Optical device with low-loss thermo-tunable closed curve optical waveguide

Abstract

The present invention relates to optical devices with low-loss thermally tunable closed curve optical waveguides, and discloses photonic structures and related methods. The structure includes a closed curve waveguide having a first height measured from a top surface of an insulating layer, and an outer curved sidewall extending substantially perpendicularly throughout the first height (e.g., to minimize signal loss). The structure includes a closed curve thermal coupler and a heating element. The closed curve thermal coupler is thermally coupled to and laterally surrounded by the closed curve waveguide and has a second height that is less than the first height. In some embodiments, the closed curve waveguide and the closed curve thermal coupler are contiguous portions of the same semiconductor layer having different thicknesses. The heating element is thermally coupled to the closed curve thermal coupler and thereby indirectly thermally coupled to the closed curve waveguide. Thus, the heating element can be used to thermally tune the closed curve waveguide through the closed curve thermal coupler to minimize any temperature-dependent resonance shift (TDRS).

Description

TECHNICAL FIELD

[0001] The present invention relates to temperature-sensitive closed-curve optical waveguides, and more particularly, to embodiments of photonic structures including optical devices having low-loss, thermally tunable, closed-curve optical waveguides. BACKGROUND

[0002] An optical ring resonator includes a plurality of optical waveguides. In the simplest optical ring resonator, the optical waveguides include a bus waveguide (i.e., an optical waveguide having discrete ends including an input end and an output end) and a closed-curve waveguide (i.e., an optical waveguide having a complete loop or a ring shape having no discrete ends) that is spatially separated from but optically coupled to the bus waveguide. An optical signal can enter the bus waveguide at the input end. Due to the optical coupling, some of the optical signal will enter the closed-curve waveguide from the bus waveguide, and some of the optical signal will enter the bus waveguide from the closed-curve waveguide. The optical signal will also exit the bus waveguide at the output end. However, within the closed-curve waveguide, the optical signal of a particular resonant wavelength of the closed-curve waveguide will repeatedly traverse the closed-curve waveguide, e.g., due to constructive interference, increasing in strength. As a result, the optical signal entering the bus waveguide from the closed-curve waveguide will significantly have the particular resonant wavelength. Thus, such a ring resonator can effectively be used as a filter. However, depending on the properties of the core material used, the closed-curve waveguide can be temperature-sensitive. That is, they can exhibit temperature-dependent resonance shifts (TDRS). For example, closed-curve silicon waveguides are known to be temperature-sensitive, and the potential TDRS can be, for example, about 70 picometers per Kelvin (pm / K) or more. To minimize the TDRS, many photonic structures including closed-curve waveguides also include a corresponding heater to thermally tune (i.e., heat) the closed-curve optical waveguide. Unfortunately, current available configurations of such photonic structures tend to exhibit performance degradation (e.g., optical power loss). SUMMARY

[0003] In view of the foregoing, disclosed are embodiments of a photonic structure. The photonic structure can include an optical device. The optical device can include one or more optical waveguides, including at least one closed curve waveguide. For example, in some embodiments, the optical device can be a ring resonator that includes at least one bus waveguide and at least one closed curve waveguide positioned laterally adjacent to the bus waveguide, more specifically, spatially separated from but optically coupled to the bus waveguide. The closed curve waveguide can have a first height, and in particular, an outer curved sidewall that extends vertically throughout the first height. The photonic structure can also include a closed curve thermal coupler. The closed curve thermal coupler can be laterally surrounded by and thermally coupled to the closed curve waveguide. The closed curve thermal coupler can also have a second height that is less than the first height of the closed curve waveguide. In some embodiments, the closed curve waveguide and the closed curve thermal coupler are contiguous portions of the same semiconductor layer (e.g., the same silicon layer) having different thicknesses. The photonic structure can also include a heating element. The heating element can be adjacent to the closed curve thermal coupler, and more specifically, thermally coupled to the closed curve thermal coupler and thereby indirectly thermally coupled to the closed curve waveguide. As such, thermal energy generated and output by the heating element can enter the closed curve thermal coupler, pass through the closed curve thermal coupler, and in turn, enter the closed curve waveguide. Thus, the heating element can be used to thermally tune the closed curve waveguide through the closed curve thermal coupler to minimize any temperature dependent resonance shift (TDRS).

[0004] The present invention also discloses embodiments of a method for forming the above-described photonic structure. The method may include forming an optical device. The process of forming the optical device may include forming one or more optical waveguides, including at least one closed-curve waveguide. For example, in some embodiments, the process of forming the optical device may include forming a ring resonator comprising at least one bus waveguide and at least one closed-curve waveguide laterally adjacent to the bus waveguide, and more specifically, spatially separated from but optically coupled to the bus waveguide. The closed-curve waveguide of the optical device may be formed having a first height, particularly having an outer curved sidewall extending substantially vertically throughout the first height. The method may also include forming a closed-curve thermal coupler. The closed-curve thermal coupler may be formed laterally surrounded by and thermally coupled to the closed-curve waveguide, and may also have a second height less than the first height of the closed-curve waveguide. In some embodiments, the closed-curve waveguide and the closed-curve thermal coupler are continuous portions of the same semiconductor layer (e.g., the same silicon layer) with different thicknesses. The method may also include forming a heating element. The heating element may be formed adjacent to the closed-curve thermal coupler, such that it is thermally coupled to the closed-curve thermal coupler and thereby indirectly thermally coupled to the closed-curve waveguide. The method may further include using a heating element to thermally tune a closed-curve waveguide via a closed-curve thermal coupler (e.g., using a heating element to generate and output thermal energy that enters and passes through the closed-curve thermal coupler and further into the closed-curve waveguide) to minimize any temperature-dependent resonant displacement (TDRS). Attached Figure Description

[0005] The invention will be better understood from the following detailed description with reference to the accompanying drawings, which are not necessarily drawn to scale and in which:

[0006] FIG. 1 This is a diagram showing the arrangement of the photon structure;

[0007] FIGS. 1A-1D yes FIG. 1 A cross-sectional view of an alternative embodiment of the photonic structure is shown in the figure;

[0008] FIG. 2 This is a diagram of another photonic structure arrangement;

[0009] FIGS. 2A-2D yes FIG. 2 A cross-sectional view of an alternative embodiment of the photonic structure is shown in the figure;

[0010] FIG. 3 This is a diagram of another photonic structure arrangement;

[0011] FIGS. 3A-3B yes FIG. 3 A cross-sectional view of an alternative embodiment of the photonic structure is shown in the figure;

[0012] FIG. 4 is a flowchart illustrating a method of forming an embodiment of a photonic structure;

[0013] FIGS. 5A-5C is a cross-sectional view illustrating exemplary process steps for forming an embodiment of a photonic structure; FIG. 1A is a cross-sectional view illustrating exemplary process steps for forming an embodiment of a photonic structure;

[0014] FIGS. 6A-6C is a cross-sectional view illustrating exemplary process steps for forming an embodiment of a photonic structure; FIG. 1B is a cross-sectional view illustrating exemplary process steps for forming an embodiment of a photonic structure;

[0015] FIGS. 7A-7C is a cross-sectional view illustrating exemplary process steps for forming an embodiment of a photonic structure; and FIG. 1C is a cross-sectional view illustrating exemplary process steps for forming an embodiment of a photonic structure; and

[0016] FIGS. 8A-8C is a cross-sectional view illustrating exemplary process steps for forming an embodiment of a photonic structure. FIG. 1D is a cross-sectional view illustrating exemplary process steps for forming an embodiment of a photonic structure. DETAILED DESCRIPTION

[0017] As noted above, closed curve waveguides (i.e., optical waveguides having a complete loop or a ring shape without discrete ends) can be thermally sensitive and, in particular, depending on the properties of the core material used. That is, they can exhibit a temperature dependent resonance shift (TDRS). For example, closed curve silicon waveguides are known to be thermally sensitive and the potential TDRS can be, for example, about 70 picometers per Kelvin (pm / K) or more. In order to minimize the TDRS, many photonic structures including closed curve optical waveguides also include a corresponding heater to thermally tune (i.e., heat) the closed curve optical waveguide. Unfortunately, current available configurations of such photonic structures tend to exhibit performance degradation (e.g., optical power loss).

[0018] In view of the foregoing, embodiments of the disclosed photonic structure include an optical device having low loss, thermally tunable, closed curve optical waveguides. More specifically, disclosed are various embodiments of a photonic structure. The photonic structure can include an optical device having one or more optical waveguides that include at least one closed curve waveguide. The closed curve waveguide can have a first height measured from a top surface of an insulating layer, and an outer curved sidewall that extends substantially vertically throughout the first height (e.g., as discussed below, to minimize signal loss and improve mode confinement). The photonic structure can also include a closed curve thermal coupler thermally coupled to and laterally surrounded by the closed curve waveguide and having a second height that is less than the first height. In some embodiments, the closed curve waveguide and the closed curve thermal coupler can be contiguous portions of the same semiconductor layer (e.g., the same silicon layer, or alternatively, the same polysilicon, germanium, or silicon germanium layer) having different thicknesses. Finally, the photonic structure can include a heating element thermally coupled (e.g., at the same design level or at a different design level) to the closed curve thermal coupler, and thereby indirectly thermally coupled to the closed curve waveguide through the closed curve thermal coupler. In such a photonic structure, the heating element can be used to thermally tune the closed curve waveguide through the closed curve thermal coupler to minimize any temperature dependent resonance shift (TDRS). Also disclosed are method embodiments for forming the above-described photonic structure.

