EP4433631A1 - Engineering doped thin-film structure and device - Google Patents

Abstract

The disclosure is directed to methods for fabricating a doped thin-film structure 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 optically active ions of a rare-earth type 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 optically active ions of the rare-earth type 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, wherein the first resonant frequency is shifted from the second resonant frequency.

Description

ENGINEERING DOPED THIN-FILM STRUCTURE AND DEVICE

RELATED APPLICATION

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

GOVERNMENT LICENSE RIGHTS

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

BACKGROUND

[0003] A same material host having different polymorphic phases are known to affect optical property of doped ions. Such effect may be utilized to implement various types of optical devices include but not limited to quantum information processing devices. Quantum information processing particularly promises advances in communication and computing. The effect of polymorphic phases on optical properties of the doped ions, for example, may be utilized in quantum memory devices to enable a quantum network capable of establishing quantum entanglement-based links over long distances.

SUMMARY

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

[0005] In one embodiment, the present disclosure describes an optical device. The optical device includes a substrate; and an thin-film structure disposed on the substrate. The thin-film structure includes a first region doped with optically active ions of a rare-earth type 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 optically active ions of the rare-earth type 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, wherein the first resonant frequency is shifted from the second resonant frequency.

[0006] In another embodiment, the present disclosure describes a method for fabricating an optical device. The method includes providing a substrate; and disposing an thin-film structure on the substrate. The thin-film structure includes a first region doped with optically active ions of a rare-earth type 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 optically active ions of the rare-earth type 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, wherein the first resonant frequency is shifted from the second resonant frequency.

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

[0008] In some other embodiments, a computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The system, device, product, and/or method described below may be better understood with reference to the following drawings and description of nonlimiting and non-exhaustive embodiments. The components in the drawings are not necessarily to scale. Emphasis instead is placed upon illustrating the principles of the present disclosure.

[0010] FIG. 1 is a schematic diagram of an example optical device.

[0011] FIG. 2 is a flow diagram of a method for fabricating an example optical device. [0012] FIG. 3A is a schematic diagram of another example optical device.

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

3A.

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

[0015] FIG. 4A is a schematic diagram of another example optical device.

[0016] FIG. 4B is a schematic diagram of another example optical device.

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

4B.

[0018] FIG. 5A is a schematic diagram of an example optical resonator device.

[0019] FIG. 5B is a schematic diagram of another example optical resonator device.

[0020] FIG. 6 shows structural characteristics of various polymorphic phases of titanium oxide (polycrystalline on Silicon, epitaxial single crystal on strontium titanate (STO) and r-sapphire (AI2O3)).

[0021] FIG. 7A shows optical characteristics of optically active erbium ions doped in TiO2 having various example polymorphic phases depending on the growth temperature.

[0022] FIG. 7B shows a spectrum from erbium ions in a rutile phase obtained after annealing anatase polycrystalline TiO2 on Silicon at 850C for 30 mins.

[0023] FIG. 8A shows comparison of inhomogeneous linewidth of optical response from various polymorphic phases of TiO2 on Silicon.

[0024] FIG. 8B shows inhomogeneous linewidths of optical response for rutile phases (on Silicon) obtained as-grown and via thermal annealing.

[0025] FIG. 8C shows inhomogeneous linewidth for various polymorphic phases obtained under various growth conditions.

[0026] FIG. 9 demonstrates a role of a bottom buffer between an optically active thin-film and a substrate in improving the inhomogeneous optical linewidth of the active thin-film. [0027] FIG. 10A shows contribution to inhomogeneous optical linewidth as a function of concentration of optically active doped ions.

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

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

[0030] FIG. 12A shows schematic diagrams of an exemplary embodiment in the present disclosure.

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

[0032] FIG. 12C shows characteristic charts of the exemplary embodiment shown in FIG. 12A.

[0033] FIG. 12D shows schematic diagrams of another exemplary embodiment in the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0034] The disclosed systems, devices, and methods will now be described in detail hereinafter with reference to the accompanied drawings that form a part of the present application and show, by way of illustration, examples of specific embodiments. The described systems and methods may, however, 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. This disclosure may be embodied as methods, devices, components, or systems. Accordingly, embodiments of the disclosed system and methods may, for example, take the form of hardware, software, firmware or any combination thereof.

[0035] Throughout the specification and claims, terms may have nuanced meanings suggested or implied in context beyond an explicitly stated meaning. Likewise, the phrase “in one embodiment” or “in some embodiments” as used herein does not necessarily refer to the same embodiment and the phrase “in another embodiment” or “in other embodiments” as used herein does not necessarily refer to a different embodiment. It is intended, for example, that claimed subject matter may include combinations of exemplary embodiments in whole or in part. Moreover, the phrase “in one implementation”, “in another implementation”, or “in some implementations” as used herein does not necessarily refer to the same implementation(s) or different implementation(s). It is intended, for example, that claimed subject matter may include combinations of the disclosed features from the implementations in whole or in part.

