Engineering doped thin film structures and devices

Abstract

The present disclosure is directed to a method for fabricating doped thin film structures and devices. The device includes a substrate and a thin film structure disposed on the substrate. The thin film structure includes a first region doped with a rare earth-based optically active ion and having a first polymorphic phase, the first polymorphic phase resulting in a first resonant frequency of the optically active ion modified by the first polymorphic phase, and a second region doped with a rare earth-based optically active ion and having a second polymorphic phase, the second polymorphic phase resulting in a second resonant frequency of the optically active ion modified by the second polymorphic phase, the first resonant frequency being shifted from the second resonant frequency.

Description

[Background technology]

[0001] (Related Applications) This application is based on and claims the benefit of priority to U.S. Provisional Application No. 63 / 281,194, filed November 19, 2021, which is incorporated by reference in its entirety herein.

[0002] Government Licensing Rights This invention was made with government support under DE-AC02-05CH11231 awarded by the Department of Energy, and N00014-18-1-2869 awarded by the Office of Naval Research. The Government has certain rights in this invention.

[0003] It is known that the same material host with different polymorphic forms affects the optical properties of doped ions. Such effects can be exploited to implement various types of optical devices, including, but not limited to, quantum information processing devices. Quantum information processing holds particular promise for advances in communications and computing. The effects of polymorphic forms on the optical properties of doped ions can be exploited, for example, in quantum memory devices, enabling quantum networks that can establish quantum entanglement-based links over long distances. Summary of the Invention [Means for solving the problem]

[0004] The present disclosure relates to methods for fabricating doped thin film structures and devices.

[0005] In one embodiment, the present disclosure describes an optical element. The optical element includes a substrate and a thin film structure disposed on the substrate. The thin film structure includes a first region doped with rare earth-based optically active ions and having a first polymorphic phase, the first polymorphic phase resulting in a first resonant frequency of the optically active ions modified by the first polymorphic phase, and a second region doped with rare earth-based optically active ions and having a second polymorphic phase, the second polymorphic phase resulting in a second resonant frequency of the optically active ions modified by the second polymorphic phase, the first resonant frequency being shifted from the second resonant frequency.

[0006] In another embodiment, the present disclosure describes a method for fabricating an optical element. The method includes providing a substrate and disposing a thin film structure on the substrate. The thin film structure includes a first region doped with a rare earth-based optically active ion and having a first polymorphic phase, the first polymorphic phase resulting in a first resonant frequency of the optically active ion modified by the first polymorphic phase, and a second region doped with a rare earth-based optically active ion and having a second polymorphic phase, the second polymorphic phase resulting in a second resonant frequency of the optically active ion modified by the second polymorphic phase, the first resonant frequency being shifted from the second resonant frequency.

[0007] In some other embodiments, the system may include the optical elements described above.

[0008] In some other embodiments, a computer readable medium comprises instructions that, when executed by a computer, cause the computer to perform the above-described methods. [Brief description of the drawings]

[0009] The systems, elements, products, and / or methods described below may be better understood with reference to the following drawings and description of non-limiting and non-exhaustive embodiments. Components in the drawings are not necessarily to scale. Emphasis is instead placed on illustrating the principles of the present disclosure.

[0010] [Figure 1] FIG. 1 is a schematic diagram of an exemplary optical element.

[0011] [Diagram 2] FIG. 2 is a flow diagram of an exemplary method for fabricating an optical element.

[0012] [Figure 3A] FIG. 3A is a schematic diagram of another exemplary optical element.

[0013] [Figure 3B] FIG. 3B is a schematic diagram for fabricating the optical element in FIG. 3A.

[0014] [Figure 3C] FIG. 3C is another schematic diagram for fabricating the optical element in FIG. 3A.

[0015] [Figure 4A] FIG. 4A is a schematic diagram of another exemplary optical element.

[0016] [Figure 4B] FIG. 4B is a schematic diagram of another exemplary optical element.

[0017] [Figure 4C] FIG. 4C is a schematic diagram for fabricating the optical element in FIG. 4B.

[0018] [Figure 5A] FIG. 5A is a schematic diagram of an exemplary optical resonator element.

[0019] [Figure 5B] FIG. 5B is a schematic diagram of another exemplary optical resonator element.

[0020] [Figure 6]FIG. 6 shows the structural characteristics of the various polymorphic forms of titanium oxide (polycrystalline on silicon, epitaxial single crystal on strontium titanate (STO), and r-sapphire (Al2O3)).

[0021] [Figure 7A] FIG. 7A shows the optical properties of optically active erbium ions doped in TiO 2 with various exemplary polymorphs depending on the growth temperature.

[0022] [Figure 7B] FIG. 7B shows the spectrum from erbium ions in the rutile phase obtained after annealing anatase polycrystalline TiO 2 on silicon at 850° C. for 30 minutes.

[0023] [Figure 8A] FIG. 8A shows a comparison of the inhomogeneity linewidth of the optical response from different polymorphs of TiO 2 on silicon.

[0024] [Figure 8B] FIG. 8B shows the inhomogeneous linewidth of the optical response for the rutile phase (on silicon) obtained as grown and via thermal annealing.

[0025] [Figure 8C] FIG. 8C shows the inhomogeneous linewidths for the different polymorphs obtained under different growth conditions.

[0026] [Figure 9] FIG. 9 demonstrates the role of the bottom buffer between the optically active thin film and the substrate in improving the inhomogeneous optical linewidth of the active thin film.

[0027] [Figure 10A] FIG. 10A shows the contribution to the inhomogeneous optical linewidth as a function of the concentration of optically active doped ions.

[0028] [Figure 10B]FIG. 10B shows an exemplary inhomogeneous optical linewidth as a function of temperature.

[0029] [Figure 11] FIG. 11 shows the spectral diffusion as a function of top and bottom buffer thickness and concentration of optically active doped ions.

[0030] [Figure 12A] FIG. 12A shows a schematic diagram of an exemplary embodiment of the present disclosure.

[0031] [Figure 12B] FIG. 12B shows a characteristic chart of the exemplary embodiment shown in FIG. 12A.

[0032] [Figure 12C] FIG. 12C illustrates a characteristic chart of the exemplary embodiment shown in FIG. 12A.

[0033] [Figure 12D] FIG. 12D shows a schematic diagram of another exemplary embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Detailed Description of the Disclosure The disclosed systems, elements, and methods will now be described in detail hereinafter with reference to the accompanying drawings, which form a part of this application and show, by way of illustration, examples of specific embodiments. However, the described systems and methods may be embodied in a variety of different forms, and therefore, the claimed subject matter covered by this disclosure is intended to be construed as not being limited to any of the embodiments. The present disclosure may be embodied as a method, element, component, or system. Thus, embodiments of the disclosed systems and methods may take the form of, for example, hardware, software, firmware, or any combination thereof.

