Switchable polarization rotators

Abstract

Structure (10) for a polarization rotator, wherein the structure (10) comprises: a substrate (16); a first waveguide core (12) over the substrate (16), wherein the first waveguide core (12) comprises a section (18, 20); a second waveguide core (40) above the substrate (16), wherein the second waveguide core (40) is positioned near the section (18, 20) of the first waveguide core (12), and the second waveguide core (40) is vertically displaced from the first waveguide core (12), and a third waveguide core (60) which is vertically displaced from the first waveguide core (12), wherein the third waveguide core (60) comprises a section (70) which is positioned in an overlapping relationship with the section (18, 20) of the first waveguide core (12), wherein the second waveguide core (40) consists of a first material having a refractive index that is reversibly variable in response to a stimulus, wherein the first waveguide core (12) consists of a second material different from the first material, and the third waveguide core (60) consists of a third material different from both the first and second material, and the section (70) of the third waveguide core (60) and the section (18, 20) of the first waveguide core (12) define a polarization region configured to rotate a polarization mode of light, and the second waveguide core (40) is configured to absorb the light when the stimulus is applied to the first material.

Description

BACKGROUND

[0001] The present invention relates to photonic chips and in particular structures for a polarization rotator and methods for manufacturing a structure for a polarization rotator.

[0002] Photonic chips are used in many applications and systems, including but not limited to data communication and computing systems. A photonic chip integrates optical components, such as waveguides, optical switches, and directional couplers, and electronic components, such as field-effect transistors, into a single platform. Integrating both types of components onto the same chip reduces layout space, costs, and operational overhead, among other factors. US Patent 6,311,004 B1 describes the use of a heat source near the active region of the waveguide to influence the temperature-dependent refractive index. JP H10-90,543 A discloses a meandering waveguide with laterally arranged grooves, where the penetration of field lines in the outer circumferential direction by the grooves in the curvilinear waveguides is suppressed.US Patent 10,996,398 B1 discloses a polarization rotator in which a first and a second waveguide core are located side by side in a coupling region, and a third waveguide core is arranged above the second waveguide core in the coupling region. The third waveguide core consists of a material with a variable refractive index, and the first and second waveguides consist of the same single-crystal semiconductor material. US Patent 2008 / 0080808 A1 discloses an integrated photonic arrangement whose waveguides represent a multilayer III-V semiconductor structure.

[0003] Polarization rotators are another type of optical component commonly found in photonic chips. A polarization rotator can be configured to receive optical signals of a given polarization state (e.g., the fundamental transverse magnetic (TM0) mode) as input and output a different polarization state (e.g., the fundamental transverse electric (TE0) mode). Polarization rotators are passive optical components with optical performance that cannot be tuned, switched, or otherwise configured.

[0004] Improved structures for a polarization rotator and methods for manufacturing a structure for a polarization rotator are needed. BRIEF SUMMARY

[0005] In one embodiment of the invention, a structure for a polarization rotator is provided. The structure comprises a substrate, a first waveguide core above the substrate, and a second waveguide core above the substrate. The second waveguide core is positioned near the section of the first waveguide core. The second waveguide core is made of a material having a refractive index that is reversibly variable in response to a stimulus.

[0006] In one embodiment of the invention, a method for forming a structure for a polarization rotator is provided. The method comprises forming a first waveguide core over a substrate and forming a second waveguide core over the substrate. The second waveguide core is positioned near a section of the first waveguide core, and the second waveguide core is made of a material having a refractive index that is reversibly variable in response to a stimulus. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The accompanying drawings, which are incorporated into and form part of this specification, illustrate various embodiments of the invention and, together with the general description of the invention given above and the detailed description of the embodiments given below, serve to explain these embodiments. In the drawings, the same reference numerals refer to the same features in the different views. Fig. Figure 1 is a top view of a structure in an initial manufacturing stage of a processing method according to embodiments of the invention. Fig. Figure 2 is a cross-sectional view of the entire structure along a line 2-2 in Fig. 1. Fig. Figure 3 is a top view of the structure at a manufacturing stage following Fig. 1. Fig. Figure 4 is a cross-sectional view of the entire structure along a line 4-4 in Fig. 3. Fig. Figure 5 is a cross-sectional view of the structure at a manufacturing stage following the Fig. 3, Fig. 4. Fig. Figure 6 is a top view of a structure according to alternative embodiments of the invention. Fig. Figure 7 is a cross-sectional view of the entire structure along a line 7-7 in Fig. 6. Fig. Figure 8 is a top view of a structure according to alternative embodiments of the invention. Fig. Figure 9 is a cross-sectional view of the entire structure along a line 9-9 in Fig. 8. Fig. Figure 10 is a top view of a structure according to alternative embodiments of the invention. Fig. Figure 11 is a cross-sectional view of the entire structure along a line 11-11 in Fig. 10. Fig. Figure 12 is a cross-sectional view of the structure at a manufacturing stage following the Fig. 10, Fig. 11. DETAILED DESCRIPTION