[0036] In general, 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 a variety of meanings that may depend at least in part upon the context in which such terms are used. In addition, the term “one or more” or “at least one” as used herein, depending at least in part upon context, may be used to describe any feature, structure, or characteristic in a singular sense or may be used to describe combinations of features, structures or characteristics in a plural sense. Similarly, terms, such as “a”, “an”, or “the”, again, may be understood to convey a singular usage or to convey a plural usage, depending at least in part upon context. In addition, the term “based on” or “determined by” may be understood as not necessarily intended to convey an exclusive set of factors and may, instead, allow for existence of additional factors not necessarily expressly described, again, depending at least in part on context.

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

[0038] In the example 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 offers paradigm shift in communication and computing. Practical quantum information processing based on optical interactions may rely on a quantum mechanical platform that simultaneously possesses long coherence times and narrow optical transitions while allowing for chip-scale integration with photonic structures. Quantum memory devices, in particular, are a key part of quantum information processing and quantum computing, particularly for a quantum network capable of establishing quantum entanglement-based links over long distances. Quantum information may be stored at the level of a small ensemble of atoms or single atom/defects, for example, rare-earth (RE) ions defects in solids. This disclosure describes example devices containing structures that provide these structural and physical characteristics suitable for practical quantum information processing. While the disclosure focuses on quantum information processing application, the devices described below are not so limited. They may be adapted to other optical processing and communications applications other than quantum information processing.

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

[0040] In various embodiments disclosed below, solids doped with RE ions may serve as quantum device/apparatus for processing quantum information, wherein RE ions may serve as quantum bits (qubits). Er doped titanium dioxide (TiO2) thin films may be selected as an exemplary platform for at least one of the following practical considerations. One consideration may be that Er has its first emission in the telecom C band (1530 - 1565 nm) such that optical emission from Er qubits can be directly transmitted over existing optical fibers with minimal losses, thereby enabling long distance quantum entanglement distribution. Another consideration may be that Er ions have narrow 4f-4f transitions, as the 4f shell is shielded by the full 5s and 5p levels which provides protection from the local environment, resulting in long coherence times. [0041] Various embodiments disclosed below include growth of erbium doped TiO2 thin films. Some general methodology of doping and/or growing Er doped thin films are described in PCT Application PCT/US20/21257 filed on March 5, 2020 and U.S. Patent Application No. 17/434,221 filed on August 26, 2021 , both of which are incorporated herein by reference in their entireties. Deposition of TiO2 thin film, for example, may use metal organic precursors through at least one of the following methods, chemical vapor deposition (CVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE). Depending on growth temperature, the growth of TiO2 thin film under various growth conditions may result in at least one of multiple polymorphic phases, for example, an anatase phase or a rutile phase. In some implementations, a thin film may include any type of the following types: epitaxial film, poly-crystalline film, or amorphous film. In some other implementations, a thin film may include more than one types, 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 main polymorphic phases (anatase and rutile phases) depending on growth conditions. These polymorphic phases maybe referred to as “phases” for brevity. Each of the polymorphic phase may corresponds to a particular crystal structure of TiO2. The two phases of TiO2 may result in two different resonant frequencies of the doped Er ions modified by the phases of TiO2. For example, Er ions doped in the anatase TiO2 thin film may have a resonant frequency corresponding to an optical wavelength of approximately 1532 nanometers (nm); and Er ions doped in the rutile TiO2 thin film may have a resonant frequency corresponding to an optical wavelength of approximately 1520 nanometers (nm). Here, “approximately x nm” refers to a range from (x-a) nm to (x+a) nm, inclusive, where a may be any number within, for example, several nanometers.

[0043] Individual erbium atoms, small ensemble or even large ensembles can function as qubits for various applications, for example for quantum information processing.

[0044] The present disclosure describes various example embodiments to achieve at least some of the multiple polymorphic phases in a single nanophotonic structure by a controlled annealing operation. As shown in further detail below, this controlled annealing operation may result in “better quality” films in term of optical homogeneity in addition to provide multiple polymorphic phases in a single device. The various embodiments may include creating at least two regions (both C-band compatible) doped with erbium having different optical resonant frequencies in the single nanophotonic structure. The different optical resonant 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 Stark effect). The various embodiments, for example, may include controlling a spatial distribution of qubits having different optical resonances by selective spatial control of the polymorphic phases, for example via controlled thermal annealing by using spatially selective laser heating or local electric heating, and/or by logographically controlling the thermal annealing process.

[0045] Referring to FIG. 1 , the present disclosure describes various embodiments including a device 100. The device 100 may be an optical device interacting with optical field for manipulating, controlling, processing, and/or reading out information in the device. The device 100 may include a portion 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 type of the following types: epitaxial film, poly-crystalline film, or amorphous film. In some other implementations, the thin film 130 may include more than one types, for example, 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 optically active ions of a rare-earth type and have a first polymorphic phase, and the first polymorphic phase may result in a first resonant frequency of the optically active ions modified by the first polymorphic phase. The second region may be doped with optically active ions of the rare-earth type and have a second polymorphic phase, and the second polymorphic phase may result in a second resonant frequency of the optically active ions 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 same polymorphic phase. In FIG. 1 , the thin-line box around the first region 131 may indicate the first region being in the anatase phase; and/or the thick-line box around the second region 132 may indicate the second region being in the rutile phase. The first region 131 and the second region 132 may be spatially separate. While FIG. 1 shows as an example that region 131 and region 132 are spatial separate in the in-plane direction of the device 100, they may alternative be spatially separate in the growth direction, or in both the in-plane direction and the growth direction (not specifically illustrated in the general depiction of the device 100, but is shown in the specific implementations of FIG. 4A and FIG. 4B below).