[0019] More specifically, disclosed are various embodiments of a photonic structure. For example, see photonic structure embodiments 100A-100D shown in the arrangement diagrams of FIG. 1 and further illustrated in the alternative cross-sectional diagrams of FIGS. 1A-1D ; see photonic structure embodiments 200A-200D shown in the arrangement diagrams of FIG. 2 and further illustrated in the alternative cross-sectional diagrams of FIGS. 2A-2D ; and see photonic structure embodiments 300A-300B shown in the arrangement diagrams of FIG. 3 and further illustrated in the alternative cross-sectional diagrams of FIGS. 3A-3B .

[0020] Each of the photonic structure embodiments 100A-100D, 200A-200D, 300A-300B can include a substrate 101, 201, 301. The substrate 101, 201, 301 can be, for example, a semiconductor substrate, such as a silicon substrate. Each of the photonic structure embodiments 100A-100D, 200A-200D, 300A-300B can also include an insulating layer 102, 202, 302 on the substrate 101, 201, 301 and an optical device 199, 299, 399 including one or more optical waveguides, and more particularly, at least one closed curve waveguide 120, 220, 320 on the insulating layer 102, 202, 302.

[0021] Each optical waveguide of the optical device 199, 299, 399 can be, for example, a semiconductor waveguide, such as a silicon waveguide, or alternatively, a polysilicon waveguide, a germanium waveguide, a silicon-germanium waveguide, or any other type of waveguide having a core material with a refractive index that varies with temperature.

[0022] Those skilled in the art will recognize that, in order to facilitate the propagation of an optical signal through an optical waveguide, the waveguide material, also referred to as the core material, should have a first refractive index and should be surrounded by a cladding material having a second refractive index that is less than the first refractive index. For example, the refractive index of silicon is a function of both temperature and wavelength. That is, assuming a wavelength of 1 micron, the refractive index of silicon can range from about 3.52 to about 3.57 over a temperature range of about 50K to about 295K, respectively. However, at a given wavelength of 5 microns, the refractive index of silicon is lower and can range from about 3.39 to about 3.43 over a temperature range of about 50K to about 295K, respectively. Thus, if the optical waveguide of the optical device 199, 299, 399 is silicon, the insulating layer 102, 202, 302 immediately adjacent to the bottom surface of the optical waveguide and will act as a cladding material, such as can be a layer of silicon dioxide having a refractive index less than 1.6, a layer of silicon nitride having a refractive index less than 2.1, or any other suitable insulating material layer having a refractive index less than the lowest refractive index of silicon.

[0023] As noted above, the optical devices 199, 299, 399 can include one or more optical waveguides, at least one of which is a closed curve waveguide. A "closed curve" waveguide refers to a waveguide having a complete loop or a loop shape without discrete ends, such that an optical signal can make repeated round trips through the waveguide. For purposes of illustration, the closed curve waveguides 120, 220, 320 are shown in the figures as having an elliptical loop shape without discrete ends. However, it should be understood that the figures are not intended to be limiting, and that the closed curve waveguides 120, 220, 320 can alternatively have any other complete loop or loop shape without discrete ends (e.g., a circular loop shape, an oval loop shape, a race track loop shape, etc.). In any case, the closed curve waveguides 120, 220, 320 can have a bottom surface proximate to a top surface of the insulating layer 102, 202, 302 and a top surface that is opposite and substantially parallel to the bottom surface. Thus, the closed curve waveguides 120, 220, 320 can have a first height 125, 225, 325 as measured from the top surface of the insulating layer 102, 202, 302.

[0024] The closed curve waveguides 120, 220, 320 can have an outer curved sidewall 121, 221, 321 that defines an outer boundary of the waveguide and extends substantially perpendicularly throughout the first height (i.e., from the top surface of the insulating layer 102, 202, 302 to the top surface of the waveguide). The closed curve waveguides 120, 220, 320 can also include an inner curved sidewall 122, 222, 322 that is opposite the outer curved sidewall 121, 221, 321 and defines an inner boundary of the waveguide. The dimensions of the closed curve waveguides 120, 220, 320 can be tailored to achieve a desired result. For example, the height and width of the closed curve waveguide can be tailored to facilitate the propagation of optical signals having wavelengths within a given wavelength range. Further, the circumference of the closed curve waveguide can be tailored to achieve a particular resonant wavelength and set the frequency at which resonance occurs. As noted above, the resonant wavelength of a closed curve waveguide refers to the wavelength of an optical signal that makes repeated round trips through the closed curve waveguide and builds in intensity. Techniques for tailoring the dimensions of a waveguide are well known in the art, and thus details are omitted from the present specification to allow the reader to focus on the salient aspects of the disclosed embodiments. It should be noted that having an outer curved sidewall that is substantially perpendicular from the top surface of the insulating layer 102, 202, 302 to the top surface of the closed curve waveguide minimizes losses for signals having a particular resonant wavelength by passing through the outer curved sidewall 121, 221, 321. Low losses are extremely important to device performance when the circumference is small, resulting in tight turns.

[0025] In some embodiments, the optical device 199, 299, 399 can be a ring resonator. As described above, a ring resonator can include a bus waveguide 110, 210, 310 laterally adjacent and optically coupled to a closed curve waveguide 120, 220, 320, as described above. A "bus" waveguide refers to a waveguide having discrete ends, including an input end 111, 211, 311 and an output end 112, 212, 312. For purposes of illustration, the bus waveguide 110, 210, 310 is shown in the figures as including a substantially linear path between the input and output ends. However, it should be understood that the figures are not intended to be limiting, and that alternatively, the bus waveguide 110, 210, 310 can include a non-linear path between the input and output ends. That is, alternatively, the bus waveguide 110, 210, 310 can include one or more curves. In any case, the bus waveguide 110, 210, 310, like the closed curve waveguide 120, 220, 320, can have a bottom face immediately adjacent a top face of the insulating layer 102, 202, 302 and a top face opposite and parallel to the bottom face. The bus waveguide 110, 210, 310 may, for example, have the same first height as the closed curve waveguide 120, 220, 320 and parallel sidewalls extending substantially perpendicularly the entire first height (i.e., from the top face of the insulating layer 102, 202, 302 to the top face of the waveguide) (as shown). Alternatively, the bus waveguide 110, 210, 310 can have a different height than the closed curve waveguide 120, 220, 320 and parallel sidewalls extending substantially perpendicularly to the different height. In any case, the bus waveguide 110, 210, 310 can be positioned laterally adjacent a portion of the outer curved sidewall 121, 221, 321 of the closed curve waveguide 120, 220, 320, and more particularly, can be spatially separated but optically coupled to the closed curve waveguide 120, 220, 320 at the portion of the outer curved sidewall 121, 221, 321.

[0026] It should be noted that the portion of the outer curved sidewall of the closed curve waveguide that is optically coupled to the bus waveguide can be a curved portion (as shown). Alternatively, the portion of the outer curved sidewall of the closed curve waveguide that is optically coupled to the bus waveguide can be a linear portion (e.g., in the case of a racetrack-shaped closed curve waveguide) (not shown).

[0027] In such a ring resonator, an optical signal can enter the bus waveguide 110, 210, 310 at the input end 111, 211, 311. Due to optical coupling with the closed curve waveguide 120, 220, 320, some of the optical signal can enter the closed curve waveguide 120, 220, 320 from the bus waveguide 110, 210, 310, and some of the optical signal can enter the bus waveguide 110, 210, 310 from the closed curve waveguide 120, 220, 320. More specifically, the bus waveguide and the closed curve waveguide can be placed close enough to create an evanescent field between the two waveguides, and the evanescent field allows some of the optical signal to pass from the bus waveguide to the closed curve waveguide, and vice versa. The creation of such an evanescent field between adjacent waveguides, and the likelihood of optical coupling therebetween (i.e., the coupling that facilitates the transfer of optical signals between adjacent waveguides), depends at least on the distance between the waveguides, the coupling length (i.e., the length of the portion of the adjacent waveguides that are in close proximity), and the refractive index of the medium between the waveguides. The closer the distance, the more likely the optical coupling; the longer the coupling length, the easier the optical coupling; and so on. In any case, the optical signal can also exit the bus waveguide 110, 210, 310 at the output end 112, 212, 312. However, within the closed curve waveguide 120, 220, 320, due to constructive interference, only the optical signal of a particular resonant wavelength of that closed curve waveguide will repeatedly traverse the closed curve waveguide, thereby building up in intensity. As a result, the optical signal that enters the bus waveguide 110, 210, 310 from the closed curve waveguide 120, 220, 320 and is output at the output end 112, 212, 312 of the bus waveguide 110, 210, 310 will primarily have the particular resonant wavelength. However, as noted above, closed curve waveguides are known to be thermally sensitive. In other words, they are known to exhibit a temperature-dependent resonance shift (TDRS). For example, the potential TDRS can be on the order of 70 picometers per Kelvin (pm / K) or more.

[0028] Accordingly, to minimize the temperature-dependent resonance shift (TDRS) in the closed curve waveguide 120, 220, 320 (i.e., to avoid temperature-dependent variations in the resonant wavelength of the closed curve waveguide 120, 220, 320), each photonic structure embodiment 100A-100D, 200A-200D, 300A-300B can also include a heating element 140, 240, 340 and a thermal coupler 130, 230, 330 that indirectly thermally couples the heating element 140, 240, 340 to the closed curve waveguide 120, 220, 320 for thermal tuning.

[0029] More specifically, each photonic structure embodiment 100A-100D, 200A-200D, 300A-300B can also include a closed curve thermal coupler 130, 230, 330 having a bottom surface proximate to the insulating layer 102, 202, 302 and a top surface opposite and substantially parallel to the bottom surface. For purposes of the present disclosure, a "thermal coupler" refers to a non-contacting passive component made of any suitable non-metallic electrically insulating thermally conductive material through which thermal energy can be transferred without the transfer of electrical current. Exemplary thermal coupler materials with respect to particular photonic structure embodiments are discussed in greater detail below.