[0008] With reference to the Fig. 1, Fig. 2 and according to embodiments of the invention, a structure 10 for a switchable polarization rotator comprises a waveguide core 12 positioned on a dielectric layer 14 and over a substrate 16. The waveguide core 12 can be made of a semiconductor material, such as single-crystal silicon from a fixture layer of a silicon-on-insulator (SOI) wafer. The silicon-on-insulator wafer further comprises a buried insulator layer encompassed by a dielectric material, such as silicon dioxide, which provides the dielectric layer 14, and the substrate 16 can be comprised of a semiconductor material, such as single-crystal silicon. The waveguide core 12 can be structured from the fixture layer of the silicon-on-insulator wafer by lithography and etching processes during front-end-of-line processing.The fixture layer of the silicon-on-insulator wafer can be fully etched to define a ridge waveguide, as shown, or alternatively only partially etched near the waveguide core 12 to define a rib waveguide.

[0009] The waveguide core 12 comprises a section 18, a section 20, and a section 22, which are connected in series and aligned along a longitudinal axis 24. The section 20 is positioned between the section 18 and the section 22. The section 20 tapers inversely (i.e., widens) in one direction from the section 18 to the section 22 and, conversely, tapers (i.e., narrows) in one direction from the section 22 to the section 18. The direction of widening can be the direction of light propagation through the structure 10. The section 20 can intersect the section 18 at one end and the section 22 at its opposite end. The width of the section 20 changes along its length from a width W1 at its intersection with the section 18 to a width W2 at its intersection with the section 22. In one embodiment, the width W2 is greater than the width W1.In one embodiment, sections 18 and 22 can be neither tapered nor straight. Section 18 can have a width W1 over the portion of its length adjacent to section 20, and section 22 can have a width W2 over the portion of its length adjacent to section 20.

[0010] With reference to the Fig. 3, Fig. 4, in which the same reference signs refer to the same features in the Fig. 1, Fig. 2, and in a subsequent manufacturing stage, a dielectric layer 32 is formed over the waveguide core 12. The dielectric layer 32 can be comprised of a dielectric material, such as silicon dioxide, which is deposited by chemical vapor deposition and planarized, for example, by chemical-mechanical polishing to remove topography and provide a flat surface for subsequent deposits. The waveguide core 12 is arranged within the dielectric material of the dielectric layer 32, which acts as a low-index cladding.

[0011] Additional dielectric layers 34, 36, and 38 can be formed on dielectric layer 32. Dielectric layer 34 and dielectric layer 38 can be encapsulated by silicon dioxide, and dielectric layer 36 can be encapsulated by silicon nitride. In one embodiment, dielectric layer 36 can be omitted from the layer stack.

[0012] A waveguide core 40 can be positioned above the substrate 16 on the dielectric layer 38. The waveguide core 40 can be formed by depositing a layer onto the dielectric layer 38 and structuring the deposited layer using lithography and etching processes. For this purpose, an etch mask is formed over the deposited layer by a lithography process, and unmasked sections of the deposited layer are etched and removed by an etching process, such as reactive ion etching. The shape of the etch mask determines the structured shape of the waveguide core 40. The etching process can be selected to stop at the material of the dielectric layer 38 after it has completely penetrated the deposited layer.