[0035] Throughout the specification and claims, terms may have subtle meanings that are suggested or implied in a context other than the explicitly stated meaning. Similarly, the phrases "in one embodiment" or "in some embodiments" as used herein do not necessarily refer to the same embodiment, and the phrases "in another embodiment" or "in other embodiments" as used herein do not necessarily refer to different embodiments. For example, it is intended that the claimed subject matter may include, in whole or in part, a combination of the exemplary embodiments. Furthermore, the phrases "in one implementation," "in another implementation," or "in some implementations" as used herein do not necessarily refer to the same or different implementations. For example, it is intended that the claimed subject matter may include, in whole or in part, a combination of the disclosed features from the implementations.

[0036] Generally, terminology may be understood, at least in part, from usage in context. For example, terms such as "and," "or," or "and / or" as used herein may include various meanings that may depend, at least in part, on the context in which such terms are used. In addition, the terms "one or more" or "at least one" as used herein may be used to describe any feature, structure, or characteristic in a singular sense, or may be used to describe a combination of features, structures, or characteristics in a plural sense, at least in part, depending on the context. Similarly, terms such as "a," "an," or "the" may be understood to convey singular use or to convey plural use, again, at least in part, depending on the context. In addition, the terms "based on" or "determined by" may not necessarily be understood as intended to convey an exclusive set of factors, but instead may allow for the existence of additional factors not necessarily explicitly described, again, at least in part, depending on the context.

[0037] The present disclosure relates to methods for fabricating doped thin film structures and the fabricated devices that may interact with optical fields and may be configured to include various optical structures, including optical resonators, for various optical processing and applications and platforms, including, but not limited to, scalable quantum memory platforms and / or quantum information processing.

[0038] In the exemplary context of the present disclosure, quantum information processing may broadly include at least one of storing quantum information, transporting quantum information, computing quantum information, reading out quantum information, initializing quantum information, controlling quantum information, or manipulating quantum information. Such quantum information processing brings about a paradigm shift in communication and computing. Practical quantum information processing based on optical interactions may rely on quantum mechanical platforms that simultaneously possess long coherence times and narrow optical transitions while allowing chip-scale integration with photonic structures. Quantum memory elements are an important part of quantum information processing and quantum computing, especially for quantum networks that are capable of establishing quantum entanglement-based links over long distances. Quantum information can be stored at the level of small populations of atoms or single atoms / defects, for example, rare earth (RE) ion defects in solids. The present disclosure describes exemplary elements, containing structures, that provide these structural and physical properties suitable for practical quantum information processing. Although the present disclosure focuses on quantum information processing applications, the elements described below are not so limited. They may also be adapted for other optical processing and communications applications besides quantum information processing.

[0039] As an exemplary optically active material, rare earth (RE) ions such as erbium (Er) doped in solids are virtually shielded from their crystalline surroundings by a closed outer shell, and feature 4f-4f intrashell transitions that allow for long spin coherence times (e.g., up to 6 hours) and narrow optical transitions (e.g., less than 1 kHz). As a non-limiting example, such rare earth ions are incorporated into various exemplary elements described in this disclosure as active optical components. In various embodiments, rare earth ions may refer to ions of rare earth elements in the periodic table, which may refer to a set of 17 metallic elements.

[0040] In various embodiments disclosed below, solids doped with RE ions may serve as quantum elements / devices for processing quantum information, with RE ions serving as quantum bits (qubits). Er-doped titanium dioxide (TiO2) thin films may be selected as an exemplary platform due to at least one of the following practical considerations. One consideration may be that Er has its primary emission within the telecommunications C-band (1,530-1,565 nm) so that optical emission from Er qubits can be transmitted directly and with minimal loss over existing optical fibers, thereby enabling long-distance quantum entanglement distribution. Another consideration may be that Er ions have narrow 4f-4f transitions because the 4f shell is shielded by full 5s and 5p levels, providing protection from the blocking local environment and resulting in long coherence times.

[0041] Various embodiments disclosed below include the growth of erbium-doped TiO2 thin films. One general methodology for doping and / or growing Er-doped thin films is described in PCT Application No. PCT / US20 / 21257, filed March 5, 2020, and U.S. Patent Application No. 17 / 434,221, filed August 26, 2021, both of which are incorporated herein by reference in their entirety. The deposition of TiO2 thin films may use metal-organic precursors, for example, through at least one of the following methods: chemical vapor deposition (CVD), atomic layer deposition (ALD), and molecular beam epitaxy (MBE). Depending on the growth temperature, the growth of TiO2 thin films under various growth conditions may result in at least one of multiple polymorphic phases, for example, anatase phase or rutile phase. In some implementations, the thin films may include any of the following types: epitaxial films, polycrystalline films, or amorphous films. In some other implementations, a thin film may include more than one type, for example, a portion of the thin film may include one type and another portion of the thin film may include another, different type.

[0042] In various embodiments disclosed below, TiO2 may crystallize into two primary polymorphic phases (anatase and rutile phases) depending on the growth conditions. These polymorphic phases may be referred to as "phases" for brevity. Each polymorphic phase may correspond to a particular crystal structure of TiO2. The two phases of TiO2 may result in two different resonant frequencies of the doped Er ions, which are modified by the phase of TiO2. For example, Er ions doped in an anatase TiO2 thin film may have a resonant frequency corresponding to an optical wavelength of about 1,532 nanometers (nm), and Er ions doped in a rutile TiO2 thin film may have a resonant frequency corresponding to an optical wavelength of about 1,520 nanometers (nm). Here, "about x nm" refers to a range of (xa) nm to (x+a) nm, inclusive, where a may be any number within a few nanometers, for example.

[0043] Individual erbium atoms, either in small or even large groups, can function as qubits for a variety of applications, for example, for quantum information processing.

[0044] This disclosure describes various exemplary embodiments for achieving at least some of the multiple polymorphs in a single nanophotonic structure by a controlled annealing operation. As shown in more detail below, this controlled annealing operation, in addition to providing multiple polymorphs in a single element, may result in a "better" film in terms of optical homogeneity. Various embodiments may include creating at least two erbium-doped regions (both C-band compatible) with different optical resonance frequencies in a single nanophotonic structure. The different optical resonance frequencies are achieved without external control (such as by applying an external electric field to one of the two regions to modify the optical resonance via the Stark effect). Various embodiments may include controlling the spatial distribution of qubits with different optical resonances, e.g., by selective spatial control of the polymorphs, via controlled thermal annealing, e.g., by using spatially selective laser heating or localized electrical heating and / or by lithographically controlling the thermal annealing process.

