Selective porosification for volumetric optical devices

Abstract

A multilayer substrate may include a selectively porosified optical writing layer. A trench through to the optical writing layer may be used to selectively position a protective agent. The protective agent may inhibit porosification of the portions of the optical writing layer below the trench and proximate to the sidewalls of the trench within the optical writing layer. After application of the protective agent, one or more layers above the optical writing layer may be removed to expose portions of the optical writing layer that lack application of the protective agent. The optical writing layer may be selectively porosified by applying porosification where the regions exposed by removal of one or more layers above the optical writing layer are porosified while porosification is inhibited in the regions on the sidewalls and / or below the trench.

Description

SELECTIVE POROSIFICATION FOR VOLUMETRIC OPTICAL DEVICESStatement regarding federally sponsored research or development

[0001] This invention was made with government support under 1935289 awarded by the National Science Foundation. The government has certain rights in the invention.PRIORITY

[0002] The application claims the benefit of U.S. Provisional Patent Application No. 63 / 678,852 filed August 2, 2024, titled SELECTIVE POROSIFICATION FOR VOLUMETRIC OPTICAL DEVICES, which is incorporated by reference in its entirety herein.BACKGROUNDTechnical Field

[0003] The disclosure relates generally to volumetric optical devices.Brief Description of Related Technology

[0004] Rapid advances in communication technologies, driven by immense customer demand, have resulted in the widespread adoption of optical communication media. As one example, many millions of miles of optical fiber provide short and long haul optical communications throughout the world. Improved interconnects, optical processing, andintegration with semiconductor based electronics will continue to increase demand.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1 shows an example porous scaffold with selectively porosified regions.

[0006] Figure 2 shows an example selective personification method.

[0007] Figure 3 shows a second example selectively porosified device.

[0008] Figure 4 shows a third example selectively porosified device.

[0009] Figure 5 shows an example coupling scenario.DETAILED DESCRIPTION

[0010] In various contexts, volumetric photonic integrated devices may be integrated with other optical and / or photonic devices. Accordingly, methods and devices for improving integration, reducing insertion loss, and improving overall photonic device quality will improve performance and drive adoption of technologies.

[0011] The various techniques and architectures discussed below provide for devices for coupling, free-space to guided-mode transitions, device writing with reduced writing artifacts and errors, waveguide bends, anti-reflective coating layers, and / or other volumetric integrated photonic device configurations and fabrication methods.

[0012] In various systems optical interconnects, e.g., optical elements interfacing one or more optical mediums to one or more other media (including other optical media), utilize specific three-dimensional forms to couple light among the media. Further, for complex and / or multiplexed routing and / or filtering, complex three-dimensional forms may be used. In some cases, processing operations on optical signals may use complex and / or specifically dimensioned forms. In addition, lensing may, in some cases, use two or three dimensional forms of a complex nature (e.g., as opposed to curved lenses or flat uncomplicated forms such as Fresnel lenses) which may be designed through computer modelling or empirical study. Such complex two- or three-dimensional forms may be impractical or impossible to create or to align accurately using standard lens manufacturing techniques alone. Accordingly, techniques and architectures that allow the creation of monolithically integrated optical devices, electro-optical devices, photonic elements, interconnects, waveguides, prisms, and / or other optics of arbitrary dimension, function, and form, such as those discussed below involving writing one or more voxels into a scaffold and / or writing an optic into such a scaffold,offer improvements over existing market based solutions. For example, the optics may include Mach-Zehnder interferometer, other interferometers, and / or other optics. The optical writing layer may be silicon in some cases. Accordingly, integration with solar cells, photodiodes, modulators, and / or other electro-optics may be readily performed. In an example, electrodes may be added proximate to a written waveguide optic to form a modulator device.

[0013] In some systems, when writing a form to a medium, lower parts of a form provide support for the upper parts of the form because the writable medium (e.g., a polymer in liquid or aqueous form) provided no physical rigidity unless hardened through exposure. Accordingly, the conventional wisdom was that when constructing via laser writing an optic (e.g., into a polymer medium), the form of the optic should be selected such that the lower portions of the form would support the upper portions. In some cases, the inclusion of a scaffold provides rigidity within the writable medium allowing forms without support from lower portions of the structure, which allows one to proceed with structure selection that may be contrary to the conventional wisdom.