[0013] In one embodiment, the waveguide core 40 can consist of an active material having a refractive index that can be tuned (i.e., switched) by applying a stimulus, such as an electrical, optical, or thermal stimulus. In another embodiment, the waveguide core 40 can consist of a material characterized by a refractive index that is reversibly variable between different states characterized by significantly different refractive indices and optical absorption properties. In yet another embodiment, the refractive index of the active material can have several different states in which the real and imaginary parts are significantly different. For example, the refractive index of the active material can have a higher real part and a lower imaginary part in one state than in another.In embodiments, the stimulus used to induce the reversible variation between the different refractive index states can be heat from a resistance heating device providing a temperature change, an electric field provided by an applied voltage, an electric current provided by an applied voltage, or optical pumping by pump light provided by an external laser.

[0014] In one embodiment, the active material contained in the waveguide core 40 can be a conductive oxide, such as indium tin oxide, where the real and imaginary parts of its refractive index can be approximately 2 and approximately zero, respectively, in one tuned state, and approximately 1 and approximately 0.3, respectively, in another tuned state. In another embodiment, the active material contained in the waveguide core 40 can be a phase-change material, such as vanadium oxide or germanium antimony telluride. For example, vanadium oxide undergoes a reversible metal-insulator phase change between its metallic and insulating states near a temperature of 68°C. In yet another embodiment, the active material contained in the waveguide core 40 can be a two-dimensional material, such as graphene or molybdenum disulfide.In one embodiment, the active material contained in the waveguide core 40 can be an electro-optical polymer.

[0015] The waveguide core 40 is positioned near the waveguide core 12 and is vertically offset from it. The waveguide core 40 is located on a different level within the structure 10 than the waveguide core 12. Specifically, the waveguide core 12 is positioned on a level between the level containing the substrate 16 and the level containing the waveguide core 40. The proximity of the waveguide core 40 to the waveguide core 12 provides sufficient spatial proximity to enable switching by light absorption.

[0016] The waveguide core 40 comprises a section 48, a section 50, and a section 52, which are serially adjacent to one another and connected with an orientation along a longitudinal axis 54 that changes direction in the different sections 48, 50, and 52. In one embodiment, section 48 of the waveguide core 40 can be positioned in an overlapping arrangement with section 18 of the waveguide core 12, and section 50 of the waveguide core 40 can be positioned in an overlapping arrangement with section 20 of the waveguide core 12. Section 50 is arranged between sections 48 and 52. Section 50 tapers (i.e., narrows) in one direction from section 48 to section 52 and, conversely, tapers (i.e., widens) in the opposite direction from section 52 to section 48. Section 50 can intersect section 48 at one end and section 52 at its opposite end.The width of section 50 changes along its length from a width W3 at its intersection with section 48 to a width W4 at its intersection with section 52. In one embodiment, the width W4 is greater than the width W3. In another embodiment, sections 48 and 52 can be neither tapered nor straight. Section 48 can have a width W3 over the portion of its length adjacent to section 50, and section 52 can have a width W4 over the portion of its length adjacent to section 50.

[0017] The waveguide core 40 can terminate at one end 42 and at an end 44 opposite end 42. Section 48 of the waveguide core 40 can extend from end 42 at an angle relative to the waveguide core 12, and section 48 can be positioned to overlap with section 18 of the waveguide core 12. Section 50 of the waveguide core 40 can be arranged to partially overlap the waveguide core 12 and can be positioned mainly over section 20 of the waveguide core 12. Section 52 of the waveguide core 40 can be angled to diverge away from the waveguide core 12 and can terminate at end 44.

[0018] In an alternative embodiment, the waveguide core 40 can be shaped with adiabatic bends. As used herein, an adiabatic bend comprises gradual changes in direction, so that optical signals can propagate without a significant reduction in light confinement.

[0019] With reference to Fig. 5, in which identical reference signs refer to identical features in the Fig. 4, and in a subsequent manufacturing stage, a dielectric layer 55 can be formed over the dielectric layer 38 and the waveguide core 40. The dielectric layer 55 can be comprised of dielectric material, such as silicon dioxide, deposited by chemical vapor deposition or atomic layer deposition. In one embodiment, the dielectric layer 55 can be comprised of a dielectric material, such as silicon dioxide, deposited by chemical vapor deposition using tetraethyl orthosilicate (TEOS) as a reactant.

