Manufacturing of doped transparent polycrystalline ceramic materials

Abstract

Solution synthesis of rare earth doped nanoparticles and subsequent flash sintering of the nanoparticles may result in rare earth doped transparent polycrystalline ceramics having beneficial optical and / or mechanical properties, such as low optical loss, high optical coherence, high refractive index controllability, and / or high mechanical strength. In one example application, high quality erbium doped yttrium oxide for quantum memory devices can be fabricated.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of priority of U.S. Provisional Application No. 63 / 421,253, filed on November 1, 2022, under 35 U.S.C. § 119, the content of which is incorporated herein by reference in its entirety. Technical field

[0003] The present disclosure relates to rare - earth - doped transparent ceramics and methods for making the same. Background art

[0004] Quantum memories have become a hot research area in applications of quantum communication and quantum computing. In quantum communication, a key research area is the creation of quantum repeater systems, and quantum memories form an important element of such systems. Another application of quantum memories, which is relevant to both quantum communication and quantum computing, is single - photon sources, which use quantum memories to store the photon states from probabilistic sources for later release upon deterministic triggering.

[0005] Erbium - doped materials are ideal candidates for use in quantum memories. Erbium memories operate using photons in the 1.5 - μm telecommunication band, and studies have shown that the memory - state lifetime (T2) can be up to the millisecond level, meeting the requirements of quantum repeater systems. In the past, the best performance of quantum - state lifetimes has typically been achieved using single - crystal materials, and more recently it has been shown that the lifetimes in polycrystalline ceramic materials can be comparable to those in single - crystal devices. For example, erbium - doped ceramic materials that can maintain long quantum - state lifetimes and are beneficially also optically transparent can be fabricated using, for example, a hot isostatic pressing (HIP) process. Although the HIP process produces high - quality materials, it is slow and requires relatively expensive equipment. In addition, the long processing times and high temperatures of HIP cause a large - scale diffusion of elements within the material, which limits the ability to fabricate ceramic materials with a composition gradient throughout the material. For example, a composition gradient may be desirable for constructing waveguides, which are useful structures for quantum systems. Brief description of the drawings

[0006] Described herein are methods for forming rare - earth - doped transparent polycrystalline ceramic materials. Various embodiments are described with reference to the accompanying drawings, in which:

[0007] Figure 1A and 1B is a flow chart of a method for forming a transparent polycrystalline ceramic material from rare - earth - doped nanoparticles by flash sintering.

[0008] Figure 2A and 2B is a flow chart of a method for synthesizing rare - earth - doped nanoparticles. Detailed implementation manners

[0009] This document describes a process for producing high-quality transparent rare-earth-doped polycrystalline ceramics using nanoparticles for flash sintering. Flash sintering belongs to the general category of field-assisted sintering techniques (FAST), which use electric or electromagnetic fields to increase the sintering rate, i.e., the rate at which particulate materials are coalesced and densified into a porous or solid mass. In flash sintering, at certain combinations of temperature and electric field strength, heat and a direct current (DC), alternating current (AC), or pulsed electric field are applied across the material simultaneously, causing a sudden increase in the conductivity of the material and the resulting current flowing through the material, which is accompanied by a sudden and significant increase in the sintering rate. Compared with other sintering techniques, which may take several hours, including other field-assisted sintering techniques, flash sintering can densify ceramic materials in an extremely short time, e.g., from a few seconds to a few minutes. Additionally, compared with other sintering techniques, flash sintering can generally be carried out at much lower temperatures (e.g., hundreds of degrees Celsius lower), and in some instances, sintering can be achieved even near room temperature. Advantageously, this sharp reduction in process time and process temperature can result in a significant reduction in energy consumption and economic costs compared with, e.g., HIP and other conventional processes.