[0014] A scaffold may be saturated or immersed in writable media to define the writable volume. The scaffold, which may be porous, may host the writable media and may be transparent both in the presence of the writable media and in cases where the writable media is removed (e.g., after writing is complete or to allow for a second writable media to replace the first for additional writing stages). The writable media may include a writable medium (e.g., a medium for which the refractive index of the medium may change if the medium undergoes a material property change, such as a change that is optically induced or driven. A voxel may be written into the scaffold to create an optic. The scaffold may be disposed on a substrate (e.g., a silicon-on-insulator (SOI) compatible substrate, a lll-V compatible substrate, or other substrate) which may allow for integration with other optics, electrical devices, and / or other systems. The scaffold may be rigid (e.g., a structure capable of holding written regions in a position in three-dimensional space at least when undisturbed and / or when exposed to ambient conditions). However, in various implementations, the scaffold may have varying degrees of flexibility when exposed to particular deforming forces.

[0015] In an example technique, a voxel is generated within a scaffold. The voxel may include a region that has undergone a material property change, such that the optical properties of the region are altered for an operational period (e.g., permanently, for multiple hours / days / months / years, over a decay period, a dissolution period, or other duration over which the written voxel may be used as (at least of portion of) an optic). The voxel may be written by focusing incident light into the voxel to cause the material property change. The material property change may include a refractive index change, a density change, acompositional change, a mechanical property change, an electrical property change, an acoustical property change, a thermal property change, or other material property change, within a writable medium through optical absorption. Further, the material property change may cause the position of the voxel to be fixed within the scaffold. One or more voxels may be used to define an optic within the scaffold.

[0016] In other words, optics may be created such that the optics are written into an at least partially optically transparent porous scaffold permeated with a writable medium. The optics may be formed and arranged in a three-dimensional pattern. The optics may include regions having an optically-driven-state-changed writable medium.

[0017] The scaffold includes at least a partially rigid region. A writable volume may be included within or overlapping with the at least partially rigid region. The writable volume is defined via a writable medium that at least temporarily permeates the scaffold. The writable medium may include a substance for which an exposure to focused incident light may cause a material property change. In some cases, the optical absorption process may include a linear absorption process. However, in some cases, the optical absorption process includes a nonlinear absorption process. For example, the process may include a two-photon, three-photon, and / or multi-photon absorption process. In some cases, use of a non-linear absorption process may allow the site of a voxel to be localized to the area exposed to the focus of a beam. In some cases, higher-order nonlinear absorption processes result in more localization than lower order nonlinear absorption processes. The higher the order of the nonlinear absorption process, the more photons that must be simultaneously absorbed to excite the process. Accordingly, the likelihood of excitation of the process, will increase dramatically (e.g., nonlinearly) near (or at) the focus of a focused incident light source (such as a laser or light emitting diode (LED)). Accordingly, a voxel may include the focal volume (or some portion of the focal volume) of the focused incident light source.

[0018] In some cases, the writable volume (e.g., the writable medium therein) may include a monomer, polymer, a photoresist, or a combination thereof. As an illustrative example, pentaerythritol triacrylate may be used. As another illustrative example, writable medium may include a self-assembled-monolayer-forming monomer. In this and other monomer examples, the monomer may form a polymer after cross-linking the monomer as a result of the optical absorption.

[0019] As discussed above, the scaffold may include a porous material. In various implementations, the porous material may include porous silicon, porous silicate, porous silica, porous gallium nitride, porous gallium arsenide, porous indium phosphide, porous lithiumniobate, other porous lll-V materials, porous high-temperature high-silica glass, any porosified nanofabrication substrate porous metallic materials, porous semiconductor materials, and / or porous dielectric materials. The particular scaffold may be selected based on the desired characteristics of the scaffold for a particular application. For example, porous silicon may be selected based on a birefringence of the porous silicon. In an example, the birefringence of the silicon may aid in phase-matching in a multi-wave mixing process.