[0020] A back-end-of-line stack 56 can be formed by back-end-of-line processing over the dielectric layer 55. The back-end-of-line stack 56 can comprise one or more dielectric layers encapsulated by a dielectric material, such as silicon dioxide or a low-k dielectric material.

[0021] In one embodiment, a heating device 58 can be positioned near the waveguide core 40. In the representative embodiment, the heating device 58 is positioned in the back-end-of-line stack 56 near the waveguide core 40. The proximity of the heating device 58 to the waveguide core 40 is sufficient to provide the stimulus for a change in the refractive index of the active material of the waveguide core 40 through temperature changes.

[0022] The heating element 58 can be comprised of a deposited and structured metal, such as nickel-chromium, tantalum nitride, or titanium nitride. The heating element 58 can be coupled via metal features 61, 62 in the back-end-of-line stack 56 to a power supply 64 for driving the heating element 58 to provide a temperature change that causes the refractive index change of the active material of the waveguide core 40. During operation, heat is transferred from the driven heating element 58 through the dielectric material of the back-end-of-line stack 56 to the waveguide core 40. The temperature of the waveguide core 40 can be varied above and below the transition temperature by selectively applying heat as a stimulus to provide the different refractive indices.

[0023] When used, optical signals propagating in a transverse magnetic (TM) mode can be guided through the waveguide core 12 to the input of the structure 10 on the photonic chip. If the active material of the waveguide core 40 is tuned to a refractive index state (e.g., by a temperature lower than a phase-change transition temperature), the waveguide cores 12 and 40 work together to invert the polarization mode of the light embodied in the optical signals from the TM mode to the transverse electric (TE) mode, which is output by the structure 10.In particular, the overlapping arrangement of section 18 of waveguide core 12 and section 48 of waveguide core 40 and / or the overlapping arrangement of section 20 of waveguide core 12 and section 50 of waveguide core 40 can provide a polarization in which the polarization mode of the light is rotated. Depending on the length of the waveguide cores 12, 40, the light emerging from the structure 10 can be polarized such that it comprises only TE mode light, or alternatively, it can comprise a polarization mixture that includes light containing a TE mode component and light containing a TM mode component. If the active material of the waveguide core 40 is tuned to a different refractive index state (e.g.,(by a temperature that is higher than a phase change transition temperature), the waveguide core 40 can effectively block light transmission through the structure 10 by absorbing and diverting the optical input signals, thereby providing a light circuit.

[0024] Structure 10 can be integrated into a photonic chip in any of the embodiments described here, comprising electronic components and additional optical components. For example, the electronic components can include field-effect transistors fabricated by CMOS front-end-of-line (FEOL) processing.

[0025] With reference to the Fig. 6, Fig. 7 and according to alternative embodiments of the invention, a waveguide core 60 can be positioned above the substrate 16 in an overlapping relationship with the waveguide core 12, and the waveguide core 40 can be laterally offset to be positioned in a non-overlapping relationship with the waveguide core 12. The waveguide core 60 can be positioned near the waveguide core 40 and at the same level within the structure 10. Consequently, both waveguide cores 40, 60 are arranged at a different level within the structure 10 than the waveguide core 12. In this respect, the waveguide core 12 is positioned at a level between a level containing the substrate 16 and a level containing the waveguide cores 40, 60.

[0026] The waveguide core 60 can be formed by depositing a layer onto the dielectric layer 38 and structuring the deposited layer using lithography and etching processes. For this purpose, an etch mask is formed over the deposited layer by a lithography process, and unmasked sections of the deposited layer are etched and removed by an etching process, such as reactive ion etching. The shape of the etch mask determines the structured shape of the waveguide core 60. The etching process can be selected to stop at the material of the dielectric layer 38 after it has completely penetrated the deposited layer. In one embodiment, the waveguide core 60 can be made of a dielectric material, such as silicon nitride. In an alternative embodiment, the waveguide core 60 can be made of polysilicon. The waveguide core 60 can be masked during the formation of the waveguide core 40.