[0030] The closed curve thermal coupler 130, 230, 330 can be smaller than the closed curve waveguide having substantially the same full loop or ring shape (e.g., a circular ring shape, an oval ring shape, an elliptical ring shape, a racetrack ring shape, etc.) and can further be laterally surrounded by the closed curve waveguide 120, 220, 320. More specifically, the closed curve thermal coupler 130, 230, 330 can be positioned laterally adjacent to the inner curved sidewall 122, 222, 322 of the closed curve waveguide 120, 220, 320 and can abut or at least be close enough to the inner curved sidewall 122, 222, 322 to thermally couple to the closed curve waveguide 120, 220, 320. More specifically, the closed curve thermal coupler can be placed immediately adjacent to or close enough to the inner curved sidewall of the closed curve waveguide such that thermal energy can be transferred from the closed curve thermal coupler to the closed curve waveguide, thereby increasing the temperature of the closed curve waveguide. The likelihood of the closed curve thermal coupler thermally coupling to the closed curve waveguide depends at least on the distance between the closed curve thermal coupler and the closed curve waveguide, the coupling length (the length of the portion of the adjacent waveguides that are spaced apart), and the thermal conductivity of any medium between the closed curve thermal conductor and the closed curve waveguide. The closer the distance, the greater the likelihood of thermal coupling and the better the effect; the longer the coupling length, the greater the likelihood of thermal coupling and the better the effect; and so on. Furthermore, the closed curve thermal coupler 130, 230, 330 can have a second height 135, 235, 335 (e.g., measured from the top surface of the insulating layer 102, 202, 302 to the top of the thermal coupler) that is smaller than a first height 125, 225, 325 of the closed curve waveguide 120, 220, 320. In exemplary embodiments, the second height 135, 235, 335 can be less than half of the first height. For example, the first height 125, 225, 325 of the closed curve optical waveguide 120, 220, 320 can be 220 nm and the second height 135, 235, 335 of the adjacent closed curve thermal coupler 130, 230, 330 can be 100 nm. Thus, in each photonic structure embodiment, the closed curve thermal coupler 130, 230, 330 is positioned laterally adjacent only to a lower portion of the inner curved sidewall 122, 222, 322 of the closed curve waveguide 120, 220, 320 such that the inner curved sidewall 122, 222, 322 extends vertically above the level of the top surface of the closed curve thermal coupler 130, 230, 330.

[0031] Each photonic structure embodiment 100A-100D, 200A-200D, 300A-300B can also include a heating element 140, 240, 340 and at least two contacts 149, 249, 349 on the heating element 140, 240, 340. For purposes of the present disclosure, a "heating element" refers to a resistor made of any suitable electrically conductive material through which an electric current flows in response to a voltage difference at the contacts and is converted into heat energy. Those skilled in the art will recognize that the direction and amount of the current will depend on the voltage difference. Furthermore, the amount of heat generated per unit length will depend on the material used and the current density (which is a function of the cross-sectional area of the heating element). Exemplary heating element materials with respect to particular embodiments will be discussed in greater detail below. In any case, the heating element 140, 240, 340 can be spatially separated from the closed curve waveguide 120, 220, 320. The heating element 140, 240, 340 can further be thermally coupled adjacent to the closed curve thermal coupler 130, 230, 330, and more particularly, can abut or at least be close enough to the closed curve thermal coupler 130, 230, 330 so that it is thermally coupled to the closed curve thermal coupler 130, 230, 330 as is the closed curve waveguide. That is, the heating element can be placed immediately adjacent to or close enough to the closed curve thermal coupler so that heat energy can be transferred from the heating element into and through the closed curve thermal coupler to increase the temperature of the closed curve thermal coupler. The likelihood of thermal coupling between the closed curve thermal coupler and the closed curve waveguide depends at least on the distance between the closed curve thermal coupler and the closed curve waveguide, the coupling length (the length of the portion of the distance between the adjacent waveguides), and the thermal conductivity of any medium between the closed curve thermal conductor and the closed curve waveguide. The closer the distance, the greater the likelihood of thermal coupling and the better the effect; the longer the coupling length, the greater the likelihood of thermal coupling and the better the effect; and so on. It is important to note that, due to the similarity of thermal coupling between the closed curve thermal coupler and the closed curve waveguide, heat energy can further be transferred from the closed curve thermal coupler into the closed curve thermal waveguide, thereby increasing the temperature of the closed curve waveguide. Thus, the heating element 140, 240, 340 is indirectly thermally coupled to the closed curve waveguide 120, 220, 320 via the closed curve thermal coupler 130, 230, 330 and can be used to thermally tune the closed curve waveguide 120, 220, 320 via the closed curve thermal coupler 130, 230, 330 to minimize any temperature-dependent resonance shift (TDRS). In other words, the heating element 140, 240, 340 can generate and output heat energy and, due to thermal coupling, can be transferred into the closed curve thermal coupler 130, 230, 330, can pass through the closed curve thermal coupler 130, 230, 330, and can pass from the closed curve thermal coupler 130, 230, 330 into the lower portion of the closed curve waveguide 120, 220, 320.That is, heat energy can be transferred from the heating element through the closed curve thermal coupler into the closed curve thermal waveguide. The amount of heat energy can be predetermined to ensure that the temperature of the closed curve waveguide 120, 220, 320 is maintained at a particular temperature or within a particular temperature range to ensure that the optical signal of a particular resonant wavelength is increased in intensity as it repeatedly traverses the closed curve waveguide 120, 220, 320.

[0032] The various photonic structure embodiments 100A-100D, 200A-200D, 300A-300B disclosed herein differ in the materials used for the different components (e.g., for the waveguide, thermal coupler, and heating element) and / or the relative positioning of the different components (e.g., waveguide, thermal coupler, and heating element).

[0033] For example, with reference to FIG. 1 and FIGS. 1A-1D In the photonic structure embodiments 100A-100D, the heating element 140 can be at the same design level as the closed curve thermal coupler 130 and the closed curve waveguide 120. Further, the closed curve thermal coupler 130 can be positioned laterally between and immediately adjacent to both the closed curve waveguide 120 and the heating element 140 to thermally couple to the closed curve waveguide 120 and the heating element 140.

[0034] More specifically, in the photonic structure embodiments 100A-100D, the heating element 140 can have a bottom surface immediately adjacent to the top surface of the insulating layer 102. The heating element 140 can be smaller than the closed curve thermal coupler 130 and substantially have the same loop or ring shape (e.g., a circular ring shape, an oval ring shape, an elliptical ring shape, a racetrack ring shape, etc.) and can be further laterally surrounded and immediately adjacent by the inner curved sidewall of the closed curve thermal coupler 130 such that the three components (i.e., the closed curve waveguide 120, the closed curve thermal coupler 130, and the heating element 140) substantially form a concentric, abutting, ring shape.

[0035] It is noted that the heating element 140 can be a complete loop or ring shape (as shown) without discrete ends. Alternatively, the heating element 140 can have at least one segment removed to have an open-loop or ring shape with discrete ends (not shown). Alternatively, the heating element 140 can be segmented with segments forming a loop or ring shape and each segment having discrete contact ends (e.g., such that different amounts of heat energy can be applied to different portions of the closed curve thermal coupler and thus to different portions of the closed curve waveguide) (not shown).

[0036] In the photonic structure embodiment 100A (see FIG. 1AIn this configuration, the closed-curve waveguide 120, the closed-curve thermal coupler 130, and the heating element 140 may comprise continuous portions 103.1-103.3 of the same semiconductor layer 103 on the insulating layer 102. That is, the closed-curve waveguide 120 may be a first portion 103.1 of the semiconductor layer 103 having a first thickness (i.e., see first height 125). The closed-curve thermal coupler 130 may be a second portion 103.2 (also referred to herein as a recessed portion or planar portion) of the semiconductor layer 103, which is continuous with the first portion 103.1 but has been recessed (i.e., etched back) to have a second thickness (i.e., see second height 135) less than the first portion (i.e., the second portion 103.2 is thinner than the first portion 103.1). The heating element 140 may comprise a third portion 103.3 of the semiconductor layer 103, which is continuous with the second portion 103.2, and may also include a metal silicide layer 145 on the third portion 103.3. Semiconductor layer 103 may be, for example, a silicon layer. Alternatively, the semiconductor layer may be some other type of semiconductor layer suitable for optical waveguide formation and having a temperature-dependent refractive index, including but not limited to polycrystalline silicon layers, germanium layers, or silicon-germanium layers. Metal silicide layer 145 may be, for example, a cobalt silicide (CoSi) layer, a nickel silicide (NiSi) layer, a tungsten silicide (WSi) layer, a titanium silicide (TiSi) layer, or any other suitable metal silicide layer. Optionally, metal silicide layer 145 may be doped with N-type or P-type dopants to reduce resistance.

[0037] In photonic structure embodiment 100B (see...) FIG. 1B The closed-curve waveguide 120 and the closed-curve thermal coupler 130 can be continuous portions 103.1-103.2 of the same semiconductor layer 103 (e.g., the same silicon layer, or alternatively, the same polycrystalline silicon, germanium, or silicon-germanium layer, as described above with respect to photonic structure embodiment 100A). However, in this case, the heating element 140 can be a resistive element 141 of some other metal or metal alloy, formed on an insulating layer 102 adjacent to the inner curved sidewall of the closed-curve thermal coupler 130. This resistive element 141 can be made of, for example, tungsten, aluminum, nickel, titanium, tantalum, cobalt, copper, or alloys thereof.