[0045] With reference to FIG. 1 , the present disclosure describes various embodiments, including a device 100. The device 100 may be an optical device that interacts with an optical field to manipulate, control, process, and / or read information in the device. The device 100 may include some or all of the following components: a substrate 110 and a thin film structure 130 disposed on the substrate. In some implementations, the thin film 130 may include any one of the following types: an epitaxial film, a polycrystalline film, or an amorphous film. In some other implementations, the thin film 130 may include more than one type, e.g., a portion of the thin film may include one type and another portion of the thin film may include another different type.

[0046] The thin film structure 130 may include a first region 131 and a second region 132. The first region may be doped with a rare earth based optically active ion and may have a first polymorphic phase, which may result in a first resonant frequency of the optically active ion being modified by the first polymorphic phase. The second region may be doped with a rare earth based optically active ion and may have a second polymorphic phase, which may result in a second resonant frequency of the optically active ion being modified by the second polymorphic phase. In some implementations, the first resonant frequency may be shifted from the second resonant frequency. In some other implementations, the first region and the second region may have the same polymorphic phase. In Figure 1, a thin line box around first region 131 may indicate a first region that may be in the anatase phase, and / or a thick line box around second region 132 may indicate a second region that may be in the rutile phase. First region 131 and second region 132 may be spatially separated. Figure 1 shows, by way of example, that regions 131 and 132 are separated in an in-plane direction of element 100, and they may alternatively be spatially separated in the growth direction or in both the in-plane direction and the growth direction (not specifically illustrated in the general depiction of element 100, but shown in the specific implementations of Figures 4A and 4B below).

[0047] In various embodiments, Figure 2 shows a flow diagram of a method 200 for fabricating an optical element. The method 200 may include some or all of the following: providing a substrate 210 and / or disposing a thin film structure on the substrate 220. The thin film structure includes a first region doped with a rare earth based optically active ion and having a first polymorphic phase, the first polymorphic phase resulting in a first resonant frequency of the optically active ion modified by the first polymorphic phase, and a second region doped with a rare earth based optically active ion and having a second polymorphic phase, the second polymorphic phase resulting in a second resonant frequency of the optically active ion modified by the second polymorphic phase, the first resonant frequency being shifted from the second resonant frequency.

[0048] In some implementations, each region may serve as at least one site for storing information, e.g., storing quantum information as at least one quantum bit (qubit). In some other implementations, thin-film structure 130 may include other regions that serve as other qubits.

[0049] For one embodiment, the substrate 110 may include at least one of silicon, sapphire, silicon-on-insulator (SOI), SiN, or strontium titanate (STO), and / or the thin film structure 130 may include at least one TiO2 thin film, and the rare earth optically active ions may include Er ions. The first polymorphic phase may include an anatase phase, and the second polymorphic phase may include a rutile phase. The Er ions in the anatase TiO2 thin film may have a resonant frequency corresponding to an optical wavelength of about 1,532 nanometers (nm), and the Er ions in the rutile TiO2 thin film may have a resonant frequency corresponding to an optical wavelength of about 1,520 nanometers (nm). Here, "about x mm" refers to a range of (xa) nm to (x+a) nm, inclusive, where a may be any number within a few nanometers, for example.

[0050] In some implementations, the first region forms a first quantum bit and the second region forms a second quantum bit. The first quantum bit interacts with a first optical field having a first resonant frequency and the second quantum bit interacts with a second optical field having a second resonant frequency. The first optical field and the second optical field may be generated by two different light sources, or the first optical field and the second optical field may be generated by the same light source, which outputs, either simultaneously or sequentially, the first optical field having the first resonant frequency and the second optical field having the second resonant frequency.

[0051] The approximately 12 nm wavelength difference between the resonant frequencies, representing the shift between the optical transitions of erbium doped in rutile TiO2 and anatase TiO2, may allow for spectral selection by the optical field between the two qubits represented by Er ions doped in the anatase TiO2 and rutile TiO2 thin films. For example, in a situation where the two qubits are spatially close to each other, either in the plane or in the growth direction, the optical field may be tuned to either about 1,532 nm or 1,520 nm to selectively interact with one of the two qubits and be transparent to the other qubit. Here, "about x mm" refers to the range of (xa) nm to (x+a) nm, inclusive, where a may be any number within a few nanometers, for example.

[0052] In some embodiments, the TiO2 thin film layer may be deposited at a growth temperature to form the anatase phase. The growth temperature may be a relatively low temperature below 600 degrees Celsius (°C), such as 550 degrees Celsius or 480 degrees Celsius.

[0053] The thermal annealing process may cause the regions in the anatase phase to transition to the rutile phase. The thermal annealing may be localized thermal annealing such that only a small portion of the thin film anatase phase is converted to the rutile phase. The annealing temperature may be higher, above 600 degrees Celsius, and the thermal annealing may be performed for a predetermined duration. The thermal annealing process may be one or a combination of electrical heating and / or optical heating by light absorption using a gate electrode. The thermal annealing may be applied locally by focused light (e.g., from a laser) or a localized electric heater. For example, the gate electrode may include a conductive layer with a specifically designed spatial pattern deposited on the thin film structure, and when an electrical voltage is applied on the gate electrode, the gate electrode may locally generate heat and provide localized thermal annealing to a specific region. Localized control through laser heating may be used to generate regions of interest by focused laser beams or by controlled laser spatial interference patterns. Such controlled annealing creates rutile areas in otherwise anatase films, thereby bifurcating the film into rutile and anatase domains: one TiO2 region essentially becomes “transparent” to the optical field with wavelength resonance to the other region.

[0054] One advantage / benefit of various embodiments in the present disclosure may include leveraging existing techniques to achieve deterministic placement. In some implementations, the accuracy of deterministic placement and / or thermal annealing may be achieved to about 1 micrometer (μm) or less.

[0055] The spatial resolution of localized thermal annealing by laser heating may, for example, also be controlled by the focused laser intensity profile. In some implementations, the absorption of the heating laser beam may be nonlinear. Thus, the higher intensity parts of the focused beam are more absorbed, which may lead to improved localized thermal annealing spatial resolution.

[0056] In some implementations, the thermal annealing may include a 30 minute anneal at 750-850 degrees Celsius, inclusive. In some other implementations, at thermal annealing temperatures above 850 degrees Celsius, the predetermined duration of the thermal annealing may be less than 30 minutes, such as as short as minutes, seconds, and / or fractions of a second.

[0057] Thin-film structure 130 may include one or more thin-film layers, and the first region and the second region may be located in the same thin-film layer or in two different thin-film layers.

