WO2017115279A1

WO2017115279A1 - A method to suspend an optical resonator in a fluid using segmented waveguides

Info

Publication number: WO2017115279A1
Application number: PCT/IB2016/058023
Authority: WO
Inventors: Anatol KHILO; Soha YOUSUF; Marcus DAHLEM
Original assignee: Masdar Institute Of Science And Technology
Priority date: 2015-12-28
Filing date: 2016-12-27
Publication date: 2017-07-06

Abstract

A method for forming a suspended micro-ring resonator includes: etching a composite material comprising a first material and a second material, wherein the second material at least partially surrounds the first material; removing the second material until the first material is exposed to an environment and until a base is formed from the second material, with the base supporting the first material in an elevated position; and forming a grating in the first material. Also a suspended micro-ring resonator comprising: a base; a disc coupled to the base; and a ring coupled to the disc using a plurality of extensions, the plurality of extensions collectively forming a grating, wherein the suspended micro-ring resonator is formed using a single lithographic step. The micro- ring resonator may be comprised within an optical sensor that further comprises: an input waveguide and an output waveguide that are positioned on opposing sides of the micro-ring resonator.

Description

A METHOD TO SUSPEND AN OPTICAL RESONATOR IN A FLUID USING

SEGMENTED WAVEGUIDES

CROSS REFERENCE TO RELATED APPLICATION

[001] This application claims the benefit of co-pending U.S. Provisional Patent Application Serial Number 62/271,773 filed on December 28, 2015, entitled A METHOD TO SUSPEND AN OPTICAL RESONATOR IN A FLUID USING SEGMENTED WAVEGUIDES, which is fully incorporated in its entirety herein by reference.

TECHNICAL FIELD

[002] The present invention relates generally to optical resonators, and more specifically to optical resonators suspended in a fluid.

BACKGROUND

[003] The silicon-on-insulator (SOI) platform has attracted significant interest for sensor applications. For example, integrated photonic structures that may be used for detecting changes in the refractive index of a surrounding medium include non-resonant structures such as Mach-Zehnder interferometers and resonant structures such as ring resonators, disk resonators, and Bragg grating-based cavities. In resonant structures, a high sensitivity and a high quality factor (Q-factor) are desirable for obtaining a low detection limit. For example, a micro-ring resonator with at least a portion of the ring exposed to the environment can be used as a basic sensor. Increasing the sensitivity of a micro-ring resonator can be accomplished by increasing the overlap between the optical field and the environment.

SUMMARY

[004] A method for forming a suspended micro-ring resonator consistent with at least one embodiment of the present disclosure includes etching a composite material comprising a first material and a second material, wherein the second material at least partially surrounds the first material. The method may further include removing the second material until the first material is exposed to an environment and until a base is formed from the second material. The base may support the first material in an elevated position. The method may additionally include forming a grating in the first material.

[005] An optical sensor consistent with at least one embodiment of the present disclosure may include a suspended micro-ring resonator. The suspended micro-ring resonator may be suspended by a base and the suspended micro-ring resonator may be formed using a single lithographic step. The optical sensor may also include an input waveguide and an output waveguide. The output waveguide and the input waveguide may be positioned on opposing sides of the suspended micro-ring resonator.

[006] A suspended micro-ring resonator consistent with at least one embodiment of the present disclosure may include a base. A disk may be coupled to the base. A ring may be coupled to the disk using a plurality of extensions. The plurality of extensions may collectively form a grating. The suspended micro-ring resonator may be formed using a single lithographic step

BRIEF DESCRIPTION OF THE DRAWINGS

[007] These and other features and advantages will be better understood by reading the following detailed description taken together with the drawings.

[008] FIG. 1 is a perspective view of a suspended micro-ring resonator positioned adjacent a waveguide, consistent with embodiments of the present disclosure.

[009] FIG. 2 A is an embodiment of a segmented waveguide, consistent with embodiments of the present disclosure.

[010] FIG. 2B is an embodiment of a segmented waveguide having multiple gratings, consistent with embodiments of the present disclosure.

[011] FIG. 3 is a top view of an embodiment of a suspended micro-ring resonator positioned between two waveguides, consistent with embodiments of the present disclosure.

[012] FIG. 4 is a perspective view of a suspended micro-ring resonator, consistent with embodiments of the present disclosure.

[013] FIG. 5A is a cross-sectional view of a waveguide, consistent with embodiments of the present disclosure.