[0038] In the photonic structure embodiment 100C (see FIG. 1C ) and 100D (see FIG. 1DIn photonics structure embodiment 100B, the closed curve waveguide 120 can include a discrete portion 103.1 of the semiconductor layer 103 (e.g., a silicon layer, or alternatively, a polysilicon, germanium, or silicon germanium layer, as described above with respect to photonics structure embodiment 100A). The closed curve thermal coupler 130 can be some other non-metallic electrically insulating thermally conductive feature 104 formed on the insulating layer 102 and positioned laterally between and abutting the closed curve waveguide 120 and the heating element 140. For example, the closed curve thermal coupler 130 can be a thin layer of polysilicon, silicon, nitride, boron nitride, silicon carbide, or diamond on the insulating layer 102 and extending laterally between and abutting the closed curve waveguide 120 and the heating element 140. The heating element 140 can include another discrete portion 103.3 of the semiconductor layer 103 positioned laterally immediately adjacent the closed curve thermal coupler 130, and can further include a metal silicide layer 145 on the discrete patterned portion 103.3 of the semiconductor layer, as described above with respect to photonics structure embodiment 100A (e.g., see FIG. 1C photonics structure embodiment 100C). Alternatively, the heating element 140 can be a resistive element 141 of some other metal or metal alloy formed on the insulating layer 102 abutting the closed curve thermal coupler 130, as described above with respect to photonics structure embodiment 100B (e.g., see FIG. 1D photonics structure embodiment 100D).

[0039] In each of photonics structure embodiments 100A-100D, the optical device 199 can be a ring resonator. The ring resonator can include at least one closed curve waveguide 120 and at least one bus waveguide 110 abutting the closed curve waveguide 120. In this case, the bus waveguide can be a discrete portion 103.4 of the semiconductor layer 103 that is adjacent to but spatially separated from (i.e., from the first portion 103.1 of the semiconductor layer 103) and optically coupled to the closed curve waveguide 120.

[0040] Referring to FIG. 2 and FIGS. 2A-2DIn photonic structure embodiments 200A-200D, heating element 240 can be at the same design level as closed curve thermal coupler 230 and closed curve waveguide 220. Further, closed curve thermal coupler 230 can be positioned laterally between closed curve waveguide 220 and heating element 240. However, in this case, closed curve thermal coupler 230 can be physically separated from closed curve waveguide 220 and / or from heating element 240 by space 250. It should be appreciated that the distance between closed curve thermal coupler 230 and closed curve waveguide 220 and heating element 240 (i.e., the width of any space 250) should be small enough to ensure that closed curve thermal coupler 230 is still thermally coupled to closed curve waveguide 220 on one side and heating element 240 on the other side.

[0041] More specifically, in photonic structure embodiments 200A-200D, heating element 240 can have a bottom surface that is immediately adjacent to the top surface of insulating layer 202. Heating element 240 can be smaller than closed curve thermal coupler 230 that has substantially the same loop or ring shape (e.g., a circular ring shape, an oval ring shape, an elliptical ring shape, a racetrack ring shape, etc.) and can be further laterally surrounded by closed curve thermal coupler 230 such that the three components (i.e., closed curve waveguide 220, closed curve thermal coupler 230, and heating element 240) substantially form concentric rings, at least two of the three rings being separated by space 250.

[0042] It should be noted that heating element 240 can be a complete loop or ring shape (as shown) without discrete ends. Alternatively, heating element 240 can have at least one section removed to have an open loop or ring shape with discrete ends (not shown). Alternatively, heating element 240 can be segmented with sections forming a loop or ring shape and each section having discrete contact ends (e.g., such that different amounts of thermal energy can be applied to different portions of the closed curve thermal coupler, and thus to different portions of the closed curve waveguide) (not shown).

[0043] For illustrative purposes, FIGS. 2A-2D Only a single space 250 between closed curve thermal coupler 230 and heating element 240 is shown. It should be appreciated that alternatively, space 250 can be between closed curve waveguide 220 and closed curve thermal coupler 230 but not between closed curve thermal coupler 230 and heating element 240, or space 250 can be between closed curve thermal coupler 230 and closed curve waveguide 220 and heating element 240.

[0044] In photonic structure embodiment 200A (see FIG. 2A), the closed curve waveguide 220 and the closed curve thermal coupler 230 can be contiguous portions 203.1-203.2 of the same semiconductor layer 203 on the insulating layer 202. That is, the closed curve waveguide 220 can be a first portion 203.1 of the semiconductor layer 203 having a first thickness (i.e., see the first height 225). The closed curve thermal coupler 230 can be a second portion 203.2 (also referred to herein as a recessed portion or a slab portion) of the semiconductor layer 203 that is contiguous with the first portion 203.1 but has been recessed (i.e., etched back) to have a second thickness (i.e., see the second height 235) that is less than the first portion (i.e., the second portion 203.2 is thinner than the first portion 203.1). The heating element 240 can include a third portion 203.3 of the semiconductor layer 230 that is physically separated from the second portion 203.2 by a space 250, and can also include a metal silicide layer 245 on the third portion 203.3.

[0045] The semiconductor layer 203 can be, for example, a silicon layer. Alternatively, the semiconductor layer can be some other type of semiconductor layer suitable for optical waveguide formation and having a temperature dependent refractive index, including but not limited to a polysilicon layer, a germanium layer, or a silicon-germanium layer. The metal silicide layer 245 can be, for example, a cobalt silicide (CoSi) layer, a nickel silicide (NiSi) layer, a tungsten silicide (WSi) layer, a titanium silicide (TiSi) layer, or any other suitable metal silicide layer. Optionally, the metal silicide layer 245 can be doped with N-type or P-type dopants to reduce the electrical resistance.

[0046] In the photonic structure embodiment 200B (see FIG. 2B ), the closed curve waveguide 220 and the closed curve thermal coupler 230 can be contiguous portions 203.1-203.2 of the same semiconductor layer 203 (e.g., the same silicon layer, or alternatively, the same polysilicon, germanium, or silicon-germanium layer, as described above with respect to the photonic structure embodiment 200A). However, in this case, the heating element 240 can be an electrically resistive element 241 of some other metal or metal alloy formed on the insulating layer 202 and physically separated from the closed curve thermal coupler 230 by a space 250. The electrically resistive element 241 can be made of, for example, tungsten, aluminum, nickel, titanium, tantalum, cobalt, copper, or alloys thereof.

[0047] In the photonic structure embodiments 200C (see FIG. 2C ) and 200D (see FIG. 2DIn the photonic structure embodiment 200B, the closed curve waveguide 220 can be a discrete portion 203.1 of the semiconductor layer 203 (e.g., a silicon layer, or alternatively, a polysilicon, germanium, or silicon germanium layer, as described above with respect to the photonic structure embodiment 200A). The closed curve thermal coupler 230 can be some other non-metallic electrically insulating thermally conductive feature formed on the insulating layer 202 that is laterally positioned between the closed curve waveguide 220 and the heating element 240 and separated from one or both by a space 250. Each space 250 can be small enough so that the closed curve thermal coupler 230 is still thermally coupled to both the closed curve waveguide 220 and the heating element 240. In this case, the closed curve thermal coupler 230 (i.e., the non-metallic electrically insulating thermally conductive feature) can be, for example, a thin layer of polysilicon, silicon, nitride, boron nitride, silicon carbide, diamond, or some other non-metallic electrically insulating thermally conductive material on the insulating layer 202 interposed between the closed curve waveguide 220 and the heating element 240. The heating element 240 can include a discrete portion 203.3 of the semiconductor layer 203 (e.g., laterally positioned immediately adjacent to the closed curve thermal coupler 230 but separated therefrom by a space 250) and can further include a metal silicide layer 245 on the discrete patterned portion 203.3 of the semiconductor layer 203, as described above with respect to the photonic structure embodiment 200A (e.g., see FIG. 2B of the photonic structure embodiment 200B). FIG. 2C Alternatively, the heating element 240 can be some other metallic or metal alloy resistive element 241 formed on the insulating layer 202 adjacent to the closed curve thermal coupler 230 but separated therefrom by a space 250, as described above with respect to the photonic structure embodiment 200B (e.g., see FIG. 2C of the photonic structure embodiment 200D). FIG. 2D Alternatively, the heating element 240 can be some other metallic or metal alloy resistive element 241 formed on the insulating layer 202 adjacent to the closed curve thermal coupler 230 but separated therefrom by a space 250, as described above with respect to the photonic structure embodiment 200B (e.g., see FIG. 2C of the photonic structure embodiment 200D).

[0048] In each of the photonic structure embodiments 200A-200D, the optical device 299 can be a ring resonator. The ring resonator can include at least one closed curve waveguide 220 and at least one bus waveguide 210 adjacent to the closed curve waveguide 220. In this case, the bus waveguide 210 can be a discrete portion 203.4 of the semiconductor layer 203 that is adjacent to but spatially separated from (i.e., separated from the first portion 203.1 of the semiconductor layer 203) and optically coupled to the closed curve waveguide 220.

[0049] Referring to FIG. 3 and FIGS. 3A-3B In the photonic structure embodiments 300A-300B, the heating element 340 can be at a different design level than the closed curve thermal coupler 330 and the closed curve waveguide 320.

[0050] More specifically, in photonic structure embodiments 300A-300B, the heating element 340 can have a bottom surface that is above and physically separated from a top surface of the insulating layer 302, as shown. For example, the heating element 340 can be a metallic or metallic alloy resistive element 345 in a back end of the line (BEOL) metal level 306. For example, the heating element 340 can be made of copper or aluminum or some other BEOL metal material. Alternatively, the heating element 340 can be below the insulating layer 302 (not shown). For example, the heating element 340 can be a metallic or metallic alloy resistive element (not shown) embedded in the substrate 301. In any case, the heating element 340 can have substantially the same loop or ring shape as the closed curve thermal coupler and the closed curve waveguide, and can be aligned with and close enough to the closed curve thermal coupler 330 to facilitate thermal coupling therewith. The heating element 340 can be a complete loop or ring shape without discrete ends. Alternatively, the heating element 340 can have at least one section removed to have an open loop or a ring shape with discrete ends (not shown). Alternatively, the heating element 340 can be segmented with sections forming a loop or ring shape and with discrete contact ends for each section (e.g., so that different amounts of thermal energy can be applied to different portions of the closed curve thermal coupler, and thus to different portions of the closed curve waveguide) (not shown). For purposes of illustration, the heating element 340 is shown as extending partially over the closed curve thermal coupler 330. However, alternatively, the heating element 340 can extend completely over the closed curve thermal coupler or can be completely offset from the closed curve thermal coupler.