[0047] In various embodiments, FIG. 2 shows a flow diagram of a method 200 for fabricating an optical device. The method 200 may include a portion or all of the following: step 210, providing a substrate; and/or step 220, disposing a thin-film structure on the substrate. The thin-film structure includes a first region doped with optically active ions of a rare-earth type 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 optically active ions of the rare-earth type 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, wherein the first resonant frequency is shifted from the second resonant frequency.

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

[0049] For one example, 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 optically active ions of a rare-earth type may include Er ions. The first polymorphic phase may include an anatase phase, and the second polymorphic phase may include a rutile phase. Er ions in the anatase TiO2 thin film may have a resonant frequency corresponding to an optical wavelength of approximately 1532 nanometers (nm); and Er ions in the rutile TiO2 thin film may have a resonant frequency corresponding to an optical wavelength of approximately 1520 nanometers (nm). Here, “approximately x nm” refers to a range from (x-a) nm to (x+a) nm, inclusive, where a may be any number within, for example, several nanometers. [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 the first resonant frequency; and the second quantum bit interacts with a second optical field having the 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 a same light source, which, either simultaneously or sequentially, outputs 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 transition of erbium doped in rutile TiC and anatase TiO2, may allow spectral selections by optical fields between two qubits represented by Er ions doped in the anatase TiCh thin film and the rutile TiO2 thin film. For example, in the situation where the two qubits are spatially close to each other either in plane or in growth direction, an optical field may be tuned to either approximately 1532 nm or 1520 nm to selectively interact with one qubit of the two qubits and being transparent to the other qubit. Here, “approximately x nm” refers to a range from (x-a) nm to (x+a) nm, inclusive, where a may be any number within, for example, several nanometers.

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

[0053] A thermal annealing process may transform a region at an anatase phase to a rutile phase. The thermal annealing may be a local thermal annealing so that only a small portion of the thin film anatase phase is transformed to the rutile phase. The annealing temperature may be a higher temperature above 600 Celsius degrees and the thermal annealing may be performed for a pre-determined duration. The thermal annealing process may be one of or a combination of electrical heating with a gated electrode; and/or optical heating by light absorption. The thermal annealing may be applied locally, by focused light (from, e.g., a laser) or local electric heaters. For example, the gated electrode may include a conductive layer with a specifically- designed spatial pattern deposited onto the thin-film structure, and when an electrical voltage is applied on the gated electrode, the gated electrode may generate heat locally to provide local thermal annealing to particular regions. The local control through laser heating may be used to create regions of interest by focused laser beams or by controlled laser spatial interference patterns. Such controlled annealing creates rutile areas in the otherwise anatase thin-film, thereby bifurcating the thin- film into rutile regions and anatase regions. One TiO2 region essentially become “transparent” to an optical field with a wavelength resonant to the other region.

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

[0055] The spatial resolution of local thermal annealing by laser heating, for example, may be further controlled by the focused laser intensity profile. In some implementations, the absorption of the heating laser beam may be nonlinear. As such, the higher intensity portion of the focused beam may be absorbed more, leading to enhanced local thermal annealing spatial resolution.

[0056] In some implementations, the thermal annealing may include a 30-minute anneal from 750 to 850 Celsius degrees, inclusive. In some other implementations, at a thermal annealing temperature higher than 850 Celsius degrees, the predetermined duration in a thermal annealing may be shorter than 30 minutes, for example, may be as short as a couple of minutes, a few seconds, and/or a fraction of a second.

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

[0058] FIG. 3A shows an embodiment of an optical device 300. The optical device 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) in 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 methods for fabricating the optical device 300, which includes the two regions having two non-overlapping optical transitions with shifted wavelengths in the same dopant-host system on a single nanophotonic device.

[0060] FIG. 3B shows one example embodiment for fabricating the optical device 300 of FIG. 3A. The method 200 of FIG. 2 may be adapted to further include a portion or all of the following.

[0061] A first step includes depositing the thin-film layer doped with optically active ions of the rare-earth type on the substrate at a growth temperature. The growth temperature may be a relatively low temperature (e.g., 550 C), thereby the thin-film layer 330 is formed in the anatase phase, including region 332a in anatase phase.

[0062] A second step includes applying local thermal annealing 340 (local laser heating or electric heating as described above) to the region 332a in the thin-film layer at an annealing temperature higher than the growth temperature to convert the region 332a of the thin film in anatase phase into rutile region 332b (second polymorphic phase) to form the second region 332.

[0063] A third step includes forming the first region 331 having anatase phase (the first polymorphic phase) within the remaining portion of the thin-film layer. In some implementations, the portion other than the second region 332 may form the first region 331 . In some other implementations, the first region 332 and the second regions 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 via, for example, lithographical means (e.g., etching, dissolving, or lifting off).