[014] FIG. 5B is a cross-sectional view of a waveguide, consistent with embodiments of the present disclosure. [015] FIG. 6A is a perspective view of a suspended micro-ring resonator positioned between two waveguides, consistent with embodiments of the present disclosure.

[016] FIG. 6B is a top view of a portion of a suspended micro-ring resonator adjacent a waveguide, consistent with embodiments of the present disclosure.

[017] FIG. 6C is another top view of a suspended micro-ring resonator adjacent a waveguide, consistent with embodiments of the present disclosure.

[018] FIG. 7 is a representation of the results of an experiment carried out with a micro-ring resonator, consistent with embodiments of the present disclosure.

[019] FIG. 8A is a top view of a suspended micro-ring resonator having a support beam, consistent with embodiments of the present disclosure.

[020] FIG. 8B is a top view of a suspended micro-ring resonator having two support beams, consistent with embodiments of the present disclosure.

[021] FIG. 8C is a top view of a suspended micro-ring resonator having three support beams, consistent with embodiments of the present disclosure.

[022] FIG. 8D is a top view of a suspended micro-ring resonator having four support beams, consistent with embodiments of the present disclosure.

[023] FIG. 9A is a top view of a suspended micro-ring resonator positioned between two waveguides having a concave portion, consistent with embodiments of the present disclosure.

[024] FIG. 9B is an enlarged view of a portion of the suspended micro-ring resonator positioned between two waveguides having a concave portion of FIG. 9A, consistent with embodiments of the present disclosure.

[025] FIG. 10A is a top view of a suspended micro-ring resonator positioned between two waveguides having a convex portion, consistent with embodiments of the present disclosure.

[026] FIG. 10B is an enlarged view of a portion of the suspended micro-ring resonator positioned between two waveguides having a convex portion of FIG. 10A, consistent with embodiments of the present disclosure.

[027] FIG. 11A is a top view of a suspended micro-ring resonator positioned between two substantially parallel waveguides having, consistent with embodiments of the present disclosure.

[028] FIG. 11B is an enlarged view of a portion of the suspended micro-ring resonator positioned between two substantially parallel waveguides of FIG. 11 A, consistent with embodiments of the present disclosure. [029] FIG. 12 is a flow chart of an example method for forming a suspended micro- ring resonator, consistent with embodiments of the present disclosure.

DETAILED DESCRIPTION

[030] The examples described below are for illustrative purposes only, and are not to be construed as limiting the present disclosure. Thus, where examples may be described in detail, or where a list of examples may be provided, it should be understood that the examples are not to be construed as exhaustive and do not limit the embodiments of the present disclosure to the examples described and/or illustrated.

[031] The use of integrated photonic structures as sensors for detecting changes in the environment (e.g., air) has become an area of increasing interest in the semiconductor field. Increasing the sensitivity of photonic sensors can be obtained by changing the mode of the input waves (e.g., TE-mode or TM-mode). However, further sensitivity benefits can be obtained by increasing the exposure of the optical field to the environment.

[032] For example, a micro-ring resonator exposed to TM-polarized light (wherein light, as used herein, can be any form of light and is not limited to the visible spectrum) will have a higher sensitivity than the same micro-ring resonator exposed to TE-polarized light. The higher sensitivity from the use of TM-polarized light is due to the larger evanescent field of TM-polarized light (assuming that the transmission core height is less than its width). Further improvement can be obtained through the utilization of slot waveguides where the optical field is highly concentrated in the slot. Slot waveguide based micro-rings may further improve the sensitivity of micro-rings. The slot waveguides may be created in a resonant cavity that includes Bragg gratings. Alternatively, sub-wavelength grating waveguides, where the mode is fully exposed to the environment in the gaps of the sub-wavelength structure, may be employed to increase the sensitivity of the sensor. A commonality between the above structural alterations to the micro-ring is that each structural alteration is made in an effort to increase the modal overlap with the environment.

[033] Increasing the modal overlap with the environment can further be achieved (and combined with any combination of the above methods) by suspending the micro-ring resonator. Some micro-ring resonators may be suspended using, for example, a thin silicon membrane within the ring created by a partial silicon etch, or by using supporting arms to form a suspension bridge, or through the utilization of multiple nano-sized support trusses, or the resonator may be what is referred to as a "wiggler" resonator, or the suspension system may comprise internal spokes such as those that support an optomechanical wheel resonator.