[0051] In photonic structure embodiment 300A (see FIG. 3A ), the closed curve waveguide 320 and the closed curve thermal coupler 330 each comprise different contiguous portions of the same semiconductor layer 303 on the insulating layer 302. That is, the closed curve waveguide 320 can be a patterned first portion 303.1 of the semiconductor layer 303 having a first thickness (i.e., see first height 325). The closed curve thermal coupler 330 can be a patterned second portion 303.2 (also referred to herein as a recessed portion or a slab portion) of the semiconductor layer 303 that is contiguous with the first portion 303.1 but has been recessed (i.e., etched back) to have a second thickness (i.e., see second height 335) that is less than the first portion (i.e., the second portion 303.2 is thinner than the first portion 303.1). The bus waveguide 310 can be a discrete patterned portion 303.4 of the semiconductor layer 303 that also has the first thickness. The semiconductor layer 303 can be, for example, a silicon layer. Alternatively, the semiconductor layer can be some other type of semiconductor layer suitable for optical waveguide formation and having a temperature dependent refractive index, including but not limited to a polysilicon layer, a germanium layer, or a silicon germanium layer.

[0052] In photonic structure embodiment 300B (see FIG. 3B ) the closed curve waveguides 320 and bus waveguides 310 can be respectively discrete patterned portions 303.1 and 303.4 of the same semiconductor layer 303. The closed curve thermal couplers 330 can be some other non-metallic electrically insulating thermally conductive features formed on the insulating layer 302 that are laterally adjacent and contiguous to the closed curve waveguides 320. For example, the closed curve thermal couplers 330 can be a thin layer of polysilicon, silicon, nitride, boron nitride, silicon carbide, or diamond on the insulating layer 302 that is laterally adjacent and contiguous to the closed curve waveguides 320.

[0053] In any case, each of the photonic structure embodiments 100A-100D, 200A-200D, 300A-300B can also include one or more dielectric material layers 105, 205, 305 covering exposed surfaces of the insulating layers 102, 202, 302 and device components thereon. For example, the dielectric material layers can cover the optical devices 199, 299, 399 (including the closed curve waveguides 120, 220, 320 and bus waveguides 110, 210, 310 (if applicable)), the closed curve thermal couplers 130, 230, 330, and the heating elements (if applicable) (e.g., see heating elements 140 and 240) and any spacers therebetween. That is, the dielectric material can fill any spaces between the bus waveguides and the closed curve waveguides, between the closed curve waveguides and the closed curve thermal couplers, and / or between the closed curve thermal couplers and the heating elements. The dielectric material 105, 205, 305, and in particular immediately adjacent to the optical waveguides (i.e., immediately adjacent to the closed curve waveguides 120, 220, 320 and bus waveguides 110, 210, 310 (if applicable)) can be any dielectric material suitable for use as a cladding material for those optical waveguides. For example, if the optical waveguides are silicon waveguides, which as noted above can have temperature and wavelength dependent refractive indices generally higher than 3.2, the dielectric material layers 105, 205, 305 can be silicon dioxide (e.g., having a refractive index less than 1.6), silicon nitride (e.g., having a refractive index less than 2.1), or any other suitable dielectric material having a refractive index less than the lowest refractive index of silicon.

[0054] In each of the photonic structure embodiments 100A-100D, 200A-200D, 300A-300B described above and shown in the accompanying drawings, the optical device 199, 299, 399 is shown as including a single loop resonator of a single bus waveguide 110, 210, 310 and a single closed curve waveguide 120, 220, 320. However, it should be understood that these figures are not intended to be limiting. Alternatively, the optical device 199, 299, 399 can be a loop resonator having some other more complex configuration (e.g., multiple closed curve waveguides optically coupled to the same bus waveguide, a single closed curve waveguide between and optically coupled to a pair of parallel bus waveguides, multiple closed curve waveguides between and optically coupled to a pair of parallel bus waveguides, etc.). In such complex loop resonator configurations, each closed curve waveguide can be indirectly thermally coupled to a respective heating element through a respective closed curve thermal coupler, as described above.

[0055] Finally, as mentioned above, in each of the photonic structure embodiments 100A-100D, 200A-200D, 300A-300B, the outer curved sidewalls 121, 221, 321 of the closed curve waveguides 120, 220, 320 extend substantially perpendicularly the entire first height 125, 225, 325 of the closed curve waveguides from the top surface of the insulating layer 102, 202, 302 to the top surface of the waveguide itself. As a result, the mode confinement is improved and signal loss is minimized. More specifically, conventional waveguides are typically either strip or rib waveguides. A strip waveguide refers to, for example, by forming a mask having a desired shape on a waveguide core material layer, then performing an anisotropic etching process to etch completely through the waveguide core material layer, such that all sidewalls of the resulting waveguide extend perpendicularly to the entire height of the waveguide. A rib waveguide refers to, for example, by forming a mask having a desired shape on a waveguide core material layer, then performing an anisotropic etching process to etch only partially through the waveguide core material layer, such that a recessed portion of the waveguide core material (also referred to as a slab portion) extends laterally away from a lower portion of the resulting waveguide, and such that sidewalls of the resulting waveguide do not extend to the entire height of the waveguide, but only perpendicularly from the recessed portion of the waveguide core material. Thus, a rib waveguide essentially has an inverted T-shape. The recessed portion of the waveguide core material in a rib waveguide allows light signal leakage. Moreover, in a closed curve rib waveguide, the likelihood of light signal leakage along the outer curved sidewalls is greater. Thus, in each of the photonic structure embodiments 100A-100D, 200A-200D, 300A-300B, by eliminating the recessed portion of the waveguide core material (e.g., silicon, or alternatively, polysilicon, germanium, or silicon germanium) at least along the outer curved sidewalls 121, 221, 321 of the closed curve waveguides 120, 220, 320, the mode confinement is improved and signal loss is minimized, even though the recessed portion 103.2, 203.2, 303.2 of the same semiconductor layer 103, 203, 303 (e.g., the same silicon layer, or alternatively, the same polysilicon, germanium, or silicon germanium layer) is used to form the closed curve waveguides 120, 220, 320 (e.g., as shown in the cross-sectional view of FIG. 1A-1B, 2A-2B, and 3A, respectively), and the closed curve thermal coupler 130, 230, 330 is continuous. FIGS. 1A-1B 、 FIGS. 2A-2B and FIG. 3A the photonic structure embodiments 100A-100B, 200A-200B, and 300A, respectively).

[0056] With reference to the flowcharts of FIG. 4 , the present disclosure also discloses embodiments of methods of forming the above photonic structure embodiments (e.g., see the photonic structure embodiments 100A-100D shown in the arrangement diagrams of FIG. 1 and the alternative cross-sectional views further illustrated in FIGS. 1A-1D , respectively; see the photonic structure embodiments 200A-200D shown in the arrangement diagrams of FIG. 2photonic structure embodiments 200A-200D shown in the plan view of FIG. 2A and in the alternative cross-sectional view of FIG. 2B, and photonic structure embodiments 300A-300B shown in the plan view of FIG. 3A and in the alternative cross-sectional view of FIG. 3B. FIGS. 2A-2D Further alternative cross-sectional view; see also the plan view of FIG. 2A and the alternative cross-sectional view of FIG. 2B. FIG. 3 Further alternative cross-sectional view; see also the plan view of FIG. 3A and the alternative cross-sectional view of FIG. 3B. FIGS. 3A-3B Further alternative cross-sectional view; see also the plan view of FIG. 2A and the alternative cross-sectional view of FIG. 2B.

[0057] Generally, the method embodiments of the present application can include forming an optical device 199, 299, 399 on the insulating layer 102, 202, 302 (see process step 402). The process of forming the optical device 199, 299, 399 can include at least forming a closed curve waveguide 120, 220, 320. For example, in some embodiments, the process of forming the optical device 199, 299, 399 can include forming a ring resonator including at least one bus waveguide 110, 210, 310 and at least one closed curve waveguide 120, 220, 320 that is laterally adjacent to and optically coupled with the bus waveguide 110, 210, 310. As noted above, for the purposes of the present application, a bus waveguide refers to a waveguide having discrete ends including an input end and an output end, while a closed curve waveguide refers to a waveguide having a complete loop or ring shape with no discrete ends. In any case, the closed curve waveguide 120, 220, 320 of the optical device 199, 299, 399 can be formed in process step 402 to have a first height 125, 225, 325 (e.g., measured from the top surface of the insulating layer to the top surface of the closed curve waveguide), outer curved sidewalls 121, 221, 321 that extend substantially perpendicularly the entire first height of the closed curve waveguide (e.g., to improve mode confinement and minimize signal loss), and inner curved sidewalls 122, 222, 322 opposite the outer curved sidewalls. Further, the dimensions of the closed curve waveguide 120, 220, 320 can be tailored to achieve a desired result. For example, the height and width of the closed curve waveguide can be tailored to facilitate the propagation of optical signals having wavelengths in a given wavelength range. Further, the perimeter of the closed curve waveguide can be tailored to achieve a particular resonant wavelength and set the frequency at which resonance occurs. As noted above, the resonant wavelength of a closed curve waveguide refers to the wavelength of an optical signal that repeatedly traverses the closed curve waveguide and builds in intensity. Techniques for tailoring the dimensions of a waveguide are well known in the art, and thus, details are omitted from the present specification to allow the reader to focus on the salient aspects of the disclosed embodiments.