[0010] In addition to providing economic advantages, flash sintering also helps to produce microstructures and beneficial material properties that cannot be achieved or cannot be achieved to the same extent by other sintering methods. The shortened process time and reduced temperature suppress crystal grain growth and pore encapsulation in polycrystalline materials, which increases transparency as well as mechanical strength. The increased transparency, in turn, can expand the range of ceramic materials suitable for quantum optics or other optical applications, e.g., allowing the use of asymmetric polycrystalline ceramics. Furthermore, the short process time and lower temperature employed in flash sintering also suppress dopant diffusion and aggregation at grain boundaries, achieving a more uniform dopant distribution, which is beneficial for optical coherence properties and for quantum state lifetimes in quantum optical applications. The greater dopant uniformity also enables better control and adjustability of the refractive index profile in the material, e.g., facilitating the fabrication of layered ceramic waveguide structures with a high refractive index contrast between the core and the cladding. Flash sintering can also produce ceramics with unbalanced compositions, i.e., dopant concentrations higher than the dopant solubility in the polycrystalline matrix material, which further increases refractive index adjustability.

[0011] The flash-sintering-based manufacturing process described herein is applicable to a wide variety of transparent polycrystalline ceramics, typically comprising a polycrystalline matrix (or lattice) of metal compounds such as metal oxides or salts. Specifically (but not exclusively) herein are covered compounds of transition metals (i.e., elements in the d-block of the periodic table), such as yttrium (Y), zirconium (Zr), or hafnium (Hf), as well as compounds of main group metals such as aluminum (Al), calcium (Ca), or magnesium (Mg). The transparent polycrystalline ceramics can include one or more rare-earth dopants distributed throughout the polycrystalline metal compound matrix. The rare-earth dopants include "inner transition metals" (i.e., elements in the f-block of the periodic table), including lanthanides (e.g., lanthanum (La) and erbium (Er)) and actinides, as well as yttrium and scandium (Sc).

[0012] The rare-earth element dopants in the lattice can have a shaped spectral structure corresponding to a superposition of states that can transition between energy levels, as well as long optical coherence, making them suitable for storing single photons in quantum memory devices (e.g., as described in U.S. Patent No. 10,304,536, which is incorporated herein by reference). The rare-earth dopants can also be used to adjust the refractive index of the material. In some applications, the polycrystalline material is co-doped with a first rare-earth dopant for photon storage and a second dopant for refractive index adjustment. An example material suitable for quantum memory applications is erbium-doped yttrium oxide (Er-Y2O3), optionally co-doped with lanthanum, e.g., used as a waveguide core in a quantum memory system. Other materials that can be used in quantum optics or other applications include, for example, rare-earth-doped cerium oxide (CeO2), yttrium orthovanadate (YVO4), yttrium aluminum garnet (Y3Al5O12), yttrium silicate (Y2SiO5), yttrium titanate (Y2Ti2O7), calcium tungstate (CaWO4), strontium tungstate (SrWO4), lanthanum trifluoride (LaF3), and lithium yttrium fluoride (LiYF4). Of course, as will be understood by those skilled in the art, the applications of the rare-earth-doped transparent polycrystalline ceramics described herein are not limited to quantum memory. Other potential application areas include, but are not limited to, gain media for solid-state lasers, scintillators, ceramic phosphors, and infrared windows.

[0013] In various embodiments, the polycrystalline ceramic is formed from rare-earth-doped nanoparticles having a narrow diameter distribution, with an average diameter less than 200 nm, less than 100 nm, or less than 50 nm (e.g., about 40 nm) and a standard deviation of no more than 20 nm. Such nanoparticles can be synthesized from compounds of base metals (e.g., transition metals) and rare-earth metals (e.g., salts or coordination complexes) of a polycrystalline matrix material by heating the metal compounds in water and mixing them with an organic precursor (e.g., urea). In some embodiments, the metal compounds, water, and organic precursor are heated together in a mixture; in other embodiments, the metal compounds and water are preheated and subsequently mixed with the organic precursor to initiate the formation of rare-earth-doped nanoparticles. The benefit of the latter case is that smaller nanoparticle diameters can often be achieved. Using small-diameter nanoparticles as starting materials and maintaining extremely limited grain growth in a flash sintering process enables transparent rare-earth-doped polycrystalline ceramic materials with grain sizes in the sub-micron range, e.g., an average grain size less than 1 µm, less than 500 nm, less than 100 nm, or even less than 50 nm.