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

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

[0066] A first step includes using a lithographical method (e.g., lithographically patterning a first patterned layer to expose the substrate at one or more certain region), depositing a first thin film doped with optically active ions of the rare-earth type on the substrate at a growth temperature in a lithographically defined region (e.g., region 332d). The lithographical method may use a photoresist or sacrificial layer patterned with opening at region 332d deposition of the first thin film. The growth temperature may be a relatively low temperature, thereby the a lithographically defined region (the region 332d) is formed in the anatase phase.

[0067] A second step includes applying thermal annealing 340 to the device such at an annealing temperature higher than the growth temperature to convert the region 332d of the thin film from the anatase phase into the rutile phase, as indicated by 332e, forming the second region 332. The thermal annealing here can but need not be local as the patterned thin film layer is only present at the second region 332.

[0068] A third step includes lithographically defining the first region within the exposed the substrate and depositing a second thin film with optically active ions of the rare-earth type at the growth temperature in the first polymorphic phase over the substrate at the first region 331 . The growth temperature may be a relatively low temperature, thereby the first region 331 is formed in the anatase phase. Lithographical 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 at the second step above, spatially patterning and etching away the photoresist or sacrificial layer over the first region 331 to expose the substrate under the first region 331 before depositing the second thin film over the first region 331 and the remaining photoresist or sacrificial layer, and then lifting-off above the photoresist layer that covers the regions other than the first region, to form the structure of the third step of FIG. 3C.

[0069] Alternatively in the example flow of FIG. 3C, the first thin-film may be deposited before lithographical patterning of region 332d at the growth temperature to form a anatase thin-film. The first thin-film may then be annealed and converted into rutile thin film. The annealed first thin-film may then be processed by lithography to form the region 332 with rutile phase, e.g., by etching the rest regions other than the region 332.

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

[0071] FIG. 4A shows an embodiment of an example optical device 400. The optical device 400 may include a substrate 410 and a thin-film structure 430. The thin-film structure may include more than one thin-film layers: a first thin-film layer 440 and a second thin-film layer 450. A first region 431 and a second region 432 may be disposed in different thin-film layers along an 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] Referring to FIG. 4A, in some embodiments, the first region and the second region in the optical device 400 may be at different in-plane portions of the two thin-film layers.

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

[0074] Various embodiments may be used to fabricate the optical device 400 or 401 , hereby creating two non-overlapping transitions with shifted wavelengths in the same dopant-host system on a single nanophotonic device.

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

[0076] A first step includes depositing a first thin film 440 doped with optically active ions of the rare-earth type on the substrate 410 at a growth temperature. The growth temperature may be a relatively low temperature, thereby the first thin-film layer 440 including region 432a formed in the anatase phase. [0077] A second step includes thermally annealing 450 the second region at an annealing temperature higher than the growth temperature to transform the region 432a from the anatase phase into the rutile phase region 432b to form the second region 432. The thermal annealing may be performed locally.

[0078] A third step includes depositing a second thin film 450 with optically active ions of the rare-earth type at the growth temperature over the first thin film and form the first region 431 in the second thin film. The growth temperature may be a relatively low temperature, thereby the second thin-film layer 450 including the first region 431 is formed in the anatase phase.

[0079] The underlying principles of device 401 of FIG. 4B above may be extended to more than two thin-film layers in the growth direction. Further, in each of the thin-film layer, the first region 431 and/or the second region 432 may expand the entire thin film in the in-plane direction. The definition of in-plain locations of the first and second regions 431 and 432, if needed, may be achieved via various lithographical processing as described above.

[0080] Without limitation, the first region and the second region in the various example devices above may be same rare-earth ions doped in different rare-earth oxide thin films. The rare-earth oxide may include titanium dioxide (TiC ), strontium titanate (SrTiOs), or yttrium oxide (Y2O3). Alternatively, the first region and the second region in the device may be different rare-earth ions doped in same rare- earth oxide thin films; or the first region and the second region in the device 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 FIGs. 4A and 4B may be any phase of the anatase phase and the rutile phase, and the second region 432 may be the other phase. The second region 332a in FIG. 3B, 332c in FIG. 3C, or 432a in FIG. 4C may be a different polymorphic phase, and upon thermal annealing, is transformed to the rutile phase.

[0082] The present disclosure further describes various embodiments of applications incorporating the devices above in any one or a combination of the above embodiments.

[0083] One embodiment may include significantly increasing the qubit density, for example, doubling the qubit density with lateral configuration and/or vertical configuration by using sequential deposition and anneal method. Another embodiment may include achieving deterministic qubit placement by polymorphic phase conversion on selected areas.

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

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

[0086] The present disclosure describes various embodiments of using thermal annealing to transform a particular region from a first polymorphic phase (an anatase phase) to a second polymorphic phase (a rutile phase). There may be a number of advantages/benefits of thermal annealing in comparison with a rutile phase directly fabricated by depositing a thin film at a higher growth temperature.

[0087] The present disclosure describes various embodiments of controlling properties of one or more qubit by sandwiching a doped layer in between undoped layers, for example but not limited, 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 a 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 qubit in the erbium doped layer may be controlled. For example, as discussed in more details in following sections, by increasing the thickness of the bottom layer, the optical properties (e.g., inhomogeneous optical linewidth) of the one or more qubit in the erbium doped layer gets narrower (i.e. , smaller).