[0027] The waveguide core 60 comprises a section 68, a section 70, and a section 72, which are connected in series and aligned along a longitudinal axis 74 that changes direction in the different sections 68, 70, and 72. Section 70 is positioned between sections 68 and 72. Section 70 tapers (i.e., narrows) in one direction from section 68 to section 72 and, conversely, tapers (i.e., widens) in the opposite direction from section 72 to section 68. Section 70 may intersect section 68 at one end and section 72 at its opposite end. The width of section 70 narrows along its length from its intersection with section 68 to its intersection with section 72. In one embodiment, sections 68 and 72 may be straight or not tapered.

[0028] The waveguide core 60 can be vertically displaced from the waveguide core 12 and laterally displaced from the waveguide core 40. In the representative embodiment, section 68 of the waveguide core 60 can be positioned in an overlapping arrangement with section 18 of the waveguide core 12, and section 70 of the waveguide core 60 can be positioned in an overlapping arrangement with section 20 of the waveguide core 12. Section 68 of the waveguide core 60 can be inclined at an angle relative to the waveguide core 12. Section 72 of the waveguide core 60 can be inclined at an angle to diverge away from the waveguide core 12.

[0029] The waveguide core 40 comprises section 48, which is laterally offset from section 68 of the waveguide core 60, and section 50, which is laterally offset from section 70 of the waveguide core 60. Sections 48 and 50 are angled relative to each other, and section 50 may not be tapered. The longitudinal axis 54 of section 48 of the waveguide core 40 may be oriented parallel or substantially parallel to the longitudinal axis 74 of section 68 of the waveguide core 60 such that section 48 and section 68 are inclined at either the same or substantially the same angle relative to section 18 of the waveguide core 12.The longitudinal axis 54 of section 50 of the waveguide core 40 can be aligned parallel or substantially parallel to the longitudinal axis 74 of section 70 of the waveguide core 60 such that section 50 and section 70 are inclined either at the same angle or substantially at the same angle relative to section 20 of the waveguide core 12.

[0030] In an alternative embodiment, the waveguide core 60 can be shaped with adiabatic bends. As used herein, an adiabatic bend comprises gradual changes in direction, so that optical signals can propagate without a significant reduction in light confinement.

[0031] When used when the active material of the waveguide core 40 is tuned to a refractive index state (e.g., by a temperature lower than a phase transition temperature), the waveguide cores 12 and 60 work together to rotate the polarization mode of the light embodied in the optical signals from the TM mode to the transverse-electrical (TE) mode output by the structure 10. Specifically, the overlapping arrangement of section 18 of waveguide core 12 and section 68 of waveguide core 60, and / or the overlapping arrangement of section 20 of waveguide core 12 and section 70 of waveguide core 60, can provide a polarization region in which the polarization mode of the light is rotated.Depending on the length of the waveguide cores 12, 60, the light emerging from the structure 10 can be polarized to include only TE mode light, or alternatively, it can include a polarization mixture containing light with a TE mode component and light containing a TM mode component. If the active material of the waveguide core 40 is tuned to a different refractive index state (e.g., by a temperature higher than a phase-change transition temperature), the waveguide core 40 can effectively block light transmission through the structure 10 by absorbing and diverting the optical input signals, thereby providing light switching.

[0032] With reference to the Fig. 8, Fig. 9 and according to alternative embodiments of the invention, the waveguide core 40 can be vertically displaced from the waveguide core 60 and positioned to have an overlapping relationship with the waveguide core 60. In this respect, the waveguide core 40 can be positioned at a different level than the waveguide core 60 within the structure 10. Consequently, each of the waveguide cores 12, 40, 60 is arranged at a different level within the structure 10, with the waveguide core 60 being positioned vertically between the waveguide core 40 and the waveguide core 12.

[0033] When used when the active material of the waveguide core 40 is tuned to a refractive index state (e.g., by a temperature lower than a phase transition temperature), the waveguide cores 12 and 60 work together to rotate the polarization mode of the light embodied in the optical signals from the TM mode to the transverse-electrical (TE) mode output by the structure 10. Specifically, the overlapping arrangement of section 18 of waveguide core 12 and section 68 of waveguide core 60, and / or the overlapping arrangement of section 20 of waveguide core 12 and section 70 of waveguide core 60, can provide a polarization region in which the polarization mode of the light is rotated.Depending on the length of the waveguide cores 12, 60, the light emerging from the structure 10 can be polarized to include only TE mode light, or alternatively, it can include a polarization mixture containing light with a TE mode component and light containing a TM mode component. If the active material of the waveguide core 40 is tuned to a different refractive index state (e.g., by a temperature higher than a phase-change transition temperature), the waveguide core 40 can effectively block light transmission through the structure 10 by absorbing and diverting the optical input signals, thereby providing light switching.