[0020] In various implementations, porous materials that have been porosified via various processes may be used. For example, materials that have been porosified by uniform or non- uniform chemical etching may be used as a scaffold. Materials that have been porosified by uniform or non-uniform physical etching may be used as a scaffold. Materials that have been porosified by uniform or non-uniform electrochemical etching may be used as a scaffold. Materials that have been porosified by lithographic etching may be used as a scaffold. Materials that have been porosified by spatially selective etching may be used as a scaffold. Materials that are porous because of deposition at a glancing angle may be used as a scaffold. Materials made porous by assembly or that are porous because of assembly (including selfassembly) may be used as a scaffold. Porous materials, in some cases with porosity exceeding 50% air by volume, and / or in some cases with above 80% air by volume, may be used as a scaffold. In some implementations, the porosity may be controlled, e.g., by controlling the electrochemical etch current density and / or the chemical composition of the electrolyte used in the etch. For example, the porosity may be selectively controlled to vary from 1% air by volume to 99% air by volume. Porous materials, in some cases, with pores below 1 micron in at least one dimension, or, in some cases, below 100 nanometers, may be used as a scaffold. In some implementations, the size of pores may be controlled, e.g., by controlling the electrochemical etch current density and / or the concentrations of the electrolyte used in the etch. For example, pores may be selectively controlled to vary in size from up to 10 nm to 50 nm or more. Further, the thickness of the pore walls may be selectively controlled to vary in size from up to 1 nm to 50 nm or more. The porous scaffold may be annealed to relieve strain by heating up and / or cooling down the scaffold. The porous scaffold may be exposed to plasma treatment to modify its surface or volumetric material properties or to perform cleaning.

[0021] Additionally or alternatively, the porosity, pore size, and thickness of the pore walls may be made to vary as a function of etch depth by varying the current density and / or electrolyte composition as a function of time. The variation of current density and / or electrolyte composition may be done continuously (e.g., with a linear or nonlinear ramp of the current density and / or by continuously flowing a new composition of electrolyte into the etchingchamber to partially or completely exchange with the existing electrolyte). Additionally or alternatively, the variation of current density and / or electrolyte composition may be performed with a fixed number of discrete steps. A look up table of the current density and electrolyte composition may be used to determine the etch rate as well as to design the necessary profiles of current density and electrolyte composition to achieve a desired set of profiles for porosity, pore size, and pore thickness. Porosity, pore size, and pore thickness affect the optical and other material properties of the scaffold (e.g., refractive index, scattering losses) as well as the optical and other material properties of the polymer that is written into the scaffold because the porosity affects the maximum amount of polymer that can infill the scaffold and thus for example the tuning range for refractive index, the pore size affects the degree to which the polymer is able to infill as well as the size of scattering discontinuities, and the pore thickness affects the smoothness of the written polymer. Therefore, the optical and other material properties of both the scaffold and of the written polymer may be controlled as a function of depth in the selectively porosified regions.

[0022] Further, a scaffold may be selected based on transparency constraints of a particular implementation. In various implementations, the transparency of the scaffold allows for exposure of the writable medium to focused incident light. In some cases, opaque scaffolds or scaffolds with limited transparency may prevent (or at least partially inhibit) exposure. Accordingly, a scaffold material may be constructed in various manners (such as any of those discussed above or other porosification processes). However, material selection to meet transparency constraints may be applied (at least in some cases) regardless of the porosification technique employed.

[0023] In some implementations, the level / intensity of the material property change may be selected. For example, the exposure may be lessened (reduced in intensity or duration) to reduce the overall change that occurs (e.g., reduce the change in refractive index either up or down). In another example, the exposure may be increased (increased in intensity or duration) to increase the overall change that occurs (e.g., increase the change in refractive index either up or down). Intensity may be increased or decreased by adjusting the input power of the incident light. Intensity may also be increased or decreased by varying the duty cycle for pulse width modulation. In some cases, intensity may be increased or decreased by adjusting the focal volume of the incident light. In some cases, the degree of the material change may be increased or decreased by varying the total exposure time.