[034] During fabrication of the suspension structure, it is beneficial to remove the oxide residing on the top and bottom surface of the waveguide. Removal of the oxide may increase the modal overlap with the environment, which may further increase the efficiency of the sensor. However, prior methods of reducing/removing the oxide on the waveguide required the use of multiple lithographic steps, increasing the complexity of manufacturing.

[035] A micro-ring resonator consistent with at least one embodiment of the current disclosure includes a ring that is formed from a segmented silicon waveguide, wherein the grating has a period small enough that it does not scatter light propagating along the waveguide. In the suspended ring, the grating may only be inside the ring, and connects the ring to a central silicon (Si) disk. With an isotropic etch, the buried oxide may be completely removed from under the ring and the grating, while some oxide may remain under the central Si disk, protected by the disk. This oxide forms a base (e.g., a pedestal) which mechanically supports the whole structure. The optical mode of the ring decays evanescently in the grating region and does not couple into the disk.

[036] The micro-ring resonator has enhanced sensitivity because the optical mode is exposed to the environment both above and below the waveguide, as well as in the grating region. For maximum sensitivity, TM-polarized light and small core widths can be used. Sensitivities of 503, 630, and 835nm/RIU are found from 3D FDTD simulations for 450, 320, and 200 nanometers (nm) core widths, respectively. For a ring with a radius of 15 micrometers (μπι) and 450nm core an intrinsic Q-factor of 75k is found. The radius may need to be increased to 50μπι for a high Q-factor in a 200nm-wide ring. These calculations assumed 450nm grating period (200nm Si + 250nm gap), 220nm Si thickness, 1.31 refractive index of the environment, and 1550nm wavelength.

[037] The suspended micro-ring resonator consistent with at least one embodiment disclosed herein provides a structure for achieving a high sensitivity and high a Q-factor. Further, the disclosed micro-ring resonator may be formed using a single lithography step (without a partial silicon etch) and with a moderate minimum feature size of 200nm. In addition to sensing, the disclosed suspended micro-ring resonator can be useful for optomechanical and mid-infrared applications. [038] Turning now to the figures, one embodiment of a suspended micro-ring resonator 100 consistent with the present disclosure is shown in FIG. 1. The suspended micro-ring resonator 100 may include a base 105, a disk 110, a ring 115, and at least one extension 120. A plurality of the extensions 120 may extend between the disk 110 and the ring 115 to collectively form a grating 130. Therefore, in some instances, the grating 130 may be generally described as being positioned between the disk 110 and the ring 115. Light 121 is transmitted to the suspended micro-ring resonator 100 using an input waveguide 125 via an optical coupling. For example, the optical coupling may be influenced by the separation distance (e.g., gap 225 of FIG. 6C) between the micro-ring resonator 100 and the input waveguide 125, a coupling length 127, and the refractive indices between the micro-ring resonator 100 and the input waveguide 125. The coupling length 127 may generally be described as a length of a waveguide (e.g., the input waveguide 125) capable of optically coupling to the micro-ring resonator 100.

[039] Once optically coupled, the transmitted light 126 passes through the micro- ring resonator 100. The ring 115 is coupled to the disk 110 using the extensions 120; however, the transmitted light 126 oscillating in the ring 115 is substantially optically isolated from the disk 110. The optical oscillations within the ring 115 are facilitated, at least in part, by the formation of a region that has a lower refractive index than the disk 110. In some embodiments, this is accomplished when the ring 115 and the extensions 120 form the grating 130 that may be a segmented waveguide.

[040] An embodiment of a segmented waveguide 135 having a backbone 140 and a plurality of extensions 145 extending from the backbone 140 is shown in FIG. 2 A. The period of the segmented waveguide 135 may be less than the first-order Bragg period. By using a period less than the first-order Bragg period, light propagates within the waveguide with substantially no scattering or reflection, which reduces the propagation losses. If TM-polarized light is used, the grating period can be larger than if TE-polarized light is used. As may be appreciated, the period of a grating for TM-polarized light is greater because TM-polarized light has a smaller effective index and, therefore, a longer first-order Bragg period than TE-polarized light.