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

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

[0090] Another advantage/benefit may include that performing the deposition at low temperature allows for using photolithographic masks directly for deposition; to lower thermal budget with only short duration with thermal anneal at high temperature in comparison with growing at high temperature for a long time duration; and/or to simplify process cycle with only one type thin film growth (low temperature growth).

[0091] Below, the present disclosure further describes detailed device fabrication methods and details related measurements of the structure and optical characteristics of the fabricated devices.

[0092] Quantum memory devices are a key part of proposed networks capable of establishing quantum entanglement-based links over long distances. The information is stored at the level of a small ensemble of atoms or single atom/defects - e.g. NV center in diamond and Rare-earth defects in YSO among many others. Solid state hosts for these qubits have emerged as a promising qubit platform, where the primary advantage comes from these systems being inherently scalable. The systems may be any one or a combination of more than one embodiment as discussed above. The potential for using semiconductor industry compatible thin films hosts can take advantage of established processes. One important driving force in this area has been key demonstrations of interaction with single atoms using nanophotonic structures - while these demonstrations are proof-of-concept they provide strong support for the potential of a thin film platforms.

[0093] The characterization of Er doped TiO2 thin films may be described in sections below. The choice of Er is motivated by the fact that Er has its first emission in the telecom C band (1530 - 1565 nm), this means that light from Er qubits can be directly transmitted over existing optical fibers with minimal losses thus enabling long distance entanglement distribution. Additionally, Er as a rare earth ion (REI) has narrow 4f-4f transitions, as the 4f shell is shielded by the full 5s and 5p levels which provides protection from the local environment.

[0094] Erbium in TiC in the nanoparticle form has been explored as a solar cell material. Thin film growth of TiO2 may use metal organic precursor on r-plane sapphire. The method is more advantageous than direct evaporation (of Ti) as the TiO2 pressure can be controlled precisely and at much lower temperatures.

Fabrication Methods and Thin-Film Characteristics

[0095] The growth of the example TiO2 film(s) may be performed using Riber oxide MBE system. Titanium tetraisoperoxide (TTIP) was used as the source of titanium. The precursor was bought from Sigma-Aldrich and had a purity of 99.999% (trace metal basis). The amount of TTIP introduced in the chamber was precisely controlled via the precursor vapor pressure via a computer-controlled needle valve. Doping in the film was performed using a metal 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 temperature (480 Celsius degrees (C) - 850 C), as measured by a pyrometer. For growth, temperature was set at 750 C for rutile films and 480 C for the anatase films. Oxygen flow was kept at 0.55 seem to get a chamber background pressure of 5E-6 Torr. The substrates used were, r-plane Sapphire (LATTICE), STO (100) and Si (100), the former two used a 350 nm thick Ta layer on the backside for improved heat transfer between the heater and substrate. For STO and sapphire, the substrate was cleaned using 20 min of O2 plasma on the substrate; for Silicon, RCA clean was followed by an HF dip (removes the native oxide) and the cleanliness of the substrate was signaled by the 2x1 reconstruction. Dopant density was varied by controlling the source temperature (800 C to 1200 C). Films grown were cooled down (from growth temperature to 200 C) with the 0.55 seem oxygen flowing.

[0096] The growth was monitored in-situ using RHEED and ex-situ film characterization was performed using X-ray diffraction (XRD) (Broker D8 discover). Surface characterization was performed using a Broker Dimension Icon atomic force microscope (AFM).

[0097] Broadband off-resonant spectroscopy was performed using a 905 nm excitation laser (QPhotonics QFLD-905-200S). Erbium photoluminescence was collected with a fiber-coupled spectrometer and a liquid nitrogen-cooled InGaAs camera (lsoPlane-320 and PyLoN-IR from Teledyne Princeton Instruments) using a 300 g/mm diffraction grating blazed for 1.2 pm light, achieving a spectral resolution of approximately 0.3 nm.

[0098] The optical characterization setup is a home-built confocal microscope using a 50x objective focused on the thin films. Light is collected/emitted from/into free-space from a single collimator into a polarization maintaining fiber. The fiber is connected to a bidirectional 2x1 acousto-optic modulator switch (Brimrose) which switches between a collection and an excitation path. During excitation, the switch routes light from a tunable C-band laser (PurePhotonics) into free-space to the sample, while during collection the switch routes the photoluminescence into a superconducting nanowire single photon detector (SNSPD). Two additional acousto- optic modulators (AA-Optoelectronic) on the excitation path helps fully attenuate any cross-talk from the excitation below the SNSPD dark noise (< 100 counts/s). Spectra are obtained by sweeping the wavelength of the tunable laser and exciting for a duration between 0.1 and 1 millisecond (ms). The photoluminescence is collected after a delay of 100 ps for a duration between 1 and 15 ms (for transients).

Homogeneous linewidth measurements are realized using an intensity electro-optic modulator (10 GHz, Thorlabs), biased at the quadratic point, to create the probe sidebands. The sideband frequency is set and swept by a driving external microwave signal generator. Film Structural Characteristics

[0099] TiO2 crystallizes into two main phases, anatase (I4i/amd, a = 3.7845 A, c = 9.5143 A; Z = 4) and rutile (P4₂/mnm a = 4.5937 A, c = 2.9587 A; Z = 2). Of these, rutile has been grown epitaxially on r-plane sapphire (012)//TiO2 (101 ), with a mismatch of 3.7% along [010]_nO2 and 6.04% along [101]_nO2.