[0024] The light source of the incident light may be a laser, a light emitting diode, a lamp, a flash lamp, an image projector, a fully or partially incoherent light source, and / or a fully orpartially coherent light source.

[0025] In various implementations, various, optics (such as ball lenses, Luneburg lenses, singlepass-written or multipass-written waveguides, end couplers, scaffold transition layers, complex Al-designed and / or algorithmically-designed lenses, stitched optics, and / or other optics) and / or optical writing techniques such a position correction lookup, multipass writing, adjusted power lookups, dither, ultraviolet 2-D writing, and / or other writing techniques may be combined with the selectively porosified devices and selective porosification architectures and techniques discussed herein. For example, the optics and optical writing techniques and architectures discussed in WIPO International Patent Application No. PCT / US2023 / 75602, filed September 23, 2023, entitled Volumetric Optical Devices, which is incorporated herein in its entirety, may be readily and optionally combined (e.g., individually or as a group) with the selectively porosified devices and selective porosification architectures and techniques discussed herein.

[0026] Selective porosification

[0027] In various implementations, porosification to generate a scaffold may provide a background for selective writing of optics and / or other nano-featured device. Thus, it may be desirable to generate a uniformly porosified area over which to write optics. Nevertheless, contrary to conventional wisdom, selective porosification of particular areas may allow for creation of particular optics and / or facilitate particular use cases. For example, where a porosified region may be created up to a sidewall of a trench, optics may be written proximate to the sidewall. In some cases, proximity to sidewalls may allow for external side coupling via conventional optical fiber or free space optical elements into the optics written in the porosified region. The external side coupling may be achieved via waveguides probing into the trench and / or after partitioning of the device along the trench for side-coupling access. In another example, selective porosification may allow two regions with different levels of porosity or other material properties to be located in close proximity to one another. In such an example, optics or waveguides may be written in proximity to the sidewalls to enable coupling between the two porous regions. In another example, the trench may be used to facilitate the separation of the selectively porosified regions via cleaving.

[0028] Accordingly, as recognized herein, selective porosification, including in regions defined through the creation of trenches, may be desirable.

[0029] Figure 1 shows an example selectively porosified device 100. The selectively porosified device 100 has one or more selectively porosified regions 112 of an optical writing layer 110. The optical writing layer 1 10 may be within a multilayer substrate 102. Them ultilayer substrate may have had one or more layers 103, 104 above the optical writing layer 1 10 that have been selectively removed, via a removal scheme. The one or more (previously removed) layers 103, 104 above the optical writing layer 110 may be used to control the locations of one or more trenches 106 extending into the optical writing layer.

[0030] The one or more trenches 106 may be used control the selective porosification boundaries within the optical writing layer 110. In other words, the one or more trenches 106 may be used to control the selection of the one or more selectively porosified regions 1 12. In the various implementations, the depth of the trench 106 is chosen to be greater than the thickness of the selectively porosified regions 1 12 in order to prevent lateral undercutting and porosification occurring underneath the bottom of the trench 106.

[0031] In an illustrative example, the one or more layers 103, 104 above the optical writing layer 1 10 may include a mask layer 103 and a process layer 104. The mask layer 103 may be applied (e.g., patterned) such that the locations of the trenches may be defined by the mask layer 103. The spaces corresponding to gaps in the mask layer 103 may be subjected to material removed during the trench formation process, such as an etching process (e.g., a Bosch etch, or other etch), a milling process, an ablation process, and / or other material removal process.

[0032] The process layer 104 may include a layer that is selectively removable from the optical writing layer 1 10 via, e.g., a removal scheme that affects the process layer 104 without removal of the optical writing layer 1 10. As an illustrative example, the process layer 104 may include an oxide layer and the optical writing layer 110 may include a silicon layer. For example, the process layer 104 may have a thickness in a range of 1.4 to 1.5 pm. In some implementations, the process layer 104 may have a thickness in a range of 0.5 to 5 pm. The example oxide layer may be removable via a chemical etch (e.g., with hydrofluoric acid (HF)) that may not necessarily affect the silicon layer. Thus, in this illustrative example and / or in other example systems), the process layer 104 may be removed (e.g., along with the mask layer 103 above it) without removal or modification of the optical writing layer 110 or its selectively porosified regions 112.