[041] Although in FIG. 2A the segmented waveguide 135 is illustrated as being substantially straight, it should be appreciated that, when the segmented waveguide 135 is used in the suspended micro-ring resonator 100, the backbone 140 forms the ring 115 and the plurality of extensions 145 form the extensions 120 and serve to couple the backbone 140 to the disk 110. Further, as should be appreciated, the segmented waveguide 135 of FIG. 2A may be formed using only half of the segmented waveguide as shown, for example, in FIG. 2B.

[042] As is shown in FIG. 3, while the transmitted light 126 is oscillating in the ring 115 a portion of the transmitted light 126 may be further transferred to an output waveguide 150. As such, the output waveguide 150 is optically coupled to the ring 115 in substantially the same manner as the input waveguide 125 is optically coupled to the ring 115. The output light 155 may be used to determine the desired environmental conditions. As should be appreciated, the more the transmitted light 126 overlaps with the environment surrounding the suspended micro-ring resonator 100 the more sensitive the sensor becomes. Therefore, the input waveguide 125, the output waveguide 150, and the micro-ring resonator 100 may be generally described as collectively forming an optical sensor 101.

[043] As previously discussed, the suspension of the suspended micro-ring resonator 100 increases the modal overlap with the environment, increasing the sensitivity of the suspended micro-ring resonator 100. As generally shown in FIG. 4, the suspension of the suspended micro-ring resonator 100 is accomplished using the base 105. The base 105 of the suspended micro-ring resonator 100 may be formed using an isotropic, or partially anisotropic, etching process. The etching time may be selected such that a desired shape for the base 105 is obtained. For example, changes in the etching time can be used to remove oxide that is present on a bottom surface 160 and a top surface 167 of the suspended micro-ring resonator 100. As was previously discussed, the removal of the oxide increases the modal overlap of the transmitted light 126 and the environment. As may be appreciated, where the base 105 meets the disk 110, the cross-sectional area may be reduced such that a majority of the oxide is removed from the bottom surface 160. However, the reduction of the cross-sectional area is limited by the structural requirement that the base 105 suspend the suspended micro-ring resonator 100.

[044] The process used to form the base 105 may also be applied to other platforms, and not only a silicon-on-insulator platform. For example, FIG. 5A shows a cross- section of a waveguide 165 for which the above etching process can be applied. The waveguide cross section may be any one or more of the input waveguide 125, the output waveguide 150, the ring 115, and/or the grating 130. In operation, the light passes through a core material 170 but, before processing, the core material 170 may be surrounded by a second material 175 (i.e., the second material 175 is both an under- cladding and an over-cladding material). However, in some instances, the second material 175 may only partially surrounds the core material 170 (e.g., the second material 175 is only one of the under-cladding or the over-cladding material). The core material 170 may, for example, be silicon and the second material 175 may, for example, be silicon dioxide. However, in other embodiments, the core material 170 may, for example, be silicon nitride or silicon dioxide. Further, in some instances, an additional layer may be present between the core material 170 and second material 175. For example, a core 170 being made of silicon may be covered by a thin layer of silicon nitride. Such additional layers may change the optical mode and as such the disclosed etching method may be applicable in this situation for removal of these additional layers.

[045] As shown in FIG. 5B, some embodiments may employ an under-cladding material 180, an over-cladding material 185, and the core material 170. As should be appreciated, the under-cladding material 180 and the over-cladding material 185 may be different. Further, in some instances, an additional layer may be present between the core material 170 and the under-cladding material 180 and/or the over-cladding material 185. For example, a core 170 being made of silicon may be covered by a thin layer of silicon nitride. Such additional layers may change the optical mode and as such the disclosed etching method may be applicable in this situation for removal of these additional layers.

[046] In operation, regardless of whether the configurations described in FIGS. 5A and/or 5B, are used the disclosed method exposes the core material 170 to the surrounding environment.

[047] As may be appreciated, the etching process may be material dependent. As such, in one embodiment the disk 110 may be made (e.g., formed) of silicon and the base 105 may be made (e.g., formed) of silicon dioxide. Therefore, the etching material may be selected such that it is capable of etching silicon dioxide and not substantially etching silicon. In other words, the etching material selected may remove silicon dioxide at a much faster rate than silicon, resulting in the formation of a base 105 connected to the disk 110 of the suspended micro-ring resonator 100. However, such a configuration is non-limiting. For example, the etching rate may be substantially the same when the etchant is applied to the disk 110 and the base 105.