[0014] Furthermore, by doping the nanoparticles themselves, rather than relying on dopant diffusion in conventional sintering processes, atomic-level homogeneous rare-earth dispersion can be achieved, and the trade-off between the homogeneity of rare-earth distribution and grain size throughout the polycrystalline matrix can be avoided. "Homogeneous distribution" of rare-earth dopants is understood herein to mean a distribution in which at least 50% of the rare-earth dopants are located away from grain boundaries within the grains of the polycrystalline matrix. In various embodiments, due to the short process time and simultaneous suppression of dopant diffusion towards grain boundaries, a significantly higher degree of homogeneity can be achieved, e.g., 80% or more, or even 95% or more of the rare-earth dopants are located within the grains and away from grain boundaries.

[0015] Turning now to the drawings, Figure 1A and 1B are flowcharts of methods 100, 102 for forming transparent polycrystalline ceramic materials with rare-earth-doped nanoparticles by flash sintering. Referring to Figure 1A , method 100 begins, for example, by using the processes described below with reference to Figure 2A and 2BSynthesize rare earth-doped nanoparticles (120) by any of the described methods 200, 202. The nanoparticles can have a narrow size distribution, for example, with an average diameter less than 200 nm, less than 150 nm, less than 100 nm, less than 50 nm, or less than 40 nm. In one example, erbium-doped yttrium oxide nanoparticles with a size of approximately 40 nm are fabricated. The nanoparticles can be pressed into pellets (122). For example, a sample of the nanoparticles can be uniaxially pressed, for instance, pressed in a ¾-inch steel die with a force of approximately 8 k pounds, and then further isostatically pressed, for example, using an isostatic sheath at approximately 25 kpsi.

[0016] Subsequently, flash sintering (124) is performed on the nanoparticle pellets. For this purpose, the pellets can be placed on a heating surface and connected between two electrodes (e.g., platinum electrodes), and the two electrodes apply a DC, AC, or pulsed voltage across the pellets to generate an electric field in the material, and the electric field, for example, has a field strength in the range from 5 V / cm to 1000 V / cm. In some embodiments, the electric field is kept constant, for example, maintained at 500 V / cm, while heating the furnace, for example, at a constant heating rate between 1 and 100 °C / minute until a current through the sample is observed, indicating a sudden increase in material conductivity and the onset of flash sintering. In other embodiments, the temperature is fixed, and in fact, the electric field ramps up, for example, at a constant rate until the current and flash sintering begin. After the onset of flash sintering, the power supply generating the electric field is switched from voltage control to current control. For example, the current can be set in the range between 10 mA and 10 A. A larger sintering current generally results in faster sintering. The current can be maintained, and sintering is allowed to continue for a period of time. Generally, depending on the sintering current and the material, sintering is completed within a time period between a few seconds and a few minutes (e.g., less than ten minutes, less than one minute, less than thirty seconds, or less than 10 seconds). Therefore, after this time, the furnace and the power supply can be turned off. Optionally, in some embodiments, conventional sintering (e.g., HIP) is performed after flash sintering, and the conventional sintering is carried out for a shorter period and / or at a lower temperature compared to the case without the previous flash sintering step.

[0017] To illustrate the performance and effects of flash sintering, in one example, pellets of 40 nm sized Er-Y2O3 nanoparticles were placed in an electric field of 500 V / cm and heated in a furnace at a heating rate of 10 °C / min. At the beginning, no current was detected on the sample. At 1218 °C, a strong current appeared, indicating the start of flash sintering. Flash sintering was allowed to continue for sixty seconds at a current of 30 mA, and then the electric field and the furnace were switched off. For comparison, a second pellet of the same type was placed in the furnace for the same thermal process, but without applying an electric field. After this process was completed, the densities of the two samples were compared. The density of the flash-sintered pellet was 26% higher than that of the pellet sintered only by heat.

[0018] Now referring to Figure 1B , method 102 similarly begins, for example, by synthesizing rare-earth doped nanoparticles (120) using any one of methods 200, 202 described below with reference to Figure 2A and 2B . Then, the nanoparticles can be dispersed into a slurry (140), for example, by adding a suitable solvent (e.g., ethanol or deionized water) to the nanoparticle sample and grinding the mixture with a suitable grinding medium (e.g., yttria-stabilized zirconia grinding medium) for about dozens of hours. The slurry can be separated from the grinding medium by filtration and mixed with one or more binders and / or plasticizers. Optionally, the slurry can be rolled to remove air. Subsequently, the slurry can be cast into a film (142) using a suitable casting process such as tape casting or spin coating. After casting, the film can be dried, for example, by drying in air with a cover for about one day, followed by drying in an oven for about half an hour. In this way, thin nanoparticle films with a thickness in the range from a few microns to dozens of microns (e.g., less than 100 µm) can be fabricated. In some embodiments, multiple nanoparticle films of different compositions can be stacked together to produce a layered structure. Finally, the nanoparticle film or the layered stack of films is flash sintered (124) in the same manner as described above with reference to Figure 1B to produce a rare-earth doped transparent polycrystalline ceramic (optionally followed by conventional sintering).