[0033] In various implementations, a protective agent 1 16 may be applied within the trench 106 at least within the optical writing layer. The protective agent 116 may be selected to protect covered regions from porosification without inhibiting the removal scheme.

[0034] For example, for a chemical etch (e.g., with hydrofluoric acid (HF)) the protective agent may be selected to have no effect. In the example, the chemical etch would remove e.g., an oxide layer without removing a silicon optical writing layer. However, duringporosification (e.g., via an HF electrochemical etch) after the removal scheme, the protective agent 116 may inhibit porosification for regions coated by the protective agent. An example protective agent may include a n-type dopant (such as a phosphorus based dopant), e.g., which may be spin coated. For p-type silicon, n-type doping may reduce the presence of holes in the n-doped area (e.g., which may be 100-5000 nanometers into the silicon layer). The presence of holes may facilitate porosification from the electrochemical etch. The reduction of such holes inhibits porosification and may prevent penetration of the etching agent into the silicon layer. Accordingly, the n-type dopant may act as a protective agent for silicon with respect to a HF electrochemical etch.

[0035] Because the removal of the process layer 104 occurs after coating with the protective agent 116, areas covered by the process layer 104 are later exposed without protective agent 1 16 after the removal. Thus, the areas covered by the process layer 104 may be selected for porosification. Areas of the optical writing layer 110 under the trench and / or proximate (e.g., within 100-5000 nanometers) to coated sidewalls of the trench may be protected from porosification.

[0036] In various implementations, the protective agent 118 is also applied to the top of another portion of the optical writing layer 110. Figure 3 shows a second example selectively personified device 300, with multiple sets of differently porosified regions 112, 320. In such implementations, optics or waveguides may be written in proximity to interface between the porosified regions 112 and the unmodified writing layer 110 to enable vertical coupling between the two regions. In such implementations, the protection layers 1 16 and 118 may be removed, e.g., via an etching process such as dry etching, via milling, ablation, and / or other material removal processes after the creation of the first set of porous regions 112, and the selective porosification process may be repeated under modified conditions using a protection agent 118 above the first set of porous regions 1 12 and trenches 106 to create a second set of porous regions 320 with different material properties (e.g., porosified to a different extent, with different size pores, different pore density, unporosified and / or otherwise unmodified, and / or other material property differences). In various implementations, the protective agent 1 18 may be selectively applied via selective removal of the layers 103, 104 above the optical writing layer 1 10. Thus, the layers 103, 104 above the optical writing layer may be removed in multiple phases allowing for the multiple different sets of porosified regions. In such implementations, optics 312, 322 or waveguides may be written in proximity to interface 324 between the first set of porosified regions 1 12 and the second set 320 of porosified regions to enable lateral coupling between the two regions. The sets of selectively porosified regions 1 12, 320 may include regions separated by trenches 106 and / or side-by-side regions, asshown. Although the second example selectively porosified device 300 is shown with two sets of selectively porosified regions 112, 320, in some implementations, three or more sets of selectively porosified regions may be used, e.g., each of the set having different material properties.

[0037] In some applications, it is desirable to separate the selectively porosified regions 1 12, 320 into individual die. The mechanical properties of the selectively porosified regions 112, 320 may make it difficult to cleave without generating unwanted debris. The trench 106 may be used to facilitate the separation of the selectively porosified regions into individual die because the unmodified writing layer underneath may be well suited for cleaving.

[0038] In some applications, the area of the porosified regions is large enough that built up strain causes unwanted wrinkling of the regions to occur. The trench 106 may be used to provide volumes for lateral expansion of the porosified regions and enable a relief of strain. The relief of strain may enable a larger total area to be porosified without wrinkling.