[048] With reference to FIGS. 6A, 6B, and 6C the optical sensor 101 may include the micro-ring resonator 100, the input waveguide 125, and the output waveguide 150. The micro-ring resonator 100 may be formed using a single lithographic step. As shown, the micro-ring resonator 100 is suspended by the base 105 such that the input waveguide 125 and the output waveguide 150 are positioned on opposing sides of the micro-ring resonator 100. As also shown, the micro-ring resonator 100 includes the disk 110, the ring 115, and a plurality of extensions 120 extending between the disk 110 and the ring 115. The plurality of extensions 120 collectively form a grating 130. Therefore, the micro-ring resonator 100 may generally be described as having the grating 130 positioned between the disk 110 and the ring 115. The ring 115 of the micro-ring resonator 100 may be separated from the input waveguide 125 and the output waveguide 150 by a gap 225.

[049] An example embodiment of a suspended micro-ring resonator 100 consistent with the present disclosure may have a ring thickness 190 that measures 220 nanometers (nm), a ring width 195 that measures 450nm, an input waveguide width 200 that measures 450nm, an output waveguide width 205 that measures 450nm, a separation distance 210 between the inner surface 215 of the ring 115 and the disk 110 that measures 3 micrometers (μπι), and a ring radius 220 that measures 15μπι. The gap 225 may measure 1.1 μιη. The grating period of the grating 130 at the inner surface 215 of the ring 115 may measure 450nm. The extensions 120 may have an extension width 216 that measures 200nm and a spacing between the extensions 120 may have an initial spacing width 217 measuring 250nm. However, as the grating approaches the center of the ring 115, the width of the extensions 120 may remain constant while the width of the spacing between the extensions 120 decreases from the initial spacing width 217. Assuming the refractive index of environment is 1.31 the theoretical quality factor is approximately 75k. At 1550nm the 5n_eff/ 5n_c was 1.22 and the sensitivity was 503nm/RIU. The above theoretical values were obtained assuming that the ring 115 comprises silicon and that TM-polarized light was used. By way of further example, when the width is decreased from 450nm to 320nm and 200nm, the sensitivity increased to 630nm/RIU and 835nm/RIU, respectively.

[050] In another example embodiment, the ring thickness 190 measures 220nm, the ring width 195 measures 506nm, the input waveguide width 200 measures 350nm, the output waveguide width 205 measures 350nm, the gap 225 measures 587nm, the grating period at the inner surface 215 of the ring 115 measures 450nm (wherein the extension width measures 200nm and the initial spacing width 217 measures 250nm), and the ring radius 220 measures 26.2μπι. FIG. 7 shows the measured drop-port and through-port transmission spectra for an experiment carried out on this example embodiment. The experiment was carried out on a non-suspended micro-ring resonator (i.e., the micro-ring resonator was fully surrounded by silicon dioxide). As shown in FIG. 7, the resonances are regularly spaced and have quality factors of 29.2k at 1537nm and 16.4k at 1588nm. [051] As such, at least one embodiment consistent with the above disclosure generally features a micro-ring resonator that may be fully suspended in a fluid (e.g., air) for a silicon-on-insulator platform. The micro-ring resonator may include a ring coupled to a disk via a grating and the disk may be coupled to a base. The grating period may below the first-order Bragg period such that light can propagate along the waveguide with very little loss. Further, as should be appreciated the above disclosure does not require a partial silicon etch and the entire structure can be fabricated in a single lithographic step. In some instances, the micro-ring resonator 100 may be fabricated using a 248nm lithography process.

[052] Further, as should be appreciated, suspended micro-ring resonators can be used for opto-mechanics. In other words, the suspended micro-ring resonator may move due to, for example, mechanical, electrical, or optical forces. However, as should be appreciated, the above suspension means is intended to prevent movement of the suspended micro-ring resonator. As such, the structure may require modification to facilitate movement. For example, as shown in FIGS. 8A, 8B, 8C, and 8D, one or more support beams 230 connect the ring 115 and the grating 130 to the disk 110. The disk 110 may be coupled to the base 105. The support beams 230 should be coupled to the grating 130 at a distance from the inner surface 215 of the ring 115 such that the amount of light reaching the point of coupling is insufficient to affect the optical mode. As should be appreciated, the use of support beams 230 allows for a less rigid structure to be created, enabling the suspended micro-ring resonator 100 to move more readily. Further, the support beams 230 can be used within a suspended micro-ring resonator 100 having any shape including, for example, ellipse shapes or oval-shapes (e.g., the oval may have one or more flat surfaces).