[00100] Referring to lower half of FIG. 6, transmission electron microscopy (TEM) images (row 1 and row 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 a preferred growth direction as evidenced from the SAD (row 3).

[00101] Similarly, for anatase, STO (100)//TiO2 (001 ) has a mismatch of 3.1 % but due to this epitaxial relationship the growth remains epitaxial. On Silicon (100) however, due to the absence of an epitaxial relationship, it was found that the phase had a strong dependence on temperature and a growth surface was thermodynamically driven - a preferred growth direction was identified for both rutile and anatase. In these growths, while no interfacial layer was seen for TiO₂ growth on r-sapphire and STO, there was a distinct amount of oxide at the interface for the silicon substrate which are thicker for the rutile grown at a higher temperature. This oxide interlayer is attributed to oxidation of Si during the growth, with the higher growth temperature leading to a thicker layer of oxide (2 vs 4 nm).

Optical Characterization

[00102] Erbium in rutile and anatase has two distinct emission peaks as seen in the photoluminescence (PL) signature. Based on this, the temperature ranges over anatase growth may be identified. For the range explored, a distinct phase/temperature dependence was seen.

[00103] As shown in FIG. 7A, the film is primarily rutile at 750 C and above, primarily anatase for 550 C and below and intermediate temperatures give a distribution between the two phases while it is expected to depend on the temperature but haven’t explored here. In FIG. 7A, the Er doped TiO2 (rutile) has a peak ~1520 nm while the anatase phase has a peak ~1532nm. The rutile phase diminishes as the growth temperature is decreased (red: 750 C, blue: 610 C; black: 550 C, cyan: 480 C). [00104] FIG. 7B shows that, for a sample grown at low temperature but annealed at 850 C (5x10E-5 torr of oxygen pressure, 850 C). the low temperature peak disappears.

[00105] When comparing the rutile and anatase phase for the single crystal growth, it’s found that the optical inhomogeneous linewidth is significantly larger for the rutile phase (~ 6x). The larger lattice mismatch between r-sapphire and rutile could be at least partially responsible for this effect. If, however, this was the only reason it might be expected that the linewidths would get closer for the polycrystalline thin films on Si (100). Instead, it gets worse with r_£nft being ~8x larger for rutile as compared to anatase, as shown in FIGs. 8A-8C. The reason is not entirely clear, and it is expected the position of erbium within the crystal structure may play a role. Because of this, polycrystalline anatase on silicon may be picked for study of the effect of the interface, concentration, and sample temperature.

[00106] FIG. 8B shows homogeneous linewidth form fitting lorentzian to the optical photoluminescence data.

[00107] In FIG. 8C, single crystal rutile’s r_inh is ~ 6x broader than single crystal anatase. For polycrystalline samples this becomes rutile is ~8x broader than anatase.

Engineering the film properties

[00108] The interface may have a significant role on the inhomogeneous linewidth (r_£n£l). Utilizing this, various embodiment may include controlling properties of one or more gubit by sandwiching an erbium doped layer in between undoped layers by changing the thickness of one or more undoped layers. For example, but not limited, the properties of the one or more gubit may include optical properties in term of inhomogeneous linewidth.

[00109] A strong dependence was seen when comparing a “bare” layer (10 nm Er: TiO2 film) with a bottom buffer of 10 nm and a top buffer of 5 nm. Increasing the bottom buffer thickness further improves r_£nft as shown in FIG. 9. The best linewidth in series of ~ 5.2 GHz at 4 Kelvin degrees (K) was found for the film stack with a bottom buffer of 60 nm. In FIG. 9, the series of experiments may demonstrate the role of the bottom buffer in improving the inhomogeneous optical linewidth. The best linewidth was obtained for the 60 nm bottom buffer (inset). [00110] Whether the linewidth was limited by concentration or temperature was also investigated. It was found that in the parameter space the concentration does not seem to be a limiting factor. However, some room for improvement remains with a lower sample temperature.

[00111] FIGs. 10A and 10B, shows the contribution to r_inh coming from concentration and temperature. It was found that the concentration is low enough to not cause major broadening however, there is still room for improvement by going to lower temperatures.

[00112] T2* is another important parameter that characterized potential qubits. Traditionally, it is evaluated via interfacing with a pulse of light. An indirect method described earlier was employed to get an upper limit on T2*. This was used to evaluate the role that top and bottom buffer and dopant concentration play. Compared to r_£nft, T2* is more sensitive to buffers and concentration as shown in FIG. 11.

[00113] Carefully designed experiments further show that the properties of erbium doped thin films may be engineered. Additionally, Si compatibility and fabricability of the film enables device fabrication directly on this platform.

[00114] The better performance of the polycrystalline can’t be fully attributed to fully relaxed nano-crystallites. However, there might be other mechanism at the play that are not fully clear at this point. As capabilities to study the homogeneous linewidth undergo further development, this in turn might also allow studying the nature of these interactions in depts.