[0058] The method of the present application can also include forming a closed curve thermal coupler 130, 230, 330 (see process step 404). The closed curve thermal coupler 130, 230, 330 can be formed to be smaller than the closed curve waveguide 120, 220, 320 having substantially the same complete loop or ring, and further to be laterally surrounded and thermally coupled by the closed curve waveguide 120, 220, 320. The thermal coupling between the closed curve waveguide 120, 220, 320 and the closed curve thermal coupler 130, 230, 330 can be achieved by forming the closed curve thermal coupler 130, 230, 330 such that it is in close proximity or close enough to the closed curve waveguide 120, 220, 320 to ensure that thermal energy from the closed curve thermal coupler 130, 230, 330 can be transferred into the closed curve waveguide 120, 220, 320. Further, the closed curve thermal coupler 130, 230, 330 can be formed to have a second height that is smaller than a first height of the closed curve waveguide 120, 220, 320. For the purposes of the present application, a "thermal coupler" refers to a non-contacting component made of any suitable non-metallic electrically insulating thermally conductive material through which thermal energy can be transferred without transferring electrical current. Exemplary thermal coupler materials will be discussed in greater detail below with respect to particular method embodiments.

[0059] The method can also include forming a heating element 140, 240, 340 to facilitate thermal tuning of the closed curve waveguide 120, 220, 320 (see process step 406). For the purposes of the present application, a "heating element" refers to a resistor made of any suitable electrically conductive material through which an electrical current flows in response to a voltage difference at the contacts and is converted into thermal energy. Those skilled in the art will recognize that the direction and amount of the current will depend on the voltage difference. Further, the amount of heat generated per unit length will depend on the material used and the current density (which is a function of the cross-sectional area of the heating element). In this case, the heating element 140, 240, 340 can be formed to be thermally coupled to the closed curve thermal coupler 130, 230, 330, thereby indirectly thermally coupled to the closed curve waveguide 120, 220, 320, as opposed to being formed to be directly thermally coupled to the closed curve waveguide 120, 220, 320 for thermal tuning. For example, the heating element can be formed at the same design level as the closed curve thermal coupler 130, 230, 330 or at a different design level. The thermal coupling between the heating element 140, 240, 240 and the closed curve thermal coupler 130, 230, 330 can be achieved by forming the heating element 140, 240, 340 such that it is in close proximity or close enough to the closed curve thermal coupler 130, 230, 330 to ensure that thermal energy from the heating element 140, 240, 340 can enter the closed curve thermal coupler 130, 230, 330. Exemplary heating element materials will be discussed in greater detail below with respect to particular embodiments.

[0060] Various method embodiments of the present application can vary according to the specific processing techniques and / or materials used in the course of process steps 402-408 to form photonic structure embodiments 100A-100D, 200A-200D, 300A-300B, and more particularly to form different components (e.g., for waveguides, thermal couplers, and heating elements).

[0061] For example, with reference to FIG. 1 and FIGS. 1A-1D In some embodiments of the method, process steps 402-408 can be performed such that heating element 140, closed curve thermal coupler 130, closed curve waveguide 120, and bus waveguide 110 (if applicable) are all at the same design level, especially immediately adjacent to the top surface of insulating layer 102, and further such that closed curve thermal coupler 130 is laterally positioned between and immediately adjacent to both insulating layer 102 and heating element 140.

[0062] FIGS. 5A-5C Exemplary process steps are shown for forming photonic structure embodiment 100A of FIG. 1A For example, a mask 501 can be formed on semiconductor layer 103 (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation technique), and an anisotropic etching process can be performed to define initial shapes of bus waveguide 110 (see portion 103.4), closed curve waveguide 120 (see portion 103.1), and heating element 140 (see portion 103.3) in semiconductor layer 103, each having a first height 125 and doing so without etching completely through semiconductor layer 103, thereby leaving a recessed portion 103.5 (also referred to as a slab portion) of semiconductor layer 103, and having a second height 135 (see FIG. 5A ) of insulating layer 102 covered between the thicker patterned portions 103.1, 103.3, and 103.4 that is less than first height 125. Mask 501 can be removed. Then, another mask 502 can be formed on the partially completed structure (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation technique), and then an anisotropic etching process can be performed to completely remove exposed portions of the recessed portion of semiconductor layer (e.g., those portions immediately adjacent to the opposite sides of the bus waveguide and immediately adjacent to the outer curved sidewalls of the closed curve waveguide), leaving the intact recessed portion 103.2 of semiconductor layer 103 for closed curve thermal coupler 130 laterally extending between and immediately adjacent to thicker patterned portions 103.1 and 103.3 of the semiconductor layer (i.e., laterally extending between and immediately adjacent to closed curve waveguide 120 and heating element 140) (see FIG. 5BMask 502 can be removed. Another mask 503 can then be formed on the partially completed structure (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask forming technique), and a metal silicide process can be performed to form a metal silicide layer 145 on the exposed surface of the thick patterned portion 103.3 of the semiconductor layer for the heating element 140. The metal silicide layer 145 can be, for example, a cobalt silicide (CoSi) layer, a nickel silicide (NiSi) layer, a tungsten silicide (WSi) layer, a titanium silicide (TiSi) layer, or any other suitable metal silicide layer. Optionally, the metal silicide layer 145 can be doped with an N-type or P-type dopant to reduce resistance. Techniques for forming metal silicide layers are well known in the art and are therefore omitted from this specification to allow the reader to focus on the prominent features of the disclosed embodiments.

[0063] As FIGS. 5A-5C As a result of the process steps shown and described above, the closed-curve waveguide 120, the closed-curve thermal coupler 130, and the heating element 140 comprise continuous portions 103.1-103.3 of the same semiconductor layer 103 (e.g., the same silicon layer, or alternatively, the same polysilicon, germanium, or silicon-germanium layer) on the insulating layer 102. That is, the closed-curve waveguide 120 is formed from a first portion 103.1 of the semiconductor layer 103, which has a first thickness (i.e., see first height 125). The closed-curve thermal coupler 130 is formed from a second portion 103.2 of the semiconductor layer 103 (also referred to herein as a recessed portion or planar portion), which is continuous with the first portion 103.1 but has been recessed (i.e. etched back) to have a second thickness (i.e., see second height 135) less than the first portion (i.e., the second portion 103.2 is thinner than the first portion 103.1). The heating element 140 includes a third portion 103.3 of a semiconductor layer 103 that is continuous with the second portion 103.2 and a metal silicide layer 145 on the third portion 103.3.

[0064] FIGS. 6A-6C It shows the method for forming FIG. 1B Exemplary process steps of photonic structure embodiment 100B. For example, a mask 601 can be formed on semiconductor layer 103 (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask forming technique), and an anisotropic etching process can be performed to define the initial shapes of bus waveguide 110 (see portion 103.4) and closed curve waveguide 120 (see portion 103.1) in semiconductor layer 103, both having a first height 125, and in doing so, not completely etching through semiconductor layer 103, thus leaving a recessed portion 103.5 (also called a planar portion) of semiconductor layer 103 with a second height 135 less than the first height 125 covering an insulating layer 102 between the thicker patterned portions 103.1 and 103.4 (see portion 103.4).FIG. 6A ). The mask 601 can be removed. Then, another mask 602 can be formed (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation technique) over the partially completed structure, and then an anisotropic etching process can be performed to completely remove specific portions of the recessed portion of the semiconductor layer (e.g., immediately adjacent to the opposite sides of the bus waveguide, immediately adjacent to the outer curved sidewalls of the closed curve waveguide, and spatially separated from the inner curved sidewalls of the closed curve waveguide), leaving the intact recessed portion 103.2 of the semiconductor layer 103 for the closed curve thermal coupler 130 directly laterally positioned to the thick patterned portion 103.1 of the semiconductor layer (i.e., laterally positioned immediately adjacent to the closed curve waveguide 120) (see FIG. 10B). FIG. 6B ). The mask 602 can be removed. Then, another mask 603 can be formed (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation technique) over the partially completed structure and a resistive element 141 of metal or metal alloy for the heating element 140 can be formed (e.g., deposited into the patterned openings in the mask) over the insulating layer 102 and immediately adjacent to the insulating layer 102 and further laterally positioned immediately adjacent to the recessed portion 103.2 of the semiconductor layer (i.e., so as to abut the closed curve thermal coupler 130). This resistive element 141 can be made of, for example, tungsten, aluminum, nickel, titanium, tantalum, cobalt, copper, or alloys thereof. As FIGS. 6A-6C As a result of the process steps shown in FIG. 10B and described above, the closed curve waveguide 120 and the closed curve thermal coupler 130 are contiguous portions 103.1-103.2 of the same semiconductor layer 103, but the heating element 140 is a discrete metal or metal alloy feature.