[0039] Figure 2 shows an example method 200 for selective porosification 200. Material may be selectively removed to form a trench 106 within multilayer substrate 102 (202). The trench 106 may extend through one or more layers of the multilayer substrate to an optical writing layer of the multilayer substrate 102. The trench formation process may include one or more of various material removal processes, such as an etching process, a milling process, an ablation process, and / or other material removal process. In various implementations, it may be desirable to select a trench formation process that may support high aspect ratio trenches, such as those with a high ratio of sidewall height (e.g., 10-500 microns) to trench width (1 -100 microns). In some implementations, sidewalls that are vertical may be desired. In some cases, power calibration and / or removal application timing may be used to control the depth of the trench created. Nevertheless, in some cases, small (0.25-2 degrees) deviations from vertical may be desirable to facilitate coating via the protective agent 1 16.

[0040] After forming the trench, a protective agent 116 may be applied to the bottom of the trench in at least the optical writing layer 110 (204). In various implementations, the protective agent 116 may be applied to the sidewall of the trench to avoid coating layers above the optical writing layer 110. However, in various implementations, the protective agent 1 16 may have little effect on the removal process for layers 103, 104 above the optical writing layer 110. Accordingly, the selective application of the protective agent 1 16 may be implemented through application of the protective agent 116 non-selectively after trench formation followed by the removal of the layers 103, 104 above the optical writing layer 110 resulting in only the areas exposed by trench formation have the protective agent 116 applied. The applied protectiveagent may be readied through additional processing in some implementations. For example, spin coated dopant may be annealed to facilitate diffusion of the dopant into the coated material. After annealing, excess spin coated dopant may be removed, e.g., so as not to contaminate later processing steps for the formation of selectively porosified device 100.

[0041] In various implementations, plasma-enhanced chemical vapor deposition (PECVD) may be used to deposit a silicon oxide (SiO2) layer (i.e., the process layer 104) with a thickness in a range of 1 .4 to 1 .5 pm, over a clean silicon wafer. The thickness of the process layer may prevent uneven pores from occurring in the silicon layer below the process layer 104.

[0042] Photolithography may be used to pattern a photoresist, such as SPR-220 4.5, (e.g., the mask layer 103) to select the regions for porosification. The mask layer 103 may be resistant to an etching process. The mask layer 103 may guide where the trenches are etched on the silicon wafer.

[0043] Reactive Ion Etching (RIE) and / or deep reactive ion etching (DRIE) may be used to etch the process layer 104 (i.e., SiOs layer), creating the trenches defined by the mask layer 103. The mask layer 103 may be removed after the etching is complete. Spinning may be used to apply a phosphorus spin-on dopant (i.e., the protective agent 1 16). The spin on-dopant may be spun onto the trenches at 3000 rpm for 40 seconds. A phosphorous doping furnace may heat the structure at 400 °C, and the temperature may be increased to 950 °C, over 50 minutes. As an illustration of the exposure process, the temperature may be maintained at 1050 °C for 2 hours and 10 minutes. The temperature may be lowered to 600 °C, over 60 minutes. In various implementations the heating time may vary range of 1 minute to 720 minutes and the temperature may vary over the range of 400 °C to 1050qC.

[0044] The stated thickness of the process layer 104 may prevent the spin-on dopant from diffusing vertically into the region of the silicon layer beneath the process layer 104 during the heating process in the phosphorus doping furnace. Dopants may still diffuse a short distance laterally through the exposed silicon along the sidewalls of the trench. The stated temperature and exposure time may create an adequately thick layer (protective agent 116) possessing an adequately high doping concentration that pores are inhibited both from forming vertically in the silicon layer beneath the trench and from forming laterally in the silicon layer adjacent to the trench sidewalls.

[0045] After the dopant is sufficiently diffused into the silicon layer to form the protective agent 116, the process layer 104 and the excess spin-on dopant outside the silicon layer may be removed using a buffered oxide etch (BOE). The selected temperature and exposure time may ensure that pores form only in the silicon layer that was previously covered by the processlayer 104.

[0046] After application of the protective agent 1 16, the layers 103, 104 above the optical writing layer may be removed (206). The removal scheme may include a material removal scheme incompatible with the optical writing layer 1 10 to prevent removal of the optical writing layer 110 along with the layers 103, 104 above the optical writing layer. For example, the removal scheme may be selected to remove a mask layer 103 and / or a process layer 104 without effect on the optical writing layer 110. In some cases, the removal scheme may be selected to be uninhibited by the protective agent 116, such that the protective agent 116 may be applied non-selectively but nevertheless only be present in the trench-exposed areas after the removal scheme is executed.