[0019] Figure 2A and 2B are flowcharts of methods 200, 202 for synthesizing rare-earth doped nanoparticles. Referring to Figure 2A, Method 200 involves mixing a compound (such as a salt or complex) of a first metal (e.g., a transition metal) that is part of a polycrystalline matrix and a second metal (or second metals) that will constitute the rare-earth dopant with water (e.g., deionized water) and an organic precursor (e.g., urea, ammonium hydroxide, or the like) to form a precursor mixture (220). Subsequently, the precursor mixture is heated (222), for example, to a temperature between 70 °C and 100 °C for a period between half an hour and several hours, to initiate the formation of precursor rare-earth-doped nanoparticles in the solution. During the heating process, thermal decomposition of the organic precursor can produce OH - and CO 3 2- ions, which react with the transition metal and rare-earth metal in the metal compound to form precursor rare-earth nanoparticles. The precursor nanoparticles can be filtered and collected (224), and then annealed (226) in air or at a high temperature (e.g., between 500 °C and 900 °C) to convert the precursor nanoparticles into the final rare-earth-doped nanoparticles, which have a crystalline structure. For example, the precursor mixture can contain yttrium chloride complex YCl 3 .6H 2 O and erbium chloride complex ErCl 3 .6H 2 O, and can be heated into Y 1-x Er x (OH)CO 3 .H 2 O precursor nanoparticles, which are then converted into (Y 1- x Er x ) 2 O 3 nanoparticles.

[0020] Figure 2B Illustrates a modified nanoparticle synthesis process 202, which begins by mixing a metal compound (such as a salt or complex) of a first metal (e.g., a transition metal) that is part of a polycrystalline matrix and a second metal (or second metals) that will constitute the rare-earth dopant in water (e.g., deionized water) to form a metal compound solution (240), and then preheating the solution (242), for example, to a temperature between 70 °C and 100 °C, and then adding an organic precursor (e.g., urea) (244). The temperature of the organic precursor at the time of mixing can be less than, the same as, or greater than the temperature of the metal compound solution. Adding the organic precursor to the solution initiates the formation of precursor rare-earth-doped nanoparticles in the solution. Compared with that by Figure 2ACompared with the precursor nanoparticles generated by process 200, preheating the metal compound solution can result in a smaller nanoparticle diameter. After the precursor nanoparticles are formed, process 202 continues similarly to process 200, filtering and collecting the precursor nanoparticles (224), followed by annealing (226) to obtain the final rare-earth-doped nanoparticles.

[0021] In either process 200 or 202, the concentration of the metal compound in the precursor mixture or solution determines the size of the nanoparticles formed. A lower concentration generally results in a smaller nanoparticle diameter. In addition, the ratio of the first (transition) metal and the second rare-earth metal compound in the solution determines the dopant level within the polycrystalline ceramic. The amount of the rare-earth metal compound (e.g., measured in grams) can be about two or three orders of magnitude lower than the amount of the matrix metal compound, resulting in a dopant level of less than 1%, but higher dopant levels are also possible, including dopant levels greater than 10%.

[0022] The described process for synthesizing rare-earth-doped nanoparticles and flash sintering them into ceramics can produce high-quality transparent rare-earth-doped polycrystalline ceramics, the energy and economic costs of which are generally lower than those of conventional sintering processes (e.g., HIP). In addition, in various embodiments, compared with the transparent rare-earth-doped polycrystalline materials produced by other similar sintering processes, the resulting ceramics benefit in various ways from smaller grain sizes, lower porosity, and / or more uniform dopant distribution.