Embodiment with local laser heating at submicron scale

[00115] In some implementations, TiO2 exists in two stable phases — anatase and rutile — and local 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 that can be equal to or smaller than 1 micrometer (micron) in length and/or width, for example 0.5 micron, which is considered as sub-micron.

[00116] In some implementations, local phase change of Er-doped TiO2 may be used to affect local environment at micron or sub-micron size level, thus, micro-scale regions enabling Er (or other REI) qubit placement in a quasi-deterministic fashion. For one example, FIGs. 12A-12D demonstrate that a focused laser may be used to get phase change over sub-micron regions by local laser heating.

[00117] Referring to FIG. 12A, panel 1210 shows a schematic drawing of a crosssection of the thin-film/wafer, whose substrate is Silicon and top layer is an Er-doped TiC>2 thin film. The Er-doped TiO2 thin film is the phase of anatase. Panel 1220 shows a schematic drawing of a region (i.e. , “Experimental region”) where the local laser heating/annealing occurs. Chromium markers is used for guiding data collection using X-ray fluorescence. After local laser heating/annealing is performed in the area indicated by the rectangle, subsequent characterization of the region is performed.

[00118] Referring to FIG. 12B, panel 1230 shows an optical image of the patterned area on TiO2 sample (anatase) where a region of 19x19um is defined using chromium fiducials to identify the area. A 500 milliWatt (mW) blue laser with about 1 micrometer (urn) spot size is used to heat small regions separated by about 5 urn with a dwell time of about 1 , 10, 100, and 500 seconds, respectively. Panel 1240 shows Er emission in anatase at approximate 1533 nanometer (nm) and in rutile at approximate 1521 nm. Photoluminescence (PL) emission shows evidence of phase change through differences in emission wavelength corresponding to the area in a rectangle (1242). PL is collected using a bandpass filter centered at the rutile peak.

[00119] In the present disclosure, “about A” may refer to a range around A as A ± 10%*A, inclusive, wherein A is a number.

[00120] Referring to FIG. 12C, panels 1250 and 1260 show more evidence by scanning the region of interest (the rectangle, 1442 in FIG. 12B) using a focused X- Ray beam (40 nm) at the Advanced Photon Source. The size of the grains is measured to be below 100 nm and their locations match the areas modified by the blue laser.

[00121] Referring to FIG. 12D, panels 1270 and 1280 shows various embodiments in the present disclosure may be used to locally define the region of interest e.g., at the anti-node of nanophotonic devices (indicated by a region 1282). As a background for reference, typical photonic waveguides have a width of about 500 nm. Panel 1270 shows a schematic drawing of a fabricated device using ErTiC on silicon-on-insulator. Panel 1280 shows a schematic drawing of phase on another fabricated devices with a local laser heating method as described in the present disclosure.

[00122] While the particular disclosure has been described with reference to illustrative embodiments, this description is not meant to be limiting. Various modifications of the illustrative embodiments and additional embodiments of the disclosure will be apparent to one of ordinary skill in the art from this description. Those skilled in the art will readily recognize that these and various other modifications can be made to the exemplary embodiments, illustrated and described herein, without departing from the spirit and scope of the present disclosure. It is, therefore, contemplated that the appended claims will cover any such modifications and alternate embodiments. Certain proportions within the illustrations may be exaggerated, while other proportions may be minimized. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive.

Claims

1. An optical device, comprising: a substrate; and a thin-film structure disposed on the substrate, the thin-film structure comprising: a first region doped with optically active ions of a rare-earth type 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 optically active ions of the rare-earth type 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, wherein the first resonant frequency is shifted from the second resonant frequency.

2. The optical device according to claim 1 , wherein the first region and the second region: are among a same thin-film layer along a growth direction; and are at different in-plane portions of the thin-film layer.

3. The optical device according to claim 2, wherein the first region and second region are formed by: depositing the thin-film layer doped with optically active ions of the rare-earth type on 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 local thermal annealing at an annealing temperature higher than the growth temperature to convert the second region of the thin-film layer from the first polymorphic phase into the second polymorphic phase; and

25 forming the first region with remaining portion of the thin-film layer.

4. The optical device according to claim 3, wherein the local thermal annealing is performed using local laser annealing.

5. The optical device according to claim 4, wherein the second region has a dimension equal to or smaller than 1 micrometer (micron).

6. The optical device according to claim 4, wherein the local laser annealing is performed with a blue laser with a spot size of about 1 micron with a power of about 500 milliWatt (mW).

7. The optical device according to claim 2, wherein the first region and the second region are deposited at different times in sequence.

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

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

10. The optical device according to claim 9, wherein the first region and the second region are formed by: depositing a first thin film doped with optically active ions of the rare-earth type on the substrate at a growth temperature in the first polymorphic phase; thermally annealing the second region at an annealing temperature higher than the growth temperature to transform the second region from the first polymorphic phase into the second polymorphic phase; and after thermal annealing, depositing a second thin film with optically active ions of the rare-earth type at the growth temperature in the first polymorphic phase over the first thin film and form the first region in the second thin film.

11 . The optical device according to claim 1 , 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.

12. The optical device according to claim 11 , wherein the first optical ring resonator and the second optical ring resonator are coupled to a same optical waveguide within the thin-film structure.