[0065] FIGS. 7A-7C An example process step for forming the photonic structure embodiment 100C of FIG. 1C may be formed (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation technique) over the semiconductor layer 103 and an anisotropic etching process can be performed to define discrete shapes of the bus waveguide 110 (see portion 103.4), the closed curve waveguide 120 (see portion 103.1), and the heating element 140 (see portion 103.3) in the semiconductor layer 103, each having the first height 125 and in such a way as to completely etch through the semiconductor layer 103, thereby exposing portions of the insulating layer 102 between the patterned portions 103.1, 103.3, and 103.4 (see FIG. 10A). FIG. 7A). Mask 701 can be removed. Then, another mask 702 can be formed (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation techniques) on the partially completed structure and a non-metallic electrically insulating thermally conductive material can be deposited into the openings in mask 702 between the patterned portions 103.1 and 103.2 of the semiconductor layer and recessed to have a second height 135 that is less than the first height, thereby forming the non-metallic electrically insulating thermally conductive feature 104 for the closed curve thermal coupler 130 (see FIG. 7B ). An exemplary non-metallic electrically insulating thermally conductive material that can be used includes, but is not limited to, polysilicon, boron nitride, silicon carbide, or diamond. Mask 702 can be removed. Then, yet another mask 703 can be formed (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation techniques) on the partially completed structure and a metal silicide process can be performed to form a metal silicide layer 145 on the exposed surface of the thick patterned portion 103.3 of the semiconductor layer for the heating element 140. As described above, the metal silicide layer 145 can be, for example, a cobalt silicide (CoSi) layer, a nickel silicide (NiSi) layer, a tungsten silicide (WSi) layer, a titanium silicide (TiSi) layer, or any other suitable metal silicide layer. Optionally, the metal silicide layer 145 can be doped with N-type or P-type dopants to lower the electrical resistance.

[0066] FIGS. 8A-8C An exemplary process step for forming the photonic structure embodiment 100D of FIG. 1D ). For example, a mask 801 can be formed (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation techniques) on the semiconductor layer 103 and an anisotropic etching process can be performed to define the discrete shapes of the bus waveguide 110 (see portion 103.4) and the closed curve waveguide 120 (see portion 103.1) in the semiconductor layer 103 each having the first height 125 and in a manner that etches completely through the semiconductor layer 103, thereby exposing portions of the insulating layer 102 between and adjacent to the patterned portions 103.1 and 103.4 (see FIG. 7B ). Mask 801 can be removed. Then, another mask 802 can be formed (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation techniques) on the partially completed structure and a non-metallic electrically insulating thermally conductive material can be deposited into the openings in mask 802 adjacent to the inner curved sidewalls of the patterned portion 103.1 and recessed to have a second height 135 that is less than the first height, thereby forming the non-metallic electrically insulating thermally conductive feature 104 for the closed curve thermal coupler 130 (see FIG. 8B). As noted above, non-metallic electrically insulating thermally conductive materials that can be used include, but are not limited to, polysilicon, silicon, nitride, boron nitride, silicon carbide, or diamond. Mask 802 can be removed. Then another mask 803 can be formed (e.g., using conventional photolithographic patterning and etching techniques or any other suitable mask formation techniques) over the partially completed structure and an electrically resistive element 141 of metal or metal alloy for heating element 140 can be formed (e.g., deposited into the patterned openings in the mask) over and immediately adjacent to insulating layer 102, and further laterally positioned immediately adjacent to non-metallic electrically insulating thermally conductive feature 104 (i.e., abutting closed curve thermal coupler 130). As noted above, such electrically resistive element 141 can be made of, for example, tungsten, aluminum, nickel, titanium, tantalum, cobalt, copper, or alloys thereof.

[0067] The descriptions in the above process steps and FIGS. 5A-5C , FIGS. 6A-6C , FIGS. 7A-7C or FIGS. 8A-8C are for illustrative purposes and are not limiting. The order of the process steps can be varied, any and / or alternative process steps can be performed to form the desired components. In any case, the process steps shown in the above and FIGS. 5A-5C , FIGS. 6A-6C , FIGS. 7A-7C or FIGS. 8A-8C may be followed by deposition of one or more dielectric material layers 105 to cover the exposed surfaces of insulating layer 102 and any device components thereon. For example, a dielectric material layer can be deposited to cover optical device 199 (including closed curve waveguide 120 and bus waveguide 110 (if applicable)), closed curve thermal coupler 130, and heating element 140. This dielectric material 105, particularly any dielectric material deposited directly onto the optical waveguides, can be any dielectric material suitable for use as a cladding material for those optical waveguides. For example, if the optical waveguides are silicon waveguides, which as noted above can have temperature and wavelength dependent refractive indices generally higher than 3.2, the dielectric material layer 105 immediately adjacent to these optical waveguides can be silicon dioxide (e.g., having a refractive index less than 1.6), silicon nitride (e.g., having a refractive index less than 2.1), or any other suitable dielectric material having a refractive index less than the lowest refractive index of silicon.

[0068] It will be appreciated that similar processes can be performed to form photonic structure embodiments 200A-200D of FIG. 2 and FIGS. 2A-2D , except that various patterning processes can be performed to ensure that closed curve thermal coupler 230 is physically separated from closed curve waveguide 220 and / or heating element 240 by space 250. In addition, it will be appreciated that similar processes can be performed to form photonic structure embodiments 200A-200D of FIG. 3 and FIGS. 3A-3BThe photonic structure embodiments 300A-300B, except for the formation of the heating element 340, will be performed only after the dielectric material 305 is deposited on the optical device 399, including the closed curve waveguide 320 and the bus waveguide 310 (if applicable), and the lower design level closed curve thermal coupler 330, following BEOL processing.

[0069] Referring again to FIG. 4 The method can also include thermally tuning the closed curve waveguide 120, 220, 320 via the closed curve thermal coupler 130, 230, 330 using the heating element 140, 240, 340 (e.g., using the heating element to generate and output thermal energy that enters and passes through the closed curve thermal coupler and further into the closed curve waveguide) to minimize any temperature dependent resonance shift (TDRS) (see process step 408). That is, during thermal tuning, the heating element 140, 240, 340 can generate and output thermal energy and, due to thermal coupling, can be transferred into the closed curve thermal coupler 130, 230, 330, can propagate through the closed curve thermal coupler 130, 230, 330, and can enter the lower portion of the closed curve waveguide 120, 220, 320 from the closed curve thermal coupler 130, 230, 330. The amount of thermal energy can be predetermined to ensure that the temperature of the closed curve waveguide 120, 220, 320 is maintained at a particular temperature or a particular temperature range to ensure that a particular resonant wavelength of optical signals is enhanced in intensity as it repeatedly traverses the closed curve waveguide 120, 220, 320.

[0070] It is noted that the method embodiments can also include, prior to forming the various components (i.e., the closed curve waveguide, the closed curve thermal coupler, and the heating element), predetermining the dimensions of the various components as well as the materials of the components and any spacing between the components to ensure that the desired resonant wavelength can be achieved by the heating element through thermal tuning of the closed curve waveguide by the closed curve thermal coupler.

[0071] The method as described above is for the manufacture of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricant in raw wafer form (that is, as a single wafer having many unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either one or more substrates with surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and / or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

[0072] In some of the above-described photonic structure and method embodiments, some components (e.g., such as...) FIG. 1A The closed-curve waveguide, closed-curve thermal coupler, and part of the heating element shown; or the closed-curve waveguide and closed-curve thermal coupler, such as FIG. 1B , FIG. 2A , FIG. 2B or FIG. 3A These are referred to as continuous portions of the same semiconductor layer. It should be understood that any two “continuous portions of the same semiconductor layer” that constitute any two components of a photonic structure will be portions of the semiconductor layer that are directly adjacent to each other but otherwise treated to be distinguishable, as described above. For example, the semiconductor layer can be formed on (e.g., deposited on) an insulating layer and subsequently treated in such a way that the two portions in question are not physically separated but have different dimensions, particularly different heights and shapes, so that they can be distinguished, as described above. Therefore, for example, in… FIG. 1A , FIG. 1B , FIG. 2A , FIG. 2B and FIG. 3A In the illustrated embodiment, closed-curve waveguides 120, 220, 320 and closed-curve thermal couplers 130, 230, 330 are continuous portions of the same semiconductor layer, wherein closed-curve waveguides 120, 220, 320 are first portions 103.1, 203.1, 303.1 of the semiconductor layer, and closed-curve thermal couplers are second portions 103.2, 203.2, 303.2 that are thinner than the first portions 103.1, 203.1, 303.1, and extend laterally from immediately adjacent to the first portions 103.1, 203.1, 303.1.

[0073] Furthermore, in the above-described embodiments of the photonic structure and method, some components (e.g., FIG. 1A , FIG. 1B , FIG. 2B and FIG. 3A Bus waveguides in; such as in FIG. 1C and FIG. 2C The bus waveguide, closed curve waveguide, and part of the heating element; and as in FIG. 1D , FIG. 2D and FIG. 3B In this context, bus waveguides and closed curve waveguides are described as discrete portions of the same semiconductor layer. It should be understood that a “discrete portion” of the semiconductor layer constituting a component of a photonic structure is a patterned portion of the semiconductor layer that is physically separated from all other portions for distinguishability, as described above. For example, the semiconductor layer may be formed on (e.g., deposited on) an insulating layer and subsequently processed in such a way that the portion in question is physically separated from all other portions and has the desired dimensions, as described above.