[0047] After removing the layers 103, 104 above the optical writing layer 1 10, the optical writing layer 1 10 may be porosified (208). The trench-exposed regions of the optical writing layer 1 10, may have protective agent 1 16. Thus, porosification may be inhibited within the trench-exposed regions, preventing porosification under the trench 106 and proximate to the sidewalls of the trench.

[0048] Figure 4 shows an example selectively porosified device 400 including grooves 402 for fiber 406 side coupling to the selectively porosified area 404. In an illustrative example, an anisotropic wet chemical etch is performed to create v-grooves for attachment of one or more single-core or multi-core optical fiber(s). For example, potassium hydroxide (KOH) may be used to etch a silicon substrate that possesses a crystal orientation of e.g., <100>. In various such implementations, the anisotropic wet etch to form the grooves is performed before the selective area porosification used to form the volumetric photonic integrated circuit. Additionally or alternatively, the anisotropic wet etch is performed after the selective area porosification. In various such implementation, the protective agent is applied to protect the v- groove. Optics may be written proximate to the sidewall and allow for external side coupling between the one or more core(s) of the one or more fiber(s). The volumetric photonic integrated circuit may consist of optics and three-dimensional waveguides that perform simple operations such as pitch conversion. Pitch conversion enables light signals on one or more single-core or multi-core input fiber(s) or a mixture of single-core and multi-core input fiber(s) to be directed to corresponding cores on a set of single-core and / or multi-core output fibers that have the same or a different spatial arrangement or a different number of output fibers. The fibers may be single-mode fibers or multi-mode fibers. Thus, the volumetric photonic integrated circuit may also enable spatial mode conversion. Similarly, the volumetric photonic integrated circuit may perform polarization conversion. Likewise, the volumetric photonic integrated circuit may perform multiplexing or de-multiplexing according to wavelength, spatialmode, or polarization.

[0049] Figure 5 shows an example coupling scenario 500. In the example coupling scenario 500, a fiber 502 with one or more cores is coupled to one or more optically-written waveguides in the selectively porosified area 404.

[0050] The present disclosure has been described with reference to specific examples that are intended to be illustrative only and not to be limiting of the disclosure. Changes, additions and / or deletions may be made to the examples without departing from the spirit and scope of the disclosure. Various implementations have been described various implementations are possible. Table 1 includes various examples.w r w l w i w p w v w w i i w tw w w v w a w w s

[0051] The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

What is Claimed is:

1. A method including: selectively removing material from a multilayer substrate to create a trench within the multilayer substrate, the trench extending through one or more layers of the multilayer substrate and having sidewalls within an optical writing layer of the substrate; applying a protective agent to the sidewalls of the trench in at least the optical writing layer, the protective agent selected to inhibit a lateral porosification of the optical writing layer adjacent to the sidewalls of the trench without inhibition of the removal of the one or more layers of the substrate above the optical writing layer; removing the one or more layers of the multilayer substrate above the optical writing layer via a removal scheme incompatible with the optical writing layer and uninhibited by any of the protective agent present on a sidewall of the trench above the optical writing layer; and porosifying the optical writing layer, where lateral porosification of the optical writing layer is inhibited via the protective agent.

2. The method of claim 1 , where porosifying the optical writing layer includes applying an electrochemical etch.

3. The method of claim 1 , where the protective agent includes an electrically- biasing dopant, such as an n-type dopant or a p-type dopant, where: a presence of the electrically-biasing dopant changes an effective bias level for an electrochemical removal scheme thereby inhibiting the electrochemical removal scheme adjacent to the sidewalls of the trench, and a diffusion process for the dopant reduces a roughness of the sidewall of the trench.

4. The method of claim 3, where reducing the roughness of the sidewall of the trench includes, wet etching, oxidation followed by selective removal of oxidizedvolumes, and / or a roughness-specific material removal method such as glancing angle etching.