[053] FIGS. 9A-9B, 10A-10B, and 11A-11B show example embodiments of the input waveguide 125 and output waveguide 150 supported by one or more support structures 235. The support structures 235 provide mechanical support for the input waveguide 125 and output waveguide 150 such that the input waveguide 125 and output waveguide 150 can each be suspended for at least a portion of their respective lengths. The support structures 235 may prevent or otherwise mitigate the deformation or failure (e.g., breaking) of the input waveguide 125 and/or output waveguide 150 during fabrication and/or use. In some instances, the support structures 235 may be formed of a material that is resistant to an oxide etching material selected to form the input waveguide 125 and output waveguide 150 (e.g., the support structures 235 may be formed of silicon). Therefore, the input waveguide 125 and output waveguide 150 may terminate at a respective support structure 235.

[054] As shown in FIGS. 9A-9B, 10A-10B, and 11A-11B the gap 225 is sized such that at least a portion of the light transmitted along the input waveguide 125 is capable being optically transmitted to the micro-ring resonator 100 and optically transmitted from the micro-ring resonator 100 to the output waveguide 150. Therefore, the optical coupling efficiency may be based, at least in part, on a size of the gap 225. In some embodiments, the gap 225 may measure approximately equal to a width 240 of an optical release window 245. In some instances, the width 240 of the optical release window 245 may generally be described as the maximum separation distance between the micro-ring resonator 100 and the input and output waveguides 125 and 150 such that an optical coupling between the input and output waveguides 125 and 150 can be formed.

[055] While the gap 225 is generally described as measuring the same between the input waveguide 125 and the micro-ring resonator 100 and the output waveguide 150 and the micro-ring resonator 100, the present disclosure is not limited to such a configuration. For example, the gap 225 may measure differently between the input waveguide 125 and the micro-ring resonator 100 and the output waveguide 150 and the micro-ring resonator 100.

[056] As shown in FIGS. 9A-9B, the input waveguide 125 and output waveguide 150 each include a concave portion 250. The concave portion 250 may have an arcuate shape. In some instances, an arcuate shaped concave portion 250 may have a curvature that corresponds to the curvature of the ring 115 of the micro-ring resonator 100. As a result, the arcuate shaped concave portion 250 may be generally described as at least partially surrounding a portion of the micro-ring resonator 100. This may result in an increased coupling length and a reduction in the excitation coefficients of higher-order modes.

[057] As shown in FIGS. 10A-10B, the input waveguide 125 and output waveguide 150 each include a convex portion 255. The convex portion 255 may have an arcuate shape. In some instances, the arcuate shape of the convex portion may have an arc radius 260 that measures substantially equal to the ring radius 220 of the micro-ring resonator 100. By including the convex portion 255 it may be possible to minimize and/or reduce the amount of the input waveguide 125 and output waveguide 150 that is suspended relative to when the input waveguide 125 and output waveguide 150 include the concave portion 250 or are substantially planar as shown in FIGS. 11A-11B. [058] As shown in FIGS. 11A-11B, the input waveguide 125 and output waveguide 150 are each substantially planar. As shown, the input waveguide 125 and output waveguide 150 are substantially parallel to each other on opposing side of micro-ring resonator 100.

[059] While the discussion of FIGS. 9A-9B, 10A-10B, and 11A-11B, has generally described the input waveguide 125 and output waveguide 150 as having the same shape, the present disclosure is not limited to such a configuration. By way of non-limiting example, the input waveguide 125 may include the concave portion 250 and the output waveguide 150 may include the convex portion 255.

[060] FIG. 12 shows a flow chart for an example method 1200 for forming the suspended micro-ring resonator 100. As shown, the method 1200 may include a step 1202. The step 1202 may include etching a composite material comprising a first material and a second material, wherein the second material at least partially surrounds the first material. In some instances, the first material may be the core material 170 of FIG. 5A and the second material may be the second material 175 of FIG. 5A. The step 1202, in some instances, may be performed using an isotropic etching material or a partially anisotropic etching material. The method 1200 may include a step 1204. The step 1204 may include removing the second material until the first material is exposed to an environment and until a base is formed from the second material, the base supporting the first material in an elevated position. The method 1200 may also include a step 1206. The step 1206 may include forming a grating in the first material.