[0023] The short process time and reduced process temperature employed in flash sintering can contribute to reducing the optical loss (e.g., a loss of less than 0.5 dB / cm) of the resulting ceramics in two aspects, which corresponds to higher transparency. On the one hand, flash sintering reduces the phenomenon of pores being encapsulated into the grains. The pores inside the grains cannot be removed by conventional sintering, and they are not conducive to the transparency of the sintered ceramics. By reducing and possibly largely eliminating such residual pores, flash sintering can produce ceramics with higher transparency.

[0024] Another effect that is beneficial to transparency is that flash sintering minimizes the growth of crystal grains, resulting in ceramics with smaller grain sizes. A sufficiently small grain size makes it possible to fabricate transparent ceramics with an asymmetric crystal structure. Previously, transparent ceramics were limited to symmetric materials with isotropic refractive indices (mostly with a cubic crystal structure). In contrast, in asymmetric polycrystalline ceramics (e.g., with a monoclinic or triclinic crystal structure), randomly oriented grains scatter light, thereby reducing transparency. By reducing the grain size to a small fraction (e.g., less than one-tenth) of the wavelength at which the rare-earth dopant is active and the device made of the ceramic material operates, such scattering can be reduced and thus transparency can be improved. Therefore, if asymmetric ceramics are fully densified at a crystal size of less than 100 nm, as may be the case for flash sintering of small (e.g., 30 nm in diameter) nanoparticles, they can be adapted to working wavelengths as low as about one micron or greater than one micron, including the 1.5 µm telecommunication band that is of great practical significance. Expanding the candidate materials to ceramics with asymmetric crystals helps to utilize the advantageous properties of such materials. An example of an asymmetric material with a monoclinic crystal structure is yttrium orthosilicate (Y2SiO5), which has been shown to be a good host material for rare-earth dopants in optical quantum research.

[0025] Due to its short process time and low temperature, flash sintering also inhibits or minimizes dopant diffusion, thereby improving optical coherence properties. Transparent ceramics for optical quantum memory applications are typically doped with rare-earth elements, which act as active substances in optical processes. Good performance corresponding to a long optical coherence time is achieved if the rare-earth dopants reside inside the crystal grains and remain well dispersed. By synthesizing rare-earth-doped nanoparticles using solution methods as described herein with reference to Figure 2A and 2B an atomic-level uniform rare-earth dispersion can be achieved within the nanoparticles. However, conventional sintering triggers dopant diffusion towards the grain boundaries, which is an energetically favorable process. The resulting aggregation of rare-earth dopants in the grain boundaries or grain surface layers will be detrimental to the optical coherence properties of the material. On the other hand, flash sintering of rare-earth-doped nanoparticles can inhibit rare-earth dopant diffusion and thus optimize the optical coherence properties of rare-earth-doped ceramics. It should be noted that an alternative method of producing rare-earth-doped ceramics by sintering a mixture of a matrix material (e.g., yttrium oxide) and rare-earth oxide particles relies on certain diffusion processes for incorporating rare-earth ions into the matrix material. Therefore, combining the synthesis of rare-earth-doped nanoparticles and flash sintering those nanoparticles into transparent ceramics according to the methods described herein achieves rare-earth-doped ceramics with good optical coherence.

[0026] Suppressing dopant dispersion associated with flash sintering is also beneficial for adjusting the refractive index and controlling the refractive index profile of transparent polycrystalline ceramic materials. For example, for the purpose of fabricating optical waveguides, the refractive index of rare-earth-doped ceramics can be adjusted by adding one or more second dopants, such as lanthanum (La), lutetium (Lu), scandium (Sc), and / or gadolinium (Gd). Similar to the primary dopants used for photon storage or some other optical process, these refractive index modifiers are added to the nanoparticles through solution synthesis (e.g., as described in references Figure 2A and 2B ) to achieve uniform dispersion. In conventional sintering processes, the diffusion of refractive index modifiers to the grain boundaries often results in a non-uniform refractive index profile and reduces the efficiency of refractive index change. By suppressing dopant diffusion, flash sintering can produce a more uniform refractive index profile and maximize refractive index tunability.