[0034] With reference to the Fig. 10, Fig. 11 and according to alternative embodiments of the invention, the waveguide core 40 can be displaced laterally from the waveguide core 12 and positioned near the waveguide core 12 in a side-by-side and non-overlapping relationship and at the same level of the structure 10 as the waveguide core 12.

[0035] With reference to Fig. 12, in which identical reference signs refer to identical features in the Fig. 10, Fig. Referring to 11, and in a subsequent manufacturing stage, the dielectric layers 34, 36, 38 can be formed over the waveguide cores 12, 40, and the waveguide core 60 can be formed on the dielectric layer 38 at a different level than the waveguide cores 12, 40. The waveguide core 60 can be vertically displaced from the waveguide core 12 and positioned in an overlapping arrangement with the waveguide core 12.

[0036] When used when the active material of the waveguide core 40 is tuned to a refractive index state (e.g., by a temperature lower than a phase transition temperature), the waveguide cores 12 and 60 work together to rotate the polarization mode of the light embodied in the optical signals from the TM mode to the transverse-electrical (TE) mode output by the structure 10. Specifically, the overlapping arrangement of section 18 of waveguide core 12 and section 68 of waveguide core 60, and / or the overlapping arrangement of section 20 of waveguide core 12 and section 70 of waveguide core 60, can provide a polarization region in which the polarization mode of the light is rotated.Depending on the length of the waveguide cores 12, 60, the light emerging from the structure 10 can be polarized to include only TE mode light, or alternatively, it can include a polarization mixture containing light with a TE mode component and light containing a TM mode component. If the active material of the waveguide core 40 is tuned to a different refractive index state (e.g., by a temperature greater than a phase-change transition temperature), the waveguide core 40 can effectively block light transmission through the structure 10 by absorbing and diverting the optical input signals, thereby providing light switching.

[0037] The processes described above are used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the manufacturer in raw wafer form (e.g., as a single wafer containing multiple unpackaged chips), as a bare die, or in a packaged form. The chip can be integrated with other chips, discrete circuit elements, and / or other signal processing devices as part of either an intermediate or a final product. The final product can be any product that incorporates integrated circuit chips, such as computer products with a central processing unit or smartphones.

[0038] References herein to expressions modified by approximation language, such as "about," "approximately," and "essentially," are not to be limited to the specified precise value. The approximation language may correspond to the precision of an instrument used to measure the value and, unless otherwise dependent on the precision of the instrument, may be + / - 10% of the specified value(s).

[0039] References herein to terms such as "vertical," "horizontal," etc., are made for illustrative purposes only and not to limit or establish a frame of reference. The term "horizontal," as used herein, is defined as a plane parallel to a conventional plane of a semiconductor substrate, regardless of its actual three-dimensional spatial orientation. The terms "vertical" and "normal" refer to a direction perpendicular to the horizontal, as just defined. The term "lateral" refers to a direction within the horizontal plane.

[0040] A feature "connected" or "coupled" to another feature can be directly connected or coupled to the other feature, or one or more intervening features can be present. A feature can be "directly connected" or "directly coupled" to another feature if no intervening features are present. A feature can be "indirectly connected" or "indirectly coupled" to another feature if at least one intervening feature is present. A feature "at" or "contacting" another feature can be directly at or in direct contact with the other feature, or one or more intervening features can be present. A feature can be "directly at" or in "direct contact" with another feature if no intervening features are present.A feature can be "indirectly related" or in "indirect contact" with another feature if at least one intervening feature is present. Different features can "overlap" if one feature extends over another and covers part of it, either through direct or indirect contact.

[0041] The descriptions of the various embodiments of the present invention are presented for illustrative purposes only and are not intended to be exhaustive or limited to the disclosed embodiments. Many modifications and variations are obvious to those skilled in the art without altering the scope and concept of the described embodiments. The terminology used herein has been chosen to best explain the principles of the embodiments, their practical application, or technical improvements over commercially available technologies, or to enable other persons skilled in the art to understand the embodiments disclosed herein.

