waveguide being at least substantially defined in a second plane which is displaced and/or rotated in relation to the first plane, the at least two waveguides defining a three dimensional structure, and the at least two waveguides are being adapted to allow for controllable coupling of energy between modes guided by the at least one LR-SPP waveguide and the other type of waveguide along a predetermined length of the LR-SPP waveguide.