[0027] Suppressing dopant diffusion is also important for waveguide structures fabricated by sintering multiple layers of nanoparticle thin films that can be prepared by tape casting or spin coating. In one example, such a layered waveguide structure can include a waveguide core layer of yttrium oxide doped with lanthanum and erbium (La-Er-Y2O3), which is sandwiched between an undoped Y2O3 lower cladding and an upper cladding. In conventional sintering processes, the diffusion of lanthanum from the core layer to the cladding layers can change the refractive index profile and reduce the refractive index difference between the core and the cladding. Compared with conventional sintering processes, flash sintering can achieve a steeper compositional gradient due to significantly restricted diffusion, thus providing better controllability of the refractive index profile.

[0028] The flash sintering process is also capable of producing doped ceramics with imbalanced compositions, i.e., compositions in which the dopant concentration in the matrix material exceeds the dopant solubility in the matrix material. For example, La-doped Y2O3 transparent ceramics can be fabricated with a lanthanum concentration higher than the solubility of La in Y2O3, about 10%, thus further increasing the refractive index tunability.

[0029] In addition to improving optical properties such as transparency, optical coherence, and refractive index tunability, flash sintering can also improve the mechanical properties of the fabricated ceramics due to smaller crystal grain sizes. This effect is particularly important for thin films with a thickness of about a few micrometers or tens of micrometers. Through HIP or other conventional sintering processes, Y2O3 / La-Er-Y2O3 / Y2O3 nanoparticle thin films can be sintered into a transparent tape with a grain size of about 1-5 micrometers, even if the starting nanoparticle size is as small as 30 nm. Such tapes are fragile and difficult to handle. Flash sintering can produce transparent ceramics with a grain size close to that of the starting nanoparticles, e.g., a grain size below 100 nm. Reducing the grain size to the nanoscale is an effective way to improve the tape strength.

[0030] The possibility of fabricating polycrystalline ceramics with nanocrystalline grain sizes also provides an opportunity to study the effect of crystal size on the optical coherence properties of rare-earth dopants. Such data can be used to guide the development of thin-film-based optical microdevices, which are typically nanoscale.

[0031] Although the invention has been described with reference to specific embodiments, it will be apparent that various modifications and changes can be made to these embodiments without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive.

Claims

1. A method of forming a rare-earth doped transparent polycrystalline ceramic material, the method comprises: synthesizing rare-earth doped nanoparticles having an average diameter of less than 200 nm; producing the rare-earth doped transparent polycrystalline ceramic material by flash sintering a sample of the rare-earth doped nanoparticles for a duration of not more than ten minutes, the flash sintering being carried out using an electric current passing through the sample generated by heating the sample in the presence of an electric field.

2. The method according to claim 1, wherein the electric current is generated by heating the sample in a furnace and gradually increasing the temperature of the furnace until the electric current starts to flow through the sample, and wherein the furnace and the electric field are disconnected at the end of the duration of not more than ten minutes.

3. The method according to claim 1, wherein the sample is flash sintered for a duration of not more than one minute.

4. The method according to claim 1, wherein the sample is flash sintered for a duration of not more than twenty seconds.

5. The method according to claim 1, wherein the temperature of the sample does not exceed 1300 °C during the flash sintering.

6. The method according to claim 1, wherein the rare-earth doped transparent polycrystalline ceramic material has a sub-micron average grain size.

7. The method according to claim 1, wherein the rare-earth doped nanoparticles have an average diameter of less than 100 nm.

8. The method according to claim 1, wherein the rare-earth doped nanoparticles have an average diameter of less than 50 nm.

9. The method according to claim 8, wherein the rare-earth doped transparent polycrystalline ceramic material has an average grain size of less than 100 nm.

10. The method according to claim 1, wherein synthesizing the rare-earth doped nanoparticles comprises: heating a precursor mixture of a first metal compound, a rare-earth metal compound, water and an organic precursor to initiate the formation of rare-earth doped nanoparticles of the precursor in solution; collecting the rare-earth doped nanoparticles of the precursor from the solution by filtration; and annealing the rare-earth doped nanoparticles of the precursor to obtain the rare-earth doped nanoparticles.

11. The method according to claim 1, wherein synthesizing the rare-earth doped nanoparticles comprises: preheating a mixture of a first metal compound, a rare-earth metal compound and water to form a heated metal compound solution; mixing the heated metal compound solution with an organic precursor to form rare-earth doped nanoparticles of the precursor in solution; collecting the rare-earth doped nanoparticles of the precursor from the solution by filtration; and annealing the rare-earth doped nanoparticles of the precursor to obtain the rare-earth doped nanoparticles.