5. The method of claim 3, where the multilayer substrate includes a process layer with a thickness in a range of 0.5 to 5 pm.

6. The method of claim 5, further comprising heating the multilayer substrate including the applied protective agent to a temperature in a range of 400 °C to 1050 °C for a time in a range of 1 minute to 720 minutes.

7. The method of claim 1 , where selectively removing material from the multilayer substrate to create the trench includes, dry etching, wet etching, laser ablation, patterning and removal, ion milling, a timed material removal, and / or a power-calibrated material removal.

8. The method of claim 1 , further including optically-writing one or more optics in the optical writing layer after porosification of the optical writing layer.

9. The method of claim 1 , further including applying the protective agent to the bottom of the trench within the optical writing layer to further inhibit porosification of the optical writing layer below the bottom of the trench.

10. The method of claim 1 , where applying the protective agent to the sidewall of the trench includes applying the protective agent to at least a portion of the sidewall having an off-vertical slope.1 1 . The method of claim 10, where the off-vertical slope includes a less than two degree deviation from vertical, a less than one degree deviation from vertical, and / or a less than one-half degree deviation from vertical.

12. The method of claim 1 , where selectively removing the protective agent after selective area porosification includes, dry etching, wet etching, laser ablation,-n -patterning and removal, ion milling, a timed material removal, and / or a power- calibrated material removal.

13. The method of claim 1 , further including partitioning the multilayer substrate along the trench to expose the sidewall for off-chip coupling.

14. The method of claim 1 , where the removal scheme includes a chemical etch, a milling scheme, or both.

15. A device including: an optical writing layer of a multilayer substrate, one or more layers of the multilayer substrate above the optical writing layer selectively removed via a removal scheme, at least a portion of the optical writing layer porosified; and a trench extending into the optical writing layer, a protective agent applied to a sidewall of the trench in at least the optical writing layer, the protective agent selected to inhibit lateral porosification of the optical writing layer adjacent to the sidewall of the trench without inhibition of the removal of the one or more layers of the substrate above the optical writing layer.

16. The device of claim 15, wherein the multilayer substrate includes a process layer with a thickness in a range of 0.5 to 5 pm.

17. A method including: selectively removing material from a multilayer substrate to create a trench within the multilayer substrate, the trench extending through one or more layers of the multilayer substrate and having a bottom and sidewalls within an optical writing layer of the substrate; for multiple iterations to generate multiple regions of the optical writing layer with different material properties: applying a protective agent to the bottom and sidewalls of the trench in at least the optical writing layer, the protective agent selected to inhibit a vertical porosification of the optical writing layer below the bottom of the trench and a lateral porosification of the optical writing layer adjacent to thesidewalls of the trench without inhibition of the removal of the one or more layers of the substrate above the optical writing layer; removing, above a current region of the optical layer for the iteration, one or more layers of the multilayer substrate above the optical writing layer via a removal scheme incompatible with the optical writing layer and uninhibited by any of the protective agent present on a sidewall of the trench above the optical writing layer; and porosifying the optical writing layer, where vertical porosification of the optical writing layer below the bottom of the trench and lateral porosification of the optical writing layer adjacent to the sidewalls of the trench are inhibited via the protective agent.

18. The method or device of any of the other claims, where the physical properties and / or the material properties of at least one of the multiple regions of the optical writing layer vary with depth.

19. The method or device of any of the other claims , where the physical properties and / or the material properties of at least one of the multiple regions of the optical writing layer are modified via oxidation during an exposure process.

20. The method or device of any of the other claims, further including partitioning the multilayer substrate along the trench to expose a sidewall of at least one of the multiple regions of the optical writing layer for side access to the at least one region.21 . The method or device of any of the other claims, where at least one of the multiple regions of the optical writing layer are transferred onto another substrate.

22. The method of claim 21 , where the transfer process includes an electropolish etch to create a suspended region of the optical writing layer that remains at least in part tethered to the multilayer substrate and / or to create a fully detached region of the optical writing layer.

3. The method of claim 21 , where the transfer process includes a wet transfer including in a solvent solution and / or a dry bonding transfer of at least one of the multiple regions of the optical writing layer.

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