[0058] 3A illustrates an embodiment of an optical element 300. The optical element 300 may include a substrate 310 and a thin film structure 330. The thin film structure may include a thin film layer in which a first region 331 and a second region 332 may be disposed perpendicular to a growth direction 398. In other words, the first region 331 and the second region 332 are at different in-plane locations (or positions or portions) within the thin film layer. In some implementations, the first region 331 may be in the anatase phase and the second region 332 may be in the rutile phase.

[0059] Various embodiments include a method for fabricating an optical element 300 that includes two regions having two non-overlapping optical transitions with wavelength shifted within the same dopant-host system on a single nanophotonics element.

[0060] Figure 3B illustrates an example embodiment for fabricating the optical element 300 of Figure 3A. The method 200 of Figure 2 may be adapted to further include some or all of the following:

[0061] The first step involves depositing a thin film layer doped with rare earth optically active ions on a substrate at a growth temperature, which may be relatively low (e.g., 550° C.), such that the thin film layer 330 is formed in the anatase phase, including regions 332 a in the anatase phase.

[0062] The second step involves applying localized thermal annealing 340 (localized laser heating or electrical heating, as described above) to regions 332a in the thin film layer at an annealing temperature higher than the growth temperature to convert regions 332a of the thin film in the anatase phase to rutile regions 332b (a second polymorphic phase) and form second regions 332.

[0063] The third step involves forming a first region 331 having anatase phase (first polymorphic phase) within the remaining portion of the thin film layer. In some implementations, the portions other than the second region 332 may form the first region 331. In some other implementations, the first region 332 and the second region 331 may not occupy the entire thin film 330 in the in-plane direction, and the portions other than the first region 331 and the second region 332 may be removed, for example, via lithographic means (e.g., etching, dissolving, or lift-off).

[0064] Some embodiments of the present disclosure may not include all of the steps above in the exemplary embodiment as shown in Figure 3B. For one example, an embodiment may include the first two steps, where a pattern is formed and then a phase transformation is performed, resulting in an optical element.

[0065] 3C shows another embodiment for fabricating an optical element. The first region and the second region may be deposited at different times in the sequence within the same in-plane thin film layer. The method 200 of FIG. 2 may be adapted to further include some or all of the following for fabricating an optical element:

[0066] The first step involves depositing a first thin film doped with rare earth-based optically active ions onto the substrate in the lithographically defined regions (e.g., region 332d) at a growth temperature using a lithographic method (e.g., lithographically patterning a first patterned layer to expose the substrate in one or more regions). The lithographic method may use a photoresist or sacrificial layer patterned with openings during deposition of the first thin film in region 332d. The growth temperature may be relatively low, whereby the lithographically defined regions (regions 332d) are formed in the anatase phase.

[0067] The second step involves applying a thermal anneal 340 to the element at an annealing temperature, which is higher than the growth temperature, as indicated by 332e, to convert regions 332d of the thin film from the anatase phase to the rutile phase, forming second regions 332. However, the thermal anneal here may not need to be localized, since the patterned thin film layer is only present in second regions 332.

[0068] The third step includes lithographically defining a first region in the exposed substrate and depositing a second thin film with rare earth-based optically active ions in the first polymorphic phase at a growth temperature over the substrate in the first region 331. The growth temperature may be relatively low, whereby the first region 331 is formed in the anatase phase. The lithographic definition and growth of the first region 331 with the first polymorphic phase may include depositing a photoresist or sacrificial layer over the structure obtained in the second step above, spatially patterning and etching away the photoresist or sacrificial layer over the first region 331, exposing the substrate under the first region 331, and then lifting off the photoresist layer upwards, covering the regions other than the first region, to form the structure of the third step of FIG. 3C.

[0069] Alternatively, in the exemplary flow of FIG. 3C, a first thin film may be deposited prior to lithographically patterning region 332d at the growth temperature to form an anatase thin film. The first thin film may then be annealed and converted to a rutile thin film. The annealed first thin film may then be lithographically processed, for example, by etching the remaining areas other than region 332, to form region 332 with the rutile phase.

[0070] Some embodiments of the present disclosure may not include all of the steps above in the exemplary embodiment as shown in Figure 3C. For one example, an embodiment may include the first two steps in which a pattern is formed, for example by etching, and then a phase transformation is performed, resulting in an optical element.

[0071] 4A illustrates an embodiment of an exemplary optical element 400. The optical element 400 may include a substrate 410 and a thin film structure 430. The thin film structure may include more than one thin film layer, namely, a first thin film layer 440 and a second thin film layer 450. The first region 431 and the second region 432 may be disposed in different thin film layers along a growth direction 498. In some implementations, the first region 431 may be in the second thin film layer in the anatase phase and the second region 432 may be in the first thin film layer in the rutile phase.

[0072] With reference to FIG. 4A, in some embodiments, the first region and the second region in the optical element 400 may be in different in-plane portions of the two thin film layers.

[0073] With reference to FIG. 4B, in some other embodiments, the first region and the second region in the optical element 401 may be in the same in-plane portion of two thin film layers.

[0074] Various embodiments may be used to fabricate optical element 400 or 401, thereby creating two non-overlapping transitions with wavelength shifted within the same dopant-host system on a single nanophotonics element.

[0075] 4C shows various embodiments for fabricating an optical element. The method 200 of FIG. 2 may be adapted to further include some or all of the following steps of FIG. 4C to fabricate an optical element 401.

[0076] The first step involves depositing a first thin film 440 doped with rare earth based optically active ions on a substrate 410 at a growth temperature. The growth temperature may be relatively low, such that the first thin film layer 440 includes regions 432a formed in the anatase phase.

[0077] The second step involves thermally annealing the second region 450 at an annealing temperature higher than the growth temperature to transform the region 432a from the anatase phase to the rutile phase region 432b and form the second region 432. The thermal annealing may be performed locally.

[0078] The third step includes depositing a second thin film 450 with rare earth based optically active ions over the first thin film at a growth temperature to form first regions 431 in the second thin film. The growth temperature may be relatively low, whereby the second thin film layer 450, including the first regions 431, is formed in the anatase phase.

[0079] The underlying principles of device 401 of Figure 4B above may be extended to more than two thin film layers in the growth direction. Furthermore, in each of the thin film layers, first region 431 and / or second region 432 may extend across the entire thin film in an in-plane direction. Definition of the in-plane locations of first and second regions 431 and 432, if required, may be achieved through various lithographic processes, as described above.

[0080] Without limitation, the first and second regions in the various exemplary elements above may be the same rare earth ions doped in different rare earth oxide thin films. The rare earth oxide may include titanium dioxide (TiO2), strontium titanate (SrTiO3), or yttrium oxide (Y2O3). Alternatively, the first and second regions in the element may be different rare earth ions doped in the same rare earth oxide thin film, or the first and second regions in the element may be different rare earth ions doped in different rare earth oxide thin films.