[061] Although the embodiments of the suspended micro-ring resonator 100 discussed herein have a generally circular shape, such a shape is not required. The teachings herein are equally applicable to, for example, ellipses and oval-shapes (e.g., the oval may have one or more flat surfaces). However, the benefit of using a ring shape is that the ring shape maximizes the free spectral range for a given amount of bent loss. Further, although the figures show the base 105 as having a flared shape and being located in the center of the suspended micro-ring resonator 100, the base 105 is not so limited. It should be appreciated that the base 105 can take any shape and can be located at any location sufficient to support the micro-ring resonator. Further, it should be appreciated that the above disclosure is not limited to suspended resonators and is applicable to suspending any bent structure. For example, suspension of bent structures is applicable in mid-infrared applications (e.g., for waveguide routing on chip) and for silicone waveguides where the grating provides mechanical stability for the bent waveguide without introducing losses.

[062] Although the above disclosure has focused on micro-ring resonators, it should be appreciated that the above teachings may extend to other applications. For example, in mid-infrared photonics, silicon dioxide, commonly used as waveguide over-cladding and under-cladding, absorbs light. For mid-infrared photonic devices, silicon dioxide needs to be removed as completely as possible from the vicinity of the waveguide core. When silicon dioxide is removed, the core needs mechanical support. Mechanical support of straight waveguides can be provided by sub-wavelength waveguides. It should be appreciated that the resonators can be supported by the gratings without compromising the quality factors. Therefore, for mid-infrared applications, the method disclosed herein can be used to produce high quality factor resonators for use in integrated photonic circuits. For example, the applications may include, but are not limited to, filters, switches, modulators, and sensors.

[063] While the disk 110 is shown as having a generally circular shape, such a configuration is not required. The disk 110 may have any shape (e.g., square-shaped, rectangle-shaped, ellipse-shaped, and/or any other suitable shape) that generally prevents the oxide positioned beneath the disk 110 from being completely removed. In other words, the disk 110 aids in the formation of the base 105. Similarly, the ring 115 is not limited to being a generally circular shape and may be any suitable shape (e.g., square- shaped, rectangle-shaped, ellipse-shaped, and/or any other suitable shape).

[064] While the principals of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.

Claims

CLAIMS What is claimed is:

1. A method for forming a suspended micro-ring resonator comprising:

etching a composite material comprising a first material and a second material, wherein the second material at least partially surrounds the first material;

removing the second material until the first material is exposed to an environment and until a base is formed from the second material, the base supporting the first material in an elevated position; and

forming a grating in the first material.

2. The method of claim 1, wherein the first material is silicon.

3. The method of claim 2, wherein the second material is silicon dioxide.

4. The method of claim 1, wherein the etching is isotropic.

5. The method of claim 1, wherein the etching is partially anisotropic.

6. The method of claim 1, wherein the composite material further comprises a third material.

7. An optical sensor comprising:

a suspended micro-ring resonator, the suspended micro-ring resonator being suspended by a base, wherein the suspend micro-ring resonator is formed using a single lithographic step;

an input waveguide; and

an output waveguide, the output waveguide and the input waveguide being positioned on opposing sides of the suspended micro-ring resonator.

8. The optical sensor of claim 7, wherein the suspended micro-ring resonator includes a disk, a ring, and a grating, the grating being positioned between the disk and the ring.

9. The optical sensor of claim 8, wherein the base is formed of silicon dioxide.

10. The optical sensor of claim 9, wherein the disk is formed of silicon.

11. The optical sensor of claim 8, wherein the grating comprises a plurality of extensions, each extension extending between the disk and the ring.

12. The optical sensor of claim 11, wherein each of the extensions have a constant width.

13. The optical sensor of claim 7, wherein the input waveguide and the output waveguide include a concave portion.

14. The optical sensor of claim 7, wherein the input waveguide and the output waveguide include a convex portion.

15. The optical sensor of claim 7, wherein the input waveguide and the output waveguide are substantially parallel.

16. A suspended micro-ring resonator comprising:

a base;

a disk coupled to the base; and

a ring coupled to the disk using a plurality of extensions, the plurality of extensions collectively forming a grating, wherein the suspended micro-ring resonator is formed using a single lithographic step.

17. The suspended micro-ring resonator of claim 16, wherein the disk is formed of silicon.

18. The suspended micro-ring resonator of claim 17, wherein the base is formed of silicon dioxide.

19. The suspended micro-ring resonator of claim 16, wherein each of the extensions has a constant width.