12. The method according to claim 11, wherein the first metal compound comprises a transition metal salt or a transition metal complex, and wherein the rare-earth metal compound comprises a rare-earth metal salt or a rare-earth metal complex.

13. The method according to claim 1, wherein the nanoparticles are erbium-doped yttrium oxide nanoparticles.

14. The method according to claim 1, wherein the rare-earth doped nanoparticles comprise a first rare-earth dopant and a second rare-earth dopant.

15. The method according to claim 14, wherein the first rare-earth dopant is erbium, and the second rare-earth dopant comprises one or more of lanthanum, scandium, lutetium or gadolinium.

16. The method according to claim 1, wherein the rare-earth doped transparent polycrystalline ceramic material comprises rare-earth dopants distributed throughout a polycrystalline metal compound matrix.

17. The method according to claim 16, wherein at least 80% of the rare-earth dopants are located away from grain boundaries inside the grains of the polycrystalline metal compound matrix.

18. The method according to claim 16, wherein at least 95% of the rare-earth dopants are located away from grain boundaries inside the grains of the polycrystalline metal compound matrix.

19. The method according to claim 1, the method further comprising compacting the sample of the rare-earth doped nanoparticles into a pellet before flash sintering.

20. The method according to claim 1, the method further comprising dispersing the sample of the rare-earth doped nanoparticles into a slurry and casting the slurry into a film before flash sintering.

21. A transparent rare-earth doped polycrystalline ceramic material, the transparent rare-earth doped polycrystalline ceramic material comprising: a polycrystalline metal compound matrix having a grain size of less than 1 µm; and rare-earth dopants distributed throughout the polycrystalline metal compound matrix, wherein at least 80% of the rare-earth dopants are located away from grain boundaries inside the grains of the polycrystalline metal compound matrix.

22. The transparent rare-earth doped polycrystalline ceramic material according to claim 21, wherein the metal compound matrix comprises yttrium oxide, and wherein the dopant comprises erbium.

23. The transparent rare-earth doped polycrystalline ceramic material according to claim 21, wherein the metal compound matrix comprises yttrium oxide, and wherein the dopant comprises erbium and one or more of lanthanum, scandium, lutetium or gadolinium.

24. The transparent rare-earth doped polycrystalline ceramic material according to claim 21, wherein the metal compound matrix comprises one of cerium oxide (CeO2), yttrium orthovanadate (YVO4), yttrium aluminum garnet (Y3Al5O12), yttrium silicate (Y2SiO5), yttrium titanate (Y2Ti2O7), calcium tungstate (CaWO4), strontium tungstate (SrWO4), lanthanum trifluoride (LaF3) or lithium yttrium fluoride (LiYF4).

25. The transparent rare-earth doped polycrystalline ceramic material according to claim 21, wherein the metal compound matrix has an asymmetric crystal structure.

26. The transparent rare-earth doped polycrystalline ceramic material according to claim 25, wherein the metal compound matrix comprises yttrium orthosilicate (Y2SiO5).

27. The transparent rare-earth doped polycrystalline ceramic material, wherein the concentration of the rare-earth dopants is higher than the solubility of the rare-earth dopants in the metal compound matrix.

28. A method of fabricating a layered ceramic waveguide structure, the method comprises: synthesizing first metal compound nanoparticles having an average diameter of less than 200 nm; synthesizing second rare-earth doped metal compound nanoparticles having an average diameter of less than 200 nm; tape casting a lower cladding layer of the first metal compound nanoparticles; tape casting a waveguide core layer of the second rare-earth doped metal compound nanoparticles over the lower cladding layer; tape casting a cladding layer of the first metal compound nanoparticles over the waveguide core layer; and flash sintering the lower cladding layer, waveguide core layer, and cladding layer to produce a polycrystalline metal compound matrix having a grain size of less than 500 nm, wherein the waveguide core layer comprises rare-earth dopants distributed throughout the polycrystalline metal compound matrix.

29. The method of claim 28, wherein the first metal compound nanoparticles comprise yttrium oxide nanoparticles, and the second metal compound nanoparticles comprise yttrium oxide nanoparticles doped with erbium and one or more of lanthanum, scandium, lutetium, or gadolinium.

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