[0081] Without limitation, in various embodiments, the first region 431 in Figures 4A and 4B may be any of the anatase and rutile phases, and the second region 432 may be the other phase. The second region 332a in Figure 3B, 332c in Figure 3C, or 432a in Figure 4C may be a different polymorphic phase and converts to the rutile phase upon thermal annealing.

[0082] The present disclosure further describes various embodiments of applications that incorporate the above elements in any one or combination of the above embodiments.

[0083] One embodiment may include using sequential deposition and annealing methods to significantly increase qubit density, e.g., double the qubit density, using lateral and / or vertical configurations. Another embodiment may include achieving deterministic qubit placement by polymorphic phase transformation on selected areas.

[0084] Referring to FIG. 5A, another embodiment may include a ring resonator-based device 500. The device may be implemented as a nanophotonic memory device or other optical processing device (such as an optical filter). FIG. 5A shows an in-plane view of the thin film structure. A first region 511 in the anatase phase may form a first optical ring resonator and a second region 512 may form a second optical ring resonator in the thin film structure. In some implementations, the first optical ring resonator 511 and the second optical ring resonator 512 may be coupled to the same optical waveguide 510 in the thin film structure.

[0085] Referring to FIG. 5B, another embodiment may include a waveguide and / or photonic crystal (PhC) based element 520. The element may be implemented as a nanophotonic memory element. The element 520 may include a mirror region 521 and a region of interest 522 in the PhC. The region of interest 522 in the PhC may include a set of first regions 531 and a set of second regions 532. The element 520 may include another mirror region (not shown in FIG. 5B), which in combination with the mirror region 521 may surround the region of interest 522 and form, for example, a Fabre-Perot cavity. The first region 531 and the second region 532 in the region of interest 522 may be arranged in any in-plane spatial manner.

[0086] This disclosure describes various embodiments in which thermal annealing is used to transition specific regions from a first polymorphic phase (anatase phase) to a second polymorphic phase (rutile phase). There may be several advantages / benefits of thermal annealing compared to the rutile phase, which is directly processed by depositing thin films at higher growth temperatures.

[0087] The present disclosure describes various embodiments that control the properties of one or more qubits by sandwiching a doped layer between undoped layers, for example, but not limited to, an erbium doped layer is deposited on a first undoped layer and capped with a second undoped layer. The first undoped layer may be a bottom buffer layer, and the second undoped layer may be a top buffer layer. By varying the thickness of the bottom buffer layer (i.e., the first undoped layer) and / or the top buffer layer (i.e., the second undoped layer), the properties of one or more qubits in the erbium doped layer may be controlled. For example, as discussed in more detail in the following section, by increasing the thickness of the bottom layer, the optical properties (e.g., inhomogeneous optical linewidth) of one or more qubits in the erbium doped layer become narrower (i.e., smaller).

[0088] As one advantage / benefit, obtaining the rutile phase by annealing from the anatase phase can help achieve better quality for the final rutile phase compared to directly deposited rutile phase, as described in more detail below.

[0089] Another advantage / benefit may include greatly improved homogeneity of physical properties for Er ions in the converted rutile phase compared to directly deposited rutile phase. For example, the heterogeneity linewidth of a directly grown rutile thin film may be in the range of 76±3 GHz (inclusive), while the heterogeneity linewidth of a rutile thin film with the same thickness obtained after thermal annealing an anatase thin film may be in the range of 48±2 GHz (inclusive). The optical heterogeneity linewidth is a metric for the heterogeneity in the local environment of the dopant ions. Each Er ion will have the same transition frequency if isolated, i.e., small differences in the local environment will shift this and the overall distribution is measured by this heterogeneity linewidth. It would be advantageous to reduce the heterogeneity linewidth.

[0090] Other advantages / benefits may include allowing the use of a photolithography mask for direct deposition, lowering the thermal budget with only a short duration with thermal annealing at high temperature compared to growth at high temperature over a long duration, and / or simplifying the process cycle with only one type of thin film growth (low temperature growth).

[0091] Below, the present disclosure further describes detailed device fabrication methods and details relating to the measurement of structural and optical properties of the fabricated devices.

[0092] Quantum memory elements are a key part of the proposed network, capable of establishing quantum entanglement-based links over long distances. Information is stored at the level of small populations of atoms or single atoms / defects, such as NV centers in diamond and rare-earth defects in YSO, among others. Solid-state hosts for these qubits have emerged as promising qubit platforms, with the main advantage coming from the inherent scalability of these systems. The system may be any one or combination of more than one embodiment as discussed above. The potential for using semiconductor industry compatible thin film hosts can take advantage of established processes. While one important driving force in the field has been the important demonstration of interaction with single atoms using nanophotonic structures, these demonstrations are proof of concept, providing strong support for the potential of thin film platforms.

[0093] The characterization of Er-doped TiO2 thin films can be described in the following sections. The choice of Er is motivated by the fact that Er has its first emission within the telecommunications C band (1,530-1,565 nm), which means that light from Er qubits can be transmitted directly through existing optical fibers with minimal losses, thus enabling long-distance interdigitated distribution. In addition, Er as a rare earth ion (REI) has a narrow 4f-4f transition, since the 4f shell is screened by complete 5s and 5p levels, which provides protection from the local environment.

[0094] Erbium in TiO2 in nanoparticle form is being explored as a solar cell material. Thin film growth of TiO2 may be performed using metal-organic precursors on r-plane sapphire. This method has advantages over direct evaporation because the TiO2 pressure can be controlled precisely and at much lower temperatures.

[0095] Processing method and thin film properties Exemplary TiO2 film growth may be performed using a Riber oxide MBE system. Tetraisopropyl orthotitanate (TTIP) was used as the source of titanium. The precursor was purchased from Sigma-Aldrich and had a purity of 99.999% (trace metals basis). The amount of TTIP introduced into the chamber was precisely controlled via the precursor water vapor pressure via a computer-controlled needle valve. Doping in the film was performed using a metallic erbium source in a high-temperature Knudsen cell. Erbium in its purified metallic form was purchased from Ames Lab in Iowa and had a purity of 5N (rare earth basis). Films were grown over a range of substrate temperatures (480 degrees Celsius (°C) to 850°C) as measured by pyrometer. For growth, the temperature was set at 750°C for rutile films and 480°C for anatase films. The oxygen flow was kept at 0.55 sccm to obtain a chamber background pressure of 5E-6 Torr. The substrates used were r-plane sapphire (LATTICE), STO(100), and Si(100), the former two using a 350 nm thick Ta layer on the back side for improved heat transfer between the heater and the substrate. For STO and sapphire, the substrates were cleaned using a 20 min O2 plasma on the substrate, and for silicon, an RCA clean was performed followed by a HF dip (to remove the native oxide) and the substrate cleanliness was signaled by a 2x1 reconstruction. The dopant density was varied by controlling the source temperature (800°C to 1,200°C). The grown films were cooled (from the growth temperature to 200°C) using a 0.55 sccm oxygen flow.