[0074] It is to be understood that the terminology used herein is for the purpose of describing the disclosed structures and methods and is not intended to be limiting. For example, as used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms "comprises", "comprising", "includes" and / or "including" are used, such terms are specifically intended to be inclusive in meaning and / or scope and not exclusive. Additionally, as used herein, terms such as "right", "left", "vertical", "horizontal", "top", "bottom", "upper", "lower", "beneath", "below", "under", "above", "over", "parallel", "perpendicular", and the like are intended to describe relative positions and orientations in the figures as oriented and illustrated and are not intended to be limiting. Also, terms such as "contact", "direct contact", "abut", "directly adjacent", "immediately adjacent" and the like are intended to mean that at least one element physically contacts another element (without other elements intervening between the elements). The term "lateral" is used herein to describe the relative position of elements and more specifically indicates that the elements are positioned to the side of another element rather than above or below another element as the elements are oriented and illustrated in the figures. For example, an element that is laterally adjacent to another element will be to the side of the other element, an element that is laterally immediately adjacent to another element will be directly to the side of the other element, and an element that is laterally surrounding another element will be adjacent to the other element and border the outer sidewall of the other element. The corresponding structures, materials, acts, and equivalents of all means or step plus function elements are intended to include any structures, materials, or acts for performing the functions in combination with other claimed

[0075] The description of various embodiments of the present application is presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A photonic structure, characterized in that, comprises: a closed curve waveguide on and immediately adjacent to the insulating layer and having a first height, wherein the closed curve waveguide has outer curved sidewalls extending substantially perpendicularly from the insulating layer to the first height; a closed curve thermal coupler on and immediately adjacent to the insulating layer and laterally surrounded by and immediately adjacent to the closed curve waveguide, wherein the closed curve thermal coupler has a second height less than the first height; and a heating element on and immediately adjacent to the insulating layer and laterally surrounded by the closed curve thermal coupler, wherein the closed curve waveguide, the closed curve thermal coupler, and the heating element are annular, the closed curve waveguide is larger than the closed curve thermal coupler and the closed curve thermal coupler is larger than the heating element; and wherein the closed curve thermal coupler is adapted to transfer thermal energy from the heating element to the closed curve waveguide.

2. The photonic structure of claim 1, wherein: wherein the closed curve waveguide and the closed curve thermal coupler have a continuous annular shape, wherein the heating element is any one of a continuous annular shape, an open annular shape, a segmented annular shape, and wherein the structure further comprises a dielectric material on and immediately adjacent to the insulating layer, the dielectric material laterally adjacent to and covering the outer curved sidewalls, on and immediately adjacent to top surfaces of the closed curve thermal coupler and the closed curve waveguide, and on and immediately adjacent to the insulating layer and laterally surrounded by and immediately adjacent to the heating element.

3. The photonic structure of claim 1, wherein, the closed curve thermal coupler is contiguous to the closed curve waveguide and separated from the heating element by the dielectric material.

4. The photonic structure of claim 1, wherein, the heating element has a third height greater than the second height.

5. The photonic structure of claim 1, wherein: the closed curve waveguide comprises a first portion of a semiconductor layer, wherein the semiconductor layer comprises any one of a silicon layer, a polysilicon layer, a germanium layer, and a silicon germanium layer, wherein the closed curve thermal coupler comprises a second portion of the semiconductor layer laterally extending from the first portion and thinner than the first portion, and wherein the heating element comprises a third portion of the semiconductor layer and a metal silicide layer on the third portion.

6. The photonic structure of claim 1, wherein: the closed curve waveguide comprises a semiconductor material, wherein the closed curve thermal coupler comprises an electrically insulating thermally conductive material different from the semiconductor material, and wherein the heating element comprises a metallic material different from materials of the closed curve waveguide and the closed curve thermal coupler.

7. The photonic structure of claim 1, wherein: the annular shapes of the closed curve waveguide, the closed curve thermal coupler, and the heating element are each one of an elliptical annular shape, a circular annular shape, a racetrack annular shape.

8. A photonic structure, characterized in that comprises: a ring resonator comprising: a bus waveguide on and immediately adjacent to an insulating layer and having a first height, wherein the bus waveguide has planar sidewalls extending substantially perpendicularly from the insulating layer to the first height; and a closed curve waveguide on and immediately adjacent to the insulating layer and laterally adjacent to and having the first height, wherein the closed curve waveguide has an outer curved sidewall extending substantially perpendicularly from the insulating layer to the first height; a dielectric material on and immediately adjacent to the insulating layer, the dielectric material laterally surrounding and immediately adjacent to the outer curved sidewall, wherein the dielectric material fills between the planar sidewall and the outer curved sidewall; a closed curve thermal coupler on and immediately adjacent to the insulating layer and laterally surrounded by and immediately adjacent to the closed curve waveguide, wherein the closed curve thermal coupler has a second height less than the first height; and a heating element on and immediately adjacent to the insulating layer and laterally surrounded by the closed curve thermal coupler, wherein the closed curve waveguide, the closed curve thermal coupler, and the heating element are annular, the closed curve waveguide is larger than the closed curve thermal coupler and the closed curve thermal coupler is larger than the heating element; and wherein the closed curve thermal coupler is adapted to transfer thermal energy from the heating element to the closed curve waveguide.

9. The photonic structure of claim 8, wherein, wherein the closed curve waveguide and the closed curve thermal coupler are continuous annular, wherein the heating element is any one of continuous annular, open annular, segmented annular, and wherein the dielectric material is on and immediately adjacent to a top surface of the closed curve thermal coupler and the closed curve waveguide and on and immediately adjacent to the insulating layer and laterally surrounded by and immediately adjacent to the heating element.

10. The photonic structure of claim 8, wherein, the closed curve thermal coupler is contiguous to the closed curve waveguide and separated from the heating element by the dielectric material.

11. The photonic structure of claim 8, wherein, the heating element has a third height greater than the second height.

12. The photonic structure of claim 8, wherein, the closed curve waveguide comprises a first portion of a semiconductor layer, wherein the semiconductor layer comprises any one of a silicon layer, a polysilicon layer, a germanium layer, and a silicon germanium layer, wherein the closed curve thermal coupler comprises a second portion of the semiconductor layer laterally extending from the first portion and thinner than the first portion, wherein the heating element comprises a third portion of the semiconductor layer and a metal silicide layer on the third portion, and wherein the bus waveguide comprises a fourth portion of the semiconductor layer.

13. The photonic structure of claim 8, wherein, the bus waveguide and the closed curve waveguide comprise a semiconductor material, wherein the closed curve thermal coupler comprises an electrically insulating thermally conductive material different from the semiconductor material, and wherein the heating element comprises a metallic material different from materials of the closed curve waveguide and the closed curve thermal coupler.

14. The photonic structure of claim 8, wherein, the annular shapes of the closed curve waveguide, the closed curve thermal coupler, and the heating element are any one of elliptical annular, circular annular, and racetrack annular.

15. A method of forming a photonic structure, characterized by, comprising: forming a closed curve waveguide on and immediately adjacent to an insulating layer and having a first height, wherein the closed curve waveguide has an outer curved sidewall extending substantially perpendicularly from the insulating layer to the first height; forming a closed curve thermal coupler on and proximate to the insulating layer and laterally surrounded by and proximate to the closed curve waveguide, wherein the closed curve thermal coupler has a second height that is less than the first height; and forming a heating element on and proximate to the insulating layer and laterally surrounded by the closed curve thermal coupler, wherein the closed curve waveguide, the closed curve thermal coupler, and the heating element are annular, the closed curve waveguide is larger than the closed curve thermal coupler and the closed curve thermal coupler is larger than the heating element; and wherein the forming of the closed curve waveguide, the forming of the closed curve thermal coupler, and the forming of the heating element are such that thermal energy generated by the heating element is transferred to the closed curve waveguide through the closed curve thermal coupler.

16. The method of claim 15, wherein, The method further includes forming a bus waveguide having a first height and adjacent the outer curved sidewall, wherein the bus waveguide is formed on and proximate to the insulating layer and has planar sidewalls that extend substantially perpendicularly from the insulating layer to the first height, and is physically separated from the outer curved sidewall.

17. The method of claim 15, wherein, Further comprising: providing a semiconductor layer on an insulating layer, wherein the semiconductor layer comprises any one of a silicon layer, a polysilicon layer, a germanium layer, and a silicon germanium layer, wherein the forming of the closed curve waveguide, the forming of the closed curve thermal coupler, and the forming of the heating element comprise patterning the semiconductor layer such that the closed curve waveguide comprises a first portion of the semiconductor layer, such that the closed curve thermal coupler comprises a second portion of the semiconductor layer that laterally extends from the first portion and is thinner than the first portion, and such that the heating element comprises a third portion of the semiconductor layer, and wherein the forming of the heating element further comprises forming a metal silicide layer on the third portion of the semiconductor layer.

18. The method of claim 15, wherein, Further comprising: providing a semiconductor layer on an insulating layer, wherein the semiconductor layer comprises a semiconductor material of any one of silicon, polysilicon, germanium, and silicon germanium, wherein the forming of the closed curve waveguide comprises patterning the semiconductor layer such that the closed curve waveguide comprises a portion of the semiconductor layer, and wherein the forming of the closed curve thermal coupler comprises depositing and patterning an electrically insulating thermally conductive material that is different from the semiconductor material.

19. The method of claim 15, wherein, the closed curve waveguide and the closed curve thermal coupler form a continuous annulus, wherein the heating element is formed to have a third height that is greater than the second height, the heating element has any one of a continuous annulus, an open annulus, a segmented annulus, and wherein the method comprises forming a dielectric material on and proximate to a top surface of the insulating layer, the dielectric material laterally adjacent to and covering the outer curved sidewall, on and proximate to top surfaces of the closed curve thermal coupler and the closed curve waveguide, and on and proximate to a top surface of the insulating layer and laterally surrounded by and proximate to the heating element.

20. The method of claim 15, wherein, Further comprising using the heating element to thermally tune the closed curve waveguide through the closed curve thermal coupler. the closed curve waveguide and the closed curve thermal coupler form a continuous annulus, wherein the heating element is formed to have a third height that is greater than the second height, the heating element has any one of a continuous annulus, an open annulus, a segmented annulus, and wherein the method comprises forming a dielectric material on and proximate to a top surface of the insulating layer, the dielectric material laterally adjacent to and covering the outer curved sidewall, on and proximate to top surfaces of the closed curve thermal coupler and the closed curve waveguide, and on and proximate to a top surface of the insulating layer and laterally surrounded by and proximate to the heating element. Further comprising using the heating element to thermally tune the closed curve waveguide through the closed curve thermal coupler.

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