Claims

[1] Structure (10) for a polarization rotator, wherein the structure (10) comprises: a substrate (16); a first waveguide core (12) over the substrate (16), wherein the first waveguide core (12) comprises a section (18, 20); a second waveguide core (40) above the substrate (16), wherein the second waveguide core (40) is positioned near the section (18, 20) of the first waveguide core (12), and the second waveguide core (40) is vertically displaced from the first waveguide core (12), and a third waveguide core (60) which is vertically displaced from the first waveguide core (12), wherein the third waveguide core (60) comprises a section (70) which is positioned in an overlapping relationship with the section (18, 20) of the first waveguide core (12), wherein the second waveguide core (40) consists of a first material having a refractive index that is reversibly variable in response to a stimulus, wherein the first waveguide core (12) consists of a second material different from the first material, and the third waveguide core (60) consists of a third material different from both the first and second material, and the section (70) of the third waveguide core (60) and the section (18, 20) of the first waveguide core (12) define a polarization region configured to rotate a polarization mode of light, and the second waveguide core (40) is configured to absorb the light when the stimulus is applied to the first material. [2] Structure (10) according to claim 1, wherein the second waveguide core (40) is positioned in an overlapping relationship with the third waveguide core (60), and the third waveguide core (60) is positioned in a vertical direction between the first waveguide core (12) and the second waveguide core (40). [3] Structure (10) for a polarization rotator, wherein the structure (10) comprises: a substrate (16); a first waveguide core (12) over the substrate (16), wherein the first waveguide core (12) comprises a section (18, 20); a second waveguide core (40) above the substrate (16), wherein the second waveguide core (40) is positioned near the section (18, 20) of the first waveguide core (12), and the second waveguide core (40) is laterally displaced from the first waveguide core (12) in a non-overlapping relationship, and a third waveguide core (60) vertically displaced from the first waveguide core (12), wherein the third waveguide core (60) comprises a section (68, 70) positioned in an overlapping relationship with the section (18, 20) of the first waveguide core (12), and wherein the second waveguide core (40) is made of a first material having a refractive index that is reversibly variable in response to a stimulus, the first waveguide core (12) is made of a second material different from the first material, and the third waveguide core (60) is made of a third material that is different from both the first and second material, wherein the section (68, 70) of the third waveguide core (60) and the section (18, 20) of the first waveguide core (12) define a polarization region configured to rotate a polarization mode of light,and the second waveguide core (40) is configured to absorb the light when the stimulus is applied to the first material. [4] Structure (10) according to claim 3, wherein the third waveguide core (60) comprises a tapered section (70) which is positioned in an overlapping relationship with a tapered section of the first waveguide core (12). [5] Structure (10) for a polarization rotator, wherein the structure (10) comprises: a substrate (16); a first waveguide core (12) over the substrate (16), wherein the first waveguide core (12) comprises a section (18, 20); a second waveguide core (40) above the substrate (16), wherein the second waveguide core (40) is positioned near the section (18, 20) of the first waveguide core (12), and a third waveguide core (60) above the substrate (16), wherein the third waveguide core (60) is positioned near the first waveguide core (12) and the second waveguide core (40), wherein the second waveguide core (40) consists of a first material having a refractive index that is reversibly variable in response to a stimulus, wherein the first waveguide core (12) consists of a second material that is different from the first material, and the third waveguide core (60) consists of a third material that is different from both the first and the second material, and a section (68, 70) of the third waveguide core (60) and the section (18, 20) of the first waveguide core (12) define a polarization region configured to rotate a polarization mode of light, and the second waveguide core (40) is configured to absorb the light when the stimulus is applied to the first material. [6] Structure (10) according to claim 1, 3 or 5, further comprising: a heating device (58) in the vicinity of the second waveguide core (40), wherein the heating device (58) is configured to selectively transfer heat as the stimulus to the second waveguide core (40). [7] Structure (10) according to claim 1, 3 or 5, wherein the second material comprises single-crystal silicon and the third material comprises silicon nitride or polysilicon. [8] Structure (10) according to claim 3 or 5, wherein the first material comprises a conductive oxide, a phase-change material, a two-dimensional material or an electro-optical polymer.

Images