[0096] Growth was monitored in situ using RHEED and in situ outer membrane characterization was performed using X-ray diffraction (XRD) (Bruker D8 discover). Surface characterization was performed using a Bruker Dimension Icon atomic force microscope (AFM).

[0097] Broadband off-resonance spectroscopy was performed using a 905 nm excitation laser (QPhotonics QFLD-905-200S). Erbium photoluminescence was collected using a fiber-coupled spectrometer and liquid nitrogen-cooled InGaAs camera (Teledyne Princeton Instruments IsoPlane-320 and PyLoN-IR) that achieves a spectral resolution of about 0.3 nm using a 300 g / mm grating blazed for 1.2 μm light.

[0098] The optical characterization setup is a home-built confocal microscope using a 50x objective focused on the thin film. Light is collected / emitted from / into free space from a single collimator into a spectrum-maintaining fiber. The fiber is connected to a bidirectional 2x1 acousto-optic modulator switch (Brimrose), which switches between the collection and excitation paths. During excitation, the switch sends light from a tunable C-band laser (PurePhotonics) into free space to the sample, while during collection, the switch sends photoluminescence into a superconducting nanowire single-photon detector (SNSPD). Two additional acousto-optic modulators (AA-Optoelectronic) on the excitation path help to completely attenuate any crosstalk from the excitation, below the SNSPD dark noise (<100 counts / sec). Spectra are acquired by sweeping the wavelength of the tunable laser and exciting for durations between 0.1 and 1 millisecond (ms). Photoluminescence is collected after a 100 μs delay (for transients) for durations ranging from 1 to 15 ms. Homogeneous linewidth measurements are realized using an intensity electro-optic modulator (10 GHz, Thorlabs), biased at the quadratic point to generate the probe sideband. The sideband frequency is set and swept by a driving external microwave signal generator.

[0099] Structural properties of the membrane TiO2 crystallizes into two major phases: anatase (I41 / amd, a=3.7845 Å, c=9.5143 Å, Z=4) and rutile (P42 / mnm a=4.5937 Å, c=2.9587 Å, Z=2). Of these, rutile has been epitaxially grown on r-plane sapphire(012) / / TiO2(101), [ka] Along with 3.7% and [ka] Along the line there is an unconformity of 6.04%.

[0100] Referring to the lower half of Figure 6, transmission electron microscopy (TEM) images (rows 1 and 2) and selective area diffraction (SAD) data (row 3) show that the growth on r-sapphire has an epitaxial relationship. However, on silicon, the growth is polycrystalline, as seen in the TEM images, but textured along the preferred growth direction, as corroborated by SAD (row 3).

[0101] Similarly, with respect to anatase, STO(100) / / TiO2(001) has a mismatch of 3.1%, but due to this epitaxial relationship, the growth remains epitaxial. However, on silicon(100), due to the absence of an epitaxial relationship, it was found that the phase has a strong dependence on temperature and the growth surface was thermodynamically driven, i.e., preferred growth directions were identified for both rutile and anatase. In these growths, no interfacial layer was observed for TiO2 growth on r-sapphire and STO, but a distinctly different amount of oxide was present at the interface with the silicon substrate, which was thicker for rutile grown at higher temperatures. This oxide interlayer is due to oxidation of Si during growth, and the higher growth temperature leads to a thicker layer of oxide (2 vs. 4 nm).

[0102] Optical characterization Erbium in rutile and anatase has two distinct emission peaks, as seen in the photoluminescence (PL) signature. Based on this, the temperature range over which anatase growth occurs can be identified. Distinct phase / temperature dependencies were observed over the range explored.

[0103] As shown in Figure 7A, the films are predominantly rutile at 750°C and above, and predominantly anatase at 550°C and below, with intermediate temperatures giving a distribution between the two phases that is expected to be temperature dependent but not explored here. In Figure 7A, the Er-doped TiO2 (rutile) has a peak at about 1,520 nm, while the anatase phase has a peak at about 1,532 nm. The rutile phase decreases as the growth temperature is decreased (red: 750C, blue: 610C, black: 550C, cyan: 480C).

[0104] FIG. 7B shows that the low temperature peak disappears for a sample grown at low temperature but annealed at 850° C. (5×10E-5 torr oxygen pressure, 850° C.).

[0105] Comparing the rutile and anatase phases for single crystal growth, it is found that the optical inhomogeneity linewidth is significantly larger (about six times) for the rutile phase. The larger lattice mismatch between r-sapphire and rutile may be at least partially responsible for this effect. However, if this were the only reason, it would be expected that the linewidths would be closer for polycrystalline thin films on Si(100). Instead, it is even worse, with Γ inh is about 8 times greater for rutile compared to anatase, as shown in Figures 8A-8C. The reason is not entirely clear, and it is expected that the location of erbium within the crystal structure may play a role. For this reason, polycrystalline anatase on silicon may be taken up for the study of interface, concentration, and sample temperature effects.

[0106] FIG. 8B shows the homogeneous linewidth regime, fitting a Lorentzian function to the optical photoluminescence data.

[0107] In Fig. 8C, Γ of single crystal rutile inh is about six times wider than for single crystal anatase. For the polycrystalline samples, this results in rutile being about eight times wider than anatase.

[0108] Membrane property engineering The interface has a non-uniform linewidth (Γ inh ) can have a significant role in determining the qubit density. Taking advantage of this, various embodiments may include controlling the properties of one or more qubits by varying the thickness of one or more undoped layers, by sandwiching an erbium doped layer between undoped layers. For example, but not by way of limitation, the properties of one or more qubits may include optical properties in terms of inhomogeneous linewidth.

[0109] A strong dependence was observed when comparing a “bare” layer (10 nm Er:TiO2 film) with a 10 nm bottom buffer and a 5 nm top buffer. Increasing the bottom buffer thickness significantly reduces Γ, as shown in FIG. inh The best linewidth of the series at 4 Kelvin (K) was found to be about 5.2 GHz for a film stack with a 60 nm bottom buffer. In FIG. 9, a series of experiments can demonstrate the role of the bottom buffer in improving the inhomogeneous optical linewidth. The best linewidth was obtained for a 60 nm bottom buffer (inset).

[0110] It was also investigated whether the linewidth was concentration or temperature limited. It was found that within the parameter space, concentration does not appear to be a limiting factor. However, some room for improvement remains for lower sample temperatures.