13. The optical device according to claim 1 , wherein the first region and the second region are formed within a same optical Fabry-Perot resonator additionally formed within the thin-film structure.

14. The optical device according to any one of claims 3, 4, 8, and 10, wherein the thermal annealing process comprises at least one of the following: electrical heating with a gated electrode; or optical heating by light absorption.

15. The optical device according to any one of claims 3, 4, 8, and 10, wherein: the annealing temperature is above 600 Celsius degrees and the thermal annealing is performed for a pre-determined duration.

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

17. The optical device according to any one of claims 1 to 13, wherein the substrate comprises at least one of: silicon; sapphire; or strontium titanate (STO).

18. The optical device according to any one of claims 1 to 13, wherein: the first polymorphic phase comprises an anatase phase, and the first resonant frequency corresponds to an optical wavelength of approximately 1532 nanometers; and the second phase comprises a rutile phase, and the second resonant frequency corresponds to an optical wavelength of approximately 1520 nanometers.

19. The optical device according to any one of claims 1 to 13, wherein: the first region forms a first quantum bit; and the second region forms a second quantum bit.

28

20. The optical device according to claim 19, wherein: the first quantum bit interacts with a first optical field having the first resonant frequency; and the second quantum bit interacts with a second optical field having the second resonant frequency.

21 . The optical device according to any one of claims 1 to 13, wherein: the thin-film structure comprises a first undoped buffer layer and a second undoped buffer layer, and the doped first region is disposed between the first and second undoped buffer layers.

22. A method for fabricating an optical device, the method comprising: providing a substrate; and disposing a thin-film structure on the substrate, the thin-film structure comprising: a first region doped with optically active ions of a rare-earth type 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 optically active ions of the rare-earth type 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, wherein the first resonant frequency is shifted from the second resonant frequency.

23. The method according to claim 22, wherein the first region and the second region:

29 are among a same thin-film layer along a growth direction; and are at different in-plane portions of the thin-film layer.

24. The method according to claim 23, wherein the first region and second region are formed by: depositing the thin-film layer doped with optically active ions of the rare-earth type on 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 local thermal annealing at an annealing temperature higher than the growth temperature to convert the second region of the thin-film layer from the first polymorphic phase into the second polymorphic phase; and forming the first region with remaining portion of the thin-film layer.

25. The method according to claim 24, wherein the local thermal annealing is performed using local laser annealing.

26. The method according to claim 25, wherein the second region has a dimension equal to or smaller than 1 micrometer (micron).

27. The method according to claim 25, wherein the local laser annealing is performed with a blue laser with a spot size of about 1 micron with a power of about 500 milliWatt (mW).

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

29. The method according to claim 28, wherein the first region and the second region are formed by:

30 lithographically patterning a first pattern to expose the substrate at the second region; depositing a first thin film doped with optically active ions of the rare-earth type at the second region on the substrate at a growth temperature in the first polymorphic phase; thermally annealing the second region at an annealing temperature higher than the growth temperature to transform the second region from the first polymorphic phase into the second polymorphic phase; and after the thermal annealing, lithographically defining the first region within the exposed the substrate and depositing a second thin film with optically active ions of the rare-earth type at the growth temperature in the first polymorphic phase over the substrate at the first region.

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

31 . The method according to claim 30, wherein the first region and the second region are formed by: depositing a first thin film doped with optically active ions of the rare-earth type on the substrate at a growth temperature in the first polymorphic phase; thermally annealing the second region at an annealing temperature higher than the growth temperature to transform the second region from the first polymorphic phase into the second polymorphic phase; and after thermal annealing, depositing a second thin film with optically active ions of the rare-earth type at the growth temperature in the first polymorphic phase over the first thin film and form the first region in the second thin film.

31

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

34. The method according to claim 22, wherein the first region and the second region are formed within a same optical Fabry-Perot resonator additionally formed within the thin-film structure.

35. The method according to any one of claims 24, 25, 29, and 31 , wherein the thermal annealing process comprises at least one of the following: electrical heating with a gated electrode; or optical heating by light absorption.

36. The method according to any one of claims 24, 25, 29, and 31 , wherein: the annealing temperature is above 600 Celsius degrees and the thermal annealing is performed for a pre-determined duration.

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

38. The method according to any one of claims 22 to 34, wherein the substrate comprises at least one of: silicon;

32 sapphire; or strontium titanate (STO).

39. The method according to any one of claims 22 to 34, wherein: the first polymorphic phase comprises an anatase phase, and the first resonant frequency corresponds to an optical wavelength of approximately 1532 nanometers; and the second phase comprises a rutile phase, and the second resonant frequency corresponds to an optical wavelength of approximately 1520 nanometers.

40. The method according to any one of claims 22 to 34, wherein: the first region forms a first quantum bit; and the second region forms a second quantum bit.

41 . The method according to claim 40, wherein: the first quantum bit interacts with a first optical field having the first resonant frequency; and the second quantum bit interacts with a second optical field having the second resonant frequency.

42. The method according to any one of claims 22 to 34, further comprising: controlling a property of the first region by depositing a first undoped buffer layer and a second undoped buffer layer, wherein the doped first region is disposed between the first and second undoped buffer layers.

33