[0111] 10A and 10B show the concentration and temperature-derived Γ inhIt was found that the concentrations are low enough not to cause major broadening, however, there is still room for improvement by going to lower temperatures.

[0112] T2* is another important parameter that has been used to characterize potential qubits. Traditionally, this is evaluated via interfacing with a pulse of light. The indirect method described above was employed to obtain an upper bound on T2*. This was used to evaluate the role played by the top and bottom buffers and the dopant concentration. Γ inh In comparison, T2* is more sensitive to buffer and concentration, as shown in FIG.

[0113] Carefully designed experiments further demonstrate that the properties of erbium-doped thin films can be engineered. In addition, the Si compatibility and processability of the films enables device fabrication directly on this platform.

[0114] The better performance of the polycrystals cannot be entirely attributed to fully relaxed nanocrystals. However, there may also be other mechanisms at play that are not entirely clear at this time. As the ability to study homogeneous linewidths undergoes further development, this in turn may also allow the nature of these interactions to be studied in greater depth.

[0115] Embodiments using localized laser heating on the submicron scale In some implementations, T i O2 exists in two stable phases, anatase and rutile, and localized heating may be used to affect a change between the two phases. Using a focused laser beam, a phase change may be defined over an area equal to or smaller than 1 micrometer (micron) in length and / or width, e.g., 0.5 microns, which is considered submicron.

[0116] In some implementations, Er-doped T iLocalized phase changes of O2 may be used to affect the local environment at the micron or submicron size level, thus enabling Er (or other REI) qubit placement in a quasi-deterministic manner in microscale regions. For one example, Figures 12A-12D demonstrate that a focused laser can be used to obtain phase changes over submicron regions by localized laser heating.

[0117] Referring to FIG. 12A, panel 1210 shows a schematic of a cross-section of a thin film / wafer, where the substrate is silicon and the top layer is an Er-doped TiO2 thin film. The Er-doped TiO2 thin film is in the anatase phase. Panel 1220 shows a schematic of the region (i.e., the "experimental region") where localized laser heating / annealing occurs. A chrome marker is used to guide data collection using X-ray fluorescence. After localized laser heating / annealing is performed in the area indicated by the rectangle, subsequent characterization of the region is performed.

[0118] Referring to FIG. 12B, panel 1230 shows an optical image of a patterned area on a TiO2 sample (anatase), where a 19×19 μm area is defined using chrome fiducials to identify the area. A 500 milliwatt (mW) blue laser with about 1 micrometer (μm) spot size is used to heat small areas separated by about 5 μm with dwell times of about 1, 10, 100, and 500 seconds, respectively. Panel 1240 shows Er emission at about 1,533 nanometers (nm) for anatase and about 1,521 nm for rutile. Photoluminescence (PL) emission shows evidence of phase change through differences in emission wavelengths, corresponding to the area within the rectangle (1242). PL is collected using a bandpass filter centered on the rutile peak.

[0119] In the present disclosure, "about A" may refer to a range around A as A±10%*A (inclusive of the end values), where A is a number.

[0120] Referring to Figure 12C, panels 1250 and 1260 show further evidence by scanning the region of interest (rectangle 1442 in Figure 12B) using a focused X-ray beam (40 nm) at an advanced synchrotron radiation facility. The size of the particles is measured to be below 100 nm, and their location matches the area modified by the blue laser.

[0121] Referring to FIG. 12D, panels 1270 and 1280 show that various embodiments in the present disclosure can be used to locally define a region of interest, for example, at an antinode (indicated by region 1282) of a nanophotonics device. For background reference, a typical photonic waveguide has a width of about 500 nm. Panel 1270 shows a schematic of a device fabricated using Er:TiO2 on silicon-on-insulator. Panel 1280 shows a schematic of the phase on another fabricated device using a localized laser heating method as described in the present disclosure.

[0122] Although a particular disclosure has been described with reference to illustrative embodiments, this description is not meant to be limiting. Various modifications of the illustrative and additional embodiments of the disclosure will become apparent to those skilled in the art from this description. Those skilled in the art will readily recognize that these and various other modifications may be made to the illustrative embodiments shown and described herein without departing from the spirit and scope of the disclosure. It is therefore envisioned that the appended claims will cover any such modifications and alternative embodiments. Certain proportions of the illustrations may be exaggerated, while other proportions may be minimized. Thus, the disclosure and figures should be considered illustrative, not restrictive.

Claims

1. An optical element, A substrate; a thin film structure disposed on the substrate, the thin film structure comprising: a first region doped with a rare earth based optically active ion and having a first polymorphic phase, the first polymorphic phase resulting in a first resonant frequency of the optically active ion being modified by the first polymorphic phase; a second region doped with an optically active ion of the rare earth system and having a second polymorphic phase, the second polymorphic phase causing a second resonant frequency of the optically active ion to be modified by the second polymorphic phase, the first resonant frequency being shifted from the second resonant frequency; A thin film structure comprising: An optical element comprising:

2. The first region and the second region are Along the growth direction, in the same thin film layer, in different in-plane portions of the thin film layer; The optical element according to claim 1 .

3. The first region and the second region are depositing the thin film layer doped with the rare earth-based optically active ions onto the substrate at a growth temperature, thereby forming the thin film layer in the first polymorphic phase; forming the second region in the thin film layer by localized thermal annealing at an annealing temperature higher than the growth temperature and converting the second region of the thin film layer from the first polymorphic phase to the second polymorphic phase; forming the first region with a remainder of the thin film layer; The optical element of claim 2 formed by

4. The optical element of claim 3 , wherein the localized thermal annealing is performed using localized laser annealing.

5. The optical element of claim 4 , wherein the second region has a dimension equal to or less than 1 micrometer (micron).

6. The optical element of claim 4 , wherein the localized laser annealing is performed using a blue laser with a spot size of about 1 micron with a power of about 500 milliwatts (mW).

7. The optical element of claim 2 , wherein the first region and the second region are deposited at different times in a sequence.

8. The first region and the second region are lithographically patterning a first pattern to expose the substrate in the second region; depositing a first thin film doped with optically active ions of the rare earth system in the first polymorphic phase at a growth temperature on the second region of the substrate; thermally annealing the second region at an annealing temperature, higher than the growth temperature, to transition the second region from the first polymorph to the second polymorph; lithographically defining the first region in the exposed substrate after the thermal annealing, and depositing a second thin film with the rare earth-based optically active ions in the first polymorphic phase and at the growth temperature over the substrate in the first region. The optical element of claim 7 formed by

9. The optical element of claim 1 , wherein the first region and the second region are in different layers of the thin film structure.

10. The first region and the second region are depositing, in the first polymorphic phase and at a growth temperature, a first thin film doped with optically active ions of the rare earth system onto the substrate; thermally annealing the second region at an annealing temperature, higher than the growth temperature, to transition the second region from the first polymorph to the second polymorph; depositing a second film with said rare earth-based optically active ions over said first film after thermal annealing in said first polymorphic phase at said growth temperature to form said first region in said second film; 10. The optical element of claim 9 formed by

11. The optical element of claim 1 , wherein the first region forms a first optical ring resonator within the thin film structure and the second region forms a second optical ring resonator.

12. The optical element of claim 11 , wherein the first optical ring resonator and the second optical ring resonator are coupled to the same optical waveguide within the thin film structure.

13. 2. The optical element of claim 1, wherein the first region and the second region are formed within the same optical Fabry-Perot cavity that is additionally formed within the thin film structure.

14. The thermal annealing process includes the following steps: Electrical heating using a gate electrode, or Optical heating due to light absorption 11. The optical element according to claim 3, 4, 8, or 10, comprising at least one of:

15. 11. The optical element of claim 3, 4, 8, or 10, wherein the annealing temperature is above 600 degrees Celsius and the thermal annealing is performed for a predetermined duration.

16. the first region and the second region comprise titanium dioxide; the rare earth ions comprise erbium ions; The optical element according to any one of claims 1 to 13.

17. The substrate is silicon, Sapphire, or Strontium titanate (STO) The optical element according to any one of claims 1 to 13, comprising at least one of:

18. the first polymorphic phase comprises an anatase phase, and the first resonant frequency corresponds to an optical wavelength of about 1,532 nanometers; the second phase comprises a rutile phase and the second resonant frequency corresponds to an optical wavelength of about 1,520 nanometers. The optical element according to any one of claims 1 to 13.

19. the first region forms a first quantum bit; the second region forms a second quantum bit. The optical element according to any one of claims 1 to 13.

20. the first quantum bit interacts with a first optical field having the first resonant frequency; the second quantum bit interacts with a second optical field having the second resonant frequency.

20. The optical element according to claim 19.

21. the thin film structure comprises a first undoped buffer layer and a second undoped buffer layer; the doped first region is disposed between the first undoped buffer layer and a second undoped buffer layer. The optical element according to any one of claims 1 to 13.

22. 1. A method for fabricating an optical element, the method comprising: Providing a substrate; disposing a thin film structure on the substrate, the thin film structure comprising: a first region doped with a rare earth based optically active ion and having a first polymorphic phase, the first polymorphic phase resulting in a first resonant frequency of the optically active ion being modified by the first polymorphic phase; a second region doped with an optically active ion of the rare earth system and having a second polymorphic phase, the second polymorphic phase causing a second resonant frequency of the optically active ion to be modified by the second polymorphic phase, the first resonant frequency being shifted from the second resonant frequency; To have A method comprising:

23. The first region and the second region are Along the growth direction, in the same thin film layer, in different in-plane portions of the thin film layer; 23. The method of claim 22.

24. The first region and the second region are depositing the thin film layer doped with the rare earth-based optically active ions onto the substrate at a growth temperature, thereby forming the thin film layer in the first polymorphic phase; forming the second region in the thin film layer by localized thermal annealing at an annealing temperature higher than the growth temperature and converting the second region of the thin film layer from the first polymorphic phase to the second polymorphic phase; forming the first region with a remainder of the thin film layer; 24. The method of claim 23, wherein the compound is formed by

25. 25. The method of claim 24, wherein the localized thermal annealing is performed using localized laser annealing.

26. 26. The method of claim 25, wherein the second region has a dimension equal to or less than 1 micron.

27. 26. The method of claim 25, wherein the localized laser annealing is performed using a blue laser with a spot size of about 1 micron with a power of about 500 milliwatts (mW).

28. 24. The method of claim 23, wherein the first region and the second region are deposited at different times in the sequence.

29. The first region and the second region are lithographically patterning a first pattern to expose the substrate in the second region; depositing a first thin film doped with optically active ions of the rare earth system in the first polymorphic phase at a growth temperature on the second region of the substrate; thermally annealing the second region at an annealing temperature, higher than the growth temperature, to transition the second region from the first polymorph to the second polymorph; lithographically defining the first region in the exposed substrate after the thermal annealing, and depositing a second thin film with the rare earth-based optically active ions in the first polymorphic phase and at the growth temperature over the substrate in the first region.

30. The method of claim 28, wherein the compound is formed by

30. 23. The method of claim 22, wherein the first region and the second region are in different layers of the thin film structure.

31. The first region and the second region are depositing, in the first polymorphic phase and at a growth temperature, a first thin film doped with optically active ions of the rare earth system onto the substrate; thermally annealing the second region at an annealing temperature, higher than the growth temperature, to transition the second region from the first polymorph to the second polymorph; depositing a second film with said rare earth-based optically active ions over said first film after thermal annealing in said first polymorphic phase at said growth temperature to form said first region in said second film; The method of claim 30, wherein the compound is formed by

32. 23. The method of claim 22, wherein the first region forms a first optical ring resonator and the second region forms a second optical ring resonator within the thin film structure.

33. 33. The method of claim 32, wherein the first optical ring resonator and the second optical ring resonator are coupled to the same optical waveguide within the thin film structure.

34. 23. The method of claim 22, wherein the first region and the second region are formed within the same optical Fabry-Perot cavity that is additionally formed within the thin film structure.

35. The thermal annealing process includes the following steps: Electrical heating using a gate electrode, or Optical heating due to light absorption 32. The method of any one of claims 24, 25, 29, and 31, comprising at least one of:

36. 32. The method of any one of claims 24, 25, 29 and 31, wherein the annealing temperature is above 600 degrees Celsius and the thermal annealing is performed for a predetermined duration.

37. the first region and the second region comprise titanium dioxide; the rare earth ions comprise erbium ions; 35. The method according to any one of claims 22-34.

38. The substrate is silicon, Sapphire, or Strontium titanate (STO) The method of any one of claims 22-34, comprising at least one of:

39. the first polymorphic phase comprises an anatase phase, and the first resonant frequency corresponds to an optical wavelength of about 1,532 nanometers; the second phase comprises a rutile phase and the second resonant frequency corresponds to an optical wavelength of about 1,520 nanometers.

35. The method according to any one of claims 22-34.

40. the first region forms a first quantum bit; the second region forms a second quantum bit.

35. The method according to any one of claims 22-34.

41. the first quantum bit interacts with a first optical field having the first resonant frequency; the second quantum bit interacts with a second optical field having the second resonant frequency.

41. The method of claim 40.

42. 35. The method of any one of claims 22-34, further comprising controlling a property of the first region by depositing a first undoped buffer layer and a second undoped buffer layer, the doped first region being disposed between the first undoped buffer layer and the second undoped buffer layer.

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